Capillary Condensation of Water between Mica Surfaces above and below Zero-Effect of Surface Ions

Abstract

We have studied the capillary condensation of water from saturated vapor below 0 degrees C in the annular wedge-pore formed around two mica surfaces in contact in a surface force apparatus. The condensed water remains liquid down to at least -9 degrees C, and the measured condensate size is close to the predictions of a recent model for the dependence of the interfacial curvature of supercooled capillary condensates on temperature and surface tension. The small deviation observed may be accounted for by assuming that solute as K(2)CO(3) from the mica-condensate interface dissolves in the condensates and gives rise to an additional depression of the freezing point apart from that caused by the interface curvature. By contrast, measurements of the interface curvature at relative vapor pressures of 0.95-0.99 at 20 degrees C confirm a significantly larger deviation from the Kelvin equation. The magnitude of the deviation is in remarkable agreement with that calculated from the results of an earlier study of capillary condensation of water from a nonpolar liquid, also at T = 20 degrees C. Evidently, additional solute from the surrounding mica surface migrates into the condensates at room temperature. We conclude that the surface diffusion of ions on mica is much slower at subzero temperatures than at room temperature.

The nature of the air-cleaved mica surface

“…This surface density of K2CO3 is also consistent with the amount dissolved in the subzero water condensates mentioned in the same section [133].…”

“…The vapour-pressure lowering due to potassium solute was then estimated to be onefifth of that due to interfacial curvature. This suggested that the rate of surface diffusion of K2CO3 is significantly smaller at subzero temperatures, which is not unexpected [133].…”

“…Curiously, quantitative measurements of the size of water condensates between K-mica surfaces were only made much later [133] -the condensate size was not explicitly mentioned by Fisher and…”

Two-step crystal nucleation via capillary condensation

“…If the saturation vapour pressure is eliminated from the equations the result is a relation describing the radius of a pore-held liquid-vapour interface in equilibrium with bulk solid and its saturated vapour. 36,42,43 Several complete derivations have been published 42,44,45 of the dependence of the condensate interfacial radius of curvature on the temperature depression DT below T m and on the relative vapour pressure p/p s . These include effects of a temperature-dependent surface free energy c (it increases with decreasing temperature) and a temperature-dependent enthalpy of fusion DH fus 44 (which decreases with decreasing temperature).…”

Microstructure changes induced by capillary condensation in catalyst layers of PEM fuel cells

“…2, whose surfaces are hydrophilic. Inside the space between the Pt/C particles, capillary condensation can occur at the equilibrium vapor pressure p below the saturation vapor pressure p 0 [13]. The relation between pand p 0 can be described by the Kelvin equation,…”