DEVELOPMENT OF NOVEL DROP DIAMETER MEASURING METHOD IN THE MANUFACTURING PROCESS OF FUNCTIONAL O/W MICROCAPSULES

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1 14 th European Conference on Mixing Warszawa, September 2012 DEVELOPMENT OF NOVEL DROP DIAMETER MEASURING METHOD IN THE MANUFACTURING PROCESS OF FUNCTIONAL O/W MICROCAPSULES K. Kanaya a, S. Akao a, R. Misumi b, K. Nishi b, and M. Kaminoyama b a Kaneka Corporation, Process Technology & Research Group, Process Technology Laboratories, 1-8, Miyamae-machi, Takasago-cho, Takasago-shi, Hyogo , Japan b Yokohama National University, Division of Materials Science and Chemical Engineering, Faculty of Engineering, 79-5, Tokiwadai, Hodogaya-ku, Yokohama-shi, Kanagawa , Japan Kento_Kanaya@kn.kaneka.co.jp Abstract. A new method for preparing functional O/W microcapsules by a process involving O/W/O emulsion as particle formation was developed. Coenzyme Q10 (CoQ10) or reduced coenzyme Q10 (QH) was used as the core substance. QH is oxidized quickly when exposed to air. O/W microcapsules were manufactured by conventional liquid phase drying method (LPD). In this study, a novel online measuring method for O/W emulsion droplet diameter during the O/W microcapsules process was developed. The amount of inner oil phase released from O/W emulsion has correlation with increased total surface area of O/W emulsion droplet caused by breaking droplet. Released rate of CoQ10 from O/W emulsion droplet to outer continuous phase under different rotational speed and emulsion viscosity was measured with an absorption spectrometer. As a result of change of released inner CoQ10 amount, droplet breakage under low emulsion viscosity was promoted by agitation speed. It is concluded that droplet dispersion state during manufacturing O/W microcapsules was evaluated well immediately by applying the developed measuring method. Keywords: Liquid-liquid dispersion; Drop size; Microencapsulation; Multiple emulsion 1. INTRODUCTION The mixing of immiscible liquids is commonly utilized in process industries, such as extraction, suspension polymerization and microencapsulation. In the microencapsulation process, drop size distribution is an important parameter to produce the microcapsule diameter that is most suitable for a product. An example of microencapsulation is reduced coenzyme Q10 (QH)-containing microcapsule which was developed by a process involving O/W/O emulsion [1]. QH is oxidized quickly when exposed to air, therefore microencapsulation is utilized to improve stability by protecting it from oxidation. To control microcapsule diameter, it is important to measure the dispersion state in a vessel. In this phenomenon it is understood that part of the inner dispersed phase is released to the outer continuous phase in multiple emulsions such as O/W/O emulsion [2]. Figure 1 shows a schematic representation of the relation between droplet surface area and released inner organic phase. It is considered that the amount of inner oil phase released from O/W emulsion has a correlation with the increased total surface area of the O/W emulsion droplets, caused by breaking droplets. In this study, in order to immediately evaluate the droplet dispersion state during the O/W microcapsules process, a simple drop size measuring method based on the increased amount of inner CoQ10 in continuous organic phase was developed under non-coalescence 193

2 conditions. Results of this novel measuring method were evaluated against data from a highspeed video camera using a suction sampling method. Figure 1. Schematic representation of drop surface area and released inner organic phase. 2. EXPERIMENTAL 2.1 Preparation of O/W microcapsules QH was encapsulated by the liquid phase drying method (LPD). Gum arabic was dissolved in water. QH was added to the solution and then emulsified with a homogenizer. The O/W emulsion was additionally suspended in edible oil added to suitable emulsifiers. The O/W/O emulsion was heated under diminished pressure and the O/W emulsion droplet was dried, then O/W microcapsules were formed. These processes are summarized in Figure 2. Microcapsules of QH were also prepared by conventional spray drying to compare the stability with that of microcapsules by LPD. The outer surface of the microcapsules was observed using a scanning electron microscope to characterize structures. Inner organic phase QH or CoQ10 Aqueous phase Gum arabic solution Outer organic phase Surfactant Edible oil Emulsification 333 K for 10 min O/W Suspension O/W/O Water evaporation 348 K, 30 kpa for 30 Figure 2. Flowchart of the preparation procedure for the O/W microcapsules Stability test of microcapsules Microcapsule The microcapsules were stored in closed brown bottles. The bottles were placed in a thermo-hygrostat, which was controlled at 313 K, and 75% RH. At appropriate intervals, the microcapsules were removed, and the amount of QH and oxidized Q10 (CoQ10) within the microcapsules was quantified by high performance liquid chromatography analysis Online drop size measuring test for O/W emulsion droplet In this study, using CoQ10 as the inner oil phase, O/W emulsion droplet dispersion state in the process of producing microcapsules was measured as a cold model without a LPD. Since QH oxidizes easily, it was not used for the drop-size measuring test. The relation 194

