1,3:2,4-di-(3,4-dimethyl)benzylidene sorbitol organogels used as phase change materials: solvent effects on structure, leakage and thermal performance

Abstract

Organogels used as shape-stable phase change materials were prepared by impregnating a low molecular mass organogelator into organic phase change materials. 1,3:2,4-di-(3,4-dimethyl)benzylidene sorbitol (DMDBS) was used as the supporting material and n-octadecane, ethyl palmitate, 1-octadecanol and 1-tetradecanol were used as the phase change materials. The thermal properties, such as phase change temperature and latent heat, were investigated using a differential scanning calorimeter (DSC). The leakage and thermal storage performance of the composite phase change materials were also investigated in detail. Rheology measurements were used to demonstrate the true gel phase of the composite phase change materials. Fourier transformation infrared spectroscopy (FTIR) and scanning electronic microscopy (SEM) were used to determine the chemical structure and microstructure of the composites. The DSC results indicated that the composites exhibited acceptable thermal properties with a high mass percentage of the phase change material. Flory–Huggins parameters (χ) were calculated to estimate the interactions between the gelator and phase change materials. The results indicated that different χ resulted in different morphologies of DMDBS, which led to different leakages and thermal storage performances of the prepared composites.

---

1. Introduction

The use of phase change materials (PCMs) for energy storage has received great attention in recent years due to increasing energy consumption. PCMs can absorb or release large amounts of latent heat at their melting points as the surrounding temperature increases and decreases.¹ PCMs are developed for various applications due to their different phase change intervals: materials that melt below 15 °C are used for keeping low temperatures in air-conditioning applications, while materials that melt above 90 °C are used to drop the temperature if there is a sudden increase in heat to avoid ignition. All other materials that melt between these two temperatures can be applied in solar heating and for heat load leveling applications.² Compared to inorganic PCMs, organic PCMs melt and freeze repeatedly without phase segregation and consequent reduction of their latent heat of fusion. However, they also have some undesirable properties, such as low thermal conductivities and the need of encapsulation for preventing leakage of the melted PCM during the phase change process, which results in extra thermal resistance and cost.³

Much research has focused on the preparation of form-stable PCMs. Several methods have been used such as microencapsulation,⁴ adsorption by a porous material⁵ and the addition of a polymer as a supporting material.⁶ Alkan et al.⁷ successfully prepared PMMA microcapsules containing n-octacosane as the core material. The composite PCMs exhibited a desirable latent heat and good chemical stability. Yarin et al.⁸ achieved the intercalation of different types of paraffins and their mixtures with triglycerides inside carbon nanotubes (CNTs). The nanoscale phase change materials (PCMs) obtained using this route can be tailored to operate in a relatively wide working temperature range, as well as with a negligible thermal response time. Meng et al.⁹ prepared a fatty acid eutectic/polymethylmethacrylate (PMMA) form-stable phase change material and the feasible maximum mass fraction of the lauric acid–myristic acid eutectic was determined to be 70%. In our previous work, the low molecular-weight organic gelators 1,3:2,4-di-(4-methyl)benzylidene sorbitol (MDBS) and 1,3:2,4-di-(3,4-dimethyl)benzylidene sorbitol (DMDBS, Scheme 1) were used to prepare paraffin and 1-tetradecanol based gelatinous PCMs, which showed lower leakage with high weight percentages of the phase change materials.^10,11 In these gelatinous form-stabilized PCMs, the phase change material was well dispersed in the three-dimensional networks formed by the gelators through intermolecular interactions such as hydrogen bonding and π–π interactions.

Scheme 1 Structure of DMDBS.

In molecular gels, solvent–gelator interactions play a key role in mediating the organogel formation and ultimately determine the properties of the gel.^12,13 Several groups have reported the effect of solvents on the gel morphology and properties. Dasgupta¹⁴ et al. reported that the supramolecular superstructure of an organogel can be significantly altered simply by adjusting the solvent type. Wu¹⁵ et al. proved that by varying the ratio of the mixed solvents, the gelation ability and thermal stability of the gel can be easily adjusted. Bielejewski¹⁶ et al. found that the morphologies of xerogels were influenced by the nature of the solvent, and the polarity of the solvent influenced the thermal stability of the gel. In this work, DMDBS was used as the supporting material in different phase change materials such as n-octadecane, ethyl palmitate, 1-octadecanol and 1-tetradecanol. By investigating the self-assembly of a model gelator in different phase materials, we hope to gain a more detailed insight into the effects of phase change materials on the DMDBS morphology, and the macroscopic properties (leakage and thermal storage performance) of composite PCMs by studying the interactions between DMDBS and the phase change materials.

