Article pubs.acs.org/Langmuir
Clay-Based Colloidosomes Mark Williams and S. P. Armes* Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire, S3 7HF United Kingdom
David W. York Procter & Gamble, Newcastle Technical Centre, Whitley Road, Longbenton, Newcastle-upon-Tyne, NE12 9TS United Kingdom S Supporting Information *
ABSTRACT: Poly(ethylene imine) (PEI) has been adsorbed onto the surface of Laponite clay nanoparticles from aqueous solution at pH 9 in order to produce an efficient hybrid Pickering emulsifier. This facile protocol allows formation of stable sunflower oil-in-water Pickering emulsions via homogenization at 12 000 rpm for 2 min at 20 °C. The effect of varying the extent of PEI adsorption on the Pickering emulsifier performance of the surface-modified Laponite is investigated for five oils of varying polarity using aqueous electrophoresis, thermogravimetric analysis, and laser diffraction studies. A minimum volume-average emulsion droplet diameter of around 60 μm was achieved at a Laponite concentration of 0.50% by mass when utilizing a PEI/Laponite mass ratio of 0.50. Such emulsions proved to be very stable toward droplet coalescence over time scales of months, although creaming is observed on standing within days due to the relatively large droplet size. These conditions correspond to submonolayer coverage of the Laponite particles by the PEI, which ensures that there is little or no excess PEI remaining in the aqueous continuous phase. This situation is confirmed by visual inspection of the underlying aqueous phase of the creamed emulsion when using fluorescently labeled PEI. These Pickering emulsions are readily converted into novel clay-based colloidosomes via reaction of the primary and/or secondary amine groups on the PEI chains adsorbed at the Laponite surface with either oil-soluble poly(propylene glycol) diglycidyl ether or watersoluble poly(ethylene glycol) diglycidyl ether cross-linkers. These colloidosomes were sufficiently robust to survive the removal of the internal oil phase after washing with excess alcohol, as judged by both optical and fluorescence microscopy. However, dye release studies conducted with clay-based colloidosomes suggest that these microcapsules are highly permeable and hence do not provide an effective barrier for retarding the release of small molecules.
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barrier.3,5 Moreover, the intrinsic free volume within amorphous vinyl polymers such as polystyrene or poly(methyl methacrylate) most likely precludes latex-based colloidosomes from offering a sufficiently impermeable barrier to prevent the premature release of small molecules. In principle, such technical problems can be overcome by using crystalline, nonspherical Pickering emulsifiers such as clay nanoparticles. Laponite is a disk-shaped synthetic smectic clay with a mean diameter of 25 nm and a mean thickness of 0.92 nm.7 The disk surface has anionic character due to partial isomorphic substitution of divalent Mg2+ by monovalent Li+ in the crystal lattice, whereas the edges possess amphoteric character. Laponite alone is too hydrophilic to act as an effective Pickering emulsifier.8 However, Ashby and Binks showed that stable toluene-in-water emulsions can be prepared using Laponite provided that sufficient background salt is added to induce flocculation.9 Similarly, Bon and co-workers reported10
INTRODUCTION Colloidosomes are an interesting class of microcapsules derived from Pickering emulsions that are further stabilized by either thermal annealing,1 polyelectrolyte adsorption,1 gel trapping,2 or covalent cross-linking3,4 in order to lock in the initial particle superstructure. Recently, the permeability of such microcapsules has been assessed via dye release studies.5 Unfortunately, the release of model water-soluble dyes from colloidosomes into the aqueous continuous phase typically occurs within a few hours,5 whereas industrial applications demand retention of actives over time scales of many months/ years for useful product shelf-lives. For spherical Pickering emulsifiers such as latexes, there is also an intrinsic geometric packing problem. Bausch et al. have shown experimentally that packing a monolayer of near-monodisperse 1 μm diameter polystyrene latex particles onto spherical oil droplets inevitably leads to the formation of at least twelve packing defects or “scars”.6 We and others have recently reported that these relatively large defects cannot be eliminated by thermal annealing of the latex particles to produce a contiguous © 2011 American Chemical Society
Received: October 9, 2011 Published: December 6, 2011 1142
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Figure 1. Reaction scheme for the formation of cross-linked colloidosomes at 50 vol % using poly(ethylene imine)-coated Laponite particles at pH 9. Route A shows colloidosomes produced from an o/w Pickering emulsion using an oil-soluble cross-linker (PPG-DGE) dissolved in sunflower oil. Route B depicts colloidosome formation using a water-soluble cross-linker (PEG-DGE) dissolved in the aqueous continuous phase. (N.B. Only monolayer coverage of the Laponite around the oil droplets is shown here for clarity. In reality, the Laponite particles most likely form multilayers at the oil droplet surface, see main text.)
