Procedure developed by George Lisensky based on the Tollens' Test
and the well-known self-assembly of thiol monolayers (SAM) on gold surfaces.
Coating silver with a monolayer of octadecanethiol produces a non-polar surface on which water beads up.
Procedure
Wear eye protection
Chemical gloves recommended
Place a clean microscope slide in a Petri dish. Place 8 small drops (or
4 large drops) of a 0.5 M glucose solution on the microscope slide.
Add 25 small drops (or 12 large drops) of an active
silver ion solution to the glucose solution. Gently agitate to mix the solution.
Wait several minutes while the solution darkens and a grayish precipitate
forms. A silver mirror is also forming on the slide, though it may be
obscured by the precipitate.
Use water from a wash bottle to wash off the precipitate and reveal
the silver mirror. Avoid contact with the solution sinced it will stain your hands.
Without touching the silver solution, remove the slide from the Petri
dish and rinse the silver mirror with water. How attracted are the water
drops to the surface? (Like attracts like.) Do water drops on silver spread
out or bead up?
The contact angle is between the side of a drop and the slide. Is
the contact angle wide (small attraction to the slide) or narrow (large
attraction to the slide)?
Wait for the surface to appear dry. (For faster drying rinse with
acetone or use a hair dryer.) Cover the silver with a few drops of a long
chain alkanethiol solution in ethanol.
Allow the ethanol to evaporate, leaving behind an alkanethiol
monolayer with the sulfur atoms bound to the silver and the hydrocarbon
tails pointing away. This effectively coats the surface with hydrocarbons.
How attracted are the water drops to the surface? (Like attracts
like.) Do water drops on the monolayer coated surface spread out
or bead up? Is the contact angle greater or less than before the
alkanethiol was added? Is the water attracted more to the plain
glass, to the silver, or to the alkanethiol monolayer-coated silver?
Materials
0.5 M glucose
Dissolve 0.90 g glucose in 10 mL of water.
0.8 M KOH
Dissolve 0.45 g KOH in 10 mL of water.
0.1 M silver nitrate
Dissolve 0.17 g AgNO3 in 10 mL of water.
15 M ammonia
active silver ion solution
Add concentrated ammonia dropwise to 10 mL of 0.1 M silver nitrate solution
until the initial precipitate just dissolves. Add 5 mL of 0.8 M KOH solution;
a dark precipitate will form. Add more ammonia dropwise until the precipitate
just redissolves. This "active silver" solution should be used
within an hour of preparation. To avoid the formation of explosive silver
nitride, discard any remaining active solution by washing down the
drain with plenty of water.
alkanethiol solution
Add a small amount of a long-chain alkanethiol,
such as octadecanethiol, to 20 mL of ethanol. When finished, dispose of
this solution by adding about 5 mL of household bleach. Let stand for several
minutes then wash solution down the sink.