2. Experimental

2.1 Materials

n-Octadecane (99 wt%), ethyl palmitate (99 wt%), 1-octadecanol (99 wt%) and 1-tetradecanol (99 wt%) were obtained from the Tianjin Guangfu Fine Chemical Research Institute. n-Hexane was obtained from Tianjin Jiangtian Chemical Technology Co., Ltd. 1,3:2,4-Di-(3,4-dimethyl)benzylidene sorbitol (DMDBS) was provided by Yantai Zhichu Synthetic Chemistry Co., Ltd. All other compounds were of analytical grade and used without further treatment.

2.2 Preparation of composite PCMs

To establish the relationship between DMDBS and the chosen PCMs, the composite PCMs were prepared by adding the desired amount of DMDBS to the different melted pure PCMs, and the mixture was heated above the melting point of the gelator, which is 275 °C. The composite PCM was obtained through slow cooling to room temperature. Four kinds of composite PCMs using n-octadecane, ethyl palmitate, 1-octadecanol and 1-tetradecanol as the phase change material were obtained, denoted as PCM-1, PCM-2, PCM-3 and PCM-4, respectively. The compositions of the phase change material and DMDBS in the composites are listed in Table 1.

Table 1 Sample identification and compositions

Samples	Compositions
PCM-1	3% DMDBS + 97% n-octadecane
PCM-2	3% DMDBS + 97% ethyl palmitate
PCM-3	3% DMDBS + 97% 1-octadecanol
PCM-4	3% DMDBS + 97% 1-tetradecanol

---

2.3 Analytical methods

2.3.1 Characterization of composite PCMs. The structural analysis of the composites was carried out using a FTIR spectrophotometer. Rheology measurements were performed on an AR 2000ex advanced rheometer (TA Instruments) using a cone plate geometry for the Peltier plate. The frequency sweep experiment to obtain the storage modulus (G′) and the loss modulus (G″) was performed under a constant strain of 0.15%. The temperature for each sample was 30 °C higher than the melting temperature of the corresponding pure phase change material. The FTIR spectra were recorded on a TENSOR 27 instrument from 400 to 4000 cm⁻¹ with a resolution of 2 cm⁻¹ at room temperature using KBr pellets. The morphology and microstructure of the composite PCMs and xerogels were observed using scanning electron microscopy (SEM S-4800, Hitachi, Japan). The xerogels were prepared as follows: the composite PCMs were added to vigorously stirred n-hexane, and the white precipitate was filtered, washed with n-hexane, and then dried in vacuum for 24 h.¹⁷ The white precipitate was dropped on the SEM plate and sputtered with Pt. The physical thermal properties of the composite PCMs were investigated using differential scanning calorimetry (DSC 204 F1, NETZSCH, Germany) under an argon atmosphere. All experimental measurements were performed at a 5 °C min⁻¹ constant heating rate.

2.3.2 Measurement of leakage. The leakage of the composite PCMs was measured using the filter sheets-sandwich method which has been described previously.¹⁸ The samples were kept at 30 °C above the melting temperature of each phase change material for 24 h. Each sample was weighed before and after the annealing step and the weights are given as M₁ and M₂, respectively. The leakage of the composite PCMs was calculated using the following equation:

Leakage (wt%) = (M₁ − M₂)/M₁ (1)

2.3.3 Thermal storage performance test. The composite PCMs and the pure phase change materials were placed into cylinders with a diameter of 3.8 cm. The environmental temperatures of the samples were changed from 10 to 40 °C for n-octadecane and ethyl palmitate, from 30 to 70 °C for 1-octadecanol, and from 20 to 50 °C for 1-tetradecanol. The data were automatically monitored using a data acquisition/switch unit (TDAM 7034).

3. Results and discussion

3.1 Thermal properties of the composite PCMs

The thermal properties of the composite PCMs, such as the melting and freezing points and the latent heats were measured using DSC. The theoretical enthalpies of the composite PCMs were calculated using the following equations:

ΔH_m^calc = W_CM × ΔH_m,CM (2)

ΔH_f^calc = W_CM × ΔH_f,CM (3)

ΔH_m^calc and ΔH_f^calc represent the theoretical enthalpies of the composite PCMs during the melting and freezing process, respectively. W_CM is the weight percentage of the phase change material in the composite PCM, ΔH_m,CM and ΔH_f,CM are the melting and freezing enthalpies of the pure phase change material, respectively.

The DSC curves of all samples show one endothermic peak upon heating and two exothermic peaks upon cooling. The endothermic peak is related to the solid–liquid transition. For the two exothermic peaks, the peak at the higher temperature is related to the liquid–solid transition, and the peak at the lower temperature may be attributed to a transition from a hexagonally packed solid phase to an orthorhombically packed solid phase (Fig. S1–4†). The phase change latent heats and temperatures of the pure phase change materials and the composite PCMs are shown in Table 2. The enthalpy calculated from the total area under the peak by numerical integration is denoted as ΔH^obs. The melting and freezing points were determined from the onset temperature points of these curves. As can be seen from Table 2, during the heating process the phase change temperatures of PCM-1, PCM-2, PCM-3 and PCM-4 are 25.0, 22.0, 55.8 and 35.7 °C and their latent heat values were determined to be 173.2, 145.8, 200.3 and 227.3 J g⁻¹, respectively. In order to examine DMDBS and whether it reduces the latent heat performance of the phase change materials, the theoretical latent heat values of the composite PCMs were calculated as 196.0, 151.5, 204.6 and 231.5 J g⁻¹ for PCM-1, PCM-2, PCM-3 and PCM-4, respectively. By comparison, it can be concluded that although the phase change enthalpies of the phase change materials in PCM 1–4 change by −11.6%, −3.7%, −2.1% and −1.8%, respectively, the composite PCMs exhibit acceptable heat capacities. This confirms that DMDBS as the supporting material does not influence the latent heats of the phase change materials obviously. Similar results can also be observed for the freezing process of the composite PCMs.

Table 2 DSC data for the pure PCMs and composite PCMs

Sample	Melting			Solidifying
	T m (°C)	ΔHm^obs (J g^−1)	ΔHm^cal (J g^−1)	T f (°C)	ΔHf^obs (J g^−1)	ΔHf^cal (J g^−1)
n-Octadecane	25.1	202.1	202.1	25.3	205.2	205.2
PCM-1	25.0	173.2	196.0	25.0	173.8	199.0
Ethyl palmitate	22.0	155.6	156.2	18.0	147.8	147.8
PCM-2	22.0	145.8	151.5	17.9	139.0	143.4
1-Octadecanol	56.0	210.9	210.9	56.0	205.4	205.4
PCM-3	55.8	200.3	204.6	55.7	196.0	199.2
1-Tetradecanol	36.4	238.7	238.7	39.9	236.1	236.1
PCM-4	35.7	227.3	231.5	40.7	226.7	229.0

---

3.2 Rheological properties and FTIR analysis

The gelation of the four DMDBS–phase change material systems at a temperature 30 °C higher than the melting temperature of the corresponding phase change material and their elasticity were probed by rheological experiments (Fig. S5†). In a typical frequency sweep experiment, the variation of storage modulus G′ and loss modulus G″ is monitored under a constant strain of 0.15%. For all of the four samples, G′ and G″ show a plateau region and G′ is higher than G″, suggesting that this is the behavior expected of a true gel phase.^19–21

FTIR spectroscopy is an effective method to investigate specific interactions in the composite PCMs. The FTIR spectra of n-octadecane, DMDBS and the composite PCM-1 are shown in Fig. 1. Fig. 1a shows the spectrum of n-octadecane. The peak at 2916 cm⁻¹ signifies the symmetrical stretching vibration of the –CH₃ groups and that at 2848 cm⁻¹ represents the symmetrical stretching vibration of the –CH₂ groups. The absorption peaks at around 1471 cm⁻¹ belong to the deformation of –CH₂ and –CH₃, and the peak at 717 cm⁻¹ represents the rocking vibration of –CH₂. Fig. 1c illustrates the spectrum of DMDBS. The peaks at 3278 and 3207 cm⁻¹ signify the asymmetric stretching vibration of the functional group O–H. The peaks at around 2923–2854 cm⁻¹ represent the stretching vibrations of –CH₂ and –CH₃. The C–O and C–O–C stretching vibrations can be seen at around 1100 cm⁻¹. The absorption bands at 857–769 cm⁻¹ represent the bending vibration of C–H in the phenyl ring.

