Concentration dependence of the film pressure of human serum albumin at the water/decane interface

Axisymmetric drop shape analysis-profile (ADSA-P) was employed to obtain accurate measurements of the concentration dependence of the film pressure of human serum albumin at a water/decane interface. Fifteen aqueous human albumin solutions ranging in concentration from 0.0001 to 5 mg ml(-1) were used. Three domains were identified in the effect of the bulk protein concentration, C, on the interfacial tension, gamma: (a) a slow change of gamma at low bulk concentrations (C < 10(-3) mg ml(-1)); (b) a sharp decline of gamma within the region of intermediate concentrations (10(-3) < C < 10(-2) mg ml(-1)); (c) a plateau region of gamma (C > 10(-2) mg ml(-1)). At the tvio lowest bulk albumin concentrations, 0.0001 and 0.001 mg ml(-1), negative interfacial pressures were observed that may be attributed to repulsive electrostatic interactions and the formation of a surface depletion layer. At all other concentrations, a positive surface pressure was measured. Moreover, based on our data on the variation of gamma with C, Gibbs' adsorption equation was used to provide an account of the dependence of the surface concentration on C. The possibility of the applicability of the Gibbs' adsorption equation exists for low bulk protein concentrations (C < 10(-3) mg ml(-1)).