(475b) Interfacial Dynamics of Straight-Chain and Branched Hexadecanol and Eicosanol Mixtures

Langmuir monolayers of mixtures of straight-chain and branched molecules of hexadecanol and eicosanol were studied using surface pressure-area isotherms, Brewster angle microscopy, interfacial rheology measurements, as well as X-ray diffraction and reflectivity. For hexadecanol mixtures below 30% branched molecules, the isotherms show a lateral shift to a decreasing area proportional to the fraction of straight chains. Above 30% branched fraction, the isotherms are no longer identical in shape. The surface viscosities of both straight and branched chains exhibit a maximum in the condensed untilted LS phase at π=20 mN/m. Adding branched chains results in a nonmonotonic increase in surface viscosity, with the maximum at 12% branched chains. A visualization of these immiscible monolayers using Brewster angle microscopy in the liquid condensed phase reveals discrete domains that initially increase in number density, then decrease with increasing surface pressure. Eicosanol mixtures exhibit different behavior from hexadecanol mixtures. The addition of branched chains results in a lateral shift to increasing area, proportional to the fraction and projected area of both straight and branched chains. A phase transition is seen for all mixtures, including pure straight chains, at π=15 mN/m up to 50% branched chains. A second transition is seen at π=25 mN/m when the isotherms cross over. Above this transition, the isotherms shift in the opposite direction with increasing branched fraction. The surface viscosities of both straight and mixed monolayers show a maximum in the L₂' phase near π=5 mN/m. The surface viscosity is constant for low branched fractions, and decays beyond 15% branched chains. For fatty alcohol mixtures at high surface pressure in the LS phase, X-ray reflectivity studies suggest that the branched chains are not expelled from the monolayer to form a second layer. In this region, the lattice spacing is independent of branched fraction, verifying that the monolayer consists of intact, ordered straight chains. The branched fractions are likely expelled partly into noncrystalline regions of the monolayer and partly into micelles in the subphase. At low surface pressures in the L₂' phase, the peak positions in Q_xy increase with surface pressure as the lattice spacing decreases, and are independent of branched fraction. These results, together with the isotherm data, suggest that the monolayer phase separates into pure straight and pure branched regions. At intermediate surface pressures, the phase behavior is not understood.