WO1999048682A1 - Tarnish resistant article, preparation, manufacture and methods of use of same - Google Patents

Abstract

The present invention is directed to an article having a metal composition (21), preferably silver or copper, and a coating (37) which is resistant to tarnishing due to being coated with a thin, robust, self-assembled monolayer which when applied to an article or substrate is non-toxic, flexible, and solderable. The method of the present invention includes an organic washing step and an etching step with sulfuric acid or its equivalent prior to alkylthiolation.

Description

TARNISH RESISTANT ARTICLE, PREPARATION, MANUFACTURE AND METHODS OF USE OF SAME

FIELD OF THE INVENTION

The present invention relates generally to a tarnish resistant article and methods of forming same. More particularly, the present invention is directed to preferred methods of enhancing the tarnish resistance of a metallic article whereby a protective coating is applied to the surface of the article. It is preferable that the metallic article be comprised of a transition metal, even more preferable that the metallic article be comprised of either silver or copper or a combination thereof, and most preferable that the metallic article or at least the portion exposed to atmospheric conditions consist essentially of silver or silver alloy. Even more particularly, the present invention is directed to the formation of a self-assembled monolayer on a surface of a silver- containing or silver-coated article which is intended to come in contact with mammalian skin (e.g., jewelry). The self-assembled monolayer is useful in preventing further oxidation and sulfidation of the underlying article or substrate.

BACKGROUND Under normal atmospheric conditions, metallic surfaces have a tendency to tarnish. This is especially true in the presence of sulfides, and for those metals which form sulfides preferentially over oxides. Accordingly, silver (Ag) and copper (Cu), in the presence of hydrogen sulfide, are especially susceptible to tarnishing due to their respective oxidative potentials. In the presence of hydrogen sulfide (H₂S), each metal forms the respective sulfide (Ag₂S and Cu₂S) preferentially over the respective oxide (Ag₂O and Cu₂O). The formation of the sulfide on the surface results in tarnishing of the metal article. Even though hydrogen sulfide is at a relatively low concentration in the atmosphere (on the order of parts per billion (ppb)) it is at a sufficient concentration over normal exposure time to render the problem of tarnish formation a major drawback when utilizing these metals; hence the large market for tarnish removers. Most consumers are familiar with this problem as it relates to jewelry and family heirlooms made of silver or brass. As the sulfide layer penetrates the metal surface, the initially light color of the sulfide film becomes darker and darker. In the case of silver sulfide the penetration of the sulfide soon turns the surface greenish brown and finally black. The tarnishing rate of silver, copper, and their respective alloys is accelerated with increasing humidity, and with an increased concentration of sulfur compounds and ammonia in the environment.

Copper and silver are widely used in the manufacture of equipment, especially in the chemical and medical fields (e.g., dentistry) and in many other fields. For example, these metals either alone or in combination, are used to manufacture mirrors, coins, silverware, bearings, circuit boards, electric contacts, jewelry and dental equipment. The widespread use of these metals is primarily due to their chemical stability, high reflectivity, and overall attractive appearance. This is especially true for silver. Silver is also generally resistant to corrosion. The tarnish resistance of silver is explained partially by its position on the EMF series or activity series of metals. There are known techniques for tarnish removal and/or prevention. Proposals have been made to improve the tarnish resistance of metallic surfaces, especially surfaces comprised of silver, by forming a conversion coating by chemical and/or electrochemical methods. However, most of the known conversion coatings either provide inadequate protection, require chemicals not readily available at practical cost, or include toxic materials, and therefore are not appropriate for their intended end use. Some other known methods for addressing tarnishing of metals include removing the tarnish by: mechanically polishing the metallic article; chemically treating the surface of the metal; and cathodic passivation of the metal surface in salts of chromium and other metals. Methods are also known for increasing the resistance to tarnishing of a metal surface by covering the surface of the metal with a film of other stable metals (i.e., rhodium, oxides of aluminum, beryllium, and zirconium). Suggestions have also been made in regard to modifying the composition of a silver alloy to increase the tarnish resistance of the metal itself. Another technique which is utilized with mixed results is the process of coating the surface of the metal with a clear "epoxy" or "enamel" layer.

The known methods are subject to undesirable side effects and often result in coated substrates which do not most effectively serve their intended purpose. For example, many of the metal coatings are toxic or hazardous to the environment, and their use both on the metallic product and during the coating process poses serious health and environmental consequences. Additionally, the coating may effect the mechanical suitability of the underlying substrate. For example, use of clear coat epoxies or enamels results in decreased flexibility and reduced luster (reflectivity) of the underlying metallic article. This is primarily due to the thickness of the coating formed (normally greater than 1 micron). Snake jewelry is an example of a type of jewelry where the clear coat thickness produces results (i.e., kinking and stiffness) which are undesirable or inappropriate for the end use of the underlying substrate.

The thickness of the coating (1 micron) decreases the flexibility of the underlying substrate. Other coatings are easily removed when the metal substrate is rubbed, worn or damaged, resulting in some areas that are more susceptible to tarnishing than others and uneven coloring.

SUMMARY OF THE INVENTION

The present invention provides a valuable improvement in the formation of tarnish resistant metals. Disclosed herein is a tarnish resistant article and a method of increasing the tarnish resistance of a metallic article by forming a self-assembled monolayer on a surface of the article. Also disclosed are preferred methods of forming a self-assembled monolayer on an article or method of processing the metallic article.

Depositing self-assembled monolayers ("SAMs") on metallic surfaces, particularly gold surfaces, for the purpose of studying specific molecular interaction or creating a predetermined pattern has been heretofore described, for example see U.S. Pat. No. 5,514,501. However, depositing SAMs to protect coated metals against tarnishing, as well as the formation of a SAM coated metal having tarnish resistance characteristics suitable for common purposes has not been described. It should be noted that silver and gold are very different, and as discussed in Ulman, which is hereby incorporated by reference herein in its entirety

(Chem. Rev. 1533-1554 (1996)), very different surface phenomena exist for gold and silver. The present invention allows any type of silver, copper, or alloy surface (e.g. cast surfaces, jewelry, etc.) to be coated by employing a precoating cleaning step which places the metal in condition to be coated with a self-assembled monolayer.

Another aspect of the present invention is an article or substrate either comprised of a metal or an article coated with a metal which has an increased resistance to tarnishing. This aspect of the present invention includes a method by which the tarnish resistance of a metal article or an article having a metal coating is increased. It is preferable that the metal be comprised of a transition metal, more preferable that the metal be comprised of a metal in column IB of the periodic chart, even more preferable that the metal be selected from the group consisting of silver and copper, and most preferably that the metal be silver or an alloy of silver. When coated with self-assembled monolayers, substantial differences may exist between gold (Au), silver

(Ag), and copper (Cu) which may be generally attributable to surface phenomenon, chemisorption, and chain-chain interplay. The SAM coatings formed on silver in accordance with the present invention appear to be more robust and stable, hence more tarnish resistant, than those formed on either gold or copper. These differences are apparent when comparing the SAM layers formed on silver and copper respectively.

