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New Method for Fabrication of Loaded Micro- and Nanocontainers: Emulsion Encapsulation by Polyelectrolyte Layer-by-Layer Deposition on the Liquid Core D. O. Grigoriev,*,† T. Bukreeva,‡ H. Mo¨hwald,† and D. G. Shchukin† Max-Planck Institute of Colloids and Interfaces, Golm/Potsdam, D-14476, Germany, and ShubnikoV Institute of Crystallography, Russian Academy of Sciences, Leninskiıˇ pr. 59, Moscow, 119333, Russia ReceiVed September 17, 2007. In Final Form: October 31, 2007 A novel approach to the emulsion encapsulation was developed by combining the advantages of direct encapsulation of a liquid colloidal core with the accuracy and multifunctionality of layer-by-layer polyelectrolyte deposition. Experimental data obtained for the model oil-in-water emulsion confirm unambiguously the alternating PE assembly in the capsule shell as well as the maintenance of the liquid colloidal core. Two different mechanisms of capsule destruction upon interaction with the solid substrate were observed and qualitatively explained. The proposed method can be easily generalized to the preparation of oil-filled capsules in various oil/water/polyelectrolyte systems important in the field of pharmacy, medicine, and food industry.
Introduction In the past decade major progress in the development, synthesis, design, and purposeful application of various types of microand nanocontainers has been achieved.1-4 Depending on the aim, protection, delivery, and/or controlled release of container load are the main goals of their applications in different systems of interest. Correspondingly, in each container preparation technique special emphasis has to be placed on the aspects determining the container features responsible for the best attainment of the final goal. For example, if one mainly needs to provide high longterm stability of the colloidal system and to protect it from aggregation, either charged or sterically branched container shells are necessary.5-7 For delivery and following release from the microcontainer the mechanical robustness simultaneously with significant permeability are of great importance.8-10 A sophisticated technique of microcontainer preparation allowing the engineering of their shells on the nanolevel was recently developed by Donath and co-workers.11 According to this approach, the layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytes (PE) on a solid colloidal template provides both a wide variety of composition (multifunctionality) and nanometric accuracy during the formation of the container shell. In turn, the possibility to control precisely the shell properties * Corresponding author. Tel.: +49-331-567-9257. Fax: +49-331-5679202. E-mail:
[email protected]. † Max-Planck Institute of Colloids and Interfaces. ‡ Russian Academy of Sciences. (1) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Nature 2001, 409, 794. (2) Sukhorukov, G. B.; Fery, A.; Brumen, M.; Mo¨hwald, H. Phys. Chem. Chem. Phys. 2004, 6, 4078. (3) Guzey, D.; McClements, D. J. AdV. Colloid Interface Sci. 2006, 128-130, 227. (4) Landfester, K. Annu. ReV. Mater. Res. 2006, 36, 231. (5) Wheatley, M. A.; Singhal, S. React. Polym. 1995, 25, 157. (6) Shchukin, D. G.; Ko¨hler, K.; Mo¨hwald, H.; Sukhorukov, G. B. Angew. Chem., Int. Ed. 2005, 44, 3310. (7) Grigoriev, D.; Miller, R.; Shchukin, D.; Mo¨hwald, H. Small 2007, 3, 665. (8) Crespy, D.; Stark, M.; Hoffmann-Richter, C.; Ziener, U.; Landfester, K. Macromolecules 2007, 40, 3122. (9) Crespy, D.; Landfester, K. Macromol. Chem. Phys. 2007, 208, 457. (10) Suslick, K. S.; Grinstaff, M. W.; Kolbeck, K. J.; Wong, M. Ultrason. Sonochem. 1994, 1, S65. (11) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; Mo¨hwald, H. Polym. AdV. Technol. 1998, 9, 759.
