The Effect of Headgroup Interactions on Structure and Morphology of Arachidic Acid Monolayers

Abstract

The structural and morphological features of arachidic acid monolayers are studied systematically between pH 2 and 13 by X-ray diffraction, Brewster-angle microscopy, and thermodynamic measurements on subsolutions free of polyvalent ions. A monotonic relationship is found between the pH and the distortion of the lattice perpendicular to the chain axes. A nonmonotonic pH dependence is found for structural parameters like lattice spacings, tilt angles, molecular area, and positional correlation length as well as phase boundaries. The results provide evidence that the fatty acid amphiphiles are hydrogen bonded at the interface and that the bonding strenth is affected by the degree of headgroup dissociation in a discontinuous way. Strongest bonding is found at pH 9, where 50% of the carboxylic headgroups are dissociated. Hydrogen bonding force hence opposes electrostatic headgroup repulsion and stabilizes a particular structure. At higher pH (12−13) a drastic pH dependence of the domain shapes and domain fusi...