Article pubs.acs.org/Langmuir
Microcapsules Composed of Cross-Linked Organoclay Yannan Cui and Jeroen S. van Duijneveldt* School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K. ABSTRACT: Polyelectrolyte-modified montmorillonite particles were used to stabilize oil-in-water Pickering emulsions, which were then bound together by an oil-soluble cross-linker to obtain microcapsules. It was determined how the morphology and rigidity of the microcapsules changed as polyelectrolyte and cross-linker concentrations were varied. Well-defined microcapsules could be formed by using a moderate concentration of polyelectrolyte, and the higher the cross-linker concentration, the more rigid the microcapsules. Dried microcapsules were observed using SEM, and it was shown that the clay platelets lie flat next to each other on the microcapsule surface, forming an armor-like structure.
electrostatic repulsion.4,10 In the meantime, the short-ranged van der Waals forces bind the colloidal particles together to form robust microcapsules because the particles are close enough to fall into the deep attraction well and form irreversible aggregates.4,10 Using a gelled core also allows the microcapsules to retain their shape when transferring from one solvent to another.5,7 An alternative approach involves chemical bonding, which can be realized using cross-linkers. If the colloidal particles have surface functional groups which could react with cross-linkers, triggering the reaction also produces robust colloidosomes.15,16 In an alkali environment, siloxanes can react with the surface of a template latex colloid and then calcining the template colloid provides another route to make robust hollow capsules.9 An aminopropyltriethoxysilanemodified kaolin was used to stabilize Pickering emulsions, and reaction with cross-linkers produced very robust microcapsules.17 Here we present a novel way of using cross-linkable polyelectrolyte-modified clay particles to stabilize a Pickering emulsion and then cross-link the clay particles from inside the emulsion droplets. The advantages of this system are first the convenience of using a Pickering emulsion method to make stable clay-coated droplets. Also the modified clay has a very low organic content and is therefore environmentally friendly, compared to latex particles. Based on our work on the adsorption mechanism of amines on clay surfaces,18 crosslinkable polymers with amine groups were used to stabilize the oil-in-water precursor Pickering emulsions. The adsorption proceeds via an ion-exchange mechanism at high pH, and the equilibrium adsorbed amount is dependent on the molecular weight according to polymer scaling theory. The effect of polymer concentration on precursor emulsion stability was
1. INTRODUCTION Microcapsules are hollow microspheres with a solid shell. If the shell is composed of a layer of colloidal particles linked together, they are also known as colloidsomes.1−3 Microcapsules have found applications in foods, agrochemicals, drugs, paints, and other coatings. Velev et al.4 published the first report on preparing hollow microcapsules using emulsion droplets as templates. The term colloidosome was first coined by Dinsmore.1 A considerable amount of work has been done to develop various methods to prepare microcapsules. The general process of producing colloidosomes includes the adsorption of particles onto droplets and the binding of the particles by either physical or chemical means. Optionally, the carrier or template droplet can be removed by either solvent extraction or calcining. A variety of colloid particles with different surface properties can be used to adsorb onto the droplet surface, for example polystyrene latex,1,4−6 polymeric microrods,7 silica particles,8,9 and clay particles.10,11 By homogenizing or shaking a colloidal suspension with another liquid, particles may adsorb onto the template droplet surface,12 forming a particle-stabilized emulsion, i.e., a Pickering emulsion.13 As a rule, such emulsions are only kinetically stable, requiring mechanical homogenization. A thermodynamically stable Pickering emulsion with small droplet size was recently reported.14 However, these droplets are stabilized by charge repulsion between the colloids so the surface coverage is rather low, which makes it difficult to bind colloid particles together to make robust capsules. Adsorbed colloidal particles need to be bound together to make robust microcapsules, which can be achieved by either physical or chemical ways. Sintering the system above the glass transition temperature of the polymer produces elastic and robust microcapsules made of latex particles.1,15 Polymers have also been used to connect particles together.4,6 Charged colloid particles can be forced to approach each other closely under high shear forces or at high salt concentrations, overcoming the © 2012 American Chemical Society
Received: October 19, 2011 Revised: December 19, 2011 Published: January 9, 2012 1753
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acceleration voltage. A drop of dilute clay suspension (0.01 wt %) was laid on a carbon-coated copper grid and air-dried. Scanning electron microscopy (SEM) was used to observe the dried microcapsules. A JEOL JSM 5600LV SEM was used at an acceleration voltage of 20 kV. All samples were dried from aqueous suspensions. For air-dried capsules, a drop of capsule suspension was dried on an aluminum stub and then coated with gold to prevent charge build-up. The freeze-dried sample was dried using a freeze drier overnight and then stuck onto an aluminum stub for gold coating and observation.
