Mixed solvents, composition

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

It was concluded that the change in the kinetics of the Cd(II)/Cd(Hg) system with mixed solvent composition may be described by different model equations in water-DMSO [61], and water-HMPA solvents [62]. 

From thermodynamic considerations and after a sequence of simplifying assumptions has been applied, including those of constant temperature and pressure, an equation for the salt effect in vapor-liquid equilibria under conditions of constant mixed-solvent composition has been derived (22,23). The equation, in its simplest form, reduces to... 

This equation relates a so-called improvement factor, the logarithm of the ratio of relative volatility with and without salt present, to the salt concentration in the liquid phase under the condition of fixed mixed-solvent composition, by a salt effect parameter k. Usually, the added salt lowers the volatility of both components in the liquid phase. If the extent of this lowering is different for... 

We are concerned here with the solvent effects on the equilibrium behavior of acids and bases in dipolar aprotic solvents, such as monoglyme in x = 10, 30, and 50 mass percent mixed solvent compositions. The following reaction is of particular interest in such a study ... 

Thus hydrobromic acid is more strongly stabilized in monoglyme + water mixtures than in water. Moreover, the negative values of AG for the experimental mixed solvent compositions support the view that water is less basic than the mixed solvents, if it is assumed that the hydration of a larger bromide ion in aqueous solution is negligible, although our data indicates that the hydration number of chloride in aqueous solution might not be zero (22). 

Fig. 1.15 Dependence of the preferential adsorption parameter X on mixed solvent composition for the ternary system. (.) PVP-2-propanol-cumene and (-) ...

Isobaric vapor-liquid equilibrium data at atmospheric pressure are reported for the four systems of the present investigation in Tables I-VI. Salt concentrations are reported as mole fraction salt in the solution, while mixed-solvent compositions are given on a salt-free basis. A single fixed-liquid composition was used for potassium iodide and sodium acetate potassium acetate used three—all chosen from the region of ethanol-water composition where relative volatility is highest. In the... 

Although Johnson and Furter (1,2), among others, observed a surprising insensitivity of k to mixed-solvent composition in many alcohol-water-inorganic salt systems, such does not appear to be the case with ammonium bromide-ethanol-water. A linear dependence of k with x was observed and is demonstrated in Figure 4. The slope of this dependence is 2.63 and the intercept with the y-axis occurs at approximately a value of unity. This extrapolated salt effect when x = 0, that is, with water as the single solvent, is consistent with Raoult s Law in that the vapor pressure of the aqueous salt solution depends directly on the salt concentration. However the same behavior has not been observed for the ammonium chloride-ethanol-water system (3) as seen in Table VIII its salt effect parameter shows essentially no dependence on the liquid composition. Therefore the two systems differ in this respect. 

The above presentation shows that the detailed analysis of the dependence of AGfr on the mixed solvent composition and on the stability and concentration of various solvated reactant species may be quite complicated, especially in the case of labile ions. The free energy of ions is dependent not only on their interaction with the solvent and with other components of the mixture [252] (for instance, ions of background electrolyte), but also on the change in solution structure [252] and on the change in hydrogen bond formation [253]. 

The change in the Lewis acidity parameter, Ej with the mixed solvent composition is either monotonie or exhibits a maximum with an value higher than those found for the pure solvents [266]. 

The Kamlet-Thft [84] parameter of the Lewis basieity (see Sec. 2.3) was also determined for 14 mixtures of solvents with methanol [267] and 12 mixtures of organic solvents with water [268]. The change of / kt with the mixed solvent composition is determined by changes occurring in the structure of methanol and water after addition of the second solvent. However, the use of these parameters for a systematic analysis of electrochemical potentials is rather limited. 

The authors [224] explained these kinetic changes qualitatively by considering the energy of activation at different mole fractions of the organic component in the mixture. The minimum on the rate constant (exchange current) - mixed solvent composition dependence occurs at the largest difference in composition between the surface layer and the first solvation sphere of zinc(II). 

Fig. 12. Dependence of the logarithm of the standard rate constant on mixed solvent composition.

The main cause of the decrease of the anodic rate constant, observed when the second solvent is added to water, consists in the preferential adsorption of the organic solvent molecules (which play the role of an inhibitor) on the electrode surface. When there is no such adsorption (at very negative potentials) the cathodic reaction of manganese(II) is independent of the mixed solvent composition. 

Fig. 16. Variation of the standard rate constant of the V(III)/V(II) system (upper panel) and the absorption spectra of V(III) (lower panel) with mixed solvent composition in terms the surface coverage of the electrode by the organic component. Solvent system curve 1, H2O-DMF curve 2, HjO-DMPU curve 3, HjO-HMPA curve 4, H2O-AN.

