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Journal of Electrostatics 73 (2015) 56e64

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Journal of Electrostatics

journal homepage: www.elsevier .com/locate/elstat

Preparation of calcium alginate microcapsules containingn-nonadecane by a melt coaxial electrospray method

Meghdad Kamali Moghaddam a, *, Sayed Majid Mortazavi a, Taghi Khayamian b

a Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iranb Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

Article history:Received 1 February 2014Received in revised form24 August 2014Accepted 27 October 2014Available online 8 November 2014

Keywords:AlginateCoaxialElectrosprayMelt spinningNonadecanePhase change material

* Corresponding author. Tel.: þ98 0311 3609014.E-mail address: Kamali@tx.iut.ac.ir (M.K. Moghadd

http://dx.doi.org/10.1016/j.elstat.2014.10.0130304-3886/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

A novel method called melt coaxial electrospray was used to produce phase change material (PCM)microcapsules with sodium alginate as the shell and n-nonadecane as the core. The effect of productionparameters on microcapsule size and structure was studied. The microcapsules were characterized usingoptical microscopy and Fourier transform infrared (FTIR) spectroscopy. The thermal behavior of PCMmicrocapsules was measured by differential scanning calorimetry (DSC). The results of FTIR and DSCconfirmed the successful melt coaxial electrospraying of PCMs. Careful control of production parametersyielded spherical PCM microcapsules with diameter lower than 100 mm and 56 ± 5% encapsulation ratio.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Phase change materials (PCMs) are among interesting energystorage materials as they can store thermal energy nearly isother-mally via phase transition, with relatively high thermal capacityand storage density and small temperature swing [1,2]. Hydrocar-bon PCMs, compared to other phase change materials, have beencommonly used due to their high latent heat storage capacity, goodthermal and chemical stability, self-nucleating behavior and lowvapor pressure [3,4]. These materials have solideliquid phasechange process. Therefore, they are encapsulated with the poly-meric or inorganic materials to prevent the leakage of them frommicrocapsules during their phase change. Microencapsulation ofPCM for thermal energy storage and heat exchanger purposes havereceived considerable attention in numerous industries such astextile [5e9], building [10e14], pharmaceutical [15], etc.

There are several methods for encapsulation like emulsionpolymerization [10], in-situ polymerization [13,16], interfacialpolymerization [17,18], suspension-like polymerization [19], com-plex coacervation [20], solegel [2] and spray draying [11]. Given thesignificant drawbacks of these methods, such as the long timeneeded, capsule size limitation, size control difficulty, additives and

am).

surfactant requiring, etc., fabrication of capsules by electrosprayand coaxial electrospray could be a suitable method for theencapsulation of PCM and different substances such as drugs,fragrant oils, dyes, etc.

Loscertales et al. (2002) are the pioneers of coaxial electro-spraying [21]. Coaxial electrospraying, compared with the con-ventional electrospray, has many advantages. In coaxialelectrospinning, two or more immiscible fluids are injectedthrough an electrified coaxial needle. The coaxial electrospinningdevices and processes are often a little complicated. The general setup of coaxial devices used bymost researchers is the same [22e27],but a modification is made in using the number of pump syringes,the coaxial capillary configurations, the arrangement of devices andfeeding rates controllers.

In the previous studies, various types of capsules have beenfabricated using electrospray and electro-coextrusion (coaxialelectrospray) [28,29], but there are few studies on electro-encapsulate PCMs, especially the hydrocarbon PCMs. Therefore,fabrication and development of electro-encapsulation PCM viameltcoaxial is important, and any study that would provide new in-sights into the process could be of value.

The mainstream research on PCMs capsules has shown that theformaldehyde constitution is the most important part of shellmaterials like melamine-formaldehyde (MF) resin [30], urea-formaldehyde (UF) resin [31] and gelatin-formaldehyde [32].However, these shell materials usually release poisonous

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e64 57

formaldehyde in the applications, causing environmental andhealth problems, especially in textile applications. For this reason,some researchers have encapsulated PCMs with other polymermaterials such as gelatinearabic gum [11], phenolic resin [33],polyurethane [34], chitosan/gelatin [35], methacrylic acid andmethacrylate [36], agaregelatinearabic gum [20], etc. Neverthe-less, encapsulation of PCM with biopolymers such as alginate hasrarely been attempted.

