J Therm Anal Calorim (2013) 113:1297–1302 DOI 10.1007/s10973-012-2928-8

Phase behavior of –tridecane mixtures confined in SBA-15

X. Yan • T. B. Wang • H. R. Pei • L. P. Wang • X. Z. Lan

Received: 30 October 2012 / Accepted: 20 December 2012 / Published online: 17 January 2013 Akade´miai Kiado´, Budapest, Hungary 2013

Abstract Phase behavior of dodecane–tridecane (n-C12H26– performed on confined binary mixtures. In an early exper- C13H28,C12–C13) mixtures in bulk and confined in SBA-15 iment, CaCl2–H2O mixtures confined in silica gel (pore have been investigated using differential scanning calorimetry. diameter, 15 nm) shows a same eutectic system with the

Bulk C12–C13 system has a complicated behavior due to special bulk, while phase diagram is shifted down by 10–30 K [11]. rotator phase. It has been found that phase diagram of C12–C13/ A similar phenomenon was observed in C6H5Br–CCl4 SBA-15 (3.8 nm) system is a straight line, C12–C13/SBA-15 system adsorbed in CPGs with pore diameter of 7.5 nm (7.8 nm) a curve, and C12–C13/SBA-15 (8.9, and 17.2 nm) a [12]. Recently, pore size has shown dramatic effect on loop line. The growth of the phase diagram shows size effect on phase behavior of n-C12H26–C14H30, n-C14H30–C16H34, phase behavior of C12–C13 mixtures. Moreover, in the range of and n-C11H24–C12H26 mixtures inside pores of SBA-15 3.8–17.2 nm melting temperatures of pure C12 and mixtures at [13–15]. At present, the influence of size and interface mole fractions xC13 = 0.1–0.5 have linear relation with inverse interaction on binary fluids is still not fully understood. pore diameter, while C13 and mixtures at xC13 = 0.6–0.9 Normal products are important in practical use. In present curved lines. The confined mixtures show a similar theoretical study, they are suitable models in research of melting behavior with pure C12,C13. complex crystallization behavior of polymer materials, sur- factants, lipids, etc. [16, 17]. In this family, dodecane (C12) Keywords Confinement Dodecane–tridecane mixtures and tridecane (C13)havemediumsizecarbonchainsandsame Size effect Phase diagram crystal structures [13, 18]. Their crystals have a triclinic unit cell belonging to space group P1( Z = 1) with dimensions ˚ (a = 4.28, b = 4.81, c = 17.32) A (C12)[18], (a = 5.104, ˚ Introduction b = 7.507, c = 36.808) A (C13)[18–23], respectively. C12– C13 bulk system shows complicated phase behavior [24]. C12 Freezing and melting of fluids confined in nanopores is and C13 are only partial miscible in solid states. important from theoretical as well as practical point of view SBA-15 is a new type of mesoporous material with [1–4]. Properties of pure liquids under confinement have cylindrical channels arranged in two-dimensional hexagon. been extensively investigated and a lot of new physical It has potential applications in many fields. The pores of phenomena have been found [5–7]. Gubbins and coworkers SBA-15 are ordered and tunable with narrow size distri- point out that melting temperature of small size pure liquid butions. Therefore, SBA-15 is an ideal adsorbent in the is driven by the ratio of wall/liquid and liquid/liquid inter- study of confinement effect on fluids [25]. action [8–10]. In the meantime, few studies have been In this paper, we propose an investigation of the phase

behavior of C12–C13 mixtures in bulk and confined in SBA-15 (pore diameters 3.8, 7.8, 8.9, and 17.2 nm) using DSC. The influence of size effect on phase behavior was investigated. The & X. Yan T. B. Wang H. R. Pei L. P. Wang X. Z. Lan ( ) results show that solid–liquid phase diagram of C –C / College of Chemistry and Materials Science, Shandong 12 13 Agricultural University, Taian 271018, Shandong, China SBA-15 depends on the pore size. Moreover, the alkane mixture e-mail: [email protected] inside pores of SBA-15 behaves seemingly as confined C12,C13. 123 1298 X. Yan et al.

