Physical Control of Phase Behavior of Hexadecane in Nanopores

Article pubs.acs.org/JPCC

Physical Control of Phase Behavior of Hexadecane in Nanopores Li Ping Wang, Jian Sui, Min Zhai, Fang Tian, and Xiao Zheng Lan* College of Chemistry and Materials Science, Shandong Agricultural University, Tai’an 271018, Shandong China 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-C16H34, C16) absorbed in porous materials are investigated. Phase transitions of C16 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 C16 is known to form a transient rotator phase RI on cooling. In this work, freezing and melting points of C16 in the nanopores are found to be depressed. C16 in the large pores of CPG (300 nm) only shows a triclinic phase. In the pores of diameters d = 7.8−17.2 nm, C16 exhibits stable or metastable rotator phase RI or RII. 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 C16 can help understanding the influence of size effect, interface interactions and pore geometry on its physical properties.

1. INTRODUCTION Nanosized confinement can profoundly influence the physical properties of fluids and thus gains much attention in the fields of adsorption, condensation, intrusion, freezing, and transport in recent years.1−8 Thermal properties of the confined fluids normally are the first to be measured. Mostly, freezing and melting points of small molecule liquids such as water, organic compounds in nanospace are depressed, often predicted by the Gibbs−Thomson equation.9,10 However, melting points of some organic materials could be lifted when strong interface interactions exist between the adsorbed molecules and the wall.11−13 In that case, the transition temperature of the confined fluid may be qualitatively described by an equation in terms of the size, the relative interactions of fluid−fluid to fluid−wall, and pore geometries.14 The existing state of the confined fluid is another interest in this field for better understanding the properties.15,16 It is thought that fluid in nanopores may be in a heterogeneous state in the form of surface layer and inner layer, varying with fluid−wall interactions.17 Polymorphism of some organic compounds such as glycine or acetaminophen could be significantly influenced as confined in anodic aluminum oxide film (AAO).18,19 The occurrence of metastable forms in the nanopores is associated with the nucleation mechanism, competition between different growing crystal species, and free path length.18 Recently, phase behaviors of the normal alkanes in dispersed states or in nanopores are investigated, showing a series of new phenomena.15,16,20 Nanodroplets of alkanes in miniemulsions exhibit new rotator phases and different nucleation kinetics to the bulk.21 A new rotator (RII) was observed when nonadecane © 2015 American Chemical Society

was in microcapsules, or in the pores of CPG (7 nm) in the cooling process.22,23 Rotator RI appears when hexadecane absorbed in CPG (10 nm) was cooled and heated, whereas the RI phase exists only in C14 in CPG (10 nm) on cooling, no rotator in C12 in CPG (10 nm) on cooling and heating.24 In our systematic work, normal alkane binary mixtures show sizedependent phase diagrams and new rotators that are not have in the bulk.25−27 For example, a new phase region RII in a narrow temperature range was found in C16−C18 binary mixtures in CPG (8.1 nm) and the phase region changes with the pore size.25 In the pores of SBA-15 (7.8 and 17.2 nm) or CPG (8.1 nm), the packing of end groups (−CH3) of the alkane lamellar structure was disturbed heavily, as evidenced by the absence of (00l) plane diffractions. In such cases, the alkane molecules effectively take 2D close-packed arrangements.23,25,28 As is known, bulk alkanes possess three unique characteristics of the chain molecules: rich polymorphism, odd−even effect in structures, and surface freezing.29−32 Five rotator phases RI to RV are the important polymorphs of the alkane solids, which may be stable, metastable, or transient.24,33,34 The polymorphs of bulk alkanes change with the chain length, composition, and temperature.30 As the phase behaviors are sensitive to the changing in the size, normal alkanes are suitable model compounds to investigate confinement effect on fluids.20−28 However, the delicate influence of size effect, pore wall attractions, and pore morphology on their phase behaviors is not completely understood. The analysis of the structures of Received: April 18, 2015 Revised: June 28, 2015 Published: July 22, 2015 18697

