Faceting of monolayer domains

The formation of facets in domains of three monolayer systems, differing in molecular structure or headgroup charge, is observed by Brewster-angle mic roscopy under different experimental conditions. Arachidic acid forms facet ed 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 graduall y 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 o rder to determine the electrostatic forces in the mixture. Reversible facet ing of domains of heptadecanoic acid. methyl eater is observed within 2 deg rees C near 30 degrees 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 als o accounts for the formation of dendrites along certain directions at domai ns of arachidic acid and the methyl ester.