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 fresh silver is then coated with a self-assembled monolayer of octadecanethiol,
, making a non-polar surface on which water beads up.
Place a clean microscope slide in a Petri dish.
Place 4 large drops of a 0.5 M glucose solution on the microscope slide.
Add 12 large drops of an active silver ion 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. Discard the solution by
rinsing with water and remove the slide from the Petri dish.
Wait for the surface to appear dry. (For faster drying use a hair dryer but try
not to overheat the sample while preparing a clean non-oxidized silver surface.)
Cover only part of the silver with a few drops of a long chain alkanethiol solution in ethanol.
One way to do this is to rest the slide at an angle. Allow the ethanol to evaporate at room temperature (no hair dryer),
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 a monolayer of hydrocarbons.
Data
Add small equal size drops of water (5 µL recommended) to the surfaces. 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 bottom of the drop. Is the contact angle wide (small attraction
to the surface) or narrow (large attraction to the surface) for each surface? Make an accurate sketch or preferably take a photo for your notebook so you can measure the angle.
Use a single edge razor blade to scrape off some of the silver and make a sharp transition between glass and coated silver. What happens if you drag a drop of water from the coated surface to the glass (left)? Can you make a virtual wall by running water down the glass to the thiol-coated silver (right)?
Use a goniometer to record the contact angle of a 5 µL water drop on the three surfaces. Directions
Conclusions
1. Estimate the contact angle for a 5 µL water drop on each of the three surfaces. Draw on the photograph to indicate which angle you are measuring.
2. Is the water attracted more to the plain glass, to the silver, or to the alkanethiol monolayer-coated silver? Classify the glass, the silver, and the alkanethiol coated surface as like or unlike the probe.
3. What happens if you drag a drop of water from the coated surface to the glass? Can you make a virtual wall by running water down the glass to the thiol-coated silver?
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.)
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.
0.006 M CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2SH (octadecanethiol) in absolute ethanol. Dissolve 0.17 g octadecanthiol in 100 mL ethanol. Dispense from a dropper bottle.
Active silver ion solution, Ag(NH3)2+
Add concentrated ammonium hydroxide 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 ammonium hydroxide 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.
Equipment
, reduces
Ag(NH3)2+ to Ag metal. The fresh silver is then coated with a self-assembled monolayer of octadecanethiol,
, making a non-polar surface on which water beads up.
Chemical gloves recommended
Click image for larger view of a 4µL water drop