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

The effect of nano-confinement on the phase transitions of heneicosane ( n -C 21 H 44 , C 21 ) were investigated using differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (XRD). When C 21 is confined within the pores of controlled pore glasses (CPGs), SBA-15 or other mesoporous materials, the solid-solid ( T s ) and solid-liquid ( T m ) transition temperatures are depressed. The depression of the phase transition temperatures (Δ T s , Δ T m ) for C 21 in CPGs and SBA-15, the degree of supercooling and enthalpy change (Δ H s , Δ H m ) for C 21 in CPGs, exhibited a linear dependence on the inverse pore size. The Δ T m 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 C 21 was disturbed in the nanopores, with the extent increasing as the pore size decreased. A new rotator phase R II is found for C 21 confined in CPG (12 nm) and SBA-15 (14.4 nm).