Table 1. Representative Optical Microscopy Images and Mean Droplet Diameters for Various Oil-in-Water Pickering Emulsions (Using Either Sunflower Oil, Toluene, Chloroform, n-Dodecane, or Isopropyl Myristate As the Oil Phase) Produced Using Various Poly(ethylene imine)/Laponite Mass Ratios at a Fixed Laponite Concentration of 0.50 wt %, As Judged by Laser Diffraction Studiesa
a
Note that, in the case of the chloroform emulsion droplets, the mean diameter is estimated from optical microscope images. Scale bar represents 200 μm and is applicable for all images.
the preparation of Laponite-coated polystyrene latex via miniemulsion polymerization of Laponite-stabilized styrene droplets prepared in the presence of electrolyte. These so-called “clayarmored” latexes were subsequently used for the two-step preparation of 5−15 μm silica microcapsules.11
There are many literature reports of various small molecule additives being used to tailor the properties of inorganic particles such as montmorillonite,12 silica,13,14 or alumina15 in order to produce effective Pickering emulsifiers. For example, Wang et al. have recently shown that the adsorption of cationic 1143
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(0.020 g) was added to the aqueous continuous phase and shaken to ensure its dissolution. This emulsion was then allowed to stand unstirred at 20 °C for 12 h to allow colloidosome formation to occur. Alcohol Challenge. An aliquot of a colloidosome sample (0.50 mL of a 50 vol % suspension; mean droplet diameter = 60 μm; using PEG-DGE) was transferred to a 5.0 mL vial, to which excess 2propanol (4.0 mL) was added and the vial was shaken vigorously. The same alcohol challenge was used to assess the corresponding PPGDGE cross-linked colloidosomes and Pickering emulsions prepared in the absence of any polymeric cross-linker. Fluorescent Labeling of Poly(ethylene imine). Poly(ethylene imine) (0.50 g) was dissolved in a sodium carbonate buffer (80 mL, pH 9). Rhodamine B isothiocyanate (8.0 mg, Aldrich) was first dissolved in DMSO (1.0 mL) and then added to the PEI/buffer solution. The copolymer solution was stirred for 24 h at 4 °C and then dialyzed against water prior to freeze-drying overnight. Encapsulation and Release of Malachite Green from Colloidosomes. Malachite green (0.020 g) was weighed into a glass sample vial and dissolved in sunflower oil (5.0 mL) before homogenization with 5.0 mL of an aqueous dispersion of as-prepared PEI-coated Laponite (0.50 wt % Laponite, 0.25 wt % PEI, pH 9) for 2 min. PEG-DGE or PPG-DGE cross-linker (0.020 g) was added to either the aqueous phase or the oil phase, respectively, as described above. Release studies were conducted using a PC-controlled PerkinElmer Lambda 25 UV−visible absorption spectrophotometer operating in time drive mode. A known volume of filtered colloidosomes (0.10 mL) was placed on top of an aqueous solution (3.0 mL, pH 4) in a disposable UV grade cuvette equipped with a miniature magnetic stirrer bar. The absorbance at 426 nm due to the released dye was monitored every 100 s as a function of time for up to 24 h at 20 °C. Since the oil droplets are less dense than water, the colloidosomes remain buoyant and hence above the cell volume sampled by the transmitted beam. As a control experiment, sunflower oil containing the same concentration of dissolved dye as that used to prepare the dye-loaded colloidosomes was carefully placed above the aqueous continuous phase to examine the rate of dye release in the absence of any physical barrier. Particle Characterization. Dynamic Light