studied, and the most stable precursor emulsions were obtained at either low or high polymer concentrations, where the clay particles are relatively hydrophilic and most effective in stabilizing oil-in-water emulsions.19 The restabilization at high polymer concentration is probably due to multilayer adsorption. Meanwhile, a high polyelectrolyte coverage is essential to bind the clay particles together. An oil-soluble cross-linker was used to connect the clay particles together to form a robust shell. The oil in the droplets was extracted by a solvent to form hollow capsules. Polymer with a higher amine group density was found to produce more robust capsules. The cross-linker concentration determines the degree of the crosslinking.
3. RESULTS AND DISCUSSION 3.1. Size Distribution of MMT Particles. To avoid the aggregation of clay particles during drying from aqueous suspension21,22 and to obtain clear images, the clay particles were treated using the method described in a previous study20 and dispersed in toluene. Discrete clay particles can be observed in Figure 1, with an average particle diameter of 266 nm and a standard deviation of 99 nm.
2. EXPERIMENTAL SECTION 2.1. Materials. Montmorillonite (MMT, Wyoming SWy2) was used. The details of the clay and purification process were described in a previous study.18 Silkflo 364 polydecene (Lipo Chemicals Inc., referred to as polydecene), diamine Jeffamine E900 (Huntsman, Belgium, referred to as ED900), Lupasol G35 polyethylenimine (BASF, Mw 2000, branched, (CH2CH2NH)n, n ≈ 47, referred to as PEI), and poly(propylene glycol) diglycidyl ether (Sigma-Aldrich, Mw 640, with two epoxide groups per molecule, referred to as PPG-DGE) were used as received. The cationic exchange capacity (CEC) of MMT is 84 mmol/ 100 g.18 In this study, the MMT concentration in the water phase is 2 wt %, and 1 CEC of charge is equivalent to 16.8 mmol/L. The concentration of polymers is expressed in weight percent, as well as relative to CEC, i.e., the amine group concentration normalized to the clay charge concentration. The concentration of the cross-linker PPGDGE is expressed as the ratio of epoxide group concentration in PPGDGE to the amine group concentration in the polymer; for example, 1:1 PPG-DGE means an equal concentration of epoxide and amine groups. 2.2. Sample Preparation. 2.2.1. Pickering Emulsion. The emulsions were prepared by mixing 3 g water and 2 g oil phase. The water phase contained 2 wt % MMT, 0.01 M NaCl, and varying concentrations of ED900 or PEI. The oil phase was composed of polydecene and PPG-DGE. The samples were homogenized at 22 000 rpm for 10 min using an Ultra Turrax IKA T18 high shear mixer with an S18N-10G dispersing tool. Drop tests were carried out, and all the emulsions were oil in water type. 2.2.2. Cross-Linked Pickering Emulsion. Reactions between amines and epoxide groups do take place at room temperature, but in our case very slowly, as they were dispersed in different phases. The precursor Pickering emulsions were heated at 80 °C for 2 h to speed the reaction. The emulsions remained stable after the heating. To test whether the cross-linking was successful, a 2-propanol challenge was carried out. Polydecene has a solubility of 15 g/100 g in 2-propanol, and water is fully miscible with 2-propanol. The emulsion was added into 2-propanol at a 1: 10 ratio and mixed on a roller mixer for 48 h. At this ratio, the three liquids in the mixture form one homogeneous phase. Therefore, the liquid interface disappeared and the un-crosslinked droplets disintegrated. An aliquot of the resultant mixture was dropped on a glass slide and observed using a DIC microscope. The remainder of the mixture was then centrifuged at 5000 rpm for 10 min. The solid sediment was separated and washed with 2-propanol three times to remove any residual polydecene. The solid was then either air-dried or freeze-dried for SEM observation. 2.3. Characterization. Differential interference contrast (DIC) microscopy was carried out using an Olympus BX51 microscope with a Pixelink PL-B625CU color CCD camera. The average droplet size was calculated by measuring at least 100 droplets in a micrograph using ImageJ 1.42q software. The morphology of the dried films made from un-cross-linked or cross-linked emulsions was also examined using DIC. The size distribution of the clay particles was examined by a JEOL JEM 1200-EX transmission electron microscope (TEM) at 120 kV
Figure 1. TEM image of organically modified20 clay particles.
