Increasing Phase Change Latent Heat of Stearic Acid via

Article pubs.acs.org/JPCC

Increasing Phase Change Latent Heat of Stearic Acid via Nanocapsule Interface Confinement Shudong Zhang, †,§ Shuangshuang Wang, †,§ Jian Zhang, Zhongping Zhang,*, † and Zhenyang Wang*, †

†

Yingchang Jiang,

†

Qi Ji,‡

†

Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei, Anhui, 230031, China School of Physics and Engineering, Zhengzhou University, Zhengzhou, Henan, 450052, China

‡

S Supporting Information *

ABSTRACT: Latent heat of phase change materials (PCMs) has long been regarded as constant. In this work, it is found that this parameter can be altered when they are in nanoscale. We designed and assembled a nanocapsule-confinement system with stearic acid (SA) sealed in silica nanoshells to investigate the thermodynamics and kinetics of their phase transition in nanoscale. It is interesting that heat storage capacity of the obtained SA@SiO2 nanocapsules (NCs) could be increased up to 374.2 kJ/kg, about 36.9% more than that of the unconfined SA (273.3 kJ/kg). This is because the high superimposed stress from the curvature effect inside SiO2 nanoshells would significantly shorten the intermolecular spacing of SA as compared to their unconfined state, which will especially strengthen hydrogen bonds of SA, forming multiple stable hydrogen bond networks. Therefore, breaking and reforming of these hydrogen bonds will no doubt contribute to latent heat of SA when they change from solid to liquid. Our results not only are helpful for understanding phase transition behaviors of phase change materials in nanocapsule interface confinement conditions but also provide a good example to develop new types of heat energy storage composite materials.

1. INTRODUCTION Latent heat storage materials, also named phase change materials (PCMs), can absorb or release large amounts of heat during phase transformation like solid−liquid, liquid−gas, or solid−solid transition.1−3 Thermal energy storage using the latent heat has recently gained considerable attention.4−6 PCMs with high energy storage density and appropriate phase change temperature are important for application in solar energy storage,7 waste heat recovery,8 smart air conditioning in buildings,9 homothermal clothing,10 and so on. The development of micro-/nanoconfined technology has brought an opportunity to solve PCMs’ low thermal conductivity and alter phase change temperature for further controlling thermal release.11−16 For instance, microencapsulated PCMs are often used to improve the heat transfer area between the PCM and the ambient by increasing the surface to volume ratio of the PCM.11,17−23 The two-dimensional interface confinement effect that significantly shortens the intermolecular spacing to control heat release has thus been achieved.24−26 Although confinement methods have been developed to overcome these difficulties, to the best of our knowledge, how to take advantage of the nanoconfinement effect to increase the heat storage capacity of PCMs for storing more thermal energy has still not been reported. Herein, we experimentally illustrate that latent heat of PCMs can be greatly enhanced. We rationally design a hydrophobic interaction driven soft templating approach to synthesize SA@ SiO2 nanocapsules (NCs) with controllable SiO2 shell sizes. © 2013 American Chemical Society

The high superimposed stress from the curvature effect inside SiO2 nanoshells should significantly shorten the intermolecular spacing as compared to the unconfined state, which will especially strengthen hydrogen bonds of stearic acid (SA), forming multiple stable hydrogen bond networks. Reforming and breaking of these hydrogen bonds will especially contribute to the latent heat of PCMs during their phase change. Therefore, the heat storage capacity of the obtained SA@SiO2 NCs could be increased up to 374.2 kJ/kg, about 36.9% more than that of the unconfined SA.