Scattering. The intensity-average hydrodynamic diameter was obtained by DLS using a Malvern Zetasizer NanoZS instrument. Aqueous solutions of 0.01 w/ v % clay dispersions were analyzed using disposable plastic cuvettes, and results were averaged over three consecutive runs. Deionized water used to dilute each dispersion was ultrafiltered through a 0.20 μm membrane to remove dust. BET Surface Area Analysis. The specific surface area of LaponiteRD was determined with a Quantachrome Nova 1000e instrument using dinitrogen as an adsorbate at 77 K. A freeze-dried sample was degassed under vacuum at 100 °C for at least 15 h prior to analysis. The surface area per molecule for N2 was taken to be 16.2 Å2. Aqueous Electrophoresis. Zeta potentials were determined for both pristine Laponite and also PEI-coated Laponite particles prepared at various PEI/Laponite mass ratios using a Malvern Zetasizer Nano ZS instrument. The solution pH was fixed at pH 9.5 in the presence of 1 mM KCl, using either dilute NaOH or HCl for pH adjustment as required. Thermogravimetric Analysis. Analyses were conducted using a Perkin-Elmer Pyris-1 TGA instrument. Prior to analysis, the PEIcoated Laponite particles were purified by centrifugation at 20,000 rpm for 45 min, carefully replacing the supernatant with mildly alkaline water (pH 9) each time followed by redispersion of the sedimented particles with the aid of an ultrasonic bath. This centrifugationredispersion cycle was repeated four times to ensure that no excess nonadsorbed PEI remained in the aqueous continuous phase. The dried PEI-coated Laponite particles were heated up to 800 °C in air at a heating rate of 20 °C min−1. The amount of PEI adsorbed onto the Laponite was determined from the weight loss observed between 200 and 650 °C. In addition, selected colloidosomes were subjected to an alcohol challenge to remove the internal oil phase. In addition, Soxhlet extraction of the isolated colloidosomes with refluxing 2-propanol was conducted for 24 h to ensure removal of residual oil, followed by
oligomeric diamines onto Laponite leads to charge neutralization and hence allows the formation of stable oil-in-water Pickering emulsions.16 Similarly, Whitby et al. examined the adsorption of n-octadecylamine onto Laponite particles. In this case synergistic interactions occur at the oil−water interface since the n-octadecylamine is located within the oil phase.17 In 2010 some of us reported that covalently cross-linked colloidosomes could be prepared using poly(ethylene imine)grafted polystyrene latex particles prepared by aqueous emulsion polymerization.18 This model system also enabled the effect of the spatial location of the bisepoxy-based polymeric cross-linker to be explored. Perhaps surprisingly, laser diffraction studies indicated that intercolloidosome fusion was negligible, regardless of whether cross-linking occurred within the droplets or in the continuous phase. In the present work, branched poly(ethylene imine) is physically adsorbed onto Laponite to produce an effective Pickering emulsifier for various oils dispersed in water (see Figure 1 and Table 1). These Pickering emulsions are then converted into robust colloidosomes using either an oil-soluble bisepoxy-functionalized poly(propylene glycol) [PPG-DGE] or a water-soluble bisepoxy-functionalized poly(ethylene glycol) [PEG-DGE] cross-linker.18 Both polymeric reagents are cheap, commercially available, nonvolatile and have relatively low toxicities. Finally, dye release studies were conducted to assess the permeability of these new clay-based colloidosomes.
