Measurements of Preferential Solvent Interactions by Densimetric Techniques

This chapter discusses densimetric techniques to measure preferential solvent interactions. The determination of preferential interactions by densimetry requires the measurement of the density increment under two sets of circumstances namely, at constant m₃ and at constant μ_3. The chapter provides the formula for density increment measured under conditions at which the molal concentrations of diffusible species are kept identical in the solvent and the macromolecule solution. This results in a partial specific volume of the macromolecule in the given solvent without any consideration of interactions between it and components of the solvent. This is referred to as the “partial specific volume at constant molality.” Operationally, this is done by measuring the densities of pure solvent and of a protein solution dissolved in it. The chapter provides formula for density increment measured under conditions at which the chemical potential of component is kept identical in the solvent and the macromolecule solution. This results in an apparent partial specific volume of the macromolecule in chemical equilibrium with solvent components. This is referred to as the “partial specific volume at constant chemical potential.” Operationally, this is obtained by bringing the macromolecule solution to dialysis equilibrium with solvent and measuring the densities of the dialyzed protein and the dialysate.