Water, surface potential

An attempt to make an a priori calculation of the air/water surface potential was made by Stillinger and Ben-Naim (1967). The water molecule was idealized as a point dipole and point quadrupole encased in a spherical shell representing its excluded volume. Classical electrostatics was applied to the determination of the electric field surrounding such a molecule in the interphasial region of the liquid and vapor phases. Prom this evaluation, the mean torque on the molecule in the interphase can be deduced and leads to a spontaneous orientation polarization at the interface. The resulting p.d. at the interface was derived for several temperatures and corresponded to a tendency for the 0 atoms of water dipole/quadruples to be oriented outwards from the bulk as is indicated indirectly from the various experimental approaches mentioned above. 

The maximum of the density profiles in both phases coincides with the location of the well depth of the (9-3) LJ water-surface potential, which is at 3 A from the surface. In the quasi-2D liquid phase, one water molecule occupies about 10 A of a surface at low temperatures. This value is approximately equal to the projection of the volume occupied by a water molecule in a bulk liquid water with p = g/cm onto the surface. Arrangement of water molecules in the quasi-2D liquid phase is shown in Fig. 15. At supercooled temperatures, the surface is covered by a dense water layer, which practically does not contain holes. Upon heating from 200 to 375 K, the density of this layer decreases by about 30%, it becomes slightly less localized (see middle panel in Fig. 14), and the holes in the layer appear. However, an infinite hydrogen-bonded water network is always present in a quasi-2D liquid water (see Section 5 for further discussion on the percolation of hydration water). 

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... 

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... 

Figure4.9 Gibbs interaction energy (in units of DLVO theory using Eqs. (4.71) and (2.67). The ksT) versus distance for two identical spherical Hamaker constant was Ah = 7 x 10 J, the particles of Rp = 100 nm radius in water, surface potential was set to i( q = 30 mV. Both...

Customarily, it is assumed that e is unity and that ]l = p,cos 9, where 0 is the angle of inclination of the dipoles to the normal. Harkins and Fischer [86] point out the empirical nature of this interpretation and prefer to consider only that AV is proportional to the surface concentration F and that the proportionality constant is some quantity characteristic of the film. This was properly cautious as there are many indications that the surface of water is structured and that the structure is altered by the film (see Ref. 37). Accompanying any such structural rearrangement of the substrate at the surface should be a change in its contribution to the surface potential so that AV should not be assigned too literally to the film molecules. 

Clean boiler water surfaces reduce potential concentration sites for caustic. Deposit control treatment programs, such as those based on chelants and synthetic polymers, can help provide clean surfaces. 

Pc- (c) Dipole density p. (d) Water contribution to the surface potential x calculated from the charge density Pc by means of Eq. (1). All data are taken from a 150 ps simulation of 252 water molecules between two mercury phases with (111) surface structure using Ewald summation in two dimensions for the long-range interactions. 

The reactivity modification or the reaction rate control of functional groups covalently bound to a polyelectrolyte is critically dependent on the strength of the electrostatic potential at the boundary between the polymer skeleton and the water phase ( molecular surface ). This dependence is due to the covalent bonding of the functional groups which fixes the reaction sites to the molecular surface of the polyelectrolyte. Thus, the surface potential of the polyion plays a decisive role in the quantitative interpretation of the reactivity modification on the molecular surface. 

This potential reflects itself in the titration curves of weak polyacids such as poly(acrylic acid) and poly(methacrylic acid) [32]. Apparent dissociation constants of such polyacids change with the dissociation degree of the polyacid because the work to remove a proton from the acid site into the bulk water phase depends on the surface potential of the polyelectrolyte. 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

The phase-11 islands have several interesting properties. First, they have a positive surface potential relative to the surrounding unperturbed water film. The potential is highest immediately after formation and decays with time to zero. Another interesting property is the shape of the islands. Their boundaries are often polygonal, bending in angles of 120°,... 

FIG. 28 Changes in contact potential of mica relative to a hydrophobic tip as a function of relative humidity. The tip-sample distance during measurements was maintained at 400 A. At room temperature the potential first decreases by about 400 mV. At -30% RH it reaches a plateau and stays approximately constant until about 80% RH. At higher humidity the potential increases again, eventually becoming more positive than the initial dry mica surface. The changes in surface potential can be explained by the orientation of the water dipoles described in the previous two figures. 

Preparation of an uncontaminated surface of an aqueous solution is very difficult. Even minute traces of adsorbable organic impurities strongly influence the surface potential of water. Cleaning of the aqueous surface (e.g., by siphoning off the surface layer) is usually necessary, while for organic solutions it is usually not needed. ... 

According to Eq. (2), the real and chemical energies of transfer differ by the term containing the difference in surface potentials of a given solvent and water (see Section XIV). 

Adsorption of a dipolar substance at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water (Fig. 11). As seen in Fig. 11, the change in compensation voltage due to adsorption is the surface potential difference, usually called the surface potential or better the adsorption potential and often indicated unnecessarily by AV. ... 

Figure 11. Influence of the adsorbed molecules B on the surface potential of water.

The models presented above have also been reviewed in Ref 18. Recently, an expression for the adsorption potential at the free water surface based on a combination of the electrostatic theory of dielectrics and classical thermodynamics has also been proposed." ... 

The popular applications of the adsorption potential measurements are those dealing with the surface potential changes at the water/air and water/hydrocarbon interface when a monolayer film is formed by an adsorbed substance. " " " Phospholipid monolayers, for instance, formed at such interfaces have been extensively used to study the surface properties of the monolayers. These are expected to represent, to some extent, the surface properties of bilayers and biological as well as various artificial membranes. An interest in a number of applications of ordered thin organic films (e.g., Langmuir and Blodgett layers) dominated research on the insoluble monolayer during the past decade. 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

The presence of adsorbed Langmuir monolayers may induce very high changes in the surface potential of water. For example, A% shifts attaining ca. -0.9 (hexadecylamine hydrochloride) and ca. + 1.0 V (per-fluorodecanoic acid) have been observed. " ... 

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