Vol. 54 No. 9 2021 517Copyright © 2021 The Society of Chemical Engineers, Japan

Journal of Chemical Engineering of Japan, Vol. 54, No. 9, pp. 517–524, 2021

Preparation of Polystyrene Microcapsules Containing Saline Water Droplets via Solvent Evaporation Method and Their Structural Distribution Analysis by Machine Learning

Sukhbaatar Batchuluun1, Hideki Matsune2, Koichiro Shiomori2, Ochirkhuyag Bayanjargal3 and Tserenkhand Baasankhuu3

1 Interdisciplinary Graduate School of Agriculture and Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-nishi, Miyazaki-shi, MIyazaki 889-2192, Japan

2 Department of Applied Chemistry, University of Miyazaki, 1-1 Gakuenkibanadai-nishi, Miyazaki-shi, MIyazaki 889-2192, Japan

3 Department of Chemical and Biological Engineering, School of Engineering and Applied Sciences, National University of Mongolia University, Street-1, Ulaanbaatar-14201, Mongolia

Keywords: Microcapsules, Solvent Evaporation, S/O/W Emulsion, Structural Distribution, Machine Learning Image Analysis

Most microcapsule preparation methods produce a population of microcapsules in a bulk solution. To control the micro-capsule preparation or obtain an optimal preparation condition, the mechanism of the microcapsule preparation should be investigated. The mechanism is estimated via structure reformation during the preparation process because diameter and wall thickness are drastically altered in the solution. Considering microcapsule applications, some important proper-ties, such as the mechanical properties of microcapsules and release rate of the encapsulated product, depend on the microcapsule structure. In this study, polystyrene microcapsules containing saline water droplets were prepared via the solvent evaporation method from a solid-in-oil-in-water (S/O/W) emulsion system. The microcapsules exhibited a speci�c structural distribution, which comprised monocore, multicore, and solidcore structures. The structural distribu-tion was altered by the preparation condition. The monocore structure was absolutely dominant owing to the increase in the amount of calcium chloride added in the organic phase. The salt concentration is not the sole controlling factor of the microcapsule structure, as the surfactant and dispersion exerted a signi�cant impact on the microcapsule structure. The structural distribution was automatically analyzed by a machine learning algorithm (MLA). The decision-making time for the microcapsules preparation was shortened by the accelerated structure determination, and the accuracy was improved by increasing the number of counting particles.

Introduction

A complex emulsion system such as a water-in-oil-in-water (W/O/W) emulsion is widely adopted in a solvent evaporation method for microencapsulation (Ijichi et al., 1997; Kiyoyama et al., 2003). �e complex emulsion system exhibits an osmotic behavior because the system is a liq-uid membrane, owing to the continuous solvent molecule evaporation and the mutual solubility of the system com-ponents. �e osmotic behavior of the complex system plays an important role in the structure of the microcapsule. In addition, an aqueous solution is widely used in various in-dustries, such as food, cosmetics, dyes, and pharmaceutical industries. �erefore, a water containing microcapsule was prepared by researchers (Kentepozidou and Kiparissides, 1995; Taguchi et al., 2019), using the W/O/W system. How-ever, the S/O/W emulsion system is not usually adopted, except in few studies, including our previous study, which

focused on the preparation and release characteristics of biodegradable microcapsules encapsulating activated carbon impregnated with pesticide using the solvent evaporation method (Shiomori et al., 2004). A multiple emulsion system is the result of double dispersion. Speci cally, in this partic-ular system, which is the S/O/W system, the S/O suspension is initially formed during the solid particle distribution in the oil phase, and then, the resulting emulsion is dispersed in the aqueous media. Here, during the second step, the S/O/W system changed to the W/O/W system because of the osmotic �ow of water. Hence, two surfactants are gener-ally required in the W/O/W system. In our previous study (Kiyoyama et al., 2003; Shiomori et al., 2004), multicore mi-crocapsules were prepared by the complex emulsion system stabilized with a cosurfactant and via the Pickering method. It is necessary to control the internal structure, especially for a given durability. For example, a microcapsule inside concrete must be broken at the time a concrete microcrack is formed, and an active agent is released from inside the microcapsule for self-healing. However the microcapsule must not be ruptured during the mixing of the concrete raw material. �erefore, the structure of microcapsules plays a crucial role in releasing the pro le, as well as in the strength