3.2. Un-Cross-Linked Pickering Emulsions. ED900 or PEI-modified MMT clay was used make stable Pickering emulsions. Previously it was found that very stable emulsions can be made using organoclay containing very little surfactant (0.02 wt %).19 However in this study, the surfactant concentration needs to be high enough to ensure that there are enough amine groups to react with the cross-linker PPGDGE. Pickering emulsions with ED900 concentration from 0.2 CEC (0.17 wt %) to 2.6 CEC (2.17 wt %) were prepared, and the stability was observed after standing overnight. Creaming occurred in all emulsions, which were not as stable as those obtained using Berol previously.19 As shown in Figure 2, the emulsion fractions range from 0.65 to 0.93. However, to achieve the cross-linking in the next step, the diamine ED900 is considered a better candidate to stabilize the precursor emulsions than the monoamine surfactant used previously. Samples with either low or high ED900 concentrations (0.2, 2.0, and 2.6 CEC ED900) produced the most stable emulsions, among which the emulsion with 2.6 CEC ED900 has the highest emulsion fraction. With intermediate ED900 concentrations, the samples have the lowest emulsion fractions, between 0.65 and 0.75. Emulsion droplet sizes distributions were observed using DIC microscopy, and the result is also shown in Figure 2. Emulsions with 0.2 and 2.6 CEC ED900 have the smallest droplet sizes, indicating a better stability against coalescence, which agrees with the emulsion fraction result. The ED900 1754
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The average droplet size and polydispersity of the emulsions are shown in Figure 3 by open circles. Relatively small (32 μm) but rather polydisperse droplets were obtained at 0.3 CEC PEI. Upon increasing the PEI concentration to above 0.7 CEC, the droplet size increased dramatically due to the increased hydrophobicity of the clay. At 4 CEC PEI, the droplet size started to decrease. At very high PEI concentrations (7 and 10 CEC), the average droplet size is reduced to about 15 μm. To achieve a high surfactant concentration and a good stability, 10 CEC PEI was chosen for further experiments. What caused the decrease of droplet size at high ED900 or PEI concentrations? Two possible reasons are (a) a second layer adsorption of surfactant on the clay surface at high surfactant concentration makes the clay particles more hydrophilic and stabilizes the o/w emulsions better; or (b) there might be free surfactant in the system after the adsorption is saturated, which could also adsorb to the water−oil interface and help stabilize the emulsions. However, we need to bear in mind that emulsions made using surfactant alone are not stable. 3.3. Cross-Linked Emulsions. The freshly made precursor emulsions were heated in an oven at 80 °C for 2 h for the crosslinking reaction to take place. A range of PPG-DGE concentrations were used in the oil phase to achieve different degrees of cross-linking. The reaction proceeds according to the following equation:
Figure 2. Droplet size (error bars represent the standard deviation) and emulsion fraction of samples with ED900 concentration of 0.2, 0.6, 1, 1.4, 2, and 2.6 CEC. All the emulsions have 2 wt % clay, 0.01 M NaCl, and an oil/water ratio of 44/56 by vol.
