JPS601314B2 - Imidazolium-betaine compound and method for preventing rust of silver metal using the compound - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an imidazolium-betaine compound and a method for synthesizing the compound, and in particular provides a compound that exhibits a rust-preventive effect on silver metal.

Additionally, it is already known that certain imidazole compounds exhibit a rust-preventing effect on copper metal.

(For example, Special Publications No. 46-17046 and No. 48-114)
(See Publication No. 5). However, known imidazole compounds such as 2-undecyl imidazole and 2-undecyl-4-methylimidazole described in the above-mentioned Japanese Patent Publications No. 17046 and No. 48-1145 are not effective for rust prevention of copper metal. However, it does not show any rust-preventing effect on silver metal. The inventors of the present invention conducted extensive research in search of an imidazole compound having a rust-preventing effect on silver, and finally discovered that an imidazolium-petaine compound represented by the following formula corresponds to it.

[However, R2 is a residue selected from the group consisting of a methyl group, an ethyl group, an undecyl group, a heptadecyl group, and a phenyl group.

R4 is a hydrogen atom or a methyl group. To date, only the following imidazo-1'ium-petane compounds obtained by the reaction of a 1-position unsubstituted imidazole compound and acrylic acid are known.

That is, J. Mr. Bian Jones et al. [Canadian
・Jom female lofChemistry, 1567-15
73 pages, vol. 48, written in 1970], 1, 3 bis (2
-carboxyethyl)-imidazolium-betaine.

1,3-Bis(2-ruboxyethyl)-imidazolium-betaine However, the above-mentioned report relates to a study conducted for the purpose of investigating the relationship between the addictive mechanism and the alkylating agent.
This was not done for the purpose of investigating the rust-preventing effect on silver.

Moreover, the below-mentioned imidazo I obtained by the method of the present invention
All of the Jumobetaine compounds are compounds that have not been described in any literature.

Next, embodiments of the synthesis of imidazolium monopetine will be described.

40 qo or more of 1 mol or more of the 1-substituted imidazole compound and 2 mol or more of acrylic acid in the coexistence of a small amount of polymerization inhibitor (for example, hydroquinone or sulfuric acid steel) in a reaction vessel equipped with a reflux condenser and a sulfuric acid machine. When heated for an appropriate time at a temperature of , the following reaction occurs and the desired product is produced.

Unsubstituted imidazole at the 1-position of acrylic acid 1,3-bis(2-carboxyethyl)-imidazolium monopetaine [wherein R2 is a residue selected from the group consisting of methyl group, ethyl group, undecyl group, heptadecyl group, and phenyl group It is.

R4 is a hydrogen atom or a methyl group. ] After the reaction is completed, the contents are purified according to a conventional method.

Although the above-mentioned reaction proceeds satisfactorily in the absence of a solvent, it is difficult to remove the reaction heat when a large amount is reacted, so it is desirable to carry out the reaction in a suitable solvent to remove the reaction heat. Furthermore, in order to suppress the generation of reaction heat within a short period of time, it is of course preferable to carry out the reaction while dropping acrylic acid little by little.

The solvent used must be inert towards the source and product systems.

For example, they are as shown below. i.e. water, methanol, ethanol, acetone, acetonitrile, benzene, toluene, dioxane, inpropyl alcohol.
Ether etc. The reaction temperature may normally be 40° C. or higher, but in consideration of shortening the reaction time, removing reaction heat, and controlling the reaction temperature, it is most preferable to set the reaction time to the reflux temperature.

For example, when acetonitrile is used as a solvent and the reaction is carried out at reflux temperature, the reaction is completed within several hours.

Of course, the desired product can also be obtained by using 2 mol or less of acrylic acid per 1 mol of imidazole in this reaction, and after the reaction is completed, unreacted imidazole in the reaction mixture is extracted and removed with a solvent.

To prevent acrylic acid from polymerizing during the reaction,
It is preferable to add a small amount of a polymerization inhibitor to the reaction system in advance.

Various commercially available polymerization inhibitors can be used.

The properties of various imidazolium betaines obtained by the method of the present invention are shown below.