Alginate is a family of anionic polysaccharides, which is ob-tained from seaweeds, and composed of un-branched homopoly-meric regions of D-mannuronate and L-guluronate. The mannuronicacid forms b (1e4) linkage while the guluronic acid forms a (1e4)linkage. Alginate becomes a spherical porous hydrogel networkwhen reacting with certain divalent cations such as Ca2þ [37].

To the best of authors' knowledge, there is no study on theencapsulation of PCMs with alginate as natural polymers by usingcoaxial electrospray method. In this study, sodium alginate and n-nonadecane were used as the shell and core material, respectively.The n-nonadecanewas found to have desirable thermal storage andrelease in a proper phase change temperature range (26e32 �C)and was suitable for textile application. The encapsulated PCMswere prepared by simple melt coaxial electrospray with controlledtemperature. The capsule size was controlled by changing the flowrates of working solution, working distance, applied electric po-tential, and the solution concentration. Optical microscopy inves-tigated the morphologies and the particle size of alginate/nonadecane microcapsules. The chemical characterization of thecapsules was done by Fourier transform infrared spectroscopymethod. The thermal behavior of the capsules was determined bydifferential scanning calorimetry.

Materials and methods

Materials

n-Nonadecane (C19H40, Mw ¼ 268.52 g/mol, Merck, Germany)as the core material and alginate (Alginic acid sodium salt, AlfaAesar, China) as the shell material was used for encapsulation. Thecalcium chloride was purchased fromMerck and used in the gellingbath.

Fig. 1. (a) Schematic of the melt coaxial electro

Electrospraying setup

The experimental setup for encapsulation is shown in Fig. 1. Twoimmiscible liquids were injected at appropriate flow rates throughtwo concentrically located needles. The flow rate of the solutionwas controlled with a syringe pump (DV-BZ: 02/m-014, Germany).Two needles with a blunt end were used for capsulation, where theinner needle had the outer/inner diameter of 0.82 mm/0.51 mm(gauge 21) and the outer needle had the outer/inner diameter of1.6 mm/1.2 mm (gauge 16 G). The outer nozzle was charged to thecathode of a high-voltage generator and a metallic ring was con-nected to the anode. The ring has a diameter of 8 cm. The heatingsyringe was constructed of a glass syringe and heating tapes with acontrollable electrical source. The syringe heating was set at 50 �Cto keep the nonadecane flowing fluidly.

Preparation of nonadecane-alginate capsules

In the experimental setup shown in Fig. 1, the alginate solutionin deionized water was loaded in the outer syringe. The non-adecane was first melted at 50 �C and then injected to the innerheating syringe. The core and shell substances were controlled witha flow rate separately by syringe pump. The distance between thenozzle tip and the container was adjusted by changing the verticalheight of the pump/nozzle set up. To prevent the inner materialfrom freezing by jamming of the needle, a heating plate heated theatmosphere around the nozzle system. The fabricated capsuleswere dripped into a container of the gelling agent of calciumchloride aqueous solution (2%w/v), and the container was kept on amagnetic stirrer with a constant stirring speed. The solution wasgently stirred for 30 min while the capsules were cured. The cap-sules were then collected and washed for analyses. The values ofexperimental parameters are shown in Table 1.

Characterization

Fourier transform infrared (FTIR) spectra of the samples wereobtained using a BOMEM (MB-Series 100, Canada) FTIR spectro-photometer at a scanning number of 8 with the KBr samplingmethod. An optical microscope (Motic Microscope-B3, UK) was

spray setup and (b) coaxial needle setup.

Table 1The variable parameters used for encapsulation using melt coaxial electrospray.

