Specific Feature of Dielectric Behavior of Water Adsorbed on Ag₂O Surface

Abstract

The dielectric behavior of Ag2O samples with different amounts of adsorbed H2O was investigated in the temperature range from 308 to 200 K and in the frequency region from 0.1 Hz to 5 MHz. A large relaxation is observed in these ranges, and it is explained by the interfacial polarization due to the heterogeneity of the system. The characteristic frequency (f m) for this relaxation is lowered with decreasing temperature until a complete monolayer is established. However, such a behavior of fm in the region beyond the surface coverage of ca. 1 reveals a new specific feature; the fm value gives a maximal frequency at around 278 K. The cause of this behavior is explored through the measurements of the differential adsorption heat, water content, and Clausius-Clapeyron plots (ln p vs 1/T). As a result, a new phenomenon observed in the Ag 2O-H2O system can be explained by considering that the continuous phase change occurs at around 278 K in the adsorbed layer higher than the monolayer; the cluster-like structure formed on the OH groups at a temperature higher than 278 K is transferred to the two-or three-dimensional network structure formed by H-bonding between adsorbed H 2O molecules in the lower temperature region.

An adsorption theory of heterogeneous nucleation of water vapour on nanoparticles

“…Nucleation experiments have Kuroda et al (1997), and the line is calculated using Eq. (8) with the A, B, and s-parameters optimized using non-linear least squares as described in the text.…”

“…However, the adsorption data (Kuroda et al, 1997) show no indication of a multilayer film formation (which should be observable in the FHH plot of Fig. 3a) as a quite abrupt change of slope, see Laaksonen (2005), although the macroscopic film thickness (see Fig.…”

“…Indeed, as shown in Fig. 3, this is the case with the adsorption data of Kuroda et al (1997). The adsorption parameters A and B were therefore determined by fitting S from Eq.…”

“…Regarding silver nanoparticles, Kupc et al (2013) showed experimentally an anomalous temperature dependence for water vapour nucleation: decreasing critical supersaturation with decreasing temperatures between 278 and 262 K (usually the temperature dependence of S * is monotonously increasing as T decreases). Very interestingly, Kuroda et al (1997) have made adsorption measurements as a function of temperature at exactly the same interval, and their data appear to have some unusual features (which the authors attribute to a continuous phase change in the adsorbed layer at around 278 K). Unfortunately they made measurements for four constant surface coverages only, which makes it somewhat challenging to tease the actual temperature dependencies of the adsorption parameters out of their data.…”

“…In order to connect the theory to adsorption experiments (from which the adsorption parameter values can be obtained), a relation is needed between the droplet size and the macroscopically observable surface coverage, N. The equations below apply for adsorbent surfaces that are sufficiently flat so that their curvature is essentially zero, as is the case with the adsorption experiments considered here (Naono et al, 1994;Every et al, 1961;Kuroda et al, 1997). In general, it is quite rare that adsorption measurements are made with nanoparticles, but in such cases, adsorbent curvature should of course be taken into account.…”