Surface freezing in n-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 (phi), using surface tension and synchrotron x-ray surface diffraction techniques. A crystalline SF monolayer, having a rotator R-II structure, was found to exist for 35 degreesC less than or equal to T less than or equal to 50 degreesC and 0.3 less than or equal to phi less than or equal to 1. The extended temperature range allowed to determine the linear-expansion coefficient of the SF monolayer, (dd/dT)/d=6.5 x 10(-4) degreesC(-1). A simple thermodynamical model based on the theory of ideal solutions is shown to account well for the phi dependence of the SF temperature T-s(phi). The study shows that the temperature range of existence of the surface frozen layer at each phi, the phi range over which SF is observed, and the bulk solidification behavior, are intimately related. All are determined by the rotator-liquid dissolution line T-dR(phi).