Water layers at hydrophilic surfaces

Near hydrophobic surfaces, density of liquid water is depleted and its structure becomes less ordered (Sections 2.3 and 3.2). Quite similar behavior is seen for other fluids (for example, U fluid) near weakly attractive surfaces (Section 3.1). More peculiar behavior of water may be expected near hydrophilic surfaces, as strong localization of molecules due to the attraction to the surface causes speciflc rearrangement of water-water H-bonds. In this section, we characterize the arrangement of water molecules in various phase states of water vapor, monolayer, bilayer, and liquid water. In particular, percolation transition, that is a continuous transition between a low density vapor and a complete mono-layer, is considered in Section 5.1. A specific orientational ordering of water near the surface and its intrusion into a bulk liquid water is analyzed in Section 5.2. 

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Figure 86 Arrangement of water molecules in the surface layer at hydrophilic surface with L/q = -4.62 kcal/mol.

An alternative procedure is to clean a microscope slide with detergent and polish with a clean cloth without rinsing away the detergent, so as to form a hydrophilic layer at the surface, which facilitates stripping. The slide is dipped in a solution of formvar in ethylene dichloride (0.3% to 0.7% depending on the film thickness required) and allowed to drain dry. The film may be floated on to a water surface and mounted on grids as before. If individual grids are required, the film may be cut into small... 

Steric Hindrance. Another form of stabilization is relatively independent of ionic strength the oil droplets are prevented from making contact by simple steric hindrance. This may take two forms, either an immobilized water layer at the interface or a solid interfacial film. Emulsion stabilization by proteins, gums, and polyoxyethylene derivatives occurs by the first mechanism. Hydrophobic parts of the stabilizers adsorb at the oil surface, but adjacent large hydrophilic segments are hydrated and form an immobilized layer on the order of 10-100 nm thick (Figure 9). As mentioned, these hydrated segments frequently interact to cause flocculation, while coalescence of the oil drops themselves is prevented. Such emulsions are frequently used as carriers for oil-soluble flavors, essences, and colorants. 

The constitution of ordered layers of water at interfaces with carbonaceous adsorbents in aqueous suspensions is governed by three major factors, namely, hydrophilic properties of the surface, porosity of the material, and the feasibility of polarization of the surface at the expense of the formation of regions carrying electric charges of opposite signs. In the general case, the thickness of an adsorbed water layer on the surface is detennined by the action radius of surface forces in whose field the orientation of electric dipoles of water molecules occurs and the formation of its surface clusters takes place. 

According to Hagymassy, Brunauer and Mikhail [36], the thickness of an adsorbed water layer at the saturation pressure of water vapor is about 6 monolayers. When the thickness of a water layer is 0.3 nm, the fraction of the pore volume of the support taken up by an adsorbed film of water can be calculated. Knijff [35] established that the fraction of the pore volume filled with 6 monolayers of water agrees well with the moisture content, at which the drying rate displays the first, slight drop below the level of the constant rate period. The rate of evaporation that is maintained during that period indicates that transport of the water film proceeds more slowly when the thickness of the water layer decreases below about 6 mono-layers. However, the drying experiments clearly indicate that liquid water can be transported not only by capillary pressure, but also as a film adsorbed on the surface of oxidic supports. A prerequisite for this mechanism of transport may be that the surface of the support is hydrophilic. 

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. 

The results presented above evidence that distortion of a bulk liquid water structure near hydrophilic surfaces does not extend on more than two surface water layers. There are three main structural features of these layers localization of molecules parallel to the wall, chain-like structures of water molecules within the layers and orientational ordering of molecules. These features strongly appear in the first layer and noticeably weaker in the second layer. Except for these two layers, the rest of liquid water is similar to bulk liquid. This observation corroborates various manifestations of the specific bound water at hydrophilic surfaces. 

Unfreezable water in the solid-liquid transition of confined water is about two layers thick (Section 4.2). There are two dead water layers in the liquid-vapor transition of water near strongly hydrophilic surfaces (Section 2.2). At mineral surfaces, one to two water layers are highly ordered [441]. Numerous experimental and simulation studies indicate quite different dynamic properties (diffusion coefficient, residence time, reorientational time, etc.) of liquid water within the first 1-2 layers. 

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