Abstract:
PREDICTING the shape of growing crystals is important for industrial
crystallization processes. The
equilibrium form of a crystal can be determined unambiguously from
a consideration of the surface free
energies of the various crystallographic faces {hkl}(1), but the grow
th morphology is determined by kinetic
factors which are harder to predict. This morphology depends on the
relative growth rates R(hld)(rel) of the
crystal faces. Several theories have been advanced(2,3) to relate R(hkl)(rel)
to geometric or energetic
characteristics of the surfaces {hkl}, but these have met with limited
success in predicting the crystal
morphologies observed. Here we present a theoretical approach to the
problem in which R(hkl)(rel) is
determined by quantities that are accessible either from kinetic models
or from computer simulations of the
solid-fluid interface. The important parameters controlling the growth
rate are the energy required to create a
step at the crystal surface and the free-energy barrier for an adsorbed
solute molecule to be incorporated into
the crystal. Both can be related to the mole fraction of adsorbed solute
molecules in dynamic equilibrium with
those in the crystal surface. When this approach is applied to the
case of urea crystals grown from aqueous
solution, we predict a needle-like shape which is consistent with experimental
observations.