Molecular Mobility of the Water Molecules in Aqueous Sucrose Solutions, Studied by 2H-NMR Relaxation

Abstract

Deuteron spin lattice relaxation times T1 of sucrose/D2O solutions are given as function of temperature, pressure, frequency and concentration. From the temperature dependence of the 2H -T1 the rotational dynamics of the hydrated sucrose complex and the water molecules are determined. For high pressure and high concentrations the temperature dependence of the water molecules is described by a Vogel-Tammann-Fulcher equation. The ideal glass transition temperatures TOH2O derived for the water molecules are at higher concentrations almost constant and smaller than the TOsuc of the sugar molecules

c , T-Dependence of the Self Diffusion in Concentrated Aqueous Sucrose Solutions

“…2 the self diffusion coefficients for the water molecules are given. Contrary to the drastic effects observed for the rotational dynamics [2], the T-dependence at constant concentration is in the whole range studied describable by a V IF equa tion. Also the ratio D H2°/D S U C shows a very slight increase with concentration only, which is almost within the limits of experim ental error.…”

“…The self diffusion co efficients D of the sucrose were taken in heavy water solutions. Details of the sample preparation have been given previously [1,2]. The self diffusion co efficients were obtained on a Bruker MSL 300 spec trom eter operating at 7.05 T at a proton frequency of 300.13 MHz in a home built probe head.…”

“…The temperature dependence of the self diffusion is at all concentrations given by the VTF equation (Eqn. (2)), which also characterizes the dependence of the ro tatoric mobility as given by x2 derived from the deuteron spin lattice relaxation times Tx [2] for sucrose concen trations c < 50% w/w. For the lower concentrations the T-dependence is given by the dynamic scaling law: stabilization of the hydration shell of the individual sucrose molecule through the removal of therm al energy or by the formation of transient aggregates between the sucrose molecules.…”

“…The analysis of the spin lattice relaxation rates [1,2] and the self diffusion coefficients that are com piled with their relevant tem perature dependencies in Tables III and IV leads to the following conclu sions: A t low concentrations the fully hydrated sucrose molecules and the bulk water molecules can be described by models assuming rigid molecules that rotate isotropically with a single correlation time for both, the dissolved sugars and the water molecules. In addition the water molecules can exchange rapidly between the hydration shells and the bulk phase and possess in the hydration shell an extra anisotropic mobility.…”

“…In two previous papers the rotational dynamics of the sucrose [1] and the water molecules [2] in con centrated aqueous solutions of sucrose were stud ied in order to evaluate the molecular mobility of the individual components of this binary system. The motional models applicable for the description of the two constituents were derived, and it was attem pted to characterize the molecular features responsible for the macroscopic behaviour ob served in the bulk solutions.…”