Surface and bulk phase behavior of dry and hydrated tetradecanol : octadecanol alcohol mixtures

Surface freezing was studied in dry and hydrated octadecanol:tetradecanol (C18OH:C14OH) mixtures, using surface tension and synchrotron x-ray surface diffraction techniques. Even small amounts of admixed C18OH were found to induce surface freezing in C14OH, which does not exhibit this effect when pure. The phase diagram of the bulk was measured by calorimetry and bulk x-ray diffraction. Upon increasing the bulk mole fraction of C18OH (phi) a sharp increase in the bulk supercooling occurs at phiapproximate to0.4 in dry mixtures, while no supercooling was observed for the hydrated mixtures. A simple thermodynamical model based on the theory of s-regular mixtures is shown to account well for the dependence of the surface freezing onset temperature of both dry and hydrated mixtures, and the hydrated bulk's freezing temperature on phi. Only a phenomenological description exists for the dry bulk's phase diagram. This study is expected to provide a baseline for the general surface and bulk behavior of long-chain alcohol mixtures. (C) 2002 American Institute of Physics.