Langmuir 2003, 19, 3071-3073
Ultrasonic Study of Swollen Microcapsule Membranes Toshiaki Dobashi,*,† Hiroshi Ishimaru,† Akio Sakanishi,† and Kimio Ichikawa‡ Department of Biological and Chemical Engineering, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan, and Fujinomiya Laboratory, Fuji Photo Film Company Limited, Fujinomiya, Shizuoka 418-8666, Japan
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interesting to study the effect of the structural change of the wall membrane on the compressibility. The present note reports an application of the ultrasonic method to suspensions of microcapsules consisting of a tricresyl phosphate (TCP) core and an outer poly(urea-urethane) membrane focusing the dependence of r on the compressibility of the membrane. We will discuss the estimation of the volume fraction of the wall membrane in the microcapsule, the swelling effect of the core medium on the density and compressibility of the wall membrane, and the relationship with the Flory-Huggins χ parameter.
Received August 12, 2002. In Final Form: January 6, 2003
Introduction The structure and physicochemical properties of the wall membrane of a microcapsule are one of the most important factors which control the functional properties on its application to drug release, temperature and pressure sensitivity, and so on.1,2 It is quite useful to combine X-ray and light scattering measurements to determine the microcapsule structure.3-5 From industrial aspects, however, it is important to extract information from concentrated suspensions where X-rays and light are often strongly absorbed. A sophisticated method to determine the dispersed phase parameters of colloidal systems is a combination of ultrasonic velocity (V) and density (F) measurements.6-11 The typical frequency of the ultrasonic waves used in laboratories is 1-10 MHz, which corresponds to a wavelength of 0.15-1.5 mm in aqueous solutions. Thus, the Rayleigh scattering approximation holds for microcapsules with the typical size of 1 µm which is much smaller than the wavelength. Then we can assume additivity by volume to evaluate the compressibility (Bs ) 1/FV2) of the microcapsule membrane,11 which is the key parameter for pressure sensitivity of microcapsules. As was recently shown by mechanical and scattering studies, the wall membrane could swell by inner and/or outer dispersing liquids in the microencapsulation process by using the interfacial polymerization method, and the structure of the wall membrane could vary depending on the core/wall-forming material weight ratio r.4,5,12-14 It is * To whom correspondence should be addressed. Telephone & Fax: (+81)(277)1477. E-mail:
[email protected]. † Gunma University. ‡ Fuji Photo Film Co. Ltd. (1) Nixon, J. R. Microencapsulation; Marcel Dekker: New York, 1976. (2) Kondo, T. In Surface and Colloid Science; Matijejevic, E., Ed.; Plenum: New York, 1978; Vol. 10, pp 1-43. (3) Dobashi, T.; Chu, B. Light Scattering Studies of Microcapsules in Suspension. In Surface Characterization Methods: Principles, Techniques, and Applications; Milling, A. J., Ed.; Surfactant Science Series 87, Marcel Dekker: New York, 1999. (4) Dobashi, T.; Yeh, F.-J.; Ying, Q.; Ichikawa, K.; Chu, B. Langmuir 1995, 11, 4278. (5) Dobashi, T.; Yeh, F.-J.; Takenaka, M.; Wu, G..; Ichikawa, K.; Chu, B. J. Colloid Interface Sci. 1996, 179, 640. (6) Urick, R. J. J. Appl. Phys. 1947, 18, 983. (7) Horvath-Szabo, G.; Hoiland, H. J. Colloid Interface Sci. 1996, 177, 568. (8) McClements, D. J. J. Phys. Chem. 1990, 94, 1712. Javanaud, C. J. Phys. Chem. 1990, 94, 1713. (9) Holmes, A. K.; Challis, R. E.; Wedlock, D. J. J. Colloid Interface Sci. 1993, 156, 261. (10) Chanamai, R.; Alba, F.; McClements, D. J. J. Food Sci. 2000, 65, 507. (11) Mitaku, S.; Ikegami, A.; Sakanishi, A. Biophys. Chem. 1978, 8, 295. (12) Dobashi, T.; Furukawa, T.; Narita, T.; Shomofure, S.; Ichikawa, K. Langmuir 2001, 17, 4525.
