Use of complementary neutron scattering techniques in studying the effect of a solid/liquid interface on bulk solution structures

By appropriate combination of neutron scattering techniques, it is possible to obtain structural information at various depths from a solid/liquid interface and thus probe in some detail how the surface structures evolve into bulk structures. We have used neutron reflectometry (NR) with a newly developed shear cell, near-surface small-angle neutron scattering (NSSANS) again in combination with the new shear cell, and regular small-angle neutron scattering (SANS) with a standard Couette shear cell to probe the structures formed in our aqueous surfactant systems and how they react to a flow field, particularly in the near-surface region of a solid/liquid interface. We present data for a 20 × 10^-3 M aqueous solution of 70% cetyltrimethyl-ammonium 3,5-dichlorobenzoate (CTA3,5ClBz) and 30% CTAB. This system forms a very viscoelastic solution containing long thread-like micelles. NR only probes to a depth of ca. 0.5 μm from the surface in these systems and clearly indicates that adsorbed onto the surface is a surfactant layer which is insensitive to shear. The depth probed by the NSSANS is of the order of 20-30 μm and is determined by the transmission of the sample, the angle of incidence and the wavelength. In this region, the rods align under shear into a remarkably well ordered hexagonal crystal. The SANS from the Couette cell averages over the entire sample, so that the signal is dominated by scattering from the bulk. While the near-surface hexagonal structure is clearly visible, these data are not consistent with the crystal structure persisting throughout the bulk, leading to the postulate that the bulk structure is a 2D liquid where the rods align with the flow, but do not order in the other two dimensions.