Structure, molecular dynamics, and interactions in aqueous xylitol solutions

The cooperative dynamics and hydration of aqueous xylitol solutions were investigated at 25 °C by broad-band dielectric spectroscopy for solute concentrations c≤ 3.251 M. The spectra could be consistently interpreted by assuming a superposition of five modes. From the combined amplitudes of the bulk-water contributions at ∼18 GHz and ∼600 GHz the effective xylitol hydration number of 5.66 ± 0.08 was derived. These hydrating H₂O molecules are retarded by a factor of ∼2.6 compared to the bulk and cause ∼85% of the relaxation resolved at ∼5-10 GHz. The remaining amplitude could be attributed to the three central -OH groups of xylitol, which exhibit slower dynamics than the more exposed distal hydroxyl groups peaking at ∼50 GHz but all having the same effective dipole moment, μ = 3.43 D. A further xylitol-specific relaxation at ∼1.5 GHz probably arises from weak xylitol aggregation.