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Effect of dispersing medium on permeability of microcapsule membrane
Colloids and Surfaces B: Biointerfaces 30 (2003) 123 /127 www.elsevier.com/locate/colsurfb
Effect of dispersing medium on permeability of microcapsule membrane Chih Pong Chang a,c, Miho Kimura a, Takao Yamamoto b, Masahiro Nobe a, Toshiaki Dobashi a,* a
Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan b Department of Physics, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan c Department of Textile Engineering, Faculty of Engineering, Chinese Culture University, Hwa Kang, Yang Min Shan, Taipei 111, Taiwan, ROC Received 6 February 2003; accepted 14 March 2003
Abstract Transfer of azo-dyes from dioctylphthalate core to binary mixtures of methanol/acetone with different compositions through poly(ureaurethane) (PUU) microcapsule membrane has been measured. The release curves were expressed by an exponential equation C (t ) /Ceq(1/exp(/t /t )) except for pure acetone, where C (t ) is the concentration of the dye in the dispersing medium, Ceq that at the equilibrium state, t the elution time and t a time constant. Ceq and t are related to the chemical potentials and the diffusion coefficient of the dye, based on a simple theoretical model. The logarithm of t was roughly proportional to the reciprocal of the absolute temperature, from which the activation energy Ea for the transfer of the dye was estimated. The value of Ea observed using acetone as the dispersing medium was much larger than that observed using methanol. In the curve of t versus percentage weight of acetone w in the dispersing medium, a minimum was observed around w /40%. The release curve for pure acetone had multi steps and did not agree with the theoretical prediction. These characteristic permeability properties were attributed to a structural change of the microcapsule membrane surface due to the interaction of the protective colloid and the dispersing medium. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Microcapsule; Release curve; Activation energy; Time constant; Permeability; Membrane
1. Introduction Microcapsules are one of the important tools for controlled release of chemical reactants when they * Corresponding author. Tel.: /81-277-30-1427; fax: /81277-30-1477. E-mail address: [email protected] (T. Dobashi).
are utilized as microreactors, through the membrane of which chemical reactions are performed efficiently [1]. To obtain the maximum efficiency, it is required to bring together the information of the effect of all the parameters on the release behavior. Chemical species of the dispersing medium is one of the practical and effective parameters for the release from microcapsules. In a series of
0927-7765/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7765(03)00079-1
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papers [2 /4], it was elucidated that the release properties of poly(ureaurethane) (PUU) microcapsules were expressed well by a simple diffusion model, using an azo-dye as a model chemical reactant and methanol as the dispersing medium. The aim of this note is to study the effect of the dispersing medium on the release properties, using binary mixtures of acetone/methanol with various percentage weights of acetone w . A completely different release behavior has been observed for pure acetone and a maximum of the release rate has been found around w /40%. These characteristics are discussed from a viewpoint of the interaction between the protective colloid and the dispersing medium on the microcapsule wall membrane surface.
2. Experiments 2.1. Microencapsulation method PUU microcapsules were prepared by an interfacial polymerization [5,6]. Takenate D-110N (75% Triisocyanate in ethylacetate) was a gift from Takeda Chemical Co. and was used as the wallforming material. Dioctyl phthalate (DOP) purchased from Wako Chemical Co. was used as the core material. Copoly(vinyl alcohol-vinyl acetate) (PVA), a gift from Kuraray Co. Ltd. was used as the protective colloid. The degree of polymerization and hydrolysis of PVA were 550/650 and 86/89 mol%, respectively. 1.0 g of D-110N was added to 3.0 g of ethyl acetate with 1.25 g of DOP containing 0.065 g of yellow oil-soluble dye, N -[2,5-bis(heptyloxy)phenyl]-2-[(2,5-dibutoxy-4-p-tolylthiophenyl) hydrazono]-3-oxobutylamide, to obtain an organic phase. Then it was poured into 10.0 g of 5% protective colloid aqueous solution and mixed vigorously at 2000 rpm (system A), 3500 rpm (system B), 5000 rpm (system C), 7000 rpm (system D), 10 000 rpm (system E) for 10 min by an emulsifier (Excel Auto, Nihon Seiki Co.) on ice bath. The resultant emulsions were incubated at 40 8C for 4 h for interfacial polymerization to obtain microcapsules. The samples for the dynamic light scattering were diluted by adding appropriate amount of pure water.