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EXPERIMENTAL SECTION
Materials. Poly(ethylene imine) (PEI) (branched, MW = 10 000 by GPC; MW = 25 000 by light scattering), sunflower oil, chloroform, toluene, n-dodecane, isopropyl myristate, poly(propylene glycol) diglycidyl ether (PPG-DGE, mean degree of polymerization = 9 according to the supplier), poly(ethylene glycol) diglycidyl ether (PEG-DGE; mean degree of polymerization = 9 according to the supplier), rhodamine B isothiocyanate, and dimethyl sulfoxide were all purchased from Aldrich and were used as received. 2-Propanol was purchased from Fisher and used as received. Laponite RD was obtained from Rockwood Additives Ltd., U.K. Deionized water was used in all experiments. Adsorption of Poly(ethylene imine) onto Laponite. Poly(ethylene imine) (0−1.0 g) was dissolved in water (50 mL). This polymer solution was then added to an aqueous dispersion of Laponite (0.50 g) dispersed in water (50 mL), and the resulting mixture was stirred for 12 h at 20 °C. Preparation of Pickering Emulsions. The following protocol is representative. Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of an aqueous suspension of as-prepared PEI-coated Laponite (5.0 mL, 0.50 wt % Laponite, 0−1.0 wt % PEI, pH 9). Emulsification was achieved at 20 °C using an IKA UltraTurrax T-18 homogenizer equipped with a 10 mm dispersing tool for 2 min at 12 000 rpm. Colloidosome Preparation using the Poly(propylene glycol) Diglycidyl Ether (PPG-DGE) Cross-Linker. PPG-DGE cross-linker (0.020 g) dissolved in sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of as-prepared PEI-coated Laponite (5.0 mL, 0.50 wt % Laponite, 0.25 wt % PEI, pH 9). Emulsification was achieved at 20 °C using an IKA Ultra-Turrax T-18 homogenizer equipped with a 10 mm dispersing tool for 2 min at 12 000 rpm. This emulsion was then allowed to stand unstirred at 20 °C for 12 h to allow colloidosome formation to occur. Colloidosome Preparation Using the Poly(ethylene glycol) Diglycidyl Ether (PEG-DGE) Cross-Linker. Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of 5.0 mL of an aqueous dispersion of as-prepared PEI-coated Laponite (0.50 wt % Laponite, 0.25 wt % PEI, pH 9). Emulsification was achieved at 20 °C using an IKA Ultra-Turrax T-18 homogenizer equipped with a 10 mm dispersing tool for 2 min at 12 000 rpm. PEG-DGE cross-linker 1144
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drying at 20 °C for 16 h prior to thermogravimetric analysis using the protocol described above. FT-IR Spectroscopy. Each sample (1.0 mg) was ground with KBr (150 mg) to afford a fine powder and compressed into a pellet by applying a pelletization pressure of 8 tonnes for 10 minutes. The FTIR spectra were recorded using a Nicolet Magna (series II) spectrometer at 4.0 cm−1 resolution, and 64 scans were recorded per spectrum. Emulsion Characterization. Conductivity Measurements. The conductivities of the emulsions immediately after preparation were measured using a digital conductivity meter (Hanna model Primo 5). A high conductivity (typically >10 μS cm−1) indicates that emulsions are water-continuous. All results were confirmed using the so-called “drop test”, whereby one drop of the emulsion was added to both pure water and oil, and its ease of dispersion was assessed by visual inspection. Relatively rapid dispersion into water was taken as confirmation that the continuous phase of the emulsion was indeed water. Optical Microscopy. A single droplet (ca. 100 μL) of a Pickering emulsion or colloidosome suspension was placed on a microscope slide and optical microscopy images were recorded using a Motic DMBA300 digital biological microscope with a built-in camera and Motic Images Plus 2.0 ML software. Fluorescence Microscopy. A single droplet (ca. 100 μL) of a Pickering emulsion or colloidosome suspension was placed on a microscope slide and viewed using an Olympus Upright Epifluorescence system with Hamamatsu ORCA-ER monochrome camera and Volocity software. This technique was used to view the fluorescently labeled poly(ethylene imine) chains within the Laponite-based colloidosomes and also to confirm whether such colloidosomes survived an alcohol challenge (using excess 2-propanol). Laser Diffraction Particle Size Analysis of Emulsion Droplets. A Malvern Mastersizer 2000 laser diffraction instrument equipped with a small volume (ca. 50 mL) Hydro 2000SM sample dispersion unit, a HeNe laser operating at 633 nm, and a solid-state blue laser operating at 466 nm was used to size both the Pickering emulsions and colloidosomes. The stirring rate was adjusted to 1000 rpm. Corrections were made for background electrical noise and laser scattering due to contaminants on the optics and within the sample. Samples were analyzed five times, and the data were averaged. A typical acquisition time was 2 min per sample after alignment and background measurements. The raw data were analyzed using Malvern software. The mean droplet diameter was taken to be the mean volume-average diameter (D4/3), which is mathematically expressed as D4/3 = Σ Di4 Ni/Σ Di3 Ni. The standard deviation for each diameter provides an indication of the width of the size distribution. After each measurement, the cell was rinsed three times with 2-propanol, followed by three times with deionized water. The glass walls of the cell were carefully wiped with a lens cleaning tissue to avoid crosscontamination, and the laser was aligned centrally on the detector.
