Procedure developed by George Lisensky based on the Tollens' Test
and the well-known self-assembly of thiol monolayers (SAM) on gold surfaces.
The aldehyde group in glucose, , reduces
Ag(NH3)2+ to Ag metal. The silver is then coated with a self-assembled monolayer of octadecanethiol,
, making 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 4 large drops (left) or 8 small drops (right)
of a 0.5 M glucose solution on the microscope slide. Add 12 large drops (left) or 25 small drops (right)
of an active silver ion solution (see below) 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 since it will stain your hands. Without touching the silver solution,
remove the slide from the Petri dish and rinse the silver mirror with water.
Wait for the surface to appear dry. (For faster drying use a hair dryer.)
Cover only part of the silver with a few drops of a long chain alkanethiol solution in ethanol (see below).
One way to cover only part of the silver is to rest the slide at an angle. 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 monolayer coated surface? To the silver surface? To the glass?
Do water drops spread out or bead up? Like attracts like. Is the water attracted more to the plain
glass, to the silver, or to the alkanethiol monolayer-coated silver?
The contact angle is between the side of a drop and the slide. Is the contact angle wide (small attraction
to the surface) or narrow (large attraction to the surface) for each surface?
Materials for 25 students
0.8 M KOH (Dissolve 0.22 g KOH in 5 mL of water.)
0.1 M silver nitrate (Dissolve 0.17 g AgNO3 in 10 mL of water.)
15 M ammonia (Concentrated aqueous ammonium hydroxide.)
Active silver ion solution, Ag(NH3)2+
Add concentrated ammonia dropwise to 10 mL of 0.1 M silver nitrate solution
until the initial precipitate just dissolves. Mix with a glass stir rod. 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. Dispense from a dropper bottle. To avoid the formation of explosive silver
nitride, discard any remaining active solution by washing down the
drain with plenty of water.
0.5 M glucose or dextrose (Dissolve 0.90 g in 10 mL of water. Dispense from a dropper bottle.) Sugar or sucrose does not work.
Alkanethiol solution. Add a very small amount (just barely visible) of a long-chain alkanethiol,
such as octadecanethiol, to 20 mL of absolute ethanol. Dispense from a dropper bottle.
, reduces
Ag(NH3)2+ to Ag metal. The silver is then coated with a self-assembled monolayer of octadecanethiol,
, making a non-polar surface on which water beads up.