DOI: 10.1021/acs.jpcc.5b03728 J. Phys. Chem. C 2015, 119, 18697−18706

Article

The Journal of Physical Chemistry C

carbon film can form a graphene cylindrical network on the inner surface of the matrix.37 The pore diameter, pore volume, and specific surface area of SBA-15, C-SBA-15, and KIT-6 were determined on an Autosorb-1 system (Quantachrome Instruments) from nitrogen adsorption branches on the basis of BET and BJH methods at 77 K, presented in Table 1. The pore parameters of SBA-15 (7.8 and 17.2 nm) were reported in details elsewhere.38,39 Transmission electronic microscopy (TEM) was performed on a JEM-1400 (JEOL) operated at 120 kV. Before analysis, a small amount of SBA-15, C-SBA-15, or KIT-6 powder was dispersed in ethanol through ultrasound sonification onto a copper grid. In Figure 1, C-SBA-15 (15.6 nm) and KIT-6 (8.6 nm) display uniform ordered channels under TEM imaging, the same as observed in the other SBA-15.38

the pore solids is still not enough. Combined techniques of thermal analysis and X-ray diffraction can be more efficient in characterization of the fluids in nanopores. In this paper, freezing and melting behaviors of hexadecane in SBA-15 (7.8 and 17.2 nm), CPG (8.1 and 300 nm), C-SBA15 (15.6 nm), and KIT-6 (8.6 nm) are investigated. The porous media provide different surface polarity with silica, silicate, or carbon surface and pore geometry: SBA-15 and C-SBA-15 have ordered 1D channels in hexagonal arrangements connected by micropores; KIT-6 has two sets of ordered interwoven channels; CPGs have 3D disordered connected pores. Bulk C16 can crystallize into a triclinic structure (T16) in a space group P (Z = 1) with unit cell parameters a = 4.29 Å, b = 4.82 Å, c = 22.36 Å, α = 84.57°, β = 67.31°, γ = 72.73°.35 The investigations focus on the complexity of polymorphs and phase transition of hexadecane under nanoconfinement. It is found that C16 shows only the T16 phase in the pores of CPG (300 nm), whereas in the small pores of SBA-15, C-SBA-15, KIT-6, and CPG, the phase behavior of C16 becomes different to the bulk with the appearance of the new rotator phase RI or RII. The interface interactions and pore geometries also present significant influence on these phase sequences.

2. EXPERIMENTAL SECTION 2.1. Materials. Normal alkane hexadecane was purchased from Aladdin Reagents Co. with a purity in a mass fraction larger than 0.98. The starting materials used in preparation of SBA-15 and C-SBA-15 include tetraethyl orthosilicate (0.999, Aladdin Reagents Co.), triblock copolymer Pluronic P123 (Sigma), 1,3,5-triisopropylbenzene (TIPB, 0.97, Xiya Reagents Co.), and 2,3-dihydroxynaphthalene (DN, 0.98, Aladdin Reagents Co.). These chemicals were used as received. Mesoporous KIT-6 powder is a product of Nanjing XFNANO Materials Tech Co., Ltd. Two types of controlled pore glasses (CPGs, Millipore) were used as confinement media with pore sizes of 8.1 and 300 nm, respectively. Before use the CPG powder was cleaned with concentrated nitric acid with a method recommended by the supplier. The treatment was reported to result in negligible influence on the pore diameter and distributions.12 The specifications of the CPGs are listed in Table 1, as provided by the manufacturer.

Figure 1. TEM images of KIT-6 (8.6 nm) (a) and C-SBA-15 (15.6 nm) (b).

The surface morphology of the porous glass CPG (300 nm) coated with gold film was characterized by field-emission high resolution scanning electron microscopy (SEM, Hitachi s4800), as shown in Figure 6 in section 3.5. 2.2. DSC and XRD Measurements. DSC samples of C16 adsorbed in CPG, SBA-15, C-SBA-15, and KIT-6 were prepared as follows. The powder of the porous material with a mass of no less than 10 mg was put into a piece of glass tube and outgassed at 150 °C under a vacuum of 10−1 Pa for at least 2 h. A certain amount of liquid C16 was transferred into the glass tube by a clean glass capillary under protection of a dried nitrogen atmosphere. The alkane introduced occupied slightly less than the pore volume of the porous material. Afterward, the glass tube was sealed on a Bunsen burner. The samples were equilibrated for more than 2 h at room temperatures (∼25 °C), after being stored in a refrigerator for a period of time. The bulk C16 was also posed to such a thermal treatment before analysis. In XRD analysis, the porous material for one sample needs a mass of about 30−50 mg. Thermal analysis of the samples was performed on a DSC Q10 (TA Instruments) under a high purity nitrogen atmosphere. In a typical procedure, the sample was cooled at a rate of around 1 °C min−1 from room temperature down to 30−50 °C below the transition temperature and then was heated with a scanning rate of 5 °C min−1. The temperature scale of the DSC instrument was calibrated using high purity indium, water, and adamantine. In most cases, transition temperatures of the bulk and the pore C16 were reproducible to within 0.5 °C. XRD analysis was carried out on a Philips X’Pert Pro MPD type diffractometer in the same temperature ranges as those for the DSC measurements. The diffractometer uses a Cu Kα (1.54 Å) radiation source at a power of 40 mA/40 kV. The sample was placed in an aluminum vessel with a size of 2 × 1.6 × 1

