Mesoscopic Phase Behavior of Tridecane–Tetradecane Mixtures Confined in Porous Materials: Effects of Pore Size and Pore Geometry

Phase behaviors of tridecane–tetradecane (n-C13H28–C14H30, C13–C14) system in bulk, and confined in SBA-15 and controlled porous glass (CPG) are investigated using differential scanning calorimetry and temperature-dependent X-ray diffraction. Bulk C13–C14 mixtures exhibit a complicated behavior featuring a special rotator phase RI. Adsorbed in SBA-15 with pore diameters (d) of 3.8 and 7.8 nm, the binary mixtures display a melting boundary of a straight line and a curve, respectively. Within SBA-15 (17.2 nm) and CPG (8.1, 31.8, 46.4, and 300 nm), the mixtures show a similar phase behavior to the bulk, especially in the larger pore CPG than 30 nm. Under confinement, the phase behavior of C13–C14 mixtures varies with the pore size as well as the temperature and composition. XRD analysis reveals that the solid alkane molecules take effectively the 2D closed-packed arrangements inside pores of diameters less than 20 nm. In the large pore CPG (d > 30 nm), the alkane molecules regain lamellar ordering in solid states. Pore geometries of SBA-15 and CPG, one-dimensional channels vs three-dimensional connected pores, may also result in much different influence on the phase behavior of the confined mixtures.