Thermodynamics of crystal nucleation in multicomponent droplets: Adsorption, dissociation, and surface-stimulated nucleation

We extend our previous work on crystallization of unary liquids [Djikaev , J. Phys. Chem. A 106, 10247 (2002)] to multicomponent systems and develop the Gibbsian thermodynamics of the crystallization of multicomponent liquid solutions (bulk as well as droplets). Our treatment takes into account the possibility of formation of a solid phase from a nonstoichiometric liquid solution (noncongruent solidification). Using the capillarity approximation and taking into account both dissociation and surface adsorption effects, we derive the reversible work of formation of a crystal nucleus in homogeneous and pseudoheterogeneous modes (the density difference between crystal and liquid phases is also taken into account). In the former mode, the nucleus forms homogeneously within the (supercooled) bulk liquid solution, while in the latter mode, it forms "pseudoheterogeneously" at the liquid-vapor interface. Comparison of the works of formation in the two modes provides an inequality which must hold in order for pseudoheterogeneous crystal nucleation to be thermodynamically favored over the homogeneous process. As for unary systems, this inequality is identical to the condition of partial wetting of at least one crystal facet by its melt, i.e., the effects of adsorption and dissociation do not explicitly alter the thermodynamic condition for surface-stimulated crystallization in multicomponent systems. These effects do play an important role in determining the crystal nucleation mode, because they contribute to the liquid-vapor and liquid-solid surface tensions that enter the condition of partial wetting. The influence of these effects on the freezing behavior of droplets is expected to depend on the droplet size. (C) 2003 American Institute of Physics.