Inclusion of Phase-Change Materials in Submicron Silica Capsules

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Inclusion of Phase Change Materials in Submicron Silica Capsules Using a Surfactant-Free Emulsion Approach Zhi Chen, Yongliang Zhao, Yue Zhao, Helga Thomas, Xiaomin Zhu, and Martin Moeller Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02435 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 12, 2018

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Inclusion of Phase Change Materials in Submicron Silica Capsules Using a Surfactant-Free Emulsion Approach Zhi Chen,†,§ Yongliang Zhao,‡,§ Yue Zhao,† Helga Thomas,† Xiaomin Zhu,†,* Martin Möller† †

DWI – Leibniz-Institute for Interactive Materials e.V. and Institute for Technical and

Macromolecule Chemistry of RWTH Aachen University, Forckenbeckstrasse 50, Aachen, 52056, Germany ‡

Shanghai Dilato Materials Ltd, Shanghai 200433, P. R. China

ACS Paragon Plus Environment

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ABSTRACT: Microencapsulation of phase change materials is of great importance for thermal energy storage applications. In this work we report on a facile approach to enclosing paraffin in mechanically strong submicron silica capsules without the addition of any classical organic surfactants. A liquid silica precursor polymer, hyperbranched polyethoxysiloxane (PEOS), is used as both silica source and stabilizer of oil-in-water emulsions due to its hydrolysis-induced interfacial activity. Hydrophobic paraffin is microencapsulated in silica with quantitative efficiency simply by emulsifying the mixture of molten paraffin and PEOS in water under ultrasonication or high-shear homogenization. The size of the capsules can be controlled by emulsification energy and rate of subsequent stirring. The silica shell, whose thickness can be easily tuned by varying the paraffin to PEOS ratio, acts as an effective barrier layer retarding significantly the evaporation of enclosed substances; meanwhile the microencapsulated paraffin maintains the excellent phase change performance. This technique offers a low-cost, highly scalable and environmentally friendly process for microencapsulation of paraffin phase change materials.

KEYWORDS: phase change materials, paraffin, microencapsulation, silica, polyethoxysiloxane, surfactant-free emulsions


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INTRODUCTION The limited reserves of fossil fuels and concerns over climate change make the efficient utilization of energy a key issue in modern society. Thermal energy storage (TES) provides an elegant and realistic solution to increase the energy efficiency in a number of domestic and industrial sectors.1 Phase change materials (PCMs) are chemicals that exhibit a high enthalpy of fusion with the ability, in a relatively small volume, to store and release large amounts of energy as latent heat during melting and solidification. The application of PCMs for TES reduces the mismatch between supply and demand, improves the performance and reliability of energy distribution networks and plays an important general role in conserving energy.2-8 PCMs can be divided into two main groups, namely organic and inorganic. Inorganic PCMs are mainly salt hydrates. They usually possess high volumetric latent heat storage capacity and high thermal conductivity, and are often non-flammable and low-cost. However, they are corrosive to most metals and suffer from decomposition and supercooling. The most known and widely used organic PCM is paraffin, which has high latent heat per unit weight and freezes without much undercooling; meanwhile is non-corrosive and chemically stable. The disadvantages of organic PCMs include low thermal conductivity, high volume change upon phase transition and flammability. Microencapsulation is a process, in which tiny particles or droplets are coated by a film-forming material.9 Microencapsulation of PCMs offers a number of advantages, such as reducing environmental sensitivity of PCMs, increasing heat transfer area, preventing their leakage during the phase change process and controlling the volume change as phase transition occurs.10-14 The capsule shell for PCMs consists mostly of organic polymer materials, and the often utilized techniques are spray drying, coacervation and in situ and interfacial polymerization that all