Parameter Value Unit

Alginate concentration 1e1.5e2 % (w/v)Flow rate of core 0.05e0.2 ml/hFlow rate of shell 0.15e0.6 ml/hApplied voltage 10e20 kVNeedle-to-collector distance 3e5e10e15e20 cm

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e6458

employed to investigate the morphology and size of the micro-capsules. A differential scanning calorimeter (DSC 2010, TA in-strument, USA) was used to evaluate the thermal properties such asenergy storage capacity, melting and crystallizing peaks of micro-capsules. The analyses were carried out from 0 to 60 �C at a heatingrate of 10 �C/min under a nitrogen atmosphere. The viscosity ofalginate solutions was determined by Brookfield-Viscometer-DVII þ Pro (USA) and their conductivity was measured by JENWAY3540 pH & conductivity meter.

Fig. 3. Effect of applied voltage on capsules size (10 cm needle to collector distance).

Table 2Viscosity and conductivity of alginate solutions.

Alginate concentration (% w/v) Conductivity (mS) Viscosity (cP)

1 3.68 24.21.5 5.02 52.12 6.61 131.7

Results and discussion

Fabrication of microcapsules

According to the literature, alginates capsules are createdthrough the formation of alginate gels in the presence of divalentcations [38e40]. Alginates consist of b-D-mannuronate (M) and a-L-guluronate (G) units (Fig. 2a). In the presence of divalent cationssuch as Caþ2, the COO� groups of the guluronate blocks formphysically cross-linked gels. They can trap the cations in a stable,continuous, and thermo-irreversible three-dimensional network(Fig. 2b).

In this research, the ability of the melt coaxial electrosprayprocess was studied to prepare spherical capsules of PCM/alginatewith controlled size and narrow size distribution. In traditional orcoaxial electrospray process, several electro-hydrodynamic pa-rameters (i.e., applied voltage, working distance, flow rate of so-lution, needle gauge, and needle length) and properties of thejetting liquids (i.e. Concentration, viscosity, conductivity, and sur-face tension) may affect the properties of the fabricated capsules.However, in this work, the effects of alginate concentrations,working distance and flow rate on the size and size distributionwere investigated.

It is well known that a compound Taylor cone is formed at theexit of the needles in a set range of values of the electrical potentialand flow rates. Moreover, the stability of the structure Taylor conesand the electrified compound jets depend on the physical proper-ties of the working fluids, the liquid flow rates, and applied volt-ages. After electrospray, the particle size was measured and the

Fig. 2. (a) Molecular structure of sodium alginate and (b) the interacti

particle diameter vs. applied voltage was plotted (Fig. 3). As shownin Fig. 3, three modes of capsule formation were observed. The firstsection was dripping mode at the voltage between 8 kV to about13 kV. At the second mode, the unstable transition in the range of13e17 kV was occurring. As the applied voltage increases up to17 kV, the jet mode was dominated. In microencapsulation, the jetmode is often considered as the suitable mode. Therefore, in thefollowing experiments, the reported applied voltage was the opti-mum value in which the cone jet was formed.

The effect of alginate concentration

The concentration and molecular weight of the polymer are theeffective parameters in electrospray process. The solution viscosityand conductivity are increased as the solution concentration isincreased [41]. Table 2 shows the viscosity and conductivity ofalginate solutions at three concentrations.

on between calcium cations and oxygen atoms of guluronic acid.

Fig. 4. Optical micrograph of the microcapsules fabricated at different alginate concentrations, (a) 1%, (b) 1.5% and (c) 2% (w/v). Operation conditions: Flow rate ofshellecore ¼ 0.3:0.1, voltage ¼ 18 kV, needle-to-collector distance ¼ 10 cm and stirrer rate ¼ 500 rpm.