Received on May 24, 2021; accepted on June 30, 2021DOI: 10.1252/jcej.21we052Correspondence concerning this article should be addressed to K. Shio-mori (E-mail address: shiomori@cc.miyazaki-u.ac.jp).

Research Paper

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or durability of that microcapsule.In a complex system, the e�ect of a surfactant is investi-

gated based on its mean characteristic interface tension. In addition, �uid properties, such as viscosity, are comprehen-sively examined. However, osmosis is not usually studied es-pecially for microcapsule structures. �e structural and size distribution analysis usually start from images of scanning electron microscopy and digital microscopy. �e droplet size distribution of the O/W emulsion containing styrene was studied using an image (Konno et al., 1982; Oshima and Tanaka, 1982). Currently, an image analysis is widely implemented by MLA. �e bene ts of the automation pro-cedure include a shorter decision-making time and higher accuracy. Considering modeling, the population balance equation is adopted in several cases, owing to the statisti-cal behavior of particle systems, such as multiple emulsion systems (Khadem and Sheibat-Othman, 2020) and aerosol (Sun et al., 2021). �e population balance equation for the osmosis dominant system is expressed as:

∂nµ�t ,vµ�∂t + ∂�Sµ�t ,vµ�nµ�t ,vµ��

∂vµ=

RNu ,µ +RCo ,µ +REs,µ

∂nM(t ,vM)∂t + ∂([SM(t ,vM)+QEs,M(t ,vM)]nM(t ,vM))

∂vM=

RBr ,M +RCo ,M +REx p ,M (1)

Here, an inner droplet distribution is in�uenced by sec-ondary nucleation (RNu,μ), which is a solid particle dis-persion, and coalescence in�uences both outer and inner droplet distributions. �e breakage (RBr,M) of outer droplets is triggered by the shear �ow inside a preparation reactor (Oshima and Tanaka, 1982).

�e objective of this study is to prepare polystyrene mi-crocapsules containing saline water droplets and investigate its osmotic �ow-driven structural evolution in a single sur-factant media via machine learning.

1.　Experimental

1.1　ReagentsPolyvinyl alcohol with 98.5% hydrolysis (PVA, polymer-

ization degree: approximately 500), polystyrene (PS, polym-erization degree: approximately 2000), calcium chloride, and 1,2-dichloroethane (DCE) were purchased from Wako Pure Chemical Industries, Ltd.

1.2　Preparation of microcapsule�e preparation scheme of the microcapsule and its

characterization are illustrated in Figure 1. �e organic phase was prepared by dissolving 8.76 wt% of polystyrene in 1,2-dichloroethane. �e S/O suspension was prepared by adding a given amount of CaCl2 to the organic phase and agitating the mixture overnight at 300 rpm. An ultrasonic (Soni er 250, Branson Corp.) was adopted as an alternative dispersion method with a duration, frequency, and am-plitude of 10 min, 20 kHz, and 50%, respectively. �e S/O suspension was added to the outer aqueous phase contain-

ing a desirable concentration of the PVA at 313 K, and the mixture was stirred at 370 rpm to produce the S/O/W emul-sion. �e S/O/W emulsion was stirred, as demonstrated in a previous paper (Shiomori et al., 2004), at 370 rpm and 313 K for 8 h, with the continuous evacuation of solvent vapor at 7.2×102 mmHg. �e microcapsules were ltered with a vacuum pump.

