Surfactant and Plasticizer Segregation in Thin Poly(vinyl alcohol) Films

The vertical depth distributions of individual additive components [cetyltrimethylammonium bromide (CTAB), deuterated pentaethylene glycol monododecyl ether (d25-C12E5), and deuterated glycerol (d-glycerol)] in PVA films have been isolated and explored by ion beam analysis techniques and neutron reflectometry. The additives display an unexpectedly rich variety of surface and interfacial behaviors 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 behavior of each surfactant in PVA was reversed when the plasticizer (glycerol) was also incorporated into the films. With increasing plasticizer content, the surface activity of d25-C12E5 systematically decreased, but remarkably, when glycerol and CTAB were present in PVA, the surface and interfacial activities of CTAB increased dramatically in the presence of glycerol. Quantification of the surface excess by ion beam analysis revealed that, in many cases, the adsorbed quantity far exceeded what could reasonably be explained by a single layer, thus indicating a wetting transition of the small molecules at the surface or interface of the film. It appears that the surface and interfacial behaviors are partly driven by the relative surface energies of the components, but are also significantly augmented by the incompatibility of the components.