opens perspectives for further application of these containers in many research and industrial fields such as medicine,12,13 pharmacy and pharmaceutical industry,14 and cosmetic and food industry.15 Unfortunately, the advantages of the discussed approach are almost counteracted by significant drawbacks in the loading of the above-mentioned micro- and nanocontainers. Core dissolution before container use requires often relatively harsh conditions at which the PE shell is also significantly modified13,16 in an ill-defined way. Additionally, the removal of the destructed core frequently cannot be achieved completely,2 influencing markedly the properties in the container interior. Further, the substance to be encapsulated should first permeate through the container shell; i.e., the latter has to be switched to the open state. After loading, the opposite change of surrounding conditions is necessary again to close the shell.17 Thus, encapsulation of hydrophobic compounds requires the repetition of open-close procedures many times upon step-bystep reduction of the dispersion medium polarity.18 This loading routine not only is tedious and time-consuming but also leads to a very low final yield of loaded containers and is less applicable from a practical point of view. On the other hand, different one-step pathways allowing the preparation of mechanically robust and already loaded microcontainers1,4,8,9,19,20 do not leave any possibilities to control and manipulate finely the shell properties. Therefore, the elaboration of methods combining the advantages of one-step container preparation techniques with the accuracy provided by PE L-b-L deposition is of high practical importance. The major goal of the (12) Schu¨ler, C.; Caruso, F. Biomacromolecules 2001, 2, 921. (13) Georgieva, R.; Moya, S.; Donath, E.; Ba¨umler, H. Langmuir 2004, 20, 1895. (14) Sukhorukov, G.; Da¨hne, L.; Hartmann, J.; Donath, E.; Mo¨hwald, H. AdV. Mater. 2000, 12, 112. (15) Caruso, F.; Fiedler, H.; Haage, K. Colloids Surf., A 2000, 169, 287. (16) Ibarz, G.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Macromol. Rapid Commun. 2002, 23, 74. (17) Antipov, A. A.; Sukhorukov, G. B.; Leporatti, S.; Radtchenko, I. L.; Donath, E.; Mohwald, H. Colloids Surf., A 2002, 198, 535. (18) Moya, S.; Sukhorukov, G. B.; Auch, M.; Donath, E.; Mo¨hwald, H. J. Colloid Interface Sci. 1999, 216, 297. (19) Cho, S. H.; Andersson, H. M.; White, S. R.; Sottos, N. R.; Braun, P. V. AdV. Mater. 2006, 18, 997. (20) Lu, G.; An, Z.; Tao, C.; Li, J. Langmuir 2004, 20, 8401.
10.1021/la702873f CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007
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work at hand is the development of a novel approach to the preparation of loaded micro- and nanocontainers based on the oil-in-water emulsion encapsulation by PE L-b-L deposition directly on the droplets of dispersed phase. Herein, the liquid colloidal particles play simultaneously the role of the template as well as of the container load. Despite the fact that the emulsion encapsulation concept to prepare the oil-loaded microcontainer possesses many advantages, there are only a few efforts in this field3,21,22 in which several results on particular emulsion systems were reported. The successful attempts were, however, stringently connected with the use of either unique chemical properties of the components employed or very special extreme physical conditions in some encapsulation steps. In contrast, our approach is rather general and could be applied to the encapsulation of a broad range of emulsions comprised of various hydrophobic substances and oil-soluble compounds. Experimental Section Materials. Didodecyldimethylammonium bromide (DODAB, purity g98%), dodecane (>99%), poly(sodium 4-styrenesulfonate) (PSS, MW ∼ 70000 g‚mol-1), poly(diallyldimethylammonium chloride) (PDADMAC, average MW ∼ 100000-200000 g‚mol-1, 20 wt % in H2O), poly(fluorescein isothiocyanate allylamine hydrochloride) (FITC-labeled PAH, MW ∼ 70000 g‚mol-1), and 5,10,15,20-tetraphenylporphin (TPP) were purchased from SigmaAldrich Chemie GmbH (Taufkirchen bei Mu¨nchen, D-82024 Germany), and chloroform was obtained from VWR International GmbH (Darmstadt, D-64295 Germany). All chemicals were used as purchased without extra purification. Milli-Q deionized water (specific electric conductivity of 18.2 MΩ·cm) was used to prepare all aqueous solutions. Solution Preparation. Because of the very low solubility of DODAB in nonpolar solvents, 30 v/v% of chloroform was added to dodecane to improve the surfactant solubility in the oil phase. On the other side, because of the relatively low oil/water partition coefficient for chloroform23 and its slight solubility in the aqueous bulk, one can expect that after emulsion preparation and the following encapsulation steps chloroform is completely washed out. All DODAB molecules will then be situated merely at the oil/water interface forming a charged monolayer around