- Email: [email protected]
organic hybrid microcapsules: Melamine formaldehyde-coated Laponite-based Pickering emulsions
Journal of Colloid and Interface Science 460 (2015) 71–80
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Inorganic/organic hybrid microcapsules: Melamine formaldehyde-coated Laponite-based Pickering emulsions Mark Williams a,⇑, Birte Olland a, Steven P. Armes a,⇑, Pierre Verstraete b, Johan Smets b a b
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, UK Procter & Gamble, Eurocor NV/SA, Temselaan 100, 1853 Strombeek-Bever, Belgium
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 July 2015 Revised 11 August 2015 Accepted 22 August 2015 Available online 22 August 2015 Keywords: Microcapsules Laponite Magnafloc Melamine formaldehyde Pickering emulsion Hybrid
a b s t r a c t A facile synthesis route to novel inorganic/organic hybrid microcapsules is reported. Laponite nanoparticles are surface-modified via electrostatic adsorption of Magnafloc, an amine-based polyelectrolyte allowing the formation of stable oil-in-water Pickering emulsions. Hybrid microcapsules can be subsequently prepared by coating these Pickering emulsion precursors with dense melamine formaldehyde (MF) shells. Employing a water-soluble polymeric stabiliser, poly(acrylamide-co-sodium acrylate) leads to stable hybrid microcapsules that survive an alcohol challenge and the ultrahigh vacuum conditions required for SEM studies. Unfortunately, the presence of this copolymer also leads to secondary nucleation of excess MF latex particles in the aqueous continuous phase. However, since the Magnafloc is utilised at submonolayer coverage when coating the Laponite particles, the nascent cationic MF nanoparticles can deposit onto anionic surface sites on the Laponite, which removes the requirement for the poly(acrylamide-co-sodium acrylate) component. Following this electrostatic adsorption, the secondary amine groups on the Magnafloc chains can react with the MF, leading to highly robust cross-linked MF shells. The absence of the copolymer leads to minimal secondary nucleation of MF latex particles, ensuring more efficient deposition at the surface of the emulsion droplets. However, the MF shells appear to become more brittle, as SEM studies reveal cracking on addition of ethanol. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding authors. E-mail address: [email protected] (S.P. Armes). http://dx.doi.org/10.1016/j.jcis.2015.08.044 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
An emulsion stabilised by solid particles is known as a Pickering emulsion [1,2]. Pickering emulsion templates can be further stabilised by thermal annealing [3,4], polyelectrolyte adsorption [3],
72
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
gel trapping [5] or covalent cross-linking [6–9] in order to stabilise the initial particle super-structure. This produces so-called ‘colloidosomes’ [3] which in principle offer potential applications in the context of microencapsulation. However, dye release studies of an encapsulated water-soluble dye within latex-based colloidosomes indicated only very limited retention: complete release into the aqueous continuous phase was observed within a few hours, which suggests rather high permeability for the latex shells [10]. Similarly, Thompson et al. also reported poor retention of actives within thermally-annealed latex-based colloidosomes [6,11]. This problem was attributed to the intrinsic geometric packing defects that inevitably arise when packing near-monodisperse small spheres on the surface of larger spheres [12]. Recently, we reported that branched poly(ethylene imine) can be physically adsorbed onto an inorganic nanoclay (Laponite) to produce an efficient hybrid Pickering emulsifier for the stabilisation of oil-in-water emulsions using various oils [13]. These Pickering emulsions could be converted into colloidosomes using either oil-soluble, or water-soluble bisepoxy-functionalised polymeric cross-linkers. These colloidosomes proved to be sufficiently robust to survive the removal of the internal oil phase after washing with excess ethanol. However, subsequent dye release studies suggested that such microcapsules are highly permeable and hence do not provide an effective barrier for retarding the release of small molecules [13]. In principle, the porosity of such microcapsules could be reduced by deposition of a dense secondary shell onto the initial layer of polymer/Laponite particles. There are several known routes to such double-shell (or even multiple-shell) microcapsules, which can yield either purely organic or inorganic microcapsules, or even inorganic/organic hybrid microcapsules. One common approach utilised to prepare double-shell hybrid microcapsules is to use inorganic particles such as titanium dioxide [14], zinc oxide [15], iron oxide [16] and silica [17] to produce an initial Pickering emulsion with a suitable vinyl monomer located within the oil phase. Subsequent polymerisation of this monomer results in a polymeric layer forming at the inner particle interface of the droplets, forming a double shell. Layer-by-layer deposition (L-b-L) [18] is another technique commonly employed to form multi-layered microcapsules. Microcapsules are constructed via sequential deposition of oppositelycharged polyelectrolytes onto colloidal particles such as melamine formaldehyde (MF), polystyrene or silica, which act as sacrificial templates [19]. These solid colloidal templates typically have narrow size distributions and hence allow precise control over the microcapsule dimensions. However, they must be removed before the desired payload can be introduced into the microcapsule cores. Core removal can affect capsule integrity and wall properties, while efficient loading can be challenging in many cases [20]. This technique has been developed further by Li et al. using Pickering emulsion templates for the L-b-L deposition of polyelectrolytes, thus allowing incorporation of oil-soluble pay-loads into the cores prior to polyelectrolyte self-assembly and removing the requirement for template removal [21]. An alternative approach is to coat an initial Pickering emulsion with a second overlayer. Thus a purely organic double-shell microcapsule was reported by Thompson et al., who deposited an ultrathin overlayer of polypyrrole onto latex-based colloidosomes in order to suppress dye release from oil cores [6]. Purely inorganic double shell microcapsules have been reported by Wang and co-workers, who showed that micron-sized food-grade calcium carbonate particles could be used to stabilise an o/w Pickering emulsion and then act as nucleation sites for the subsequent deposition of a crystalline calcium carbonate shell [22]. As well as purely organic and inorganic hybrid shells, the formation of inorganic/organic hybrid capsules using a Pickering