Table 1. Specifications of Controlled Pore Glasses, SBA-15, C-SBA-15, and KIT-6 product name

mean pore diameter (nm)

pore size distribution (%)

specific pore volume (cm3/g)

specific surface area (m2/g)

CPG75 CPG3000 SBA-15 SBA-15 C-SBA-15 KIT-6

8.1 300 7.8 17.2 15.6 8.6

9 6

0.5 1.1 0.9 1.3 0.6 0.6

>120 10 623.0 306.5 186.6 338.9

Mesoporous materials SBA-15 (7.8 and 17.2 nm) and CSBA-15 (15.6 nm) were synthesized with the reference methods.36,37 For SBA-15, the polymer P123 was used as a template for pore formation and TIPB as a micelle expander for the large pores. The black powder of C-SBA-15 was prepared through thermal pyrolysis of a bonded DN monolayer onto the pore surface of SBA-15 with pore diameter d = 16−17 nm. The 18698


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Figure 2. (a) Paired DSC curves of C16 in the bulk and confined in SBA-15 (7.8 and 17.2 nm), CPG (8.1 and 300 nm), C-SBA-15 (15.6 nm), and KIT-6 (8.6 nm), recorded in heating and cooling processes. (b) Depression of melting point (ΔTm) and supercooling (ΔTH) of the pore C16 as a function of the reverse pore diameter (1/d): in SBA-15 (7.8 and 17.2 nm) (□), CPG (8.1 and 300 nm) (○), C-SBA-15 (15.6 nm) (Δ), and KIT-6 (8.6 nm) (☆). The black dot is the ΔTH of bulk C16. Error bars refer to the mean deviations of corresponding results, no larger than 0.2 °C. The ΔTm value of C16 in CPG (300 nm) is not included due to its different calculation method with the others.

mm3. Diffraction patterns were recorded at a rotating angle of 2θ = 5−40° with steps of Δ(2θ) = 0.02° at selected temperatures during cooling and heating processes at a rate of 1 °C min−1. Before each measurement, the sample was equilibrated for 10 min. The signals of the samples were reproducible at the cooling rate of 1−5 °C min−1 in DSC analysis, and at the cooling and heating rates of 1 °C min−1 in XRD measurements. The Cu Kα backgrounds were subtracted from the raw data in presentation of the diffraction patterns. In Figure 4 panel a, several scans in XRD analysis of C16 in C-SBA-15 (15.6 nm) were performed in the diffraction experimental station (4B9A) of the Beijing Synchrotron Radiation Facility (BSRF), using the six circle diffractometer Huber 5020 and the scintillation crystal detector Huber 9910, with a synchrotron radiation wavelength of 1.54 Å. Data were collected in steps of Δ(2θ) = 0.02° also at the rate of 1 °C in the cooling process.

Table 2. Depression of Melting Point (ΔTm) and Supercooling (ΔTH) of C16 in Nanopores matrix