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e64 59

The particle size and shape are two important parametersaffecting the physical properties of microcapsule. Fig. 4 shows thatthe capsules size was increased with alginate concentration. Atsodium alginate concentration of 1% (w/v), the mean diameter ofmicrocapsule was 315 ± 33 mm, while the size capsule at 1.5% (w/v)and 2% (w/v) was 326 ± 35 mm and 453 ± 72 mm, respectively. It canbe seen that the prepared capsule (Fig. 4b) had a uniform sphericalshape with a uniform structure. The particles were smooth withoutedges or sharp dents. Mazzitteli et al. [42] also found that thesmaller mean diameter and homogenous dimensional distributionwere obtained using an alginate concentration 1.5% (w/v). Asshown in Fig. 4c, when an alginate solutionwith high concentration

Fig. 5. Optical micrograph of the surface of microcapsules synthesized at the needle-to-concentration ¼ 1.5% (w/v), feed rate of shell ¼ 0.3 ml/h, feed rate of core ¼ 0.1 ml/h, appl

(2% w/v) was used, the microcapsules were polydispersed (CV% ¼ 44). The high viscosity of solution (2% w/v, 131.7 cP) may havebeen the reason for the lack of a balance of force leading to anunstable Taylor cone.

The effect of working distance

Variation in the distance between the nozzle tip and thecontainer has a direct influence on the strength of the electric fieldand the flight time of the polymer jet. Electric field intensity has aninverse relation with the working distance (Eq. (1)) [43].

collector distance (a) 20, (b) 15, (c) 10 and (d) 5 cm. Operation conditions: alginateied voltage ¼ 18 kV, stirrer rate ¼ 500 rpm.

Table 3Flow rate of shell and core solutions.

Shell and core flow rate (ml/h) S/C ratio

0.6e0.2 3:10.45e0.15 3:10.3e0.1 3:10.15e0.05 3:1

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e6460

EC ¼ 2V

rc ln�4drc

� (1)

where V is the voltage, rc is the outer radius of the nozzle and d isthe distance between the nozzle tip and the container. Clearly, theelectric field (EC) increases with the decrease of working distance(d).

Fig. 5 shows the effect of working distance on capsules size byvarying the flight time of capsules before being gelled in calciumchloride solution. The produced microcapsules have particle sizedistribution with relatively broad interval (310e590 mm,290e510 mm, 310e480 mm, and 210e350 mm for the preparedmicrocapsules with needle-to-collector distance of 20, 15, 10, and5 cm, respectively). It was found that the decrease of workingdistance from 20 to 5 cm led to decrease the capsules diameterfrom 480 ± 53 mm to 275 ± 32 mm. The capsules appeared to bespherical, smooth and homogeneously distributed without anycollapsed particles at a needle-to-collector distance of 5 and 10 cm(Fig. 5c and d). The encapsulation of PCM was done at lower flowrates of coreeshell solutions (0.15:0.05 ml/h) and at a workingdistance in the range of 3e10 cm to carefully study the influence ofneedle-to-collector distance on capsule size.

Fig. 6 shows the photograph of fabricated microcapsules atdifferent needle-to-collector distance. The results suggested thatwith decreasing the needle-to-collector distance, the capsule sizecould be decreased. The results showed that microcapsules pro-duced at 3 cm needle-to-collector distance had a spherical shapewith a nearly uniform structure and diameters lower than 100 mm.This might be because as the needle-to-collector distance wasdecreased, the electrical field strength and jet speed wereincreased, thereby leading to the reduction of the droplet diameter;therefore, the size of the capsules was decreased. The results werein agreement with those other reports which revealed that particlesize of nanoparticles was decreased when needle-to-collector dis-tance was decreased [44,45].

Flow rate of solutions

In the current coaxial electrospray, it is possible to control themicrocapsule diameter and the shell membrane thickness of thefabricated microcapsules. In a coaxial system using distinct

Fig. 6. Optical micrograph of microcapsules synthesized at the needle-to-collector distanceflow rate of shell ¼ 0.15 ml/h, flow rate of core ¼ 0.05 ml/h, applied voltage ¼ 18 kV, and

polymer solutions, the relative volumetric flow ratios of the solu-tions considerably influence the shape and overall structure. In thisstudy, we used different flow rates of shell and core (with the sameratio) to investigate the effect of these flow rates on capsulation(Table 3).

