Water-Surface Interactions

Figure 1. Comparison of ST2 water-surface interactions computed from Equations 7 or 8 using parameters for the Lennard-Jones 9-3 potential in Table II and the delocalized charge magnitude for smectite and mica surfaces in Table III.

Ideally, models of vicinal water should eventually "explain all established experimental facts. There is a long way to go However, some general observations have been made. One is that, against a variety of hydrophobic phases (silver iodide, mercury, air) water molecules appear to be oriented with the negative ends of the molecules pointing outward (sec. 3.9). In other words, the polarization of water adjacent to silver iodide and mercury is similar to the spontaneous polarization of water surfaces. The implication is that near such surfaces water-water interactions play at least an important role as water-surface interactions. Another observation, relevant for the interpretation of electroklnetic phenomena, is that tangentially immobile surface layers do occur near both hydrophilic and hydrophobic surfaces. 

FIGURE 4.30. A schematic diagram illustrating stepwise water-surface interactions with freshly cleaved or fractured quartz. (Reprinted from Dove and Rimstidt. 1994 by permission of the Mineralogical Society of America.)... 

By comparing almost 100 different silicas, Zhuravlev [86] has shown that the surface density of hydroxyl groups is a physicochemical constant for a fully hydroxylated surface (5.0 -OH per nm ). As the parameter cji is related mainly to this surface density, this means that the dispersion of water-surface interactions is also a constant value to a greater or smaller extent. Only in such a case peaks on the heat capacity curves of water adsorption on various silicas should be generally observed, as, in fact, the case is. 

The presence of these small pores can also result In non-freezing water. Hence, the non-freezing water In pulp and paper appears as the result of both the physical presence of pores and strong water-surface Interactions. 

Note that the extensive HB network is compromised near both the hydrophilic and the hydrophobic surfaces, but differently. In the case of the hydrophilic surface, the enthalpic gain from the water-surface interaction compensates for the twin losses of enthalpy and the entropy of water arising from the molecular rearrangement imposed by the surface. However, for a hydrophobic surface, such a compensation is not present. Therefore, the chemical potential of a water molecule near a hydrophobic surface is higher than that in a bulk. 

Remarkably, an apparent mass loss is found for the second immersion cycle (Adv-fdc-2) reflective of a hydrophobic surface (0adv2 = 106°). Now, water-hydrogen bonding interactions are stronger than water-surface interactions and water resists wetting the surface. Contact angles measured by drop profile analysis are close to those observed for DCA. 

Differences in the pressure of the layering transitions should be attributed first to the different strength of the water-surface interaction. This strength should correlate with the isosteric heat of adsorption q at... 

All four coexistence curves, shown in Fig. 13, are very similar, which indicates a weak sensitivity of the first layering transition to the pore size. Besides, the layering transitions in the slit-like pore and in the cyhndrical pore with the same strength of the water-surface interaction are also quite similar (Fig. 16, left panel). The critical temperature of the layering transition of water is just by a few degrees lower in case of the slit-like pore. The degree of the localization of water molecules near the surface is also determined solely by the value of Uq. The density profiles of a quasi-2D... 

With the strengthening of the water-surface interaction, the critical temperature of the layering transition starts to decrease. When the water-surface potential Uq changes from -4.62 to -7.70 kcal/mol, T drops from 400 to 360 K, whereas the surface density of a water monolayer... 

The critical temperature of the second layering transition is always lower than the critical temperature of the first one, and it was found between 0.48 T and 0.59 Tc. Similar behavior was found by density functional calculations for a strongly associative LJ fluid in pores [201-203]. Even near strongly hydrophilic surfaces, there are only two noticeable density oscillations of liquid water when it is in equilibrium with a saturated vapor. Water properties (e.g., orientational ordering) in the third and subsequent layers are close to the bulk ones [205-208]. Therefore, the third and subsequent layering transitions of water should not be expected. When the water-surface interaction weakens, the critical temperature of the second layering transition drops down by about 50°, and the triple point, where 2D gas coexists with water monolayer and with water bilayer, may be seen (lower right panel in Fig. 21). 

