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 $\Delta n$, the difference in the components' carbon numbers. For small $\Delta n$, the variation with temperature and concentration is continuous. For large $\Delta n$, 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.