Surface ion-pair

The offset of 1.89 indicates that surface ion pairing in membranes is about 100 times greater than that of octanol. Scherrer suggested that comparisons of pK°ct to pA em may be more predictive [276]. Indeed, this is true for the phenols, as indicated in Fig. 5.9. It is remarkable that the relation for the phenols... 

Another class of synthesis reaction is deprotonation of a hydrido metal carbonyl cluster on a basic surface. For example, [H4Os4(CO)i2] reacts with MgO or with y-Al203 to give [H3Os4(CO)i2], which is part of a surface ion pair on the support (Budge, Scott, and Gates, 1983). 

As already noted, the restabilization of some colloids at high ionic strength found in previous experiments20 can be explained in the traditional framework of the additivity between double layer and hydration forces, by a slight increase of the hydration repulsion caused by the increase in surface ion pair (dipole) density with electrolyte concentration. However, the increase in repulsion due to this mechanism is much too low to explain the strong increase of the second virial coefficient. 

The adsorption of the ions of the electrolyte changes the charge of the surface and generates simultaneously surface ion pairs (dipoles). Let us first assume that only the cation is adsorbed. The electric field induced by a surface dipole on the first layer ofwater molecules is given by6... 

A new theory was proposed in which the adsorption of the ions of the electrolyte generates surface ion pairs (dipoles), which induce an electric field. In addition, the neighboring dip oles in water also generate a field in water. The restabilization of the colloidal dispersion at sufficiently high ionic strengths appears to be an effect of these fields. The charge inversion produced by the adsorption of the ions of the electrolyte plays also a role particularly for the high valency ions. 

Partitioning experiments have been carried out using liposomes, where neutral log P values from liposomes tend to be very similar to those measured in octanol, but the log P values of the ion pair differ. The "surface ion pair" log P is found to be much higher in bases, zwitter ions, and amphophiles. The values for... 

Compared to the constant-capacitance model, the Stem model allows variation of ionic strength via (1) the consideration of surface ion pairs with the electrolyte ions and (2) the difluse layer. The ion pairs permit a variation of the electrolyte in terms of composition (i.e., the electrolyte binding constant can be adjusted for each ion). The difluse layer is not ion-specific and, in principle, restrictions apply which have been pointed out for model c. Compared to a purely diffuse-layer model, violation of these restrictions will have less severe consequences for the surface-charge density because the Stem layer usually decreases the difluse-layer potential compared to the purely diffuse-layer model. There is one adjustable parameter pertaining to the electrostatic model (the capacitance). For the 1-pK formalism, in addition to the site density parameter, two electrolyte-binding constants are required. 

Surface structure of alkaline earth oxides was investigated using UV absorption and luminescence spectroscopies. High surface area MgO absorbs UV light and emits luminescence, which is not observed with MgO single crystal. UV absorption, corresponds to the following electron transfer process involving surface ion pairs. 

Luminescence corresponds to the reverse process of UV absorption, md the shape of the luminescence spectrum varies with the excitation light frequency and with adsorption of certain molecules. Luminescence involves surface ion pairs of low coordination numbers. 

Four active site types on alkaline earth oxides are proposed OH groups. Sites I — III, and are summarized in Tkble 3.3. Appearance of OH group, Site I, II, and III with increasing pretreatment temperature is schematically illustrated in Fig. 3.13. Correspondence of these sites to the surface ion pairs in the model structure is also included in Table 3.3. 

The close similarity of the liquid water profile in Fig. 7 with the 13 mbar spectrum of H2O on NaCl(OOl) in Fig. 8 is good evidence that the thin film is liquid-like. Use of the optical constants of liquid water and the Beer-Lambert relation has enabled the determination of the coverage values as listed on Fig. 8 and the construction of the isotherm in Fig. 9. (A monolayer, (9 = 1, corresponds to each Na" Cl" surface ion pair covered, on average, by an H2O molecule.) In order to provide a context with an ambient environment, the water pressure needed to produce a monolayer on NaCl(lOO) corresponds to 40% relative humidity [77,78], a rather arid condition. In addition, the pristine appearing surfaces of the salt crystals in Fig. 2, photographed at 50% RH, were coated with several water layers. 

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