2. EXPERIMENTAL SECTION 2.1. Materials. Stearic acid (SA), sodium dodecyl sulfate (SDS), nitric acid (HNO3), ammonia−water (NH3·H2O), and ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent (China), Co., Ltd. Phenyltrimethoxysilane (PTMS) was purchased from Sigma-Aldrich corporation, and the above agents were analytically pure. Chemical pure n-pentanol (nC5H14OH) was commercially supplied by Shanghai Chemical Reagents, China. All chemical reagents were used without further purification. 2.2. Synthesis of SA@SiO2 Core−Shell Nanocapsules. Stearic acid (SA, 0.5 g) was melted at 80 °C in a flask. Then, 20 mL of sodium dodecyl sulfate (SDS, 0.09 M) and nitric acid Received: August 24, 2013 Revised: September 29, 2013 Published: October 2, 2013 23412

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Scheme 1. Schematic Formation of the SA@SiO2 NCs

(HNO3, 6.6 mM) solution was poured into the above flask under continuously stirring conditions. Subsequently, 0.8 mL of n-pentanol (n-C5H14OH) was added drop by drop, and a transparent and stable microemulsion was formed after stirring for 60 min. Then, phenyltrimethoxysilane (PTMS) with different concentration was added dropwise into the microemulsion, and the prehydrolysis occurred on the surface of stearic acid microemulsion in about 5 min. Condensation procedures were initiated by dripping of NH3·H2O aqueous solution to increase the pH value of the microemulsion. The hydrolysis and condensation procedures were kept at 80 °C for 6 h. The resultant products were purified by centrifugation several times to remove unreacted reagents and dried at 40 °C for 24 h in a vacuum oven. 2.3. Characterization of SA@SiO2 Core−Shell Nanocapsules. The morphology, structure, and composition of the as-synthesized samples were investigated by field emission scanning electronic microscopy (FESEM; FEI Sirion200) equipped with X-ray energy-dispersive spectrometry (EDS), field emission transmission electron microscopy (TETEM; JEM-2100F), and X-ray diffraction (XRD; Philips X’Pert Pro) with Cu Kα radiation (1.5418 Å). Fourier transform infrared (FTIR) absorption spectra were recorded on a Thermo-Fisher Nicolet is10 FTIR spectrometer from 4000 to 500 cm−1 on a KBr sampling sheet. Differential scanning calorimetry (DSC) was performed using a NETZSCH DSC Q2000 instrument, and all measurements were carried out under a nitrogen atmosphere at a heating or cooling rate of 10 °C/min with a sample weight of around 10 mg. TGA was carried out on a thermal analyzer (SHIMADZU TGA-50H).

3. RESULTS AND DISCUSSION The synthesis approach of SA@SiO2 NCs is simple and illustrated in Scheme 1. The SA was first melted and dispersed in an acidic aqueous solution containing sodium dodecyl sulfate (SDS) as emulsifier, which resulted in a stable O/W microemulsion. Considering the hydrophobic nature of SA, phenyltrimethoxysilane (PTMS) with hydrophobic chains of the benzene ring are chosen as a Si resource to fabricate SiO2 nanocapsules, and the hydrophobic chains of benzene ring are oriented into the oil droplets and hydrophilic groups out of the oil droplets. The hydrophilic segments are associated with the water molecules and trimly cover the surface of the oil droplets of SA with the hydrolysis of PTMS. Finally, the silicamicroencapsulated stearic acid was formed by the microcapsule shell fabricated onto the surface of the stearic acid droplet through an in situ hydrolysis and condensation of PTMS (more details in Experimental Section). Figure 1 shows the typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-prepared SA@SiO2 NCs. By casting the NC colloid onto a silicon substrate, the overview SEM observation in Figure 1 shows the SA@SiO2 NCs are highly monodispersive and have an average particle size of ∼100−110 nm (Figure 1a, c, e), whose core−shell structure was clearly revealed by a TEM image (Figure 1b, d, f). Both pure SA and SiO2 NCs were not found as carefully examined by TEM, suggesting the highly selective growth of SiO2 on the surface of SA cores. The thickness of SiO2 shells increased with the amount of PTAS, and they are tunable from ∼50 to 10 nm for a fixed ∼80 nm SA core (Figure S1, Supporting Information, and Figure 1b, d, and e). Meanwhile, it is clear that the core−shell NCs become larger and rounder with the increase of SiO2 shell thickness. In addition, prehydrolysis experiments have been performed to 23413

