Are water-xylitol mixtures heterogeneous? An investigation employing composition and temperature dependent dielectric relaxation and time-resolved fluorescence measurements

Abstract

Aqueous xylitol solutions at six different concentrations were studied employing dielectric relaxation (DR) and time-resolved fluorescence (TRF) measurements in the temperature range 295–323 K. The focus was to explore the solution heterogeneity aspect via monitoring the viscosity coupling of the average relaxation rates at various temperatures. TRF measurements were done using both hydrophobic and hydrophilic probes to explore the preferences, if any, for solute locations in these binary mixtures. Energy-selective population excitations and the corresponding fluorescence emissions did not suggest any significant spatial heterogeneity in solution structure within the lifetimes of these probes. DR measurements and TRF experiments indicated mild deviations from the hydrodynamic viscosity dependence of the measured relaxation rates. All these suggest mild spatiotemporal heterogeneity for these water-xylitol mixtures in the temperature range considered. In addition, DR timescales appear to originate from reorientational and H-bond relaxation dynamics, excluding the possibility of full molecular rotations. Heterogeneity in water-xylitol mixtures was investigated employing dielectric relaxation (DR) and Time-resolved fluorescence (TRF) spectroscopic techniques. Time-resolved anisotropy and DR response were found to depend both on xylitol concentration and temperature. Signature of mild heterogeneity was detected along with presence of water molecules that were slower than bulk-like water. Slower than bulk DR response vanishes with an increase of solution temperature.