Surfactant and Plasticiser Segregation in Thin Polyvinyl Alcohol Films

The vertical depth distribution of individual additive components (cetyltrimethylammonium bromide (CTAB) and deuterated pentaethylene glycol monododecyl ether (d25-C12E5) and deuterated glycerol (d-glycerol) in PVA film have been isolated and explored by ion beam analysis techniques and neutron reflectometry. The additives display an unexpectedly rich variety of surface and interfacial behaviour in spin-cast films. In separate binary films with PVA, both d-glycerol and CTAB were evenly distributed, whereas d25-C12E5 showed clear evidence for surface and interfacial segregation. The behaviour of each surfactant in PVA was reversed when the plasticiser (glycerol) was also incorporated into the films. With increasing plasticiser content, the surface activity of d25-C12E5 systematically decreased, but remarkably when glycerol and CTAB were present in PVA, the surface and interfacial activity of CTAB is dramatically increased by the presence of glycerol. Quantification of the surface excess by ion beam analysis reveals that in many cases the adsorbed quantity far exceeds what could reasonably be accounted for by a single layer; therefore indicates a wetting transition of the small molecules at the surface or interface of the film. It appears that the surface and interfacial behaviour is partly driven by the relative surface energies of the components, but is also significantly augmented by the incompatibility of the components.