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shell thickness increase significantly relative to those of the pristine SA. However, for the SA@SiO2 NCs with ∼10 nm shell thickness, only one exothermal peak centered at 68.2 °C shows in the exothermal curve (Figure 2d), while two extra small peaks appeared for the other three kinds of SA@SiO2 NCs with different shell thickness (∼62 °C and ∼85 °C, as shown in Figure 2a, b, and c). The latent heats of melting (ΔH) were 59.66, 84.59, 133.2, 49.64, and 273.3 J/g for sample S1, S2, S3, S4, and SA, respectively, shown in Table 1. To test the thermal Table 1. SA Encapsulation Ratios (M), Melting Temperatures (Tm), and Melting Enthalpy Values (ΔH) for Pure SA and Samples S1, S2, S3, and S4, Respectively sample

PTMS:SA

Tm (°C)

M (%)

ΔH (J/g)

ΔH/M

S1 S2 S3 S4 SA

1:1 1:1.5 1:2 1:2.5 0:1

84.1 85.0 84.1 84.9 70.8

21.6 29.0 35.6 17.3 100

59.66 84.59 133.2 49.64 273.3

276.2 291.7 374.2 286.9 273.3

Figure 1. SEM and TEM images of SA@SiO2 NCs with different shell thickness: (a, b) ∼30, (c, d) ∼20, and (e, f) ∼10 nm.

energy storage stability of SA@SiO2 NCs, we performed the heat storage−release cycling. More importantly, there is no obvious change in both endothermal and exothermal curves of the same sample after cycling up to 100 times (Figures S6 and S7, Supporting Information). The thermal storage capabilities of nanocapsules were calculated by eq 1.

confirm the critical role in the formation of SA@SiO2 NCs (Figure S2, Supporting Information). More experimental observations, including the condensation reaction time, the effects of temperatures, and composition of the as-synthesized samples are given in detail in the Supporting Information (Figures S3−S5). Heat storage capacity is a very important reference index for measurement of the PCM heat storage performance. Differential scanning calorimetry (DSC) is very suitable technique to study heat storage capacity of a PCM confined inside the nanocapsules. Figure 2 shows the melting−freezing DSC curves

ΔHSA@SiO2

η=

M

ΔHSA

× 100%

(1)

where η is the thermal storage capability rate of the microcapsule; ΔHSA@SiO2 is the enthalpy values of nanocapsules; ΔHSA is the enthalpy values of pure SA; and M is the encapsulating ratio of SA.12,27 The quantification of SA molecule content in the SA@SiO2 NCs was easily confirmed by normalizing the thermogravimetric analysis (TGA) curves of the nanocapsules to those of the respective pure SA (Figure S8, Supporting Information). These TGA results (Table 1) clearly demonstrate that high content of SA could be encapsulated into the interior of the SiO2 NCs, and encapsulation ratios for the samples S1, S2, S3, and S4 were 21.6%, 29.0%, 35.6%, and 17.3%, respectively. Thermal storage capabilities of samples S1, S2, S3, and S4 are calculated as 276.2, 291.7, 374.2, and 286.9 J/g, respectively, and thermal storage capability rate (η) is 101.06%, 106.85%, 136.92%, and 104.98% (Table 1), respectively. Those results demonstrated that the heat energy storage density of obtained SA@SiO2 nanocapsules is obviously higher than the pure SA. As well-known, in the nanoconfinment effect, weak interactions, such as H-bonding, π−π stacking, van der Waals forces, and electrostatic forces, were enhanced by shortening the intermolecular spacing due to high pressure and electric charge density caused by a defined environment.13,28−30 So in our case, the nanoconfinment effect has dual roles: (1) the nonpolar tails of the SDS in the SiO2 inner surface are oriented toward the center of the nonpolar droplet, as sketched in Figure 3. These tails form a pattern on the inner surface of the droplet which could act as a crystal template, facilitating the orientation of SA molecules in their crystal pattern. (2) According to the Young−Laplace equation (eq 2)

Figure 2. DSC thermal spectra of SA@SiO2 NCs with different shell thickness. (a) ∼50, (b) ∼30, (c) ∼20, and (d) ∼10 nm. Heat rate 10 K min−1.