emulsion template has also been reported. For example, Strohm and Lobmann demonstrated that composite titania (TiO2)/polymer latex shells could be prepared by the deposition of TiO2 onto polystyrene latex-stabilised 1-octanol-in-water Pickering emulsions [23]. Li et al. have reported the electrostatic adsorption of an anionic polymeric ATRP initiator onto cationic silica nanoparticles to stabilise an oil-in-water emulsion. Addition of water-soluble cross-linking monomers to the aqueous continuous phase with subsequent polymerisation at the oil/water interface resulted in silica nanoparticle–polymer composite shells [24]. Melamine–formaldehyde (MF) has been widely utilised for microencapsulation applications [25–27] because of its good mechanical [28] and thermal stability [29,30]. MF microcapsules are typically prepared via polycondensation, with the MF prepolymer being initially soluble in the continuous aqueous phase, and the hydrophobic component being present in the form of dispersed droplets. As the polymerisation starts, the soluble oligomers start to adsorb at the surface of the droplets. Once on the surface, further polymerisation and crosslinking occurs, which results in the formation of a solid impermeable MF shell [31]. In 2009 Long et al. conducted in situ polymerisation to produce 15 lm MF microcapsules containing a consumer fragrance as the oil phase [32]. An MF precondensate was added to an aqueous solution of poly (acrylamide-co-sodium acrylate) prior to homogenisation of the oil phase, with generation of the MF shell occurring on heating. The same team subsequently prepared novel double-shell inorganic–inorganic microcapsules comprising a MF inner shell and an outer shell of CaCO3 nanoparticles. In particular, it was demonstrated that the rate of release from these double-shell microcapsules was lower than that observed for the respective single shells [33]. Herein we report the adsorption of a commercial cationic polyelectrolyte, Magnafloc, onto Laponite nanoparticles to produce an effective Pickering emulsifier for sunflower oil droplets at pH 4. We show that novel inorganic/organic hybrid microcapsules can be formed by the in situ polymerisation of melamine formaldehyde onto the surface of the Pickering emulsions. Moreover, such hybrid double-shell microcapsules can be generated either in the presence or absence of poly(acrylamide-co-sodium acrylate) (see Fig. 1). 2. Experimental section 2.1. Materials Sunflower oil, n-dodecane, benzyl benzoate and isopropyl myristate were all purchased from Aldrich and used as received. MF precondensate (supplied as an 70% w/w aqueous solution; a. k.a. ‘Beetle resin’) with a formaldehyde-to-melamine molar ratio of 0.20 was kindly supplied by British Industrial Plastics Ltd. (Birmingham, UK) and was used without further purification. Poly(acrylamide-co-sodium acrylate) (MW = 2 105 g mol 1) was supplied by Polysciences (Pennsylvania, USA) and used without further purification. Laponite RD was obtained from Rockwood Additives Ltd., UK. Magnafloc (50% w/w poly(dimethyla mine-co-epichlorohydrin-co-ethylenediamine)) was kindly supplied by Vesuvius (Chesterfield, UK). Ethanol was purchased from Fisher (UK) and used as received. Deionised water was used in all experiments and all solution pH adjustments using HCl/NaOH were monitored using a pH meter. 2.2. Adsorption of Magnafloc onto Laponite Magnafloc (0–2.0 g, 50% w/w) was dissolved in water (48–50 mL). This copolymer solution was then added to a 1.0% w/w aqueous dispersion of Laponite (50.0 g) and the resulting
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
73
Fig. 1. Reaction scheme for the formation of sunflower oil-loaded inorganic–organic double-shell hybrid microcapsules. Route A involves the deposition of a cross-linked melamine formaldehyde (MF) shell onto Magnafloc/Laponite stabilised sunflower oil-in-water Pickering emulsion droplets in the presence of poly(acrylamide-co-sodium acrylate) at pH 4 via a two-step protocol. Route B involves the deposition of a cross-linked melamine formaldehyde (MF) shell onto Magnafloc/Laponite stabilised sunflower oil-in-water Pickering emulsion droplets in the absence of poly(acrylamide-co-sodium acrylate) at pH 4 in a one-pot protocol. MF polymerisation was achieved by heating at 65 °C for 4 h in each case.
mixture was stirred for 12 h at 20 °C so as to allow electrostatic adsorption of the cationic Magnafloc chains onto the anionic Laponite particles at pH 10. 2.3. Preparation of Magnafloc/Laponite 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 dispersion of Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc). The solution pH was adjusted from pH 3 to pH 12 using 0.1 M HCl or NaOH as required. Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool.
2.4. Preparation of MF/Laponite hybrid microcapsules in the presence of poly(acrylamide-co-sodium acrylate) Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of an aqueous suspension of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4). Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool. This Pickering emulsion precursor was then added to an aqueous solution of MF precondensate (2.50 g) and poly(acrylamide-co-sodium acrylate) (0.58 g) in water (70 mL) that had been stirred for 105 min at pH 4 (adjusted using HCl) at room temperature. This suspension was heated to 65 °C and stirred for 4 h. The resulting suspension of
74
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
microcapsules was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. The same protocol was repeated with the Magnafloc/Laponite particles being replaced with PEI/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.25% w/w poly(ethylene imine), pH 9) as reported previously [13]. In this case emulsification was conducted at pH 9, followed by lowering the solution pH to pH 4. 2.5. Preparation of MF/Laponite hybrid microcapsules in the absence of poly(acrylamide-co-sodium acrylate) Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of an aqueous dispersion of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4). Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool. This emulsion was added to an aqueous solution of MF precondensate (2.50 g) in water (70 mL) at pH 4 (adjusted with HCl) and this solution was then heated to 65 °C and stirred for 4 h. The resulting microcapsule suspension was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation using aqueous 1 M NaOH. The same protocol was also conducted using PEI/Laponite particles in place of the Magnafloc/Laponite particles (5.0 ml, 0.50% w/w Laponite, 0.25% w/w poly(ethylene imine), pH 9) as reported previously [13]. In this case the emulsification was conducted at pH 9, followed by lowering the pH to pH 4. 