Solvation in mixtures

E. Matteoli, A study on Kirkwood-Buff integrals and preferential solvation in mixtures with small deviations from ideahty and/or with size mismatch of components. Importance of a proper reference system, J. Phys. Chem., B lOI (1997) 9800-9810. 

Relatively little is known about competitive solvation in mixtures of nonaqueous solvents. Complex formation between Na+ and THF in solutions of Na+[AlBu4] in hexane at molar ratios 1 1 (solvated contact ion pair) and 1 4 (solvent-separated ion pair) was reported by Schaschel and Day with proton NMR as well as IR and conductivity measurements (83, 84). Preferential solvation of alkali metal ions by DMSO in 1-pentanole and by acetone in nitromethane was observed by Popov et al. 69, 85). 

Tetrahydrofuran and cosolvents. A detailed study of the preferential solvation in mixtures of tetrahydrofuran. A, and cosolvents, B, was reported by Marcus (2006a). For B = n-hexane and c-hexane at 303 K, n-heptane and /-octane at 298 K, the selfassociation of tetrahydrofuran is preferred bx ACmax) = 0.025, 0.030, 0.025, and 0.034, respectively. The mutual interaction is small the extrema in 8xab are -0.010, 0, 0.004, and 0.004, that is, hardly significant. For B = benzene, toluene, ethylbenzene, dichloromethane, chloroform, tetrachloromethane, and 1-chlorobutane at 303 K, there is essentially no preferential solvation, both 8xaa and 8xab being less than 0.01. The smallness of the preferential solvation parameters for mixtures of tetrahydrofuran and tetrachloromethane has already been noted by Ben-Naim (1990b)... 

The standard molar Gibbs energies of transfer of an ion from a source solvent (arbitrarily selected as water, W) to a target solvent, A, G (I, W S), described in Section 4.3.2.1, is the basis for a rough estimation of its preferential solvation in mixtures of these two components. However, the mutual interactions of the two... 

When the solvent is a good solvater, the determination of the solvation number b is difficult, unless the dependence of the extractant concentration on the solvent can be obtained. Solvation numbers can be obtained in mixtures of a solvating extractant and an inert diluent like hexane. Further, in these systems the extraction of the metal commonly requires high concentrations of salt or acid in the aqneons phase, so the activity coefficients of the solutes must be taken into acconnt. 

Solvatochromism and piezochromism of a range of pentacyanoferrates(II) have been examined in binary aq ueous solvent mixtures, " and their solvatochromism in micelles and reversed micelles. The solvatochromism of [Fe(CN)5(nicotinamide)] has been established in several ranges of water-rich binary solvent mixtures, " of [Fe (CN)5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.The solvatochromism of [Fe(CN)5(4Phpy)] and [Fe(CN)5(4Bu py)] has been proposed as an indicator of selective solvation in binary aqueous solvent mixtures. ... 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

Hexamethylphosphoramide, which is a liquid under ambient conditions, is able to solvate electrons. Mixtures of this solvent with up to 21 % ethanol are effective for the electrochemical Birch type reactions. The strong hydrogen bonding between the two solvents suppresses hydrogen evolution at die cathode [42], Benzene is reduced at constant current in this solvent to a mixture of hydrocarbons, cyclohexane being formed early in the process [43,44],... 

Recent NMR study suggested that PhLi exists as a mixture of tetramer and dimer in diethyl ether and that the addition of a stoichiometric amount of a coordinating solvent such as THF, dioxolane, DME or TMEDA induces complete conversion of the mixture to dimeric solvates. In a THF solution PhLi exists as a mixture of dimer and monomer. It was noted that the addition of 12-crown or HMPA increases the reactivity and decreases the selectivity of PhLi in THF. 

Calculations at the 6-3IG level indicate that in the gas phase, 2//-l,2,3-triazole is more stable than 1//-1,2,3-triazole by about 4.5 kcal moC. In solution, the IH isomer becomes the more stable species because the large difference in dipole moments favors the more polar tautomer. The triazolium ion (75) is predicted to be more stable than (76) by about 13.5 kcal mol <89Mi40i-0i>. 2//-1,2,3-Triazole represents more than 99.9% of the equilibrium mixture in the gas phase. However, the ab initio calculated proton affinity of 1//-benzotriazole is 10.2 kcal mol larger than that of 2//-benzotriazole, which is consistent with ICR measurements (1-methylbenzotriazole is 10 kcal mol more basic than 2-methylbenzotriazole). Measurements of enthalpies of solution, vaporization, sublimation and solvation in water, methanol and DMSO confirm the predominance of the IH tautomer in solution <89JA7348>. The energy difference between the tautomers of 1,2,3-triazole has also been estimated at the 6-31G (MP2)//3-21G level including zero-point effects. The... 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

