Salt-boundary effect

In addition, when water is used as eluent and the sample applied contains electrolyte, then a salt-boundary effect can occur (Posner, 1963). A typi-... 

FIGURE 5. Gel permeation chromatography of sodium humate on Sephadex G-lOO using water as eluent but with NaCI added to the sample showing the salt-boundary effect. 

In order to treat the combined effects of added salt and dielectric boundaries on a manageable level, we use screened Debye-Hiickel (DH) interactions between all charges. In the presence of a dielectric interface, the Green s function can in general not be calculated in closed form [114] except for (i) a metallic substrate (with a substrate dielectric constant e =oo) and (ii) for e =0 (which is a fairly accurate approximation for a substrate with a low dielectric constant). For two unit charges at positions r and r one obtains for the total electrostatic interaction including screening and dielectric boundary effects... 

Inorganic salts Aqueous tion on emulsion boundary surfaces Hydrogen bridges Salting-out effect... 

T (solution). Salt-in or salt-out effects with other solutes (9,12,26) demonstrate that the boundary between salt-in or salt-out effect depends also on the organic solute. There seems to be a rivalry between H-bonds of water-water, water-ions and water-organic solute, which depends on bond strength to the organic solute and whether it is a H-bond acceptor or donor. 

The two-phase boundary surface and the pvT-behavior in the supercritical region has been extensively investigated (12,13,14). In analogy to "salting out" effects at low temperature, one should expect that a dissolved salt as a third component also affects the miscibility of CO2 and H2O in the critical region. This is indeed the case as was shown some years ago (15). Recently extended measurements with the H20-C02-NaCl system has been performed at several concentrations and the results are shown in Fig. 5. 

Sihcate solutions of equivalent composition may exhibit different physical properties and chemical reactivities because of differences in the distributions of polymer sihcate species. This effect is keenly observed in commercial alkah sihcate solutions with compositions that he in the metastable region near the solubihty limit of amorphous sihca. Experimental studies have shown that the precipitation boundaries of sodium sihcate solutions expand as a function of time, depending on the concentration of metal salts (29,58). Apparently, the high viscosity of concentrated alkah sihcate solutions contributes to the slow approach to equihbrium. 

The continuum treatment of electrostatics can also model salt effects by generalizing the Poisson equation (12) to the Poisson-Boltzmann equation. The finite difference approach to solving Eq. (12) extends naturally to treating the Poisson-Boltzmann equation [21], and the boundary element method can be extended as well [19]. 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

Other reactions in which cations other than protons are catalyti-cally effective are esterification and acetal formation, catalyzed by calcium salts,277 and the bromination of ethyl cyclopentanone-2-carboxylate, catalyzed by magnesium, calcium, cupric, and nickel, but not by sodium or potassium ions.278 One interpretative difficulty, of course, is the separation of catalysis from the less specific salt effects. The boundary line between salt effects (medium effects) and salt effects (catalysis) is not sharp either in concept or experimentally. 

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