Binary solvent mixtures, effect

The effect of salts on the vapor-liquid equilibrium of solvent mixtures has been of considerable interest in recent years. Introduction of a salt into a binary solvent mixture results in a change in the relative volatility of the solvents. This effect can be used to an advantage where the separation of the solvents is of interest. Furter and co-workers have demonstrated the potential importance of salts as separating agents in extractive distillation (J, 2, 3). 

Tables IV-XVI show that the tetraalkylammonium salts have a large effect on both solvents in the binary solvent mixture, especially the larger tetraalkylammonium bromides, i.e., (n-C3H7)4NBr and (n-C4Hg)4NBr. This can be seen from consideration of the boiling temperature alone. This observation is also borne out by the surface tensions and solubilities at 25°C of the individual salts studied, the results of which are tabulated in Table XVII in water, in ethanol, and in an ethanol-water mixture at x = 0.206. For the higher homologs of the R4NBr series, these salts exert a large effect on the surface tensions of the solvent systems studied and show a marked increase in their solubility in ethanol.

In Table XVIII, there are several trends that can be noted in k if one proceeds through the R4NBr series. First of all, k tends to decrease as the size of the TAA cation increases and, in fact, tetra-n-butylammonium bromide shows a large salting-in effect. This trend is emphatically demonstrated by Figure 13, which shows the smoothed salt effects of the various salts studied in the ethanol-water system at x = 0.206. Secondly, it appears that there is a larger salting-out effect as the mole fraction of ethanol increases in the binary solvent mixture. 

Pauls (83) examined the effect of the composition of binary solvent mixtures upon the selectivity and resolution of olive oil TG components. Separation factors (a values) and resolution were calculated for the linoleyldiolein (LOO)-linoleyl-palmitylolein (LPO) and triolein (OOO)-palmityldiolein (POO) pairs in olive oil. Five strong solvents (isopropanol, dichloromethane, chloroform, tetrahydrofuran, and acetone) as well as two weak solvents (methanol and acetonitrile) were employed. 

Kinetic effects were determined by measurements of dissolution and penetration rates. A constant penetration velocity was observed for almost all compositions for both binary solvent mixtures. In all studies, case II transport assumptions provided good agreement with experimental results. For MEK-IPA, penetration rates increased with increasing MEK concentration. For MIBK-methanol, however, a maximum penetration rate was observed at a 60 40 MIBK/methanol ratio. 

In general, to explain the observed cosolvent effects, the preferential adsorption phenomena have been invoked. Flowever few topics in the physical chemistry of polymers have evoked so many theories but so little consensus as preferential adsorption. When a polymer is dissolved in a binary solvent mixture, usually one of the solvents preferentially solvates the polymer. This solvent will then be found in a greater proportion in the proximities of the macromolecule with respect to the bulk solution composition. This variation of the solvent composition can cause interesting phenomena such as cosolvency as was discussed before, [11, 91, 92] non - cosolvency [93, 94], and some times variation of the unperturbed polymer dimensions [95,96]... 

A simple example of modifier-solute selectivity effects is shown in figure 3. The retention of five different functional groups is compared for three different binary solvent mixtures with water. The acetonitrile and tetrahydrofuran concentrations have been choosen so as to provide the same retention for benzene, a k of 4.7. Different solvent modifiers are strongest for different solutes. The shifting of peaks relative to each other is quite pronounced among the different solvents. 

Different methods for the study of selective solvation have been developed [118, 120] conductance and Hittorf transference measurements [119], NMR measurements (especially the effect of solvent composition on the chemical shift of a nucleus in the solute) [106-109], and optical spectra measurements like IR absorption shifts [111] or UV/Vis absorption shifts of solvatochromic dyes in binary solvent mixtures [124, 249, 371]. Recently, the preferential solvation of ionic (tetralkylammonium salts) and neutral solutes (phenol, nitroanilines) has been studied particularly successfully by H NMR spectroscopy through the analysis of the relative intensities of intermolecular H NOESY cross-peaks [372]. 

A binary solvent mixture exhibits synergistic effects on a physicochemical solute property P if for some mixtures this property P has a value higher or lower than either properties Pi and P2 corresponding to the neat solvents Si and S2 [379]. For example, the empirieally determined solvent polarity of binary mixtures of HBD and HBA solvents is often larger than the polarities of the two neat components. Clearly, the formation of hydrogen-bonded 1 1 complexes between HBD ad HBA solvent molecules leads to a new, more polar medium [124, 249, 377] (see Chapter 7 for a definition of the term solvent polarity). 

