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42364-AC7
Glass Transition and Dynamics of Hydrogen-Bonded Liquids

Ranko Richert, Arizona State University

Dielectric relaxation experiments are common approaches towards structure and dynamics of hydrogen bonded liquids, where the prominent dielectric signals have been associated with structural relaxation, while hydrogen-bonded structures were derived from Kirkwood correlation factors. This project investigates the relations among various relaxation modes, casting serious doubts on the previous interpretation of dielectric behavior. The focus is on glass-forming alcohols, where the features of interest are particularly accessible and obvious.

For the prominent dielectric peak of many hydrogen-bonding systems, the following features have been observed: This dielectric signal occurs in monohydroxy alcohols and several amides, not in diols and polyols, in mixtures with other liquids at high alcohol content. In binary alcohol systems, the logarithmic relaxation time of a Debye peak follows ideal mixing laws. The factor by which the Debye peak is slower than the true structural relaxation at T_g depends on the particular liquid, but it approaches 100 for all systems at higher temperatures. Additionally, the glass transition temperature T_g derived from the Debye signal is not consistent with calorimetric T_g's. There is always another much smaller dielectric feature that displays all typical properties of structural relaxations.

Numerous observations have hinted towards the smaller and faster dielectric relaxation mode matching the calorimetric T_g and the mechanical relaxation times. A decisive answer came from a recent collaboration with Chr. Schick (Rostock, Germany) using isothermal scans of the frequency resolved dynamic heat capacity, C_p' + iC_p'', which are compared directly to the dielectric loss, ε''. The clear result is that calorimetric times match those of the smaller and faster dielectric peak, while there is no calorimetric mode corresponding to the Debye peak. This verifies our earlier evidence, but the disconnection of energy and dielectric polarization fluctuations has not been observed this unambiguously. Dynamic and thermodynamic behavior has also been studied regarding other possible anomalies of alcohols, but even monohydroxy alcohols display much of the typical behavior of other molecular liquids. We have tested a well established link between thermodynamic and dynamic fragility that reads m = 40 C_p(T_g)/S_m, where m is the fragility defined as dlogτ/d(T_g/T) evaluated at T_g, C_p(T_g) is the heat capacity step at T_g, and S_m is the melting entropy. Alcohols and related systems are no exception regarding this quantitative correlation. A similar result is obtained by studying the relation between the boiling temperature T_b and the kinetic T_g. For many liquids, T_b and T_g are positively correlated. Within a series of isomers, however, these quantities are negatively correlated, and this novel feature is observed equally for alkanes and monohydroxy alcohols. These studies emphasize the absence of anomalies regarding true measures of structural relaxation such as calorimetry and mechanical relaxation.

Instead of using a 3ω type calorimetry, we decided to apply a novel technique which exploits supplying energy directly to the slow degrees of freedom and thus reverting the direction of energy flow compared with standard calorimetry. For regular liquids, this allowed us to measure the configurational heat capacities and quantify their contribution to the total excess heat capacity. In the case of the monohydroxy alcohols, the Debye process would absorb energy from a high external electric field and the time scale of heat flow is assessed by time resolved dielectric experiments using period-by-period Fourier analysis of current and voltage signals. Contrary to the behavior of dielectric loss peaks of other polar liquids, we find that the Debye peak of alcohols fails to sustain any configurational energy. In other words, the energy absorbed by this mode is transferred much more rapidly to the phonons than the reciprocal Debye frequency. Consistent with the above comparison of C_p'' and ε'', these non-linear dielectric results demonstrate that these liquids have no thermal relaxations corresponding to the Debye peak. This has interesting implications on the mechanisms involved in microwave heating and microwave chemistry, as the energy flow in alcohols and related materials differs from that in generic liquids.

Our experiments have not yet revealed the microscopic origin of the Debye peak, but the detailed observations resulting from this project provide stringent bounds and tests of a model of the structural and dynamic properties of alcohols, which is expected to emerge in the near future with the help of computer simulations. The main challenge to be resolved is that the strong dipoles of these molecules fail to generate an accordingly strong dielectric signal on the time scale of the calorimetric and mechanical modes, and that the prominent dielectric signal that could match the dipole strength is so much slower. Our recent results have already led several other groups to study alcohols with their respective techniques. The project has steered my research towards issues more relevant to today's society. The results achieved were also instrumental in the postdoctoral fellow obtaining a professorial position while involved in this project.

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