Solvents composition of mixed

Fig. 55. Dependence of square root of excess LS for solutions of (a) polyethyleneglycol methacrylate and (b) polymethyl methacrylate in mixed solvents comprising 2,2,3,3-tetrafIuoropro-panol (volume fraction ) and benzyl alcohol. (T = 25 °C, 0 = 546 nm). Broken lines indicate composition of mixed solvent yielding no excess scattering from the polymer in each case1 S2 ...

Part A of the scheme represents the initial state. Two parts of the system are separated by semi-permeable membrane. Polymer (represented by ) is surrounded by molecules of monomer, M, and molecules of solvent, S. Composition of mixed solvent (solvent and monomer) is initially uniform. If the interaction between monomer and polymer is stronger than that between polymer and solvent, the diffusion through the membrane takes place. Monomer molecules are associated with macromolecules while molecules of solvent are displaced to the right part of the vessel. 

The compositions of mixed solvents are expressed by mole fraction (x), mass fraction (w) or volume fraction (0). If a mixed solvent consists of nA moles of solvent A and nB moles of solvent B, xA, wA and 0A for component A can be expressed by xA=nA/ nA+nB), wA=nA/(nA+nBMB/MA) and 0A = nA/(nA+ BVB/VA). Here, M is the molecular weight of component i and V the molar volume of i before mixing. It is assumed that the volume of the mixed solvent is equal to the sum of the volumes of the two constituent solvents, although some volume contraction may occur by mixing. 

The composition of mixed solvent (free of salt) was calculated from the amounts of methanol and water before mixing. The reproducibility of the preparation of the mixed solvent was within 0.001 mole fraction over the whole composition range. 

In specifying the composition of mixed solvents, the following abbreviations are used ... 

Fig. 16. Dependence of the intrinsic viscosity on the composition of mixed solvent (A-B) for PAA-methanol + KI (0.01 N) + DMSO (1), PAA-PVP-methanoI + DMSO (2), PMAA-DMF + DMSO (3), PMAA-PVP-DMF + DMSO...

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. 

Volume fraction is frequently used to define the composition of mixed solvent systems, or to express the solubility of one solvent in another. However, since the volumes of solutions exhibit a dependence on temperature, the expression of concentrations in terms of volume fraction requires a simultaneous specification of the temperature. In addition, since volume defects may occur during the mixing of the solvents, and since these will alter the final obtained volume, defining the solubility of a solution in terms of volume fraction can lead to inaccuracies that can be avoided through the use of other concentration parameters. 

Braude ei al. studied the dependence of rates of acid-catalyzed isomerization of of-ethynyl-y-methylallyl alcohoF and a-phenyl-y-methylallyl alcohol on the composition of mixed solvents such as aqueous ethanol and aqueous dioxane. If allowance is made for the effect of solvent on the concentration of the reactive conjugate acid of the alcohol, these data show that the rate of isomerization of the conjugate acid is nearly independent of solvent composi-tion as would be expected for a unimolecular rate-limiting dissociation of the conjugate acid to a solvated carbonium ion, Kwart and Herbig reached a... 

The tra j -labilising effect of sulphite ligands is illustrated by the rates and activation parameters for aquation of the complexes [CoX(S03)(DMG)] and [CoX(S03H)(DMG)]. Variation of rates with composition of mixed solvents indicates an associative mechanism for these aquations. ... 

Concentration dependencies of permittivity, viscosity, density (molar volume) and conductivity described here permit to select with certainty the composition of mixed solvent, characterized by any value of mentioned properties. 

Figure 9.10. Selective solvation of Nal in mixed solvent cyanomethane-methanol (a) and DMFA-methanol (b, curvel), DMFA - cyanomethane (b, curve 2) Xg - composition of mixed solvent Xg - composition of solvate shell in molar parts of the second component.

Equation [9.9 Ic] developed for the ideal solution E-A-B permits us to establish the relationship between the composition of mixed solvent Xg and the solvate shell composition x. For the special case of equimolar solvates, the expression of resolvation constant is written in the form ... 

Strengthening or weakening interaction (ion-dipole interaction or dipole-dipole interaction) of universal solvation leads to re-distribution of molecules in the mixed solvate and to the change of the composition of solvate shell in contrast to the composition of mixed solvent. 

Specific rate constants for the hydrolysis of D-mannono-1,4-lactone have been determined, " and the rate of acid-catalysed hydrolysis of the 4,6-dideoxy-L-hexono-1,5-lactones at 35°C in water, aqueous p-dioxan, and aqueous acetone was shown to decrease along the series xylo, lyxo, ribo, and arabino. The effects of the composition of mixed-solvent systems on the rate of hydrolysis were also investigated. 

For each polymer, theta solvents are given in alphabetical order no attempt was made to convert commonly used solvent names into systematic names (e.g. dioxane = 1,4-dioxane). In mixed solvents, the liquid Hsted first may thus be a solvent or a nonsolvent. Compositions of mixed solvents are given in vol./vol. unless otherwise noted (ex w/w indicates weight per weight). 

---