Endothermicity or exothermicity of water/alcohol mixtures

Abstract

Excess enthalpies of mixtures of water with methanol, ethanol, 1- and 2-propanol, 1-, 2- and tert-butanol were determined by dilution calorimetry in the accessible range of mole fractions at 25-degrees-C. Ab initio RHF calculations were performed on the alcohols, their dimers and complexes with water. A full optimisation at the 6-31G level was used to obtain the properties of interest of these systems. The calculated H-bond energies are similar for all the studied alcohols. Nevertheless, a systematic stabilisation of the mixed dimers is found, especially for the secondary and tertiary alcohols. The formation of mixed H-bond chains is exothermic and does not follow the rules of the geometric mean. To a first approximation, the experimental excess enthalpies DELTAH(e) for all the systems follow the equation DELTAH(e) = C6/1X(W)6X(A) + C1/1X(W)X(A) + C1/2X(W)X(A)2 The calorimetric results may be explained by the presence in water of two kinds of H-bonds differing by about 8.3 kJ mol-1 in energy. Upon dilution in water, the alcohol is inserted in a weak chain and renders the adjacent water-bond strong. As a consequence, the dilution of an alcohol in water is always exothermic. The dilution of water in an alcohol causes stabilisation of the H-bonds but has a negative effect on the non-specific cohesion. The effect of the non-specific forces increases in the homologous series, making the dissolution of water in the alcohols endothermic starting from propanol. The stabilisation of H-bonds formed by water at high dilution in the alcohols increases according to the sequence: primary < secondary < tertiary.