Experimental Results and discussion

Materials Phase behavior of bulk C12–C13,C12–C13/SBA-15 (3.8, 7.8, 8.9, and 17.2 nm) Dodecane (mass fraction purity C0.98), tridecane (mass fraction purity C0.98), and tetraethyl orthosilicate (TEOS, Phase behavior of bulk C12–C13 system was examined mass fraction purity 0.999) were purchased from Aladdin using differential scanning calorimetry to compare with the Reagents Co., Shanghai. 1, 3, 5-Triisopropylbenzene (TIPB, confined system. DSC curves of the mixtures at various mass fraction purity 0.97) was bought from Xiya Reagents mole fractions of tridecane (xC13) are shown in Fig. 1. The Co., Chengdu. Triblock copolymer Pluronic P123 was a multi-peak curves of the mixtures indicate a complicated product of Sigma. All chemicals were used as obtained behavior of the bulk. On basis of the DSC measurements, without further purification. solid–liquid phase diagram of C12–C13 system was estab- lished according to ‘‘shape factor method’’ [29], shown in Fig. 2, which closely is reproduced the results in Ref. [24]. Synthesis and characterization of SBA-15 Phases in the phase diagram are marked according to the literature [24]. The bulk system shows two invariants, a SBA-15 of pore size 3.8, 7.8, 8.9, and 17.2 nm was syn- peritectic and a eutectic point, which is related to the thesized according to literature methods, in which triblock special rotator phase of carbon chains in normal . copolymer Pluronic P123 was chosen as a template and In Fig. 3a, DSC scans show melting of C12–C13 mix- TIPB as a micelle expander for large pore SBA-15 tures confined in SBA-15 of pore size 3.8 nm, the smallest (17.2 nm) [26, 27]. After degassed at 433 K for 6 h, SBA- in the four mesopores. Only one endothermic melting peak 15 powder was measured at 77 K using an Autosorb-1 was observed in every composition, including C13 which system [28] for nitrogen adsorption and desorption iso- possesses a solid–liquid (s–l) and solid–solid (s–s) phase therms. Transmission electron microscopy (TEM) images transition in the bulk. To confirm these phenomena, part of were recorded on a JEM-1400 (JEOL) operated at 120 kV. the samples had been cooled down about 80 K below the Before imaging, SBA-15 powder was dispersed in ethanol corresponding melting point; however, no more thermal through sonication and then deposited on a carbon-coated anomaly was found. The experimental phase diagram of copper grid. Surface area, pore diameter, pore volume, and C12–C13/SBA-15 (3.8 nm) system drawn according to the TEM of the synthesized SBA-15 had been described in onset points of each melting peak was shown in Fig. 3b. In detail previously [14, 15]. the diagram, the s–l boundary is fitted to a straight line from the data, which increases from *228 K (C12/ SBA-15) to *239 K (C13/SBA-15) with nearly equal step, *1 K per DxC13 = 0.1. According to classification of Sample preparation and DSC measurements phase diagrams in bulk system, this is a system of miscible

SBA-15 powder with a mass of about 10 mg were put in a glass -1 x = 1 tube and outgassed at 423 K under a vacuum of 10 Torr for C13 0.9 2h.ThenacertainamountofC12,C13, or their mixtures in a small bottle was transferred into the tube at pure nitrogen 0.8 atmosphere using the commercial glass capillary. The volume 0.7 of alkanes was taken as 90–100 % the pore volume of SBA-15 0.6 in order to avoid the interference of excess liquid. At last, the 0.5 glass tube was sealed and equilibrated at room temperature for 0.4 *6h. 0.3 SBA-15 with the alkane adsorbed was transferred into Endo Heat flow/mW 0.2

DSC aluminum pan and sealed immediately. The sample 0.1 was analyzed on a TA DSC Q10 under a high purity 0 nitrogen atmosphere at a scanning rate of 5 K min-1. The temperature scale was calibrated by high purity indium and water for DSC Q10. The mass of samples for DSC analysis 220 240 260 280 were about 1–3 mg and measurement at least repeated T/K three times. In most cases the melting temperature was Fig. 1 DSC curves of C12–C13 bulk system at different mole reproducible to within 0.5 K. fractions of C13 (xC13) 123 Phase behavior of dodecane–tridecane mixtures in SBA-15 1299