Fig. 1 FTIR spectra of (a) n-octadecane, (b) PCM-1, and (c) DMDBS.

As shown in Fig. 1b, the absorption peaks of n-octadecane at 2916, 2848, 1471 and 717 cm⁻¹ also appear in the spectrum of the composite PCM-1. It is also observed that there is no shift of the absorption peaks of the PCM-1 composite when compared with the spectrum of n-octadecane. This result indicates that there is no chemical interaction between the functional groups of n-octadecane and DMDBS. n-Octadecane was easily retained in the network of DMDBS by capillary and surface tension forces, and leakage of the melted n-octadecane from the composite could be prevented.²² Similar observations were obtained for the other composite PCMs (ethyl palmitate, 1-octadecanol and 1-tetradecanol, Fig. S6–8†).

3.3 Morphology characterization of the form-stable PCMs

Fig. 2 presents the morphologies of the different form-stable PCMs and their xerogels. Fig. 2a1–d1 represent the morphologies of PCM-1, PCM-2, PCM-3 and PCM-4, respectively. The morphologies of the composites were slightly different as a result of the varying phase change material type. There was no obvious demarcation between DMDBS and the phase change material, indicating that the phase change materials were dispersed in the networks of DMDBS. When the environmental temperature was above the melting point of the phase change material, DMDBS acting as the supporting material can effectively immobilize the liquid phase so as to prevent leakage of the melted phase change material. Fig. 2a2–d2 show the xerogels prepared by removing the phase change materials and three-dimensional networks could be clearly observed in these images, which is in good agreement with the theoretical conclusion.

Fig. 2 SEM images of the composite PCMs and DMDBS xerogels: (a1) PCM-1, (a2) DMDBS xerogel obtained from PCM-1; (b1) PCM-2, (b2) DMDBS xerogel obtained from PCM-2; (c1) PCM-3, (c2) DMDBS xerogel obtained from PCM-3; (d1) PCM-4, (d2) DMDBS xerogel obtained from PCM-4.

By investigating the xerogels obtained from the different composite PCMs, there are some slight differences between the samples. As shown in Fig. 2a2–d2, the widths of the fibrils were 0.34–0.52 µm for PCM-1, 0.17–0.32 µm for PCM-2, 0.18–0.28 µm for PCM-3 and 0.10–0.21 µm for PCM-4. It was also observed that the sample with thinner fibres has the denser network. These results indicated that the self-assembly of DMDBS is affected by the different phase change materials. Feng²³ has reported that the ability of a gel to immobilize solvent was affected by the network formed by the gelator. So the different morphologies of the DMDBS xerogels may lead to different leakage performances.

3.4 Leakage of phase change material from the composite PCMs

The leakage percentages of phase change material from the composite PCMs were measured at 30 °C higher than the melting point of the corresponding phase change material and the results are presented in Fig. 3. As shown in Fig. 3, the leakage of the phase change material decreased rapidly when the amount of DMDBS additive was 3 wt%. The leakage percentages of PCM-1, PCM-2, PCM-3 and PCM-4 were 36.3, 34.4, 14.9 and 13.6%, respectively. The degree of the decrease for the four kinds of composite PCMs was in the trend of PCM-1, PCM-2, PCM-3 and PCM-4. The obvious decrease in leakage is attributed to the three-dimensional network formed by DMDBS. As described previously, when the additive amount was 3 wt%, the three-dimensional networks became thinner and denser in the trend of PCM-1, PCM-2, PCM-3 and PCM-4. In molecular gels, the networks formed by the gelator immobilize the liquid component by surface tension and capillary forces.²⁴ The denser three-dimensional networks have more fibres, which might make the gels stronger and therefore more efficient in preventing the flow of the melted phase change material, which is the main reason for the leakage. So, the sample that has a denser network would have lower leakage. This distinction also existed for other additive amounts (Fig. 3). When the amount of additive exceeded 7 wt%, there was only a slight decrease in leakage for all the composites. A probable reason is that as the amount of DMDBS increased, the capillary forces of the networks became stronger, and when the amount was 7 wt%, the network was strong enough to immobilize most of the phase change material. So, the leakage decreased little when more DMDBS was added.

Fig. 3 Relationship between the leakage of the composite PCMs and the amount of DMDBS additive.

3.5 Thermal storage performance of the composite PCMs

The thermal conductivity of the composite PCMs was studied by comparing the heat storage and release curves of the pure phase change materials with those of the corresponding composite PCMs (Fig. S9–12†). Four couples of samples were tested; they were n-octadecane and PCM-1, ethyl palmitate and PCM-2, 1-octadecanol and PCM-3, 1-tetradecanol and PCM-4. As seen from the endothermic curves, the melting times for pure n-octadecane, ethyl palmitate, 1-octadecanol and 1-tetradecanol were 903, 490, 2544 and 1990 s, respectively (Table 3). When DMDBS was added, the melting times for PCM-1, PCM-2, PCM-3 and PCM-4 were 1457, 1108, 5950 and 5461 s. This result indicated that the heat transfer rate of the composite PCMs decreased in the melting process compared with that of the corresponding pure phase change materials. This is because in the case of heating, natural convection plays an important role in the heat transfer. As the phase change material undergoes phase transition in the network of solid DMDBS, the natural convection is weakened. The weakening of the natural convection overwhelms the increase in conductivity, so the heat transfer rate in the heating process is reduced.²⁵ The endothermic times are extended by 61.4, 126.1, 133.9 and 174.4% for PCM-1, PCM-2, PCM-3, and PCM-4, respectively. The variations in the endothermic time are in good correlation with the density of the networks, indicating that the denser the networks formed in the composite PCMs, the higher the effect on weakening the natural convection. This disadvantage in the heating process could be solved by adding a small quantity of exfoliated graphite, which has been reported in our previous work.^10,11 In the solidifying process, the cooling times were 1284, 2140, 1428 and 2199 s for the four pure phase change materials, and were changed to 858, 1865, 1089 and 2102 s for PCM-1, PCM-2, PCM-3, and PCM-4, respectively. This was attributed to the higher thermal conductivity of DMDBS than that of the phase change material and the heat transfer is enhanced in the conduction dominated solidification process.

Table 3 Endothermic and exothermic times of the pure and composite PCMs^a

Sample	Endothermic times (s)	Difference (%)	Exothermic times (s)	Difference (%)
a Temperature acquisition range: n-octadecane: 23.1–29.7 °C for melt and 32.1–25.7 °C for solidification; ethyl palmitate: 21.3–27.6 °C for melt and 25.5–19.5 °C for solidification; 1-octadecanol: 47.8–59.6 °C for melt and 59.1–50.5 °C for solidification; 1-tetradecanol: 30.5–39.1 °C for melt and 38.4–31.2 °C for solidification.
n-Octadecane	903	61.4	1284	−33.2
PCM-1	1457		858
Ethyl palmitate	490	126.1	2140	−12.9
PCM-2	1108		1865
1-Octadecanol	2544	133.9	1428	−23.7
PCM-3	5950		1089
1-Tetradecanol	1990	174.4	2199	−4.4
PCM-4	5461		2102

---

3.6 Solubility parameter analysis

In order to establish a method for estimating the effects of interactions between the gelator and phase change material on leakage and the thermal storage performance, we attempt to rationalize the thermal performance data using solubility parameter analysis. The Flory–Huggins parameter (χ) is used to propose the interactions between gelator and solvent.^26,27 So, we decided to consider the Flory–Huggins parameter to discuss the DMDBS performance in the four tested phase change materials. The χ value could be estimated using the Hilderbrand solubility parameters of the solvent (δ₁) and gelator (δ₂) and the solvent molar volume (V₁), as shown in eqn (4).

χ₁₂ = V₁(δ₂ − δ₁)²/RT (4)

The Hilderbrand solubility parameters of DMDBS and the phase change material can be calculated according to Fedors’ group contribution method (Table S1–5†). This method predicts the cohesive energy with a mean accuracy of about 10%.

The value of the Flory–Huggins parameter of DMDBS in each phase change material was calculated as described above and a strong correlation between the fiber diameter, leakage, the extension of the endothermic time and χ was found (Table 4). When χ decreased, the fiber diameter and leakage of the corresponding composite PCM decreased gradually and the percentage of the endothermic time extension increased. The Flory–Huggins parameter is a polymer–solvent interaction parameter; when the value of χ increases, the tendency toward dissolution decreases, which means the gelator–gelator interaction increases, so thick fibres would be formed in the higher-χ sample. The n-octadecane based composite has the highest χ value, indicating that the interaction between DMDBS and n-octadecane is the weakest. According to the experimental results, a loose three-dimensional network with relatively thick fibres was obtained in PCM-1, which had the weakest capillary force to immobilize the melted n-octadecane, resulting in higher leakage and a lower extension of the endothermic time. The minimum χ value of PCM-4 resulted in the densest network of DMDBS, so it has the minimum leakage value and the maximum extension of the endothermic time. Similar conclusions could also be drawn for the experimental results of PCM-2 and PCM-3.

Table 4 Effects of χ on the fiber diameter and thermal performance of the composite PCMs

Sample	χ	Fiber diameter (µm)	Leakage (%)	Endothermic difference (%)
PCM-1	10.14	0.34–0.52	36.30	61.35
PCM-2	7.81	0.17–0.32	34.36	126.12
PCM-3	4.45	0.18–0.28	14.91	133.88
PCM-4	3.09	0.10–0.21	13.55	174.42

---

4. Conclusion

In conclusion, four gelatinous shape-stable phase change materials were successfully prepared and the morphology, leakage, thermal properties and thermal conductivity were investigated in detail. By introducing a low molecular mass organogelator into the PCM, most of the melted phase change material was immobilized in the three-dimensional network formed by DMDBS. It is encouraging that relatively low concentrations of DMDBS had a significant effect on reducing leakage. The heat transfer rates in the composite PCMs were slower than those in the pure phase change materials, which was caused by the weakened natural convection in these samples. Flory–Huggins parameters were calculated for the different composites. The results are helpful to understand the interactions between the gelator and phase change materials. The different composites have different interactions, which affect the self-assembly of the gelators to form three-dimensional networks, thus lead to different leakage percentages and thermal conductivity performances. In terms of applications of gelatinous soft materials, it is important to estimate the influence of the gelator–solvent interactions on aggregate structures which affect the structure, leakage and thermal conductivity of the materials.

Acknowledgements

We are grateful for the financial support of the National Natural Science Foundation of China (21276188).