The electron will be solvated in a region where the solvent molecules are appropriately arranged. There must be a cluster of electrons of a size of 4-5 to support the formation of the solvated electron from the results of Gangwer et al., [23], Baxendale [24,25], and Kenney-Wallace and Jonah [16]. This behavior does not depend on the specific alcohol or alkane and even occurs in supercritical solutions, as has been shown in experiments done using mixtures of supercritical ethane-methanol mixtures [19]. Experiments have also shown that the thermodynamically lowest state might not be reached. For example, the experiments of Baxendale that measured the conductivity of the solvated electron in alcohol-alkane mixtures showed that when there was a sufficient concentration of alcohols to form dimers, there was a sharp decrease in the mobility of the electron [24,25]. This result showed that the electron was at least partially solvated. However, the conductivity was not as low as one would expect for the fully solvated electron, and the fully solvated electron was never formed on their time scale (many microseconds), a time scale that was sufficiently long for the electron-alcohol entity to encounter sufficient alcohols to fully solvate the electron. Similarly, the experiments of Weinstein and Firestone, in mixed polar solvents, showed that the electron that was observed depended on the initial mixture and would not relax to form the most fully solvated electron [26]. 

Reaction of K[4-MePh-form] with [Au2(2,6-Me2Ph-form)2] in 1 1 molar ratio in THF leads to the crystallization of [Au2(2,6-Me2Ph-form)2][Au4(4-MePh-form)4]-2THF, which contains a mixture of dinuclear and tetranuclear derivatives. Compound [Au2(2,6-Me2-form)2]-2[Hg(CN)2]2-2THF (Figure 2.54) is obtained by reaction of [Au2(2,6-Me2-form)2] and [Hg(CN)2], It crystallizes as a tetrahydrofuran solvate in a... 

Attempts have been made to treat selective ion solvation in mixed solvents as ligand exchange reactions [36 e]. We express ion X (cation or anion) existing in solvent A by XA and in solvent B by XB,-. Here, nf=ri if the molecular size of A is very different from that of B or if A is unidentate and B is bidentate (e.g. PC and DME in the solvation of Li+). Otherwise, it is usual that n=n. Then, the ligand exchange reaction in the mixture of A and B will proceed as follows with the increase in the concentration of B ... 

Grunwald, Baughman and Kohnstam (GBK), in an appendix to a classic paper on vapor pressure studies of solvation in dioxane and water mixtures (7), presented an outline of a thermodynamic treatment of the solvation of ions in a mixed solvent. It is convenient to start from their general treatment but to adopt a different nomenclature, used previously (3). 

Since anions are much less solvated in dipolar aprotic solvents (23) than in water, the hydrogen ion will be more highly solvated in the mixed solvent because it is preferentially solvated by monoglyme in the monoglyme-water mixtures rather than in the pure aqueous medium. The selective solvation is an important factor in an understanding of solute-solvent interactions in mixed solvent systems. Unfortunately, the detailed compositions of the primary solvation shell and the secondary mode of solvation (ion-dipole interaction) in mixed solvents are not yet clearly understood. 

Time-resolved fluorescence from sub-picosecond to the nanosecond time-scale of dye molecules like coumarins has been widely used to study solvation dynamics in liquids [1], As the dye is photoexcited, its dipole moment abruptly changes. Then by monitoring the time-dependent fluorescence energy one can have access to the solvent dynamical response to the charge reorganization in the dye. The microscopic interpretation of these experiments has greatly benefited from Molecular Dynamics (MD) studies [2], Recently, few experimental [3-5] and theoretical [6,7] works have been performed on solvation dynamics in liquid mixtures. A number of new phenomena can arise in mixtures which are not present in pure solvents, like association, mutual diffusion and preferential solvation [6]. We present here a... 

In this work we presented the results of Molecular Dynamics simulations performed to study the solvatochromism and the dynamic stokes-shift of coumarin 153 in mixtures of solvents. We showed the ability of MD to reproduce available data of the time-dependent Stokes-shifts. Moreover, MD allowed us to interpret these dynamics in benzene-acetonitrile mixtures in terms of motions of benzene around the coumarin or rotation of acetonitrile. The role of benzene in the solvation process of Cl53 seems to be more important than usually assumed. 

---