Bulk solvent mixture

Preferential solvation is not restricted to ions of electrolytes dissolved in multi-component solvent systems. Even for dipolar nonelectrolyte solutes the composition of the solvation shell can deviate from that of the bulk solvent mixture, as shown for P-disulfones [255] and A-methylthiourea [256]. 

Fig. 2-11. NMR chemical shift of Na as a function of the mole fraction of dimethyl sulfoxide (DMSO) in a binary mixture of DMSO and acetone (according to [295]). Straight line ideal case without preferential solvation, primary solvation shell of the same composition as the bulk solvent mixture. Curved line real case with preferential solvation of Na by DMSO and isosolvation point at xoMso/(cmol mol ) 0.21, that is, the mole fraction of the bulk solvent for which the solvated ion chemical shift is the average of the shifts obtained in the pure solvents (A<5 = <5dmsO - Acetone)-...

There is a subtlety in assigning the value of y, for implicit in our model (but treated more fully in reference 1) is the condition fliat they of eq. [5.5.14] is the surface tension of the cavity surface at its equilibrium composition. But this is the composition of the solvation shell immediately adjacent to the molecule, and this is in general different from the composition (xi, Xj) of the bulk solvent mixture. Let fi and fj be the equilibrium mean fractional concentrations of water and cosolvent, respectively, in the solvation shell, so f, -l- f2 = 1. These fractions are defined... 

Fluorinated surfactants are both hydrophobic and lipophobic. For example, potassium per-fluorooctanesulfonate—an industrially important surfactant —forms a third phase with octanol and water, and it is impossible to determine its octanol-water partition coefficient. " Similar to fluorocarbon-hydrocarbon bulk solvent mixtures, mixed binary systems containing a perfluorocarbon surfactant and a structurally related hydrocarbon surfactant are known to behave nonideally, that is, exhibit phase separation in insoluble monolayers at the air-water interface or form two types of micelles simnltaneously in solution—one type is fluorocarbon-rich and the other is hydrocarbon-rich. This nonideal behavior of fluorocarbon-hydrocarbon surfactant mixtures is used in firefighting foams and powders—an important technical application of fluorinated surfactants. " ... 

The bulk properties of mixed solvents, especially of binary solvent mixtures of water and organic solvents, are often needed. Many dielectric constant measurements have been made on such binary mixtures. The surface tension of aqueous binary mixtures can be quantitatively related to composition. ... 

ODS3 is a "bulk type" reverse phase (the meaning of which will be discussed later) which has a fairly high capacity and is reasonably stable to small changes in pH. The column was 25 cm long, 4.6 mm in diameter and the mobile phase a methanol water mixture containing acetic acid. In this particular separation the solvent mixture was programmed, a development procedure which will also be discussed in a later chapter. 

The silica gel surface is extremely polar and, as a result, must often be deactivated with a polar solvent such as ethyl acetate, propanol or even methanol. The bulk solvent is usually an n-alkane such as n-heptane and the moderators (the name given to the deactivating agents) are usually added at concentrations ranging from 0.5 to 5% v/v. Silica gel is very effective for separating polarizable materials such as the aromatic hydrocarbons, nitro hydrocarbons (aliphatic and aromatic), aliphatic ethers, aromatic esters, etc. When separating polarizable substances as opposed to substances with permanent dipoles, mixtures of an aliphatic hydrocarbon with a chlorinated hydrocarbon such as chlorobutane or methylene dichloride are often used as the mobile... 

This process is highly suitable for rubbers with poor solubility. In this process, the rubber sheet is soaked in TEOS or quite often in TEOS-solvent mixture and the in situ sUica generation is conducted by either acid or base catalysis. The sol-gel reaction is normally carried out at room temperature. Kohjiya et al. [29-31] have reported various nonpolar mbber-silica hybrid nanocomposites based on this technique. The network density of the rubber influences the swelling behavior and hence controls the silica formation. It is very likely that there has been a graded silica concentration from surface to the bulk due to limited swelling of the rubber. This process has been predominantly used to prepare ionomer-inorganic hybrids by Siuzdak et al. [48-50]. 

In this case, a moderately water-soluble amphiphilic N-vinylcaprolaclam (NVC1) played the role of a fl-unit, and a well-water-compatible N-vinyl-imidazole (NVIAz) served as a P-unil. The polymerization was carried out in a medium of 10% aqueous dimethylsulfoxide (DMSO). The addition of DMSO to the reaction solvent was necessary because of insufficient NVC1 solubility in pure water. It was also shown that in this solvent mixture, the NVCl-homopolymers and NVCl/NVIAz-copolymers retained their LCST-behaviour [26,28]. Hence, the DMSO in the reaction solvent did not significantly suppress the hydrophobic interactions of the NVC1 units. The polymerization was initiated by the redox system (N,N,N, N -tetramethylethylenediamine (TMEDA) + ammonium persulphate (APS)) and was carried out at 65 °C (1st step). This condition was very important, since admittedly the temperature was higher than the phase separation threshold of the reaction bulk when the polymeric products were formed that is, under these thermal conditions, hydrophobically-induced folding as the NVCl-blocks appear was ensured. After completion of the reaction, the... 