each emulsion droplet, which can be considered as a mobile fluid template. Thus, making use of the data on the DODAB monolayers at the air/water interface24-26 and supposing a certain average size of emulsion droplets, one can easily estimate the amount of DODAB needed for the formation of a monolayer with high interfacial density and charge. Assuming that the molecular area is 50 Å2, the concentration of dispersed phase is 2 v/v%, and the mean droplet size (diameter) is 2 µm, one obtains an oil phase DODAB concentration of about 6 mg/mL. For an emulsion with droplets of 5 µm the total interfacial area is reduced and the corresponding surfactant concentration is decreased to about 2.5 mg/1 mL of “oil”. Here, the DODAB concentration of 2 mg/mL was used. For the preparation of an oilin-water emulsion with fluorescent labeling of the dispersed phase, the above-mentioned DODAB solution was additionally doped by 0.12 mg/mL of oil-soluble fluorescent dye TPP and sonicated a couple of minutes for its faster dissolution. The expected total interfacial area in 2 v/v% oil-in-water emulsion considerably exceeds the corresponding area at the PE L-b-L deposition on the solid colloidal template.11 Indeed, the particulate (21) Tjipto, E.; Cadwell, K. D.; Quinn, J. F.; Johnston, A. P. R.; Abbott, N. L.; Caruso, F. Nano Lett. 2006, 6, 2248. (22) Nilsson, L.; Bergenståhl, B. J. Colloid Interface Sci. 2007, 308, 508. (23) Handbook of Chemistry and Physics, 84th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2003-2004. (24) Shapovalov, V.; Tronin, A. Langmuir 1997, 13, 4870. (25) Gonc¸ alves da Silva, A. M.; Roma˜o, R. S.; Lucero Caro, A.; Rodrı´guez Patino, J. M. J. Colloid Interface Sci. 2004, 270, 417. (26) Gonc¸ alves da Silva, A. M.; Viseu, I.; M.; Campos, S. C.; Rechena, T. Thin Solid Films 1998, 320, 236.
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Figure 1. Schematic representation of several steps during L-b-L polyelectrolyte emulsion encapsulation. For details see description in text. concentration of solid cores with diameter 0.64 µm reported in ref 11 was 2.2 × 108 mL-1. Total interfacial area of particles dispersed in 1 mL of this suspension is then about 2.8 cm2. In our case of 2 v/v% emulsion with estimated mean droplet diameter of 2 µm, the corresponding area lies at about 600 cm2, which is approximately 200 times higher than that in ref 11. Therefore, to avoid too large amounts of polyelectrolyte solutions during PE deposition, the 10 times higher polyelectrolyte concentrations (20 mg/mL) than those commonly applied11,27 were used in the present work. pH values of polyelectrolyte solutions and dispersion medium of initial emulsion were kept at 6.5. Emulsion Preparation. The initial oil-in-water emulsion was prepared in two steps. At the beginning, 4 mL of DODAB solution in a dodecane/chloroform mixture were preliminarily mixed with 196 mL of chloroform-saturated water by shaking in a high-density polyethylene container. This coarse mixture was then thoroughly emulsified for 4 min at 17500 rpm by the Ultra Turrax Homogenizer (IKA, Germany). The ready-to-use emulsion is a milky liquid with quite polydisperse (Z-average diameter is 2.3 µm; corresponding PDI is 0.4) and highly positively charged (ζ-potential is +91 mV) droplets of dispersed phase. Assembly of Polyelectrolyte Multilayers on the Surface of Emulsion Droplets. The multilayer assembly was done by subsequent adsorption of polyelectrolytes from their solutions in Milli-Q water without salt addition at pH 6.5. Strongly positively charged initial emulsion was left overnight to achieve cautious and gentle separation (creaming) of less dense emulsion droplets from an aqueous dispersion medium. Centrifugation even at low rpm values cannot be considered as a reasonable alternative to accelerate the separation because of simultaneous deformation and strong coalescence of disperse phase droplets in spite of strong electrostatic repulsion. The creamed upper layer of the emulsion was added dropwise to 40 mL of the oppositely charged polyelectrolyte solution (PSS) upon continuous stirring at approximately 700 rpm as depicted in Figure 1. After all of the emulsion was brought into polyelectrolyte solution, the mixture was stirred for an additional hour to accomplish the binding of polyelectrolyte at the surfaces of the initial emulsion droplets and to ensure overcharging. At the next step the remaining free polyelectrolyte was washed out by collection and transfer of the droplet-enriched upper layer of coated emulsion into a separate water-filled vessel. The washing procedure was repeated three times and then left overnight in between to achieve the spontaneous gentle separation via creaming in the same way as for the initial noncoated emulsion. The dilution factor was each time not less than 15. The further encapsulation step was carried out with an aqueous solution of (27) Teng, X.; Shchukin, D. G.; Mo¨hwald, H. AdV. Funct. Mater. 2007, 17, 1273.