2.2. Preparation of microcapsule film [2] One milliliter of the microcapsule suspension with the percentage weight of microcapsules being 20% was dropped on a polyethylene terephthalate (PET) film, spread homogeneously with a wire bar and dried completely by a dryer. The protective colloid, PVA, played the role of a binder between the PET sheet and the microcapsule thin layer. The film was then cut into squares (1 cm /1 cm). 2.3. Release profile measurement Ten sheets of the square films were soaked in glass vessels containing 20 ml of binary mixtures of acetone/methanol, and the mixtures were gently stirred at controlled temperatures of 30, 35, 40, 45, 50 8C. The optical density (OD) of the dispersing medium was measured at the wavelength of l / 430 nm and converted into the concentration of the dye using a calibration curve. The time course of the concentration of the dye in the dispersing media was measured for acetone (percentage weight of acetone w /100%) using the films for the system A /E at various temperatures and measured for other mixtures (w B/100%) using the film for the system C at 40 8C. 2.4. Dynamic light scattering Dynamic light scattering of microcapsule suspensions was measured by a laboratory-made light scattering apparatus [7]. Data analysis was made by the CONTIN method in order to obtain an average line width (G ). The translational diffusion coefficient (D ) was determined from the equation D /G /q2, where q is the magnitude of the scattering wave vector. The concentration of microcapsules in the sample is very low. Therefore, the z average hydrodynamic radius (Rh) of a microcapsule is obtained by the Stokes/Einstein relation Rh /kBT /(6phD ), where kB is the Boltzmann constant; T is the absolute temperature; h is the solvent viscosity. Regarding the hydrodynamic radius Rh as the geometrical radius R , we obtained the average microcapsule radius. The size distribution of the microcapsules was derived from the distribution of the line width G and the average
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125
radius for the systems A, B, C, D and E was determined as 4.70, 3.57, 2.30, 1.83 and 1.60 mm, respectively.
3. Data analysis The dye release curve for microcapsules with a narrow size distribution is expressed by an equation of exponential form as: C(t)Ceq (1et=t )
(1)
where Ceq is the concentration at the equilibrium state; t is the time; t is the time constant expressed as:
Fig. 1. The release curves for microcapsules using acetone as the dispersing medium at 40 8C for the system A (k), B (I), C (^), D (2) and E (\).
1
step of the release curve at each temperature was expressed fairly well by Eq. (1). Then the parameters t and Ceq in Eq. (1) were determined by the least-squares method. The logarithm of t is plotted against the reciprocal of the absolute temperature by open circles in Fig. 3. The ratio Ea/kB was determined by the least-squares fit for temperatures less than 45 8C as 2.2 /104 K , which is larger than that obtained using methanol as the dispersing medium, 1.7 /104 K [2]. Fig. 4 shows the release curves for the system C measured using binary mixtures of acetone/methanol with different percentage weight of acetone w at 40 8C. The release curves except for pure acetone (w /100%) were well expressed by Eq. (1). The concentrations
t
3 Rl
Dm
m?c m?m
(2)
where l is the thickness of the wall membrane, Dm the diffusion constant of the dye in the membrane, m?c and m?m the concentration derivatives of the chemical potential of the dye in the core and the wall membrane, respectively [2]. The temperature dependence of the time constant t is related to the activation energy Ea as: tAeEa =kB T
(3)
where A is a positive constant.
4. Results Fig. 1 shows the release curve for the system A / E measured using acetone as the dispersing medium at 40 8C. The symbols (k), (I), (^), (2) and (\) correspond to the system A, B, C, D and E, respectively. The release profiles have multi steps, so Eq. (1) is not applicable to these systems when acetone is used as the dispersing medium. The release rate increases with increasing the average radius. This tendency is the opposite of that predicted from the theoretical Eq. (2). Fig. 2 shows the release curve for the system C using acetone as the dispersing medium measured at various temperatures. The release curves have multi steps, similarly to Fig. 1. The release rate increases with raising the temperature. The initial
Fig. 2. The release curves for microcapsules for the system C measured using acetone as the dispersing medium at various temperatures (2), 30 8C; (I), 35 8C; (^), 40 8C; (k), 45 8C and (/), 50 8C.
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Fig. 3. The logarithm of time constant for dye release against the reciprocal of the absolute temperature for pure acetone (k) and pure methanol (m).
at the equilibrium-state, Ceq and the time constant t were determined by the least-squares method. The values of Ceq agree with each other within the experimental error. The plot for t versus w is shown in Fig. 5. The time constant t for pure acetone (w /100%) is much larger than that for pure methanol (w /0%) and a minimum of t is observed around w /40%.