If 7, is positive, component-1 is selectively adsorbed on the polymer and if polymer is added to the system, the concentration of component-1 must also be increased, if the activity of component-1 is to remain constant. A helpful schematic illustration has been provided by Kratochvfl13I) and is shown in Fig. 39. The example relates to a binary solvent composition of 1 1 for the bulk solvent 1 — solvent 3 mixture. 

Fig. 39. Selective adsorption to a polymer chain from a bulk solvent medium comprising an equimolar mixture of liquid 1 and liquid 3. Only molecules of liquid 1 (circled) in excess of the equimolar ratio in the solvated shell are part of the selective adsorption complex131)...

A considerable volume of literature has accumulated on conductance measurements in mixtures of solvents. Ion mobilities and association constants have been measured over a range of bulk dielectric constants with the aim of correlating bulk solvent properties with mobilities, ion association, and ion size parameters. An example of a widely used solvent mixture is water and 1,4-dioxane, which are miscible over all concentrations, providing a dielectric constant range of 2 to 78. The data obtained in systems containing two or more solvents must be treated with circumspection, as one solvent may interact more strongly with a given species present in solution than the other, and the re-... 

In studies on solvent effects involving variation in the composition of two component mixtures, similar types of outer-sphere interactions yield preferential solvation wherein the solvent composition of the outer-sphere may differ markedly from the bulk solvent composition. Supporting electrolyte species and buffer components may also participate in outer-sphere interactions thereby changing the apparent nature (charge, bulk, lability) of the reacting solvated metal ion or metal complex as perceived by a reacting ligand in the bulk solvent. 

The success of the Potts-Guy equation led many authors to advocate a single mechanism as the rate determining step for permeation through the skin barrier for all or at least a wide range of solutes diffusion was assumed to occur primarily via the interkeratinocyte lipids of the stratum corneum, a mixture of ceramides, fatty acids, and sterols. While from a macroscopic point of view these lipids may be modeled as a bulk solvent, on a microscopic scale they... 

Several other nmr procedures have been used for the determination of fractionation factors. These have advantages in some systems. Instead of determining the effect of the concentration of an exchanging site on the averaged chemical shift, the effect on the averaged relaxation rate of water protons can be used in a very similar way (Silverman, 1981 Kassebaum and Silverman, 1989), For example, addition of the enzyme Co(ii)-carbonic anhydrase to an aqueous solution increases the observed value of XjT because the proton-relaxation rate is the average of that for the bulk solvent (cfl. 0.3 s ) and that for water bound to the cobalt ca. 6x 10 s ). The average is different in an H2O/D2O mixture if the bulk solvent and the Cobound solvent have different deuterium contents, and it has been used to determine a value for the fractionation factor of Co-bound water molecules in the enzyme. 

When the salt concentration is high enough (> 0.1 M) some of the salt will precipitate in proportion as the cosolvent concentration is increased and the temperature of the mixture decreased. While in such conditions the precipitating effect of the cosolvent toward proteins will increase (Edsall, 1947 Singer, 1962) and a new equilibrium of ions between the crystals (/ ") and the bulk solvent medium (/" ) will be established, modifying both the Donnan and electrostatic terms. In the presence of salt (say sodium chloride), the distribution of the hydrogen ions is correlated with that of the chloride ions by the Donnan relationship ... 

To illustrate this a model transesterification reaction catalyzed by subtilisin Carls-berg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure has been studied (Decarvalho et al., 1996). To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents. Water sorption isotherms were measured for the enzyme in each solvent. Catalytic activity as a function of enzyme hydration was measured, and bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents were obtained. The activity maxima were different in all media, being much higher in propane than in either CO2 or the mixtures with 50 and 10% CO2. Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. The results suggest that CO2 has a direct adverse effect on the catalytic activity of subtilisin. 

Amino acid and peptide derivatives, commonly used as substrates in peptide synthesis, form low-melting-point mixtures when combined together in the absence of bulk solvents [66]. 

An additional consideration in the cleaning of XAD resins is their flexible structure. The resin beads swell on contact with hydrophilic or polar solvents such as methanol (29). The nonpolar surface of the resin is repelled by polar solvents and attracted by nonpolar solvents. This effect causes the internal pore diameters to increase or decrease, respectively. The cleaning solvent or mixture of solvents must be polar to keep the internal pores open so that the contaminants will diffuse faster from the interior of the beads to the bulk solvent. However, the resin contaminants themselves are nonpolar, as shown in Table IV, and are not very soluble in polar solvents. The choice of an optimum resincleaning solvent should therefore be a compromise between diffusion and solubility. In addition, the solvent used after the water backwash must be miscible with water to remove the water from the resin pores. 

The bulk solvent strength of binary and ternary supercritical mixtures, measured with the solvatochromic dye Nile Red (E ),... 

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