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involve the formation of emulsions. The spray-drying process is based on an aqueous emulsion, in which the PCM is dispersed in a concentrated solution of the shell material. This emulsion is then pulverized with a hot gas, evaporating instantly water and allowing PCM droplets to be trapped inside a film of the shell material. In the coacervation technique a polymer solute is phase-separated in form of small liquid droplets (coacervate droplets), which are then deposited onto the surface of dispersed PCM particles. These droplets slowly unite to form a continuous layer around the core.15 The interfacial polymerization delivers the shell material of the capsules via polycondensation of two monomers dissolved separately in aqueous and organic phases at the interface.16 In the in situ polymerization the shell material is formed in the aqueous phase where all reactants are located.17 Besides organic polymer shells inorganic materials can also be employed to encapsulate PCMs. Although inorganic materials have a number of attractive features, there are only very few studies in this field. Amorphous silica is the most promising inorganic shell material due to its availability, good thermal conductivity, chemical inertness, superior thermal stability, high mechanical strength, non-flammability and biocompatibility.18 Sol-gel technology, which allows shaping silica to thin films under mild and low energy condition,19 is used to encapsulate PCMs. In such a process the PCM is first emulsified in water to yield an oil-in-water (O/W) emulsion, and then a silica precursor, tetraethoxysilane (TEOS)20-23 or sodium silicate,24 is added to coat the PCM droplets with silica. Surfactant molecules20,22-24 or silica particles (Pickering emulsions)21 are required to stabilize the emulsions. In our previous work we reported a novel way to microencapsulate hydrophobic liquid in closed all-silica colloidosomes by linking silica nanoparticles at the oil-water interface using a silica precursor polymer – hyperbranched polyethoxysiloxane (PEOS)25 in a Pickering emulsion.26 It


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was later demonstrated that PEOS can solely stabilize O/W emulsions without any additional emulsifiers due to its hydrolysis-induced amphiphilicity and interfacial activity.27-30 The formation of mechanically strong silica shell, on the one hand, can be catalyzed by methylfunctionalized silica nanoparticles.27 On the other hand, polymerization of the oil phase accelerates the phase separation and subsequently the migration of PEOS to the O/W interface, thus promoting the conversion of PEOS.28 Paraffin is a perfect material to be encapsulated in silica using PEOS technology. It is a highly hydrophobic liquid at temperature above the melting point, and its crystallization upon cooling may further enhance the phase separation in the paraffin/PEOS mixture to form a robust silica shell. Furthermore, since the hydrophobic oil/PEOS mixture forms a miniemulsion in water,27,28 the capsule size can be easily adjusted by varying the homogenization intensity.31 In this work, we aim at developing a new technology for microencapsulation of paraffin in silica using PEOS as both emulsifier and silica precursor. The influence of preparation conditions such as paraffin/PEOS ratio, pH of aqueous medium, homogenization intensity on the capsule morphology and properties are studied.

EXPERIMENTAL SECTION Materials. Tetraethoxysilane (TEOS, GPR RECTAPUR, VWR), acetic anhydride (ACS reagent, ≥ 98.0%, Sigma-Aldrich), n-docosane (ReagentPlus, 99%, Sigma-Aldrich) and titanium trimethylsiloxide (ABCR) were used as received. Deionized water was used for all experiments. PEOS was synthesized according to the method published elsewhere.25 The resulting PEOS had the following characteristics: degree of branching 0.54, SiO2 content 49.2 %, Mn 1740 and Mw/Mn 1.9 (measured by gel permeation chromatography in chloroform with evaporative light scattering detector calibrated using polystyrene standards).


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Table 1. Recipes for microencapsulation of n-docosane in silica with PEOS in 30 g of water

Run

PEOS (g)

n-Docosane (g)

pH

Emulsification method

Mean capsule sizea (nm)

1

1.8

1.2

7.0

Ultrasonication

450 ± 25

2

1.2

1.2

7.0

Ultrasonication

390 ± 100

3

0.6

1.2

7.0

Ultrasonication

540 ± 200

4

1.2

1.2

4.0

Ultrasonication

430 ± 100

5

1.2

1.2

9.4

Ultrasonication

540 ± 220

6

1.2

1.2

10.5

Ultrasonication

122 ± 20

8

1.2

1.2

7.0

Ultra-Turrax, 15000 rpm

2412 ± 930

9

1.2

1.2

7.0

Ultra-Turrax, 20000 rpm

1003 ± 393

a

Mean capsule size was determined by averaging diameter of 500 particles in electron micrographs.

