Interfacial Rheology and Structure of Straight-Chain and Branched Hexadecanol Mixtures

Langmuir monolayers of mixtures of straight-chain and branched forms of hexadecanol were studied using surface pressure-area isotherms, Brewster angle microscopy, and interfacial rheology measurements. For mixtures containing less than 30% branched molecules, the isotherms show only a lateral shift to a lower area that is proportional to the percentage of straight chains. Above this fraction, the isotherms are qualitatively different. The surface viscosities of both straight and mixed monolayers show a maximum in the condensed untilted phase near (pi)= 20 mN/m. The addition of branched molecules results in a non-monotonic increase in surface viscosity, with the maximum occurring near 12% branched molecules. Visualization of these immiscible monolayers using Brewster angle microscopy in the liquid condensed phase reveals the formation of discrete domains that first increase in number density and then decrease as surface pressure is increased.