Figure 2. Effect of varying the poly(ethylene imine)/Laponite mass ratio on the adsorbed amount of poly(ethylene imine) and the zeta potential (at pH 9.5) of the resulting poly(ethylene imine)-coated Laponite particles. The Laponite concentration was fixed at 0.50 wt % in each case.
mass ratio of around 0.40 and charge reversal being observed thereafter (see Figure 2). The actual adsorbed amount of PEI, as determined by thermogravimetry, mirrors this electrophoretic behavior and suggests Langmuir-type adsorption, as expected for a cationic polyelectrolyte adsorbed onto an oppositely charged colloidal substrate.19 Figure 3 illustrates how the mean droplet diameter of the sunflower oil-in-water emulsion (obtained after homogenization at 12 000 rpm for two minutes at 20 °C) varies according
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RESULTS AND DISCUSSION Initially, the effect of varying the extent of PEI adsorption onto Laponite on its Pickering emulsifier performance was investigated. FT-IR spectroscopy was used to verify that the PEI had in fact adsorbed onto Laponite (see Figure S1, Supporting Information). The FT-IR spectrum of PEI alone has two characteristic bands that are not found in the FT-IR spectrum obtained for pure Laponite. The 2900 cm−1 band is characteristic of the C−H stretch and the band at 3300 cm−1 is assigned to the N−H stretch. These two features are clearly present in the spectrum recorded for PEI-coated Laponite, suggesting that adsorption had been successful. The amount of PEI added to a 0.50 wt % aqueous dispersion of Laponite was systematically varied from zero up to 1.0 wt % (i.e., a PEI/ Laponite mass ratio of 2.0), see Figure 2. The original negative zeta potential of the uncoated Laponite particles was gradually reduced, with an isoelectric point obtained at a PEI/Laponite
Figure 3. (a) Mean droplet diameter of sunflower-oil-in-water Pickering emulsions as a function of poly(ethylene imine)/Laponite mass ratio (Laponite concentration = 0.50 wt %), as judged by laser diffraction studies. (b) Droplet size distributions obtained for sunflower oil-in-water emulsions prepared at 20 °C as a function of poly(ethylene imine)/Laponite mass ratio. 1145
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emulsifier performance to be tuned for a given oil. Presumably, this parameter allows the contact angle to be adjusted, in addition to the effective surface charge. PEI/Laponite mass ratios of 0.10, 0.50, and 2.0 were selected for the adsorption of PEI onto Laponite that corresponded to submonolayer, close to monolayer and well above monolayer coverage, respectively. As indicated earlier in Figure 3, the mean diameter of the sunflower oil droplets is systematically reduced as the PEI/ Laponite mass ratio is increased up to 0.50. Perhaps surprisingly, this trend is not observed for the other four oils. Nevertheless, it is emphasized that stable Pickering emulsions can be formed in each case given an appropriate PEI/Laponite mass ratio, which suggests that this combined emulsifier approach offers a convenient generic, albeit empirical, approach to emulsification. Like sunflower oil, close to monolayer coverage of Laponite particles by the PEI is required to stabilize either toluene or chloroform emulsion droplets. Unlike sunflower oil, however, demulsification occurs below this critical PEI/Laponite mass ratio. For both n-dodecane and isopropyl myristate, the optimum PEI/Laponite mass ratio appears to be approximately 0.10, since this value produces the smallest, most stable emulsion droplets. Larger droplets are obtained both above and below this mass ratio, which leads to relatively rapid creaming of the droplet phase on standing. As discussed above, all cross-linking studies were conducted at a fixed PEI/Laponite mass ratio of 0.50 and a constant Laponite concentration of 0.50 wt % using sunflower oil as the droplet phase. The two cross-linking strategies outlined in Figure 1 involve the same epoxy-amine chemistry18 and differ only in the spatial location of the polymeric cross-linker. Using the PPG-DGE cross-linker ensures that cross-linking is confined within the oil droplets, whereas the oil-insoluble PEG-DGE cross-linker is solely located in the aqueous continuous phase. The potential advantage of the former route is the minimization of intercolloidosome fusion when cross-linking is performed at high solids. On the other hand, using a water-soluble cross-linker may be beneficial if the crosslinking chemistry is incompatible with the colloidosome payload, which may be the case for certain fragrances or drugs. According to the laser