concentration determines its adsorbed amount on the clay surface and the hydrophobicity of the organoclay. The relatively hydrophilic particles stabilize o/w emulsions better.19,23 According to a previous study, the single layer equilibrium adsorbed amount of ED900 on MMT surface is about 0.4 CEC.18 The emulsion with 0.2 CEC ED900 has only half the surface coverage and is therefore more hydrophilic than the ones with 0.5 CEC ED900 concentration, which have a higher coverage.18 On further increasing ED900 concentration to above 1 CEC, it is thought that a second layer adsorbs and renders the clay particles hydrophilic again.24 To achieve a combination of both high surfactant concentration and good stability, 2.6 CEC ED900 was used for further study. Emulsions were also prepared using PEI at concentrations ranging from 0.3 CEC (0.021 wt %) to 10 CEC (0.7 wt %). The stability of the emulsions was investigated after 24 h standing. Emulsion fractions were calculated by dividing the height of emulsion in the vial by that of the total liquid. Filled triangles in Figure 3 are emulsion fractions, ranging from 0.68
All the cross-linked ED900 emulsions were fluid-like after heating and remained stable to creaming after 24 h. However, the viscosity of PEI emulsions increased after cross-linking. At 1:1 PPG-DGE, the emulsion became more viscous after heating. At 2:1 and 2.5:1 PPG-DEG, the emulsions turned into crumbly soft solids. The emulsions were allowed to cool to room temperature for the 2-propanol challenge. 3.4. 2-Propanol-Challenged Emulsions. The 2-propanol challenge was performed for both un-cross-linked and crosslinked emulsions. The morphology of the challenged emulsions was observed using a DIC microscope, as shown in Figure 4. In Pickering emulsions, colloidal particles are held together at the droplet surface as a result of the tendency to decrease the area of the fluid interface by replacing interface with particles.23 As the interface disappears upon dissolution of the oil phase, most un-cross-linked particles disassemble and form irregular aggregates, as shown in Figure 4a,e. The variation in the density of amine groups in the polymers also makes a remarkable difference. The un-cross-linked emulsion with ED900 failed to produce robust droplets, and only clay aggregates were observed after the 2-propanol challenge. The emulsions cross-linked by 1:1 and 2:1 PPG-DGE were not robust enough either to survive without a water/oil interface. Only the droplets using 5:1 PPGDGE managed to maintain some integrity after the challenge, as shown in Figure 4d. ED900 molecules only have two amine groups. While being able to offer a certain degree of bridging between clay particles, they are not sufficient in binding the clay particles together in the absence of the oil/water interface. Robust droplets were only produced at high concentrations of PPG-DGE, which reacts with ED900 at the interface.
Figure 3. Droplet size, distribution, and emulsion fraction of samples with PEI concentration of 0.3, 0.7, 1, 1.3, 4, 7, and 10 CEC. All the emulsions have 2 wt % clay, 0.01 M NaCl, and an oil/water ratio of 44/56 by vol.
to 0.87. A bottom water layer separated out in each sample. Samples with either a low or high PEI concentration (0.3, 7, and 10 CEC) have a high emulsion fraction while a thick water bottom layer separated out in the samples with PEI concentration from 0.7 to 4 CEC. This tendency is also observed in ED900 samples. 1755
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Figure 4. DIC images of 2-propanol-challenged cross-linked Pickering emulsions using 2.6 CEC ED900 (a−d) or 10 CEC PEI (e−h) with different PPG-DGE concentrations. (a) 2.6 CEC ED900, 0 PPG-DGE; (b) 2.6 CEC ED900, 1:1 PPG-DGE; (c) 2.6 CEC ED900, 2:1 PPG-DGE; (d) 2.6 CEC ED900, 5:1 PPG-DGE; (e) 10 CEC PEI, 0 PPG-DGE; (f) 10 CEC PEI, 1:1 PPG-DGE; (g) 10 CEC PEI, 2.5:1 PPG-DGE; (h) a close-up image of panel f (10 CEC PEI, 1:1 PPG-DGE) with enhanced contrast. The scale bars represent 50 μm.
