Water-dioxane binary mixture

Dioxane is a cyclic diether forming a six-membered ring [20]. Thus it is a nearly nonpolar symmetric molecule. 1,4-Dioxane is an extraordinary solvent, capable of solubilizing most organic compounds, and water in all proportions, and many inorganic compounds. The self-diffusion coefficient of dioxane is 1.1 x 10 cm /s, about half that of a water molecule. The effective diameter of dioxane is 5.5 A - about twice that of a water molecule. One should not forget that a water-dioxane mixture narrowly avoids a lower critical consolute point. However, the effects of criticality are reflected in the values of ffie mutual diffusion coefficient and viscosity. Note that binary mixtures are often chosen so that they are mixable (do not phase separate). Thus, the two components interact attractively and strongly. 

Along with the above stmctural view the low-frequency Raman spectra of dioxane aqueous solutions have been analyzed from the dynamic aspect of the water stmcture. 

The reduced Raman spectra of dioxane aqueous solutions are well distinguished by a superposition of three characteristic modes of water and one Gaussian mode of dioxane. The concentration dependence of the spectra indicates that the mode of water disappears below about 0.8 molar fraction of water molecules in fact this molar fraction corresponds to a ratio of four water molecules to one dioxane. This result suggests that the well-defined tetrahedral stmcture consisting of five water molecules is largely disrupted by the penetration of dioxane molecules above a 0.2 mole fraction of dioxane, according to the Raman spectral experiments [21]. 

There are plenty of examples where different dynamics and thermodynamic properties show remarkable anomalies in such an amphiphilic binary mixture. Nevertheless it is very difficult to gather them all together in one chapter in a book like this. However, for a water-dioxane mixture we discuss one such property to point out the anomalous region that can be further followed by the interested reader. 

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Casassas, E. Fonrodona, G. de Juan, A., Solvatochromic parameters for binary mixtures and a correlation with equilibrium constants. 1. Dioxane-water mixtures, J. Solut. Chem. 

In binary mixtures of MeOH, AN and NM having an almost constant Er, /-AmaBuN salts are dissociated but the variation of the Walden product with solvent composition suggests specific ion-solvent interactions. D Aprano and Fuoss observed specific effects on the conductance of the Pic", Br" and BPh of Bu4N in isodielectric mixtures of dioxan with water, MeOH, AN and / -nitroaniline (PNA). and the... 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the... 

Examples of important amphiphilic binary mixtures inelude water and dimethyl sulfoxide (DMSO), water-methanol, water-ethanol, water-tertiary butyl aleohol (TEA), water and glycerol, water and dioxane, to name a few. These amphiphilic... 

Interestingly, mostly those aqueous binary mixtures that contain a marked amphiphilic character towards water are found to be particularly important in chemistry and biology. This is clearly manifest in all useful aqueous binary mixtures such as DMSO, methanol, ethanol, TBA, acetone, and dioxane to name a few. Some of these solvents (DMSO, EtOH) are used at high concentration as effective denatur-ants of protein. At low concentration they can exhibit a reversal of role and serve as a promoter of stability. In this low-concentration regime the binary mixtures also promote the catalytic activity of enzymes, as discussed above. Recent experimental, theoretical, and simulation results exhibit the phase-transihon-hke scenarios discussed in this chapter. These systems need to be studied in great detail as much remains to be understood. 

FIGURE 3.5 Volume-corrected preferential solvation parameters 8 ( ) for water-water interactions and 8 ws ( ) for water-solute interactions in the first solvation shell of aqueous mixtures with solutes identified in the label of the abscissa (THF = tetrahydrofuran, Pip = piperidine, Py = pyridine). Empty symbols for tetrahydrofuran and dioxane pertain to second solvation shell, for pyridine to data from a second source. (From Y. Marcus, 2001, Preferential Solvation in Mixed Solvents, Part 10, Completely Miscible Aqueous Co-Solvent Binary Mixtures at 298.15 K, Monatshefte fur Chemie, 132, 1387, by permission of the publisher, Springer.)... 

FIG U RE 10.4 Comparison between experimental (o) and calculated (solid lines) solubilities of phenacetin (S is the mole fraction of phenacetin) in the mixed solvent water/dioxane is the mole fraction of dioxane) at room temperature. The solubility was calculated using Equation 10.29. 1-activity coefficients expressed via the Flory-Huggins equation, 2-activity coefficients expressed via the Wilson equation. (From C. Bustamante, and P. Bustamante, 1996, Nonlinear Enthalpy-Entropy Compensation for the Solubility of Phenacetin in Dioxane-Water Solvent Mixtures, Journal of Pharmaceutical Sciences, 85, 1109. Reprinted from E. Ruckenstein, and I. L. Shulgin, 2003c, Solubility of Drugs in Aqueous Solutions. Part 2 Binary Nonideal Mixed Solvent, International Journal of Pharmaceutics, 260, 283, With permission from Elsevier.)... 

