Physical Control of Phase Behavior of Hexadecane in Nanopores

As an important and interesting question, the delicate influence of confinement effect on properties of fluids in nanopores still needs deeper understanding. To this regard, phase behaviors of normal hexadecane (n-C16H34, C-16) absorbed in porous materials are investigated. Phase transitions of C-16 in SBA-15 (7.8 and 17.2 nm), CPG (8.1 and 300 urn), C-SBA-15 (15.6 nm) (carbon coated), and KIT-6 (8.6 nm) were scanned using differential scanning calorimetry (DSC) and temperature-dependent powder X-ray diffraction (XRD) in cooling and heating processes. The bulk C-16 is known to form a transient rotator phase R-1 on cooling. In this work, freezing and melting points of C-16 in the nanopores are found to be depressed. C-16 in the large pores of CPG (300 urn) only shows a triclinic phase. In the pores of diameters d = 7.8-17.2 nm, C-16 exhibits stable or metastable rotator phase R-I or R-II. The stabilization of the rotators by nanoconfinement is more obvious in smaller pores or with stronger pore surface interactions due to the significant contributions of interface energy of the nanosized crystals. From structural analysis, hexadecane solid in the large pores retains a complete lamellar structure, whereas the packing of molecules is heavily disturbed in the smaller pores (d < 20 nm). The phase information on the pore C-16 can help understanding the influence of size effect, interface interactions and pore geometry on its physical properties.