2.6. Preparation of MF latex in the presence of poly(acrylamideco-sodium acrylate) An aqueous solution of MF precondensate (2.50 g) and poly (acrylamide-co-sodium acrylate) (0.58 g) in water (70 mL) was stirred for 105 min at pH 4 (adjusted using HCl) at 20 °C. This suspension was heated to 65 °C and stirred for 4 h. The resulting milky suspension was cooled to 20 °C, and the solution pH was raised to 10 to quench the MF polymerisation by adding 1 M NaOH. 2.7. Preparation of MF latex in the absence of poly(acrylamideco-sodium acrylate) An aqueous solution of MF precondensate (2.50 g) was diluted in water (70 mL), heated to 65 °C and stirred for 4 h at pH 4 (adjusted using HCl). The resulting milky suspension was cooled to 20 °C, and its pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.8. Preparation of MF latex in the presence of Laponite nanoparticles An aqueous solution of MF precondensate (2.50 g) was dispersed in water (70 mL) and added to a 0.5% w/w aqueous dispersion of Laponite (10.0 g) stirred for 105 min at pH 4 (adjusted using HCl) at room temperature. This suspension was heated to 65 °C and stirred for 4 h. The resulting milky suspension was cooled to 20 °C, and its pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.9. Preparation of MF latex in the presence of Magnafloc/Laponite particles An aqueous solution of MF precondensate (2.50 g) was dispersed in water (70 mL) followed by the addition of an aqueous dispersion of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc). The solution was stirred for 105 min at pH 4 (adjusted using HCl) at 20 °C. This suspension was heated to 65 °C and stirred for 4 h. The resulting
milky suspension was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.10. Ethanol challenge Pickering emulsions and MF/Laponite hybrid microcapsules (synthesised in the presence or absence of poly(acrylamideco-sodium acrylate)) were diluted in turn using ethanol (50 mL) to obtain a final concentration of 2.0% w/w oil in ethanol and the sealed vial was shaken vigorously to break the emulsion. 2.11. Dynamic light scattering The intensity-average hydrodynamic diameter was obtained by DLS using a Malvern Zetasizer Nano ZS instrument. Dilute aqueous dispersions (0.01% w/v) were analysed using disposable plastic cuvettes and the data were averaged over three consecutive runs. The deionised water used to dilute each dispersion was ultrafiltered through a 0.20 lm membrane prior to use to remove extraneous dust. 2.12. Aqueous electrophoresis Zeta potentials were determined at pH 10 for both pristine Laponite and Magnafloc/Laponite particles prepared at various Magnafloc/Laponite mass ratios using a Malvern Zetasizer Nano ZS instrument. Zeta potential vs. pH curves were also determined for pristine Laponite and Magnafloc, as well as Magnafloc/Laponite particles (5.0 mL, prepared using 0.50% w/w Laponite and 0.05% w/w Magnafloc). Zeta potential vs. pH curves were also determined for Laponite, Magnafloc and Magnafloc/Laponite particles, as well as MF latex prepared in the presence of poly(acrylamideco-sodium acrylate). The solution pH was varied between pH 3 and pH 12 in the presence of 1 mM KCl, using either dilute NaOH or HCl for pH adjustment as required. 2.13. Thermogravimetric analysis Analyses were conducted using a Perkin–Elmer Pyris-1 TGA instrument. Prior to analysis, the Magnafloc/Laponite particles were purified by centrifugation at 20,000 rpm for 45 min, carefully replacing the supernatant with mildly alkaline water (pH 10) each time followed by redispersion of the sedimented particles with the aid of an ultrasonic bath. This centrifugation–redispersion cycle was repeated four times to ensure that no excess non-adsorbed Magnafloc remained in the aqueous continuous phase. The dried Magnafloc/Laponite particles were heated up to 800 °C in air at a heating rate of 20 °C min 1. The amount of Magnafloc adsorbed onto the Laponite particles was calculated from the weight loss observed between 200 °C and 650 °C. 2.14. Conductivity measurements Conductivities of emulsions were recorded immediately after their preparation using a digital conductivity meter (Hanna model Primo 5). High conductivities (>10 lS cm 1) indicated an aqueous continuous phase, i.e. formation of an oil-in-water emulsion. These 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 in each liquid was assessed by visual inspection. Relatively rapid dispersion into water was taken as confirmation that the continuous phase of the emulsion was indeed water.
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
75
A single droplet (ca. 100 lL) of an o/w Pickering emulsion was placed on a microscope slide and digital images were recorded using a Motic DMBA300 digital biological microscope equipped with a built-in camera and Motic Images Plus 2.0 ML software. The same protocol was utilised to image the MF latex particles and the MF/Laponite hybrid microcapsules.
CaCO3/MF hybrid double-shell microcapsules [33]. Thus the MF precondensate is added to an aqueous solution containing a water-soluble polymeric stabiliser, poly(acrylamide-co-sodium acrylate), to produce a partially cross-linked precursor that is subsequently deposited onto the Magnafloc/Laponite-stabilised Pickering emulsion droplets. In contrast, Route B depicts a facile onepot route in which the MF precondensate is added to the Pickering emulsions without any initial cross-linking step.
2.16. Laser diffraction particle size analysis of emulsion droplets
3.1. Magnafloc adsorption on Laponite
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 solidstate blue laser operating at 466 nm was used to size the Pickering emulsions. 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 analysed five times, and the data were averaged. A typical acquisition time was 2 min per sample after alignment and background measurements. The raw data was analysed using Malvern software. The mean droplet diameter was taken to be the mean volumeaverage diameter (D4/3), which is mathematically expressed as D4/3 = RD4i Ni/RD3i 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 ethanol, followed by three rinses with deionised water. The glass walls of the cell were carefully wiped with lens cleaning tissue to avoid crosscontamination, and the laser was aligned centrally on the detector.
Prior to MF deposition, the electrostatic adsorption of Magnafloc onto Laponite required systematic investigation to establish the optimum Magnafloc/Laponite mass ratio for the optimum Pickering emulsifier performance. Accordingly, the concentration of Magnafloc added to a 0.50% w/w aqueous dispersion of Laponite particles at pH 10 was varied from zero up to 1.0% w/w (i.e. up to a Magnafloc/Laponite mass ratio of 2.0). The aqueous dispersion of Laponite used in this study had a mean hydrodynamic diameter of 65 nm (see Fig. S1) and a specific surface area of 353 m2 g 1, as determined by BET analysis. As shown in Fig. 2, the initial negative zeta potential of the pristine Laponite particles is gradually reduced with increasing adsorption of the cationic Magnafloc; an isoelectric point is observed at a Magnafloc/Laponite mass ratio of around 0.20 and surface charge reversal occurs thereafter. The actual adsorbed amount of Magnafloc determined by thermogravimetry is consistent with the observed electrophoretic behaviour and suggests Langmuir-type adsorption.
