No abstract

The adsorption of a series of nonionic surfactants, C n H2 n +1(OC2H4) m OH (C n E m ), from aqueous solution onto a self-assembled monolayer (SAM) before and after chemical modification has been studied by neutron reflection. On a SAM consisting of a layer of undecenyl trichlorosilane with the ethylenic group at the aqueous interface all the surfactants adsorbed to form a monolayer whose thickness compared with the length of the molecule suggests that the molecules are tilted at angles in the region of 60° away from the surface normal. There was some penetration of the SAM layer by the alkyl chains of the surfactants. When the SAM was hydroxylated, the pattern of adsorption was found to be quite different and, for all the surfactants, the layer was only about 4 Å thick with a segmental surface density constant within experimental error, and there was no penetration of the SAM. The structure of this layer is attributed to hydrogen bonding between the ethylene glycol groups in the surfactant and the OH groups on the surface of the SAM. For comparison the adsorption of the same set of surfactants was studied on the bare silica surface. In all cases the adsorption was in the form of a defective bilayer, quite different again from the adsorption pattern on the two SAMs.

Spun-cast layers of polystyrene have been transferred to large silicon crystals as substrates for neutron reflectivity studies and to prisms for infrared-attenuated total reflection measurements of surfactant adsorption. This paper describes an investigation of the adsorption from aqueous solutions of sodium dodecyl sulfate (SDS) to this interface of aqueous solution and polystyrene. The structure and amount of adsorbate is described and discussed. The adsorption isotherm shows that both the thickness and volume fraction of the SDS layer increase to plateau values above the critical micelle concentration (cmc) where the area per molecule is approximately 42 Å2. Above the cmc, there is a decrease in the total amount of adsorbed material unless SDS of very high purity is used. This unusual behavior is explained by the presence of dodecan-1-ol from hydrolysis of SDS which is highly surface active but can be solubilized in micelles of SDS.

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.

The technique of neutron reflection has been used to investigate the adsorption of alpha-enriched gelatin from aqueous solution onto spun polystyrene substrates. Neutron reflection can provide information about the distribution of material perpendicular to an interface as well as total adsorbed amounts. The adsorbed layers were found to have maximum density at the surface, decaying with distance into solution. The adsorbed amount, layer thickness, and density were all seen to increase with solution concentration. Temperature was found to have little effect on adsorption. Thicker, less dense layers were observed at high pH and thinner, denser layers were observed at low pH, but the total adsorbed amount did not change significantly. The presence of sodium chloride had little effect on the adsorbed layers. The results are discussed in the context of other studies and the known amino acid sequence of alpha-gelatin.