pore diameter d, nm

ΔTm, °C

ΔTH, °C

bulk C16 SBA-15 SBA-15 C-SBA-15 CPG CPG KIT-6

∞ 7.8 17.2 15.6 300 8.1 8.6

0 12.6 6.1 5.3 1.0 6.7 12.1

1.5 0.8 1.6 1.2 1.6 2.3 0.7

different trend of ΔTm in SBA-15 and CPG can be attributed to the influence of the pore geometry, where the highly connected 3D pores of the porous glasses may provide a larger space than the nominal diameter for the alkane solids.12,14,26 The slight smaller ΔTm value in C-SBA-15 than in SBA-15 is due to the larger pore surface attractions of carbon film than silica to the alkane molecules.10 The ΔTm value in KIT-6 is almost on the line of trend for that in SBA-15, which indicates the influence of dimensions of the pores (1D or 3D) is not prominent in the silica-based media. Figure 2b, bottom panel, shows the supercooling of C16. In reference to the bulk, the pore C16 supercools closely as in CPG (300 nm) and SBA-15 (17.2 nm), though it is lower in SBA-15 (7.8 nm), C-SBA-15, and KIT-6. In CPG (8.1 nm), a slightly higher supercooling is observed. These ΔTH data are much smaller than the large supercooling (22 °C) in miniemulsion drops of C16 (d = 208 nm) whose crystallization was dominated by homogeneous nucleation.21 As the transient rotator favors the freezing of the bulk phase, the pore C16 transforms from melt into the solid phase via the same help of the rotator phase, which results in small supercooling.24,40 Indeed, one or two types of rotator phase occur in the cooling process of the pore C16 as observed in XRD measurements, described in sections 3.3 and 3.4. The above thermal analysis only reveals the melting or freezing of C16 in the bulk and under confinement. In the following sections, new solid−solid transition and structures of the solids will be characterized in temperature-dependent XRD experiments.

3. RESULTS AND DISCUSSION 3.1. Melting Point Depression and Supercooling. Shown in Figure 2a, freezing and melting of C16 in bulk and confined in SBA-15, CPG, C-SBA-15, and KIT-6 were scanned using DSC. During thermal cycling, the samples show exothermic and endothermic peaks with thermal hysteresis, a characteristic of the first-order transition. The pore C16 freezes and melts at lower temperatures than the bulk at each size, with a greater extent as the pore diameter decreases. The pore C16 peaks also become smaller and broader than the bulk. From the thermal analysis, depression of the melting point (ΔTm = Tm,bulk − Tm,pore) and supercooling (ΔTH = Tm − Tf) of the pore C16 are depicted as a function of the reverse pore diameter (1/d), as shown in Figure 2b. Here, ΔTm takes the temperature difference of the onset point of the bulk and peak point of the pore C16 in the heating process, except in CPG (300 nm) both values are the onset temperatures; ΔTH of a sample measures a span of its onset temperature of the melting to the freezing peak. The data measured are listed in Table 2. As seen in Figure 2b, upper panel, ΔTm values of C16 in SBA15 show a linear relation with 1/d, as described by the Gibbs− Thomson equation. The ΔTm point of C16 in CPG lies under those in SBA-15. As discussed in the previous work, this 18699


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(101), (013), and (111) found in the PDF cards.41 In the heating process, the same sample also shows only the T16 structure from the previous results.25 Temporarily, the phase sequence of C16 in CPG (300 nm) may be viewed as the melt to triclinic (L → T) on cooling and T → L on heating. This is similar to the case in the bulk except that a transient RI rotator appears upon cooling of bulk C16 with a lifetime of mostly 10− 20 s.33 Comparing with the bulk, the pore C16 displays somewhat weaker diffraction intensities, while it still keeps the characteristic lamellar structure of the normal alkane, as seen from the reflections of (00l) planes. Considering the large size of 300 nm, it is reasonable to observe the similar DSC response, stable structure, and phase behavior of the pore C16 as the bulk. However, the confinement in CPG (300 nm) can bring about significant change in the phase diagram of C16−C18 binary mixtures in reference to the bulk.25 3.3. Phase Transition of C16 in Midsized Pores of CSBA-15 and SBA-15. The diffraction patterns of C16 in CSBA-15 (15.6 nm) and SBA-15 (17.2 nm) are shown in Figure 4, recorded in cooling and heating processes. With widened peaks and amorphous backgrounds, the triclinic phase of C16 can be recognized from its characteristic peaks with indexes of (010), (011), (012), (013), and (111). The RI phase may be found from its in-plane reflections at 2θ = ∼ 21.14° (110) and 23.14−23.22° (200) on cooling and at 2θ = ∼21.18° (110) and 22.31−22.62° (200) on heating in a temperature range of ∼2− 15 °C, with the (200) plane moving to high and small angles,

3.2. Phase Transition of C16 in Large Pores of CPG (300 nm). In Figure 3, the diffraction patterns of C16 in CPG

Figure 3. Temperature-dependent XRD patterns of C16 in CPG (300 nm), recorded in the cooling process. Here in the figures and afterward, the hump from 2θ = 5.5 to 8.5° is the background of the XRD instrument and the diffractions of melt are not shown. The arrow points to the direction of temperature variation in the measurements.

