Excess molar heat capacities, Cz , throughout the entire concentration range have been determined at 25 "C for the following oxaalkanealkane systems : 2,5,8,11,14-pentaoxapentadecane (tetraglyme) with decane, nonane, octane and hexane ; 2,5,8,11-tetraoxadodecane (triglyme) with hexadecane, pentadecane, tetradecane, dodecane, decane and hexane; 2,5,8-trioxanonane (diglyme) with 2,6,10,15,19,23-hexamethyltetracosane (squalane), 2,6,10,14-tetramethylpentadecane (pristane), hexadecane and decane ; 23-dioxahexane (monoglyme) with squalane, pristane, decane and heptane ; and 1,4-dioxacycIohexane @-dioxane) with cyclohexane. For all these tures CE has a W-shaped concentration dependence (two minima separated by a maximum). By increasing the molecular size of either component, the maximum in Ck is enhanced and displaced towards high concentrations of the smaller component. C; behaviour correlates with degrees of non-randomness in the mixtures as quantified by the concentration-concentration correlation function S, , which is calculated using the Flory-Huggins theory with a group-interaction model and assuming interaction between molecular surfaces.

No 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.

No abstract

Apparent heat capacities have been measured for phenol, substituted phenols and aromatic alcohols in dilute n-heptane at 25 "C. The alcohols are phenol, o-cresol, rn-cresol, p-cresol, 2,6-dimethylphenol, 3,5-dimethylp he no I, 3,5-d i -tert-b u ty I p he n o I, 2,6-d i-tert-b u t y I p he n o I, 2,4,6-t r i-tert-bu ty I p h e no I, 2,6-d i -tert-b u ty I -4-met h y I p hen o I, 2-te rtb u t y I p he no I, 4-te rtb u t y I p h e n o I, 2-e t h y I p h e no I , 2 -prop y I p h e no I, 2-p h en y I p h e no I, be n z y I a I c o h o I , 2phenylpropan-2-01 and phenethyl alcohol. In addition, cyclohexanol, 3-ethylpentan-3-01 and hexan-1-01 were also studied. The associational part of the apparent molar heat capacity @,(assoc) shows a maximum against concentration which decreases and moves to higher alcohol concentration as the hydroxyl group on the alcohol becomes increasingly hindered, effectively reducing the self-association capability. For the 2,6-disubstituted phenols the @, maximum disappears, indicating the absence of self-association. The Treszczanowicz-Kehiaian theory for associated liquids was used to obtain the volumetric equilibrium constants and the enthalpy of hydrogen-bond formation (AH'). For ptienols AHo is lower than for other alcohols, indicating weaker hydrogen bonding. In contrast to alkan-1-ols, where tetramers are the predominant species, for the present alcohols as the steric hindrance increases tetramer population is severely reduced, dimers and trimers being more important.