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

Apparent molar heat capacities phiC(p) and volumes phiV of seven monosaccha rides {D(-)ribose, D(-)-arabinose, D(+)-xylose, D(+)-glucose, D(+)-mannose, D(+)-galactose, D(-)-fructose}, seven disaccharides {sucrose, D(+)-cellobi ose, 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) mo l . kg(-1) aqueous urea solutions at T = 298.15 K from specific heat and de nsity measurements employing a Picker how microcalorimeter and a vibrating- tube densimeter, respectively. By combining these data with the earlier rep orted partial molar heat capacities C-p,2(o) and volumes V-2(o) in water, t he 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 bee n estimated. Both the C-p,2,tr(o) and V-2,tr(o) values have been found to b e positive for all the sugars and to increase with increase in concentratio n of the cosolute (urea), suggesting that the overall structural order is e nhanced in aqueous urea solutions. This increase in structural order has be en attributed to complex formation between sugars and urea molecules throug h hydrogen bonding and to a decreased effect of urea on water structure. Th e transfer parameters have been rationalized in terms of solute-cosolute in teractions using a cosphere overlap hydration model. Pair, triplet and high er-order interaction coefficients have also been calculated from transfer f unctions and their sign and magnitude have been discussed. (C) 2000 Academi c Press.