Interfacial energetics of globular-blood protein adsorption to a hydrophobic interface from aqueous-buffer solution

Adsorption isotherms of nine globular proteins with molecular weight (MW) spanning 10-1000 kDa confirm that interfacial energetics of protein adsorption to a hydrophobic solid/aqueous-buffer (solid-liquid, SL) interface are not fundamentally different than adsorption to the water air (liquid-vapour, LV) interface. Adsorption dynamics dampen to a steady-state (equilibrium) within a 1 h observation time and protein adsorption appears to be reversible, following expectations of Gibbs' adsorption isotherm. Adsorption isotherms constructed from concentration-dependent advancing contact angles θ_a of buffered-protein solutions on methyl-terminated, self-assembled monolayer surfaces show that maximum advancing spreading pressure, II_a^max, falls within a relatively narrow 10<II_a^max <20 mN m^-1 band characteristic of all proteins studied, mirroring results obtained at the LV surface. Furthermore, II_a isotherms exhibited a 'Traube-rule-like' progression in MW similar to the ordering observed at the LV surface wherein molar concentrations required to reach a specified spreading pressure II_a decreased with increasing MW. Finally, neither Gibbs' surface excess quantities [Γ_sl -Γ_sv] nor Γ_lv varied significantly with protein MW. The ratio {[Γ_sl-Γ_sv]/Γ_lv} ∼ 1, implying both that Γ_sv∼ 0 and chemical activity of protein at SL and LV surfaces was identical. These results are collectively interpreted to mean that water controls protein adsorption to hydrophobic surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different composition.