1.3　Observation and characterization of the microcapsule

�e microcapsules were prepared via a solvent evapora-tion method. �e morphologies of the microcapsules were observed via scanning electron microscopy (SEM, TM-1000, Hitachi High-Tech Corp.) and digital microscopy (DM, VHX-600 system, KEYENCE Corp.). Elemental mappings were conducted by an energy dispersive X-ray (EDX) ana-lyzer (EDAX Genesis-APEX2, AMETEK Inc.) with SEM (SU3500, Hitachi High-Tech Corp.), under 20 kV of an ac-celerating voltage. �e microcapsule sample was directly obtained from a reactor using a micropipette. �e images were carefully taken without overlapping via DM. Via MLA, 321 images with 1,600×1,200 pixels were collected with 100 times magni cation for the automatic classi cation of the diameter measurement. �e Hough transform, histograms of oriented gradients (HOG), and support vector machine (SVM) are the main algorithms in image segmentation, fea-ture extraction, and classi cation sections, respectively, in the developed MLA. A more comprehensive information on

Fig. 1 Preparation scheme of microcapsules by solvent evaporation in the S/O/W emulsion system

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the pipeline of structure and diameter detection is available in our previous work (Batchuluun et al., 2021).

For thickness measurements, the microcapsule sample were mixed in glue media at 373 K. In addition, 179 images with 3,200×2,400 pixels were used with 400 times magni -cation. �e inner and outer diameters were manually mea-sured. �e thicknesses were calculated from the diameter measurements.

2.　Results and Discussion

�e structural distribution was changed by the prepara-tion condition. �e monocore structure was dominant by increasing the amount of water soluble solid particles, which include the calcium chloride particles added in the organic phase. �e calcium chloride particles in the oil droplets create osmotic pressure by their dissolution (Colinart et al., 1984). In a liquid membrane system, water is permeated

to the oil phase. In addition, because the organic solvent is evaporated, the wall material is solidi ed, and the nal structures of the microcapsules are formed. Typical DM and SEM images of the surface morphologies and internal struc-ture of the microcapsules controlled by calcium chloride are presented in Figure 2. �e CaCl2 particles were found inside the microcapsule via SEM-EDX mapping, which is pre-sented in Figure 2. Hence, the cores inside a microcapsule are formed by the osmotic pressure of salt. �e e�ects of the dilution, surfactant concentration, and dispersion methods were investigated.

2.1　E�ect of dispersion methodA dispersion method is an essential experimental condi-

tion for the emulsion preparation. Dispersion methods such as stirring and ultrasonication were examined. �e disper-sion factors such as the geometry of vessels and impeller, stirring rate, and duration were maintained as stable as pos-sible. In multiple emulsion systems, particles are dispersed twice as mentioned in the introduction. �e dispersion discussed in this section is the dispersion of solid particles in the organic phase. �e results of two kinds of disper-sion methods are shown in Figure 3 and Figure 4 for the S/O suspension preparation. Microcapsule morphology is di�erent in two dispersion methods but the size distribu-tion is the same except in the case of low dispersion with 3 g (WS/O=4.2%) of the initial amount of CaCl2. �is high amount of salt leads to a bimodal distribution due to the explosion of microcapsules which is the solidcore appearing in Figure 4.

During ultrasonication, the CaCl2 is broken into small particles and is evenly distributed in the oil phase which creates microdroplets more than the stirring. �us this high amount of microdroplets created dark microcapsules. Also, the size distribution was changed from bimodal to unimodal at 3 g (WS/O=4.2%) of initial concentration.

�e optical microscopy characterization in transparent media has advantages such as an observation of the internal

Table 1　Summary of experimental conditions (W(S/O)/W=6.8 wt%, Na=370 rpm)

ID Sample name Dispersion method CPVA [%] WS/O [%]

0.1HD High disp. 0.1 Sonication 0.05 0.1463.0HD High disp. 3.0 Sonication 0.05 4.200.1LD Low disp. 0.1 Stirring 0.05 0.1463.0LD Low disp. 3.0 Stirring 0.05 4.20