For purposes of discussion herein, the term "about" means +/- 10%. For example, "about 20" means in the range of 18-22. If the term is being used in the context that a whole number is required, the number derived from the above definition of "about" should be rounded up to the next highest integer. One aspect of the present invention is a method of treating or protecting a metallic surface, specifically a metallic surface which contains metallic silver or copper, and preferably a metallic surface comprised of metallic silver. The method preferably entails forming a self-assembled monolayer (SAM) on the metallic surface to be protected. The SAM is preferentially formed from an alkylthiol. The alkyl group of the alkylthiol can be either saturated or unsaturated and preferably has a carbon atom chain length of less than about 26 carbon atoms, even more preferably less than about 20 carbon atoms. It is even more preferable that the alkyl chain have a length of about 10 to about 18 carbon atoms, more preferably about 12 to about 16 carbon atoms, and even more preferable that the chain length of the alkyl portion of the alkylthiol be in the range of about 14 to about 16 carbon atoms long. It is even more preferable that the SAM be formed from a saturated alkylthiol having an alkyl chain length of about 14 carbon atoms. Alkylthiols having alkyl groups of the preferred chain lengths appear to form a very "compact" self-assembled monolayer on the underlying metal substrate and therefore provide the most suitable protective coat. Increased compactness is a desirable characteristic in the SAM coated article.

The protective coating of the present invention is best formed when the surface of the metal is degreased and cleaned. The degreasing step includes treating the metal surface with an organic solvent (e.g., acetone). The degreasing step is in fact a cleaning step, and if one started with a "clean" surface, the organic degreasing or cleaning step may be skipped. The cleaning step (sometimes referred to herein as an etching or acid etch step) includes exposing the surface of the metal to an effective concentration of an acid. In a preferred embodiment of the present invention, the article of the present invention is comprised of a substrate having a surface layer containing silver with a tarnish resistant coating formed thereon. The coating is preferably formed as a self-assembled monolayer, and more preferably, as an alkylthiol self-assembled monolayer wherein the alkylthiol can be adapted to satisfy the end use or function of the substrate. By example and not limitation, if the substrate is to be flexible as thin a protective coat as possible is preferentially formed. It is preferable to utilize an alkanethiol with its alkyl group having a carbon chain length less than about 26 carbon atoms, more preferably less than about 20 carbon atoms, more preferably in the range from about 10 to about 18 carbon atoms, even more preferably in the range of about 12 to about 16 carbon atoms, and most preferably about 14 carbon atoms in length. By way of further example and not limitation, if a shell or hard coating is desired, a polymerizable or cross linkable function located in, or at the end, of the alkyl group is preferred. In this embodiment of the present invention, alkylthiols containing one or more diacetylene groups (e.g., HS(CH₂)io C≡CC≡C(CH₂)₁o COOH) are utilized to form SAMs. Select diacetylenic containing alkylthiols and SAMs formed therefrom as well as methods of forming crosslinked polymerized SAMs are described by Kim et al. "Polymeric Self- Assembled Monolayers," (J. Am. Chem. Soc. 119, 189-193 (1997)) which is hereby incorporated herein in its entirety by reference thereto. Once deposited, the crosslinking or polymerization reaction may be initiated and propagated throughout the SAM to harden the coating via the SAM coating being crosslinked. Therefore, the self- assembled monolayer may include a functional moiety within the body of the alkylthiol chain, which when activated crosslinks with another functional moiety of the self-assembled monolayer, or acts as a template for the formation of a polymer based overcoating. It appears for purposes of crosslinking that diacetylenic and polydiacetylenic alkylthiols may be best suited for purposes of crosslinking. It is also contemplated herein that a functional group at the end of the alkylthiol may function as a template or linking point for further chemical modification of the coating.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

Fig. 1 is a schematic illustration of a preferred method of forming a SAM on an article in accordance with the present invention;

Figs. 1A - Fig. 11 are alternative methodologies useful in forming a SAM layer on a metallic article;

Fig. 2 is a graphical representation of a surface layer of a metal coated with a self-assembled monolayer;

Fig. 3 is a schematic illustration of an accelerated atmospheric tarnish test employed in testing an article of the present invention;

Fig. 4 illustrates schematically the results of the accelerated tarnish test on coated and uncoated articles of the present invention;

Fig. 5 illustrates an alternative accelerated atmosphere test useful in testing an article of the present invention;

Fig. 6 illustrates a bar graph comparison of 2-stage and biphasic deposition using dodecanethiol;

Fig. 7 illustrates a bar graph comparison of 2-stage and biphasic deposition using fluorinated thiol; and Fig. 8 illustrates a bar graph comparison of biphasic methods and 2-stage deposition of neat thiol;

Fig. 9 schematically illustrates the concept of "contact angle" useful in testing an article of the present invention;

Fig. 10 is a schematic illustration of a vapor chamber accelerated tarnish test;

Fig. 11 illustrates effect of different carrier gases on tarnish reaction using the gas wash tarnish test;

Fig. 12 illustrates reaction rate data from the vapor chamber accelerated tarnish test;

Fig. 13 illustrates reaction rate data from the gas wash accelerated tarnish test;

Fig. 14 illustrates reproducibility of the accelerated tarnish tests;

Fig. 15 illustrates schematically the use of an ocean optics spectrometer to quantify the degree of tarnishing by a decrease in reflectivity;

Fig. 16 illustrates a difference in impedance of thiol coated samples when compared to uncoated samples;

Fig. 17 illustrates that the maximum impedance occurs at 2-

4 minutes for the 92% thiol; Fig. 18 illustrates that maximum phase angle and impedance occurs at 60 minutes in this particular test with 2% thiol;

Fig. 19 illustrates effect of surface treatment on water contact angle;

Fig. 20 illustrates tarnish protection of SAM coated silver;

Fig. 21 illustrates optimum thiol concentration required for 10 minute deposition time; and

Fig. 22 illustrates optimum time for the thiol coating formation at respective concentrations.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

One aspect of the present invention is a tarnish resistant metal article comprised of a metal, preferentially silver, copper, and their alloys, having an alkylthiol self-assembled monolayer (SAM) formed on the surface thereof. The SAM layer can be formed from a thiol molecules, such as substituted alkylthiols and disulfides (e.g., alkanethiols or thiophenols may be used). The term alkylthiol is used interchangeably herein to describe the material to be used to form the SAM and the SAM once it is formed on the metallic article. The term is actually generic to a number of potential metallic surface -coating interactions. For example, the oxidation state of the sulfur atom may be other than a thiol (i.e., a thiolate). It is to be understood that for purposes of discussion herein, that the term "alkylthiol" means any sulfur containing coating which interacts with a metallic surface to form an organized tarnish resistant coating, and that the alkyl portion of the term refers to a hydrocarbon which may be a straight chain, branched chain, saturated, unsaturated, functionalized chain or any combination thereof.

Generally, the alkylthiol molecules correspond to the general formula R-(CH2)/.-SH, where SH is the sulfur or thiol head group, n may represent any desired integer depending on the desired character of the layer to be formed and R represents the terminal functional group (e.g. HOOC-, CH3, NH2, CF3, halogen, unsaturated hydrocarbon, etc.). Preferably n is less than about 25 carbon atoms, more preferably n is in the range of about 9 to 17, more preferably from 11 to 15, and most preferably from 13 to 15 and the R group is a terminating methyl group (CH3). However, R may be used to confer a desired characteristic on the SAM coating, depending on the intended use (i.e., crosslinking capability). Similarly, as will be discussed further herein, the chain may be functionalized to confer a desired characteristic. Other types of compounds which are capable of producing alkylthiol monolayers and are useful in the invention include dialkyl sulfides and dialkyl disulfides.