Small but significant effects of solvent polarity were found in the autoxidation of a variety of alkenes and aralkyl hydrocarbons [216-220] (styrene [216, 218, 219], ethyl methyl ketone [217], cyclohexene [218], cumene [218, 219], tetralin [219], etc.). An extensive study on solvent effects in the azobisisobutyronitrile (AIBN)-initiated oxidation of tetralin in a great variety of solvents and binary solvent mixtures was made by Kamiya et al. [220],... 

HP he study of the behavior of electrolytes in mixed solvents is currently arousing considerable interest because of its practical and fundamental implications (1). Among the simpler binary solvent mixtures, those where water is one component are obviously of primary importance. We have recently compared the effects of small quantities of water on the thermodynamic properties of selected 1 1 electrolytes in sulfolane, acetonitrile, propylene carbonate, and dimethylsulfoxide (DMSO). These four compounds belong to the dipolar aprotic (DPA) class of solvents that has received a great deal of attention (2) because of their wide use as media for physical separations and chemical and electrochemical reactions. We interpreted our vapor pressure, calorimetry, and NMR results in terms of preferential solvation of small cations and anions by water and obtained... 

The mechanism of the opening in acidic medium is not so simple. Although a general reaction scheme can be written, the pathway is influenced to a great extent by the structure of the oxirane, the steric and electronic effects of the substituents, and the solvent also plays an important role. Ever-increasing attention is being paid to the concept of the microscopic structure of solvation in binary solvents. In binary solvent mixtures, the structure of the solvent around the molecule in the solvent shell differs from that of bulk solvent. 

Consequently, the fractionation of heterogeneous polymers by the method of fractional precipitation mostly effects sharp, quantitative separations. Although, as a rule, chemically dissimilar polymers cannot be combined in one solvent since they will mutually precipitate each other, starch affords an example of the few exceptions to this rule, as its aqueous solutions actually contain two chemically dissimilar polymers, namely, amylose and amylopectin. Nevertheless, until 1950, there were no reports of any systematic studies on the effects caused by the gradual addition, to aqueous, starch dispersions, of water-soluble substances possessing non-solvent character for whole starch. Likewise, no work seems to have been done on establishing the relative difference in solubility between amylose and amylopectin in binary solvent mixtures. ... 

Solvent Effects of Binary Solvent Mixtures Molecular... 

Mancini PME, Fortunato G, Adam C, Terenzani A, Vottero LR (2002) Specific tmd nonspecific solvent effects on aromatic nucleophilic substitutions. Kinetics of the reaction of 1-fluoro-2,6-dinitrobenzene and homopiperidine in binary solvent mixtures. J Phys Oig Chem 15 258-269... 

The foundations for almost all quantitative discussions of solvent effects upon electron spin resonance spectra were laid down by the theory of Gendell, Freed and FraenkeF (G.F.F. theory), who assumed that changes in hyperfine splitting constants were solely due to redistribution of a radical s spin density. This redistribution was assumed to accompany the formation of complexes between the radical and the solvent molecules. In formulating a model for the complexes, Gendell et al. restricted their attention to radicals containing polar substituents and postulated that each substituent was able to form a localised complex with one solvent molecule. Thus for a radical RX dissolved in a binary solvent mixture A B, two solvates of the radical are postulated to be in equilibrium with each other according to eqn. 4.16.1... 

General analysis of the binary solvent mixtures formed by two solvate active components (these solvents are often used in analytical and electrochemistry) was conducted to evaluate their effect on H-acids. The analysis was based on an equation which relates the constant of ion association, K, of the solvent mixture and constants of ion association of the acid Kj and K of each component of the mixed solvent, using equilibrium constants of scheme [9.105] - heteromolecular association constant, ionization constant of the... 

From the above and similar investigations [Mo 71, Be 73], it could be seen that, in various binary solvent mixtures containing water, acetonitrile, methanol and dimethyl sulphoxide, the solvating effects of the individual components are generally proportional to the effects displayed by the pure solvents. An exception is the H2O-DMSO mixture, in which the silver ion occurs only in the forms of its pure aquo complex and its pure DMSO complex. 

Sisson, R.M. Giddings, J.C. Effects of solvent composition on polymer retention in thermal field-flow fractionation Retention enhancement in binary solvent mixtures. Anal. Chem. 1994, 66, 4043 053. 

One type of solvent-effect study considers diverse solvents such as water, methanol, acetonitrile, toluene, dimethyl sulfoxide (DMSO), and so on. These studies showed a variation in the alpha effect but, as may have been expected, did not point to a single factor such as solvent polarity as causing the rate enhancement. Interestingly, it was the variation in solvent composition of binary solvent mixtures, such as DMSO-H O and MeCN-HjO, that gave the most valuable clues concerning the origin of this solvent effect of alpha nucleophiles. 

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