BENDING ELASTIC-MODULUS OF MONOLAYERS AT OIL-WATER INTERFACES

Double scattering of light from interfaces with small slope roughness (studied by ellipsometry at the Brewster angle) gives information on the spectrum of the roughness at ultramicroscopic scales. This technique applied to a monomolecular film of verv low surface tension at the oil-water interface (the roughness is due to thermal fluctuations) allows for the measurement of the bending elastic constant K of the monolayer. K has been measured for an ionic surfactant (AOT) at the oil-water interface for a sequence of saturated oils of linear chains C(N). It has been found that for small chain length oils (N < 11) that penetrate the film (the oil is a good solvent for the aliphatic chains of AOT) the bending elastic constant is high (K/k(B)T approximately 1). For long chain lengths (N > 11) that do not penetrate the film (the oil is a bad solvent for the aliphatic chains of AOT), the bending elastic constant is five to ten times smaller than in the previous case.