Layering, prewetting, and wetting transitions of water near hydrophilic surfaces

2 Layering, prewetting, and wetting transitions of water near hydrophilic surfaces 

Adsorption of water from the air on hydrophilic surfaces occurs in various natural processes on the earth. Certain amount of water vapor is always present in the air. About 25 g of water per 1 kg of air corresponds to the 100% relative humidity at ambient conditions. This corresponds to the dew point, where condensation of water vapor into a liquid occurs in a bulk. At these conditions, which exist locally and temporarily on 

With increasing humidity, growth of the amount of water adsorbed may occur in a continuous way or via the surface phase transitions, such as layering and prewetting, described in Section 2.1. Obviously, the presence of water clusters, water layer(s), or macroscopic water film on the surface essentially modifies the system properties. To predict water behavior near various surfaces, it is, therefore, important to analyze in a systematic way all possible scenarios of water adsorption and to relate them with the thermodynamic conditions and with the properties of a surface. Analysis of the surface phase transitions of water at hydrophilic surfaces (this section) and at hydrophobic surfaces (Section 2.3) will be finalized by constructing the surface phase diagram of water in Section 2.4. 

In the adsorption isotherm, surface phase transition (layering or prewetting) should appear as a sharp vertical step at some pressure of a water vapor below the saturated value (Fig. 10). At this particular pressure, two water phases coexist on the surface, and relative fraction of these phases depends on the average surface coverage. Experimental studies of water adsorption on various surfaces give information about the occurrence of the surface phase transitions. In some cases, the corresponding step in the 

Apart from the technical limitations of the experimental techniques used in the studies of the phase transitions, there are several physical reasons that cause smearing out of the phase transition. First, due to the occurrence of the metastable states, the transition during adsorption and desorption may occur at different pressures. Such hysteresis indicates the first-order character of the phase transition but strongly complicates its localization. Chemical or structural heterogeneity of the 

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