0.1HS High surf. 0.1 Stirring 0.40 0.1461.5HS High surf. 1.5 Stirring 0.40 2.100.1LS Low surf. 0.1 Stirring 0.05 0.1461.5LS Low surf. 1.5 Stirring 0.05 2.10

CE0 Calcium e�ect 0 Stirring 0.05 4.20CE1 Calcium e�ect 1 Stirring 0.05 0.146CE2 Calcium e�ect 2 Stirring 0.05 0.729CE3 Calcium e�ect 3 Stirring 0.05 0.0364CE4 Calcium e�ect 4 Stirring 0.05 0.0182CE5 Calcium e�ect 5 Stirring 0.05 0.00911

disp.: dispersion, surf.: surfactant

Fig. 2 Microcapsule structure. S: SEM, D: DM, 0: Multicore, 1: Monocore, 2: Solidcore, Ca: Calcium EDX mapping, Cl: Chloride EDX mapping

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structure without breakage and a determination of thickness without a cutting. But the appearance of the microcapsule is dark which makes it dicult for structure determination in case of the high dispersion for an optical microscopy characterization because of microdroplet formation in the microcapsule. �e microcapsule internal structure is eas-ily observed from images of microcapsules prepared by the stirring dispersion method. �e stirring was used as the dis-

persion method for accurate optical observation in further investigation.

2.2　E�ect of surfactantSurfactants have a great role in emulsion stability. Even if

surfactant has a low concentration, it can stabilize the drop-lets. Since it located on the interface between the oil phase and the water phase, a low amount of surfactant is required to stabilize. With a low volume fraction of dispersed phase, a reduction of the interfacial tension is a factor as stability de-termining (Shaw, 1992). �us the surfactant concentration was chosen based on interfacial tension. �e concentrations of PVA above 0.25% were determined as the low di�erence zone of the surface tension (Bhattacharya and Ray, 2004). Although this value is not directly obtained from an in-terface between DCE and water, it was an optimal starting point because the non polar nature of air and DCE are the same. �e microcapsule population pattern was the same with low variance at all experimental conditions Figure 5. �e di�erence of average variances (v) was 0.07 in two dif-ferent concentrations of the surfactant. Because emulsion was stable, 0.05% of PVA was used. �e reason for the low concentration of surfactant is to minimize cost, and it is as-sumed that the probability of coalescence increased between inner droplets.

In fact, PVA has a very low equilibrium concentration in polystyrene (Tadros et al., 2004); hence, the low amount of PVA in our study is practical.

2.3　E�ect of calcium chloride concentrationExperimental conditions and core percentages are pre-

sented in Table 2.

Fig. 4 Optical microscopy image of the different dispersion methods. CaCl2 (in 0.1 and 3.0 amounts) [g], LD: low dispersion, HD: high dispersion (W(S/O)/W=6.8 wt%, Na=370 rpm, CPVA=0.05 wt%)

Fig. 3 Relationship between dispersion state and size distribution. CaCl2 (in 0.1 and 3.0 amounts) [g], LD: low dispersion, HD: high dispersion (W(S/O)/W)=6.8 wt%, Na=370 rpm, CPVA= 0.05 wt%)

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2.3.1　E�ect on size distribution　It was observed that a bimodal distribution is populated at WS/O=4.20% (Figure 7). �e monocore is absolutely dominated among the big population (D≈300 µm CE0 in Figure 7). However, all three kinds of structures, which include the monocore, multicore, and solidcore, exist below diameters of 200 µm. It is pos-sible to explain the formation of the solidcore droplets that

appeared in the bimodal distribution in two ways. One is a result of the initial distribution of the S/O suspension, which is directly formed during the (S/O)/W emulsi cation step. �e other way is the explosion of big monocore aer over-swelling, which is a result of the osmotic gradient. In this case, small solidcore structured microcapsules are formed, which increase the number of small solidcores. Owing to the high osmotic gradient, monocore and solidcore microcap-sules are dominant below diameter of 200 µm. �e coales-cence of the inner droplets is assumed because only 1.10% of the multicore is measured on WS/O=4.20%.

