On the Nature of the Heterogeneity of Real Solid Surfaces

Water Vapor Adsorption on the Sol−Gel Composites Prepared Using Ethyl Silicate 40 as a Silica Precursor

“…Examination of this table reveals that for samples of the KCa series all values of the characteristic energy E are much the same, in the range of 3.3−4 kJ/mol, while the values of the exponent r are about 1.0 and hence in fair accord with the one ( r = 1) usually used in modeling of adsorption on mesoporous materials. , In fact the latter values increase slightly from 1.0 to 1.2 with an increase in chloride content from 10 to 30 wt %, respectively, and this trend, similar to that observed before, is a clear indication of the more intensive adsorption of water on the surfaces with a larger chloride content. Quite surprisingly, the r values pertaining to water adsorption on LiBr-doped samples are in the range of 1.5−3.0, that is, usually connected with adsorption on microporous active carbons, ,, while the values of E are equal to about 8 kJ/mol, and again they appear to be independent of the dopant content. These larger values of both r and E on the latter samples as compared with those depicting water adsorption on the CaCl 2 -doped composites are a clear indication 15 of the larger intensity of adsorption (of water) in the pertinent relative pressure interval ( p / p o < 0.2).…”

“…In the previous studies we used the most common expression, i.e., the Dubinin−Astakhov (DA) equation where E denotes the characteristic energy of adsorption and r is a measure of the heterogeneity of the system. In general larger r values correspond to narrower pore size distributions; , e.g., for active carbons r is in the range of 1.5−3, and for mesopore adsorbents r ≈ 1. However, the value of r also depends on the nature of the adsorbent surface .…”

“…An adsorption potential distribution J ( A ), where A = RT ln( p o / p ) is the aggregative quantitative characteristic of the heterogeneity effects, may provide a deeper insight into the complex adsorption process. In terms of the condensation approximation , J ( A ) is equal to −d v ( A )/d A , where v ( A ) denotes the adsorbate volume. The extensive literature provides a number of expressions that may be used to quantify adsorption of gases on heterogeneous surfaces.…”

Water Vapor Adsorption on the SiO2−CaCl2 Sol−Gel Composites

“…The total adsorption in the porous solids which exhibit a bimodal structure (micro- and mesopores) may be expressed as where A = RT ln( p 0 / p ) is the adsorption potential at temperature T and relative pressure p / p 0 . In terms of the condensation approximation, , the adsorption potential distribution J ( A ) is equal to −d v ( A )/d A . Thus J ( A ) can readily be determined provided the pertinent adsorption expressions v ( A ) are available.…”

“…On the whole, one may thus expect the process of water adsorption on the hybrid materials to be quite complex. An adsorption potential distribution (APD) is the aggregate quantitative characteristic of both structural and chemical heterogeneity , and hence a function of considerable interest. This distribution function can be directly determined from the experimental adsorption isotherm.…”