Why Do Some Molecules Form Hydrates or Solvates?

The discovery of solvates (crystal structures where the solvent is incorporated into the lattice) dates back to the dawn of chemistry. The phenomenon is ubiquitous, with important applications ranging from the development of pharmaceuticals to the potential capture of CO2 from the atmosphere. Despite this interest, we still do not fully understand why some molecules form solvates. We have employed molecular simulations using simple models of solute and solvent molecules whose interaction parameters could be modulated at will to access a universe of molecules that do and do not form solvates. We investigated the phase behavior of these model solute-solvent systems as a function of solute-solvent affinity, molecule size ratio, and solute concentration. The simulations demonstrate that the primary criterion for solvate formation is that the solute-solvent affinity must be sufficient to overwhelm the solute-solute and solvent-solvent affinities. Strong solute-solvent affinity in itself is not a sufficient condition for solvate formation: in the absence of such strong affinity, a solvate may still form provided that the self-affinities of the solute and the solvent are weaker in relative terms. We show that even solvent-phobic molecules can be induced to form solvates by virtue of a p Delta V potential arising either from a more efficient packing or because of high pressure overcoming the energy penalty.