Surface freezing inn-alkane solutions: The relation to bulk phases

Surface freezing (SF) was investigated in tricosane-dodecane alkane solutions as a function of temperature (T) and molar concentration of tricosane (φ), using surface tension and synchrotron x-ray surface diffraction techniques. A crystalline SF monolayer, having a rotator RII structure, was found to exist for 35^∘C<~T<~50^∘C and 0.3<~φ<~1. The extended temperature range allowed to determine the linear-expansion coefficient of the SF monolayer, (dd/dT)/d=6.5×10^-4^∘C^-1. A simple thermodynamical model based on the theory of ideal solutions is shown to account well for the φ dependence of the SF temperature Tₛ(φ). The study shows that the temperature range of existence of the surface frozen layer at each φ, the φ range over which SF is observed, and the bulk solidification behavior, are intimately related. All are determined by the rotator-liquid dissolution line TdR(φ).