1,3-bis(2-carboxyethyl)-2-methylimidazolium-betaine structural formula colorless crystal.

m. p. 211-214q0 (methanol). Acidic. Easily melts in water. Soluble in methanol and ethanol. TLC (Silica G, EtOH, 12 colors): Rfo. 08~0.0
0 foundation snow:・72o (first absorption), 1590'1520, 1
433, 1375, 1323, 1265, (3rd absorption)
,1240,116&1120,1045,975,9
40,860,805,780 (first absorption) NMR (D
20): 67.40, s, (4th and 5th protons); 6
4.33,t, crab (=N-C ratio-);62,80,t,
4th (-CH2-COO); 62.67, s, crude (methyl group) Mass: m/e26 (M+), 154 (M 1172), 109 (154 1 COOH), 95 82,8
1,73 72,55 541,3-bis(2'-carboxyethyl)-2-ethylimidazolium monopetaine structural formula colorless crystal.

Acidic. m. p. 195-197℃ Easily soluble in water. Dissolved in methanol and ethanol. TLC (Silica G, EtOH
, 12 color development): Rfo. 08~0.00 approx.:17oo
, 1525, 1441'1 side (first absorption)'12351
112,960, (first absorption), 820,730 (second absorption)
Absorption) NM eaves (D20): 87.39, s, 2 days (4,
5th position proton); 64.37, t, Yamabi (=N1C Koichi); 63.10, q, (2nd position methylene); 62.85
, t, Misaki (-CH2-COO); 81.29, t, Carp (
2nd position terminal methyl) Mass: m/el95 (M1145
), 168 (M+172), 123 (168-COOH
), 95 81,72,55,451,3-bis(2'
-Carboxyethyl)-2-undecyl imidazolium-betaine Structural formula: Colorless crystals.

m. p. 148-149.5°C (acetone). Acidic. Easily soluble in water. TLC (Silica G, EtOH, 12 colors): R
fo05~000re Toru Hadahata: 167o (1st absorption), 1
527,1470'795'772 (second absorption) NMR
(CDC13): 67.54, br. s, is (4. milk standing proton); 4.26, br. s, Cape (=N-CH2-
); 3.08, br. s, 2 days (》-CH2-); 2.
78, br. s, 4 days (1C ratio - COO); 1.24,
br. s, 1 thin (polymethylene); 0.88, t, 3 days (terminal methyl) Mass: m/e1294 (M-72)
,223,222(imidazole)1,3-bis(2'
-Ruboxyethyl)-2-heptadecylimidazolium-petaine Structural formula: Colorless crystals.

Acidic. m. p. 148-150 ko○ (acetone) Refractory to water, methanol, ethanol, acetone, and DMSO. Easy port to acetic acid. TLC (Silica G, EtOH, 12 colors):
Rfo05~000 lacquer horse mackerel:・72o, 1670, (1st absorption), 1585, 1525, 1470, 1380,
775 (second absorption) NM liver (CF3C〇〇H): 67.
44, S, 2 days (4, 5 ~ protons); 4.52,
t, cape, (=N-CH2-); 3.14, m, group (1C
H2-COOH+o-position methylene); 1.33, m, 30
day (long chain methylene); 0.90, t, pan (terminal methyl)
Mass: m/e 378 (M+-72), 306 (2-heptadecylimidazole) 1,3-bis(2-carboxyethyl)-2-phenylimidazolium-petaine Structural formula Colorless oil.

Acidic. Easily melts in water and methanol. Soluble in ethanol. Difficulty with acetone. TLC (Silica G, EtOH, 12 colors): Rfo. 10~0.00re Hina board: 172o, 158
o'15oo'140o'12oo' side NMR (D20
): 67.60, m, 7 days (phenyl proton 14. kalten proton); 4.20 t, 4 days (=N-CH2-)
;2.64, t, 4th (=NC Koichi COO-) Mas
s: m/e 288 (M+), 216 (M+-acrylic acid), 171 (M-11 acrylic acid-COOH), 144 (
2-phenylimidazole), ..., 1,3-bis(2'
-Carboxyethyl)-[2-ethyl-4(5}-methylimidazolium]-petaine Structural formula Colorless oil.