3 between the amount of released CoQ10 W [mg] from O/W emulsion drops and increased total drops surface area A [m 2 ] caused by breaking drops are expressed by Eq. (1). dw dt dw da = k (1) dt dt Where k [mg/min] is the proportionality constant of the increased total drops surface per unit time. In the term on the left side, the CoQ10 release rate from O/W emulsion is corrected using the amount of released CoQ10 W [mg] at the equilibrium state; because a small amount of inner CoQ10 is released at a constant rate by diffusion after the dispersion droplet has reached the equilibrium state. By using the viscosity of dispersed phase μ [Pa s], W is determined by Eq. (2) from the preliminary test. dw ' = 0.037μ dt 0.85 (2) In the first step for measuring O/W emulsion droplets, W was measured with an absorption spectrometer and A was calculated from Eq. (1). The 60 min D 32 [μm] in each condition from the actual measurement was used as an initial condition. In the second step, the mean drop diameter D 32 corresponding to the sphere was estimated from A and the volume fraction of the dispersed phase Apparatus and condition for developed drop size measurement Figure 3 shows a schematic diagram of the experimental apparatus. The set-up for measuring drop sizes consisted of a cylindrical glass vessel equipped with four equally-spaced baffles and a sixbladed Rushton turbine impeller. The droplet dispersion state assumed during manufacturing O/W microcapsules was measured by the following methods. The dispersed and continuous phases were prepared as shown in Table 1. Melted CoQ10 (melting point: 322 K), which heated above 323 K, was used as the inner oil phase. Gum arabic solution was used as the aqueous phase. edible oil was used as the continuous outer organic phase. Melted CoQ10 and gum arabic solution were placed in a homogenizer and stirred at 10,000 rpm for 10 min at 333 K to prepare an O/W emulsion. The weight fraction of CoQ10 for the O/W emulsion was set at 25 wt%. For preparing the O/W/O emulsion, the O/W emulsion was suspended in edible oil with 0.5 wt% surfactant (0.5 wt% outer oil phase) as a stabilizer to avoid coalescence. The volume fraction of the O/W emulsion was set at 25 vol%. (1) Rushton turbine (2) Baffle (3) Motor (4) Sampling tube 50 mm 110 mm The condition of rotational speed n [rpm], power input per unit volume P v [kw/m 3 ] and O/W emulsion viscosity µ are shown in Table 2. O/W emulsion drop size measurements were made at a rotational speed of 360 rpm and at another condition where it suddenly changed from 360 to 420 rpm at 30 minutes after starting the test. Viscosity of the O/W emulsion as the disperse phase was carried out on the two conditions, 0.53 Pa s and 0.16 Pa s, by preparing the gum arabic concentration of gum arabic solution. (7) (2) (3) (5) (1) (4) (6) (8) 90 mm (5) Observation cell (6) High-speed video camera (7) Absorption spectrometer (8) Water bath Figure 3. Schematic diagram of the experimental apparatus. 195

4 Table 1. Material and components Table 2. Experimental conditions Material component volume Temp n P v μ No. Gum arabic solution 75 wt% [K] [rpm] Dispersed phase 150 ml [kw/m 3 ] [Pa s] CoQ10 25 wt% Tricaprylin 99.5 wt% Continuous phase 450 ml Soybean lecithin 0.5 wt% In order to evaluate the O/W emulsion drop diameter, the time evolution of the increased amount of CoQ10 in the continuous phase from O/W emulsion droplets was measured with an absorption spectrometer. For measuring with an absorption spectrometer, the supernatant of the O/W/O mixture sampled at each proper time was filtrated. Simultaneously, the O/W emulsion drop size was measured using a suction sampling method with a high-speed video camera for validation [3]. The transparent glass sampling tube (2.0 mm inner diameter) was connected to a vacuum pump. The sampling glass tube was mounted in a filled rectangular glass cell. The O/W/O emulsion was sucked up from the stirred vessel through a sampling tube. The O/W emulsion droplet shape was then recorded through the rectangular glass cell using a high-speed video camera. 3. RESULTS AND DISCCUSION 3.1. Comparison of stability and characteristics of manufactured microcapsules Figure 4 shows the stability of QH bulk and QH encapsulated with gum arabic by LPD and spray drying in closed bottles. QH encapsulated by LPD was very stable and hardly oxidized. QH encapsulated by spray drying oxidized more quickly than that of LPD. Figure 5 shows SEM images of QH microcapsules prepared with gum arabic by LPD and spray drying. While the surfaces of the microcapsules prepared by spray drying were irregular, the microcapsules prepared by LPD were spherical. The smaller specific surface area and higher packing density of the wall material of the microcapsules prepared by LPD is the most likely reason for the high stability against oxidation. 120 Fraction of QH [%] t [days] Figure 4. Stability of QH encapsulated (Ο) LPD, ( ) spray drying, ( ) QH bulk. 600 μm 30 μm (a) LPD (b) Spray drying Figure 5. SEM images of the microcapsules Results of measuring drop dispersion state Figure 6 shows the plot of all data about A and W-W per unit time determined by measured amount of CoQ10 in the continuous phase and calculated by Eq. (1) and (2). It can be seen that the relationship between A and W-W were reasonably estimated from the measurement results, in spite of the fact that n and µ were changing. This good correlation means that increased A expanded with occurring breakage, and could be traced accurately from the amount of CoQ10 released in the continuous phase. As a result, it is suggested that Eq. (1) is valid. 196