252 270 x = 1 [R1, L] C13 [L] 0.9 [T , L] 12 0.8 260 0.7 248 0.6 [R1] L 0.5 /K T /K 250 0.4 T 0.3 [T12, R1] 244 Endo Heat flow/mW 0.2 S 240 [T ] 12 [R1, O13] 0.1 0

[O13] a b 230 240 200220 240 260 280 300 0.0 0.2 0.4 0.6 0.8 1.0 [T12, T13] T/K Mole fraction of tridecane 0.0 0.2 0.4 0.6 0.8 1.0

Mole fraction of tridecane Fig. 4 a DSC curves of C12–C13 system confined in SBA-15 (7.8 nm) as a function of mole fractions of C13 (xC13). b Experimental Fig. 2 Experimental solid–liquid phase diagram of bulk C12–C13 phase diagram of C12–C13 system confined in SBA-15 (7.8 nm) system. T represents triclinic crystal, R rotator phase, O orthorhombic system

x = 1 240 C13 L x = 1 C13 0.9 0.9 0.8 240 0.8 L 0.7 0.7 236 0.6 S1

0.6 /K 0.5 T 0.5 /K 220 T 0.4 0.4

Endo Heat flow/mW 0.3 232 0.3 S 0.2

Endo Heat flow/mW 0.2 0.1 200 S 0.1 a 0 b 2 0 228 a b 200220 240 260 280 0.0 0.2 0.4 0.6 0.8 1.0 200220 240 260 280 0.0 0.2 0.4 0.6 0.8 1.0 T/K Mole fraction of tridecane T/K Mole fraction of tridecane Fig. 5 a DSC curves of C12–C13 system confined in SBA-15 Fig. 3 a DSC curves of C12–C13 system confined in SBA-15 (pore (8.9 nm) as a function of mole fractions of C13 (xC13). From 200 to diameter 3.8 nm) as a function of mole fractions of C13 (xC13). 230 K, the solid–solid transitions are enlarged as dashed lines, next to b Experimental phase diagram of C12–C13 system confined in SBA-15 the main DSC curves at xC13 = 0.3–1 in aid of observation. (3.8 nm) b Experimental phase diagram of C12–C13 system confined in SBA- 15 (8.9 nm). Open circle experimental melting points at mole fractions xC13, filled circle solid–solid transition temperatures both in solid and liquid state. From Figs. 2 and 3b, C12–C13 mixtures under confinement melt in much lower tempera- C12–C13/SBA-15 (7.8 nm) system is lifted *10–12 K tures than those indicated by solidus line in the bulk above C12–C13/SBA-15 (3.8 nm). system. DSC scans of C12–C13/SBA-15 (8.9 nm) system are When the pore size becomes 7.8 nm, C12–C13 mixtures shown in Fig. 5a. The endothermic peaks in the temperature adsorbed in SBA-15 displays a similar behavior to that in region of higher than 240 K correspond to melting of pore pores of 3.8 nm; only one melting peak emerges as shown alkanes. In the low temperature range, some weak thermal in Fig. 4a in all compositions. Experimental phase diagram anomalies are observed among mixtures of xC13 = 0.3–1, of C12–C13/SBA-15 (7.8 nm) system was decided by onset which are enlarged as dashed lines close to the main curves in temperatures of the peaks shown in Fig. 4b, fitted as a aid of observation. They are ascribed to some kinds of s–s curved line. In the diagram, melting point begins at phase transitions. This type of s–s transition has been found

*241 K (C12/SBA-15) and goes higher with an average in C12–C14/SBA-15 (15.9 nm) [13], C14–C16/SBA-15 step *1 K per DxC13 = 0.1. As mentioned above, this kind (17.2 nm) [14], and C11–C12/SBA-15 (17.2 nm) [15] sys- of diagram also means a system of completely miscible both tems. However, they have never been observed in such a in solid and liquid state according to classification of phase small size of SBA-15 (8.9 nm), including C13/SBA-15 diagrams of bulk system. Comparing the melting points, (8.9 nm). Experimental phase diagram is constructed from 123 1300 X. Yan et al. onset temperatures of the s–l and s–s transitions, shown in complicated shape with the pore size increasing. Herein,

