The adsorption of concentrated solutions of di-2-ethylhexylsodium sulfosuccinate (Aerosol-OT or AOT) at hydrophilic solid/aqueous and air/aqueous solution interfaces was followed by neutron specular reflection. Reflectivity profiles were recorded of protonated AOT in D2O at concentrations of 2 and 5 wt % and over a range of temperatures. Sharp diffraction peaks characteristic of a lamellar phase were observed, and the repeat spacing of the peaks was very sensitive to temperature. The bulk phase diagram in this concentration range over the temperature range 0−65 °C was also studied using neutron small angle scattering. For the bulk solution containing 5 wt % AOT, the lamellar phase at 5 °C had a repeat distance of 370 ± 30 Å, which dropped to about half that value at 35 °C, and to only 115 Å at 65 °C. The lamellar spacing decreased as the AOT concentration was lowered; for example, from 325 Å at 5 wt % to 225 Å at 1 wt %, at 15 °C. Over some of the concentration and temperature range narrow diffraction peaks were observed for the lamellar phase adsorbed at both interfaces, indicating long-range order in the direction normal to the surface. The repeat spacing in the adsorbed lamellar phase was generally 10−20% smaller than in the bulk solution. Although addition of small amounts of electrolyte (0.1 and 1 mM NaBr) had only a small effect on the small angle scattering from the bulk lamellar phase, it caused the disappearance of the adsorbed lamellar phase. These observations are consistent with an overall attraction of the lamellar phase to the surface in the absence of added electrolyte.

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.