Although for most applications it is preferable that the alkylthiol chain be saturated, an unsaturated alkylthiol such as an isoprenoid (i.e., a Cj.5 farnesyl) may provide enhanced stacking and/or the ability to later functionalize the SAM coating. In this way it may be possible to use a longer chain side chain with functional groups which affect solubility. The functional groups depicted for example in Fig. 2, specifically, Ri, R2, R3, R4 and R5 of the self-assembled monolayer may be functionalized (e.g. HOOC-, NH2, CF3, halogen, unsaturated hydrocarbon, etc.) to enable crosslink formation between the alkyl side chains 41 or to provide a template for future modification. In the embodiments described in detail herein R2, R3, R4 and R5 are preferably hydrogen or fluorine atoms and R_ is a terminating methyl group. Therefore the alkyl side chain 41 is a straight chain. The functionalization of R2, R3, R4 and R5 is not limited to the structural representation shown in Fig. 2, rather Fig. 2 illustrates that functional groups may exist in non-terminal alkyl chain positions. For example, the entire chain length may be functionalized with fluorine atoms instead of hydrogen atoms.

The foregoing are merely representative examples, and there are many other compounds which could be used in the invention. The important characteristic is the ability to form a robust and stable thiol monolayer on a metallic substrate (Me in Fig. 2). It is preferable that the metallic substrate contain either copper or silver and even more preferable that the metallic substrate be comprised of silver. Although not to be bound by theory, the thiol monolayer can be represented as Ag-S-

( CΗ.2)n-R. Although the precise binding or type of bond formed at the silver surface is unknown, this description and the representation of the alkylthiol coated substrate 30 illustrated in the attached figures provided as models in order to more fully convey the concept of forming an alkylthiol SAM on the surface of a metallic substrate 21, preferably a silver substrate 21. For example, in the figures, the self-assembled monolayer 37 is illustrated as being formed only on an outer upper surface of the metal substrate 21. Of course, the SAM 37 is formed on all surfaces exposed to the alkylthiol solution.

A common and easy test to determine if a SAM is present on the silver surface is the contact angle test. The contact angle test measures the angle that is formed between a drop of liquid and a solid surface. This test is effective in detecting the SAM because a coated sample will be more hydrophobic than bare metal. Fig. 9 displays how the contact angle increases when a SAM is applied. The bare silver surface is somewhat hydrophilic, which allows water to relax on the surface, while a

SAM coating is hydrophobic causing water droplets to bead on the surface. However an increase in contact angle beyond 100° does not necessarily correlate with increased tarnish protection. Therefore different techniques have been developed to evaluate the coated sample's resistance to tarnishing.

As the contact angle with H2O increases, the "compactness" increases. Up to a point, a higher contact angle means that steric interaction between the side chains is minimized and the density of the coating is optimized. However, with contact angles above 100 degrees, there may not be a direct correlation with increased tarnish resistance.

For this reason, as discussed herein, different technologies have been developed to test a sample's resistance to tarnish. Accordingly, the contact angle with H₂O is only one criteria by which the suitability of the self-assembled monolayer 37, or more precisely the suitability or tarnish resistance of a coated article 30, may be measured.

Referring now to Fig. 1, a method of the present invention is illustrated. This method employs a stepwise processing of substrate 21. However, as will be discussed more fully herein, one or more steps may be combined and/or eliminated. Advantageously the method of the present invention can be carried out at room temperature. In the degreasing system 10, the metal substrate or metal coated substrate 21, preferably comprised of silver, is immersed in an organic or aprotic solvent 23 such as acetone in a beaker or container 18 so as to degrease or wash the organic soluble contaminants from the surface of the metal substrate 21. It is preferable that the organic solvent 23 be a three stage solvent bath (trichlore thane, acetone, ethanol) or that alternatively the solvent 23 be a hot alkaline bath like that described in Kim et al. "Polymeric Self- Assembled Monolayers," J. Am. Chem. Soc. 119, 189-193 (1997) which is hereby incorporated herein in its entirety by reference thereto. It should be noted that this degreasing step may be avoided if starting with a freshly deposited metallic surface or otherwise "clean" surface. In a first rinsing system 11 the degreased metal substrate 21 is rinsed with a hydrophilic solvent 28 such as water, preferably de-ionized distilled water. After rinsing, the metal substrate is processed through an acid etch step 12, preferably by immersion in a sulfuric acid solution 25. The acid solution 25, for reasons set forth below, is preferably H₂SO . The H₂SO solution 25 concentration can vary from 1 weight % to about 90 weight % H2SO4, more preferably from about 2 weight % to about 40 weight % H2SO4, even more preferably from about 3 weight % to about 30 weight % H2SO-1, and more preferably about 4 weight % to about 20 weight % H2SO4, and most preferably about 5 weight % to about 10 weight %. The time for adequate preparation in the acid etch system 12 may vary depending upon the concentration of the acid solution 25 and the time of exposure.

It is noted that an adequate preparation of the silver substrate 21 does not appear dependent on the pH of the solution, but rather on the type of acid employed in the cleaning step. As can be seen from Table 1 below, a much more robust and stable SAM coated surface is obtained when using H2SO-1 solution 25 or acetic acid as opposed to other common acids.

TABLE 1

Etchant Acid SAM formed Contact Angle with H2O

20% H₂SO₄ C12 110 degrees

20% HC1 C12 90 degrees

20% HNO3 surface very rough; loss of luster

20% Acetic Acid C12 109 degrees

No Acid but C12 71 degrees

Degreased

From Table 1 it can be seen from the acids employed that nitric acid (HNO3) is the least suitable acid to use in etchant step 12. The contact angle with H2O in Table 1 is indicative of the "compactness" of the self-assembled monolayer 37 on the surface 35 of the silver substrate 21. From Table 1, it can also be seen that it is preferable to utilize a suitable acid etchant in the acid etchant system 12, selected from the group consisting of H2SO.., HC1, and CH3CO2H, even more preferable to utilize an acid etchant solution from the group consisting of H2SO₄and CH3CO2H, and most preferable to utilize an acid etchant solution comprised of H2SO-1. Another acid which may be suitable for the acid etch step is p- toluenesulfuric acid. P-toluenesulfuric acid is particularly useful when the acid etch step is done in an organic solvent.

Not intending to be bound thereby, it is speculated herein that the use of H2SO4 and acetic acid better dissolves adsorbed salts and metal oxides (i.e., silver oxides) present on the surface. It may be possible to employ UV radiation in place of the H2SO or acetic acid etchant system 12. If this were done, it would still be preferable to clean with an acidic solution like those listed in Table 1 (in order to more thoroughly clean the metal substrate 21). Additionally, it is more preferable to utilize H2SO4 for solvent compatibility reasons.

After the metal substrate 21 is exposed to the H2SO4 solution 25, the cleaned metal substrate 21 is preferably rinsed in a second rinsing step 13, preferably with de-ionized water and ethanol 29. In the example used to prepare the samples of Fig. 4, the silver substrate 21 was immersed in a 10% H2SO4 solution 25 for 1-2 minutes and washed with water and ethanol. After exposure to the H2SO4, the silver substrate samples 21 were immersed overnight in 50 mM alkylthiol solution 27 in an alkylthiol monolayer exposure system 14.

The molar concentration of the alkylthiol solution 27 is important in relationship to the time of exposure. A lower concentration (i.e, ImM) is acceptable if the time of exposure to the solution 27 is lengthened (see Fig. 22). After being removed from the alkylthiol solution 27, the tarnish resistant article 30 is preferably rinsed thoroughly with fresh ethanol, followed by de-ionized water (not shown). The tarnish resistant article 30 is comprised of the silver substrate 21 and self- assembled monolayer 37. The self-assembled monolayer 37 is comprised of a sulfur atom 33 in contact with the surface 35 of the silver substrate 21 and an alkyl chain 41. Although the nature of the silver surface 35 - sulfur 33 interaction is not clearly defined, it is clear that the present invention provides a robust bond between the sulfur atom 33 and the surface 35 of the metal substrate 21. This method involves coating the metal 21 using neat hexadecanethiol (92% technical grade) or a dilute solution (2 vol.% hexadecanethiol in trichloroethylene) with a process that involves transferring the sample between several solutions.

