Solvent isotope effect on sol-gel transition of methylcellulose studied by DSC

Methylcellulose (MC), a hydrophobically modified cellulose derivative, in an aqueous solution undergoes sol-to-gel and gel-to-sol transitions on heating and cooling, respectively. Using differential scanning calorimetry, MC in light (H₂O) and heavy (D₂O) water solutions has been investigated to elucidate the solvent isotope effect on the transitions. As a result, their transition temperatures are higher in H₂O by about 4 °C than D₂O. This phenomenon is rationalized in terms of the strength of the hydrophobic attractive interaction; the strength is enhanced by D₂O. We discuss the reason for the enhancement and the difference in the isotope effect between MC and a poly(N-isopropylacrylamide) polymer which shows an opposite trend to MC.