References

1. S. Raoux, W. Welnic and D. Lelmini, Phase change materials and their application on nonvolatile memories, Chem. Rev., 2010, 110, 240–267.
2. G. Y. Fang, H. Li, F. Yang, X. Liu and S. G. Wu, Preparation and characterization of nano-encapsulated n-tetradecane as phase change material for thermal energy storage, Chem. Eng. J., 2009, 153, 217–221.
3. Z. H. Chen, F. Yu, X. R. Zeng and Z. G. Zhang, Preparation, characterization and thermal properties of nanocapsules containing phase change material n-dodecanol by miniemulsion polymerization with polymerizable emulsifier, Appl. Energy, 2012, 91, 7–12.
4. X. X. Zhang, Y. F. Fan, X. M. Tao and K. L. Yick, Crystallization and prevention of supercooling of microencapsulated n-alkanes, J. Colloid Interface Sci., 2005, 281, 299–306.
5. G. Y. Fang, H. Li, Z. Chen and X. Liu, Preparation and properties of palmitic acid/SiO₂ composites with flame retardant as thermal energy storage materials, Sol. Energy Mater. Sol. Cells, 2011, 95, 1875–1881.
6. J. L. Zeng, J. Zhang, Y. Y. Liu, Z. X. Cao, Z. H. Zhang, F. Xu and L. X. Sun, Polyaniline/1-tetradecanol composites form-stable PCMs and electrical conductive materials, J. Therm. Anal. Calorim., 2008, 91, 455–461.
7. A. Sari, C. Alkan, A. Karaipekli and O. Uzun, Microencapsulated n-octacosane as phase change material for thermal energy storage, Sol. Energy, 2009, 83, 1757–1763.
8. S. S. Ray, R. P. Sahu and A. L. Yarin, Nano-encapsulated smart tunable phase change materials, Soft Matter, 2011, 7, 8823–8827.
9. L. J. Wang and D. Meng, Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage, Appl. Energy, 2010, 87, 2660–2665.
10. X. X. Zhang, P. F. Deng, R. X. Feng and J. Song, Novel gelatinous shape-stabilized phase change materials with high heat storage density, Sol. Energy Mater. Sol. Cells, 2011, 95, 1213–1218.
11. T. Tian, J. Song, L. B. Niu and R. X. Feng, Preparation and properties of 1-tetradecanol/1,3:2,4-di-(3,4-dimethyl)benzylidene sorbitol gelatinous form-stable phase change materials, Thermochim. Acta, 2013, 554, 54–58.
12. L. B. Xing, B. Yang, X.-J. Wang, J.-J. Wang, B. Chen, Q. Wu, H.-X. Peng, L.-P. Zhang, C.-H. Tung and L.-Z. Wu, Reversible Sol-to-Gel Transformation of Uracil Gelators: Specific Colorimetric and Fluorimetric Sensor for Fluoride Ions, Langmuir, 2013, 29, 2843–2848.
13. D. Khatua and J. Dey, Spontaneous formation of gel emulsions in organic solvents and commercial fuels induced by a novel class of amino acid derivatized surfactants, Langmuir, 2005, 21, 109–114.
14. D. Dasgupta, S. Srinivasan, C. Rochas, A. Ajayaghosh and J.-M. Guenet, Sovent-mediated fiber growth in organogels, Soft Matter, 2011, 7, 9311–9315.
15. Y. P. Wu, S. Wu, G. Zou and Q. J. Zhang, Solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel, Soft Matter, 2011, 7, 9177–9183.
16. M. Bielejewski, J. Kowalczuk, J. Kaszynska, A. Lapinski, R. Luboradzki, O. Demchuk and J. Tritt-Goc, Novel supramolecular organogels based on a hydrazide derivative: non-polar solvent-assisted self-assembly, selective gelation properties, nanostructure, solvent dynamics, Soft Matter, 2013, 9(31), 7501–7514.
17. M. Suzuki, H. Saito and K. Hanabusa, Two-component organogelators based on two L-amino acids: effect of combination of L-lysine with various L-amino acids on organogelation behavior, Langmuir, 2009, 25, 8579–8585.
18. U. Beginn, Applicability of frozen gels from ultra high molecular weight polyethylene and paraffin waxes as shape persistent solid/liquid phase change materials, Macromol. Mater. Eng., 2003, 288, 245–251.
19. F. Delbecq, Y. Masuda, Y. Ogue and T. Kawai, Salt complexes of two-component N-acylamino acid diastereoisomers: self-assembly studies and modulation of gelation abilities, Tetrahedron Lett., 2012, 53, 6588–6593.
20. K. Q. Liu and J. W. Steed, Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons, Soft Matter, 2013, 9, 11699–11705.
21. P. K. Sukul and S. Malik, Removal of toxic dyes from aqueous medium using adenine based bicomponent hydrogel, RSC Adv., 2013, 3, 1902–1915.
22. H. Li and G. Y. Fang, Experimental investigation on the characteristics of polyethylene glycol/cement composites as thermal energy storage materials, Chem. Eng. Technol., 2010, 33, 1650–1654.
23. H. Q. Xu, J. Song, T. Tian and R. X. Feng, Estimation of organogel formation and influence of solvent viscosity and molecular size on gel properties and aggregate structures, Soft Matter, 2012, 8, 3478–3486.
24. M. George and R. G. Weiss, Molecular organogels. Soft matter comprised of low molecular-mass organic gelators and organic liquids, Acc. Chem. Res., 2006, 39, 489–497.
25. M. Xiao, B. Feng and K. C. Gong, Thermal performance of a high conductive shape-stabilized thermal storage material, Sol. Energy Mater. Sol. Cells, 2001, 69, 293–296.
26. S. J. Liu, W. Yu and C. X. Zhou, Solvent effects in the formation and viscoelasticity of DBS organogels, Soft Matter, 2013, 9, 864–874.
27. D. W. Van Krevelen, Properties of polymers, Elsevier Scientific Publishing, New York, 1976.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15420g

---