HYDROGEN-BOND EQUILIBRIUM AND THE ENTHALPY AND ENTROPY RELAXATIONS INA NONPOLAR STATE OF VITRIFIED 2-METHYL-3-HEPTANOL

The spontaneous decrease of the enthalpy and entropy of the variously vitrified 2-methyl-3-heptanol, an octanol isomer which behaves like a nonpolar liquid as its molecular dipole vectors mutually cancel on int ermolecular hydrogen bonding, has been studied by differential scannin g calorimetry. The effect of annealing for different periods at a fixe d temperature, and at different temperatures for a fixed period, has b een investigated. The structural relaxation is close to exponential, a nd its characteristic time, in contrast with other glasses, does not i ncrease with the annealing time. These characteristics are different f rom those found for other materials and correspond to a first-order ra te of a chemical process or an exponential decay of the thermal functi ons. The exponential character and the low activation energy (=37.7 kJ /mol) of the process, which is comparable to the energy for the format ion of two H-bonds and to the enthalpy (-36 kJ/mol) of ring dimerizati on data available in the literature, indicate that the enthalpy decrea se observed on annealing is mainly a reflection of an increase in the concentration of the nonpolar ring dimers formed by H-bond association . The effect is attributed to the increase in the reaction quotient to ward the equilibrium constant value of the intermolecular hydrogen-bon d association reaction in the liquid's structure, as the fictive tempe rature decreases. The increase in the number of H-bonds and in the rea ction quotient, as it approaches the equilibrium constant for the reac tion, monomer <----> ring dimer or lineardimer <----> ring dimer, as w ell as the equilibrium concentration of the dimers, has been calculate d from the enthalpy and entropy decrease observed on annealing. This i s combined with a similar calculation from the dielectric data availab le in the literature. It is proposed that such effects occur also in m aterials with abundant H-bonding, as in hydrated proteins and carbohyd rates and those in which ion association and isomerization reactions o ccur, and that the availability of configurational and conformational degrees of freedom is necessary for their occurrence. Hence, we introd uce the terms for discussing spontaneous enthalpy and entropy decrease of an H-bonded glass in terms of a chemical fictive temperature corre sponding to the H-bond and ion-pair formation.