Physical Control of Phase Behavior of Hexadecane in Nanopores

Abstract

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-C 16 H 34 , 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 nm), 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 I 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 nm) 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.

Polymorphism of a hexadecane–heptadecane binary system in nanopores

“…In a large composition between the T p and the O i one-phase region, there exist the O p or M dci phase or their mixture, which is difficult to discern them due to the same reection patterns in the small pores. At T ¼ À (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Clearly, there is a signicant changing in the phase behaviour of the C 16 -C 17 system in CPG (8.1 nm) in reference with the bulk. From the absence of (00l) reections in the high and low temperature domains, the layered ordering of the arrangements of the alkane molecules in CPG (8.1 nm) has been heavily perturbed.…”

Rotator phases in hexadecane emulsion drops revealed by X-ray synchrotron techniques

“…[1][2][3] Formation of rotator phases (denoted hereafter by R) has been observed experimentally in alkanes with chain lengths between 16 and 50 Catoms in bulk, in surface layers and/or in various micro-and nanoconfinements. [9][10][11][12][13][16][17][18][19][20][21][22][23][24] The molecules in the rotator phases are characterized by limited positional freedom, while being able to rotate and/or oscillate around their long axis. [9][10][11] Three types of rotator phases were defined in the literature, depending on their stability: thermodynamically stable R phases are observed in a certain temperature range on both cooling and heating; thermodynamically metastable R phases exist in a certain range only upon cooling, whereas they are not observed upon heating; the least stable, transient rotator phases are observed only upon cooling for a very short period of time.…”

Toward Tailoring Chemistry of Silica-Based Phase Change Materials for Thermal Energy Storage

“… (B) Scheme of polymorphism of hexadecane in SBA-15, CPG, C-SBA-15, and KIT-6. Adapted with permission from ( Wang et al., 2015 ). Copyright 2015, American Chemical Society.…”

“…In addition, the hydrogen bonding between PCMs and SiO 2 can adjust the strength of the interfacial interactions, and thus affect the phase change behaviors. Wang et al (2015) also systematically studied the influence of different pore sizes of SiO 2 on the thermal property of hexadecane, including SBA-15 (7.8 and 17.2 nm), controlled porous glass (CPG) (8.1 and 300 nm), carbon-coated SBA-15 (C-SBA-15, 15.6 nm), and KIT-6 (8.6 nm), as shown in Figure 7B. Surprisingly, hexadecane exhibits an only triclinic phase in (Mitran et al, 2015).…”