PEI is more effective in binding the clay particles together at the interface. A few empty circles were observed in the uncross-linked emulsion in Figure 4e, indicating that PEI alone at high concentration (10 CEC) to some extent can lead to capsule formation, presumably due to this polymer bridging across different clay platelets. Similar observations were made using polystyrene particles and Tween 20.4 The presence of PPG-DGE gives the droplets more mechanical strength, which could be measured using micromanipulation techniques.25 Although the mechanical strength was not measured quantitatively here, Figure 4f−h shows capsules with noticeably better integrity than that in Figure 4e. The morphology of the droplets after 2-propanol challenge can be found in the DIC micrographs in Figure 4f,g. The circles with a thin wall in the micrographs are cross-linked shells from the droplets, which can be clearly seen in the contrast-enhanced close-up image shown in Figure 4h. The oil originally inside them has been mostly dissolved so the droplets have been extracted to become robust hollow spheres. The DIC images show that the refractive index difference between inside and outside the droplet has disappeared, which proves the dissolution of the oil originally in the droplet. A higher concentration of PPG-DGE was expected to produce more robust walls, but this cannot be judged from the DIC images. However, too much PPG-DGE leads to the bridging between droplets and the formation of droplet aggregates. Inside the aggregates, the morphology of individual hollow spheres was retained as shown in Figure 4g, which further demonstrates their mechanical strength. PEI produces more robust droplets than ED900 because it has 47 amine groups in one molecule, which can bind together clay particles more effectively. 3.5. Dried Emulsions. Drops of emulsions were allowed to dry in a vacuum oven at room temperature on glass slides. The dried samples were observed using a DIC microscope. Uncross-linked droplets ruptured and left a layer of oil homogeneously on the surface, where no individual clay shell was observed. The cross-linked droplets also ruptured, but the clay shells remained visible in DIC micrographs, forming a walllike structure between droplets, as shown in Figure 5. The shape of the dried droplets in Figure 5 is not perfectly circular because the droplets collapse upon drying and squeeze the droplets around them. Such structures have also been observed upon drying an emulsion stabilized by silica particles.8 3.6. Observation of Dried Hollow Capsules. It is impossible to observe the surface morphology of the micro-
Figure 5. DIC micrographs of dried cross-linked emulsion using 10 CEC PEI and 1:1 PPG-DGE.
capsules when oil is still inside the capsules. The oil film formed during drying will cover the clay shell and make the observation very difficult. After extraction of the oil using 2-propanol, the capsules were dispersed in water, dried in air, and observed using SEM. However, the capsules did not survive this drying process, presumably because of capillary forces. To avoid these, an aqueous suspension of the capsules was freeze-dried, yielding a fluffy powder. Hollow capsules were observed using SEM. Although some broke into pieces, half of them retained the spherical shape. A hollow microcapsule is shown in Figure 6a. The size of the capsule is about 20 μm, in good agreement with DIC observation. Because of the higher density of the capsules compared to water, the capsules settled to the bottom of the suspension during freezing. The pressure from adjacent capsules deformed each other, and they are not perfectly spherical, as can be seen in Figure 6a. Also the volume of water inside the capsules increased upon freezing, and the capsules burst open. Cracking of microcapsules was also observed after freeze-drying by Yeo and Park.26 However, the fact that most of the capsule survived the freeze-drying demonstrated the mechanical strength of the capsules. The cracking of microcapsules caused by freeze-drying may be avoided by using critical point drying.27 This has not been attempted here. The open capsule in Figure 6a enables us to observe the inside of the capsules. The inside of the shell is composed of thin layers of clay or clay aggregates. The shell itself consists of several layers of clay particles. Figure 6b is a close-up of part of the capsule in Figure 6a, which shows the surface morphology of the capsule. Holes can be observed in 1756
