The relative preference of a molecular species to solvate (interact) with another molecular species is quantified using the preferential interaction coefficient, Γ Γ . Lets consider a solution that consists of the solvent (water), solute, and cosolute. The relative (effective) interaction of water with the solute is related to the preferential hydration coefficient, Γ Γ W , which is positive if the solute is "preferentially hydrated". In the Kirkwood-Buff theory frame-work, and in the low concentration regime of cosolutes, the preferential hydration coefficient is: