Melting behavior of n-alkanes in anodic aluminum oxide (AAO) nanopores using Flash differential scanning calorimetry

Flash differential scanning calorimetry is used to study nanoconfinement effects on the melting and solid–solid transitions of n-hexadecane (C₁₆H₃₄) and n-nonadecane (C₁₉H₄₀) in an anodic aluminum oxide (AAO) nanoporous membrane with pore diameters of 20 and 55 nm. The size-dependent melting behavior of n-hexadecane (C₁₆) was investigated as a function of pore fullness from underfilled to overfilled. A comparison between the bulk, underfilled, and overfilled pores indicates a melting point depression (ΔT_m) of 4.20 ± 0.60 °C in 55 nm AAO pores and 6.01 ± 0.24 °C in 20 nm AAO pores, independent of pore fullness. Nanoconfined n-nonadecane (C₁₉), shows a melting point depression of 2.46 ± 0.40 °C and 4.2 ± 0.51 °C in 55 and 20 nm AAO pores, and its solid–solid transition is depressed by 1.94 ± 0.15 °C and 3.01 ± 0.29 °C in 55 and 20 nm AAO pores, respectively. The size-dependent melting behavior (ΔT_m vs 1/d) for both C₁₆ and C₁₉ was found not to extrapolate to the bulk melting point at infinite pore size, indicating that a different crystal structure, perhaps a nematocrystalline state may be formed in AAO nanopores, as backed up by X-ray diffraction.