diffraction data and optical microscopy images presented in Figure 4, there is little evidence for significant intercolloidosome fusion when using either polymeric cross-linker, since the mean droplet diameter obtained in each case is comparable to that observed for the corresponding Pickering emulsion alone. Similar results were recently reported by Walsh et al. for colloidosomes prepared using PEI-stabilized polystyrene latex with the same two polymeric cross-linkers.18 To verify the presence of the PEI/Laponite hybrid particles at the surface of the oil droplets, the PEI was partially functionalized using rhodamine B isothiocyanate (Aldrich). A typical image obtained from fluorescence microscopy studies is shown in Figure 5a. Clearly, the rhodamine label is exclusively associated with the oil droplets, which confirms their expected Pickering emulsion character. Moreover, fluorescence microscopy studies confirm that these colloidosomes remain intact (albeit partially collapsed) after being subjected to excess alcohol, which completely removes both the oil phase and the water phase (see Figure 5, panels c and d). In contrast, only illdefined debris is observed after an alcohol challenge of the corresponding Pickering emulsions (see Figure 5b). After creaming of the emulsion droplets on standing, visual inspection confirms that the lower aqueous phase is trans-
to the PEI/Laponite mass ratio at this optimized concentration of 0.50 wt % Laponite. A minimum volume-average diameter of around 60 μm was achieved at an approximate PEI/Laponite mass ratio of 0.50. At this mass ratio, the adsorbed amount of PEI on Laponite is approximately 0.81 mg m−2, whereas full monolayer coverage is estimated to be 1.05 mg m−2, as judged by thermogravimetry. The Pickering emulsions prepared under such conditions proved to be of the oil-in-water type in all cases, as judged by conductivity measurements and drop test studies. Emulsions prepared at PEI/Laponite mass ratios of less than 0.50 exhibited relatively poor stability toward coalescence, with the gradual development of an upper oil phase after standing for 24 h at 20 °C. In contrast, an emulsion prepared using a PEI/Laponite mass ratio of 0.50 proved to be very stable toward droplet coalescence over time scales of months (although creaming was observed within days due to the relatively large droplet size). Following optimization of the Laponite/PEI mass ratio, the effect of varying the Laponite concentration (at a fixed PEI/Laponite mass ratio of 0.50) on the mean oil droplet diameter was investigated. Again, a limiting volume-average diameter of around 60 μm was achieved at a Laponite concentration of 0.50% by mass (see Figure S2, Supporting Information). Thus all cross-linking studies were conducted at a PEI/Laponite mass ratio of 0.50 and a fixed Laponite concentration of 0.50 wt %. Since these conditions correspond to submonolayer coverage of the Laponite particles by the PEI, this should ensure that there is little or no PEI remaining in the aqueous continuous phase. Experimental evidence is presented below which suggests that this is indeed the case. As mentioned earlier, Binks and Lumsdon found that Laponite alone was too hydrophilic to act as a Pickering emulsifier.8 This finding was confirmed in the present study, since rapid demulsification always occurred when using Laponite as the sole Pickering emulsifier in the absence of added electrolyte. In contrast, using 0.25 wt % PEI alone leads to stable sunflower-oil-in-water emulsions of approximately 91 ± 59 μm diameter as judged by laser diffraction studies. However, if such emulsions are prepared using rhodaminelabeled PEI (see Experimental for synthesis details) and the oil droplets are allowed to cream on standing, then the lower aqueous phase is strongly fluorescent, which indicates the presence of excess polymeric stabilizer. Importantly, if Pickering emulsions are prepared at an optimized PEI/Laponite mass ratio of 0.50, significantly smaller oil droplets of 59 ± 28 μm are obtained. Moreover, there is no evidence for excess PEI or Laponite being present within the aqueous phase after creaming of the emulsion droplets. Thus, although PEI alone can act as a Pickering emulsifier, much more efficient adsorption at the droplet interface is achieved when this polymer is utilized in combination with Laponite (see Figure S3, Supporting Information). In summary, using a combination of PEI and Laponite to prepare Pickering emulsions appears to offer a genuine synergistic advantage. Table 1 shows the effect of varying the oil type on the longterm stability and mean oil droplet diameter of Pickering emulsions. Toluene, chloroform, n-dodecane and isopropyl myristate were evaluated as model oils for comparison with sunflower oil. Only sunflower oil emulsion droplets are stabilized by PEI alone: rapid phase separation always occurs for the other four oils. However, the judicious combination of PEI and Laponite enables stable emulsions to be obtained for all five oils, with the PEI/Laponite mass ratio allowing 1146