of the SA@SiO2 NCs with different shell thickness of ∼50, ∼30, ∼20, and ∼10 nm (Figure 2a, b, c and d), respectively. The DSC curves (Figure S6, Supporting Information) for the pristine SA present an endothermic peak centered at 70.8 °C and an exothermic peak centered at 62.8 °C, corresponding to the melting point (Tm) and freezing point (Tf) of the pure SA. For the SA@SiO2 NCs with different shell thickness, it is surprising to find that the endothermic and exothermic peaks are centered at 84−85 °C and 75−76 °C, respectively, which means that the Tm and Tf of the SA@SiO2 NCs with different 23414

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classical thermodynamic approach by the so-called Gibbs− Thomson equation (eq 3). ΔTm = Tm,confined = Tm,bulk = 2

2γ R

Hλm,bulk

(3)

According to the Gibbs−Thomson thermodynamic equation (eq 3),3,27,39 the shift of the phase transition temperature ΔTm is inversely proportional to the pore radius H and is proportional to the difference of the surface tension γwf − γws, where γwf and γws are the wall−fluid surface tensions; ν is the molar volume of the liquid phase; and λm,bulk is the bulk latent heat of fusion. So in our case, SA macromolecules were rearranged, and the high superimposed stress from the curvature effect inside SiO2 nanoshells would significantly shorten the intermolecular spacing of SA or the occurrence of phase transition among SA polymorphs40 as compared to their unconfined state, which will especially strengthen hydrogen bonds of SA, forming multiple stable hydrogen bond networks. Breaking and reforming of these hydrogen bonds will no doubt contribute to increased latent heat and melting temperature of SA when they change from solid to liquid. Variable-temperature infrared spectra provide a good technical means for detecting phase change microstructure information of nanocapsules. The temperature dependence of the infrared absorption spectrum was carried out to study the phase change process of the synthetic SA@SiO2 samples (Figure 5). From 75 to 100 °C (Figure 5a), the stretching vibration of CO of SA has been shifted from originally at ∼1723 cm−1 to a lower frequency of ∼1712 cm−1 in the variable-temperature FTIR spectra, and at 85 °C, dual peaks coexist. Those peak signals show that the molecules’ thermal motion was increased by increasing the temperature to lead to destroying of the SA intermolecular H-bonding. On the contrary, when the temperature decreased (Figure 5b), the stretching vibration of −CO of SA shifted from a lower frequency of ∼1712 cm−1 to originally at ∼1723 cm−1. Intermolecular interactions are very sensitive to C−H stretching vibration at 3000−2800 cm−1.41 At lower temperature (Figure 5c), the shoulder peak at 2955 cm −1 demonstrates strong molecular interaction among −CH3 groups sitting at the end of SA. At high temperature, the shoulder peak disappeared, and molecular interaction among −CH3 groups was reduced (Figure 5d). Those results further verified that SA phase change is a reversible process with the temperature fluctuation. Meanwhile, at room temperature, chemical characterization of the unconfined solid SA was further carried out by Fourier transform infrared (FT-IR) spectroscopy analysis (Figure S9, Supporting Information). The peaks around 2922 and 2849 cm−1 signify the stretching vibrations of −CH3 and −CH2 groups in SA molecules, respectively. However, the stretching vibration of CO (1700 cm−1) in the FT-IR spectrum of the pure stearic acid decreased markedly, compared with that of the SA@SiO2 composites (1723 cm−1). The observable frequency shifts of the main groups of SA are due to that there are some interface interactions to induce modification of conformation and the occurrence of a phase transition in the closed and high-pressure system. The similar observable frequency shift was also reported in the SA−GO composite system.42 Therefore, the nanoconfinement effect for increasing latent heat storage capacity of SA is attributed to two contributions: (1) The

Figure 3. Nonpolar tails of the SDS in the SiO2 inner surface are oriented toward the center of the nonpolar droplet. Curvature effect results in high superimposed stress to form multiple stable hydrogen bond networks.

