  Salt, glycerol and dimethyl sulfoxide (DMSO) are used to modify the
properties of protein solutions. We experimentally determined the effect of
these additives on the phase behavior of lysozyme solutions. Upon the addition
of glycerol and DMSO, the fluid-solid transition and the gas-liquid coexistence
curve (binodal) shift to lower temperatures and the gap between them increases.
The experimentally observed trends are consistent with our theoretical
predictions based on the thermodynamic perturbation theory (TPT) and the
Derjaguin-Landau-Verwey-Overbeek (DLVO) model for the lysozyme-lysozyme pair
interactions. The values of the parameters describing the interactions, namely
the refractive indices, dielectric constants, Hamaker constant and cut-off
length, are extracted from literature or are experimentally determined by
independent experiments, including static light scattering to determine the
second virial coefficient. We observe that both, glycerol and DMSO, render the
potential more repulsive, while sodium chloride reduces the repulsion.
