Phase Behaviors Of N-C₁₂-C₁₆,C₁₆-C₁₈,C₁₆ Under Nanoconfinement

Phase behavior of nanoconfinement n-alkanes in porous materials is significantly influenced by the pore size, interface interaction and pore geometry. The existing state, phase change temperature and enthalpy of pure and binary mixtures under different size, interface interaction and pore geometry are the key points of understanding the thermodynamic properties of the confined n-alkanes. Ordered mesoporous silicates( SBA-15, KIT-6),carbon-walled silicates(C-SBA-15) and controlled porous glasses(CPGs) with different pore sizes act as the media for adsorbing n-alkanes, thereby the gradient interface interactions can be formed between pore wall and the molecules in weak, moderate and strong strength.For pure n-alkanes and the representative mixture, the melting and freezing temperatures and the enthalpies during the transitions have be measured confined in pores of 3 ? 300 nm at different pore geometry and interaction modes by low temperature DSC method. Combined with XRD diffraciton patterns for crystlline structure of alkanes freezing layer may be deduced. The global phase diagram of pure alkanes can be obtained in respect to different size and interface interactions. For binary alkanes of 3 nm to microns scale, phase diagram under confinement have be determined under different size and interface interactions. The evolution of the binary phase diagram of n-alkane systems from the simple type under nanoconfinement to that of the bulk system have be studied. The research may help understanding the basic thermodynamic properties for low dimensional fluids and provides useful basic thermodynamics data for related research fields such as adsorption, condensation, freezing,transport and nanomaterials fabrication. The main work of this study is as follow:(1) Phase behaviors of dodecane?hexadecane(n-C12H26?C16H34, C12?C16) binary mixtures in bulk and confined in SBA-15(pore diameters 3.8, 9.5, and 17.2 nm)are investigated using differential scanning calorimetry(DSC). According to the thermal analysis, the bulk mixtures belong to a system of partial miscibility with two solid solutions and a eutectoid in the range of mole fraction xC16 = 0.1?0.8. Under confinement, phase behavior of C12?C16mixtures is distinct from the bulk. In comparison with those of chain length difference of pure components of two carbon atoms or less, C12?C16mixtures exhibit different phase behavior not only in the bulk but that of the confined state. The new phase behaviors are discussed with respect to confinement effect and influence of chain length difference.(2) Solid–liquid phase behaviors of hexadecane–octadecane(n-C16H34–C18H38, C16–C18)system in the bulk, and confined in controlled porous glass(CPG)are investigated using DSC and temperature-dependent powder X-ray diffraction(XRD). 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. These new phase behaviors of the confined alkanes could be attributed to the size effect and the interface interactions.(3) Confinement effect on phase behaviors of even normal alkane hexadecane(n-C16H34,C16)under different porous materials are investigated. In the cooling and heating processes,phase transitions of C16 absorbed in SBA-15(7.8 and 17.2 nm), CPG(8.1 and 300 nm),C-SBA-15(15.6 nm)(with carbon film), and KIT-6(8.6 nm)were scanned using DSC and XRD. The melting points of C16 in the nanopores are depressed, which varied largely with the pore size and the nature of the pore wall, the polar or none-polar. Inside CPG(300 nm), C16 shows only the triclinic phase as the bulk. While in SBA-15(7.8 nm), CPG(8.1 nm), and C-SBA-15(15.6 nm)rotator phase RI and RII occurred on cooling and heating, except that RII was not observed on heating in CPG(8.1 nm). The presence and the existing temperature range of these new rotator phases RI and/or RII(as stable or metastable)forms are associated with the pore diameter, interface interaction and pore geometry.