Mixed solvents, surface charging

The solution chemistry of nonaqueous solvents is very different from that of water-rich mixed solvents. pH measurement in nonaqueous solvents is difficult or impossible. Salts often show a limited degree of dissociation and limited solubility (see [132] for solubility of salts in organic solvents). Ions that adsorb nonspecifically from water may adsorb specifically from nonaqueous solvents, and vice versa. Therefore, the approach used for water and water-rich mixed solvents is not applicable for nonaqueous solvents, with a few exceptions (heavy water and short-chain alcohols). The potential is practically the only experimentally accessible quantity characterizing surface charging behavior. The physical properties of solvents may be very different from those of water, and have to be taken into account in the interpretation of results. For example, the Smoluchowski equation, which is often valid for aqueous systems, is not recommended for estimation of the potential in a pure nonaqueous solvent. Surface charging and related phenomena in nonaqueous solvents are reviewed in [3120-3127], Low-temperature ionic liquids are very different from other nonaqueous solvents, in that they consist of ions. Surface charging in low-temperature ionic liquids was studied in [3128-3132]. 

Pays special attention to correlations of the PZC and IEP with other physical quantities and properties, surface charging in mixed and nonaqueou,s solvents, surface charging at high ionic strengths, and ion-specificity in I-l electrolytes... 

We shall start from methods similar to that previously described, characterized by the use of the apparent surface charge (ASC) description of the electrostatic interaction term Ve/, passing then to consider other continuum methods, which use a different description of Ve/.To complete the exposition we shall introduce, where appropriate, methods not based on the solution of a Schrodinger equation, and hence not belonging to the category of continuum effective Hamiltonian methods. We shall pass then to a selection of methods based on mixed continuum-discrete representation of the solvent, to end up with the indication of some approaches based on a full discrete representation of the solvent. 

The compilation of PZC in this book involves results obtained in temperature range 15 0°C. In some studies the temperature was not controlled or measured room temperature) or at least the temperature is not reported. Detailed discussion of temperature effects on the PZC is not intended but a few examples of studies reporting the temperature dependence of the PZC are presented to show the general trends. Surface charging in mixed solvents and nonaqueous media, especially in polar solvents and water-organic mixtures is not much different from that in aqueous solution. A few results of such studies are presented as the last section of Chapter 3. 

Alumina was studied in aqueous alcohols [925], aqueous dioxane [666,963], aqueous dimethylsulfoxide (DMSO), aqueous glycerol, and aqueous heavy water [963]. Fe2O3 was studied in aqueous alcohols [1375,1386,1434,1456], aqueous dioxane [1388], and aqueous DMSO [1411]. Goethite was studied in aqueous acetone and aqueous methanol [1521]. Silica was studied in aqueous alcohols [1838, 1910,1911] and in other water-organic mixtures [1838]. Silica capillary was studied by electro-osmosis in 50 50 mixtures of organic solvents with water in the presence of a phosphate buffer [2927]. Surface charging of silica in mixed solvents is reviewed in [3110]. Titania was studied in aqueous alcohols [220,550,1986, 1988,2059,2115], aqueous dioxane [666,963], aqueous DMSO, aqueous glycerol, and aqueous heavy water [963]. Yttria was studied in aqueous alcohols [220]. 

Kosmulski, M. and Plak, A., Surface charge of anatase and alumina in mixed solvents. Colloids Surf. A, 149,409, 1999. 

Mustafa, S., Tasleem. S.. and Naeem, A., Surface charge properties of Fe2O3 in aqueous and alcoholic mixed solvents, J. Colloid Interf. Sci., T15, 523, 2004. 

Kosmulski, M. and Matijevic. E.. Formation of the surface charge on silica in mixed solvents, Colloid Polym. Sci., 270, 1046. 1992. 

The complexity of MLC is much greater dmn that of conventional RPLC with aqueous-organic solvents, because of Ihe number of possible interactions with both mobile and stationary phases (Fig. 5.1). Hie solutes in the mobile phase can interact electrostatically with the charged outer-layer of ionic micelles, and hydrophobically with their lipophilic interior. The steric factor can also be important. The modification of the stationary phase by adsorption of surfactant monomers, which creates a "micelle-like" surface, gives rise to similar interactions with the solutes. The combination of these interactions cannot be duplicated by any traditional pure or mixed solvent system. While micellar solutions will never totally replace traditional aqueous-organic eluents, they offer several interesting alternatives to separation work. 

Surface Charge and Conductance in Dispersions of Titania in Nonaqueous and Mixed Solvents... 

Keywords mixed solvent conductance zeta potential surface charge adsorption surface sites... 

Both linear and cross-linked monodisperse latexes of polystyrene in the size range 0.1 - 1.2y have been prepared by persulfate-initiated emulsion polymerization (6,7,8), and the size and size distributions of the polymer spheres detennined by electron microscopy. Free electrolyte was removed by a mixed-bed ion exchange resin, and surface charge measured by conductometric titration against standard base. Redispersion in organic media was effected by successive dialyses, first with methanol and finally against the desired solvent. 

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