Vapor Sorption into Pure and Mixed Fatty Acid Multilayers

Abstract

The sorption of polar and nonpolar vapors into Langmuir-Blodgett multilayers of pure arachidic acid (C 2 0 ) was compared to that in films formed of a 1:1 mixture of behenic acid (C 2 2 ) and stearic acid (C 1 8 ). The mass of vapor sorbed was quantified using a quartz crystal microbalance. Film swelling was determined using multiple beam interferometry as implemented in the surface forces apparatus. We find that the solubility of water in the pure and the mixed films is the same. Moreover, the absorption of water is ideal in the sense that the film swelling observed is consistent with the incorporation of water at its bulk molar volume. In contrast, the solubilities of pentane, decane, and pentanol were higher in the mixed fatty acid films than in the pure film. The additional free volume available in the mixed films (due to alkane chain mismatch) also results in nonideal absorption of these organic vapors. Vapor sorption kinetic data support the hypothesis that the transport rate is governed primarily by large pitlike defects spanning multiple layers within the film structure.