5. Discussion The results for size dependence of the release curve shown in Fig. 1 indicate that the theoretical
Fig. 4. Release curves for microcapsules measured using various dispersing media. (m), Acetone 100%; (%), acetone 90%; ("), acetone 87.5%; (j), acetone 80%; ('), acetone 75%; (\), acetone 70%; (I), acetone 50%; (k), acetone 40%; (^), acetone 25%; (2), acetone 10%; (/), acetone 0%.
Fig. 5. Time constant for dye release as a function of percentage weight of acetone.
scheme as summarized in Section 3 can not be applied to the present experiment when acetone is used as the dispersing medium. This is in contrast to the fact that the release behavior from PUU microcapsules when methanol is used as the dispersing medium was well explained by the theory [2 /4]. The dye has no functional groups for chemical reactions with the microcapsule constituents. Thus we need to examine the effect of the dispersing medium on the physicochemical properties of the membrane and the dye. The small release rate for acetone in comparison with that for methanol as shown in Figs. 2 /5 suggests that the inconsistency cannot be attributed to the affinity of azo-dye with acetone, since acetone is the best solvent of the dye among the solvents of acetone, methanol and DOP. PUU swells in both methanol and acetone, while the protective colloid of PVA is insoluble in acetone and slightly soluble in methanol. Thus, it may be presumed that PVA shrinks into a globular state at the interface between the PUU microcapsule wall and the dispersing acetone to make a thin solid layer. The newly formed layer should be more or less inhomogeneous, since the soaking process of the microcapsule films is too fast to reach the equilibrium state, and the inhomogeneity may increase with the surface area of microcapsules. In fact, an inhomogeneity with the order of several nanometers was found at the interface of microcap-
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Fig. 6. Atomic force micrograph of microcapsule surface when acetone is used as the dispersing medium.
sules and dispersing acetone with an atomic force microscope (Digital Nanoscope III) on the tapping mode, as shown in Fig. 6. In contrast, no such inhomogeneities were observed when methanol was used as the dispersing medium (not shown). This picture can explain the experimental results for the release behavior when acetone is used as the dispersing medium in the following way: Since the dye permeates through the coarsest specks on the thin PVA layer most effectively, the release rate should be higher for larger microcapsules, which is consistent with the result of Fig. 1. The multi step release for pure acetone observed in Figs. 1 and 2 could not be attributed to a simple mechanism with two time constants, since the multi step behavior could only be resulted from phenomena which appear at different times on the way of release. At this stage it is only speculated that this behavior is also attributed to some effects of the inhomogeneity of the PVA thin layer. Because of the appearance of the extra thin solid
127
layer, the activation energy for the permeation may be raised when pure acetone is used in comparison with that when pure methanol is used, which is consistent with the results of Figs. 2 and 3. For binary mixtures, a small amount of methanol could loosen the PVA network in the thin layer and the thin loose layer could not strongly disturb the dye permeation. The higher solubility of the dye in acetone competes with the disturbance by the thin layer due to the PVA shrink, resulting in the maximum permeability at a certain composition of the mixture. The minimum of the time constant in Fig. 5 could be attributed to this competitive action of microcapsules to the dispersing medium. The wide range of t observed for the different dispersing media suggests that the dispersing medium is one of effective controllable parameters for changing the release rate.
Acknowledgements We are grateful to Dr Kimio Ichikawa for his valuable discussion.
References [1] W. Ehrfeld, V. Hessel, H. Lowe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH, Weinheim, 2000. [2] T. Sato, T. Yamamoto, S. Shibako, K. Ichikawa, T. Dobashi, J. Membr. Sci. 213 (2003) 25. [3] T. Yamamoto, T. Dobashi, M. Kimura, C.P. Chang, Colloids Surf. B: Biointerf. 25 (2002) 305. [4] C.P. Chang, T. Yamamoto, M. Kimura, T. Sato, K. Ichikawa, T. Dobashi, J. Control. Release 86 (2003) 207. [5] K. Ichikawa, J. Appl. Sci. 54 (1994) 1321. [6] S. Shimofure, S. Koizumi, K. Ichikawa, H. Ichikawa, T. Dobashi, J. Microencapsulation 18 (2001) 13. [7] T. Isojima, S. Fujii, K. Kubota, K. Hamano, J. Chem. Phys. 111 (1999) 9839.
