Question

A researcher measures the equilibrium separation between two flat hydrophilic mica plates in a solution of non-ionic surfactant. When the surfactant concentration is below CMC the surfaces touch at 0 nm distance without any separation (i.e., there is no surfactant adsorption and repulsion).

a) The researcher increases the surfactant concentration much above CMC and finds out that the system has equilibrium separations at 0 nm, 4 nm, 8 nm, 12 nm and some at larger distances. Draw schematics and explain the reason for this effect.

b) The researcher hydrophobizes the mica surfaces and again dips them in the first solution with surfactant concentration below CMC. When these surfaces are brought in contact, they remain separated at a distance of 4 nm (no equilibria at larger separations). Draw a schematic and explain the reason for this observation with hydrophobic surfaces.

Answer 1