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Figure 2. Microstructures of the initial uncoated (a) emulsion as well as the encapsulated emulsion at various numbers N of PE layers in the capsule shell: 1 (b), 2 (c), 3 (d), and 4 (e). Black scale bar corresponds to 10 µm.
Figure 3. Alternation of ζ-potential of PE coated emulsion droplets with increase of total number N of polyelectrolyte layers in the shell. positively charged polyelectrolyte, PDADMAC, in the same manner (Figure 1). Successively, alternating deposition of oppositely charged polyelectrolyte layers was performed until the desired number was assembled. Particle Size Analysis. For the determination of the size distribution and average size of droplets in the initial emulsion, dynamic light scattering measurements (DLS) were carried out by HPPS5001 (Malvern Instruments Ltd, UK). ζ-Potential Measurements. The ζ-potential of bare and encapsulated emulsion droplets was measured by a Zetasizer Nano-Z (Malvern Instruments Ltd, UK). Each value was obtained as an average from three subsequent runs of the instrument with at least 10 measurements. Optical Microscopy. The morphologies of initial and encapsulated emulsions were determined by optical microscopy (Jenaval Microscope, Carl Zeiss Jena, Germany). Emulsions were gently shaken in Eppendorf safe-lock test tubes to homogenize them before observation. A drop of emulsion was then placed on a microscope slide and observed at different magnifications between 400× and 1000×. The corresponding images were acquired using digital color Microcular in VGA resolution and image-processing software (Unlead Photo Explorer ver.7). Confocal and Fluorescence Microscopy. Confocal images were taken with a Leica TCS SP confocal laser scanning system (Wetzlar, Germany) equipped with a 100× oil-immersion objective operating in the fluorescence and bright-field mode. SEM (Scanning Electron Microscopy). Samples for SEM analysis were prepared by putting a drop of the encapsulated emulsion on a glass substrate and then drying overnight. Then samples were sputtered with gold and measurements were conducted using a Gemini Leo 1550 instrument at an operation voltage of 3 keV.
