Surface energy of oxides and silicates

Abstract

Published data and the author's own data on the surface energy of hydrophilic oxides, silicates, and hydrophobic adsorbents based on them are reviewed. The prospects of using the combined Gibbs-Helmholtz-Young equation to obtain data on the surface pressure, heat of wetting, and wetting contact angle of hydrophilic and hydrophobic adsorbents are demonstrated. These data are used to estimate the thermodynamic characteristics of the surface and interfacial regions at the boundary between the materials and water. It is shown that the boundary layers of water close to the hydrophobic surfaces are more ordered while those close to the hydrophobic surfaces are less ordered than with liquid water.

Adsorption-calorimetric determination of the surface energy of dispersed oxides

“…In most cases the values of the wetting angle given in the literature were determined in air q a , although it is the q V angle that is thermodynamically sound [2,9]. If the wetting angles are used it is also necessary to take account of the fact that the characteristics w, q, and p reflect the interaction of the vapor of the liquid or of the liquid itself directly with the degassed surface of the solid.…”

“…Therefore, a specific value of the angle q V is affected both by the characteristics of the substrate itself and by the characteristics of the adsorbed, in our case, aqueous film of 4-5 molecular layers. For hydrophobic surfaces, such as Teflon, or hydrophilic-hydrophobic surfaces, such as modified kaolinite [2] or Silochrome (see the Table 1), on the surface of which an aqueous film is not formed or is formed with hydrophobic defects, there is a correlation between the values of q V and the values of w, q, and p.…”

“…In the case of hydrophilic aluminum oxide, the film of water on the surface of which is under the influence of three types of active centers (see above), the q V values do not correlate with w, q, and p. The angles q V (1) presented in the Table 1 were calculated by means of the Gibbs-Helmholtz-Young equation [2], while the angles q V (2) were calculated by means of the Young equation s S -s SL = p + s LV ×cos (q V ), where s S is the surface free energy, s SL is the interfacial free energy, and s LV is the surface energy (surface tension) of the wetting liquid at its boundary with the saturated vapor. The values of q V (1) and q V (2) in the Table 1 coincide with each other and agree satisfactorily with the experimental values: For silica gel q = 26.8° [10], for a-Al 2 O 3 , q = 26°-44° [11,12].…”

“…2. The points for muscovite and graphite on the correlation curve are given according to the experimental values of s S , s SL , and q [2]. For quartz s S and q were determined experimentally.…”

Kinetics of Congruent Vaporization of ZnO Islands

“…[11] This dependence is contrary to the general characteristic of most materials, where the surface energy negatively depends on temperature, due to the increasingly strong effect of entropy at elevated temperature. [14] We speculate that the temperature-dependent surface energy is attributed to our experimental condition -a low pressure and a high temperature: this could intensify the effect of surface charging, [15] because the amount of ZnO evaporated into Zn + and O -is larger than for high P and low T. Specifically, this charging effect could appear when unequal number of the two species are present on the surface. We postulate that Zn + left behind during evaporation is incorporated into the surface of ZnO bulk, as indicated in prior reports.…”