Salt Effects on the Adsorption of Organic Molecules on Nonionic and Zwitterionic Polymer Brushes

We investigate the adsorption of organic molecules on polymer brushes of nonionic poly(2-hydroxyethyl methacrylate) and zwitterionic poly(carboxybetaine methacrylate) in aqueous solutions of 0.0 and 0.6 mol kg^-1 NaCl using all-atom molecular dynamics simulations. Experimental studies have attributed the enhancement and suppression of adsorption caused by salts to salt-induced collapse and extension of the polymer chains, respectively. Such structural changes are absent in our simulations due to the use of very high density polymer brushes. Nevertheless, the amount of adsorption in the 0.6 mol kg^-1 solution is different from that in the 0.0 mol kg^-1 solution. This is because Na⁺ and Cl^- ions alter the Gibbs energy of the adsorbate in the solution but not in the polymer brush. We interpret experimental results of two nonionic polymers and four zwitterionic polymers reported in the literature, taking this mechanism into account. The very good antifouling performance in salt water found for two zwitterionic polymers cannot be explained without the salt-induced extension of the polymer chains. However, in the other cases, the change in the Gibbs energy of the adsorbate in the bulk solution seems to play a major role in the salt effect on the antifouling performance.