Theoretical Analysis Let us denote the volume fraction of microcapsules in a suspension as φM, that of the membrane in a microcapsule as φm, and the dry weight concentration of microcapsules in the suspension as CM. The subscripts M and m denote microcapsule and microcapsule membrane, respectively. We have
φM/CM ) VM/(FMVM - Wmδ)
(1)
where FM, VM, Wm, and δ are the density of the microcapsules defined by FM ) (WM + Wmδ)/VM, the volume of the microcapsules in the suspension, the weight of the microcapsule membrane in the suspension, and the hydration of the microcapsule membrane (grams of hydrated water for one gram of the membrane), respectively, and WM is the dry weight of microcapsules in the suspension. Here we define intrinsic values of density F, ultrasonic velocity V, and adiabatic compressibility B as11
(Fs - F0) CMf0 F0CM
(2)
(Vs - V0) CMf0 V0CM
(3)
(Bs - B0) B0CM
(4)
[F] ) lim
[V] ) lim
[B] ) lim
CMf0
where the subscripts s and 0 denote the suspension and the dispersing medium. Using the relationship B ) 1/(FV2), we have
[B] ) -[F] - 2[V]
(5)
For microcapsules consisting of a core medium with the density Fi and a membrane with the density Fm, using eq 1 combined with the additivity of density by volume, eq 2 is rewritten as
[F] ) VM[Fmφm + Fi(1 - φm) - F0]/{[Fmφm + Fi(1 - φm)]VM - Wmδ}F0 (6) As the hydration δ of the poly(urea-urethane) membrane (13) Dobashi, T.; Furukawa, T.; Ichikawa, K.; Narita, T. Langmuir 2002, 18, 6031. (14) Ichikawa, K. J. Appl. Polym. Sci. 1994, 54, 1321.
10.1021/la0207116 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/12/2003
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Langmuir, Vol. 19, No. 7, 2003
Notes
used in this study is negligible,15 we have
[F] ) {1/F0 - 1/[Fmφm + Fi(1 - φm)]}
(7)
Thus, if we can estimate φm, Fm is calculated from experimentally determinable quantities of [F], F0, and Fi as
Fm ) [1/(1/F0 - [F]) - Fi(1 - φm)]/φm
(8)
For microspheres (no core medium (φm ) 1)), eq 8 is reduced to
Fm ) F0/(1 - F0[F])
(9)
Correspondingly, if the compressibilities of the core medium and the membrane are denoted as Bi and Bm, respectively, we have
[B] ) [Bmφm + Bi(1 - φm) - B0]/B0/[Fmφm + Fi(1 - φm)] (10) Bm ) {[B]B0[Fmφm + Fi(1 - φm)] - Bi(1 - φm) + B0}/φm (11) in the case for δ ∼ 0. For microspheres (φm ) 1), we have
Bm ) [B]B0Fm + B0
(12)
The parameter φm is not always calculated from the overall composition of wall-forming materials and core materials, since the wall membrane could be swollen by the core material or the outer dispersing medium in the microencapsulation process.12,13 Scanning electron microscopy and dynamic mechanical thermal study showed that the poly(urea-urethane) microcapsules containing TCP did not form a core-shell structure but formed a swollen microsphere when the weight ratio of the core and the wallforming material, r, was less than the critical value rcritical ) 0.37,12,14 which corresponds to the condition that the weight fraction of the core medium in the sum of the wallforming material and the core medium, ω, is less than the critical value ωcritical ) 0.27. Here we assume that all the TCP is absorbed in the microcapsule membrane to form a microsphere when ω e ωcritical, and the excess amount of TCP remains in the inner core to form a microcapsule when ω g ωcritical. In the former case, φm ) 1. In the latter case, if we denote the density of the microcapsule at ω ) ωcritical as Fcritical, the volume fraction of the wall membrane in a microcapsule is calculated as