Fig. 5b. In the diagram, pore C12 melts at *243 K and the the behavior of pore C13 may partially reflect the property melting point of mixtures increase with a step of *0.3 K per of the whole system. Although bulk C13 has an s–l and s–s DxC13 = 0.1. This increment is smaller than those confined phase transition at different temperatures, only s–l transi- in SBA-15 (3.8, 7.8 nm). Mixtures in the region of tion is observed for C13 confined in SBA-15 of pore size xC13 = 0–0.3 do not show s–s transitions, which might result B7.8 nm, indicating that the monoclinic to cubic (s–s) from incomplete crystallization [7]. The s–s transition tem- transition suppressed. When the pore is up to 8.9 nm or peratures increase from *199.5 K (xC13 = 0.3) to xC13 = 1 larger, the s–s transition emerges for pore C13. This means with an average step of *1.5 K per DxC13 = 0.1, larger than a critical size of crystal should be in the range of those in the corresponding melting process. 7.8–8.9 nm for the presence of C13 s–s transition. Mean- As the pores of SBA-15 expand to 17.2 nm, the confined while, a similar phase behavior is found in the confined

C12–C13 mixtures show similar behavior as that in pores of C12–C13 mixtures: in the size of 7.8 nm or less, only s–l SBA-15 (8.9 nm). In Fig. 6a, DSC scans show the melting melting is observed; in the pores of 8.9 nm or larger, s–s of pore alkanes in higher temperature region while s–s transitions come up in most of the compositions in addition phase transitions come out in compositions of xC13 = to the s–l melting. By Figs. 5a and 6a, s–s transitions cover 0.2–1, enlarged in dashed lines close to the complete a wider xC13 range inside the larger pores, e.g., from curves. Experimental phase diagram is determined from the xC13 = 0.3–1.0 (8.9 nm) to 0.2–1.0 (17.2 nm). The corre- onset points of s–l and s–s phase changes as shown in sponding s–s transition temperatures are also lifted among

Fig. 6b. Melting of the confined system begins from C12/ them. The above evolution of phase behavior should be SBA-15 (17.2 nm) at *249 K and gains a total increment correct considering a normal diagram achieved in bulk of *10 K up to xC13 = 1. The s–s phase transition covers a scale, showing all the specific characteristics of rotator temperature range of *213–233 K in xC13 = 0.2–1, which phase in the system [24]. is wider than that in SBA-15 (8.9 nm) (xC13 = 0.3–1). The cross section of pure normal alkanes (r) is around Compared with C12–C13/SBA-15 (8.9 nm) system, s–l and 0.49 nm in the most stable configuration [22]. Therefore, s–s boundary in C12–C13/SBA-15 (17.2 nm) locates in SBA-15 of pore diameter (3.8, 7.8, 8.9, and 17.2) nm has a higher temperature region with an average gap of 9 and 14, channel of about 8, 16, 18, and 35r in radial direction. respectively. Probably, alkane molecules in pores 3.8 and 7.8 nm could partially crystallize on cooling in consideration of a crite-

Comparison of the phase behavior of C12–C13/SBA-15 rion of 10–15r for complete crystallization [7, 30]. The (3.8, 7.8, 8.9, and 17.2 nm) alkane in pores of size 8.9 and 17.2 nm could crystallize completely in consideration of a size of 20 nm for com- From Figs. 3b, 4b, 5bto6b, the phase diagram of the plete crystallization [7]. In any reason, the difference in the confined C12–C13/SBA-15 system varies from a simple to a size of alkane leads to distinct phase behavior among them.

xC13 = 1 0.9 255 L Melting points of C12,C13 and the mixtures 0.8 as a function of pore size 0.7