Mild acidity. Soluble in water and methanol. Dissolved in acetone. T
LC (Silica G, EtOH, 12 colors): Rfo. 10
~0.00re Minato board: ・73o, 1585 (first absorption),
1395 (second absorption), 1190, .・・・． ...NM old (D20): 67.12, br. s, and 67.02,
br. s, total IH (4. Scattered protons); 64.27
, 2 t, 4 days (N-CH2-) 6300, m, 2
Sun (2nd place methylene); 62.70, m, Cape (-CH2
COO); 62.30 and 2.24, 2 s, Chihito (4
．． 5th methyl); 61.30, 2 t, insect day (2nd methyl) Mass: m/e 254 (M 10), 209 (M +
-COOH), 182 (M+-72), 137 (182
-COOH), 110, 109, 95,...・・１、３
-Bis(2'-carboxyethyl)-[2-undecyl-4'5'-monomethylimidazolium]-betaine Structural formula Colorless oil.

Acidic. Elutriate in water, chloroform, methanol, ethanol, acetic acid and acetone. TLC (Silica G, EtOH,
12 colors): Rfo. 10~○-00 lacquer board. :340
0,1740,1580, (first absorption, ruboxylate), 1410 (ruboxylate) NM liver (CD30
D): 67.32, S, IH (proton at 5th position); 4.3
6, m, 4 days (N-CH2-); 3.16, m, 2 days (Q-position methylene); 2.95, m, 4 days (-C ratio - CO
O-); 2.35, S, plaque (4-position methyl); 1.29,
m, 19 days (long chain methylene); 0.89, t, mass (terminal methyl) Mass: m/e 308 (M+-acrylic acid)
, 263 (308-COOH), 236 (2-undecyl-4-methylimidazole), 72 (acrylic acid),
55(CH2=CH.CO.), 45(COOH)1,
Colorless crystals of 3-bis(2'-carboxyethyl)-[2-phenyl-4'5i-methylimidazolium]-betaine structural formula.

Acidic. m. p. 170-17ro. Easily soluble in water and acetic acid.
Soluble in methanol. Dissolved in acetone. TLC (Silica G
, EtOH, 12 colors): Rfo. 10~○-00 skin field r:, skin, skin 0,1410,940,770NM liver (
D20): around 67.44, m, spots (phenyl group); 7
34, S, IH (4th position proton); 4.21 and 416,
2 t, 4 days (white N-CH2-): 2.64 and 252
, 2 t, Cape (-C ratio-COO-); 2.38, S,
Crude (5th methyl). Mass: m/e230 (M+-7
2), 185 (230-COOH), 158 (230~
72), 10472 (acrylic acid), 55, 45 (1C
OOH) Next, an embodiment of the rust prevention method for silver metal according to the present invention will be described.

The imidazolium-betaine is dissolved in a solvent to form a solution, and the solution is brought into contact with a polished silver surface at room temperature (
(Coating, misting, or immersion methods are acceptable) Then, the solvent adhering to the surface is evaporated. The surface of silver metal treated in this way maintains its brightness even when left in the air for a long time, and shows no tendency to darken. In carrying out the present invention, it is possible to prepare a paste by mixing the solution with a suitable abrasive, and it is also possible to obtain a rust-preventing effect by polishing the silver surface with the paste. Since this means that the solution is also in contact with the fibers, it goes without saying that it is within the scope of the present invention. The present invention will be explained in detail below using examples.

Example 1 0.05 mol (4.1 g) of 2-methylimidazole, 0.11 mol (7.1 g) of acrylic acid, 2 g of hydroquinone (polymerization inhibitor), and 3 units of acetonitrile were mixed in a reflux condenser and Nada Azusa. The reaction mixture was heated to reflux for 10 minutes in a reaction vessel equipped with a vacuum cleaner, and the reaction mixture was concentrated under reduced pressure to remove residual crystals. I got sustenance.

The properties of this crystal are as follows. Melting point 200-204
℃. TLC (Silica G, EtOH, methyl red color)
:Rfo. 55 to 0.40 (very light, equivalent to acrylic acid), 0.15 to 0.00 (dark, equivalent to the target product).
TLC (silica, EtOH, 12 colors): Rfo. 90
~0.70 (extremely thin, equivalent to hydroquinone), 0
．． 55 to 0.40 (very light, equivalent to acrylic acid),
0.15 to 0.00 (dark. Corresponding to the target material). The crystals were extracted with 300 methanol using a Soxhlet extractor, the extract was concentrated to a concentration of 100, and then cooled.
The precipitated crystals were dried in a furnace to obtain 7 g (yield 62%) of the target product 1.3-bis(2'-carboxyethyl)-2-tylimidazolium-betaine (melting point 211-214°C). Ta. Example 2 0.05 mol of 2-ethylimidazole (4. total), 0.11 mol of acrylic acid (7. Shigeru), 48 hg of hydroquinone as a polymerization inhibitor, and 4.8 mol of acetonitrile were combined into 1
After heating under reflux for an hour, 19% of acetone was added to the reaction mixture, the reaction mixture was cooled, and the precipitated crystals were dried in an oven.
-Pis(2'-carboxyethyl)-2-ethylimidazolium-betaine (melting point 193-195°C)10. Spots (crude yield: %) were obtained.