Δp = pα − pβ =

(γwf − γws)ν

(2)

where pα and pβ are the internal and external pressures of the spherical surface; γ is surface tension; and R is its radius. The Young−Laplace equation shows that the pressure inside a spherical surface is always greater than the pressure outside, and the pressure difference increases if the radius becomes smaller and tends to infinite when R tends to zero. Therefore, a small curvature effect results in high superimposed stress in nanometer-scale spheres compared with the reported microcapsules. The high superimposed pressure inside SiO2 should significantly shorten the intermolecular spacing as compared to the unconfined state, which will be especially strong for chargeassisted hydrogen bonds,31 forming multiple stable hydrogen bond networks compared to the unconfined solid SA with three kinds of equilibrium states (Figure 4),32,33 in which the short

Figure 4. Solid stearic acid equilibrium under the hydrogen bonding effect.

−CO···H−O− contacts can be generated. It is speculated that the confinement of SiO2 nanoshells changes some geometric factors of the encapsulated SA, leading to the enhancement of weak interactions and the high stability of asformed crystals. Therefore, the melting temperature (Tm) of the encapsulated SA was increased to ∼85 °C (Figure 2), which means that the Tm of SA@SiO2 NCs increases significantly relative to those of the pristine SA. The previous works have reported that the nanoconfinement effect can result in the increase of melting temperature of ionic liquids.13,28−30 Meanwhile, many polymorph transitions of SA crystals will appear and occur under the high pressure.33−37 The high superimposed pressure inside SiO2 could possibly result in the occurrence of phase transition among SA polymorphs. The exothermal curves for SA@SiO2 NCs (∼50, ∼30, and ∼20 nm shell thickness) show two extra small peaks (Figure 2a, b, and c), which possibly appeared and induced phase transition among SA polymorphs by the high superimposed pressure. The small confined particles have lower phase change point (melting point) than bulk materials due to an increased proportion of surface atoms as the size of the particles decreases.11,22,23,25,26,38 The size-dependent phase change point depression of confined nanoparticles has been described in a 23415

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ASSOCIATED CONTENT

S Supporting Information *

XRD, TGA, and additional material characterization. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

S. D. Zhang and S. S. Wang contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the National Basic Research Program of China (No. 2009CB939902) and National Natural Science Foundation of China (No. 51202253, 51002159).

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REFERENCES

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Figure 5. Variable-temperature IR spectroscopy of SA@SiO2 NCs at different temperatures. In (a), from 75 to 100 °C, the stretching vibration of CO of SA has been shifted from originally at ∼1723 cm−1 to a lower frequency of ∼1712 cm−1. On the contrary, in (b), the temperature decreased, and the stretching vibration of CO of SA shifted from a lower frequency of ∼1712 cm−1 to originally at ∼1723 cm−1. At lower temperature (c), the shoulder peak at 2955 cm−1 demonstrates strong molecular interaction among −CH3 groups siting at the end of the SA. At high temperature (d), the shoulder peak disappeared, and molecular interaction among −CH3 groups was reduced.

high superimposed pressure inside SiO2 shells should significantly shorten the intermolecular spacing as compared to the unconfined state to form multiple stable hydrogen bond networks. (2) The interaction energy from the nonpolar chains of the SA.

4. CONCLUSIONS In this work, we rationally design a hydrophobic interactiondriven soft templating approach to have synthesized SA@SiO2 NCs with controllable SiO2 shell size. Heat storage capacity of the obtained SA@SiO2 NCs could be increased up to 372.4 kJ/ kg, about 36.9% more than that of the unconfined SA. This result is from the formed multiple stable hydrogen bond networks under high superimposed stress from a curvature effect inside SiO2 nanoshells, which significantly shorten the intermolecular spacing as compared to the unconfined state. Meanwhile, the interaction energy from the nonpolar chains of the SA partly contributed to an increase in heat storage density of the obtained SA@SiO2. Our results not only provide a useful way to increase the latent heat of phase change nanomaterials but also are helpful to understand the thermodynamics of phase change under nanoscale. 23416

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dx.doi.org/10.1021/jp408478h | J. Phys. Chem. C 2013, 117, 23412−23417