Effect of nanopore confinement on the thermal and structural properties of heneicosan

The effect of nano-confinement on the phase transitions of heneicosane (n-C21H44, C21) were investigated using differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (XRD). When C21 is confined within the pores of controlled pore glasses (CPGs), SBA-15 or other mesoporous materials, the solid-solid (Ts) and solid-liquid (Tm) transition temperatures are depressed. The depression of the phase transition temperatures (ΔTs, ΔTm) for C21 in CPGs and SBA-15, the degree of supercooling and enthalpy change (ΔHs, ΔHm) for C21 in CPGs, exhibited a linear dependence on the inverse pore size. The ΔTm values were insensitive to the polarity pore wall but varied with the pore geometry. XRD analysis revealed that the lamellar ordering characteristic for the solid C21 was disturbed in the nanopores, with the extent increasing as the pore size decreased. A new rotator phase RII is found for C21 confined in CPG (12 nm) and SBA-15 (14.4 nm).