Association methanol-water

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

MW) is the molar concentration of methanol/water associate, and (k) is the "association" constant... 

R v) is the molar refractivity of water, and (Rmw) is the molar refractivity of the methanol/water associate. 

Taking known values for the molar refractivities of water and methanol, and again assuming a range of values for the equilibrium constant (k) and the refractive index (ni) of the methanol/water associate, the actual values that fit the equation for these... 

It is seen that the three values for the equilibrium constant (k) range from 0.00443 to 0.00565 with an average value of 0.00504. The two values for the densities of the methanol/water associate are in reasonable agreement and have a magnitude that would be expected for the hydrogen bonded associate. 

Using the average value for the equilibrium constant, the distribution concentration of the different components of a methanol water mixture were calculated for initial methanol concentrations ranging from zero to 100%v/v. The curves they obtained are shown in Figure 28. The molar refractivities of 11.88 is also in accordance with that expected since the molar refractivity s of water and methanol are 3.72 and 8.28 respectively. The refractive index of the associate of 1.3502 is, as would be expected, higher than that of either water or methanol. 

Katz, Lochmuller and Scott also examined acetonitrile/water, and tetrahydrofuran (THF)/water mixtures in the same way and showed that there was significant association between the water and both solvents but not nearly to the same extent as methanol/water. At the point of maximum association for methanol, the solvent mixture contained nearly 60% of the methanol/water associate. In contrast the maximum amount of THF associate that was formed amounted to only about 17%, and for acetonitrile the maximum amount of associate that was formed was as little as 8%. It follows that acetonitrile/water mixtures would be expected to behave more nearly as binary mixtures than methanol/water or THF/water mixtures. 

Figure 30. Curves Relating Volume Fractions of Methanol, Water and Methanol/Water Associate to Volume Fraction of Methanol before Mixing for Three Different Temperatures...

Considering the hexadecane/water-methanol system the same arguments and treatment can be afforded to the methanol/water mixture on the assumption that it is a ternary mixture containing methanol, water and methanol associated with water. Thus, the equation used for the system of Katz et al. reduces to... 

Kmw) is the distribution coefficient of the solute between methanol associated with water and n-hexadecane... 

The association of methanol and water was examined by Katz, Lochmiiller and Scott (11) using volume change on mixing and refractive index data and established that the methanol/water solvent system was indeed a complex ternary system. 

Atkinson, G. Kor, S. K. (1965). The kinetics of ion association in manganese sulphate solutions. I. Results in water, dioxane-water mixtures, and methanol-water mixtures at 25 °C. Journal of Physical Chemistry, 69, 128-33. 

Huber and associates have refined the structure of trypsinogen in methanol-water, inidally at 213 K (Singh et al., 1980) and subsequently... 

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). 

The effectiveness with which the solvent promotes migration decreases in the order methanol > ethanol water. The extremely low stability of complexes in water can be explained by the relatively great tendency of metal ions to associate with water molecules.70 The difference between the rate in ethanol and that in methanol can be attributed, at least partly, to the fact that salts are more highly dissociated into free ions in methanol71 a higher concentration of free cations would permit a higher concentration of positively charged carbohydrate species. 

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