Optical probe studies of relaxation processes in viscous liquids

The question of relaxation time distributions in viscous liquids is examined by observing time-dependent behavior of optically active elements of the liquid structure, both intrinsic and otherwise, in response to perturbations from equilibrium states. As a suitable medium for this study the polyalcohol sorbitol C₆H₈(OH)₆ is adopted. Equilibrium distributions of intact and broken hydrogen bonds in the pure sorbitol have been determined using near IR spectra, and the loss of, and re-establishment of, this equilibrium, during down and up temperature ramping processes, has been monitored and analyzed. The hydrogen-bond equilibrium is established on essentially the same time scale and with essentially the same distribution of relaxation times as for the enthalpy. (The latter has been monitored by differential scanning calorimetry). Analysis shows that the kinetics of the bond-breaking process are well accounted for by a relaxation function, near equilibrium, of the form φ_e(t) = e ^-[(t/τ0)β], with β = 0.55±0.03. As extrinsic probes, two carbocyanine dyestuffs have been introduced in low concentrations into the sorbitol, and the equilibrium distribution of monomer and dimer species has been determined as a function of temperature. The loss and recovery of the equilibrium has also been monitored during down and up temperature ramping experiments. It is found that these equilibria are lost and recovered at temperatures far above that at which solvent structure equilibrium is established on the same time scale, showing that equilibration times for the monomer-dimer equilibrium are many orders of magnitude longer than for solvent structure. In this case the probe equilibrium is established with a single relaxation time, which depends on concentration, but which is not consistent with second order reaction kinetics. Finally, the distribution of Co ²⁺ ion probes between octahedral Co(-OH)₆²⁺ and CoCl₄^2- sites in sorbitol + pyridinium chloride solutions has been studied at equilibrium and then monitored during temperature ramping processes. In this case the probe ion octahedral-tetrahedral equilibrium is established on the same overall time scale as the enthalpy and the hydrogen bond equilibrium, but the "spectrum of times" is distinctly narrower, i.e., the β parameter in the relaxation function is much closer to unity. This is consistent with previous observations for local structure processes in a purely ionic solvent. All observations are consistent with the idea that the spectrum of relaxation times which is observed for a given relaxation process depends on the volume of the system subspace which must be explored to relax most of the imposed stress.