Effect of dispersing medium on permeability of microcapsule membrane Chih Pong Chang a,c, Miho Kimura a, Takao Yamamoto b, Masahiro Nobe a, Toshiaki Dobashi a,* a
Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan b Department of Physics, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan c Department of Textile Engineering, Faculty of Engineering, Chinese Culture University, Hwa Kang, Yang Min Shan, Taipei 111, Taiwan, ROC Received 6 February 2003; accepted 14 March 2003
Abstract Transfer of azo-dyes from dioctylphthalate core to binary mixtures of methanol/acetone with different compositions through poly(ureaurethane) (PUU) microcapsule membrane has been measured. The release curves were expressed by an exponential equation C (t ) /Ceq(1/exp(/t /t )) except for pure acetone, where C (t ) is the concentration of the dye in the dispersing medium, Ceq that at the equilibrium state, t the elution time and t a time constant. Ceq and t are related to the chemical potentials and the diffusion coefficient of the dye, based on a simple theoretical model. The logarithm of t was roughly proportional to the reciprocal of the absolute temperature, from which the activation energy Ea for the transfer of the dye was estimated. The value of Ea observed using acetone as the dispersing medium was much larger than that observed using methanol. In the curve of t versus percentage weight of acetone w in the dispersing medium, a minimum was observed around w /40%. The release curve for pure acetone had multi steps and did not agree with the theoretical prediction. These characteristic permeability properties were attributed to a structural change of the microcapsule membrane surface due to the interaction of the protective colloid and the dispersing medium. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Microcapsule; Release curve; Activation energy; Time constant; Permeability; Membrane
1. Introduction Microcapsules are one of the important tools for controlled release of chemical reactants when they * Corresponding author. Tel.: /81-277-30-1427; fax: /81277-30-1477. E-mail address: [email protected] (T. Dobashi).
are utilized as microreactors, through the membrane of which chemical reactions are performed efficiently [1]. To obtain the maximum efficiency, it is required to bring together the information of the effect of all the parameters on the release behavior. Chemical species of the dispersing medium is one of the practical and effective parameters for the release from microcapsules. In a series of
0927-7765/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7765(03)00079-1
124
C.P. Chang et al. / Colloids and Surfaces B: Biointerfaces 30 (2003) 123 /127
papers [2 /4], it was elucidated that the release properties of poly(ureaurethane) (PUU) microcapsules were expressed well by a simple diffusion model, using an azo-dye as a model chemical reactant and methanol as the dispersing medium. The aim of this note is to study the effect of the dispersing medium on the release properties, using binary mixtures of acetone/methanol with various percentage weights of acetone w . A completely different release behavior has been observed for pure acetone and a maximum of the release rate has been found around w /40%. These characteristics are discussed from a viewpoint of the interaction between the protective colloid and the dispersing medium on the microcapsule wall membrane surface.
2. Experiments 2.1. Microencapsulation method PUU microcapsules were prepared by an interfacial polymerization [5,6]. Takenate D-110N (75% Triisocyanate in ethylacetate) was a gift from Takeda Chemical Co. and was used as the wallforming material. Dioctyl phthalate (DOP) purchased from Wako Chemical Co. was used as the core material. Copoly(vinyl alcohol-vinyl acetate) (PVA), a gift from Kuraray Co. Ltd. was used as the protective colloid. The degree of polymerization and hydrolysis of PVA were 550/650 and 86/89 mol%, respectively. 1.0 g of D-110N was added to 3.0 g of ethyl acetate with 1.25 g of DOP containing 0.065 g of yellow oil-soluble dye, N -[2,5-bis(heptyloxy)phenyl]-2-[(2,5-dibutoxy-4-p-tolylthiophenyl) hydrazono]-3-oxobutylamide, to obtain an organic phase. Then it was poured into 10.0 g of 5% protective colloid aqueous solution and mixed vigorously at 2000 rpm (system A), 3500 rpm (system B), 5000 rpm (system C), 7000 rpm (system D), 10 000 rpm (system E) for 10 min by an emulsifier (Excel Auto, Nihon Seiki Co.) on ice bath. The resultant emulsions were incubated at 40 8C for 4 h for interfacial polymerization to obtain microcapsules. The samples for the dynamic light scattering were diluted by adding appropriate amount of pure water.