TLC (Silica G, EtOH, 12 colors): Rfo. 8
0 to 0.70 (very light, equivalent to hydroquinone),
0.10 to 0.00 (dark, target). A portion of the crystals was further recrystallized with ethanol decahydrate (3:ol. ratio) to obtain an identified sample (melting point 195-1970). Example 3 2-undecylimidazole 0.05 mol (11.1 g
), acrylic acid 0.12 mol (8 g), hydroquinone 11 g, and acetonitrile 11 g were heated under reflux for 2 hours, then 44 g of acetone was added, and the reaction mixture was cooled to precipitate crystals. was dried in an oven to obtain crude 1,3-bis(2'-carboxyethyl)-2-undecylimidazolium-betaine (melting point 138-140°C. 15.5 g
(assembly yield 85%) was obtained.

TLC (Silica G, EtOH, 12): Rfo. 60～
0.50 (very light, equivalent to acrylic acid), 0.10
~0.00 (dark. Equivalent to the target object). A part of the crystals was extracted with acetone using a Soxhlet extractor, and the precipitated crystals in the extract were collected in a furnace to obtain an identified sample (melting point 148-149.5°C).
And so. Example 42-Heptadecylimidazole 0.
05 mol (15. mol), 0.14 mol (10 mol) of acrylic acid
．． female), 15 grams of Hyde-o-quinone and 1 serving of acetonitrile
5's 4 people 4. The reaction mixture was heated to reflux for an hour, the reaction mixture was cooled, and the precipitated crystals were dried in an oven to give 20.7 g of 1.3-bis(2-carpoxycetyl)-2-heptadecyl imidazolium-betaine (melting point 93-96 qo) (lees yield 92 %)
I got it.

TLC (Siri force G, EtOH, 12): Rfo. 20~
0.00 (very light, impurity), 0.00 (dark, target). A part of the crystals was extracted with acetone using a Soxhlet extractor, and the precipitated crystals in the extract were collected in a furnace and used as an identification sample (
The melting point was 148-150°C). Example 5 0.05 mol of 2-phenylimidazole (7.0%), 0.12 mol of acrylic acid (8.0%), 7.0mg of hydroquinone, and 7M of acetonitrile were mixed in 2.2%. After heating under reflux for an hour, the reaction mixture was concentrated under reduced pressure to leave a bottle-shaped residue. & got.

TLC of the residue is as follows. TLC (Silica G, EtOH, 12 colors): Rfo. 80~0.60(
Very thin. hydroquinone equivalent), 0.50 to 0.
20 (extremely thin, equivalent to acrylic acid), 0.10 to 0
．． 00 (Dark. Equivalent to the target object). This one is therefore coarse 1,
It is determined to be 3-bis(2'-carboxyethyl)-2-phenylmethylimidazolium-betaine. The residue 1
4. The mixture was heated to reflux with 140% acetone, and after cooling, the lower layer was separated and collected.Additionally 86% acetone was added to the sampled liquid, and heated under reflux again.After cooling, the lower layer was separated and collected, and the collected liquid was concentrated under reduced pressure. An oily residue (crude yield: 69%) was obtained. The TLC of this product is as follows. TLC (Silica G,
Day E, 12 colors): Rfo. 80-0.55 (extremely thin, equivalent to hydroquinone), 0.35-0.20 (
Very thin. (equivalent to acrylic acid), 0.10 to 0.00 (
dark. As shown by TLC (corresponding to the target substance), the residue was found to be insufficiently purified to be used as an identification sample, so it was further dissolved in ethanol, passed through an activated Shirashi layer, and the adsorbed layer was purified with hot water. The refined solution was concentrated under reduced pressure to obtain an oily residue [TLC (Silica G, EtOH, 12 color development): R
fo. 10 to 0.00] was used as the identification sample. Example 6
2-ethyl-4-methylimidazole 0.05 mol (5
．． Acid), 0.125 mol of acrylic acid (9.5 mol), 20 hg of hydroquinone, and 5 mol of acetonitrile were heated under reflux for 2 hours, 10 mol of acetone was added, and after cooling, the upper layer was removed, and the lower layer was Concentration under reduced pressure yields an oily residue12.
Oni (group yield 98%) was obtained.