2.15. Optical microscopy
2.17. Scanning electron microscopy SEM images were obtained using a FEI inspect F FEG instrument operating at 20 kV. All samples were sputter-coated with a thin overlayer of gold prior to inspection to prevent sample charging problems. 2.18. BET surface area analysis The specific surface area of Laponite-RD 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 and a linear fivepoint adsorption isotherm was constructed.
3.2. Magnafloc/Laponite stabilised Pickering emulsions As in the case of the PEI/Laponite particles [13], physical adsorption of Magnafloc onto Laponite particles enabled stable Pickering emulsions to be obtained for a range of oils, with systematic variation of the Magnafloc/Laponite mass ratio allowing emulsifier performance to be tuned for a given oil (see Fig. S3). In the case of sunflower oil-in-water emulsions, Magnafloc/Laponite particles prepared using a relatively low level of Magnafloc (0.50% w/w Laponite, 0.05% w/w Magnafloc; i.e. well below the plateau region of the adsorption isotherm, see Fig. 2), conferred optimal Pickering emulsifier performance. Moreover, an acidic reaction solution (e.g. pH 4) is required for the MF synthesis, so the precursor Pickering emulsion must be stable at this pH in order to prepare the inorganic/organic hybrid double-shell microcapsules. Magnafloc/Laponite particles prepared using a Magnafloc/Laponite mass ratio of 0.10 can be used to stabilise Pick-
3. Results and discussion Fig. 1 shows the two synthetic routes used to produce inorganic–organic hybrid microcapsules via deposition of an MF overlayer onto Laponite-stabilised Pickering emulsions. Magnafloc is a commercial water-soluble copolymer comprising secondary amine repeat units: it is adsorbed onto Laponite particles to facilitate the stabilisation of sunflower oil-in-water Pickering emulsions. Recently, we reported that a similar cationic polyelectrolyte, poly (ethylene imine) or PEI, adsorbs onto Laponite and modifies its surface wettability, producing an effective PEI/Laponite Pickering emulsifier. Systematic variation of the PEI/Laponite mass ratio allowed fine-tuning of the Pickering emulsifier performance, enabling stable oil-in-water emulsions to be produced for a range of model oils [13]. However, attempts to coat such PEI/Laponite Pickering emulsions with MF using either of the two methods summarised in Fig. 1 invariably led to aggregation of the emulsion droplets, and a non-contiguous MF coating (see Fig. S2). Route A in Fig. 1 is based on a previously reported protocol used to prepare
Fig. 2. Effect of varying the target Magnafloc/Laponite mass ratio on: (a) the zeta potential ( ) recorded at pH 10 for the resulting Magnafloc/Laponite particles and (b) the adsorbed mass of Magnafloc per unit surface area of Laponite ( ). In each case the Laponite concentration was fixed at 0.50% w/w.
76
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
Fig. 3. Zeta potential vs. pH curves recorded for aqueous dispersions of melamine formaldehyde (MF) particles prepared: (i) in the absence of poly(acrylamide-cosodium acrylate) (d); (ii) in the presence of poly(acrylamide-co-sodium acrylate) ( ); (iii) in the presence of Laponite alone ( ); (iv) in the presence of Magnafloc/ Laponite particles (Magnafloc/Laponite mass ratio = 0.10) ( ).
ering emulsions with a volume-average droplet diameter of 54 ± 24 lm that exhibit good long-term stability at pH 10. On adjusting the pH from pH 10 to pH 4, these emulsion droplets remained essentially unchanged in size and their long-term stability was not compromised (see Fig. S4). Thus MF deposition onto this colloidal template to produce inorganic–organic hybrid microcapsules could now be investigated. 3.3. Melamine formaldehyde interaction with Laponite Route A involves stabilisation of the MF precondensate using poly(acrylamide-co-sodium acrylate) prior to polymerisation, see
Fig. 1. This interaction is likely to be merely electrostatic in nature, although we cannot rule out the possibility of amide bond formation between the anionic carboxylate groups on the copolymer and the amine groups of the precondensate. In control experiments, MF latex particles were synthesised both in the presence and absence of poly(acrylamide-co-sodium acrylate) in order to examine the influence of this stabiliser. Fig. 3 shows the corresponding aqueous electrophoretic data in the form of two zeta potential vs. pH curves. MF particles prepared in the absence of poly(acrylamide-cosodium acrylate) are cationic at pH 4, and exhibit an isoelectric point at around pH 8.5. In contrast, MF particles prepared in the presence of poly(acrylamide-co-sodium acrylate) possess negative zeta potentials at pH 4, indicating electrostatic adsorption of this anionic stabiliser. Furthermore, dynamic light scattering studies of the latter MF particles indicate a hydrodynamic diameter of around 400 nm, whereas the former MF particles were much larger and more polydisperse (10 ± 5 lm diameter, as judged by laser diffraction). This significant difference in particle size is confirmed by optical microscopy studies, see Fig. 4A and B. Such observations suggest that the copolymer confers steric stabilisation. In contrast, Route B in Fig. 1 does not involve this poly(acrylamide-co-sodium acrylate) stabiliser. The surface of the Magnafloc/Laponite particles prepared using a relatively low adsorbed amount of Magnafloc has both anionic character and secondary amine functionality. The former feature allows electrostatic adsorption of the nascent cationic MF nuclei, while the latter most likely enables in situ chemical grafting to occur. To test this hypothesis, an MF polymerisation was conducted in the presence of bare Laponite particles and the zeta potential data obtained for the resulting MF particles are shown in Fig. 3. The addition of Laponite resulted in weakly cationic particles at pH 4, with an isoelectric point being observed at pH 5.5. Moreover, at pH 4 the electrophoretic mobility is characterised
Fig. 4. Optical microscopy images recorded for melamine formaldehyde (MF) particles prepared in the presence of: (A) no additive; (B) poly(acrylamide-co-sodium acrylate) stabiliser; (C) Laponite; and (D) Magnafloc/Laponite particles (Magnafloc/Laponite mass ratio = 0.10).