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PEI/Laponite mass ratio of 0.50 used to prepare these colloidosomes and further suggests that both components are efficiently adsorbed at the oil/water interface. According to BET surface area analysis, Laponite has a specific surface area of 353 m2 g−1. Given a Laponite concentration of 0.50 wt % and an aqueous phase volume of 5.0 mL), the total available surface area due to Laponite is calculated to be approximately 8.8 m2. This value is more than an order of magnitude higher than the total surface area of the emulsion droplets, which is estimated to be 0.50 m2 for a typical 50 vol % emulsion comprising oil droplets of 60 μm diameter. This strongly suggests that the Laponite is adsorbed as multilayers on the oil droplets, rather than as a single monolayer. If it is assumed that all the Laponite disks adsorb flat onto the oil droplets and there is no excess Laponite present in the aqueous phase, we estimate that the oil droplets are coated with approximately nine Laponite monolayers. This suggests an overall shell thickness of around 10−30 nm (depending on the thickness of the adsorbed PEI layer on each Laponite particle). Clearly, this calculation represents a minimum shell thickness: if the Laponite is adsorbed as a highly disordered layer, the shell thickness could be much greater. Indeed, there is good evidence to believe that the latter scenario is correct, since DLS studies of PEI/Laponite mixtures in aqueous solution suggest that addition of relatively low levels of PEI causes significant flocculation of the Laponite disks (see Figure S4, Supporting Information). In principle, this hybrid PEI/Laponite shell could offer enhanced retention of encapsulated molecular species compared to, say, latex-based colloidosomes, since it may provide a tortuous pathway for dye diffusion and also should not contain the pentagonal packing defects that are an intrinsic feature of packing small spheres on large spheres.6 In order to test this hypothesis, the permeability of these clay-based colloidosomes was evaluated by performing dye release studies. The model dye was chosen to be Malachite Green (pKa ∼ 6.9), which is sufficiently oil-soluble in its unprotonated form to ensure its efficient encapsulation at high pH. On lowering the pH of the external aqueous phase this dye becomes protonated and watersoluble, which leads to its diffusion from the oil droplets. Thus oil-in-water colloidosomes were prepared at pH 9 with encapsulated Malachite Green within the oil cores, and dye release was subsequently triggered at 20 °C by adjusting the pH of the aqueous continuous phase to pH 4. Detection of the released protonated dye at 426 nm was readily achieved using visible absorption spectroscopy, using essentially the same experimental protocol described by Thompson and coworkers.3 Unfortunately, the colloidosome walls proved to be highly permeable, with essentially all of the dye being released within 6−7 h at 20 °C (see Figure 6). Similar rates of dye release were observed for both types of cross-linkers, which were only marginally slower than that found for the equivalent Pickering emulsion. Moreover, these release profiles were actually more rapid than that observed in a control experiment, whereby a macroscopic layer of dye-loaded sunflower oil was carefully placed on top of the aqueous phase (i.e., when there were no colloidosome walls present). Presumably, this apparent anomaly simply reflects the much higher interfacial surface area of the colloidosomes (5000 cm2) compared to that of the macroscopic oil phase (1 cm2). Overall, these observations are consistent with a relatively disordered (and hence porous) hybrid PEI/Laponite colloidosome wall, rather than a highly ordered “bricks and mortar” structure that would be more likely
Figure 4. (A) Optical microscopy images of sunflower oil-in-water emulsions prepared at 20 °C in the absence and presence of a bisepoxy polymeric cross-linker (either water-soluble PEG-DGE or oil-soluble PPG-DGE, see Figure 1). (B) Mastersizer droplet size distributions. Conditions: Laponite concentration = 0.50 wt % and poly(ethylene imine)/Laponite mass ratio = 0.50.