(300 nm) are recorded in the range 2θ = 5−40° with slow cooling from +15 to −60 °C. All the solids belong to the same triclinic crystalline system as the bulk T16, determined from the characteristic planes of (002), (003), (010), (011), (012),

Figure 4. Temperature-dependent XRD patterns of C16 confined in C-SBA-15 (15.6 nm) and SBA-15 (17.2 nm) recorded in the cooling (a and c) and heating (b and d) processes. In panel a, the diffraction signals by a synchrotron source without humps in 2θ = 5.5−8.5° are inserted at T = −15 to −25 °C. Indexes of the rotator phase RI (●) and T16 (★) are labeled to the representative diffraction patterns. 18700


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Figure 5. Temperature-dependent XRD patterns of C16 in SBA-15 (7.8 nm), CPG (8.1 nm) and KIT-6 (8.6 nm) recorded in the cooling (a, c, and d) and heating (b and e) processes. Indexes of the rotator phase RI (●) and T16 (★) are marked to the representative diffraction patterns.

in a bit disordered packing of the molecules, as evidenced from the absence of (00l) planes, at temperatures near the melting. On further cooling, the transition of the RII to pure RI can be attributed a reduction of rotational freedom of the molecules.30,43,44 Then, the coexistence of the RI and T16 phases reveals that the pore surface attractions occur only in the molecules of the RI phase close to pore wall at those temperatures, whereas the core molecules transform into a more dense state of the triclinic phase than the RI. With decreasing temperature, the RI phase continues to transform into the T16 phase still driven by the more dense packing density of the triclinic form.30 The similar reasoning above can explain the coexistence of the RI and T16, and finally the transition of the RI to T16 of C16 in SBA-15 (17.2 nm). However, the appearance of the pure RII and RI of C16 in CSBA-15 (15.6 nm) but not in SBA-15 (17.2 nm) is ascribed to the stronger attractions of the carbon pore surface than a weakly polar silica to the nonpolar hexadecane molecule. In the heating process followed, the transformation from T16 to the pure RI and then RII of C16 in C-SBA-15 (15.6 nm) again reflects the significant influence of the carbon pore surface on the chain stacking, covering up all the molecules at this stage. The transitions between the pore solids are on the basis of the weakened lamellar coupling and the defects in the interlayer surface or inside the chains already formed in the cooling process.24,45 With the input of thermal energy, the defects in the chains are increased. And the triclinic phase loses its

respectively. In a narrow temperature range, the RII phase in CSBA-15 (15.6 nm) can be observed from a typical single reflection peak at 2θ = ∼ 21.1° (110). Coexistence of RI and T16 phases in SBA-15 (17.2 nm) occurs in a range of ∼10 °C in the thermal cycling. Accordingly, C16 has a stable rotator RI in both the media and a stable RII in the C-SBA-15. The phase sequence of C16 in the C-SBA-15 is L → RII → RI → [RI + T16] → T16 on cooling (panel a) and T16 → RI → RII → L on heating (panel b). The phase sequence of C16 in SBA-15 (17.2 nm) is L → [RI + T16] → T16 on cooling (panel c) and T16 → [RI + T16] → L on heating (panel d). It should be noted that (00l) reflections of C16 in both cases are not observed, which means the lamellar structures are heavily perturbed.23 Upon freezing, the hexadecane molecules in the nanopores cannot stack regularly as in the bulk or in a free environment due to the pore surface attractions in the region near the pore wall.17,23 In the process, the random adsorption (immobilization) of the molecules onto the rough pore surface can bring about an irregular stacking of the molecular chains, which results in the weakly coupled 2D layers as evidenced from the absence of (00l) reflections.23,25,26,28 This somewhat disordered packing of the chain molecules, analogous to a situation of a bulk binary mixture, allows or induces more voids, end-gauche defects, or even kink defects in the middle the chains.25 This just meets the conditions for the formation of rotator RI or RII as known in the bulk alkanes.42,43 Accordingly, it is understandable to see the rotator RII in C16 in C-SBA-15 (15.6 nm) 18701


