Interfacial energetics of protein adsorption from aqueous buffer to surfaces with varying hydrophilicity

Adsorption isotherms constructed from time-and-concentration-dependent advancing contact angles θ, show that the profound biochemical diversity among ten different blood proteins with molecular weight spanning 10-1000 kDa has little discernible effect on the amount adsorbed from aqueous phosphate-buffered saline (PBS) solution after 1 h contact with a particular test surface selected from the full range of observable water wettability (as quantified by PBS adhesion tension τ_a = γ_1v cos θ_a where γ_1v is the liquid-vapor interfacial tension and θ_a is the advancing PBS contact angle). The maximum advancing spreading pressure, Π_a^max, determined from adsorption isotherms decreases systematically with τ_a for methyl-terminated self-assembled monolayers (CH₃ SAM, τ° = -15 mN/m), polystyrene spun-coated onto electronic-grade SiO_x wafers (PS, τ = 7.2 mN/m), aminopropyltriethoxysilane-treated SiO_x surfaces (APTES, τ - 42 mN/m), and fully water wettable SiO_x (τ = 72 mN/m). Likewise, the apparent Gibbs' surface excess [Γ_sl - Γ_sV], which measures the difference in the amount of protein adsorbed Γ (mol/cm²) at solid-vapor (SV) and solid-liquid (SL) interfaces, decreases with tau; from maximal values measured on the CH₃ SAM surface through zero (no protein adsorption in excess of bulk solution concentration) near τ = 30 mN/m (θ_a, = 65°). These latter results corroborate the conclusion drawn from independent studies that water is too strongly bound to surfaces with τ > 30 mN/m to be displaced by adsorbing protein and that, as a consequence, protein does not accumulate within the interfacial region of such surfaces at concentrations exceeding that of bulk solution (Γ[ _sl - Γ_sv] = 0 at τ = 30 mN/m). Results are collectively interpreted to mean that water controls protein adsorption to surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different amino acid compositions.