Figure 5. Fluorescence microscopy images of (A) noncross-linked Laponite-based colloidosomes prepared using rhodamine B-labeled PEI; (B) noncross-linked Laponite-based colloidosomes prepared using rhodamine B-labeled PEI following an alcohol challenge using excess 2-propanol; (C) PEG-DGE cross-linked Laponite-based colloidosomes prepared using rhodamine B-labeled PEI following an alcohol challenge using excess 2-propanol; (D) PPG-DGE cross-linked Laponite-based colloidosomes prepared using rhodamine B-labeled PEI following an alcohol challenge using excess 2-propanol.
parent, which indicates that essentially all the Laponite particles are adsorbed onto the oil droplets (along with the PEI chains). Furthermore, thermogravimetric analysis of dried colloidosomes (isolated after washing with excess alcohol, followed by Soxhlet extraction to remove all traces of the oil phase) indicated incombustible residues of approximately 63% at 600 °C. Assuming that the Laponite is thermally stable and the PEI chains are completely pyrolyzed, this is consistent with the 1147
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of PEI. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
ACKNOWLEDGMENTS We thank P & G Technical Center (Newcastle-upon-Tyne, U.K.) for an Industrial EPSRC CASE award to support M.W.
Figure 6. Dye release curves obtained at pH 4 for a Malachite Green dye diffusing from a 60 μm diameter sunflower-oil-in-water Pickering emulsion prepared using hybrid PEI/Laponite particles (□; PEI/ Laponite mass ratio = 0.50; Laponite concentration = 0.50% by mass), and the two corresponding colloidosomes prepared using either PEGDGE (Δ) or PPG-DGE (▲) cross-linkers. A control experiment using pure sunflower oil (○) is also shown. In this case relatively slow dye release was observed, presumably due to the much lower surface area (1 cm2 vs 5000 cm2).
to offer a tortuous pathway. However, it is perhaps worth mentioning that lowering the pH of the aqueous phase may well disrupt the colloidosome wall structure, since the PEI chains will presumably also become highly protonated under these conditions.
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CONCLUSIONS Poly(ethylene imine) (PEI) has been utilized to modify the surface properties of Laponite clay nanoparticles in order to produce an effective hybrid Pickering emulsifier. Systematic variation of the PEI/Laponite mass ratio enables emulsifier performance to be fine-tuned and allows stable emulsions to be produced for a range of five model oils. The synergistic advantage of using PEI in combination with Laponite is clearly demonstrated, since the former is an inefficient stabilizer when used alone while the latter is a completely ineffective sole stabilizer (unless used in the presence of added salt). In the case of sunflower oil, these Pickering emulsions can be readily cross-linked to produce novel clay-based colloidosomes that are sufficiently robust to withstand an alcohol challenge. Such covalent stabilization is based on epoxy-amine chemistry between the physically adsorbed PEI chains and either an oilsoluble or a water-soluble bisepoxy-based polymeric crosslinker. However, studies of the rate of dye release from such colloidosomes indicate that their hybrid PEI/Laponite walls are highly permeable and unfortunately seem to offer no effective means of retaining small molecules within the oil droplets over long time scales.
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ASSOCIATED CONTENT
S Supporting Information *
FT-IR spectra for PEI-coated Laponite, variation of mean droplet diameter as a function of Laponite concentration for sunflower oil at a fixed poly(ethylene imine)/Laponite mass ratio, digital optical micrographs of selected Pickering emulsions and colloidosomes, dynamic light scattering studies of aqueous dispersions of Laponite in the presence and absence 1148
dx.doi.org/10.1021/la2046405 | Langmuir 2012, 28, 1142−1148