�e bimodal distribution changes to unimodal under WS/O<0.15%. In addition, all three kinds of structures are mutually distributed in the prepared microcapsules. By fur-ther decreasing the concentration of the salt, the multicore was increased in the microcapsule population. �e maxi-mum fraction of the multicore structure was observed at WS/O=0.73%.

Fig. 5 Size distributions with di�erent amount of surfactant (W(S/O)/W=6.8 wt%, Na=370 rpm)

Table 2 E�ect of the calcium chloride concentration on the water content and core percentage

ID WS/O [%] P0 [%] P1 [%] P2 [%] WH2O [%]

CE0 4.20 83.92 1.10 14.98 88.28CE1 0.146 87.49 8.53 3.98 49.81CE2 0.729 70.00 24.00 6.00 30.38CE3 0.0364 73.30 22.78 3.92 25.49CE4 0.0182 76.76 16.18 7.06 21.19CE5 0.00911 74.67 14.22 11.11 20.27

Fig. 6　Optical microscope images of microcapsule with di�erent thicknesses in glue media (W(S/O)/W=6.8 wt%, Na=370 rpm, CPVA=0.05 wt%)

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�e solidcore-structured microcapsule is dominant at a small size, owing to breakage and explosions. In other words, the absence of large-sized solidcore particles indi-cates the successful distribution of salt.

2.3.2　E�ect on thickness of monocore microcapsule　 �e wall thickness decreases with the further increase in the amount of calcium chloride, which exhibits a reverse depen-dence, as illustrated Figure 8. Assume that the thickness of wall is Z1 and the diameter is D1 at WS/O=0.00911%. If WS/O increases, then the osmotic �ow increases and the water content inside the microcapsule rises accordingly (Table 2). Hence, we can hypothesize that provided the volume remains constant in the organic phase, such as the coales-

cence, then thickness should decrease (Z1>Z2), and the di-ameter should increase (D1<D2) because WS/O is increased. No direct relationship exists between the WS/O and diameter in Figure 9; therefore, the assumption is rejected. �e as-sumption is not true then the coalescence of outer droplets occurs during preparation.

�e internal structure of the microcapsule, which is pre-pared by di�erent salt concentrations, is clearly illustrated in Figure 6. �e water droplets of the multicore and monocore microcapsules were observed in the center of the microcap-sule. �erefore, a centrifugal force in�uenced the internal structure of the microcapsule, or droplets near the surface escaped during the preparation.

2.4　Mechanism of structure formation�e polystyrene microcapsule formation is elucidated,

based on the experimental results (Figure 10).�e structural formation processes (ki,j, i∈ I, II, III, IV,

j∈ α, β, γ) have up to four steps and three di�erent path-ways, which depend on dispersion methods and salt concen-tration.

In step I (ki,j), S/O/W emulsion is formed. Droplets in aqueous media have di�erent contents of solid particles that are prepared with dispersion methods and the concentration in the S/O suspension preparation.

A general solid particle distribution case is presented in last column of Figure 10. Owing to the dispersion methods, the solid particle distribution inside initial droplets is di�erent.

If the sonication method is used, then a more uniform distribution occurs and only the β path is assumed to be possible. �e experimental fact is that the bimodal distri-bution at a low dispersion state (3.0LD) was changed to unimodal distribution at high dispersion state (3.0HD), as illustrated in Figure 3.

A wider distribution is formed when stirring is adopted as

Fig. 7 Relationship between dilution and structure distribution of the prepared microcapsules (W(S/O)/W=6.8 wt%, Na=370 rpm, CPVA=0.05 wt%)

Fig. 8 Relationship between dilution and thickness (W(S/O)/W= 6.8 wt%, Na=370 rpm, CPVA=0.05 wt%)

Fig. 9 Relationship between dilution and diameter (W(S/O)/W= 6.8 wt%, Na=370 rpm, CPVA=0.05 wt%)

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a dispersion method. In this case, all pathways are possible; however, the probability of pathways di�ers, depending on the concentration of solid particles.

