Dielectric behaviour of water sorbed on silica gels. Part 2.—Purified silica gel and samples calcined at different temperatures

Abstract

Sorption isotherms, reversible dielectric isotherms (over a frequency range from 2.5 Kc/sec to 1 Mcjsec) and corresponding reversible loss curves are reported for water, both adsorbed and desorbed, on a purified commercial silica gel and on two samples calcined at 610 and 810"C, respectively. The first linear, frequency-independent, region in each dielectric isotherm corresponded to water molecules adsorbed on surface silanol groups. Dispersion phenomena observed in the second region, up to 500 Kclsec, was attributed to Wagner-type dispersion due to the heterogeneous nature of the system and to the conductivity of the adsorbed film after monolayer completion.Differences in the dispersion phenomenon exhibited by the uncalcined and calcined gels could be explained on the basis of progressive reduction in the conductivity of the adsorbed f i l m on increasing the calcination temperature, partly due to progressive removal of ions still present on the surface of the uncalcined gel. At 1 Mc/sec, no dispersion is observed and then each dielectric isotherm consists of two linear regions, with the second (extending to near saturation) being of smaller slope than the first, and apparently characteristic of the dielectric behaviour of polar adsorbates on porous adsorbents.

Water as a tuneable solvent: a perspective

“…113 The evolution of the permittivity and 1 H NMR spectra with water content, temperature and pressure enables deconvolution of the dielectric properties of water and the confining matrix. [160][161][162][163] Evolution of the X-ray scattering will reveal changes in the local water structure and density and is part of a global NMR crystallography approach. [163][164][165] Combination of in situ sum frequency generation (SFG) vibrational spectroscopy and FTIR spectroscopy will offer additional insight in water structures and dynamics, 95,166 water-induced structural changes of the matrix material and the solid-liquid interfaces inside the pores.…”

Dielectric constant of sandstones, 60 kHz to 4 MHz

“…A number of examples in the literature focus on the role of surface water in the dielectric behavior of solid materials containing relatively small amounts of water (McIntosh et al, 1951;Kamiyoshi and Odake, 1953;Kurosaki, 1954;Thorp, 1959;Nair andThorp, 1965a, 1965b;McCafferty and Zettlemoyer, 1971). The dielectric mechanism in each of these studies is interpreted either in terms of the rotational molecular mobility of individual water molecules or in terms of the larger scale conductivity heterogeneities in the water-solid s-ystem.…”

“…With such a mechanism, it is not the rotational mobility of the water molecules in the surface water that is important, but the translational mobility, or the conductivity of the surface layer. A conductive mechanism has been proposed by Kamiyoshi and Odake (1953) and Nair andThorp (1965a, 1965b), for dielectric dispersion in systems in which water coated the surfaces of a solid. Sen (1980) and Kenyon (1984) use a Maxwell-Wagner effect, modified by the shape of the grains, to explain the dielectric response of water-saturated sedimentary rocks.…”

The Specific Activity of Silica Supported Platinum for the Catalysis of Hydrogen – Water Deuterium Exchange

“…Three possible explanations suggest themselves for this behavior, although no definite conclusions can be reached. The heats of adsorption of water on silica gel vary from 5 to 1 kcal/mol for gels degassed at 150 and 810 "C, for 50 mg water adsorbed per g of gel (30). Thus the gels degassed at higher temperatures are less hydrophilic, and, if water adsorbed on the gel plays any part in the kinetics, lower rates may result.…”