Results and Discussion Figures 2 and 3 evidence the successful PE emulsion encapsulation. Figures 2a-2e exhibit the microstructures of the initial uncoated emulsion as well as the encapsulated emulsion at various numbers of PE layers N in the capsule shell in accordance with the scheme presented in Figure 1. These pictures confirm the maintenance of disperse phase particlessemulsion
dropletsson each subsequent stage of encapsulation. Together with Figure 3 showing the classical zigzag-shaped character of ζ-potential dependence on N,11 they unambiguously prove the formation of PE assemblies on the emulsion droplets as mobile colloidal template. Successful deposition of PE layers on each individual coating step is also illustrated by Figure 4a. FITC-labeled PAH was used for deposition of the fourth PE layer on the emulsion droplets instead of PDADMAC that was the standard cationic PE in this work. As one can see, the intense fluorescence from the capsule shell indicates the location of the dye-labeled PE and is additional confirmation of the PE assembly formation around each emulsion droplet. The fluid core of encapsulated emulsion plays simultaneously, as was stressed in the Introduction, the role of the capsule load. On demand, dodecane used in our paper as a model oil could be easily replaced either by other oils of interest or by the solution of the desirable oil-soluble substance. In our case, the oil-soluble dye, TPP, was added to the dispersed oil phase to show the stability of the fluid colloidal template during deposition of PE layers and to mimic a loaded oil-soluble substance that could be delivered and controllably released by means of encapsulated emulsion droplets. Figure 4b shows the intensive red fluorescence inside the liquid capsule core whereas no fluorescence is observed from the outside. There can be no doubt, therefore, that during the subsequent emulsion encapsulation steps when the PEs were introduced from the aqueous phase, the fluid oil core remains unchanged and no loss of loaded substance occurred. The oil-loaded capsules, especially those with a few PE layers in the shell, are not as robust mechanically as PE capsules synthesized on the solid colloidal core.28 Although the liquid oil core may be considered as incompressible it is, in contrast to the solid one, mobile and therefore easily deformable. Then if the encapsulated emulsion particles are transferred onto a glass substrate, the interaction between this substrate and the outer PE layer in the capsule shell can be strong enough to deform and finally destroy the capsule. Two different mechanisms of capsules destruction were observed. If the first mechanism is realized, the capsule suddenly bursts and the capsule load forms the oily spot around the place where the capsule was situated before. Figures 5a-5d show the described mechanism for the capsules with PDADMAC outer layer (four PE layers in total) transferred together with the drop of dispersion medium on the glass substrate. Substrate and the outermost capsules shell are oppositely charged and interact more and more intensively with the time resulting in the capsule burst. The fast mechanism of capsule destruction is, in our opinion, only possible when the PE shell was formed not homogeneously and has some significant defects. Then the interaction with the substrate leads to further deformation of shell and to the simultaneous increase of pressure in the capsule interior. As a result, the capsule bursts and the encapsulated content forms a visible thick oil spot at the former capsule site (Figure 5c). This spot, however, disappears very fast because of oil spreading at the substrate/dispersion medium interface (Figure (28) Dubreuil, F.; Elsner, N.; Fery, A. Eur. Phys. J. E 2003, 12, 215.
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Figure 4. Fluorescence and transmission micrographs of (a) droplets of encapsulated emulsion with FITC-labeled PAH in the outer fourth PE layer and (b) droplets of emulsion with TPP-labeled oil core coated by three PE layers.
Figure 5. Fast destruction of microcapsule on the oppositely charged glass substrate (shown by arrow) obtained after deposition of four PE layers on the oil-in-water emulsion droplets. Time from the beginning of video-recording: (a) 0 s; (b) 0.067 s; (c) 0.133 s; (d) 0.233 s. Black scale bar corresponds to 10 µm.
Figure 6. Slow destruction of microcapsule on the oppositely charged glass substrate (shown by arrow) obtained by deposition of four PE layers on the oil-in-water emulsion droplets. Time from the beginning of video-recording: (a) 0 s; (b) 18.53 s; (c) 31.87 s; (d) 42.03 s; (e) 55.87 s; (f) 73.53 s; (g) 84.93 s. Black scale bar corresponds to 10 µm.
5d). The second, slow mechanism of capsule destruction due to interaction with the oppositely charged substrate is presented in Figures 6a-6f. In this case the capsule shrinks gradually, showing with the time more and more concave sites but remains intact before complete destruction of the shell (Figures 6e and 6f). The process of complete capsule destruction takes now essentially
more time than in the case of fast destruction. The possible mechanism of slow destruction can be explained in a fashion similar to the fast one with the only difference being that the capsules with fewer shell defects undergo this destruction mechanism. Hence, the slow destruction takes essentially more time than the fast one; the release of oil from the capsule during
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Figure 7. SEM images of a dried emulsion encapsulated by deposition of four PE layers at different magnifications: (a) low, scale bar denotes 20 µm; (b) medium, scale bar denotes 10 µm; (c) high, scale bar denotes 2 µm. The randomly shaped dodecane-coated fields around collapsed capsules are slightly darker (arrows).
