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 pos sible to obtain structural information at various depths from a solid/ liquid interface and thus probe in some detail how the surface structu res evolve into bulk structures. We have used neutron reflectometry (N R) 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 s ystems and how they react to a flow field, particularly in the near-su rface region of a solid/liquid interface. We present data for a 20 x 1 0(-3) M aqueous solution of 70% cetyltrimethylammonium 3,5-dichloroben zoate (CTA3,5ClBz) and 30% CTAB. This system forms a very viscoelastic solution containing long thread-like micelles. NR only probes to a de pth of ca. 0.5 mu m from the surface in these systems and clearly indi cates that adsorbed onto the surface is a surfactant layer which is in sensitive to shear. The depth probed by the NSSANS is of the order of 20-30 mu m and is determined by the transmission of the sample, the an gle of incidence and the wavelength. In this region, the rods align un der shear into a remarkably well ordered hexagonal crystal. The SANS f rom the Couette cell averages over the entire sample, so that the sign al is dominated by scattering from the bulk. While the near-surface he xagonal structure is clearly visible, these data are not consistent wi th the crystal structure persisting throughout the bulk, leading to th e postulate that the bulk structure is a 2D liquid where the rods alig n with the flow, but do not order in the other two dimensions.