When the water-surface interaction weakens further, the layering transitions disappear and the appearance of the prewetting transition may be expected. Prewetting transition is a first-order phase transition, which occurs (similarly to the layering transitions) at some undersaturated vapor pressure and indicates condensation of a water film on the surface (see Section 2.1). The line of the prewetting transition meets the bulk liquid-vapor transition in a triple point, where bulk vapor and bulk liquid coexist with a water film. The temperature of this triple point is a... 

Figure 22 Coexistence curves of water in cylindrical pores with = 25 A and different strength of the water-surface interaction. Surface phase transitions (layering and prewetting) are shown by open circles. Coexistence between vapor and liquid phase is shown by open squares. Coexistence between the water film and the hquid water is shown by closed circles.

The temperature of the wetting transition is sensitive to the used water model and to the pore geometry. The phase diagram of ST2 water in slit-like pore of 24 A width and with Uq = -4.62 kcal/mol is shown in Fig. 25. The temperature of the wetting transition is by about 40° higher than in the case of TIP4P water in cylindrical pore with the same water-surface interaction. In parallel, the critical temperature of the prewetting... 

Drastic changes in the phase diagram of water occur when the water-surface interaction weakens by just 1 kcal/mol (from —3.08 to... 

Drying transition may occur in a liquid phase upon heating along the liquid-vapor coexistence curve (see Section 2.1). This transition has drastic effect on the liquid-solid interface above the temperature Tj of a drying transition, the liquid is separated from the solid surface by a macroscopic vapor layer. However, even below Tj and out of the liquid-vapor equilibrium, distant etfect of the drying transition may noticeably affect the liquid density profile. Therefore, it is important to know the temperature of the drying transition of water and its sensitivity to the water-surface interaction. This allows description of the density profiles of liquid water near hydrophobic surfaces at various thermodynamic conditions. 

Figure 29 Density profiles of liquid water in cylindrical pores (i p = 25 A) with smooth surfaces of various strengths Uq of the water-surface interaction.

When the profiles of the local diameters are normalized by the bulk diameter at the same temperature, they do not collapse on a single master curve, as it happens with the profiles of the local order parameter (Fig. 49, right panel). This nonuniversality may be caused by the long-range water-surface potential. As behavior of water near a surface with short-range water-surface interaction is not yet studied, this idea remains speculative. The local diameter pd calculated in the surface layer vanishes upon increasing temperature much faster the bulk diameter (Fig. 50). It is... 

We would expect that the amplitude B of the leading singular term in equation (13) should not depend on the water-surface interaction potential, at least in the first approximation. This term arises from the bulk order parameter, whose amplitude Bq is determined by the water-water interaction only. Therefore, we believe that the water-water interaction gives a major contribution to the amplitude B. In contrast, the parameters of the asymmetric terms in equation (13) should strongly depend on the water-surface interaction. In particular, Pc in the surface layer is essentially below the bulk critical density, when a weak fluid-wall interaction provides preferential adsorption of voids, whereas pc may exceed the bulk critical density in the case of a strong water-surface interaction. It is difficult to predict the values of the temperature-dependent terms in the asymmetric contribution, as the surface diameter reflects interplay between the natural asymmetry of liquid and vapor phases, described by the bulk diameter, and preferential adsorption of one of the component (molecules or voids). 

Distribution of the water molecules in vapor phase at low temperature and low density is determined mainly by water-surface interaction. Close to the triple point temperature, water vapor shows adsorption even at the strongly hydrophobic surface. In this regime, the vapor density profiles py(Az) can be perfectly described by the Boltzmann formula for the density distribution of ideal gas in an external field ... 

A further increase in the temperature (density) of the saturated vapor promotes the effect of missing neighbors, and at some thermodynamic state, it may be roughly equal to the effect of surface attraction. The signature of such balance is an almost flat density profile. For the water-surface interaction with a weU depth Uq = —0.39 kcal/mol, this happens at T 475 K and 0.02 g/cm (right-upper panel in Fig. 52). At the more hydrophilic surface, the flat density profile may be found at higher temperature. One may expect that at some level of hydrophilicity, the flat density profile of water may appear at the bulk critical point only. 

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