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Solvent dependence of the step free energy on side faces of naphthalene crystals

1988; Taylor & Francis; Volume: 64; Issue: 2 Linguagem: Inglês

10.1080/00268978800100193

1362-3028

M. Elwenspoek,

Phase Equilibria and Thermodynamics

The phenomenon of kinetic roughening is used to study the step free energy on the (110)-faces of naphthalene crystals growing form a ternary solution of naphthalene, toluene and n-hexane. Keeping the mole fraction of naphthalene in the solution constant—which implies that the heat of dissolution divided by the temperature is constant—we varied the toluene-n-hexane composition. In order to calculate the supersaturation from the undercooling the relevant part of the phase diagram of the ternary system is studied and discussed in the framework of the regular solution model. The classical Jackson model for the solid-liquid interface predicts that the step energy is proportional to the heat of dissolution. In contradiction to this the step free energy increases with increasing hexane concentration in our experiment. We show that it is impossible to repair the Jackson model by inclusion of the (usually neglected) wetting phenomena. A model for the solid-liquid interface is proposed in which the flat aromatic molecules in the liquid (naphthalene and toluene) are ordered with respect to their orientation in the first few layers adjacent to the solid surface. The consequence of the order is the good fit of the liquid to the crystal surface and the step free energy is reduced. The irregular shaped n-hexane molecules diminish the order parameter of the smectic-like layer covering the crystal which leads to an increase of the step free energy. The dependence of the step free energy on the mole fraction of n-hexane can be described by a mean field theory for an Ising model of the smectic-like layer.