Adsorption and Wettability of Branched Betaine and Cationic Surfactants on a Poly(methyl methacrylate) Surface

The wetting properties of the poly(methyl methacrylate) (PMMA) surface by aqueous solutions of the branched zwitterionic surfactants, hexadecanol glycidyl ether glycine betaine and hexadecanol polyoxyethylene(3) glycidyl ether glycine betaine, and branched cationic surfactants, hexadecanol glycidyl ether ammonium chloride and hexadecanol polyoxyethylene (3) glycidyl ether ammonium chloride, were investigated using sessile drop analysis. The influence of the surfactant type, structure, and concentration on contact angle was explored. The results indicate that the PMMA surface was slightly hydrophobically modified with the bulk concentration of surfactant before reaching the critical micelle concentration (cmc) because the adsorbed surfactant molecules are parallel to the substrate surface through hydrogen bonding and the hydrophilic groups are close to the surface. At this stage, the contact angle stays almost constant because of the simultaneous decrease in the surface tension and adhesional tension. However, at concentrations higher than the cmc, the surfactant molecules can adsorb on the PMMA surface through the hydrophobic interactions and the hydrophilic groups toward the bulk phase of solution. This leads to the increase in the hydrophilic character of the PMMA surface. The contact angle decreases dramatically with the increase in bulk surfactant concentration. The variation in the surfactant type and the introduction of ethylene oxide units has little effect on the contact angle because the alkyl chain in these four surfactant molecules is branched in the same way.