Adsorption of Globular Proteins at the Air/Water Interface as Measured via Dynamic Surface Tension: Concentration Dependence, Mass-Transfer Considerations, and Adsorption Kinetics

Brian C. Tripp, Jules John Magda, and Joseph D. Andrade

The pendant drop technique was used to measure the dynamic surface tension (DST) of eight globular proteins at the air/water interface as a function of protein bulk concentration (C_b) at C_b values of 0.01, 0.1, and 1.0 mg/ml. Initial periods of negligible decrease in the DST ("induction times") were observed for many of the proteins at C_b = 0.01 mg/ml, but were less common at higher C_b values. DST kinetic parameters varied by up to several orders of magnitude for different proteins, even those with similar bulk diffusion coefficients. All eight proteins achieved initial values of the mesoequilibrium surface tension (MST) within 15 h at C_b = 1.0 mg/ml, although only four of the proteins attained steady-state surface tension within this time. The 15-h MST value at C_b = 1.0 mg/ml did not vary much among the eight proteins, with an average value of 47 ± 6 mN/m. The DST data were numerically modeled by a 4-parameter Hua-Rosen equation and analyzed with respect to protein surface hydrophobicity, conformational stability, bulk depletion effects, and the apparent vs theoretical diffusion-limited rate of adsorption.