Polyurethane (PU) PV membranes were reported by Schauer et al. (1999). PU membranes were prepared by the reaction of toluene-2,4-diisocyanate with hydroxyl-terminated oligomers. Oligomers were either liquid polybutadiene (PB) (MW 3000) or propylene oxide-based PEs (MW 420 and 4800). The prepared membranes were used in PV of binary mixtures of water-EtOH, water-dioxane, and EtOH-toluene, respectively. Membranes obtained from the polymer quatemized poly[3-(N, N -dimethyl)aminopropylamide-co-acrylonitriles] showed selective separation of water from aqueous EtOH solution by PV (Yoshikawa et al. 1991). The separation factor toward water reached over 15,000. Membrane performance showed a good correlation to membrane polarity. DSC melting endotherms of the water-swoUen membranes were studied to clarify the state of water in the membrane. The resnlts suggested that there were two states of water in the membrane bound and free. The higher the fraction of bound water in the membrane, clearly, the more preferentially was water permeated. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

In those binary solvents in which selective sorption of the more polar component on the polymer chain may take place, an increased polarity of microenvironment could be observed, compared with the polarity of the binary mixture used. In aU the binary solvents studied here (alcohol/, dioxane/, and acetone/water) a qualitative eement was found between the polarity of the microenvironment of PHEMA in solution determined from the energy of the CT absorption band of pyridinium-betaine (SB) and values obtained from the energy of the emission transition of PHEMA-bound DNS fluorophore (Figure 11). 

The study of solute-solvent and solvent-solvent interactions in mixed solvents has been gaining significance in recent years61-64, because of the increasing application of these solvents. Casassas and collaborators67 have used the Kamlet-Taft multiparametric equation for the correlation of dissociation constants of acids in 1, 4-dioxane-water mixtures. They found that when the main solvent is retained the property does not involve significant changes in the cavity volumes and, in those cases, the pK in binary solvents can be described by equation 8 ... 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

Many papers concerning salt effect on vapor-liquid equilibrium have been published. The systems formed by alcohol-water mixtures saturated with various salts have been the most widely studied, with those based on the ethyl alcohol-water binary being of special interest (1-6,8,10,11). However, other alcohol mixtures have also been studied methanol (10,16,17,20,21,22), 1-propanol (10,12,23,24), 2-propanol (12,23,25,26), butanol (27), phenol (28), and ethylene glycol (29,30). Other binary solvents studied have included acetic acid-water (22), propionic acid-water (31), nitric acid-water (32), acetone-methanol (33), ethanol-benzene (27), pyridine-water (25), and dioxane-water (26). 

Fig. 5-13. Correlation between Ig A i [40] and the Kirkwood function (cr — l)/(2 r + 1) for the solvolysis of 2-chloro-2-methylpropane in binary 1,4-dioxane/water (A), ethanol/water (B), acetone/water (C), and methanol/water (D) mixtures at 25 °C cf. Eq. (5-13) in Section 5.3.1.

Figure 5. Solvent composition dependence of the polarity (E> of the binary solvents dioxane/water (1) and acetone/water (2), and the polarity of the PHEMA microenvironment in these mixtures (3), (4). For a definition of the symbols E-p and u see Figure 3.

The electrical conductivity of mixtures of electrolytes and non-electrolytes are scarce. The only systems studied above 200 °C are KCl - Ar (Hartmann and Franck, 1969), NaCl-dioxane (Yeatts and Marshall, 1972) and Nal - methanol (Korobkov and Mikhilev, 1970). In these cases the system can be considered as a binary electrolyte solution in a mixed solvent. Therefore, the specific and molar conductivity decreases with the increase of the non-electrolyte in water due to the reduction of the dielectric constant of the mixed solvent. 

The separation of solvent effects on reactivities into constituent initial-state and transition-state effects by the use of appropriate kinetic and thermodynamic data has been successfully carried out for several organic reactions. Thus, for example, the solvolysis of t-butyl chloride and the Menschutkin reaction were treated in this manner some time ago a recent organic example is afforded by the solvolysis of isopropyl bromide in aqueous ethanol. For inorganic reactions, this approach was early used for reactions of tetra-alkyltin(iv) compounds with mercury(ii) halides. A more recent analysis of reactions of low-spin iron(n) complexes with hydroxide and with cyanide in binary aqueous mixtures was complicated by the need to make assumptions about single-ion values in such ion+ion reactions. Recent estimates of thermodynamic parameters for solvation of complexes of the [Fe(phen)3] + type are helpful in this connection. However, it is more satisfactory to work with uncharged reactants when trying to undertake this type of analysis of reactivity trends. A suitable system is provided by the reaction of [PtClaCbipy)] with thiourea. In dioxan-and tetrahydrofuran-water solvent mixtures, reactivity is controlled almost entirely... 

The cases where in a binary aqueous solutions water molecules fmm the solvate shell arc more often. This is observed for Li and Na in H2O + H2O2 mixtures, for Li, Na, Cs n, Ca, Mn, and Co in H2O + CH3OH mixtures, and for Na, Cs, Ca, Co, and La in H20-dioxane mixtures. 3. Sometimes the solvate shell is formed by the second component (not H2O). This is precisely the second component... 

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