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
by a single signal (see Fig. S5), which lies between that of bare Laponite and MF latex prepared in the absence of Laponite. This intermediate mobility indicates that an electrostatic interaction between the cationic MF latex and the anionic Laponite particles leading to the formation of inorganic–organic hybrid particles. Furthermore, the optical microscopy image shown in Fig. 4C indicates ill-defined non-spherical aggregates of MF particles prepared in the presence of Laponite. Since this particle morphology differs significantly from the spherical MF latexes shown in Fig. 4A and B, this supports the hypothesis of an electrostatic interaction between the cationic MF and the anionic Laponite. When MF was synthesised in the presence of Magnafloc/Laponite particles (see Figs. 3 and 4D), the zeta potential of the resulting hybrid particles was 16.2 mV at pH 10. This value is close to that obtained for the Magnafloc/Laponite particles (see Fig. 2), indicating their adsorption onto the surface of the MF particles. On lowering the solution pH, an isoelectric point is observed at pH 6.5, which is similar to the electrophoretic behaviour of the Magnafloc/Laponite particles alone (see Fig. S6). The optical microscopy image shown in Fig. 4D suggests that the particle size of the MF/Magnafloc/Laponite aggregates is comparable to those formed in the presence of bare Laponite alone. This is confirmed by laser diffraction studies which indicate volume-average diameters of 35 ± 13 lm and 30 ± 11 lm, respectively. SEM images for the four types of particles shown in Fig. 4 are provided in the supporting information, see Fig. S7. Previously, we reported that emulsion droplets stabilised by the PEI/Laponite particles do not promote efficient MF deposition [13]. In the light of the present study, we suggest that this is because the PEI/Laponite mass ratio employed corresponded to almost monolayer coverage, thus there are relatively few anionic sites remaining on the Laponite surface available for electrostatic adsorption of the cationic MF particles. In contrast, utilising a Magnafloc/Laponite mass ratio that corresponds to submonolayer coverage enables a strong interaction between the Laponite and the MF components.
77
3.4. Formation of inorganic/organic hybrid microcapsules Having established experimental evidence for a favourable electrostatic interaction between MF and the Magnafloc/Laponite particles, inorganic–organic hybrid double-shell microcapsules were prepared by the two routes summarised in Fig. 1. Fig. 5 shows the particle size distributions obtained using laser diffraction for a Magnafloc/Laponite Pickering emulsion (prepared at pH 4) and MF/Laponite hybrid microcapsules prepared in the presence and absence of the poly(acrylamide-co-sodium acrylate) stabiliser. The volume-average diameter of the MF/Laponite hybrid microcapsules produced by either Route A or Route B is comparable to that of the precursor Pickering emulsion. However, the presence of the polymeric stabiliser leads to a somewhat broader particle size distribution. Moreover, there is a significant difference in the appearance of the underlying aqueous phase after the oil droplets are allowed to cream on standing, as indicated by the digital photo images shown in Fig. 5. The aqueous phase obtained for microcapsules prepared in the presence of poly(acrylamide-co-sodium acrylate) via Route A is milky-white, which suggests the formation of sterically-stabilised MF latex particles. In contrast, the corresponding aqueous phase obtained for microcapsules prepared in the absence of poly(acrylamide-co-sodium acrylate) via Route B is transparent, suggesting that all of the MF has been deposited onto the Pickering emulsion droplets. This is presumably because secondary nucleation is suppressed in the absence of poly (acrylamide-co-sodium acrylate). Furthermore [13], since the Magnafloc is employed at submonolayer coverage when coating the Laponite particles (see Fig. 2), there are little or no non-adsorbed Magnafloc chains remaining in the aqueous continuous phase, which further reduces the probability of MF particle nucleation occurring in solution. To test this hypothesis, the underlying milky-white aqueous phase formed when preparing microcapsules in the presence of poly(acrylamide-co-sodium acrylate) was isolated and compared to that of MF latex particles synthesised in the presence and absence of poly(acrylamide-co-sodium acrylate) in control experiments (see Fig. 3). Dynamic light scattering studies of the MF particles formed in the presence of poly(acrylamideco-sodium acrylate) and the MF particles present in the underlying milky aqueous phase indicated similar hydrodynamic diameters of around 400 nm. Optical microscopy and SEM images of these two types of MF particles are also comparable (see Fig. S8). Finally, the electrophoretic footprint of the MF particles present in the underlying milky aqueous phase is very similar to that of the reference MF latex prepared in the presence of poly(acrylamide-cosodium acrylate), see Fig. S9. Thus it seems that the aqueous supernatant isolated after formation of microcapsules in the presence of poly(acrylamide-co-sodium acrylate) is indeed contaminated with excess MF latex particles formed via secondary nucleation. Nevertheless, Routes A and B both lead to the formation of inorganic/ organic hybrid microcapsules that exhibit good long-term stability due to a combination of both steric and electrostatic stabilisation conferred on the microcapsules by the dense MF shell and the presence of the anionic copolymer stabiliser. 3.5. Alcohol challenge of inorganic/organic hybrid microcapsules
Fig. 5. Mastersizer droplet size distributions and corresponding digital photographs recorded for: (i) an uncoated Magnafloc/Laponite-stabilised Pickering emulsion; (ii) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamideco-sodium acrylate); (iii) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate). Conditions used for preparation of the Pickering emulsifier particles: 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4.
Fig. 6 shows optical microscopy images obtained for Magnafloc/ Laponite stabilised Pickering emulsions and MF/Laponite hybrid microcapsules prepared in the presence and absence of poly (acrylamide-co-sodium acrylate), both before and after a challenge with excess ethanol. Ethanol is miscible with both sunflower oil and the aqueous continuous phase, hence addition of excess ethanol can be used to assess the strength and permeability of the microcapsules after complete removal of both phases. Both optical microscopy and scanning electron microscopy were used to exam-
78
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
Fig. 6. Optical microscopy images recorded for: (A) a Magnafloc/Laponite-stabilised Pickering emulsion; (B) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamide-co-sodium acrylate); (C) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate); (D) a Magnafloc/Laponitestabilised Pickering emulsion after an ethanol challenge; (E) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamide-co-sodium acrylate) after an ethanol challenge; (F) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate) after an ethanol challenge. In each case the final sunflower oil concentration after ethanol addition was 2.0% w/w.
