  Ethanol is a common protein crystallization agent, precipitant, and
denaturant, but also alters the dielectric properties of solutions. While
ethanol-induced unfolding is largely ascribed to its hydrophobic parts, its
effect on protein phase separation and inter-protein interactions remains
poorly understood. Here, the effects of ethanol and NaCl on the phase behavior
and interactions of protein solutions are studied in terms of the metastable
liquid-liquid phase separation (LLPS) and the second virial coefficient $B_2$
using lysozyme solutions. Determination of the phase diagrams shows that the
cloud-point temperatures are reduced and raised by the addition of ethanol and
salt, respectively. The observed trends can be explained using the extended law
of corresponding states as changes of $B_2$. The results for $B_2$ agree
quantitatively with those of static light scattering and small-angle X-ray
scattering experiments. Furthermore, $B_2$ values calculated based on
inter-protein interactions described by the Derjaguin--Landau--Verwey--Overbeek
(DLVO) potential and considering the dielectric solution properties and
electrostatic screening due to the ethanol and salt content quantitatively
agree with the experimentally observed $B_2$ values.