Preparation of paraffin@SiO2 capsules. Solid n-docosane was added into a 100 mL flask, which was charged with 30 mL of water and heated to 60 °C. After complete melting of ndocosane, PEOS was added. The mixture was then emulsified at 60 °C using either ultrasonication for 15 min (Branson Sonifier 450 cell disrupter, 3 mm microtip, 0.9 time circle, 247 W output) or a rotor-stator homogenizer (IKA T 18 digital Ultra-Turrax®) operating at different circumferential speeds. The resulting milk-like emulsion was magnetically stirred with a speed of 750 rpm at 60 °C for 24 h. The capsules were isolated by centrifugation, rinsed 3 times with water, and then dried by freezing-drying. The recipes for the preparation of capsules are summarized in Table 1. Dynamic Light Scattering (DLS) Measurements. Hydrodynamic diameter was measured with a Malvern Zetasizer Nano Series at a scattering angle of 173° at 25 °C in water. Before


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measurements, the stock dispersions were diluted to a particle concentration of approximately 1.5 wt.-‰. Field-Emission - Scanning Electron Microscopy (FE-SEM). FE-SEM measurements were performed on a Hitachi S4800 high-resolution field emission scanning electron microscope with an accelerating voltage of 1.5 kV. Before measurements, the capsule dispersion in water prior to freezing-drying was diluted to a desired concentration and spin-cast on a silicon wafer substrate. In the case of cryogenic FE-SEM measurements, the electron microscope was equipped with a liquid-nitrogen-cooled sample preparation and transfer unit. A drop of an emulsion was placed on a sample holder and quickly frozen by liquid nitrogen. Afterwards it was transferred into a cooled sample chamber of -160 °C, and then into the microscope chamber. Transmission Electron Microscopy (TEM). TEM measurements were carried out on a Zeiss Libra 120 transmission electron microscope. The accelerating voltage was set at 120 kV. The samples were prepared by placing a drop of the diluted capsule dispersion on a Formvar-carboncoated copper grid with 200 meshes. Fourier-Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra of freeze-dried samples were recorded on a Nicolet 60 SXR FT-IR spectrometer using KBr pellet technique. Thermogravimetric Analysis (TGA). TGA measurements were conducted on a Perkin Elmer STA 6000 unit operating under nitrogen atmosphere with a flow rate of 100 mL·min-1. 5-10 mg of freezing-dried capsule powder was placed in a standard Perkin Elmer alumina 85 µL crucible and heated with a rate of 10 °C/min or under isothermal conditions. Differential Scanning Calorimetry (DSC). DSC measurements were performed using a Netzsch DSC 204 unit with a heating or cooling rate of 10 °C/min under a nitrogen atmosphere.


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For the measurements 5-10 mg of freeze-dried capsule powder was enclosed in a standard Netzsch 25 µL aluminum crucible.

RESULTS AND DISCUSSION In this work n-docosane with a melting temperature range from 42 to 45 °C was used as a model paraffin PCM. When n-docosane and PEOS are added into water of 60 °C, a three-phase fluid mixture is formed, where the aqueous phase is sandwiched by the molten paraffin layer on the top and the PEOS layer with a density of 1.14 g/mL on the bottom, showing that PEOS and molten n-docosane are not miscible at this temperature. A homogenous milky emulsion is obtained after high-shear homogenization e.g. unltrasonication. After stirring the resulting emulsion at 60 °C for 24 h an aqueous dispersion of n-docosane@SiO2 microcapsules is prepared. The morphology of the microcapsules was studied by FE-SEM and TEM. Figure 1 shows the electron micrographs of n-docosane@SiO2 microcapsules prepared using the weight ratio of PEOS to n-docosane of 1.5:1 and ultrasonic emulsification. The core-shell structure can clearly be observed by TEM (Figure 1b) due to the high difference in density between ndocosane (0.778 g/cm3) and hydrated silica (1.65 g/cm3). In this case, spherical capsules with an average diameter of 450 nm and a narrow size distribution are obtained. Silica forms a continuous shell around the n-docosane core, and the thickness of the silica shell determined from TEM data is approximately 20 nm. The FT-IR spectrum of the capsules appears to be a superposition of the spectra of n-docosane and hollow spheres comprised of pure silica, indicating the full conversion of PEOS to silica (Figure 2). Considering the density of both ndocosane and hydrated silica, capsule size and silica content in PEOS one can easily calculate the thickness of the silica layer to be 21 nm, which is very close to the value determined by TEM,