ine the microcapsule integrity after this ethanol challenge. Prior to ethanol addition, the precursor Pickering emulsion comprised spherical, polydisperse droplets and exhibited good long-term stability towards coalescence (Fig. 6A). The MF/Laponite hybrid microcapsules prepared in the presence of poly (acrylamide-co-sodium acrylate) are buckled and collapsed, resulting in loss of their original spherical morphology (Fig. 6B). In contrast, the MF/Laponite hybrid microcapsules prepared in the absence of poly(acrylamide-co-sodium acrylate) retain their spherical morphology and are much less prone to deformation (Fig. 6C). Addition of excess ethanol breaks the precursor Pickering emulsion and also removes the encapsulated oil from within the hybrid microcapsules. In the former case, no remnants of the original morphology can be detected by optical microscopy (Fig. 6D). In contrast, MF/Laponite hybrid microcapsules are much more robust and largely survive the ethanol challenge (see Fig. 6E and F). However, it is difficult to judge whether these ethanol-washed microcapsules contain any residual oil or merely comprise empty hollow shells. Fig. 7 shows representative SEM images obtained for MF/Laponite hybrid microcapsules prepared in the presence and absence of poly(acrylamide-sodium acrylate) following an ethanol challenge in each case. Fig. 7A and B depict MF/Laponite
hybrid microcapsules prepared using poly(acrylamide-co-sodium acrylate). Under the ultrahigh vacuum (UHV) conditions required for SEM studies, these microcapsules appear to be quite flexible: following the ethanol challenge they collapse without rupturing, which suggests that the poly(acrylamide-co-sodium acrylate) may act as a plasticiser for the walls of the microcapsules. This would also account for the partially buckled appearance of these spherical microcapsules when viewed by optical microscopy, see Fig. 6B. Furthermore, the presence of excess MF latex particles is confirmed, not only in the background, but also on the surface of the microcapsules. In contrast, Fig. 7C and D show MF/Laponite hybrid microcapsules prepared in the absence of poly (acrylamide-co-sodium acrylate). These microcapsules are much more rigid than those prepared in the presence of poly (acrylamide-co-sodium acrylate): they remain free-standing even under UHV conditions and retain their original spherical morphology. There is no evidence for any MF latex and the shell walls appear to be relatively smooth. However, many of these microcapsules have ruptured, revealing their hollow interior. This observation suggests that they may be rather more brittle than microcapsules prepared in the absence of poly(acrylamideco-sodium acrylate). Thus, although this water-soluble stabiliser
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
79
Fig. 7. Scanning electron microscopy images recorded after an ethanol challenge followed by drying in air at 20 °C for: (A and B) MF/Laponite hybrid microcapsules prepared in the presence of poly(acrylamide-co-sodium acrylate); (C and D) MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate).
is not essential for the production of hybrid microcapsules, its inclusion appears to result in more durable microcapsule walls, which may be important for certain microencapsulation applications. 4. Conclusions The formation of novel inorganic–organic hybrid microcapsules based on coating Magnafloc/Laponite stabilised Pickering emulsions with a melamine–formaldehyde (MF) overlayer is described. Empirically, it was found that Magnafloc/Laponite particles prepared using a relatively low amount of Magnafloc (10% by mass based on Laponite, i.e. well below the ‘knee’ of the adsorption isotherm) conferred optimal Pickering emulsifier performance. Moreover, these particles stabilised Pickering emulsions at pH 4, which is the solution acidity required for MF deposition. Inorganic– organic hybrid microcapsules were prepared in the presence and absence of poly(acrylamide-co-sodium acrylate). The presence of this water-soluble polymeric stabiliser led to robust microcapsules that can survive both an alcohol challenge and also the UHV conditions required for scanning electron microscopy studies. However, this copolymer also facilitates the formation of excess MF latex in the aqueous continuous phase, since it can electrostatically adsorb onto these particles and act as a steric stabiliser. Provided that the Magnafloc is employed at submonolayer coverage, the precipitating nascent cationic MF nuclei can adsorb onto the partiallycoated anionic Laponite particles, which removes the requirement for the poly(acrylamide-co-sodium acrylate). In the absence of this copolymer, the secondary amine groups on the Magnafloc chains most likely react with the MF, leading to chemical grafting. Thus secondary nucleation of MF latex particles can be avoided, which
allows more efficient MF deposition at the surface of the emulsion droplets. However, the resulting MF shells appear to be more brittle, as SEM studies reveal that extensive rupture of the microcapsules occurs after addition of excess alcohol. However, such embrittlement may actually be beneficial for ‘consumer-activated’ fragrance release in certain laundry products. Thus we conclude that microcapsule preparation via Route B appears to offer a number of advantages over Route A. Future work will focus on establishing the advantages (if any) of encapsulation of various hydrophobic actives within these new hybrid inorganic/organic microcapsules using Route B. Acknowledgments We thank P & G Technical Center (Newcastle-upon-Tyne, UK) for an Industrial EPSRC CASE studentship to support MW and also for permission to publish this work. The three reviewers of this manuscript are thanked for their helpful comments.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.08.044. References [1] W. Ramsden, Separation of solids in the surface-layers of solutions and ’Suspensions’ (observations on surface-membranes, bubbles, emulsions, and mechanical coagulation). Preliminary Account, Proc. Royal Soc. London 72 (1903) 156–164. [2] S.U. Pickering, Emulsions, J. Chem. Soc. 91 (1907) 2001–2021.