φm )
(1 - ω + ωcritical)/Fcritical (1 - ω + ωcritical)/Fcritical + (ω - ωcritical)/Fi
(13)
where Fi is the density of the core medium. Materials and Methods
Figure 1. Reduced density (a) and compressibility (b) as a function of the weight concentration of microcapsules, CM, for system A (b) and system E (O). grams of triisocyanate monomer solution was added to 20 g of ethyl acetate with a given amount of TCP to obtain an organic phase. The weight ratio of TCP and triisocyanate monomer was varied from 0 to 1.5 (r ) 0, 0.37, 0.73, 1.01, and 1.48 for systems A, B, C, D, and E, respectively). Thus, system B has the critical value of r for forming a core-shell structure. The organic solutions were poured into an aqueous solution containing 3 wt % of protective colloid and immersed vigorously by a mechanical emulsifier. The resultant emulsions were stirred under atmospheric pressure at 40 °C for 4 h to obtain microcapsule suspensions. The sample of the aqueous phase containing free colloidal polymers was obtained from the supernatant phase by centrifuging the suspension at 12 000 rpm for 30 min and used for determining the density and compressibility of the dispersing medium. The dry weight concentration of the sample WM was determined by drying the microcapsule suspension at 105 °C in a circulation type drying oven for 1 h. The mean diameter of the microcapsules was 1.2-1.4 µm determined by a static light scattering (Horiba LA910) measurement assuming the refractive index of the microcapsule to be 1.18. The ultrasonic velocity was measured by using a laboratory-made automatic measuring system mainly consisting of a sequential pulse oscillation velocimeter at 3 MHz.16 The temperature was kept at 25 °C with the precision of (0.02 °C. The density was measured by using a vibrational densimeter DMA602 (Anton Paar Co., Ltd.). The density and compressibility of the dispersing medium and the core medium were determined as F0 ) 1.002 g/cm3, B0 ) 4.408 × 10-11 cm2/dyn and Fi ) 1.1685 g/cm3, Bi ) 3.801 × 10-11 cm2/ dyn, respectively.
Triisocyanate monomer (75%) in ethyl acetate (Takenate D110N, Takeda Chemical Ind. Ltd.) and tricresyl phosphate (Tokyo Kasei Co., Ltd.) were used as a wall-forming material and a core medium, respectively. Copoly(vinyl alcohol-vinyl acetate), abbreviated as PVA, used as a protective colloid is a gift from Kraray Co., Ltd. The degree of polymerization and hydrolysis were 1700 and 85 mol %, respectively. These materials were used without further purification. A typical interfacial polymerization method was used for the microencapsulation.14 Twenty
The dependencies of the reduced density and compressibility on the weight concentration of microcapsules, CM, are shown in Figure 1a,b. The results clearly show that the intrinsic density and compressibility are determined from the slope of the straight lines in the experimental concentration range. Parts a and b of Figure 2 are the
(15) Private communications from Prof. Shin Yagihara at Tokai University.
(16) Dobashi, T.; Sanda, Y.; Akaiwa, R.; Sakanishi, A. Biorheology 1988, 25, 527.
Results and Discussion
Notes
Langmuir, Vol. 19, No. 7, 2003 3073
χ ) -[ln(1 - φm) + φm + V1n(φm1/3 - φm/2)]/φm2
Figure 2. Intrinsic density (a) and compressibility (b) for poly(urea-urethane) microcapsules containing TCP as a function of the weight ratio of the core medium and the wall-forming material, r.
