Size-Dependent Phase Behavior of the Hexadecane-Octadecane System Confined in Nanoporous Glass

Solid liquid phase behaviors of the hexadecane-octadecane (n-C16H34-C10H30, C-16-C-18) system in the bulk, and confined in controlled porous glass (CPG) are investigated using differential scanning calorimetry and temperature-dependent powder X-ray diffraction. Bulk C-16-C-18 mixtures show the special rotator phase R-I, and two intermediate phases with monoclinic (M-dcp) and orthorhombic (O-p) structures. Inside CPG (8.1 and 300 nm), the R-I, and O-p phases could be found in wider compositions of the solid mixtures than the bulk, however the M-dcp form was not observed. Unexpectedly, under the confinement of CPG (8.1 nm) not only the rotator R-I appeared in the two pure components, C-16 and C-18, but a rotator phase R-II occurred in the pure C-18 and most of the mixtures upon heating. These new phase behaviors of the confined alkanes could be attributed to the size effect and the interface interactions. According to XRD analysis, in the small pores of CPG (8.1 nm) the alkane molecules effectively take the 2D arrangements. While in the large pores of CPG (300 nm), they take ordered packing to some extent both in the lamellar and lateral directions.