80
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
[3] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Colloidosomes: selectively permeable capsules composed of colloidal particles, Science 298 (2002) 1006–1009. [4] S. Laib, A.F. Routh, Fabrication of colloidosomes at low temperature for the encapsulation of thermally sensitive compounds, J. Colloid Interface Sci. 317 (2008) 121–129. [5] O.J. Cayre, P.F. Noble, V.N. Paunov, Fabrication of novel colloidosome microcapsules with gelled aqueous cores, J. Mater. Chem. 14 (2004) 3351– 3355. [6] K.L. Thompson, S.P. Armes, J.R. Howse, S. Ebbens, I. Ahmad, J.H. Zaidi, D.W. York, J.A. Burdis, Covalently cross-linked colloidosomes, Macromolecules 43 (2010) 10466–10474. [7] A. Walsh, K.L. Thompson, S.P. Armes, D.W. York, Polyamine-functional sterically stabilized latexes for covalently cross-linkable colloidosomes, Langmuir 26 (2010) 18039–18048. [8] L.A. Fielding, S.P. Armes, Preparation of Pickering emulsions and colloidosomes using either a glycerol-functionalised silica sol or core–shell polymer/silica nanocomposite particles, J. Mater. Chem. 22 (2012) 11235–11244. [9] Q. Yuan, O.J. Cayre, S. Fujii, S.P. Armes, R.A. Williams, S. Biggs, Responsive core– shell latex particles as colloidosome microcapsule membranes, Langmuir 26 (2010) 18408–18414. [10] H.N. Yow, A.F. Routh, Release profiles of encapsulated actives from colloidosomes sintered for various durations, Langmuir 25 (2009) 159–166. [11] K.L. Thompson, S.P. Armes, From well-defined macromonomers to stericallystabilised latexes to covalently cross-linkable colloidosomes: exerting control over multiple length scales, Chem. Commun. 46 (2010) 5274–5276. [12] A.R. Bausch, M.J. Bowick, A. Cacciuto, A.D. Dinsmore, M.F. Hsu, D.R. Nelson, M. G. Nikolaides, A. Travesset, D.A. Weitz, Grain boundary scars and spherical crystallography, Science 299 (2003) 1716–1718. [13] M. Williams, S.P. Armes, D.W. York, Clay-based colloidosomes, Langmuir 28 (2012) 1142–1148. [14] T. Chen, P.J. Colver, S.A.F. Bon, Organic–inorganic hybrid hollow spberes prepared from TiO2-stabilized pickering emulsion polymerization, Adv. Mater. 19 (2007) 2286–2289. [15] W. Chen, X. Liu, Y. Liu, H.-I. Kim, Synthesis of microcapsules with polystyrene/ ZnO hybrid shell by Pickering emulsion polymerization, Colloid Polym. Sci. 288 (2010) 1393–1399. [16] H. Liu, C. Wang, Q. Gao, X. Liu, Z. Tong, Magnetic hydrogels with supracolloidal structures prepared by suspension polymerization stabilized by Fe2O3 nanoparticles, Acta Biomater. 6 (2010) 275–281. [17] Q. Gao, C. Wang, H. Liu, C. Wang, X. Liu, Z. Tong, Suspension polymerization based on inverse Pickering emulsion droplets for thermo-sensitive hybrid
[18]
[19]
[20] [21] [22]
[23]
[24]
[25] [26] [27] [28] [29]
[30]
[31] [32]
[33]
microcapsules with tunable supracolloidal structures, Polymer 50 (2009) 2587–2594. E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Mohwald, Novel hollow polymer shells by colloid-templated assembly of polyelectrolytes, Angew. Chem.-Int. Ed. 37 (1998) 2202–2205. A.P.R. Johnston, C. Cortez, A.S. Angelatos, F. Caruso, Layer-by-layer engineered capsules and their applications, Curr. Opin. Colloid Interface Sci. 11 (2006) 203–209. C.S. Peyratout, L. Dahne, Tailor-made polyelectrolyte microcapsules: from multilayers to smart containers, Angew. Chem.-Int. Ed. 43 (2004) 3762–3783. J. Li, H.D.H. Stover, Pickering emulsion templated layer-by-layer assembly for making microcapsules, Langmuir 26 (2010) 15554–15560. X. Wang, W. Zhou, J. Cao, W. Liu, S. Zhu, Preparation of core–shell CaCO3 capsules via Pickering emulsion templates, J. Colloid Interface Sci. 372 (2012) 24–31. H. Strohm, P. Lobmann, Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised Pickering emulsions, J. Mater. Chem. 14 (2004) 2667–2673. J. Li, A.P. Hitchcock, H.D.H. Stover, Pickering emulsion templated interfacial atom transfer radical polymerization for microencapsulation, Langmuir 26 (2010) 17926–17935. S. Luzius, Process for the Encapsulation of Dispersible Materials, US 3594328 A, 1971. P.L. Foris, R.W. Brown, P.S. Phillips Jr., In situ Polymerization, Microencapsulation, US 4100103 A, 1978. Y. Lvov, A.A. Antipov, A. Mamedov, H. Mohwald, G.B. Sukhorukov, Urease encapsulation in nanoorganized microshells, Nano Lett. 1 (2001) 125–128. G. Sun, Z. Zhang, Mechanical properties of melamine–formaldehyde microcapsules, J. Microencapsul. 18 (2001) 593–602. X.X. Zhang, X.M. Tao, K.L. Yick, X.C. Wang, Structure and thermal stability of microencapsulated phase-change materials, Colloid Polym. Sci. 282 (2004) 330–336. X.X. Zhang, Y.F. Fan, X.M. Tao, K.L. Yick, Fabrication and properties of microcapsules and nanocapsules containing n-octadecane, Mater. Chem. Phys. 88 (2004) 300–307. K. Hong, S. Park, Melamine resin microcapsules containing fragrant oil: synthesis and characterization, Mater. Chem. Phys. 58 (1999) 128–131. Y. Long, D. York, Z. Zhang, J.A. Preece, Microcapsules with low content of formaldehyde: preparation and characterization, J. Mater. Chem. 19 (2009) 6882–6887. Y. Long, B. Vincent, D. York, Z. Zhang, J.A. Preece, Organic–inorganic double shell composite microcapsules, Chem. Commun. 46 (2010) 1718–1720.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Inorganic/organic hybrid microcapsules: Melamine formaldehyde-coated Laponite-based Pickering emulsions Mark Williams a,⇑, Birte Olland a, Steven P. Armes a,⇑, Pierre Verstraete b, Johan Smets b a b
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, UK Procter & Gamble, Eurocor NV/SA, Temselaan 100, 1853 Strombeek-Bever, Belgium
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 10 July 2015 Revised 11 August 2015 Accepted 22 August 2015 Available online 22 August 2015 Keywords: Microcapsules Laponite Magnafloc Melamine formaldehyde Pickering emulsion Hybrid
a b s t r a c t A facile synthesis route to novel inorganic/organic hybrid microcapsules is reported. Laponite nanoparticles are surface-modified via electrostatic adsorption of Magnafloc, an amine-based polyelectrolyte allowing the formation of stable oil-in-water Pickering emulsions. Hybrid microcapsules can be subsequently prepared by coating these Pickering emulsion precursors with dense melamine formaldehyde (MF) shells. Employing a water-soluble polymeric stabiliser, poly(acrylamide-co-sodium acrylate) leads to stable hybrid microcapsules that survive an alcohol challenge and the ultrahigh vacuum conditions required for SEM studies. Unfortunately, the presence of this copolymer also leads to secondary nucleation of excess MF latex particles in the aqueous continuous phase. However, since the Magnafloc is utilised at submonolayer coverage when coating the Laponite particles, the nascent cationic MF nanoparticles can deposit onto anionic surface sites on the Laponite, which removes the requirement for the poly(acrylamide-co-sodium acrylate) component. Following this electrostatic adsorption, the secondary amine groups on the Magnafloc chains can react with the MF, leading to highly robust cross-linked MF shells. The absence of the copolymer leads to minimal secondary nucleation of MF latex particles, ensuring more efficient deposition at the surface of the emulsion droplets. However, the MF shells appear to become more brittle, as SEM studies reveal cracking on addition of ethanol. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
