Structure of water at zwitterionic copolymer film–liquid water interfaces as examined by the sum frequency generation method

A copolymer film composed of zwitterionic carboxymethylbetaine (CMB) and n-butyl methacrylate (BMA), Poly(CMB-r-BMA), was cast on a flat plane of an octadecyltrichlorosilane (ODS)-modified fused quartz prism with a semi-cylindrical shape. CH stretching of the polymer film and OH stretching of water at the surface of the film were examined using the sum frequency generation (SFG) technique. The CH stretching band of the cast film, indicating a gauche defect of the film, was affected by the contact medium including dry nitrogen, water vapor-saturated nitrogen and liquid water. In contrast, the CH stretching of an octadecyl group introduced onto the quartz prism for stable attachment of the cast film was not significantly changed by the contact medium. The OH stretching band indicated that water molecules at the surface of the Poly(CMB-r-BMA) film in contact with liquid water were not greatly oriented in comparison with those at the surfaces of a bare prism, an ODS SAM-modified prism, and a prism covered with a PolyBMA film or a copolymer film of BMA and methacrylic acid or 2-(dimethylamino)ethyl methacrylate. A similar small perturbation of the structure of water was previously observed in the vicinity of water-soluble zwitterionic polymers and zwitterionic copolymer films using Raman and attenuated total reflection infrared spectroscopies, respectively. A distinct effect of charge neutralization to diminish the perturbation of the structure of interfacial water around polymer materials was suggested.