In an alternative methodology, the phase compatibility or incompatibility of the various steps are utilized to combine one or more steps. Referring to Figs. 1A through II, alternative methodologies of the present invention are described. Each of these alternative methods rely on phase separation of the various solvent systems being used in order to accomplish a minimization of exposure of the metal substrate or metal article or metal coated substrate 21 to air. This phase separation may be accomplished by physically separating miscible phases (e.g., by partitioning walls) or by the immiscibility of the respective solvents being used. It is most preferable that exposure to air be avoided between acid etching and alkylthiol deposition.

Fig. 1A illustrates a method of the present invention wherein the organic or aprotic solvent 23 is in a biphasic relationship with acid solution 25. In this system, the organic solvent is less dense than the sulfuric acid enabling the metal article to be lowered first through the organic solvent phase 23 where it is degreased and then into the sulfuric acid layer phase 25 for the acid etch step without exposure to air. After acid etching, the organic solvent can be drawn, poured, or decanted off through decanter valve 61 which is preferably disposed at the solvent-acid phase interface. Alternatively, metal substrate 21 can also be brought back up through the organic solvent 23, but this is less preferred than decanting the organic solvent 23. After being removed from the sulfuric acid solution 23, the metal substrate 21 can be optionally rinsed in rinsing step 11 (as is shown by the dotted lines) or directly processed into the thiol-containing or alkylthiol monolayer exposure system 14. As discussed above, in formation of a stable SAM, the alkylthiol concentration of the solution 27 is important in relationship to the time of exposure. A lower concentration of alkylthiol 27 is acceptable if the time of exposure to the solution 27 is lengthened. After being removed from the alkylthiol solution in Fig. 1A, the tarnish-resistant article 30 can either be rinsed thoroughly with fresh ethanol 63 in rinsing step 11 or processed directly into an ethanol/de-ionized water bath 31. An optional additional washing step is shown as washing or rinsing step 13 wherein article 30 is washed with e.g., de-ionized water. Article 30 is then left to dry resulting in article 30 having a self-assembled monolayer 37, as shown in Fig. 2.

Another embodiment of a method useful in the present invention is shown in Fig. IB wherein the acid 25 and alkylthiol- containing solution 27 are accomplished in the same container. In Fig. IB the metallic article 21 is rinsed in ethanol, acetone or fluorinated liquid 23 and optionally permitted to dry and/or rinsed and allowed to dry in rinsing step 11. Article 21 is then lowered or processed into acid- containing solution 25, preferably a sulfuric acid solution. In this step the metal article 21 is etched with an aqueous acid solution, preferably a 10% sulfuric acid solution, from 1-5 minutes (permitting etching of the metal article 21 surface). The metal article 21 is then lowered from the less dense acid phase 25 through a phase partitioning region into the alkylthiol solution 27, in this embodiment preferably a fluorinated alkylthiol solution 27. In this step, the alkylthiol monolayer is applied to the metal article 21. It is preferable that the thiol be dissolved in an environmentally benign solvent such as a fluorinated liquid containing minimal concentrations of the thiol and the exposure time should be within the range of about 1-20 minutes or preferably in the range of 2-15 minutes and even more preferably in the range of 4-10 minutes, thereby allowing sufficient time for the thiols to self-assemble on the metal article 21. The metal article 21 can then be removed from this biphase mixture by pulling it up through the sulfuric acid layer 25 or the sulfuric acid layer 25 could be decanted off the top of the alkylthiol solution. The metallic article 21, now coated with a self-assembled monolayer, results in tarnish resistant article 30 which may be washed with acetone or water individually or together and optionally can be processed on into the ethanol bath 31. After removing the metallic article 30 from the ethanol bath 31, the tarnish resistant material 30 can be either washed again in washing or rinsing step 210 or dried resulting in a final product 30.

Fig. IC illustrates another method of processing metallic article 21. Metallic article 21 is put into or exposed to an organic or aprotic solvent 23. After exposure to the organic solvent 23 the metallic article is either dried or optionally washed then dried and deposited into an acid etch system which includes acid solution 25. The metallic article 21 is allowed sufficient time to be acid etched preferably by sulfuric acid. Metallic article 21 is then removed from the acid etch solution. After exposure to the sulfuric acid, it is best to immediately transfer the metallic article 21 to a wash bath 13 or directly to the next step so that the sulfuric acid does not have time to undesirably corrode the underlying metallic surface. Fig. IC illustrates a biphasic combination of the alkylthiolation and wash step. The metallic article 21 as shown in Fig. IC is lowered through a wash solvent 31 such as ethanol/water down into an alkylthiol solution 27 which is where the self-assembled monolayer is formed upon the metallic article 21 resulting in alkylthiololated tarnish resistant article 30. In the embodiment shown in Fig. IC, tarnish resistant article 30 is raised back up through the ethanol/water solution 31 to remove any excess alkylthiol and is optionally washed in washing step 210 or dried resulting in the final product article 30.

In the embodiment in Fig. ID, a two-phase liquid system is shown made up of less dense organic degreasing solvent 23 and a more dense sulfuric acid solution 25. The metallic article 21 is lowered into the organic solvent 23 where it is degreased. Then upon sufficient degreasing, article 21 is lowered down into the sulfuric acid solution 25 where it is acid etched. Concentrated alkylthiol or fluoroalkyl-thiol is then carefully added to the already present organic solvent 23 resulting in a solution containing organic solvent 23 and alkylthiol 27 or fluoroalkyl-thiol 27. The metallic article is then raised up through the biphasic partitioning layer into the layer 27, now including alkylthiol. Upon withdrawal of the tarnish resistant article 30 out of the alkylthiol-containing layer, it is further processed by treatment with an ethanol bath 31 or washed with de-ionized water and/or ethanol in washing step 13, resulting in final product 30 which has formed thereon the tarnish resistant self-assembled monolayer.

In the alternative embodiment shown in Fig. IF, the system utilizes a tank (e.g., a U-shaped flask) having a vertical partition 64. As shown in Fig. IF, metallic article 21 is degreased in the organic solvent 23 as discussed above and either washed (step 11) or dried and then processed into sulfuric acid solution 25. After sufficient etching, the metallic article 21 is then lowered into the thiolalkylation layer 27 wherein the alkylthiol is deposited as a self-assembled monolayer on surfaces of the metallic article 21. The metallic article 21 being coated with the self-assembled monolayer results in a tarnish resistant article 30 which can be removed out the opposite side of the divided upper portion of the flask or reaction tank without removal of the organic solvent 23 by, for example, decanting. Upon removal, the tarnish resistant article 30 may be optionally washed or dried as described above and then bathed in an ethanol acetone solution 31 to remove any excess alkylthiol. After removal of the excess alkylthiol, the tarnish resistant metallic article is then dried and/or goes through a washing stage 210 and results in a final tarnish resistant article 30 which is shown in Fig. 2.

