Nucleation Theory and the Cloud-point *

Nucleation theory [l-4] deals with that general class of phenomena which can be mapped onto the familiar condensation of a supercooled vapor and, in its present form, it heavily rests’upon the concept of clusters (droplets, grains, etc.). Although a set of basic ideas is now rather well-established, the theory still uses many phenomenological ideas and approximations so that, in order to minimize the lack of precision, it can be better discussed in relation with simple, well-defined models. A convenient model for this purpose is the lattice-gas version of the Ising model where, for instance, precise definitions for a cluster are possible [5]. The lattice-gas model [6] consists of a regular lattice, say a simple cubic lattice, whose sites are associated to a set of occupation variables each capable of only two values, n, = 1 (particle) or 0 (hole). The system presents a (configurational) energy given by