Abstract
We developed a transparent, low-viscosity molten mixture employing sorbitol, urea and water, and investigated its physical properties, molecular-level interactions and dynamics through different experiments. The measured viscosity coefficient ( \(\eta\) ) of the molten mixture varies from ~73 to 8 cP within the temperature range 288 K \(\le T\le\) 338 K. Temperature dependent (303 K \(\le T\le\) 343 K) dielectric relaxation (DR) measurements in the frequency window \(0.2\le \nu /{\rm GHz}\le 50\) reveals presence of multi-Debye relaxation dynamics in this medium with average relaxation time spanning from ~400 to 100 ps within this temperature range. The high static dielectric constant \(({\varepsilon}_{s} \sim 80)\) of the molten mixture, like water, makes it excellent at dissolving things, even though its dynamics are much slower than water. Steady-state emission studies using fluorescent dyes coumarin 153 (C153) and coumarin 343 (C343) demonstrate spatial homogeneity in the medium, though signs of mild dynamic heterogeneity are evident from the fractional viscosity dependency of the rotational dynamics of the dye molecules ( \({\tau}_{r} \propto {\left[\frac{\eta }{T}\right]}^{p}\text{w}\text{i}\text{t}\text{h} \, p\approx 0.7)\) . In contrast, DR dynamics captures less dynamical heterogeneity of the system, with \({\tau}_{r} \propto {\left[\frac{\eta }{T}\right]}^{p} \text{w}\text{h}\text{e}\text{r}\text{e}\) \(p \approx 0.87\) . Further, the rotational dynamics exhibit an Arrhenius-type temperature dependence. The obtained activation energy for C153 and C343 rotation are ~30 kJ mol–1 and 26 kJ mol–1, respectively, whereas the viscosity and average DR times activation energies are ~36 kJ mol–1 and ~32 kJ mol–1. These results indicate that the reorientation of dipolar molecules of the molten mixture felt higher friction than the rotation of C153 and C343 in the same medium.
Graphical abstract
The graphical abstract presents the dielectric relaxation response of the (sorbitol + urea + water) molten mixture over a frequency range \(0.2\le \nu /{\rm GHz}\le 50\) at various temperatures. The spectra reveal multiple relaxation modes arising from complex molecular motions and hydrogen-bond rearrangements. Decomposition of the dielectric loss profiles highlights cooperative dipolar dynamics, while the medium maintains high polarity together with comparatively low viscosity.