In coaxial electrospray, it has been accepted that the core flowrate should be lower than the shell flow rate. In other words, thecontinuous incorporation of the core into the shell does not occur atthe low shell flow rate, and the shell does not encapsulate the coreuniformly when core flow rate is too high [46,47].

Fig. 7 shows an optical micrograph of fabricated microcapsules.These microcapsules display a regular spherical shape without anyclear deformity of their surfaces. The size of the capsule wasincreased with the volumetric flow rate of the core-shell solution.However, the capsule size was not changed significantly with adecrease in the flow rate of shell and core solution from 0.3:0.1 (ml/h) to 0.15:0.05 (ml/h). The fabricated calcium alginate microcap-sules were relatively transparent, and the texture of the alginatemembranes and the inner oil droplets could be seen clearly. Asshown in Fig. 7a and b, some fabricated capsules did not have corematerial. It is observed that the shell encapsulates the core uni-formly with lower flow rates.

Structure of microcapsules

The chemical characterization of alginate/nonadecane micro-capsules was carried out using Fourier transform infrared spec-troscopy method. The FTIR spectra of the PCM microcapsules, n-nonadecane, and sodium alginate are shown in Fig. 8. According tothe literature [10,16,33], in the pure aliphatic paraffin spectrum, thepeaks at 2960e2850 cm�1 are CeH stretching peaks, the peaks at1490e1460 cm�1 are associated with the CeH bending, and thepeak at 1377 cm�1 represents CeH and CeC bending in a plane. The

(a) 3, (b) 4, (c) 5, (d) 10 cm. Operation conditions: alginate concentration ¼ 1.5% (w/v),stirrer rate ¼ 500 rpm.

Fig. 7. Generated microcapsules under different flow rate of the shell and core (mL/h) (a) 0.6: 0.2, (b) 0.45: 0.15, (c) 0.3: 0.1, (d) 0.15: 0.05. Operation conditions: alginateconcentration ¼ 1.5% (w/v), needle-to-collector distance ¼ 10 cm, applied voltage ¼ 18 kV, and stirrer rate ¼ 500 rpm.

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e64 61

in-plane rocking vibration and bending in a plane showed thatethylene groups could be seen at 730e700 cm�1.

Fig. 8a shows the absorbance spectra of the characteristic peaksfor nonadecane, alginate, and microcapsule in the range of4000e400 cm�1. As shown in Fig. 8b and c, the absorption peaks ofthe pure n-nonadecane appeared at 2920, 2850.5, 1467.7, and721.32 cm�1. The absorption bands from FTIR spectrum of alginatewere assigned as follows: 1731.9, 1635.5 and 1458 cm�1 corre-sponded to the asymmetrical band of the carboxylate ion, and3444.6 cm�1 was attributed to hydroxyl band [31]. The FTIR spectraof these microcapsules also displayed the in-plane rocking vibra-tion of the methylene group at 713.6 cm�1and the CeH wasbending vibration in themethylene group at 2920 and 2850.5 cm�1.This means that the nonadecane was microencapsulated insidealginate shell, successfully.

Phase change property

Differential scanning calorimetry investigated the phase changebehavior of the n-nonadecane and microcapsule and their thermo-grams (Fig. 9). As can be seen from Fig. 9a, the nonadecane had twoabsorbing heat peaks that referred to the rotator phase transitionfrom a fully ordered crystal to a crystal that retained translationalorder, but exhibited a dynamic orientational disorder(21.68e27.30 �C) and solideliquid phase transition (27.30e37.86 �C).

Thermal properties evaluated from the curves showed that n-nonadecane was melted at 21.68 �C and frozen at 29.67 �C. In theheating process, a small peak showed solidesolid transition and thesignificant peak signified solideliquid melting process. On theother hand, the big peak denoted the liquidesolid solidification andthe small peak indicated the solidesolid transition process in the

cooling process. As depicted in Fig. 9b, the solideliquid melting andliquidesolid solidification temperatures of microcapsule were31.12 �C and 30.20 �C, respectively. Noticeably, the latent heat ofmicrocapsule (DH ¼ 81.67 J/g) was smaller than that of pure n-nonadecane (DH ¼ 146.3 J/g). This value showed that not all phasechange materials of the initial recipe had been encapsulated insidethe microcapsule. Therefore, the results of DSC data confirmed thesuccessful encapsulating of n-nonadecane in the melt coaxialelectrospray process. The phase transition characteristics of non-adecane and microcapsules are summarized in Table 4.