Using the divided tank or flask system (e.g., U-shaped flask), of Fig. IF, Fig. 1G illustrates an alternative system whereby the metallic article 21 is deposited in an organic solvent 23 to degrease and is then either dried or washed (step 11). Degreased metallic article 21 is lowered into acid solution 25, preferably sulfuric acid solution 25. Subsequent immersion which occurs directly below the acid solution 25 into a thiolalkylation layer 27 is then performed. The resulting tarnish resistant article 30 is then brought back up through the hydrophilic or water-based bathing solution 31. The acid solution 25 is divided and separated from a hydrophilic or water-based bathing solution 31 by divider wall 64. Therefore, in this step, the etch step, the monolayer deposit step and the rinse step may be combined so that the resulting tarnish resistant metallic article 30 can be passed between these steps without exposing metallic article 21 to air and then removed and either dried or simply washed resulting in a final tarnish resistant metallic article 30 as shown in Fig. 1G and in more detail in Fig. 2. A subsequent step is a rinse 210 that removes any excess thiol from the metal surface. This is to ensure the desired result of a single monolayer on the surface, thereby not affecting the visual properties of the metal. The rinse 210 can be conducted by immersing the thiol-coated silver in a separate vessel containing water, neat ethanol, acetone, or fluorinated liquid. If a fluoroalkane liquid is used for the alkylthiol or fluoralkyl- thiol, the water can be placed directly upon the fluoroalkane on the right-hand side of the vessel. Utilization of the described one-flask system would minimize evaporative losses of the relatively expensive fluoroalkane. If a hydrofluoroether liquid is used a thin layer of water should be placed upon the hydrofluoroether (ethanol and acetone will dissolve in hydrofluoroether, but water will not). Therefore three phases (sulfuric acid, fluorinated liquid-thiol solution, and rinse fluid) would be used in the U-shaped vessel, with the silver having no exposure to air between any of the acid wash, thiolalkylation, or rinse steps. It should be noted in the system utilizing a partitioned tank or flask that the level of the bottom layer should extend above the partition wall 64 sufficiently to prevent interaction of the solutions contained in the upper quadrants or chambers.

Fig. IE shows a system wherein each of the degreasing solution 23, acid etch solution 25 and thiol depositing solution 27 are combined in one solution and the metallic article 21 is deposited therein. Upon removal from this system, the tarnish resistant metallic article 30 can be washed in ethanol acetone bath 31 or optionally with de-ionized water to remove any excess alkylthiol resulting in final product 30 which is tarnish resistant. In the method illustrated in Fig. IE, the preferable type of solvent system used would be organic due to the limited solubility of long alkylthiols. Therefore, it would be preferable to use an organic acid system which would have the acid etching capability of sulfuric acid or nearly the acid etching capability of sulfuric acid. These types of acids are, for example, acetic acid and/or p-toluenesulfonic acid.

In Fig 1H a triphasic liquid system using a fluorous phase is illustrated wherein the organic solvent 23 which is used to degrease the metallic article 21 first comes in contact with the metallic article. The metallic article 21 is then lowered into the next aqueous phase which includes a acid aqueous phase 25, which phase includes a sulfuric acid concentration sufficient to acid etch the metallic article 21. After acid etching, the metallic article 21 is lowered into the fluorous phase 27 where fluoroalkylation occurs. Upon fluoroalkylation the metallic article can be removed directly by either decanting the phases from the flask or by raising the tarnish resistant metallic article 30, which is coated with the self-assembled monolayer, up through the various solutions. It would appear that this triphasic system would be limited to utilization of a fluorous phase due to the typical existence of a two-phase system as opposed to a three-phase system. However, the use of fluoroalkylated thiols may be particularly useful in this embodiment due to the fact that they may be soluble in the fluorous phase. The use of various fluorous systems, particularly solvent systems, are described in U.S. Patent No. 5,859,247 to Curran and U.S. Patent No. 5,777,121, both of which are hereby incorporated by reference in their entirety, as well as in a review article by Dennis P. Curran entitled "Strategy-Level Separations in Organic Synthesis: From Planning to Practice," (Angew. Chem. Int. Ed.

1998, 37, 1174-1195), which is also incorporated herein by reference thereto. Finally, in Fig. II a biphasic liquid system is illustrated having an organic soluble acid 23/25 shown as the top layer. In this system, the degreasing and etching step are combined resulting in a system which may clean and etch the metallic article 21. The metallic article is then processed into the alkylthiol solution 27, which forms the second layer in the flask, for deposition of the self-assembled monolayer.

A two step test procedure is preferred in determining the relative effectiveness of SAM coatings for tarnish prevention. The first step is exposure of the sample to a chemically harsh environment that causes tarnishing. The second step is measurement of the sample's reflectivity to quantify the degree of tarnish. A fiber optic spectrometer can be used to quantitatively measure the intensity of reflected light from a sample, shown for example in Fig. 15. The sample's reflectivity (or luster) decreases as it corrodes. Additionally, the samples reflectivity may be quantified as a measure of the amount of tarnish that the metal has accumulated.

Fig. 15 shows a fiber optic spectrometer 300 (for example, a Ocean Optics S2000) that can be used to measure the reflectivity of the metal samples. An integrating sphere 301 is designed to shine the visible spectrum of light onto the metal sample 310, and then record the amount of light that is reflected from it. The instrument is calibrated by first recording two standards into the spectrometer's memory. A ight' sample is recorded by analyzing a new piece of silver and setting its value equal to one hundred percent reflectivity. A 'dark' sample is recorded by analyzing total darkness and setting its value equal to zero percent reflectivity. Analysis is then performed on the experimental samples and a quantitative measure 320 of their reflectivity with respect to the two standards is displayed. The analysis could be performed for example by a computer with an Ocean Optics Inc. 001-ADC500 REV A analog digital card.

The SAM may also be evaluated by Electrochemical

Impedance Spectroscopy (EIS). An EIS test is performed by passing a small sine wave voltage through a sample and measuring the resulting current. The AC resistance of a system is called the impedance, /Z/. The lag between the maximum current and the maximum voltage is known as the phase shift, θ. These parameters, the phase shift, θ, and the impedance, IZl, generated from EIS testing may be used to estimate the monolayer quality and thickness. Fig. 16 shows impedance data from one uncoated sample ("clean"), and from three samples coated with Ciβ thiol for the optimum times.

The EIS data can be used to estimate the quality and thickness of a monolayer. The part of the curve that gives the most information about the quality of the monolayer lies at a low frequency, 0.1 Hertz. The data at 0.1 Hz can show the presence of pinholes in the monolayer by a flattening of the impedance or a decreasing phase angle. The quality of the monolayer is maximized at the point where the impedance, IZl, and the phase shift, θ, are at their maximum. This process was used in determining the deposition time required for applying a complete monolayer.

The thickness of the monolayer coating can also be estimated from the capacitance of the coating by approximating the monolayer as a parallel plate capacitor at frequencies where the phase angle approaches - 90°. With this approximation the following equation can be utilized in order to determine the thickness of the monolayer:

d = ε_iε₀A2πf(Z - Z_n) (5)

where d= thickness of layer, εi = dielectric constant of the thiol (2.6 to 3.0), εo = 8.854x10-12 F/m (dielectric of free space), A= surface area (cm²), f = frequency (Hz), Z= impedance (Ω) at frequency f, and Zo = impedance at high frequency (solution resistance)

As illustrated in Table 2 below, the length of the alkyl side chain 41 of the self-assembled monolayer 37 when it is a saturated alkane is preferably less than about 20 carbon atoms, more preferably in the range of 10 to about 18 carbon atoms, even more preferably about 12 to about 16 carbon atoms, and most preferably in the range of about 14 carbon atoms to about 16 carbon atoms.

