2H NMR study of 2D melting and dynamic behaviour of CDCl3 confined in ACF nanospace

Abstract

Two-dimensional melting of trichloromethane (chloroform) confined in activated carbon fibre was investigated using differential thermal analysis and ²H NMR techniques. Differential thermal analysis revealed a thermal anomaly with an endothermic peak at 269 K, which was distributed from 250 K to 287 K on the heating direction. This anomaly was also observed upon cooling at the same temperature. Furthermore, ²H NMR revealed that slow motion such as molecular hopping and/or diffusion of CDCl₃ in ACF affected the spectral line width. The temperature dependence (Arrhenius plot) of the spectral line width showed an inflection point at 227 K. The activation energy of molecular motion of CDCl₃ in ACF was 4 kJ mol⁻¹ at temperatures greater than 227 K and 7.7 kJ mol⁻¹ at temperatures less than 227 K. Reduction of the activation energy suggests that the average intermolecular distance between CDCl₃ molecules enlarges above the inflection point. The difference of activation energy (3.7 kJ mol⁻¹) is close to the enthalpy of fusion in typical plastic crystals. These results reveal that the thermal anomaly and the transition of dynamic process correspond respectively to melting of CHCl₃ in ACF and the pre-melting phenomenon.