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this unlikely happens in the straight channels of SBA-15 with small pore size. If it is the case, at least a part of the molecules in CPG tend to possess somewhat ordered arrangements such as in the interlayer regions; however, this trend is against the formation of a pure RI and RII phase.30 The stronger diffraction signals of hexadecane molecules in CPG than in SBA-15 also correspond to the more ordered tiny particles. Moreover, the transition temperatures listed in Table 3 may provide another clue to the effect of the pore geometry: for a same kind of transition, a higher transition temperature in CPG (8.1 nm) than in SBA-15 (7.8 nm) should originate from the more dense or less disordered packing of the molecules, corresponding to a lower energy state. The pore C16 in KIT-6 displays a phase sequence different from those in SBA-15 and CPG (d ≈ 8 nm) although they have the close pore diameters. Especially, C16 in KIT-6 has a large temperature range of coexistence of T16 and RI phases on both cooling and heating. In such small pores, it is reasonable to observe the obvious stabilization effect on RI phase because of the interface interactions. However, it is still not well understood for this difference in the three media, probably associated with the pore geometry and the chain flexibility. Table 3 summarizes the transition temperature (Ttrs) of C16 confined in the six kinds of porous materials on cooling and heating. Note that melting and freezing of the melt phase were measured with DSC and solid−solid transition temperatures were obtained from the XRD scans. 3.5. General Consideration of Confinement Effect. The phase behaviors of the pore C16 in the several porous materials are schemed in Figure 6, panel a. As a feature of the new phenomena, the appearance of rotator phases reflects the influence of the size effect, interface interactions, and pore geometry together with the chain molecule character and thermal excitation.5,10,16,18,23,30 In view of the structure, the hexadecane molecules in the nanopores (d < 20 nm) cannot stack into a regular layered structure as the bulk but with some defects such as in the RI or RII phase (the right in panel a). The influence of the confinement becomes more prominent in the small pores or in carbon filmed C-SBA-15. Meanwhile, the pore geometry is another important factor underlying the new phase behavior of the pore C16. In reference to the RI phase observed in CPG (10 nm),24 two rotators RI and RII occurring in C16 in CPG (8.1 nm) clearly show the size effect of the same matrix, where the stabilization of the rotator phase is more obvious in smaller pores. Moreover, comparing the case of C16 in CPG (8.1 nm) with C14 (a metastable RI) and C12 (no rotator) in CPG (10 nm),24 the influence of the chain length or molecular flexibility on the phase behavior may be estimated, where the stabilization of the rotator phase by the nanoconfinement tends to be more noticeable as the chain length increases. In view of energy, phase transformation may be interpreted as the variation of the relative free energies of the phases involved with temperature. According to Beiner et al., the interface energy Gσ contributes largely to the total free energy G(T,A) on the basis of the bulk phase G∞ in the nanosized system.6 Obviously, the smaller the pore diameter d or the larger interface energy is, the greater the Gσ contributes to the G(T,A) value, and thereby the more likely the phase behavior may be changed. This is qualitatively consistent with the results shown in Figure 6, panel a. Figure 6, panels b−e, show schemes of relative free energies versus temperature of the representative transitions, which

advantage of the packing density of the molecules and gives way to the rotator phase at a small expense of energy for the partially (RI) or fully (RII) rotations of the chains. In SBA-15 (17.2 nm), the partial change of the T16 to RI phase also indicates the heterogeneity of its existing state of the C16 molecules at this stage. As the above two porous media have similar pore geometries, the interface interactions are responsible for the difference in the phase sequence of the pore C16 in them. 3.4. Phase Transition of C16 in Small Pores of SBA-15, CPG, and KIT-6. As seen in Figure 5, the diffraction patterns of C16 in SBA-15 (7.8 nm), CPG (8.1 nm), and KIT-6 (8.6 nm) are recorded in the cooling and then heating process. In such small pores, especially SBA-15 (7.8 nm), the reflection peaks of hexadecane molecules are even wider than in the midsized and large pores.25,26 Here, three phases T16, RI, and RII, can be recognized in the solids. The rotator RII shows its characteristic in-plane reflection of (110) plane at 2θ = ∼20.96°. The RI phase exhibits the typical diffractions of the (110) plane at 2θ = ∼20.96° and the (200) plane at 2θ = 21.93−23.43°, the latter shifting to a higher angle on cooling and to the lower angle on heating. The triclinic phase displays the reflections of (010), (011), (012), (013), and (111) planes. Coexistence of RI and T16 phases appears in a temperature span of ∼25, 15, and 40 °C on cooling in the SBA-15, CPG, and KIT-6, respectively. Stable RI or RII occurs to C16 inside the three media. The pore C16 in SBA-15 (7.8 nm) has a reversible phase sequence of L ↔ RII ↔ RI ↔ [RI + T16] ↔ T16 on cooling (panel a) and heating (panel b). In CPG (8.1 nm), C16 displays a sequence of L → RII → RI → [RI + T16] → T16 as in SBA-15 (7.8 nm) on cooling (panel c) but is T16 → [RI + T16] → L on heating (see ref 25). In KIT6 (8.6 nm), the transition of the pore C16 is a reversible order of L ↔ [RI + T16] ↔ T16 (panels d and e). As in the midsized pores, C16 in the small pores does not show (00l) reflection planes of the lamellar arrangements of the molecules. Again, this means the interlayer regions are heavily disturbed, resulting in 2D-like structures with weakened interlayer coupling.23 Similar to the discussion in the midsized pores, the appearance of the pure RII phase of C16 in SBA-15 and CPG (d ≈ 8 nm) on cooling is ascribed to the defects of the chain stacking in the interlayer region and amidst the long chains at a higher temperature, initiated from the pore surface attractions. Also, the transition of the RII to the pure RI phase is realized among the weakly coupled chain layers while at lower temperatures than in the midsized pores. As the temperature decreases, the transition of the RI to the T16 phase is driven again by the more dense packing density of the molecules in the latter modification. The wider temperature range of the coexistence of RI and T16 phases at present than in the larger pores indicates a larger portion of the molecules affected by the pore surface attractions. At the even lower temperatures, the dense packing density in the T16 molecules can compensate for the interface interactions of the molecule−pore wall in the RI phase, so that T16 prevails finally. In the heating process, the pore C16 experiences the transitions from the T16 to RI and RII in SBA-15, or to [T16 + RI] in CPG with the general reasons of the pore surface attractions and thermal energy excitation, as discussed in section 3.3. Here, the different phase sequence of C16 in SBA-15 and CPG, the reversible versus irreversible, is associated with the influence of the pore geometry, as discussed in section 3.1 and the previous work.25,26 Probably, the highly connected CPG pores allow the molecules to stack in 3D directions, whereas 18702