TABLE 2

Length of Carbon Hours to Discoloration with Chain Accelerated Tarnish Test

Uncoated sample 0.33 hours (i.e., 20 minutes

C5 layer 0.50 hours (i.e., 30 minutes)

Cβ layer 4.5 hours

C12 layer 9 hours

C14 layer > 11.5 hours

Ciβ layer > 11.5 hours

Ciβ layer* 6 hours

* Due to limited solubility in ethanol, the C_8 layer was deposited using CH₃C1. Fig. 2 is a graphical representation of the self-assembled monolayer. As shown in Fig. 4 and Table 2, the Cs - C carbon chain length has the best tarnish resistance capabilities. While not wishing to be bound by any particular theory, these capabilities are believed to be due to the protection that a long chain aliphatic coating provides against unwanted oxidation of the silver surface 35 and optimal stacking (increased compactness-minimized steric interplay) of the self-assembled monolayer 37. The results of the different chain lengths illustrated in Table 2 appears to indicate that a "trade-off between chain length and chain compactness of the self assembled monolayer 37 exists. An alkyl chain 41 chain length of 14 to 16 carbon atoms, inclusive, appears to have better tarnish resistance (>11.5 hours in an accelerated tarnish resistant test), than does a carbon length of 12 carbon atoms (about 9 hours). Although this is somewhat surprising due to the fact that C_2 has a better contact angle, this may be the result of trade off between the distance that a H₂S molecule needs to travel and the compactness of the self-assembled monolayer 37. The factor regarding longer chain length apparently outweighs the benefit of the compactness of a C12 self-assembled monolayer 37. It is speculated that the dodecane (C1.2) side chain 41 allows the largest number of sulfur 33 - surface 35 interactions while being of sufficient length to form an organized SAM membrane on the surface 35 of the silver substrate 21. Fluorinated alkylthiols may also be used (e.g., Ri, R₂, R3, R4 and/or R5 are fluorinated individually or separately or collectively). The alkylthiol may be partially fluorinated or entirely fluorinated.

To test the SAMs and their resistance to tarnishing, very rapid analytical tests were developed. Fig. 3 schematically illustrates the accelerated atmospheric tarnish test utilized to rapidly replicate atmospheric tarnishing of a metal substrate 21. A sulfide buffer solution comprised of a IM NaOH and IM Na2S aqueous solution 42 can be placed in a container 48, generally a glass vessel (i.e., a beaker or the like). The metal substrate 21 is suspended above the solution 42 and the beaker 48 sealed with a stopper 46. The solution 42 is then heated with a hot plate 47 which may be controlled by thermocouple 45 to a temperature of about 50° C +/- 1° C and the samples were observed. The solution is very similar to that used by Randin et al. in Werkstoffe und Korrosion 43, (48-55 (1992)), which is incorporated herein in its entirety by reference.

Although Randin used a similar solution, a gold substrate was repeatedly immersed in the solution to accelerate tarnishing. Although other immersion type tests may be similarly utilized, the accelerated test used herein relied upon the formation of H₂S and H₂O vapor 44 to more accurately simulate the tarnishing environments that a metallic substrate 21 would encounter under air exposure. The advantages of this embodiment of the test is that it is fast, inexpensive, compact, and does not require a fume hood. During the tarnishing test, no odor was detected, which indicates that the vapor 44 concentration of H2S within the container 48 was less than about lppm. Most other tests require multiple days to complete (i.e., "flower of sulfur" test and "thioacetamide" test) or require at least a fume hood. While the immersion test discussed in Randin only requires about 3 hours to complete, it requires elaborate equipment, produces galvanic effects, and results in non-uniform discoloration of the sample metal substrate 21.

Fig. 4 is a schematic representation of the results obtained upon exposing various metallic articles comprised of silver to the accelerated tarnishing system described above. After three hours, all of the samples show substantial tarnishing, except those coated with SAMs. Silver substrate 21 was mill annealed and coated with a self-assembled monolayer 37 which has an alkyl side chain 41 that is 12 carbon atoms long. Substrate 21α is mill annealed and coated with a self-assembled monolayer 37α which has an alkyl side chain 41 which is 16 carbon atoms long; surface 356 of silver article 216 is uncoated and mill annealed; surface 35c of silver article 21c uncoated with a 0.05 μm polish; article 21c is uncoated with a #600 grit dry polish; surface 35e of article 21e is uncoated with a #600 grit wet polish; and surface 35/ of silver article 21/ is uncoated mill-annealed and solvent cleaned. The silver substrate 21 coated with dodecane thiol (C1.2) and the silver substrate 21α coated with a hexadecanethiol (Ciβ) showed little, if any, tarnishing in the accelerated tarnishing test. Each of the other articles (216-21/) showed substantial discoloration or tarnishing (depicted by shading) after only 3 hours.

An alternative tarnish test is executed by suspending metal samples over a solution of sodium sulfide (Na2S) in alkaline water, contained in a closed chamber. As the solution is heated, sulfur compounds are released into the chamber atmosphere, and are circulated with a small fan, and react to tarnish the suspended metal sample (Fig.

10). This test method can tarnish silver to approximately thirty percent of its original reflectivity in one hour. This test tarnishes silver, copper and brass.

The tarnish solution is a 0.5 N NaOH + 0.1 M Na₂S solution heated to 40-45°C. A 100 ml solution is made by starting with 97 ml of distilled water, adding 2.5 ml of 50% NaOH solution, stirring, then adding 2.4 g of Na2S.9H2O. Upon heating to 40-45°C, the solution gives off a sulfur odor, most likely H2S, which is mixed in the chamber by the fan. The chemical reactions are believed to be the following:

Na₂ S + 20H^~ → 2NaOH + S⁼ (l)

S⁼ + 2 H⁺ → H₂ S (2)

2 H₂0 → 2 H⁺ + 20H^" (3)

Na₂S + 2 H₂0 → 2NaOH + H₂ S (4)

The sodium sulfide (Na2S) should never be added to neutral or acidic water because large quantities of hydrogen sulfide gas (H2S) can be given off, which is poisonous at 10 ppm. The human nose can detect 1 ppm of H₂S, but becomes dull with exposure so a person working in the lab should never try to tolerate an obnoxious sulfur odor. For disposal of the solution, the solution should be mixed slowly with 60 ml of 5% NaOCl

(household bleach) for one hour, then diluted and disposed.

The chamber was a Bel- Art Product Catalog #F42010-000, with a white polypropylene base and a clear polycarbonate dome. The mixing fan was a Radio Shack 12 VDC brushless fan, Catalog #273-240. The fan was bolted to the support plate in the chamber and the holes in the support plate were enlarged to increase vapor circulation. The fans tended to corrode in this atmosphere, so longer life was obtained from the fans by first operating them in a chamber of hexadecanethiol vapor. After each test, the fans should also be run until dried. In Fig. 5, an alternative gas wash accelerated tarnish test was developed. In this test a stream of suspended sulfur compounds was passed over a metal sample. A lOmM sodium sulfide solution is used as the supply of sulfur for the system. The test is executed by placing a surfactant (a few drops of Ivory™ dish washing soap) in the sulfur solution to immobilize the sulfur with a carrier gas, which can be transported to the metal surface. This test method can tarnish silver to approximately twenty percent of its original luster in five minutes. Fig. 5 displays the apparatus used for the test. Prior to the test, a 5-10mM solution of sodium sulfide is made in a 0.1N NaOH solution.

Approximately 75 milliliters of this solution and 2 ml of industrial surfactant (a few drops of Ivory Dishwashing Soap which acts as a carrier of the sulfur agent) are placed in a 125 milliliter gas-washing bottle 51. A metal sample is hung in a chamber 53 or simply above the solution. A carrier gas such as nitrogen, oxygen, or air is used to transfer the sulfur containing agent with the surfactant to the metal 21. The test runs for between five and ten minutes. After the test, the samples are placed on a spectrometer to measure their reflectivity compared to control metal surface. 100% reflectivity is that of an untarnished metal and 0% reflectivity is that of a dull black surface. The spectrometer measures absorbance throughout the visible spectrum. Intensities at 500 and 600 nanometers are recorded for each sample. An untreated sample will tarnish to a reflectivity of about 20% ( about 2%) in the five-minute test with air as the carrier gas.

