Thermodynamics and structure in nonelectrolyte solutions

Abstract

Thermodynamic quantities, particularly second-order are indicators of structure both in the pure components and in solution. Three solution structures are treated: (1) Non-randomness or concentration fluctuations due to "antipathy" between the components. This gives rise to a W-shape concentration dependence of the excess heat capacity CpE. Systems containing polyethers and alkanes illustrate the effect of molecular size on the W-shape CpE and its relation with a quantitative measure of nonrandomness calculated from group-solution models, the concentration-concentration correlation function, See.(2) Complexation between alcohol solute OH groups and proton-acceptor (PA), groups e.g. ketone or ester, in PA solvents or PA-inert solvent mixtures. The apparent molar heat capacity of the alcohol at infinite dilution changes with PA group concentration giving the AH and equilibrium constant for the complex.(3) Another "structure" is thought to arise when a highly flexible molecule, e.g. octamethylcyclotetrasiloxane comes into contact with small solvent molecules, e.g. benzene or dioxane. Thermodynamic excess quantities, supported by spectroscopy, suggest that these small molecules can intercalate between methyl groups of the dimethylsiloxane chain increasing the frequency of low-frequency high-amplitude modes of the flexible molecule. This in turn decreases free volume and affects a range of excess quantities.

Temperature and composition dependence of excess Gibbs energy in the system of tetraglyme (1) + decane (2): Evaluation by simultaneous correlation of limiting activity coefficients, liquid–liquid equilibrium, and calorimetric data

“…The results showing that the "W-shape" is enhanced as the temperature approaches the critical solubility temperature were previously published (e.g. [2][3][4][5]8]). This phenomenon has been explained qualitatively by an enhancement of non-randomness and local fluctuations in a liquid solution that is close to the critical solubility point.…”

“…After the regression, the composition of the coexisting phases was calculated from the parameters of Eq. (8). This reverse calculation showed that the maximum deviation between the calculated and experimental composition was 0.008 in the mole fraction, and the average deviation was about 0.003.…”

“…This means that the agreement between the values of partial molar excess enthalpy at an infinite dilution derived from the experiment and calculated from Eq. (8) or (22) is worse. A similar trend was found in the case of the limiting activity coefficient of tetraglyme in decane.…”

“…This phenomenon has been explained qualitatively by an enhancement of non-randomness and local fluctuations in a liquid solution that is close to the critical solubility point. The concentration correlation function S CC defined by Bathia and Thornton [9] was used for its quantitative description [5,8,10,11]. Information on the second derivative of Gibbs mixing energy G M with respect to composition is needed to calculate S CC , and this fact places extreme demands on the quality of the experimental phase equilibrium data.…”

A new approach for prediction of the permittivity of mixtures

“…That expression is symmetric with respect to each component of the mixture and ε e (v i = 1) = ε i . Table 1 shows the comparison of measured equivalent permittivity and values calculated using equation (20) for a liquid mixtures of an apolar solvent, n-nonane, with a polar solute, tetra(ethylene glycol) dimethyl ether (also named tetraglyme) for a complete range of composition and at T = 298.15 K. This mixture presents a consolute point close to T = 288 K. Then one can reasonably expect that above this temperature, the mixture presents local aggregates of its pure components as Andreoli-Ball (10) has suggested from thermodynamic arguments. Experimental results were obtained in our laboratory.…”