The change from inhibition to acceleration of the rate of electrode reaction occurs exactly at the mixed solvent composition at which the process of reactant resolvation begins in the bulk of the solution. It appears that even partial resolvation of vanadium(III) ions (Avv(in)>0) initiates the increase of the rate of reaction in the surface phase, when the concentration of the organic solvent is considerably higher there than in the bulk. Such behavior is observed in mixtures of water with solvents of Lewis basicity lower (AN) and also higher (HMPA) than water. 

This result may indicate that the ionic depolarizer reacts with two molecules (trimers) of water from the surface phase. This conclusion may not be strictly valid since the shape of the k versus mixed solvent composition dependence, calculated for the Eu(III)/Eu(II) system by means of Eq. (73), is only very slightly dependent on the b, p and q parameters. 

The application of Eq. (75) to the analysis of the Zn(II)/Zn(Hg) electrode reaction in several mixed solvents, shown in Fig. 18, illustrates a good way of determining the mixed solvent composition range, where the organic solvent acts as an inhibitor of the electrode reaction (see Eq. (68)). 

A practicaly linear dependence between and AGtr for lead(II) was observed in mixtures of water with DMSO [225]. A linear dependence between logAr and AGtr was also found for the Zn(II)/Zn(Hg) and Li(I)/Li(Hg) systems in mixtures of PC with DMSO [216], and the electroreduction of Mn(II) in H2O-DMF mixtures [228]. These linear dependences may be explained by the occurrence of the studied reactions at quite negative potentials, where the organic component of the mixture should be desorbed and the composition of the surface phase may be similar to the bulk composition over a large range of mixed solvent compositions. 

The fact that is a linear function of AG,r for some of the mixed solvent composition range may suggest that the adsorption energy of the activated complex, AGa(js> is also a linear function of AG of the reactant. 

Recently Jaworski and coworkers [303] have analyzed mixed solvent influence on surface intramolecular reactions. The authors tried to explain the change in the rate constant with the H2O-DMF mixed solvent composition in terms of the change in the estimated longitudinal relaxation time. A linear correlation was found between k for various H2O-DMF compositions and Tl . 

The solubility of drugs in aqueous mixed solvents often exhibits a maximum in the curve solubility versus mixed solvent composition. This enhancement in solubility often greatly exceeds the solubilities not only in water, which is quite natural, but also in nonaqueous cosolvents. Such a dependence could not be explained by simple equations like the log-linear model for the solubility in a mixed solvent (Yalkowsky and Roseman, 1981)... 

However, Eq. (23) cannot describe the maximum in the curve of solubility versus mixed solvent composition which was frequently observed experimentally for the solubilities of drugs in aqueous mixed... 

The solubility of drugs in aqueous mixed solvents often exhibits a maximum as a function of the mixed solvent composition. The higher solubility of a solid solute in a mixed solvent than in either of the pure solvents, was frequently observed (Acree, 1984 Prausnitz et ah, 1986) and is not an exception as it seemed several decades ago. 

Two limitations are involved in the derivation of the above equation (1) the compositions of mixed solvents (points c and d) should be close enough to each other for the trapezoidal mle used to integrate the Gibbs-Duhem equation to be valid, (2) the solubility of the solid should be low enough for the activity coefficients of the solvent and cosolvent to be taken equal to those in a solute-free binary solvent mixture. In addition, the fugacity of the solid phase in Eq. (4) should remain the same for all mixed solvent compositions considered. 

Using the above equations, the solubility of the solute was calculated for small changes in the mixed solvent composition (2.5 mol%) ... 

The solubility of sulphamethoxypyridazine in ethanol-water mixtures represents a rare kind of drug solubility in an aqueous mixed solvent, because it exhibits two solubility maxima on the curve solubility versus mixed solvent composition (Escalera et al., 1994). It is of interest to verify if such behavior satisfies the thermodynamic consistency criterion. The values of D were calculated using Eq. (11), and... 

It should he pointed out that Equations 9 and 10 cannot represent the composition dependence of the soluhilities when the deviation from ideality is large. For example, they cannot provide a maximum in the soluhility versus mixed solvent composition. However, such cases are frequently encountered (see the examples listed in refs 14, 22, 23, 27). To represent the large deviations from ideal behavior, such as a maximum, on the solubility curve, the ideal molar volume of the mixed solvent (Equation 6) will be replaced in Equation 9 by... 

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