  The properties of the solvent affect the behavior of the solution. We propose
a model that accounts for the contribution of the solvent free energy to the
free energy of globular proteins in solution. For the case of an attractive
square well potential, we obtain an exact mapping of the phase diagram of this
model without solvent to the model that includes the solute-solvent
contribution. In particular we find for appropriate choices of parameters upper
critical points, lower critical points and even closed loops with both upper
and lower critical points, similar to one found before [Macromolecules, 36,
5845 (2003)]. In the general case of systems whose interactions are not
attractive square wells, this mapping procedure can be a first approximation to
understand the phase diagram in the presence of solvent. We also present
simulation results for both the square well model and a modified Lennard-Jones
model.
