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

Adsorption isotherms constructed from time-and-concentration-dependent advancing contact angles theta(a) 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 I h contact with a particular test surface selected from the full range of observable water wettability (as quantified by PBS adhesion tension tau(o)(a) = gamma(o)(lv) cos theta(o)(a); where gamma(o)(lv) is liquid-vapor interfacial tension and theta(o)(a) is the advancing PBS contact angle). The maximum advancing spreading pressure, Pi(max)(a), determined from adsorption isotherms decreases a systematically with tau(o)(a) for methyl-terminated self-assembled monolayers (CH3 SAM, tau(o) = -15 mN/m), polystyrene spun-coated onto electronic-grade SiOx wafers (PS, tau(o) = 7.2 mN/m), aminopropyltriethoxysilane-treated SiOx surfaces (APTES, tau(o) = 42 mN/m), and fully water wettable SiCx (tau(o) = 72 mN/m). Likewise, the apparent Gibbs' surface excess [Gamma(sl) - Gamma(sv)], which measures the difference in the amount of protein adsorbed Gamma (mol/cm(2)) at solid-vapor (SV) and solid-liquid (SL) interfaces, decreases with tau(o) from maximal values measured on the CH3 SAM surface through zero (no protein adsorption in excess of bulk solution concentration) near tau(o) = 30 mN/m (theta(a) = 65 degrees). These latter results corroborate the conclusion drawn from independent studies that water is too strongly bound to surfaces with TO -: : 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 ([F,l - F,,] = 0 at TO = 30 mN/m). Results are collectively interpreted to mean that water controls protein adsorption to surfaces and that the mechanism of protein adsorption can