TLC (silica G, E day, 12 colors): Rfo. 60
~0.40 (extremely thin, acrylic acid), 0.10~
0.00 (Dark.Objective). This one is therefore roughly 1.3
-bis(2'-carboxyethyl)-[2-ethyl-4
'5' monomethylimidazolium] 1petine. As shown by TLC, the residue was judged to be insufficiently purified to be used as an identified sample, so it was further adsorbed on a silica gel column, washed with ethanol, the adsorbed layer was washed with hot water, and the debriefing solution was concentrated under reduced pressure. oily residue [TL
C (silica G, E day, 12 colors): Rfo. 10-0
．． 00] was used as the identification sample. Example 7 0.05 moles of 2-undecyl-4-methylimidazole (11.0%), 0.11 moles of acrylic acid (7.00%), 12.0mg of hydroquinone, and 12.0mg of acetonitrile were combined into 2.0% of the above four. The reaction mixture was heated under reflux for a period of time and concentrated under reduced pressure to obtain one patch of oily residue (100% crude yield).

TLC (Silica G, EtOH, 12 colors): Rfo. 8
0 to 0.65 (very light, equivalent to hydroquinone),
0.35-0.15 (very light, equivalent to acrylic acid)
, 0.15 to 0.00 (dark, equivalent to target). This is therefore 1.3-bis(2-carboxyethyl)-[2
-Undecyl-4'5}1-methylimidazolium]1-petaine. As shown by TLC, the residue was judged to be insufficiently purified to be used as an identification sample, so 470 ml of acetone was added to the residue, heated to reflux,
After cooling, the acetone solution portion was collected with a slant, concentrated under reduced pressure to form an oily substance, dissolved in 120 mm of ethanol, and subjected to column chromatography using silica gel 7 mm. Wash with hot water, concentrate the refined liquid under reduced pressure, and remove the oily substance 11. Foundation (assembly yield 62%)
I got it. TreC (Silica G, EtOH, 12 colors): R
fo. 80-0.70 (very thin, equivalent to hydroquinone), 0.15-0.10 (very thin, impurities)
, 0.10-0.00 (dark, target). As shown by TLC, the oily substance was judged to be insufficiently purified as an identified sample, so a portion of this was taken, dissolved in methanol, passed through an activated clay layer, and the clay layer was rinsed with aqueous ammonia. , an oily substance obtained by concentrating the washing liquid under reduced pressure [T
LC (Silica G, EtOH, 12 colors): Rm. 10~
0.00) was used as the identification sample. Example 8 0.05 mol of 2-phenyl-4-methylimidazole (
7.9 g), 0.11 mol of acrylic acid (8.9 g), 5 g of hydroquinone, and 8 mol of acetonitrile.
After heating under reflux for an hour, the reaction mixture was dried under reduced pressure.
．． One piece of 3-bis(2'-carboxyethyl)-[2-phenyl-4(5)-monomethylimidazolium]-betaine (compound yield 99%) was obtained.

TLC (Silica G, EtOH, 12 colors): Rfo. 8
0 to 0.65 (extremely thin, equivalent to hydroquinone),
0.55-0.20 (extremely thin, equivalent to acrylic acid)
, 0.12 to 0.00 (dark. Corresponding to the target material). This product was recrystallized from 60' acetone and had a melting point of 158-160.
9. Crude crystals at °C. (assembly yield 61%) was obtained. A portion of this material is dissolved in methanol and passed through an activated clay layer.
The clay layer was washed with aqueous ammonia, and the pig washing liquid was concentrated under reduced pressure to obtain an identified sample (melting point: 170-171°C). Example 9 0.05 mol (4.1 g) of 2-methylimidazole, 0.11 mol (7.1 g) of acrylic acid, and 2 g of hydroquinone as a polymerization inhibitor were prepared using a reflux condenser and a drying machine. The reaction vessel was heated for 20 minutes through an aluminum block overflowed to 120°C.

