Thermochemical study of the solvation of alkanes in mixtures of methanol with butyl alcohol isomers

Abstract

A calorimetric method was applied at 25 ~ to measure the enthalpies of dissolution of cyclohexane, heptane, and deeane in the methanol--n-butanol mixed solvent and hexadecane in mixtures of methanol with n-, iso-, and tert-butyl alcohols. The standard cnthalpies of dissolution of alkanes were determined. It was shown that the equation proposed in the literature for calculation of the enthalpies of dissolution of alkanes in mixtures with nonspecific intermoiecular solvent--solvent interactions describes satisfactorily the enthalpies of dissolution of aikanes in mixtures of methanol with n-and iso-butyl alcohols. It was suggested that there is no preferential solvation of alkanes by one of the mixed solvent components in the MeOH--BunOH and MeOH--BuiOH mixtures; in the MeOH--ButOH system, the composition of alkane solvation shell differs slightly from the solvent composition in the bulk.Key words: cnthalpy of dissolution, enthalpy of transfer, excess cnthalpy of mixing; alkanes, methanol, butyl alcohol isomers, mixed solvent.Isomeric butyl alcohols have virtually equal molar volumes, enthalpies of vaporization, and refraction indices. Since the degrees of association of butyl alcohols differ by no more than 10%, it can be assumed that the bond energies in the isomeric self-associates are also similar. I However, the empirical parameters characterizing the solvent polarity of the isomers differ appreciably. 2,3 Thus BuaOH is a stronger proton donor but a weaker proton acceptor than ButOH. 3 Thus, isomerization has a substantial influence on intermolecular interactions in solutions in these alcohols. Previously, 4 various excess properties of MeOH mixtures with BunOH, BuiOH, and ButOH have been measured. Among the series of excess properties, excess enthalpy of mixing of alcohols (HE) is of interest. Since the dispersion forces in these systems are virtually identical, the differences between their excess properties are related 4 mostly to different strengths and numbers of the H-bonds formed between the alcohol molecules. Since the proton-acceptor capacity of alcohols increases in the sequence BunOH--BuiOH <<ButOH, it can be assumed that the formation of a mixed associate of MeOH with ButOH is energetically more favorable than that of MeOH with BuaOH. This is confirmed by the magnitudes of the excess enthalpies of mixing of these systems. In the MeOH--BunOH and MeOH--BuiOH systems, the crucial role is played by the destruction of hydrogen bonds between the molecules, as indicated by positive H E values. The negative H E values found for the MeOH--ButOH system are apparently 4 due to the fact that in this system, the formation of H-bonds predominates. Note. R is the correlation coefficient; S is tile error of approximation. The AH and H values are expressed in kJ tool -i.