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thus confirming the full incorporation of silica into the capsules. It is also worth noting that a grainy rather than smooth outer surface is observed from the magnified FE-SEM image (inset of Figure 1a) as well as the TEM micrograph (Figure 1b). In the literature such kind of characteristic structure was attributed to the phase separation of unhydrolyzed TEOS.32,33 In the PEOS/paraffin system, this structural feature can be attributed to the phase separation between hydrolyzed (hydrophilic) and unhydrolyzed (hydrophobic) PEOS macromolecules that are present during the conversion of PEOS to silica. For microencapsulation of PCM it is desirable to enclose as much as possible active substance in the shell in order to reach high latent heat density. This can be achieved by decreasing the PEOS amount in the present system. Hence the microencapsulation was carried out with reduced weight ratios of PEOS to n-docosane, and the representative FE-SEM and TEM images of the formed n-docosane@SiO2 microcapsules are presented in Figure 3. When the PEOS/n-docosane ratio is 1:1, capsules of submicron size are also obtained; however, the size distribution is higher than that with the ratio of 1.5:1. The thickness of the silica shell measured by TEM is 13.8 nm. With further decreasing the ratio to 0.5:1, both size and size distribution increase (Table 1), and the shell thickness is reduced to 8.3 nm as measured by TEM. Importantly, although the silica shell is very thin, the capsules can still keep the mechanical integrity even under high vacuum and high voltage electron irradiation, indicating good mechanical strength of the shell. When the ratio of PEOS to n-docosane is reduced further, the resulting emulsion becomes instable and many broken capsules are observed by FE-SEM.


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Figure 1. (a) FE-SEM and (b) TEM images of typical n-docosane@SiO2 capsules prepared with the PEOS/ndocosane ratio of 1.5:1 and pH 7 (cf. Table 1, run 1). The scale bars correspond to 500 nm.

Figure 2. FT-IR spectra of PEOS, pure silica, free n-docosane, and n-docosane@SiO2 capsules (cf. Table 1, run 1).


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Figure 3. FE-SEM images of core-shell capsules prepared at pH 7 and with different weight ratios of PEOS to ndocosane: (a) 1:1, (b) 0.5:1 (cf. Table 1, runs 2 and 3). The insets show the corresponding TEM images. The scale bars correspond to 500 nm.

DLS was employed to monitor the process of microcapsule formation (Figure 4). After ultrasonication submicron emulsion droplets are formed, though the size distribution is quite broad. During the course of the conversion of PEOS the size of the droplets continuously decreases, and the size distribution becomes narrower. In comparison with PEOS-assisted miniemulsion polymerization of styrene,28 the size distribution of the emulsion droplets is much broader in the case with paraffin and there are also ultrasmall particles present. In Figure 4 the evolution of particles size distribution is compared between different PEOS/n-docosane ratios. The size and size distribution of the emulsion droplets as well as the final capsules obtained at


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the ratio of 1.5:1 is smaller than that with the ratio of 0.5:1, and this result is in good agreement with the electron microscopy data. With lower PEOS content the reduction of size and size distribution is less significant, and the final size distribution is reached after a shorter time period.

Figure 4. Evolution of particle size distribution during conversion of PEOS at 60 °C after ultrasonic emulsification of PEOS/n-docosane mixture in water measured by DLS: (a) PEOS/n-docosane ratio 1.5:1, run 1 in Table 1; (b) PEOS/n-docosane ratio 0.5:1, run 3 in Table 1.


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Figure 5. Cryo-FESEM images of an emulsion of PEOS/n-docosane (ratio 1.5:1) mixture in water stirred with a speed of 750 rpm at 60 °C. (a) Immediately after ultrasonic emulsification; (b) 1 h after emulsification; (c) 3 h after emulsification.