The reaction mixture showed a white mass.

After cooling, the lumps were crushed and extracted with 300 methanol using a Soxhlet extractor, and the extract was diluted with 100 methanol.
After concentrating until it becomes
Methylimidazolium-betaine (melting point: 202-20: 0) was obtained (yield: 80%). Example 10 2 hg of hydroquinone of Example 9 was added to sulfuric acid steel crystal 010
The same procedure as in Example 10 was carried out except for replacing the target with hg.
-carpoxyethyl)-12-methylimidazolium-betaine was obtained (yield: 80%).

Example 111,3-bis(2'-carpoxyethyl)
0.50WV of 12-methylimidazolium-betaine
)% aqueous solution was prepared.

Next, after polishing the surface, the right half of a pure silver plate (width 1.5cm, length 10cm) that had been thoroughly washed with acetone was immersed in sand at room temperature for 6 nights in the aqueous solution, and then the silver plate was removed from the aqueous solution. It was lifted up, shaken lightly to remove any droplets on the surface, and left indoors. While the left half gradually began to discolor after being left alone, the right half continued to maintain its luster. After 90 days, the right half remained glossy, while the left half had already completely turned black.

Example 12 The same procedure as in Example 11 was carried out using a 0.5 M/V)% aqueous solution of 1,3-bis(2'-hydroxyethyl)-2-ethylimidazolium-betaine.

As I left it alone, the left half began to gradually discolor.
The right half remained shiny. After 21 days, the right half had maintained its luster, while the left half had already turned blackish brown.

Example 13 The same treatment as in Example 11 was carried out using a 0.50y'V)% aqueous solution of 1,3-bis(2'-carboxyethyl)-2-undecylimidazolium-petaine.

After 40 days, the right half remained glossy, while the left half had already completely turned black.

Example 14 1,3-bis(2'-carboxyethyl)-12-hef.

0.5W/V of Tadecyl Imidazo IJ Umu Betaine)
Exactly the same treatment as in Example 11 was carried out using a warm aqueous solution of %. After 40 days, the right half remained glossy, while the left half had already completely turned black.

Example 15 0.0% of 1,3-pis(2'-carboxyethyl)-2-phenylimidazolium-betaine. Further, the same treatment as in Example 11 was performed using a W/V)% aqueous solution.

After 14 days, the right half had maintained its luster, while the left half had already turned yellow.

Example 16 0 of 1,3-bis(2'-carboxyethyl)-[2-ethyl-4(5)-monomethylimidazolium]-betaine
．． The same treatment as in Example 11 was carried out using a 9W'V)% aqueous solution.

After 30 days, the right half remained glossy, while the left half had already turned black.

Example 17 0 of 1,3-bis(2'-carboxyethyl)-[2-undecyl-4■-monomethylimidazolium]-betaine
．． The same treatment as in Example 11 was carried out using a 5W'V)% warm aqueous solution.

After 21 days, the right half had maintained its luster, while the left half had already turned blackish brown.

Example 18 The same treatment as in Example 11 was carried out using a 0.8W'V)% aqueous solution of 1,3-bis(2'-carboxyethyl)-[2-phenyl-4{51-methylimidazolium]-betaine. went.

After 14 days, the right half had maintained its luster, while the left half had already turned yellow.

Claims (1)

[Claims] 1 Structural formula ▲ Numerical formula, chemical formula, table, etc. ▼ [However, R_2 is a residue selected from the group consisting of a methyl group, an ethyl group, an undecyl group, a heptadecyl group, and a phenyl group, and R_4 is hydrogen Represents an atom or a methyl group. ] 1,3-bis(2'-carboxyethyl)
-Imidazolium-betaine compounds. 2 Structural formula ▲ There are numerical formulas, chemical formulas, tables, etc. ▼ [However, in the formula, R_2 and R_4 are the same as above. ] 1,3-bis(2-carboxyethyl)-
A method for preventing rust of silver, which comprises bringing a solution of an imidazolium-betaine compound into contact with the surface of a silver metal, and volatilizing a solvent attached to the surface of the silver metal.