0.6 240 In Fig. 7, melting points (T ) are plotted as a function of S1 m,d 0.5 pore diameter for C12,C13, and their mixtures confined in

0.4 /K T SBA-15 (3.8, 7.8, 8.9, and 17.2 nm). The depression of 0.3

Endo Heat flow/mW 0.2 225 melting point of C12, C13 may be understood qualitatively 0.1 on basis of the work of Gubbins and coworkers (Eq. 1) 0 [7, 9, 10] and a modified G-T equation [31] (Eq. 2): S2 a b 210 200220 240 260 280 300 0.0 0.2 0.4 0.6 0.8 1.0 Ttr f ðH ; aÞð1Þ T/K Mole fraction of tridecane 2VTm;bulkðccw clwÞ Fig. 6 a DSC curves of C –C system confined in SBA-15 DTm ¼ Tm;bulk Tm;r ¼ ð2Þ 12 13 rDHm (17.2 nm) as a function of mole fractions of xC13. From 210 to 246 K, the solid–solid transitions are enlarged as the dash lines, next to the Eq. (1) describes phase transition (freezing/melting) main DSC curves at xC13 = 0.2–1. b Experimental phase diagram of temperatures T as a function of the reduced pore size C –C system confined in SBA-15 (17.2 nm). Open circle exper- tr 12 13 H* = H/r and the ratio of the wall/fluid (wf) to the fluid/ imental melting points at mole fractions xC13, filled circle solid–solid transition temperatures fluid (ff) interactions a * Cqwewf/eff, where H is the pore 123 Phase behavior of dodecane–tridecane mixtures in SBA-15 1301

width, r is the diameter of pore liquid molecule, qw and e where the pore diameter d of SBA-15 is 3.8–17.2 nm; units -1 are the density of wall atoms and the potential well depth, of C1 and C2 is K and K nm , respectively. As for C12, and C is a constant depending on the wall geometry, C1 = 254.22, C2 = 98.79, R & 0.9992. For compositions respectively. In Eq. (2), Tm,bulk represents bulk melting of xC13 = 0.6–1, Tm,d of the mixtures (xC13 = 0.6–1) and point; Tm,r is melting point of particle with radius r; Vm C13 can be fitted by the following expression (4): represents the crystal molar volume; ccw and clw are C2 C3 crystal-wall and liquid-wall interaction energy; DH is the Tm;d ¼ C1 þ ð4Þ m d d2 enthalpy of fusion of the crystal. As it is known, SBA-15 -1 -2 comprises silica tetrahedrons with nearly neutral or weakly where units of C1, C2 and C3 is K, K nm and K nm , polar pore wall because of surface hydroxyl groups. When respectively. In the case of C13: C1 = 266.77, C2 = SBA-15 adsorbs the nonpolar alkane molecules, an 139.34, C3 = 128.787, R & 0.9998. intermediate strength interaction exists between the pore Melting points of C12–C13/SBA-15 (3.8, 7.8, 8.9, and wall and alkane. In this case, the value of a in Eq. (1) is less 17.2 nm) as a function of composition than 1, i.e., a \ 1, resulting in depression of melting points The experimental melting temperatures of C12–C13/ SBA-15 (3.8, 7.8, 8.9, and 17.2 nm) systems, indicated for C12,C13 confined in SBA-15 [9, 10]. Meanwhile, this in Fig. 3b, 4b, 5b, and 6b, are fitted to the following also means ccw is larger than clw, i.e., ccw [ clw, which leads to the depletion of transition temperatures too, expressions: determined by Eq. (2). x : A a. C12–C13/SBA-15 (3.8 nm) system: Tm;d ¼ x Tm;dþ In the size range of 3.8–17.2 nm, T of the confined m,d ð1 xÞ: TB ; alkanes shows a linear relation with 1/d in x = 0–0.5, a m;d C13 x A b. C12–C13/SBA-15 (7.8 nm) system: T ¼ x T þ curved line in xC13 = 0.6–1. In both cases, Tm,d of the P m;d m;d B i alkanes increases with the increase of xC13 at a same pore ð1 xÞTm;d aix ; size, and with d for a given composition xC13, just as the a1 = 4.7365, a2 =-8.1303, a3 = 1.1687, pure C12 or C13 does. In this sense, melting behavior of 2 a4 = 2.2560, R = 0.9967. C12–C13 mixtures is indeed like the pure alkane qualita- tively in the above size range. c. C –C /SBA-15 (8.9 nm) system: Tx x TA 12 13 P m;d ¼ m;dþ Tm,d of C12 and the mixtures (xC13 = 0–0.5) may be B i ð1 xÞTm;d aix ; fitted as a function of d by the following Eq. (3): a1 = 0.5954, a2 =-1.3621, a3 =-3.0564, C 2 T ¼ C 2 ð3Þ a4 = 3.7936, R = 0.9357. m;d 1 d d. C –C /SBA-15 (17.2 nm) system: Tx x TA 12 13 P m;d ¼ m;dþ B i ð1 xÞTm;d aix ;