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Since DLS measurements may not deliver accurate particle size distribution, cryo-FE-SEM was used to study the formation mechanism of silica capsules. The particles size evolution in the emulsions measured by cryo-FE-SEM shows a similar trend as the DLS data. The size distribution of the droplets in the emulsion immediately after ultrasonic treatment is very broad. Besides big microsized droplets there are also very small nanoparticles present. These small particles seem to disappear with time, and completely vanish after 24 h, meanwhile, the size of the big particles decreases significantly. Finally, nearly monodisperse particles are formed. For a better understanding of the reaction mechanism the PEOS/n-docosane(1.5:1)-in-water emulsion was stirred at different speeds. According to the obtained DLS data (Figure 6), the size distribution of the resulting silica capsules depends strongly on the stirring speed. Without stirring the size distribution curve of the capsules almost resembles the one of the emulsion with the shift of the maximum to the smaller size. Interestingly, the size becomes smaller and the size distribution narrows with the increase of the stirring speed.

Figure 6. Dependence of particle size distribution in the system with PEOS/n-docosane ratio 1.5:1 on the speed used to stir emulsions.


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Based on the experimental data we propose the mechanism of formation of paraffin@SiO2 capsules as shown in Scheme 1. Although both PEOS and molten n-docosane are highly hydrophobic, they are not miscible. Nevertheless, a stable emulsion is formed after high-energy homogenization. The emulsion droplets are stabilized by a layer comprised of partially hydrolyzed PEOS; meanwhile, the rest of PEOS forms small droplets with an outmost layer of hydrolyzed PEOS in water. PEOS molecules are adsorbed onto the paraffin particle surface from the aqueous phase in form of small droplets; on the contrary PEOS migrates from the oil phase to the interface in the PEOS/styrene system.28 It seems that the PEOS-stabilized microsized paraffin droplets break up even by weak shearing force generated by magnetic stirring. This unusual behavior can be explained by the fact that hydrolyzed PEOS is located on the water side of the interface, so a gradient of interfacial tension occurs during shearing that slows down the motion of the liquid inside the droplet and hence diminishes the amount of energy needed to deform and break up the droplet.34 Furthermore, during the course of the reaction more amphiphilic PEOS molecules are formed via hydrolysis, and they are continuously adsorbed onto the newly formed droplet surface to restore the equilibrium and to thicken the silica layer in the case of excess. This system differs significantly from the PEOS/styrene one where PEOS and styrene are miscible. The phase separation in that case takes place during the polymerization of styrene and conversion of PEOS, and the latter is promoted by the former by accelerating the migration of PEOS macromolecules to the interface. Without polymerization the conversion of PEOS is not complete even after a long reaction time.28 In the PEOS/n-docosane system PEOS macromolecules are located on the surface of the paraffin core as well as in the form of small droplets in the aqueous phase because of their immiscibility with molten paraffin, thus have much better access to water and a much higher emulsifying efficiency. Therefore, the PEOS


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conversion is completed within a short time. The smaller size and narrower size distribution of the capsules obtained with a higher PEOS/paraffin ratio can be accounted for by a higher amount of active emulsifier.

Scheme 1. Illustration of formation mechanism of paraffin@SiO2 microcapsules and chemical structure of hyperbranched polyethoxysiloxane (PEOS)

TGA was used to measure the silica content of the n-docosane@SiO2 capsules prepared at different weight ratio of PEOS to n-docosane as well as their thermal stability, and the results are shown in Figure 7. Given the fact that the silica content of PEOS is 49.2%, the amount of residue silica after complete evaporation of n-docosane coincides well with the feeding ratio of PEOS to n-docosane shown in Table 1, confirming further the full incorporation of silica into the capsules. As can be seen from the dynamic TGA curves, the onset degradation temperature of ndocosane@SiO2 capsules is higher than that of free n-docosane for nearly 50 °C, showing that even a very thin silica shell already retards significantly the evaporation of the encapsulated paraffin by acting as an efficient barrier layer. The improved thermal stability is of great