2.2. Preparation of microcapsule film [2] One milliliter of the microcapsule suspension with the percentage weight of microcapsules being 20% was dropped on a polyethylene terephthalate (PET) film, spread homogeneously with a wire bar and dried completely by a dryer. The protective colloid, PVA, played the role of a binder between the PET sheet and the microcapsule thin layer. The film was then cut into squares (1 cm /1 cm). 2.3. Release profile measurement Ten sheets of the square films were soaked in glass vessels containing 20 ml of binary mixtures of acetone/methanol, and the mixtures were gently stirred at controlled temperatures of 30, 35, 40, 45, 50 8C. The optical density (OD) of the dispersing medium was measured at the wavelength of l / 430 nm and converted into the concentration of the dye using a calibration curve. The time course of the concentration of the dye in the dispersing media was measured for acetone (percentage weight of acetone w /100%) using the films for the system A /E at various temperatures and measured for other mixtures (w B/100%) using the film for the system C at 40 8C. 2.4. Dynamic light scattering Dynamic light scattering of microcapsule suspensions was measured by a laboratory-made light scattering apparatus [7]. Data analysis was made by the CONTIN method in order to obtain an average line width (G ). The translational diffusion coefficient (D ) was determined from the equation D /G /q2, where q is the magnitude of the scattering wave vector. The concentration of microcapsules in the sample is very low. Therefore, the z average hydrodynamic radius (Rh) of a microcapsule is obtained by the Stokes/Einstein relation Rh /kBT /(6phD ), where kB is the Boltzmann constant; T is the absolute temperature; h is the solvent viscosity. Regarding the hydrodynamic radius Rh as the geometrical radius R , we obtained the average microcapsule radius. The size distribution of the microcapsules was derived from the distribution of the line width G and the average
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125
radius for the systems A, B, C, D and E was determined as 4.70, 3.57, 2.30, 1.83 and 1.60 mm, respectively.
3. Data analysis The dye release curve for microcapsules with a narrow size distribution is expressed by an equation of exponential form as: C(t)Ceq (1et=t )
(1)
where Ceq is the concentration at the equilibrium state; t is the time; t is the time constant expressed as:
Fig. 1. The release curves for microcapsules using acetone as the dispersing medium at 40 8C for the system A (k), B (I), C (^), D (2) and E (\).
1
step of the release curve at each temperature was expressed fairly well by Eq. (1). Then the parameters t and Ceq in Eq. (1) were determined by the least-squares method. The logarithm of t is plotted against the reciprocal of the absolute temperature by open circles in Fig. 3. The ratio Ea/kB was determined by the least-squares fit for temperatures less than 45 8C as 2.2 /104 K , which is larger than that obtained using methanol as the dispersing medium, 1.7 /104 K [2]. Fig. 4 shows the release curves for the system C measured using binary mixtures of acetone/methanol with different percentage weight of acetone w at 40 8C. The release curves except for pure acetone (w /100%) were well expressed by Eq. (1). The concentrations
t
3 Rl
Dm
m?c m?m
(2)
where l is the thickness of the wall membrane, Dm the diffusion constant of the dye in the membrane, m?c and m?m the concentration derivatives of the chemical potential of the dye in the core and the wall membrane, respectively [2]. The temperature dependence of the time constant t is related to the activation energy Ea as: tAeEa =kB T
(3)
where A is a positive constant.
4. Results Fig. 1 shows the release curve for the system A / E measured using acetone as the dispersing medium at 40 8C. The symbols (k), (I), (^), (2) and (\) correspond to the system A, B, C, D and E, respectively. The release profiles have multi steps, so Eq. (1) is not applicable to these systems when acetone is used as the dispersing medium. The release rate increases with increasing the average radius. This tendency is the opposite of that predicted from the theoretical Eq. (2). Fig. 2 shows the release curve for the system C using acetone as the dispersing medium measured at various temperatures. The release curves have multi steps, similarly to Fig. 1. The release rate increases with raising the temperature. The initial
Fig. 2. The release curves for microcapsules for the system C measured using acetone as the dispersing medium at various temperatures (2), 30 8C; (I), 35 8C; (^), 40 8C; (k), 45 8C and (/), 50 8C.
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Fig. 3. The logarithm of time constant for dye release against the reciprocal of the absolute temperature for pure acetone (k) and pure methanol (m).
at the equilibrium-state, Ceq and the time constant t were determined by the least-squares method. The values of Ceq agree with each other within the experimental error. The plot for t versus w is shown in Fig. 5. The time constant t for pure acetone (w /100%) is much larger than that for pure methanol (w /0%) and a minimum of t is observed around w /40%.
