Molecular thermodynamics of precipitation

Using a simple theory for fluids and a simple theory for a solid, it is pos sible to construct a semi-quantitative corresponding-states phase diagram w here a reduced temperature is plotted as a function of a reduced density. T he reducing parameters are molecular size (sigma(3)) and molecular potentia l energy (epsilon/k(B)); the phase diagram includes both low-density and hi gh-density fluid regions and the solid region. These calculations apply to a pure substance or, of more interest, to a solute dissolved in a continuou s solvent. The qualitative nature of the phase diagram depends strongly on the range of attractive intermolecular forces as indicated by an exponentia l parameter n; when coordination number z = 8 and n is about 6, we obtain t he usual phase diagram where the fluid-solid region lies to the right of th e fluid-fluid coexistence curve. But when n is about 7 or 8, the fluid-soli d region lies above the fluid-fluid coexistence curve. Applications are dis cussed for aqueous solutions of a colloid or a globular protein that may al so contain a salt or a polymer to induce precipitation. (C) 1999 Elsevier S cience S.A. All rights reserved.