Surface freezing in binary mixtures of alkanes: New phases and phase transitions

Surface freezing of a crystalline monolayer has been observed at the free surface of liquid binary mixtures of normal alkanes by x-ray and surface tension measurements. Two dramatically different behaviors are found for the monolayer properties depending on Δn, the difference in the components' carbon numbers. For small Δn, the variation with temperature and concentration is continuous. For large Δn, the variation is discontinuous, exhibiting surface segregation and 2D structural phase transitions. A theory based on competition between entropic mixing and a repulsive interaction due to chain length mismatch accounts well for the observed phenomena