Colloidal stability of protein-polymer systems: A possible explanation by hydration forces

In this paper the stability domains of immunoglobulin (IgG fragment) F(ab′${)}_2$-polymer systems have been examined using a low-angle scattering technique. The rates of aggregate formation are expressed in terms of a stability ratio as a function of electrolyte concentration. After the usual rapid aggregation achieved at a certain ionic strength (critical coagulation concentration), an abnormal stabilization is observed with increasing ionic strength. This exceptional stability at high electrolyte concentration cannot be explained by the Derjaguin, Landau, Verwey, and Overbeek [B. V. Derjaguin and L. Landau, Acta Physicochim. USSR 14, 633 (1941); E. J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids (Elsevier, Amsterdam, 1952), Vols. 1 and 2] theory, which attributes the colloidal stability to the London–van der Waals attraction and the electrostatic repulsion. Effects of electrolyte concentration, counterion valence, pH, protein coverage, and time on the experimental stability are investigated. A possible explanation based on the so-called ``hydration forces'' is proposed.