its collapse is carried out gradually without formation of a visible oil spot. In spite of the significant interaction between capsules and substrate, of relatively small N and of mobile fluid core, the majority of capsules remains quite stable and can be easily dried for SEM measurements. Figures 7a-7c show the samples of dried encapsulated emulsions at different SEM magnification. As was shown by Leporatti and co-workers,29 the drying process leads to a collapse of the capsule that, in turn, results in a release of loaded oil outward from the capsule. One can see this liberated oil in Figure 7a as dark fields surrounding the capsule rest. At higher magnification, however, the border between water- and oil-covered substrate surfaces exhibits less contrast (Figure 7b). On the other side, high magnification (Figure 7c) reveals fine details of the collapsed containerssskinlike shells with numerous folds confirming once more the formation of PE multilayer assemblies on the colloidal liquid templates. Further repetition of the alternating coating steps leads to the formation of capsules with desired shell thickness depending on the particular final demand. Hence, the proposed approach to prepare loaded PE containers via emulsion encapsulation can be envisaged as a general one that depends neither on the nature of the used oil nor on the specific behavior of an oil-soluble capsule load. It is noteworthy that several details of this approach were used to some extent also in previous works,3,21,22 as was briefly mentioned in the Introduction. All of them are, however, restricted either by employing quite specific substances or by applying unique preparation conditions. For example, in the work of Tjipto et al.,21 a compound able to form the nematic liquid crystals, 4′-pentyl-4-cyanobiphenyl (5CB), was used to form the droplets of dispersed phase in the model oil-in-water emulsion. However, this substance cannot be considered as a typical “oil” because of its amphiphilic molecular structure, its significant charge in a broad pH range,21 and especially a specific elastic mechanism of emulsion stabilization found recently for droplets of liquid crystals in the nematic state.30,31 Biopolymers such as proteins in alternation with bioemulsifiers can also be effectively used for the emulsion encapsulation, as was shown on diverse food-relevant systems by McClements and co-workers.3,32-39 But amounts of hydrophobic bioemulsifiers (29) Leporatti, S.; Voigt, A.; Mitlo¨hner, R.; Sukhorukov, G.; Donath, E.; Mo¨hwald, H. Langmuir 2000, 16, 4059. (30) Heppenstall-Butler, M.; Williamson, A.-M.; Terentjev, E. M. Liq. Cryst. 2005, 32, 77. (31) Tixier, T.; Heppenstall-Butler, M.; Terentjev, E. M. Langmuir 2006, 22, 2365. (32) Moreau, L.; Kim, H. J.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2003, 51, 6612. (33) Surh, J.; Gu, Y. S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2005, 53, 4236. (34) Klinkesorn, U.; Sophanodora, P.; Chinachoti, P.; McClements, D. J.; Decker, E. A. J. Agric. Food Chem. 2005, 53, 4561.