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importance for high temperature processing of PCMs, e.g. spray drying, blending with polymer melts, etc. It is worth mentioning that the silica capsules remain intact after evaporation of ndocosane, indicating that the encapsulated substance is released through the pores in the silica shell rather than via the burst of the capsules. A slight weight loss of ca. 2 wt.-% is observed upon heating the n-docosane@SiO2 capsules from ambient temperature to 150 °C. This is ascribed to the loss of adsorbed water molecules on silica surface, which can be confirmed by a strong and broad band at 3430 cm-1 and a weak band at 1631 cm-1 in the FT-IR spectrum (cf. Figure 2). The evaporation of n-docosane was further investigated by isothermal TGA. As compared with the free substance the microencapsulated n-docosane exhibits a significantly reduced evaporation rate. The evaporation can be further slowed down by increasing the silica shell thickness (Figure 7b). The activation energy of evaporation was determined by summarizing the isothermal release profiles at different temperatures using Arrhenius equation (Figure 7c). It is 50.7 kJ/mol for free n-docosane. For n-docosane encapsulated in silica using the PEOS/n-docosane ratio 0.5:1, 1:1 and 1.5:1 it is calculated to be 62.5, 68.5 and 73.7 kJ/mol, respectively. It can clearly be seen that the activation energy of n-docosane enclosed in silica microcapsules is higher than that of free n-docosane, and increases further with the increase of the thickness of the silica shell. In contrast to SiO2 microcapsules prepared in this work, the polymer shell does not retard the evaporation of the encapsulated n-docosane35 possibly due to the plasticization by the molten paraffin as well as decomposition of the polymer shell at high temperature. The evaporation can simply be treated as a chemical reaction,36 and its rate constant je may be written as je = B exp(-∆Ev/RT)

(1)


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where ∆Ev is the molar activation energy of evaporation, R is the molar gas constant, T is the absolute temperature, and B is the as yet undetermined frequency factor. ∆Ev is related to the heat of evaporation ∆Hv according to the following equation ∆Hv = ∆Ev + NA ps (vg – vf)

(2)

NA ps (vg – vf), where NA represents the Avogadro number, is the external work done during the expansion of one mole from the free volume per molecule of vf in the condensed state to the volume per molecule vg in the gaseous state at the vapor pressure ps. The increase of ∆Ev with the thickening of the silica shell can be accounted for by the reduced ps.


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Figure 7. TGA curves of n-docosane@SiO2 microcapsules prepared with different weight ratios of PEOS to ndocosane (cf. Table 1, runs 1 - 3) obtained (a) by heating with a rate of 10 °C/min and (b) in an isothermal process (free n-docosane at 175 °C and n-docosane@SiO2 microcapsules at 200 °C). (c) Arrhenius plot for evaporation of ndocosane from SiO2 microcapsules (cf. Table 1, run 1).


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Figure 8. DSC curves of free n-docosane and microencapsulated in silica n-docosane prepared with different PEOS/n-docosane ratios (cf. Table 1, runs 2 and 3) during cooling (a) and second heating (b) at a rate of 10 °C/min. (c) DCS curves of n-docosane@SiO2 microcapsules prepared with PEOS/n-docosane ratio 0.5:1 recorded during 60 heating and cooling cycles.

The most important characteristic of PCMs is their phase transition behavior. In this work the phase change temperature and enthalpy of the microencapsulated n-docosane were measured by DSC (Figure 8). On the DSC curve of free n-docosane obtained during cooling two exothermic peaks at 38.3 and 35.3 °C are present. It is known from the literature that upon cooling the isotropic melt of n-docosane is transformed to a hexagonally packed RII rotator phase, which is


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then converted to a face-centered orthorhombic RI rotator phase, and finally a stable triclinic crystalline phase emerges.37-39 The transition from RII to RI phase can hardly be observed due to a very small transition enthalpy. The exothermic peaks on the DSC cooling curve of free ndocosane can therefore be attributed to the isotropic melt to rotator phase and rotator to triclinic crystalline phase transitions. The crystallization behavior of the microencapsulated n-docosane is quite different from the free one. First, a small exothermal peak with the peak temperature of 43.5 °C is observed, which corresponds to the surface freezing of alkanes.40 Second, the crystallization peaks become broad and complex, however, they differ significantly from ndocosane microencapsulated in microsized melamine-formaldehyde capsules.41 For the ndocosane@SiO2 capsules prepared with the PEOS/n-docosane ratio of 1.5:1 and 1:1 the peak temperature of the transition from the isotropic melt to the RII phase is very close to that of free n-docosane, meanwhile, n-docosane enclosed in the capsules prepared with the ratio of 0.5:1 exhibits a lower transition temperature. The further phase transitions of the microencapsulated ndocosane occur at much lower temperature than that of free n-docosane. According to Figure 6b the melting temperature shows another trend, i.e. n-docosane enclosed in the thinner but bigger silica capsules prepared with the ratio of 0.5:1 exhibits a melting temperature similar to that in bulk, while the melting temperature of n-docosane in the thicker but smaller silica capsules prepared with the PEOS/n-docosane ratio of 1.5:1 and 1:1 is a bit lower. The phase change enthalpy of the microencapsulated n-docosane is summarized in Table 2. The encapsulation ratio is calculated using the DSC data as the ratio of the melting enthalpy of the n-docosane@SiO2 capsules to that of free n-docosane.23,24 It is lower than the load content of n-docosane measured by TGA (Table 2), implying the reduced crystallinity of the microencapsulated n-docosane. It is well documented that confinement has a strong influence on melting and crystallization of