The fusion and crystallization temperature (Tm and Tc) and theencapsulation ratio (R) are the important parameters in the phasechange behavior of PCMmicrocapsules. The encapsulation ratio canbe computed by the following:

R ¼ jDHMicro�PCMjjDHPCMj � 100 (2)

where jDHMico�PCMj is the enthalpy of microcapsules (J/g) andjDHPCMj is the enthalpy of n-nonadecane. According to DSC datashown in Table 4, the enthalpy of the microcapsules was 81.67 J/g,thereby revealing the microcapsules containing 56 wt% of n-nonadecane.

Conclusion

The objective of this study was to fabricate microcapsules usingmelt coaxial electrospray, a unique method used for the encapsu-lation of phase change materials. Sodium alginate as the shellmaterial and n-nonadecane as the core material were employed inthis research. Process parameters, such as alginate concentration,

Fig. 9. DSC thermogram of (a) n-nonadecane and (b) alginate/n-nonadecane microcapsule.

Fig. 8. FTIR spectra of alginate/nonadecane microcapsule, (a) 400e4000 cm�1, (b) 400e2000 cm�1, and (c) 2000e4000 cm�1.

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e6462

Table 4Differential scanning calorimetry data of pure n-nonadecane and microcapsule.

Heating

Solidesolid transition Solideliquid melting

Tonset(�C)

Tpeak(�C)

Tendset(�C)

DH(Jg�1)

Tonset(�C)

Tpeak(�C)

Tendset(�C)

DH(Jg�1)

n-Nonadecane 21.68 22.75 27.30 47.60 31.12 33.12 37.86 146.3Microcapsule 21.00 22.46 24.00 18.63 30.00 32.10 37.12 81.67

Cooling

Liquidesolid solidification Solidesolid transition

Tonset(�C)

Tpeak(�C)

Tendset(�C)

DH(Jg�1)

Tonset(�C)

Tpeak(�C)

Tendset(�C)

DH(Jg�1)

n-Nonadecane 29.67 27.90 23.99 47.90 20.39 18.68 16.79 146.50Microcapsule 30.20 28.71 23.15 18.68 19.32 18.52 16.00 81.88

M.K. Moghaddam et al. / Journal of Electrostatics 73 (2015) 56e64 63

working distance, and flow rate of solution, affecting the diameterof capsules were investigated. The PCM microcapsules were char-acterized using optical microscope, Fourier transformed infraredspectroscopy, and differential scanning calorimetry. The experi-mental results showed that the capsules size was increased withalginate concentration. The smaller mean diameter and homoge-nous dimensional distribution were obtained with an alginateconcentration of 1.5% (w/v). Our findings suggested that bydecreasing needle-to-collector distance from 20 to 5 cm, the meancapsule size was found to decrease from 480 to 275 mm. Never-theless, decreasing working distance up to 3 cm led to producing aspherical shape with a nearly uniform structure and diameterslower than 100 mm. The size of the capsules was increased withincreasing the volumetric flow rate of the coreeshell solution.However, the capsules size was not changed significantly with adecrease in the shell and the core solution flow rates (ml/h) from0.3:0.1 to 0.15:0.05. FTIR and DSC results confirmed the successfulmelt coaxial electrospraying of PCMs. The microcapsules with di-ameters lower than 100 mm and n-nonadecane encapsulation ratioof 56 ± 5% was obtained under the experimental conditions. Basedon all these results, it can be concluded that this technique offersgood results regarding the encapsulation of phase change mate-rials; nonetheless, it is highly desirable to study the coaxial elec-trospray parameters such as core needle protrusion, shell and corenozzle diameter, and shell to core flow ratio to enhance the effi-ciency and uniformity of capsules.

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