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Neutron reflection has been used to measure the adsorption of gelatin, in combination with the surfactants sodium dodecyl sulfate and hexadecyltrimethylammonium bromide, to polystyrene. For gelatin/ surfactant systems, a mixed adsorbed layer is observed with a similar diffuse density profile to gelatin alone. The relative proportion of each component at the interface depends on the sequence of adsorption. Adsorption of a mixture to a fresh surface produces a layer with a high proportion of surfactant. Addition of gelatin or a gelatin/ surfactant mixture to an existing surfactant layer displaces surfactant from the interface. Adsorbed gelatin is not removed on addition of a surfactant solution; some surfactant penetrates, but the layer remains rich in gelatin. © Springer-Verlag 1999.

We have used neutron reflection to study the effects of pH and added electrolyte on the structure of a weak polyelectrolyte brush anchored by means of a hydrophobic block to a hydrophobic surface. The copolymer was poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) (poly-(DMAEMA-b-MMA)) containing 70% DMAEMA and having a molecular weight of 10000. The MMA block and the hydrophobic octadecyltrichlorosilane (OTS) layer were deuterated in order to highlight the DMAEMA fraction of the layer. The MMA block was found to bind strongly and irreversibly to a self-assembled monolayer of octadecyl trichlorosilane (OTS) on silica. The copolymer distribution could be described in terms of two uniform layers, a thin layer next to the surface containing all the MMA block and variable amounts of the DMAEMA residues with a copolymer volume fraction of about 0.8 and a more diffuse block corresponding to the polyelectrolyte brush. The presence of DMAEMA residues in the thin layer and the variation of this amount with pH and added electrolyte show that, near the anchor layer of the MMA block, the DMAEMA block is not ionized, in agreement with the model of a grafted weak polyelectrolyte proposed by Israels et al.

We have studied the adsorption of lysozyme layers at a hydrophobic silicon water interface using specular neutron reflection. The hydrophobic surface was obtained by self-assembly of a densely packed monolayer of octadecyltrichlorosilane (OTS) onto the natural silica layer on the smooth surface of a (111) silicon block. The effect of pH on the adsorbed lysozyme layer was examined at a constant lysozyme concentration of 0.03 g dm-3 and at a constant ionic strength of 0.02 M. Reflectivity profiles at different pH show that adsorption is irreversible with respect to pH, the composition and structure of the final layer being dependent on the route by which the pH was achieved. The adsorbed protein layer was found to divide into approximately two regions, a densely packed thin layer next to the OTS surface and a diffuse thicker layer extending into the bulk solution. None of the dimensions of this structure corresponds to those of the globular protein in solution, suggesting that, unlike its adsorption at the hydrophilic silica/water interface, lysozyme is denatured at the OTS/water surface. The irreversible adsorption is explained by the combined interaction of the hydrophobic attraction of the hydrophobic fragments in lysozyme to the OTS surface and electrostatic repulsion within the adsorbed layer. The hydrophobic surface induces the exposure of hydrophobic fragments from the lysozyme assembly. The thickness of the dense layer suggests that the denatured protein adsorbs in the form of peptide chains with the hydrophobic amino acid side chains attached to the OTS surface with the hydrophilic side chains extending into the bulk solution. Since lysozyme is more stable at pH 7 than at pH 4, the difference in initial adsorption is dominated by the greater relative stability of lysozyme to denaturation at the higher pH. A change of pH from 7 to 4 reduces the stability of the protein to unfolding and results in more adsorption than when the pH is changed in the opposite direction. Solution pH also affects the net charges within the hydrophilic tail region and the structural distribution of the tail region was found to vary with pH. Copyright 1998 Academic Press.

The level of the background scattering in neutron reflection experiments limits the range of momentum transfer over which a specular reflectivity profile may be measured and also makes it difficult to observe off-specular scattering. A further limitation of neutron reflection, which applies to studies of the solid/liquid interface, restricts accessibility to a small range of solid surfaces. The restriction of specular reflection measurements to below a relatively small value of κmax means that the intrinsic resolution of specular neutron reflection is low. We show how selective isotopic labelling can be used to enhance the resolution beyond the limit determined by κmax and illustrate the method using the structure of hexadecyl trimethyl ammonium bromide as an example. For the range of coverages typical of a surfactant monolayer at the air/water interface the structure can be resolved to the level of a group of three to four carbon atoms. The difficulty of extending neutron reflection at the solid/liquid interface to different solids is that the large size of solid sample and the requirement that the solid usually be crystalline makes the cost of the solid material prohibitively high except for a small range of materials which include silicon, amorphous silica and crystalline quartz. We give examples that show how self-assembled monolayers with different chemical functionality on their outer surfaces can be used to create a wide variety of different surfaces with quite different adsorptive behaviour. This should greatly extend the range of neutron reflection experiments on the important solid/liquid interface. The lamellar phase of the surfactant Aerosol-OT adsorbs preferentially from a mixture of lamellar phase and isotropic solution at both air/liquid and solid/liquid interfaces. Strong off-specular scattering from fluctuations in the lamellae is observed which is enhanced by Bragg diffraction from the multilayer structure. This indicates that there must be partial correlation of the fluctuations in different lamellae. © 1998 Elsevier Science B.V. All rights reserved.