Free energies of protein adsorption onto mineral particles — from the initial encounter to the onset of hysteresis

The kinetics and energetics of human serum albumin (HSA) adsorption onto silica and talc particles are compared, from the earliest moment of contact, up to 24 h exposure. The measured affinity constants are used to obtain the free energies of interaction (ΔG). The free energies of interaction are also obtained via the measured surface properties of HSA, water, silica and talc. Comparison between the ΔG-values obtained by the two different approaches makes it possible to deduct the manner and conformation of HSA adsorption at the earliest moment, as well as after onset of hysteresis. Two different modes of hysteresis are identified — one allows slow dissociation and the other approaches apparent irreversibility. The mechanisms of both are elucidated, as is the mechanism of desorption at high pH and by displacement with identical or quasi-identical molecules.