Time-dependent behavior of insoluble monomolecular films: Fatty acids and some derivatives

Abstract

Saturated fatty acids ranging from 15 to 20 carbons in alkyl chain length and some stearic acid derivatives were spread as insoluble monomolecular films and compressed at a constant velocity. As these surface films assume close packing, they exhibit a loss in surface pressure when held at constant area below an apparent collapse pressure which is not due to dissolution into the subphase solution. The higher the surface pressure to which the film is compressed, the greater the number of repeated compressions to a given surface pressure after periods of noncompression, and the longer the time period between successive compressions, the greater the rate and extent of instability. Increasing chain length increases film stability, but different behavior is noted for odd- and even-carbon acids in a manner analogous to their melting behavior. From these studies it is concluded that three-dimensional phase separation occurs when the film is compressed to surface pressures approaching the liquid condensed-solid condensed transition point. The relative rates of molecular expulsion from the surface, and of lens formation and growth determine the rate of surface pressure loss when compression is stopped. Similar studies with methyl stearate, 2-hydroxystearic acid, methyl 2-OH stearate and methyl 9-OH stearate are consistent with these conclusions.