Competition between Solute‐Solvent and Solvent‐Solvent Hydrogen Bonds: Pyridines in Alcohols and in Water

Abstract

Calorimetry / Mobile OrderPartial enthalpies of mixing of pyridines in methanol, ethanol, I-butanol, water and cyclohexane were determined calorimetrically at 25°C. From these data their molar enthalpies of transfer, at infinite dilution, from the inert solvent to the H bonded solvents were calculated. They were compared with the enthalpies of H bond formation between the pyridines and the alcohols in CC14 and the enthalpies of the selfassociation bonds in the pure liquids. Although in the pure alcohols more than 95% of the molecules are at a given time involved in H bonds with an energy of about 25 kJmol-' which is markedly larger than the energy of the pyridine-alcohol bonds in CCI,, the molar enthalpy of transfer of the pyridines from an inert solvent to an H bonded one remains exothermic. A quptitative treatment of the competition between the two types of H-bonds is presented. It is based on the thermodynamics of mobile order that was recently developed. It leads to an equation that reasonably predicts the transfer enthalpies. According to this treatment the competition reduces the complexation constant by a factor KjB/(KiBKiA), K i B and K i A being the stability constants of the H bonds in the absence of competition. In water the exothermic effect is enhanced because the second 0 -H group of the molecule forms much weaker selfassociation H-bonds than the first one.Solutions / Thermodynamics

Surface properties of diluted solutions of cyclohexanol and cyclopentanol in ethylene glycol

“…The activity coefficients for the solute (γ 2 ) were obtained from mobile order-disorder theory (MOD) [22][23][24][25].…”

Enthalpy of cooperative hydrogen bonding in complexes of tertiary amines with aliphatic alcohols: Calorimetric study

“…Question about the value of solvent reorganization enthalpy existed for a long time [29][30][31]. Authors of work [29] supposed that the contribution of reorganization is not to be constant but depended on the structure of alcohol associated species (linear or cyclic).…”

“…Because alcohol molecule has oxygen atom with two lone electron pairs and only one acid proton, a quantity of free and H-bonded protons determines equilibrium between free and H-bonded molecules of alcohol. The process of specific interaction of proton acceptor with aliphatic alcohol in its environment is complicated [29][30][31][32][33]. Initially, when solute molecule is being dissolved in associated solvent, the solvent reorganization (breaking solvent-solvent hydrogen bonds) takes place.…”

“…The concentration ratio of free and bonded molecules changes, since B forms hydrogen bonds with O-H groups. This phenomenon was discussed in works [29][30][31][32]. It can be concluded, that determination of the enthalpy of cooperative hydrogen bonding of solute molecule with associated solvent clusters requires an accurate evaluation of reorganization term.…”

Surface thermodynamic functions of dilute solutions of methylcyclohexanols in ethylene glycol

“…The activity coefficients for solute (γ 2 ) were obtained from mobile order-disorder theory (MOD) [26][27][28][29].…”