Small angle neutron scatteirng investigations of water sorption in porous silica and ceria gels

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Abstract

The processes of adsorption and capillary condensation of water in silica and ceria gels having different pore sizes have been investigated by small angle neutron scattering (SANS). These gels have been prepared by a sol—gel method and are composed of monosized spherical particles packed with a defined short-range order. Mean pore sizes, rp, were controlled by the sol particle size and were ∼ 2.9 and ∼ 2.0 nm for silica gels and < 2.0 nm for the ceria gel. SANS measurements have been made at progressive stages of equilibrium water uptake by exposing the gels to increasing vapor pressures. By selecting the water composition (H2OD2O ratio) to give a contrast match with the oxide adsorbent, it has been made possible to analyze changes in the SANS behavior in terms of the development of an adsorbed film and growth of a capillary-condensed meniscus at the points of particle contact. Such an analysis is satisfactory for the two mesoporous silica gels but does not accord with the behavior of the microporous ceria gel. Here there is evidence that a volume filling process occurs, where the density of the sorbate is appreciably less than that in the bulk. This lower density is tentatively ascribed to a difference in the ordering of water molecules when they are confirmed in the micropores compared to that imposed by H bonding in the bulk liquid.

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    Present address: AWRE, Aldermaston, Berkshire, United Kingdom.

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