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Chem. 66,989 (1988).Apparent heat capacities have been measured for fifteen branched and cyclic alcohols in dilute n-decane solution at 25OC. The alcohols were 2-methyl-2-propanol, cyclohexanol, 3-methyl-3-pentanol, trans-, cis-, and mixed isomer 2-methylcyclohexanol, I-methylcyclohexanol, 3-ethyl-3-pentanol, cyclooctanol, 3,7-dimethyl-I-octanol, 5-decanol, 4-propyl-4-heptanol, cyclododecanol, 5-butyl-5-nonanol, and 8-hexadecanol (in n-hexane). Excess heat capacities cPE throughout the concentration range were measured at 25OC for: 1-hexanol + n-hexadecane (11-CI6) and + 2,2,4,4,6,8,8-heptamethylnonane (br-C16), 4-propyl-4-heptanol, and 1-decanol + n-decane, 3-methyl-3-pentanol + 11-c~~ and + br-C16 and at 27OC for cyclohexanol + 1 1 -c~~ and + br-CI6 Also, for 3-methyl-3-pentanol + n-decane CpE was measured at 10, 25, 40, and 50°C. For a series of isomeric alcohols, the apparent molar heat capacities show 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 alcohol self-association capabilities. This situation is also reflected by the heat capacities of the pure alcohols which increase strongly in magnitude in going from a linear I-alcohol to an isomeric alcohol which has its hydroxyl group on a quaternary carbon atom. cPE of the mixtures are negative at low alcohol concentrations turning positive at increasingly higher alcohol concentrations as the steric hindrance on the hydroxyl group increases. Throughout most of the concentration range cPE for the branched or cyclic alcohols is considerably more positive than for the corresponding isomeric 1-alcohol. For the highly hindered 3-methyl-3-pentanol c P E (~) passes through a maximum. All of the above behaviour is explained by the Treszczanowicz -Kehiaian model for self-associated liquids + inert solvents which has been applied to the present data. Equilibrium constants have been obtained for alcohol association and are sensitive to alcohol structure. At low alcohol concentrations, while for the linear I -alcohols tetramers are the predominant species and dimer are almost absent, for the corresponding isomeric alcohols the concentration of tetramers is severely reduced and the lower species, i.e. trimers and dimers, are more important. For the highly hindered alcohols, monomers are the predominant species in dilute solution reflecting the decrease in self-association ability that steric hindrance of the hydroxyl group imposes on them.MERCEDES CACERES-ALONSO, MIGUEL COSTAS, LINA ANDREOLI-BALL et DONALD PATTERSON. Can. J. Chem. 66, 989 (1988).OpCrant i 25"C, on a mesure les capacitis calorifiques apparentes de quinze alcools ramifiCs et cycliques en solutions diluCes dans le n-dCcane. I1 s'agit des alcools suivants : methyl-2 propanol-2, cyclohexanol, methyl-3 pentanol-3, les isomtres cis-et trans-du mCthyl-2 cyclohexanol, methyl-l cyclohexanol, Cthyl-3 pentanol-3, cyclooctanol, dimethyl-3,7 octanol-1, dCcanol-5, propyl-4 heptanol-4,...

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.

The molar excess heat capacity ( C i ) has been obtained through the concentration range at 25°C for mixtures of alkan-1-01s: methanol with butanol, hexanol and decanol and decanol with butanol and hexanol. C i is negative and small, increasing in magnitude with difference in chain length of the alkan-1-01s to a maximum of -3 J mol-' K-' for methanol-decanol. C i has also been measured at 25°C for methanol and other alkanols mixed with iso-, sec-and tert-butyl alcohol (I), 2-methylbutan-2-01 (ll), 3-methylpentan-3-01 (111) and 3-ethyl-pentan-3-01 (IV). With increasing steric crowding of the tertiary OH, CF becomes extremely large, -18 J mol-' K-' for methanol-lV at equimolar concentration, the apparent molar C, of methanol in IV being negative at low concentration. The negative CE(x) for methanol-l is of normal positive curvature, but with II, CE has a W-shaped concentration dependence exhibiting two regions of positive Ci(x) curvature separated by a region of negative curvature. An extension of the Treszczanowicz-Kehiaian association model has been made to alcohol-alcohol mixtures with consideration of multimers mainly confined to linear tetramers. Assuming equal enthalpies and equilibrium constants for H-bonding between like alkan-1-01s (AA and BB) and between unlike alkanols (AB) leads to positive C; predictions. The experimental negative CF values are associated with a lowering of the equilibrium constant for H-bond formation in the alkan-1-01 with increasing chain length coupled with relatively stronger AB bonding. The equilibrium constant, K,, , for H-bonding between tertiary alcohol hydroxyls is found to be lowered by steric crowding of the tertiary hydroxyl, raising the C, of the pure liquid. The constant, K,, , for bonding between primary (A) and tertiary (B) hydroxyls in the multimer chain is less affected by the tertiary crowding resulting in lower solution C, and large negative C i . The S-and W-shape concentration dependences require modifications of KA, owing to the proximity in the tetramer chain of either A or B alcohols resulting in an effective concentration dependence of KA, .