Polymorphism of a hexadecane–heptadecane binary system in nanopores

Abstract

Inspired by the interest in the confinement effect on polymorphism of alkane mixtures, phase behaviors of a hexadecane–heptadecane (n-C₁₆H₃₄–C₁₇H₃₆, C₁₆–C₁₇) binary system in the bulk and in nanopores of controlled porous glasses (CPGs) are investigated using differential scanning calorimetry (DSC) and temperature-dependent powder X-ray diffraction (XRD). As reported, bulk C₁₆–C₁₇ mixtures form two intermediate phases with monoclinic (M_dci) and orthorhombic (O_p) structures in a low temperature range and rotator phase R_I in high temperature range. In the pores of CPG (300 nm), these solid phases appear in the temperatures slightly below the bulk system. The R_I one-phase region extends a bit in temperature range in the mixtures of high C₁₇ components. In CPG (8.1 nm), the solid phases come up in much lower temperatures than the bulk. The rotator R_I is found in a larger range of the temperature and composition than the bulk and in CPG (300 nm). A new phase, the rotator R_II, is observed in a narrow temperature range of high C₁₇ composition just before melting. XRD analysis shows diffractions from the alkane layered ordering in CPG (300 nm) with a more roughened layer surface than the bulk. Lamellar stacking of the solids in CPG (8.1 nm) is heavily perturbed. The variation in the polymorphism of C₁₆–C₁₇ mixtures in the nanopores is associated with the reduced ordering of the molecule arrangements. These results can help to understand the influence of confinement effect on binary mixtures of alkanes, the chain molecules.