Hydrophobic Protein−Polypyrrole Interactions:  The Role of van der Waals and Lewis Acid−Base Forces As Determined by Contact Angle Measurements

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Adsorption of human serum albumin (HSA) onto conducting polypyrrole powders, doped with chloride (PPyCl), dodecyl sulfonate (PPyDS), and tosylate (PPyTS), has been monitored in 0.1 M phosphate buffer saline (PBS) and pH 7.4 using UV−visible spectroscopy in conjunction with the depletion method. The decreasing trend of adsorption was PPyTS > PPyDS > PPyCl and was interpreted in terms of hydrophobic interactions. Electrochemically synthesized PPyCl, PPyDS, and PPyTS films were used as model surfaces for contact angle measurements. Both static, advancing, and receding water contact angle (θ W ) suggested that the PPyTS is the most hydrophobic polymer among the three under test. The simple measure of θ W permitted qualitative interpretation of the adsorption trend in terms of hydrophobic protein−PPy interactions. The van Oss−Good−Chaudhury (VOGC) method was further used to determine the dispersive, acidic, and basic components of the surface free energy (γ S d, γ S +, and γ S -, respectively) of the conducting polypyrroles. These components show that polypyrrole generally behaves as a strong Lewis acid. The three surface free energy components were subsequently used to assess the absolute hydrophobicity of the substrates (Δ G 1W1 ), that is, the PPy−PPy interaction in water, the trend of which is that of protein adsorption. More importantly, the VOGC theory permitted determination of Δ G 1W2, the free energy of protein−PPy in water, that is, the extent of hydrophobic interaction forces. The decreasing trend of Δ G 1W2 values (absolute) was found to be PPyTS > PPyDS > PPyCl. This is a quantitative evidence for the role of hydrophobic interactions at the protein−PPy interface. In the case of PPyTS and PPyDS, the acid−base force contribution was much more important than the van der Waals one. In contrast, for the HSA−PPyCl system, the van der Waals forces predominantly contributed to Δ G 1W2 .