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Figure 6. SEM images of freeze-dried hollow microcapsules: (a) a hollow microcapsule, (b) a close-up of panel a, and (c) bridging between microcapsules. The scale bars represent 5 μm. (8) Gao, Q. X.; Wang, C. Y.; Liu, H. X.; Wang, C. H.; Liu, X. X.; Tong, Z. Polymer 2009, 50, 2587. (9) Bon, S. A. F.; Chen, T. Langmuir 2007, 23, 9527. (10) Subramaniam, A. B.; Wan, J. D.; Gopinath, A.; Stone, H. A. Soft Matter 2011, 7, 2600. (11) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. Adv. Mater. 2001, 13, 500. (12) Kralchevsky, P. A.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 145. (13) Binks, B. P.; Kirkland, M. Phys. Chem. Chem. Phys. 2002, 4, 3727. (14) Sacanna, S.; Kegel, W. K.; Philipse, A. P. Phys. Rev. Lett. 2007, 98, 158301. (15) Walsh, A.; Thompson, K. L.; Armes, S. P.; York, D. W. Langmuir 2010, 26, 18039. (16) Thompson, K. L.; Armes, S. P.; Howse, J. R.; Ebbens, S.; Ahmad, I.; Zaidi, J. H.; York, D. W.; Burdis, J. A. Macromolecules 2010, 43, 10466. (17) Gittins, D. I.; Mulqueen, P. J.; Taylor, P.; Pilrip, T.; Deibit, I. G. Making microcapsules used e.g. for combating pests, involves forming solution of cross-linker in liquid, forming slurry of surface-modified particulate inorganic material in aqueous medium, and dispersing solution in slurry. WO2009063257-A2, May 22, 2009. (18) Cui, Y. N.; van Duijneveldt, J. S. Langmuir 2010, 26, 17210. (19) Cui, Y. N.; Threlfall, M.; van Duijneveldt, J. S. J. Colloid Interface Sci. 2011, 356, 665. (20) Leach, E. S. H.; Hopkinson, A.; Franklin, K.; van Duijneveldt, J. S. Langmuir 2005, 21, 3821. (21) Michot, L. J.; Bihannic, I.; Porsch, K.; Maddi, S.; Baravian, C.; Mougel, J.; Levitz, P. Langmuir 2004, 20, 10829. (22) Paineau, E.; Bihannic, I.; Baravian, C.; Philippe, A. M.; Davidson, P.; Levitz, P.; Funari, S. S.; Rochas, C.; Michot, L. J. Langmuir 2011, 27, 5562. (23) Binks, B. P.; Horozov, T. S. Colloidal particles at liquid interfaces; Cambridge University Press: Cambridge, 2006. (24) Cui, Z. G.; Yang, L. L.; Cui, Y. Z.; Binks, B. P. Langmuir 2010, 26, 4717. (25) Sun, G.; Zhang, Z. Int. J. Pharm. 2002, 242, 307. (26) Yeo, Y.; Park, K. AAPS PharmSciTech 2004, 5, 52. (27) Dubey, R.; Shami, T. C.; Rao, K. U. B.; Yoon, H.; Varadan, V. K. Smart Mater. Struct. 2009, 18, 025021.
the capsule surface. Although it is not clear if the hole is only in the surface layer or it penetrates into the inside of the shell, this might affect its application in controlled release. Another feature of the capsule surface is that it consists of clay particles of about 1 μm diameter lying flat next to each other, forming an armor-like structure. This is consistent with the observation of the air-dried sample. Figure 6c shows the bridging among microcapsules. Although oil-soluble cross-linker was used, bridging still happened among adjacent droplets during the cross-linking at 80 °C. Probably the close proximity of the droplets and the high temperature enable the cross-linker to penetrate through the shell surface and cross-link between droplets.
4. CONCLUSIONS By adjusting the particle hydrophobicity, stable Pickering emulsions were prepared using amine polymer-modified montmorillonite. More hydrophilic particles stabilize oil-inwater emulsions better, which was clearly shown when varying polymer concentration. Polymer with a higher amine group density binds the clay particles together more effectively. Microcapsules were only produced at high enough cross-linker concentration when using ED900. Well-defined PEI-MMTstabilized microcapsules capsules were made when cross-linker was present. Too much cross-linker turned the microcapsules into bridged capsules and the system into a soft solid. The surface of the microcapsules is composed of clay platelets lying flat next to each other, forming an armor-like structure.
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ACKNOWLEDGMENTS Y.C. is funded by a Dorothy Hodgkin Postgraduate Award (Engineering and Physical Sciences Research Council grant EP/P504368/1), cosponsored by AkzoNobel. We thank Miss Francesca Speranza for kindly providing PEI and Huntsman for a gift of Jeffamines. We also thank Miss Ling Zhu for the help with TEM.
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REFERENCES
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