Effect of molecular structure on the conductivity of amorphous carbohydrate-water-KCl mixtures in the supercooled liquid state.

The effect of carbohydrate structure on the conductivity of low water content amorphous carbohydrate-water, and carbohydrate-water-KCl mixtures, has been measured using both direct current and alternating current techniques at temperatures in the supercooled liquid and glassy range, ranging from -40 to 80 degrees C. The structures included homologous mono-, di- and trisaccharides (glucose, maltose and maltotriose), a monosaccharide with no exocyclic hydroxymethyl group (xylose) and a second trisaccharide (raffinose). The KCl-mixtures contained 9.3% w/w water and 0.74% w/w KCl which resulted in calorimetric glass transition temperatures, T(g), in the range -29-19 degrees C. At this concentration conduction due to KCl dominated that due to intrinsic conductors originating from the carbohydrates and water. In the supercooled liquid region, as temperature, T, is reduced to T(g), the activation energy of the molar conductivity of KCl, Lambda(m), increased as described by a Vogel-Tamman-Fulcher-type equation, Lambda(m)=Lambda(m0)exp[B/(T-T(0))], where Lambda(m0), B and T(0) are constants. Comparison of the molar conductivity of KCl in the carbohydrate mixtures at T(g) with that in aqueous solutions showed that conductivity is, to varying extents, uncoupled from viscosity. The uncoupling increased in the order D-xylose<D-glucose<maltose<maltotriose and raffinose. The results suggest that the primary structural characteristic determining conductivity is molecular weight, though the presence of the exocyclic hydroxymethyl group in the monosaccharide also has an effect. Whilst at T(g) the D-xylose mixture had the lowest conductivity, at a particular temperature the trisaccharide mixtures of maltotriose and raffinose had the lowest conductivities.