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T → RI + T −34.0 RI + T → RI −27.5 RI → RII −6.0 RII → L 0.3 L → RII −0.5 RII → RI −5.5 RI → RI+T −23.5 RI + T → T −52.5

involve a transient RI phase in the bulk (b),33 a stable RI (c), a stable (d) and a metastable RII (e) phase of the pore C16, respectively. From the results in sections 3.3 and 3.4, the stable RI phase is observed in C16 in SBA-15 (7.8 and 17.2 nm), CSBA-15 (15.6 nm), KIT-6 (8.6 nm), and CPG (8.1 nm). The stable RII phase is found in SBA-15 (7.8) and C-SBA-15 (15.6 nm) whereas it is metastable in CPG (8.1 nm). In a tunable manner, C16 in the nanopores behaves as the pure oddnumbered alkane (such as C17) or even-numbered members with carbon numbers 22 ≤ n ≤ 26 in the bulk.29 This unique phase behavior also can be observed in the bulk binary alkanes with Δn ≤ 2.20,25,48 Previously, the pore C16 in CPG (10 nm) is estimated to have a large decrease in the free energy of the rotator and melt phase whereas a slight reduction in the triclinic phase in reference to the bulk melt.40 As a result, the transient RI in the bulk C16 turns out to be stable under the nanoconfinement. An estimation of free energies of the pore phases is useful for understanding the phase behaviors yet there is technical difficulty for accurate determination of related parameters such as the surface energy, latent heat, and vapor pressure. In contrast to the transient RI in the bulk, C16 in the nanopores (d < 20 nm) can transform between the melt and triclinic phase via the help of the rotator. The stabilization of the rotators under nanoconfinement is favored by a better match of the plastic and defected rotator phase than the rigid triclinic phase with the pore shape.40 Following this, the interface energy (σ) of the melt (l), rotator, and triclinic phase to pore wall (w) should have an order of σ(l-w) < σ(RII-w) < σ(RI-w) < σ(T16-w). As this is the phase sequence in the cooling process, the crystallization of the pore C16 should be favored by the lower interface energy in the polymorphs. Meanwhile, the above order of the interface energy might also be the trend of the nucleation energy at each stage of the freezing.24,33,40 If it is the case, the multiple steps in solidification of the pore C16 follow a path of both thermodynamically and kinetically. In any case, the polymorphism under confinement helps a deeper understanding of the physical origin of Ostwald’s step rule of stages,47 which is still not resolved thoroughly because of the unstable nature of the metastable forms in the early stages of crystallization.6 In the previous work, nanoconfinement often exhibits a complicated influence on the thermodynamic and dynamic properties of a large scope of materials such as the small molecules or polymers, the organic or inorganic, glass former, or crystals. The pore counterparts may exist in a nonfreezing, glass, or crystalline state.49 In many cases, as the pore C16 in this work, the influence of nanoconfinement can be understood from the size effect, interface interactions, and pore geometry.5,10,16,18,23 Recently, the study on the glass transition behaviors of the capped nanometer polymer (PS) films reveals a close relation with the free volume at the interface of the film and the substrate.50,51 The shift of the Tg value of the nanoscale film shows dependence on annealing time besides the thickness and interface interactions, where the free volume will be diminished through the relaxation of the entangled chains of the polymer. In consideration of the crystalline dominated system of the present work, C16 molecules are much shorter than the chains of the polymer and thus they can fit well onto the inner surface of the porous media used. In the case, the free volume in the interface region can be negligible; otherwise, the molecules would change their conformations to be adsorbed onto the pore wall with a compensation of the energy released

