Williams, Graham David; (1998) Diffraction studies of alkyl-ammonium vermiculites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The aim of the research was to measure the structure developed at the interface between an aqueous liquid and a charged surface (the electrical double layer, EDL). Despite its importance in e.g. colloids, it had not previously been measured due to the lack of a suitable system. Macro scopically swollen vermicuhte containing propyl- ammonium counterions was identified as a suitable system for such a measurement. Vermicuhte swelling behaviour was mvestigated as a flmction of salt concentration (c) and temperature, using neutron and X-ray diffraction. Small angle neutron scattering showed that the swollen vermicuhte gels behave as typical colloids, exhibiting a layer spacing proportional to c-1/2 Furthermore, it was discovered that the gels could be prepared with a layer spacing of ~40A, sufficiently small to allow measurement of the EDL structure by diffraction. The EDL structure m the vermiculite gels was measured using time-of-flight neutron diffraction. Gels were prepared with several isotopic compositions and aligned with the scattering vector perpendicular to the clay layers. Difference analysis allowed location of both water molecules and counterions along the clay c* axis. This was a novel treatment, allowing a principally Uquid structure to be described by a single particle distribution flmction. It was found that two distinct layers of water molecules are associated with each of the clay surfaces and that the counterion density is greatest midway between the clay layers, in contrast to predictions based on the primitive model. The same difference method was used to determine the interlayer structure in hydrated, unswollen crystals of butyl- and methyl-ammonium vermicuhte, in order to clarify why swelling occurs only in propyl- and butyl-ammonium vermicuhte. Butyl- ammonium ions are found to stand perpendicular to the clay surface while methyl- ammonium ions he parallel. It is proposed that the larger counterions force the clay layers apart, weakening the clay-counterion Coulombic interactions.

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