Partial molar heat capacities and volumes of transfer of some saccharides from water to aqueous urea solutions at T = 298.15 K

Abstract

Apparent molar heat capacities φC_p and volumes φV of seven monosaccharides {d( - )-ribose,d( - )-arabinose,d( + )-xylose,d( + )-glucose,d( + )-mannose,d( + )-galactose,d( - )-fructose}, seven disaccharides {sucrose,d( + )-cellobiose, lactulose,d( + )-melibiose hemihydrate,d( + )-maltose monohydrate,d( + )-lactose monohydrate,d( + )-trehalose dihydrate} and one trisaccharide {d( + )-raffinose pentahydrate} have been determined in (0.5, 1.0, 1.5, and 3.0)mol · kg ^-1 aqueous urea solutions at T = 298.15 K from specific heat and density measurements employing a Picker flow microcalorimeter and a vibrating-tube densimeter, respectively. By combining these data with the earlier reported partial molar heat capacitiesC_p,2^o and volumes V₂^oin water, the corresponding partial molar properties of transfer (C_{p,2, tr}^o and V_{2, tr}^o) from water to aqueous urea solutions at infinite dilution have been estimated. Both the C_{p,2, tr}^o and V_2,tr^o values have been found to be positive for all the sugars and to increase with increase in concentration of the cosolute (urea), suggesting that the overall structural order is enhanced in aqueous urea solutions. This increase in structural order has been attributed to complex formation between sugars and urea molecules through hydrogen bonding and to a decreased effect of urea on water structure. The transfer parameters have been rationalized in terms of solute-cosolute interactions using a cosphere overlap hydration model. Pair, triplet and higher-order interaction coefficients have also been calculated from transfer functions and their sign and magnitude have been discussed.