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materials.42,43 The decrease of melting temperature in the confined geometry is generally ascribed to the existence of a non-freezing layer of liquid near the pore walls. The crystallization of n-alkanes in microsized capsules was well studied.44 In general the formation of a surface freezing monolayer is enhanced that induces the transition from the isotropic liquid to the rotator phase,45 however, during the solid-to-solid phase transition homogeneous nucleation, which leads to significant supercooling,46 is dominant. In our case the similar phenomena are observed. Nevertheless, the effect of confinement is more pronounced as reflected on the DCS traces, since the size of our silica capsules lies in the range of 400 – 600 nm. In future temperature-dependent X-ray diffraction measurements will be performed to address the evolution of crystalline morphology during the crystallization process in these submicron capsules. The phase change characteristics of n-docosane enclosed in the silica microcapsules were examined for 60 heating and cooling cycles using DSC,47 and the results are shown in Figure 8c. After so many cycles the mass loss is negligible and no significant change is observed in the melting and crystallization behavior, indicating the outstanding thermal stability of the n-docosane@SiO2 microcapsules that is important for PCM applications. Table 2. Phase change characteristics of microencapsulated n-docosane prepared with different PEOS/n-docosane ratios (cf. Table 1, run 1, 2 and 3)

PEOS/n-docosane ratio

Melting enthalpy, ∆ࡴ࢓

Crystallization enthalpy, ∆ࡴࢉ

Encapsulation ratioa)

n-Docosane content in capsulesb)

0:1

317.0 J/g

311.6 J/g

-

-

0.5:1

218.9 J/g

215.1 J/g

69.1 %

78.5 %

1:1

185.5 J/g

183.9 J/g

58.5 %

68.3 %

1.5:1

145.0 J/g

141.6 J/g

45.7 %

59.3 %

a)

Encapsulation ratio is calculated as the ratio of the melting enthalpy of the capsules to that of free n-docosane.

b)

n-Docosane content in capsules is determined by TGA shown in Figure 7a.


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From our previous work it is known that the interfacial activity and conversion of PEOS in both O/W and water-in-oil (W/O) emulsions depend strongly on pH of the aqueous media.26-28,30,48 In the W/O Pickering emulsions, colloidosomes with a well-defined silica shell can be prepared by gluing silica particles with PEOS at the oil-water interface in the pH range of 1-4.48 In the case of O/W Pickering emulsions, colloidosomes are formed only at the pH value close to 9.26 When PEOS is used solely, silica capsules, for example polystyrene@SiO2 nanoparticles, with a welldefined core-shell structure can be obtained in a wide pH range from 3 to 10, and pH has an influence on the capsule size. In this work, hydrochloric acid or ammonia solution was used to tune the aqueous pH from acidic to basic conditions. Well-defined n-docosane@SiO2 microcapsules can be prepared in the pH range of 3 to 10. In a wide pH range the capsule size remains similar (Figure 9 a,b), however, it drastically decreases to 150 nm when the pH is raised to 10.5 (Figure 9c). This phenomenon can probably be attributed to the high amphiphilicity of partially hydrolyzed PEOS due to high dissociation degree of silanol groups at high pH. Interestingly, capsules with a smooth surface are obtained at pH other than 7. At the neutral pH the hydrolysis rate is lower than that under both acidic and basic conditions,19 therefore, the phase separation between the unhydrolyzed and hydrolyzed PEOS is much more pronounced at pH 7 where the grainy surface structure is observed.