a Refer to melting or freezing of the melt, measured on DSC with average deviations of mostly within 0.5 °C. In the bulk C16, Tf = 15.9 °C, Tm = 17.6 °C. bRefer to transitions between solid phases with average deviations mostly ∼2.5 °C, a half of the temperature span in the X-ray diffraction scans.

H

T → RI + T −47.5 RI + T → L 1.3 T → RI + T −7.5 RI + T → L 8.8 L → RII 6.6 RII → RI 3.5 RI → RI + T −2.5 RI + T → T −22.5 T→L 16.7 L→T 15.7 T → RI −3.5 RI → RII 3.0 RII → L 8.0 L → RII 6.9 RII → RI 0.5 RI → RI + T −12.5 RI + T → T −17.5 T → RI + T −4.0 RI + T → L 8.0

heating 7.8 nm cooling

L → RI + T 6.4 RI + T → T −17.5

H 300 nm C H 15.6 nm C H 17.2 nm C

L → RI + T 0.6 RI + T → T −42.5

8.6 nm C

8.1 nm

H

C

KIT-6 CPG C-SBA-15 SBA-15

Table 3. Phase Transition Temperatures (Ttrs, s−la and s−sb) of C16 in SBA-15 (7.8 and 17.2 nm), CPG (8.1 and 300 nm), C-SBA-15 (15.6 nm), and KIT-6 (8.6 nm) Measured in the Cooling (C) and Heating (H) Processes


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Figure 6. (a) Schematic illustration of polymorphism of C16 in the bulk and confined in SBA-15 (7.8 and 17.2 nm), CPG (8.1 and 300 nm), C-SBA15 (15.6 nm), and KIT-6 (8.6 nm) on cooling (C) and heating (H), with the molecular arrangements in the bulk (T16) and RI and RII phases (right side). Subscript “o” represents the ordered channels; “d”, the disordered pores or channels. The structure of KIT-6 is drawn after the reference with interwoven channels in blue and orange.46 (Bottom) scheme of the relative free energy versus temperature for a transient RI phase (b), a stable RI phase (c), a stable RII phase (d), and a metastable RII phase (e). The dotted lines represent the equilibrium free energy curves for the three representative phases. The heavy (blue) and light (red) solid lines are the paths taken on cooling and heating, respectively.

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in the process. In addition, the directly intrusion of the melt of C16 into the nanopores and the following thermal treatment on the samples in the preparation can eliminate the free volume encountered in the capped nanofilm. However, the free volume and the nonequilibrium factor should be evaluated in investigation of phase behaviors of the confined alkanes with fair long chains.

AUTHOR INFORMATION

Corresponding Author

*X. Z. Lan. E-mail: [email protected]. Tel: +86-5388247753. Fax: +86-538-8242251. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from National Natural Science Found of China (No. 21273138) and 4B9A stations in BSRF for assistance with X-ray diffraction measurements.

4. CONCLUSION Using C16 as a model compound, we have shown the significant influence of nanoconfinement on its phase behavior. C16 in the large pores of CPG (300 nm) keeps the layered structure as complete nanosized crystals. Different from the transient RI in the bulk, C16 in the midsized pores (d < 20 nm) shows RI and even RII rotator as stable or metastable forms on cooling and heating. The appearance of the new rotators is accompanied by the changing in the packing of the molecules, from ordered into the weakened coupled layers. The stabilization of the rotators becomes more obvious in the small pores (d ≈ 8 nm) or in carbon-filmed SBA-15 because of the significant contribution of the interface energy of the nanosized crystals. The new phase behaviors of the pore C16 show a dependence on the pore size, interface interactions, and pore geometry. Further investigation on more normal alkanes should provide a deeper understanding of the nanoconfinement effect on phase behaviors of the chain molecules.

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Physical Control of Phase Behavior of Hexadecane in Nanopores

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