used in the form of aqueous dispersions (up to 3.5 wt % in Klinkesorn et al.34) and the preparation conditions (room temperature and short sonication) allow the assumption that the primary emulsion was stabilized either by multilamellar liposomes in the solid state, i.e., by very small solid particles forming a so-called “Pickering emulsion”40,41 or by emulsifier multilayers occurring due to liposomes fusion at the droplet surfaces.42 Anyhow, all further steps of such an emulsion encapsulation should be considered as L-b-L assembly on the solid-like colloidal template. On the other side, the use of water-soluble surfactants like SDS to produce the primary emulsion with template properties35 is also quite questionable because their stabilizing effect on the primary emulsion is determined by the density of surfactant molecules at the oil/water interface, which, in turn, is closely related to the variable bulk concentration. Being mixed with the solution of protein or polyelectrolyte which forms the first stable layer at the droplet interface, the surfactant solution is diluted and therefore the equilibrium surface density is changed, leading to an ill-defined condition for L-b-L deposition. Moreover, the presence of free surfactant molecules in the aqueous phase during addition of an oppositely charged coating substance will unambiguously lead to the formation of surfactant/protein clusters43 in the solution bulk. These clusters effect usually bridging, reducing strongly the colloidal stability of the system.33 In addition, the clusters are the second colloidal species in the system and, therefore, they can influence or even completely mask the properties of the colloidal system of interest, i.e., encapsulated oil-in-water emulsion. Proteins constitute a special class of compounds, which are very suitable for L-b-L emulsion encapsulation. They are, on the one hand, natural weak polyelectrolytes but, on the other hand, several hydrophobic groups in their structure make these compounds interfacial-active. Thus, being applied as emulsifiers proteins simultaneously charge the adsorption interface36-39 and therefore enable the following steps of L-b-L deposition either by other biopolyelectrolytes or by conventional PEs. Nilsson and co-worker22 also used the charged amphiphilic precursor DODAB to impart to the dispersed phase the function (35) Aoki, T.; Decker, E. A.; McClements, D. J. Food Hydrocolloids 2005, 19, 209. (36) Gu, Y. S.; Decker, E. A.; McClements, D. J. Langmuir 2004, 20, 9565. (37) Gu, Y. S.; Decker, E. A.; McClements, D. J. J. Agric. Food Chem. 2004, 52, 3626. (38) Gu, Y. S.; Decker, E. A.; McClements, D. J. Langmuir 2005, 21, 5752. (39) Gu, Y. S.; Decker, E. A.; McClements, D. J. Food Hydrocolloids 2005, 19, 83. (40) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001. (41) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (42) Launois-Surpas, M. A.; Ivanova, Tz.; Panaiotov, I.; Proust, J. E.; Puisieux, F.; Georgiev G. Colloid Polym. Sci. 1992, 270, 901. (43) La Mesa, C. J. Colloid Interface Sci. 2005, 286, 148.
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of the template. But the conditions at which they have dispersed the precursor in the oil phase (160 °C) and the amount of DODAB used for this purpose (3% w/w) make it evident that during emulsion preparation the initially dissolved DODAB was either crystallized again within oil droplets and then precipitated on their surface or directly crystallized at the surface forming a solid charged crust. In any case, the following steps of L-b-L deposition are therefore performed already on the solid core. In contrast, as one can see from the results presented here the developed new approach seems to have fewer restrictions than many of the methodologies discussed above and can be extended to emulsion encapsulation for a wide variety of oil-auxiliary solvent-surfactant combinations depending on the final application of oil-loaded containers. Different classes of surfactants can be used as interfacial precursorssfatty acids or their salts, long-chain amines, charged phospholipids (in the biologically relevant cases), etc. Similarly, in the cases when the application of harmful solvents like chloroform is not suitable, they can be replaced by more environmentally friendly solvents such as benzyl alcohol, butyl lactate, or triacetin.44 Furthermore, the oppositely charged proteins show an example of weak natural polyelectrolytes for the subsequent L-b-L deposition. The oil core may (44) Trotta, M.; Gallarate, M.; Pattarino, F.; Morel, S. J. Controlled Release 2001, 76, 119.
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be composed of different types of natural or artificial fats or oils like fish-oil, soya-bean oil, triglycerides, hydrophobic vitamins (like vitamin B12) or drugs dissolved in oil, etc. depending on the demands of the container application in the fields of biology, medicine, pharmacy, or food industry.
Conclusions A novel approach to layer-by-layer emulsion encapsulation is proposed and experimentally proven on a model oil-in-water emulsion. The data collected evidence the successful formation of polyelectrolyte multilayer assemblies on the liquid colloidal template. The encapsulated emulsion droplets remain stable in the bulk of the dispersion medium whereas the interaction with oppositely charged solid substrate (glass) leads to their destruction. Two different mechanisms of capsules destruction are observed, which depend on the quality of the previously formed capsule shell. The developed approach is quite general and can be, on demand, easily extended on other emulsion systems of interest. Acknowledgment. The research was supported by the NanoFutur program of the German Ministry for Education and Research (BMBF) and DAAD/GRICES Germany/Portugal collaboration project (D/06/12931). LA702873F