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Figure 9. FE-SEM images of n-docosane@SiO2 capsules prepared with the n-docosane/PEOS 1:1 at different pH: (a) 4.0, (b) 9.4, and (c) 10.5 (cf. Table 1, runs 4, 5 and 6). The insets show the corresponding TEM images.


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Figure 10. FE-SEM images of n-docosane@SiO2 capsules prepared by emulsifying a mixture of n-docosane and PEOS (1:1) in water of pH 7 using a rotor-stator homogenizer operating at different rotation speeds: (a) 15000 and (b) 20000 rpm (Table1, runs 8 and 9).

The process described so far was performed by means of ultrasonication. A rotor-stator homogenizer was also employed for the emulsification of the PEOS/n-docosane system in water, and n-docosane@SiO2 capsules have been successfully prepared as well. The size of the resulting capsules is quite big lying in the micrometer range (Figure 10); nevertheless, it can be easily tuned by varying the rotation speed (Table 1).


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CONCLUSION In summary, a surfactant-free emulsion technique has been developed for incorporation of paraffin phase change materials in submicron silica capsules using PEOS as both silica source and emulsifier. In this process, the mixture of molten paraffin and PEOS is emulsified in water to form an emulsion. Paraffin is then microencapsulated in silica after the conversion of PEOS to silica to yield quantitatively paraffin@SiO2 microcapsules, and the paraffin content in these capsules can reach up to 80 wt.-%. The mechanism of formation of paraffin@SiO2 microcapsules, which differs significantly from the styrene/PEOS system, has been proposed based on the DLS and cryo-FE-SEM data. The capsules size decreases with the increase of the emulsification energy and strong stirring of the resulting emulsions is required to obtain microcapsules with smaller size and narrower size distribution via further breaking up the droplets. The thickness of the silica shell can be easily adjusted by varying the weight ratio of PEOS to paraffin. According to the TGA measurements the evaporation of paraffin is significantly retarded by the silica shell, and is further slowed down by increasing the shell thickness as indicated by the increase of activation energy. The DSC results show that the encapsulated paraffin maintains excellent phase change performance and remains intact after many heat-cooling cycles, though the melting and crystallization are affected by the confinement. We believe that this technique offers a low-cost, highly scalable and environmentally friendly process for the microencapsulation of paraffin phase change materials, which is important for various TES applications.


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AUTHOR INFORMATION Corresponding Author. *Dr. Xiaomin Zhu: telephone +49-241-8023341; fax +49-2418023301; e-mail [email protected]. Author Contributions. §These authors contribute equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources. The research project IGF-No. 19620 N of the research association Forschungskuratorium Textil e.V. was provided via Arbeitsgemeinschaft industrieller Forschungsvereinigungen

e.V.

(AiF)

within

the

promotion

program

of

Industrielle

Gemeinschaftsforschung und –entwicklung (IGF) of the Federal Ministry of Economics and Technology due to a resolution of the German Bundestag. This work was performed in part at the Center for Chemical Polymer Technology CPT, which is supported by the EU and the federal state of North Rhine-Westphalia (grant no. EFRE 30 00 883 02). Y.Z. thanks Shanghai Pujiang Talent Program (no. 18PJ1432900) for financial support.

Notes. Some portion of the results presented in this paper form a part of a patent submitted by Y.Z., X.Z., and M.M.

ACKNOWLEDGMENT. The authors thank Dr. Walter Tillmann for performing the FT-IR measurements, Dr. Rostislav Vinokur for the DSC measurements and Ms. Sabrina Malmann for cryo-FESEM measurements.

ABBREVIATIONS. PEOS, hyperbranched polyethoxysiloxane; TES, thermal energy storage; PCM, phase change material; O/W, oil-in-water; W/O, water-in-oil; TEOS, tetraethoxysilane; DLS, dynamic light scattering; FE-SEM, field-emission - scanning electron microscopy; TEM, transmission electron microscopy; FT-IR, Fourier-transform infrared; TGA, thermogravimetric analysis; DSC, differential scanning calorimetry.


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Inclusion of Phase-Change Materials in Submicron Silica Capsules

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