Experimental and theoretical study of thermodynamic excess properties of associating liquid alkanol + 4-picoline mixtures based on quantum mechanical ab initio calculations

Abstract

Experimental data of the molar enthalpy of mixing H m E of 6 alkanol + 4-picoline systems obtained by flow calorimetric measurements have been compared with statistical–mechanical calculations based on the ERAS-model. The original version of the model has been extended by accounting for results obtained by quantum mechanical ab initio calculations of energetic and structural properties of about 50 molecular clusters, each containing up to 4 molecules associated by hydrogen bonding. The essential results obtained from these calculations are cooperative effects concerning the hydrogen bonding energies of linearly associated alkanol clusters and the quantitative specification of particularly stabilized cyclic alkanol tetrameric clusters. Furthermore, results for the structure and energy of cross-associating clusters containing alkanols and 4-picoline molecules have revealed that the cross-associating energy of the alkanol + 4-picoline hydrogen bonding exceeds the self associating energy of alkanol molecules also leading to an additional cooperative energetic effect when more than two molecules are involved. These quantum mechanical results have directly been incorporated as molecular parameters into the extended ERAS-model without increasing the number of freely adjustable parameters compared to previous versions of the model. An excellent description of the thermodynamic mixture properties has been achieved. The molar excess enthalpy H m E is negative for all 1-alkanol + 4-picoline mixtures while isomeric 2-alkanol + 4-picoline mixtures show positive H m E values quantitatively described by the theory. An explanation of this effect is given on the basis of the ab initio results.