Thermodynamics of the interaction of globular proteins with powdered stearic acid in acid pH.

Adsorption isotherms of different globular proteins and gelatin on strearic acid particles have been studied as a function of biopolymer concentration, ionic strength of the medium, and temperature. The effect of neutral salts including CaCl2, Na3PO4, and urea on the adsorption isotherms has been also investigated. It is observed that the extent of adsorption (Gamma2(1)) increases in two steps with the increase of biopolymer concentration (C2) in the bulk. Gamma2(1) increases with an increase of C2 until a steady maximum value Gamma2(m) is reached at a critical concentration C2(m). After initial saturation, Gamma2(1) again increases from Gamma2(m) without reaching any limiting value due to the surface aggregation of the protein. The values of the standard free energy change for adsorption have been calculated on the basis of the Gibbs equation. The standard entropy and enthalpy changes are also calculated.