THERMODYNAMICS AND STRUCTURE IN NONELECTROLYTE SOLUTIONS

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 non-randomness calculated from group-solution models, the concentration-concentration correlation function, Scc. (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. © 1990 De Gruyter