intrinsic density [F] and the intrinsic elastic modulus [K] ) -[B] (the elastic modulus K is related to the compressibility B as K ) 1/B), respectively, as a function of weight ratio of the core TCP and the wall-forming triisocyanate, r. Both [F] and -[B] decrease with increasing r. The decreases in [F] and -[B] at r ∼ 1.5 are as large as 22% and 40%, respectively. The density and the compressibility of the wall membrane were determined by a combination of eqs 9 and 12 as Fm ) 1.296 g/cm3, Bm ) 1.026 × 10-11 cm2/dyn for system A and Fm ) 1.261 g/cm3, Bm ) 1.67 × 10-11 cm2/dyn for system B. Using the value of Fm ) 1.261 g/cm3 for system B as Fcritical, Fm and Bm for the other systems are calculated from eqs 8, 11, and 13 as Fm ) 1.259 g/cm3, Bm ) 1.73 × 10-11 cm2/dyn for system C, Fm ) 1.252 g/cm3, Bm ) 1.74 × 10-11 cm2/dyn for system D, and Fm ) 1.255 g/cm3, Bm ) 1.75 × 10-11 cm2/dyn for system E. Here, the experimental errors are estimated to be as large as ∆Fm ) (0.005 g/cm3 and ∆Bm ) (0.05 × 10-11 cm2/dyn because of a decrease in the significant figures in eqs 8 and 11. The constant values for Fm and Bm for systems B-E are consistent with the assumption used in the Theoretical Analysis that the membrane contains a saturated amount of core medium calculated from ωcritical, when ω g ωcritical. The difference of the values of Fm and Bm between system A and systems B-E manifests the effectiveness of TCP as a plasticizer. The considerable increase in Bm by TCP corresponds to the large decrease in the glass transition temperature of the microcapsule wall membrane reported in ref 14. Equilibrium swelling is reached when the entropic elastic force balances with the free energy decrease in dilution. The significant effect of TCP could be discussed by the equilibrium swelling theory of Flory and Rehner:
(14)
where χ is the Flory-Huggins polymer-solvent interaction parameter, V1 is the molar volume of the core medium, and n is the cross-link density representing the number of active network chain segments per unit volume of the wall membrane.17 The quantity n equals Fm/Mc, where Mc is the molecular weight between the cross-links of the wall membrane.18 The isocyanate monomer used in the present study was synthesized by reacting xyllilane diisocyanate with trimethylolepropane to form a trifunctional structure. The isocyanate moieties react with each other forming a urea bond during the microencapsulation process; therefore, the wall membrane is considered to be a three-dimensional cross-linked structure. The value of Mc of the wall membrane could be evaluated from the ideal structure assuming the chemical reaction mentioned above. In the present case, the value of Mc is roughly estimated to be 460, which is 2/3 of the molecular weight of the triisocyanate monomer used. The Flory-Huggins χ parameter for the microcapsule containing TCP is estimated as -0.21 using the values obtained in the present study. The structural formation mechanism of the poly(urea-urethane) microcapsules prepared using the same isocyanate monomer has been investigated by scanning electron microscopy analysis. The parameter χ for a microcapsule containing dibutyl phthalate (DBP) as a core material has been reported as 0.03, and the solubility parameter of DBP is 10.5 cal1/2 cm3/2 (ref 19), which is the same as that of TCP. Although the value of χ depends on the estimation of Mc (a 5% decrease in Mc results in χ ) -0.15 for microcapsules containing TCP), the sign and the relative magnitude of χ for different systems are not affected. The negative value of χ shows the high affinity of TCP for the poly(urea-urethane) wall membrane. Recently, single-particle light scattering and related measurements were employed to determine the structure of poly(ureaurethane) microcapsules containing a series of phthalates.12,13 The results showed a coupling of cross-linking, swelling, and phase separation, depending on the affinity between the wall-forming materials and the core media. Due to swelling, a homogeneous elastic membrane is formed with organic solvents that are compatible with the wall-forming monomers, whereas due to phase separation the membrane becomes brittle and heterogeneous with less compatible solvents.13 The dye release rate from the microcapsules could be varied by more than two decades by varying the affinity.20 From the estimated value of χ for poly(urea-urethane) microcapsules containing TCP, only the coupling of cross-linking and swelling in the microencapsulation process should be taken into consideration. This coupling results in the considerable increase in Bm. From the aspect of the molecular structure, a strong polar interaction with hydrogen bonds between TCP and the wall membrane should result in the strong plasticizing effect and considerable swelling. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science under Grant 11555170. LA0207116 (17) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (18) Sperling, L. H. Introduction to Physical Polymer Science, 27th ed.; John Wiley & Sons: New York, 1992. (19) Matsunami, Y.; Ichikawa, K. Int. J. Pharm. 2002, 242, 147. (20) Furukawa, T.; Hung, S. C.; Yamamoto, T.; Terao, K.; Dobashi, T. Unpublished.