The effective amounts of the sulfur-bearing alkyl compounds utilized may vary with the nature of the medium in which they are applied and the particular composition of the surface to be protected, and with the mode of application. Concentrations of 0.01 to 1.5% by weight are preferred in liquids in which the silver-surfaced objects are to be dipped or immersed, while more viscous and paste-like polishing compositions may require 0.1 to 5%.

Fig. 6 provides a summary of accelerated tarnishing results as measured by the modified accelerated tarnish test outlined in Fig. 5. The results compare (a) untreated silver surfaces; (b) silver surfaces coated with dodecane thiol in a 2-step or 2-stage application; and, (c) silver surfaces coated with dodecane thiol in a two-phase system that eliminates exposure to air between the acid etching and thiol application. These results clearly demonstrate that dramatic improvements in tarnish resistance are attained when the 2-step application of the present invention is used. Reflectivity for untreated silver surfaces (a) is 19-21%, while for dodecanethiol-coated silver surfaces coated with the 2-step application reflectivity is 73-81%. The tarnish resistance increases even more dramatically when the two-phase system is used, with the reflectivity values of (c) being 95-98%, approaching that of the original silver (100%).

Fig. 7 provides a comparison of reflectivity for fluoronated thiols, as measured by the accelerated tarnish test or gas wash accelerated tarnish test shown in Fig. 5. The graph compares (a) untreated silver surfaces; (b) silver surfaces coated with fluoroalkyl-thiol in a 2-step application; and, (c) silver surfaces coated with fluoroalkyl- thiol in a two-phase system. In parallel to those shown in Fig. 6, these results also demonstrate that dramatic improvements in tarnish resistance are attained when a 2-step fluoroalkyl-thiol application is used. Reflectivity for untreated silver surfaces (a) is 19-21%, while for fluoroalkyl-thiol coated silver surfaces coated with the 2-step application reflectivity is 57-83%. The tarnish resistance for fluoroalkyl-thiol is further enhanced when the two-phase system is used, with the reflectivity values ranging from 95-97%.

Fig. 8 demonstrates that the preferred two-phase system of applying thiols (i.e. fluoroalkyl-thiol or alkylthiol), yields results comparable to the 2-stage system employing neat thiols. Because neat thiol solutions are extremely expensive and dangerous, the multi-phase. methods of the present invention are preferred. The embodiment that uses dilute concentrations of thiol immersed in a phase below the wash phase, reduces the amount of thiol required, the odor of thiols, the flammability of neat thiol solution and the expense of neat thiols.

Fig. 11 shows that different carrier gases effect the rate of tarnishing. This data from the gas wash accelerated tarnish test, reinforces the belief that oxygen is a necessary participant in the tarnishing process.

The above-described accelerated tarnish tests, although different in operation, yield similar results. Figures 12 and 13 compare reaction rate data from each test using reflectivity. The vapor method

(Fig. 12) tarnishes slower than the gas wash test (Fig. 13). Fig. 14 displays a comparison of successive silver samples tarnished with the two tests. This graph shows that although both tests yield acceptable results, the gas wash test gives higher reproducibility. The gas wash test uses a simpler solution but requires constant attention throughout the test, and can produce an unpleasant odor in small unventilated areas. The vapor chamber gives poorer reproducibility because of the non-uniform circulation of the air in the chamber. (Also one or two of samples in the vapor test may have been previously contaminated with thiol vapor which slowed their tarnishing). Although a caustic solution is used in the vapor test, it is less labor intensive than the gas wash test. The vapor test also accelerates tarnishing by using atmospheric corrosion which is a more typical exposure for silver.

The recommended time for running the vapor test is 3-4 hours. The tarnish solution appears to store well at room temperature, but it decays in effectiveness over time when it is heated and releases sulfur compounds. Therefore fresh tarnish solution is used for each test. The bubble test is run at room temperature for five minutes.

The quality of the articles formed by the present invention compare favorably with those tarnish resistant prevention techniques known in the related art. The SAM coated tarnish resistant article 30 was compared with common tarnish resistant preventative chemical treatment known as Blitz™ polish (available commercially from Blitz Mfg. Co., Inc., P.O. Box 846, Jeffersonville, IN 47131). When an article 21 containing silver was treated with Blitz™ and then subjected to the accelerated tarnish test described in Fig. 3 above, the treated sample began to substantially tarnish at 4 hours. As can be seen from Table 2 above, this is not as favorable as even a carbon chain length of eight atoms (4.5 hours). Beyond the fact that the SAM coated silver article 30 has as good or better tarnish resistant characteristics than those identified, the SAM coated metal of the present invention is non-toxic and flexible and therefore does not detrimentally affect the intended end use of the metal substrate 21.

In another embodiment of the present invention, the metallic article 21 may also be comprised of copper or a copper coated metallic article. A type of substrate 21 comprised of copper which is particularly susceptible to tarnishing is brass. Copper and silver are similar in a number of ways. They are transition metals in the same column on the periodic chart (IB) and they spontaneously form an oxide. The differences between copper and silver, if any, result from an interplay of chemisorption and chain-chain interaction. Table 3 illustrates contact angle data and accelerated tarnish test results of self-assembled monolayer 37 formed on a substrate 21 comprised of copper.

TABLE 3

LENGTH OF CONTACT ANGLE HOURS TO CARBON CHAIN OF H2O DISCOLORIZATION

No Acid but 71 degrees less than about 0.5 Degreased hours C₁₂ layer 114 degrees about 7 hours C₁₄ layer 120 degrees about 7 hours Ci6 layer 138 degrees about 7 hours C.8 layer* 130 degrees about 7 hours

* Due to its limited solubility in ethanol, the Cis layer was deposited using

It would appear from Table 3 that each of the chain lengths are essentially equivalent in terms of tarnish resistance, with Ciβ forming the best theoretical self-assembled monolayer 37 on a copper substrate 21. Any layer above Ciβ would be less desirable than Cβ-Cis due to solubility and capability issues.

Another advantageous feature of the present invention is that the SAM layer 37 formed has a thickness in the range from about 0.25 to about 7 nm, more preferably in the range of 0.5 nm to about 5nm

(Fig. 2) and most preferably a thickness of about 14 to about 16 straight chain unsaturated carbon side chain in length. This thin layer offers the same or better protection as thicker epoxy or polymer layers. Advantageously the article 30 has a coating 5 with a thickness in the range of 1-100 nanometers which does not impact the flexibility of the metal substrate 21, as does the relatively thick epoxy coatings (>1 micron) of the art. Also advantageously, the thin layer formed by the alkylthiol does not significantly effect the natural reflectivity of the underlying metal substrate 21 as do thicker coatings.

