Preparation Temperature Dependence of the Dissociation Enthalpy of Hydrophobic Binding in N-isopropylacrylamide Gels

Abstract

The thermal properties of N-isopropylacrylamide (NIPA) gels in water have been investigated with differential scanning calorimetry (DSC). The dissociation enthalpy of hydrophobic bonding per NIPA monomer, ΔH_N and the number of water molecules associated with the dissociation, (n-n₀), were evaluated as a function of the gel preparation temperature, T_prep, where n and n₀ are the number of water molecules per NIPA mole before and after dissociation, respectively. It was found that ΔH_N is independent of T_prep. On the other hand, both n and n₀ increase with increasing T_prep, while the difference, i.e., (n-n₀) is constant irrespective of T_prep. These experimental findings suggest that the enthalpy and the number of water molecules associated with the dissociation are independent of T_prep although the number of trapped water molecules increases with T_prep in both the swollen and shrunken states.