Faceting of monolayer domains

Abstract

The formation of facets in domains of three monolayer systems, differing in molecular structure or headgroup charge, is observed by Brewster-angle microscopy under different experimental conditions. Arachidic acid forms faceted domains on increasing the pH of the subphase from pH 12.0 to 12.3 or by adding traces of cholesterol at pH 12.0. The degree of faceting is gradually varied in a mixture of 1-monopalmitoyl-rac-glycerol and 1-O-hexadecyl-rac-glycerol. The surface potentials of the pure substances were measured in order to determine the electrostatic forces in the mixture. Reversible faceting of domains of heptadecanoic acid methyl ester is observed within 2 °C near 30 °C. Faceting in monolayers is explained on the basis of fundamental forces that govern domain texture and shape, including line tension anisotropy and electrostatic repulsive forces. This explanation also accounts for the formation of dendrites along certain directions at domains of arachidic acid and the methyl ester.