Theory of cooperative phenomena in monolayers of hydroxyhexadecanoic acid isomers

Abstract

Kellner and Cadenhead have obtained surface pressure/area isotherms for monolayers of several hydroxyhexadecanoic acid isomers. They find considerable differences in behaviour, depending on the position of the hydroxyl group on the carbon chain, and divide the isomers into “monopolar” and “bipolar” types. A lattice fluid model is introduced here with monomer states to represent erect conformations of the molecules, a dimer state to represent conformations parallel to the interface and vacant sites or “holes”. Using a Flory–Huggins type approximation the main behaviour types are reproduced. There are second-order (slope-discontinuity) transitions on some isotherms for all cases and first-order (liquid–gas) transitions in addition for the “monopolar” case with the monomer states energetically favoured. For “bipolar” cases with the dimer state energetically favoured the isotherms are more extended with nearly horizontal portions on the high-density side of the transition at some temperatures. With appropriate interaction parameters there is a region of negative thermal expansion on the low-density side of the transition similar to that found experimentally in 9-HHA monolayers.