Wettability of a polymethylmethacrylate surface in the presence of benzyl-substituted alkyl betaines

The adsorption and wettability of benzyl substituted alkyl carboxylbetaine (BCB) and alkyl sulfobetaine (BSB) at polymethylmethacrylate (PMMA) surfaces have been studied by contact angle measurements and the adsorption parameters such as adhesional tension, adsorption amount, work of adhesion and PMMA-liquid interfacial tension (γSL) have been calculated. The results show that the values of contact angle of betaines keep constant before CMC, which can be explained well by the compensation effect of the decrease of surface tension and increase of γSL with an increase of bulk concentration. However, only the contact angle of BCB solution begins to decline when the concentration is far higher than CMC, indicating the re-adsorption. Both the hydrophilic head and benzyl of BCB molecules interact with PMMA surface, which decreases adsorption amount but improves ability of hydrophobic modification for PMMA. On the other hand, the benzyl may trend to stretch towards aqueous for BSB, instead of interacting with PMMA, because of the steric effect of larger hydrophilic head of BSB. The unique arrangement of BSB molecules at PMMA surface makes it difficult to re-adsorb onto monolayer at higher bulk concentration.