5. Discussion The results for size dependence of the release curve shown in Fig. 1 indicate that the theoretical
Fig. 4. Release curves for microcapsules measured using various dispersing media. (m), Acetone 100%; (%), acetone 90%; ("), acetone 87.5%; (j), acetone 80%; ('), acetone 75%; (\), acetone 70%; (I), acetone 50%; (k), acetone 40%; (^), acetone 25%; (2), acetone 10%; (/), acetone 0%.
Fig. 5. Time constant for dye release as a function of percentage weight of acetone.
scheme as summarized in Section 3 can not be applied to the present experiment when acetone is used as the dispersing medium. This is in contrast to the fact that the release behavior from PUU microcapsules when methanol is used as the dispersing medium was well explained by the theory [2 /4]. The dye has no functional groups for chemical reactions with the microcapsule constituents. Thus we need to examine the effect of the dispersing medium on the physicochemical properties of the membrane and the dye. The small release rate for acetone in comparison with that for methanol as shown in Figs. 2 /5 suggests that the inconsistency cannot be attributed to the affinity of azo-dye with acetone, since acetone is the best solvent of the dye among the solvents of acetone, methanol and DOP. PUU swells in both methanol and acetone, while the protective colloid of PVA is insoluble in acetone and slightly soluble in methanol. Thus, it may be presumed that PVA shrinks into a globular state at the interface between the PUU microcapsule wall and the dispersing acetone to make a thin solid layer. The newly formed layer should be more or less inhomogeneous, since the soaking process of the microcapsule films is too fast to reach the equilibrium state, and the inhomogeneity may increase with the surface area of microcapsules. In fact, an inhomogeneity with the order of several nanometers was found at the interface of microcap-
C.P. Chang et al. / Colloids and Surfaces B: Biointerfaces 30 (2003) 123 /127
Fig. 6. Atomic force micrograph of microcapsule surface when acetone is used as the dispersing medium.
sules and dispersing acetone with an atomic force microscope (Digital Nanoscope III) on the tapping mode, as shown in Fig. 6. In contrast, no such inhomogeneities were observed when methanol was used as the dispersing medium (not shown). This picture can explain the experimental results for the release behavior when acetone is used as the dispersing medium in the following way: Since the dye permeates through the coarsest specks on the thin PVA layer most effectively, the release rate should be higher for larger microcapsules, which is consistent with the result of Fig. 1. The multi step release for pure acetone observed in Figs. 1 and 2 could not be attributed to a simple mechanism with two time constants, since the multi step behavior could only be resulted from phenomena which appear at different times on the way of release. At this stage it is only speculated that this behavior is also attributed to some effects of the inhomogeneity of the PVA thin layer. Because of the appearance of the extra thin solid
127
layer, the activation energy for the permeation may be raised when pure acetone is used in comparison with that when pure methanol is used, which is consistent with the results of Figs. 2 and 3. For binary mixtures, a small amount of methanol could loosen the PVA network in the thin layer and the thin loose layer could not strongly disturb the dye permeation. The higher solubility of the dye in acetone competes with the disturbance by the thin layer due to the PVA shrink, resulting in the maximum permeability at a certain composition of the mixture. The minimum of the time constant in Fig. 5 could be attributed to this competitive action of microcapsules to the dispersing medium. The wide range of t observed for the different dispersing media suggests that the dispersing medium is one of effective controllable parameters for changing the release rate.
Acknowledgements We are grateful to Dr Kimio Ichikawa for his valuable discussion.
References [1] W. Ehrfeld, V. Hessel, H. Lowe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH, Weinheim, 2000. [2] T. Sato, T. Yamamoto, S. Shibako, K. Ichikawa, T. Dobashi, J. Membr. Sci. 213 (2003) 25. [3] T. Yamamoto, T. Dobashi, M. Kimura, C.P. Chang, Colloids Surf. B: Biointerf. 25 (2002) 305. [4] C.P. Chang, T. Yamamoto, M. Kimura, T. Sato, K. Ichikawa, T. Dobashi, J. Control. Release 86 (2003) 207. [5] K. Ichikawa, J. Appl. Sci. 54 (1994) 1321. [6] S. Shimofure, S. Koizumi, K. Ichikawa, H. Ichikawa, T. Dobashi, J. Microencapsulation 18 (2001) 13. [7] T. Isojima, S. Fujii, K. Kubota, K. Hamano, J. Chem. Phys. 111 (1999) 9839.