Molecular Properties of Surfactants

I designed this exercise to demonstrate visually the two structures formed by surfactants in water: surface monolayers and micelles. It works equally well as a classroom demonstration.

Surface Monolayer: In a 250-mL beaker nearly filled with water, students sprinkle pepper on the water's surface and record what happens (the pepper floats). They then dip a piece of soap (could be the soap they made) into the middle of the surface and record what happens (the surface clears of pepper around the soap). Finally, they sprinkle more pepper into the clear space and record observations.

There are two equally valid and, indeed, mutually dependent ways to explain the clearing of the water surface of pepper. One is that the monolayer of soap molecules pushes the pepper away as it spreads. The other is that residual surface tension outside the region occupied by the monolayer pulls the pepper away from the monolayer.

Micelles: In separate containers (beakers or test tubes), students put either water or a fairly concentrated detergent solution. Into each, they put a drop of vegetable oil and mix well (mixing is improved if the solutions are warmed) and record observations. They then compare water, original detergent solution, and their oil/detergent mixture with a Tyndall apparatus. We use an overhead projector covered with a piece of cardboard which has about an eighth-inch hole in it to let a light beam pass. There is an obvious Tyndall effect with the oil dispersion, but also with the detergent solution, which is more unexpected since it appears clear. In my yet limited experience, uncolored commercial dishwashing detergent seems to work best here. I colored my oil with enough oil red O dye to make it deep red, so it is more visible.

Both the Tyndall effect with the detergent solution and the (nearly) uniform dispersal of the added oil demonstrate the presence of micelles.

The exercise concludes with questions asking the students (1) whether, from all their observations, pepper is more or less dense than water, (2) what causes the pepper to float on water, (3) why the clear area formed, (4) why the second pepper added didn't float, (5) whether oil is more or less dense than water, (6) why oil doesn't dissolve in water, (7) why the oil disperses well in the detergent solution, and (8) to explain their observations of the Tyndall test.

As I mentioned above, this can be readily demonstrated in the classroom with the aid of an overhead projector.

Revised 9/1/06