ABSTRACT
The vertical depth distributions of amine oxide surfactants, N,N-dimethyldodecyl amine N-oxide (DDAO) and N,N-dimethyltetradecyl amine N-oxide (DTAO), in poly(vinyl alcohol) (PVA) films were explored using neutron reflectometry (NR). In both binary and plasticized films, the two deuterated surfactants formed a single monolayer on the film surface with the remaining surfactant homogeneously distributed throughout the bulk of the film. Small-angle neutron scattering and mechanical testing revealed that these surfactants acted like plasticizers in the bulk, occupying the amorphous regions of PVA and reducing its glass-transition temperature. NR revealed little impact of plasticizer (glycerol) incorporation on the behavior of these surfactants in PVA. The surfactant molecular area in the segregated monolayer was smaller for DTAO than for DDAO, indicating that the larger molecule was more densely packed at the surface. Surface tension was used to assess the solution behavior of these surfactants and the effect of glycerol incorporation. Determination of molecular area of each surfactant on the solution surface revealed that the structures of the surface monolayers are remarkably consistent when water is placed by the solid PVA. Incorporation of glycerol caused a decrease of molecular area for DDAO and increase in molecular area for DTAO both in solution and in PVA. This suggests that the head group interactions, which normally limit the minimum area per adsorbed molecule, are modified by the length of the alkyl tail.
ABSTRACT
The blooming of sodium dodecyl sulfate (SDS) and the influence of plasticizer (glycerol) on the surfactant distribution in poly(vinyl alcohol) (PVA) films have been explored by neutron reflectometry (NR) and ion beam analysis techniques. When in binary films with PVA, deuterated SDS (d25-SDS) forms a surface excess corresponding to a wetting layer of the surfactant molecules at the film surface. The magnitude of this surface excess increased significantly in the presence of the plasticizer, and the surfactant was largely excluded from the PVA subphase. NR revealed smectic nanostructures for both SDS and glycerol components within this surface excess in plasticized films. This combined layer comprises surfactant lamellae, separated by interstitial glycerol-rich layers, which is only formed in the plasticized films and persists throughout the surface excess. Atomic force microscopy micrographs of the film surfaces revealed platelike structures in the plasticized PVA, which were consistent with the rigid defects in the surfactant-rich lamellae. The formation of these structures arises from the synergistic surface segregation of SDS and glycerol, evidenced by surface tensiometry. Cloud point analysis of bulk samples indicates a transition at â¼55% water content, below which phase separation occurs in ternary films. This transition is likely to be necessary to form the thick wetting layer observed and therefore indicates that film components remain mobile beyond this point in the drying process.
ABSTRACT
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.