5 Figure 7 shows the result of time evolution of W-W and A. In high µ conditions, changing rotational speed hardly affected amount of CoQ10 released or the droplet surface area. On the other hand, in low µ conditions, the amount of CoQ10 released and the droplet surface area were easily influenced by rotational speed. Accordingly, it is indicated that the measuring method developed could express a rapid change of a drop dispersion state in a vessel. dw/dt -dw'/dt [mg/min] da/dt [m 2 /min] Figure 6. Plot of all data about A and W-W per unit time. W-W' [mg] a) Time evolution of W-W b) Time evolution of A Figure 7. Result of time evolution of W-W and A. The plot of (Ο), ( ) changed rotational speed suddenly from 360 to 420 rpm at 30 minutes. A [m 2 ] Results of drop size distribution with the high speed video camera Results of drop size distribution, for both high and low μ, from the high-speed video camera are shown in Figure 8. In high μ conditions, drop size distribution become multimodal at the early dispersion stage. The distribution become narrow and shifted to smaller sizes as time progressed. However, no significant effect of rotational speed was found. In case of low μ conditions, drop size distribution was found to be uni-modal and comparatively sharp. Furthermore, by rapidly increasing rotational speed, it shifted to a smaller size, and distribution became sharper. Volume frequency [-] min 15 min 30 min 60 min Volume frequency [-] min 15 min 30 min 60 min D d [mm] a) μ = 0.53 Pa s b) μ = 0.16 Pa s Figure 8. Results of drop size distribution from the high-speed video camera. Both cases of rotational speed were changed suddenly from 360 to 420 rpm at 30 minutes D d [mm] 197

6 3.4. Comparison of the two techniques Figure 9 shows the comparison of mean drop diameter from both drop size measuring techniques. Drop size measurement with the developed method showed good agreement with the data of the photographic measurement with a high-speed video camera. Particularly, the change of the mean drop diameter by rapidly changing rotational speed can also be expressed correctly at low μ conditions. D 32 [μm] D 32 [μm] a) Developed measuring method b) High-speed video camera Figure 9. Comparison of mean drop diameter from both drop size measuring techniques. 4. CONCLUSION QH was encapsulated by LPD, which involves O/W/O emulsion, to improve the stability against oxidation. QH encapsulated by LPD had excellent stability compared with that encapsulated by spray drying. A simple online drop size measuring method, based on the released inner tracer amount to continuous organic phase, was developed. This study was carried out under noncoalescence conditions by utilizing a stabilizer and in the absence of any reactions for microencapsulation. The relationship between total drop surface area and the amount released by an inner tracer were reasonably estimated from the developed method. This method was applicable for a wide range of rotational speeds and viscosity of dispersed phase. Moreover, the developed measuring method can estimate the variation of the time evolution of the total droplet surface area and mean drop diameter resulting from different dispersed phase viscosities. The drop size estimated in the developed method showed good agreement with the data obtained by the photographic measurement with a high-speed video camera. Therefore, this developed method is useful for real-time estimation of the dispersion state of industrial O/W microcapsules. 5. REFERENCES [1] Akao, S., Ueda, T., Ueda, T., Development of a Reduced CoenzymeQ10-Containing Microcapsule with High Stability against the Oxidation in Air, 17 th Int. Symp. on Microencapsulation (Nagoya, Sep), Abstract Book, pp [2] Kanaya, K., Sato, M., Development of Enteric-Coated S/O Microcapsules Utilizing Edible Fats, J. Chem. Eng. Japan., 45, [3] Misumi, R., Tsukada, K. M., Nishi, K., Kaminoyama, M., Suspension Property of Highly Concentrated Solid Particles Settling in a Draft-Tube Stirred Vessel, J. Chem. Eng. Japan., 41,