For example, the γ pathway has a higher probability than the α pathway when the concentration of salt is suciently enough. If the concentration of salt is suciently low, then the α pathway has a higher probability than the γ pathway. In both cases, the β pathway has the highest probability.

In step II (kII,j), a conversion from S/O/W emulsion to W/O/W emulsion starts and the multicore structure of the W/O/W emulsion is formed. �e osmotic gradient of solid particles triggers the conversion. �e microcapsule structure is formed by the water droplets. �e nucleation of the water droplets is secondary because the growth of the inner drop-lets is initiated by salt. �e dispersion methods represent the mean factor for the seeding, as structure is more related to geometry or space.

In step III (kIII,j), the monocore microcapsules are formed. �e coalescence of the inner droplets occurs, as the osmotic �ow is not terminated, owing to the high concentration of salt. When the osmotic �ow reaches equilibrium, the co-alescence is terminated. �e surfactant is another important in�uencing factor. �e low concentration of the surfactant and nature of the polysurfactant in�uence the coalescence.

In step IV (kIV,j), an explosion of the monocore mi-crocapsule occurs and the result of the explosion is an additional solidcore formation. In the high concentration (WS/O=4.2%) case, the solidcore is formed by the γ pathway, rather than the α pathway.

Because the mechanism of the structure formation is il-lustrated by using the rate constants (ki,j), it is also possible to elucidate it in terms of serial processes. In this case, only one pathway is sucient for the elucidation. For example, in the γ pathway (ki,γ), the multicore and monocore structured microcapsules are formulated as intermediate products, and the solidcore structured microcapsule is formulated as the

nal product. It is possible to t the core percentage by changing the rate constants. However, the proof of the experimental results, which are the time evolution of the structure distribution, is required. From this perspective, other experimental conditions including a kinetic experi-ment are favorable for further investigations.

�e nucleation of inner droplets starts from the particle distribution in the S/O suspension space. Further, coales-cence can occur between droplets that are geometrically close to each other. Finally, the cluster-based structure is more practical for the microcapsule structure in this system.

�erefore, the salt concentration is not the sole in�u-encing factor of the microcapsule structure, as the surfac-tant and dispersion signi cantly in�uence the microcapsule structure.

Conclusions

Monocore, multicore, and solidcore structures were ob-served on prepared microcapsules. �e structures were au-tomatically detected by the Hough transformation, and were classi ed by SVM. In addition to the water content, the size and structural distribution of the microcapsules were con-trolled by the weight ratio of the solid to the organic phase. �e bimodal distribution changed to unimodal distribution via the dispersion methods and/or the weight ratio of the solid to the organic phase.

Acknowledgement

�e research reported in this paper was supported by “Functional materials based on Mongolian natural minerals for environmental en-gineering, cementitious, and �otation processes (J11A15),” under the Higher Engineering Education Development Project.

Fig. 10 Schematic illustration of the structural formation in the osmosis dominant multiple emulsion system. HD: high dispersion, LLD: low dispersion with low concentration, HLD: low dispersion with high concentration

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Nomenclature

C = concentration [%]D = average diameter [µm]M = weight [g]n = droplet number density [m−3]Na = stirring speed of reactor [rpm]P = particle fraction [%]Q = shrinkage rate [m3 s−1]R = rate [number m−3 s−1]S = swelling rate [m3 s−1]t = time [s]v = volume [m3]W = weight fraction [%]Z = average thickness [µm]ν = variance of diameter [—]

‹Subscripts›CaCl2 = calcium chloride0 = multicore1 = monocore2 = solidcoreBr = breakageCo = coalescenceEs = escapeExp = explosionM = outer dropletNu = nucleationPVA = polyvinyl alcoholS/O = solid/oil emulsion(S/O)/W = solid/oil/water emulsionα, β, γ = pathwaysI, II, III, IV = stepsμ = inner droplet

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