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

Abstract

Apparent molar heat capacities, ϕ_C, and volumes, ϕ_V, have been measured for methanol, hexan-1-ol and decan-1-ol in n-alkane solvents between x_ROH= 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 dϕ_C/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, C^E_p, and dC^E_p/dT of the mixtures are of different sign in the following approximate concentration ranges: (I) for x_ROH > 0.01, C^E_p and dC^E_p/dT > 0, (II) for 0.005 < x_ROH < 0.01, C^E_p > 0 and dC^E_p/dT < 0 and (III) for x_ROH < 0.005, C^E_p and dC^E_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. ϕ_C and ϕ_V 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.