Self-association of alcohols in inert solvents. Apparent heat capacities and volumes of linear alcohols in hydrocarbons

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Abstract
Apparent molar heat capacities, ϕC, and volumes, ϕV, have been measured for methanol, hexan-1-ol and decan-1-ol in n-alkane solvents between xROH= 0.001 and 0.2 at 10, 25 and 40 °C. The apparent molar heat capacities show a maximum against concentration which increases and moves to lower alcohol concentrations as the temperature decreases. This leads to a negative dϕC/dT at low alcohol concentrations, changing sign at higher alcohol concentrations. The Treszczanowicz–Kehiaian model for self-associated liquids + inert solvents explains these concentration and temperature dependences in terms of alcohol self-association through hydrogen bonds. Tetramers are the predominant species, dimers being almost absent even at very low alcohol concentrations. The excess heat capacity, CE p, and dCE p/dT of the mixtures are of different sign in the following approximate concentration ranges: (I) for xROH > 0.01, CE p and dCE p/dT > 0, (II) for 0.005 < xROH < 0.01, CE p > 0 and dCE p/dT < 0 and (III) for xROH < 0.005, CE p and dCE p/dT < 0. This behaviour is explained quantitatively by the Treszczanowicz–Kehiahian theory and is believed to occur in all associated + inert liquid systems. The apparent molar volumes increase rapidly as the concentration of alcohol decreases, corresponding to a destruction of the tetramers. ϕC and ϕV have also been measured for methanol dissolved in an active solvent, methyl acetate. The radically different results indicate that hydrogen bonding between the alcohol and the solvent has replaced self-association as the predominant influence on the thermodynamics.

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