Figures 17 and 18 display the EIS data used to choose the optimum deposition times. This data led to a final choice of 2 min. for 92% thiol and 30-60 min. for a 2% thiol solution because these times correspond to the high impedance and phase angle. Other tests, not shown here, have demonstrated good coatings for the 2% thiol ranging from 30-600 minutes immersion. The calculated thicknesses of the SAM coating can vary from 2 nm (one monolayer) to 8 nm (4 monolayers). The SAM coating deposited by the 92% thiol for 2 minutes appears to be identical to the SAM coating deposited by 2% thiol for 30 minutes. Likewise, Fig. 19 shows similar water drop contact angles, and Fig. 20 shows similar reflectivity after the gas wash tarnish test. In principle, it should be possible to use almost any concentration of thiol if one first determines the optimum time window. Fig. 21 displays EIS data used to find the solution concentration needed to run a process designed for a 10 minute deposition time. This data suggests that a 10 minute SAM deposition can be made at a concentration of 7.5 vol.% thiol. The optimum time versus concentration window is shown in Fig. 22. The hexadecanethiol has a high vapor pressure and the vapor can disperse from an open vessel and coat everything in the vicinity. Uncoated silver samples placed in the same chamber as thiol coated samples can also be coated with thiols and exhibit reduced tarnishing rates. Care must be taken to prevent the thiol vapor from contaminating other surfaces. Contaminated surfaces may be cleaned with a bleach solution.

Stripping of the hexadecanethiol SAM can be accomplished by heating in air to vaporize/oxidize the coating. Pinholes begin developing in the coating after 30 minutes at 100°C, while complete vaporization/oxidation of the thiol occurs after 30 minutes at 150°C. The thiol may also be removed by soaking for several minutes in a household bleach solution (5% NaOCl) or in 50% NaOH solution, but these solutions will discolor the silver surface, turning it grayish-black.

Self-Assembled-Monolayers (SAMs) can be applied to silver in order to reduce the atmospheric effects of tarnishing without effecting the appearance of the sample. Accelerated tarnish tests have been developed to test tarnish resistance, which can be used for quality control. Electrochemical Impedance Spectroscopy (EIS) was used in the research phase to optimize the SAMs overall quality. The method of applying SAMs to silver consists of a multi-step process, with emphasis on the third step of thiol deposition. It is possible, if desired, to combine steps of the multi-step process to avoid exposure to air (see Fig. 1A and Fig. IB). The thiol deposition step can be tailored to fit a desired deposition time within a window from two minutes for a 92% thiol to several hours for a dilute thiol solution.

Dipping, washing or rinsing compositions may contain water, ethanol, dioxane, trichloroethylene and other liquid media, also addition agents some of which may improve the effectiveness of the compounds of the invention or increase the useful life of the composition.

Dialkylphosphates, such as dilauryl phosphate, and dialkyl sulfosuccinates, such as dilauryl sulfosuccinate, have been found to be effective surfactants in this method.

Liquid polishing pastes and polishing emulsions employed in the method of the invention may contain mild abrasives, such as calcined magnesia or chalk powder. Solid polishing pastes, in addition to containing the alkylthiol compounds described above may include known abrasives, for example, rouge, French chalk or pumice, and a binder which may be a fat or grease, wax, paraffin, or an alcohol of high molecular weight having the required paste-like consistency.

The fact that the SAM coated metal article can be used in the formation of electrical contacts provides an unexpected and surprising benefit. Specifically, the metal article 30, if an electrical contact, can be coated with a SAM (alkylthiol or fluorinated alkylthiol) and can be soldered without interference from the SAM coating. While not wishing to be bound by theory, it appears that the heating of the SAM coated metal article (in this case an electric contact) results in sufficient degradation of the coating to allow the metal article to blend together and form a soldered joint. Advantageously, it does not appear that the portion of the article which is not exposed to heat loses any of the tarnish resistance imparted by SAM formation. The electric contact after soldering may be again exposed to more alkylthiols for additional coating with a desired SAM.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A method of treating a metallic surface to prevent tarnishing, said method being comprised of: forming a self-assembled monolayer on the metallic surface.

2. The method of claim 1, wherein the metallic surface is comprised of metallic silver.

3. The method of claim 2, wherein the self-assembled monolayer is comprised of an alkylthiol.

4. The method of claim 3, wherein said alkylthiol is comprised of a fluorinated alkylthiol.

5. The method of claim 3, further including the step of cleaning the silver surface prior to forming the self-assembled monolayer thereon.

6. The method of claim 5, wherein the step of cleaning the surface includes immersing the silver surface in an acid.

7. The method of claim 6, wherein the step of cleaning the surface further includes washing the silver surface with an organic solvent.

8. The method of claim 1, wherein the metallic surface is comprised of metallic copper.

9. The method of claim 6, wherein the acid is sulfuric acid at a concentration of about 10 weight % H2SO4 to about 40 weight % H₂SO₄.

10. The method of claim 9, wherein the sulfuric acid is at a concentration of about 5 weight % H₂SO₄ to about 20 weight % H2SO4.

11. The method of claim 3, wherein said alkythiol is comprised of an alkyl group having a chain length of less than about twenty-six carbon atoms.

12. The method of claim 11, wherein said chain length is less than about twenty carbon atoms.

13. The method of claim 11, wherein said chain length is in the range of about twelve carbon atoms to about sixteen carbon atoms.

14. A tarnish resistant article comprised of: a substrate; a silver surface layer covering said substrate; and a self-assembled monolayer formed on said silver surface layer.

15. The tarnish resistant article of claim 14, wherein said self-assembled monolayer is comprised of an alkylthiol monolayer.

16. The tarnish resistant article of claim 15, wherein said self-assembled monolayer has a primary chain length of less than about twenty carbon atoms.

17. The tarnish resistant article of claim 16, wherein said alkanethiol monolayer has a carbon chain length in the range of about eight carbon atoms to about eighteen carbon atoms.

18. The tarnish resistant article of claim 17 wherein said self-assembled monolayer has a primary chain length in the range of about twelve carbon atoms to about sixteen carbon atoms.

19. The tarnish resistant article of claim 14, wherein said substrate is a flexible material.

20. The tarnish resistant article of claim 14 wherein said substrate is selected from the group consisting of jewelry, silverware, coins, mirrors, and dental equipment.

21. The tarnish resistant article of claim 14, wherein said self-assembled monolayer includes a functional moiety which when activated crosslinks with another functional moiety of the self-assembled monolayer.

22. A method of forming a tarnish resistant article comprised of immersing a metal article in an alkylthiol containing solution, said alkylthiol solution being at a predetermined concentration.

23. The method of claim 22, said concentration of said alkylthiol being dependent upon immersion time in said alkylthiol solution.

24. The method of claim 23, wherein said method results in said article having a self-assembled monolayer.

25. The method of claim 24, wherein said metal article is comprised of a transition metal and self-assembled monolayer formed over said surface layer.

26. The method of claim 25, wherein said transition metal is selected from the group consisting of silver and copper.

27. The method of claim 25, wherein said self-assembled monolayer is comprised of an alkanethiol layer.

28. The method of claim 27, wherein said alkanethiol has a carbon chain length in the range of about eight carbon atoms to about eighteen carbon atoms.

29. The method of claim 25, wherein said metal article is a flexible material.

30. The method of claim 25, wherein said alkylthiol is an unsaturated alkylthiol.

31. The method of claim 25, wherein said alkylthiol is halogenated alkylthiol.

32. The method of claim 25, wherein said self-assembled monolayer includes a functional moiety which when activated crosslinks with another functional moiety of the self-assembled monolayer.

33. A method of forming a self-assembled monolayer comprised of exposing a surface to a predetermined concentration of an alkylthiol for a predetermined exposure time, said predetermined exposure time being dependent on said predetermined concentration.

34. The method of claim 33, wherein said predetermined exposure time and said predetermined concentration are within about the optimum coating range illustrated in Fig. 22.

35. The method of claim 33, wherein said predetermined exposure time and said predetermined concentration are within an optimum coating range as illustrated in Fig. 22.