a1 = 10.7851, a2 =-23.6346, a3 = 10.8971, 260 2 a4 = 2.0147, R = 0.9924.

x A B where Tm;d, Tm;d,andTm;d represent melting point of the mixture 250 (x), pure C13 (A) and C12 (B) confined in SBA-15 (d); x is the A B mole fraction of C13; i = (1, 2,…, n). Obviously, T and T /K m;d m;d T are 239.03 and 228.39 K for SBA-15 (3.8 nm) system; 250.95 240 and 241.10 K for SBA-15 (7.8 nm) system; 252.88 and 243.21 K for SBA-15 (8.9 nm) system; 259.10 and 248.77 K for

SBA-15 (17.2 nm) system. At the same time, coefficients ai in 230 i Rai x were determined by fitting deviation of melting point of the mixture to the straight line defined by the first two terms. From 0.00 0.05 0.10 0.15 0.20 0.25 1/d (nm–1) these expressions, melting points of the confined mixtures can be easily estimated in this system.

Fig. 7 Melting points of pure C12 and C13,C12–C13 system (xC13 = 0.1–0.9) confined in SBA-15 with variation of pore diameter d = (3.8, 7.8, 8.9, and 17.2) nm. Filled square C12, filled circle C13. Conclusions Mole fraction xC13: open square (0.1), filled up pointing triangle (0.2), filled down pointing triangle (0.3), filled diamond (0.4), filled left pointing triangle (0.5), filled right pointing triangle (0.6), open circle Physical size has a great influence on phase behavior of (0.7), star (0.8), open up pointing triangle (0.9) C12–C13 binary mixtures confined in pores of SBA-15. In 123 1302 X. Yan et al. the scale of 3.8–17.2 nm, the solid–liquid phase diagram 14. Pei HR, Yan X, Liu WB, Lan XZ. Phase behavior of - grows from a straight line (3.8 nm), a curve line (7.8 nm) mixtures confined in SBA-15. J Therm Anal Calorim. doi:10.1007/s10973-012-2557-2. to a loop line comprising a curved solid–liquid and solid– 15. Yan X, Pei HR, Wang TB, Liu WB, Lan XZ. Phase behavior of solid boundary (8.9 and 17.2 nm). Melting temperatures of -dodecane mixtures confined in SBA-15. E-J Chem. C12, C13, and the mixtures confined in SBA-15 show a Article ID 476236. simple relation in DT versus 1/d. The mixtures of C and 16. Fu D, Liu Y, Liu G, Su Y, Wang D. Confined crystallization of m 12 binary n-alkane mixtures: stabilization of a new rotator phase by C13 have a similar melting behavior as that for pure C12, enhanced surface freezing and weakened intermolecular interac- C13 under confinement, varying with mole fraction and tions. Phys Chem Chem Phys. 2011;13(33):15031–6. pore diameter of SBA-15. 17. Fu DS, Liu YF, Gao X, Su YL, Liu GM, Wang DJ. Binary n-Alkane mixtures from total miscibility to phase separation in Acknowledgements We thank the financial support from National microcapsules: enrichment of shorter component in surface Natural Science Found of China (no. 21273138) and Natural Science freezing and enhanced stability of rotator phases. 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