⇑ Corresponding authors. E-mail address: [email protected] (S.P. Armes). http://dx.doi.org/10.1016/j.jcis.2015.08.044 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
An emulsion stabilised by solid particles is known as a Pickering emulsion [1,2]. Pickering emulsion templates can be further stabilised by thermal annealing [3,4], polyelectrolyte adsorption [3],
72
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
gel trapping [5] or covalent cross-linking [6–9] in order to stabilise the initial particle super-structure. This produces so-called ‘colloidosomes’ [3] which in principle offer potential applications in the context of microencapsulation. However, dye release studies of an encapsulated water-soluble dye within latex-based colloidosomes indicated only very limited retention: complete release into the aqueous continuous phase was observed within a few hours, which suggests rather high permeability for the latex shells [10]. Similarly, Thompson et al. also reported poor retention of actives within thermally-annealed latex-based colloidosomes [6,11]. This problem was attributed to the intrinsic geometric packing defects that inevitably arise when packing near-monodisperse small spheres on the surface of larger spheres [12]. Recently, we reported that branched poly(ethylene imine) can be physically adsorbed onto an inorganic nanoclay (Laponite) to produce an efficient hybrid Pickering emulsifier for the stabilisation of oil-in-water emulsions using various oils [13]. These Pickering emulsions could be converted into colloidosomes using either oil-soluble, or water-soluble bisepoxy-functionalised polymeric cross-linkers. These colloidosomes proved to be sufficiently robust to survive the removal of the internal oil phase after washing with excess ethanol. However, subsequent dye release studies suggested that such microcapsules are highly permeable and hence do not provide an effective barrier for retarding the release of small molecules [13]. In principle, the porosity of such microcapsules could be reduced by deposition of a dense secondary shell onto the initial layer of polymer/Laponite particles. There are several known routes to such double-shell (or even multiple-shell) microcapsules, which can yield either purely organic or inorganic microcapsules, or even inorganic/organic hybrid microcapsules. One common approach utilised to prepare double-shell hybrid microcapsules is to use inorganic particles such as titanium dioxide [14], zinc oxide [15], iron oxide [16] and silica [17] to produce an initial Pickering emulsion with a suitable vinyl monomer located within the oil phase. Subsequent polymerisation of this monomer results in a polymeric layer forming at the inner particle interface of the droplets, forming a double shell. Layer-by-layer deposition (L-b-L) [18] is another technique commonly employed to form multi-layered microcapsules. Microcapsules are constructed via sequential deposition of oppositelycharged polyelectrolytes onto colloidal particles such as melamine formaldehyde (MF), polystyrene or silica, which act as sacrificial templates [19]. These solid colloidal templates typically have narrow size distributions and hence allow precise control over the microcapsule dimensions. However, they must be removed before the desired payload can be introduced into the microcapsule cores. Core removal can affect capsule integrity and wall properties, while efficient loading can be challenging in many cases [20]. This technique has been developed further by Li et al. using Pickering emulsion templates for the L-b-L deposition of polyelectrolytes, thus allowing incorporation of oil-soluble pay-loads into the cores prior to polyelectrolyte self-assembly and removing the requirement for template removal [21]. An alternative approach is to coat an initial Pickering emulsion with a second overlayer. Thus a purely organic double-shell microcapsule was reported by Thompson et al., who deposited an ultrathin overlayer of polypyrrole onto latex-based colloidosomes in order to suppress dye release from oil cores [6]. Purely inorganic double shell microcapsules have been reported by Wang and co-workers, who showed that micron-sized food-grade calcium carbonate particles could be used to stabilise an o/w Pickering emulsion and then act as nucleation sites for the subsequent deposition of a crystalline calcium carbonate shell [22]. As well as purely organic and inorganic hybrid shells, the formation of inorganic/organic hybrid capsules using a Pickering
emulsion template has also been reported. For example, Strohm and Lobmann demonstrated that composite titania (TiO2)/polymer latex shells could be prepared by the deposition of TiO2 onto polystyrene latex-stabilised 1-octanol-in-water Pickering emulsions [23]. Li et al. have reported the electrostatic adsorption of an anionic polymeric ATRP initiator onto cationic silica nanoparticles to stabilise an oil-in-water emulsion. Addition of water-soluble cross-linking monomers to the aqueous continuous phase with subsequent polymerisation at the oil/water interface resulted in silica nanoparticle–polymer composite shells [24]. Melamine–formaldehyde (MF) has been widely utilised for microencapsulation applications [25–27] because of its good mechanical [28] and thermal stability [29,30]. MF microcapsules are typically prepared via polycondensation, with the MF prepolymer being initially soluble in the continuous aqueous phase, and the hydrophobic component being present in the form of dispersed droplets. As the polymerisation starts, the soluble oligomers start to adsorb at the surface of the droplets. Once on the surface, further polymerisation and crosslinking occurs, which results in the formation of a solid impermeable MF shell [31]. In 2009 Long et al. conducted in situ polymerisation to produce 15 lm MF microcapsules containing a consumer fragrance as the oil phase [32]. An MF precondensate was added to an aqueous solution of poly (acrylamide-co-sodium acrylate) prior to homogenisation of the oil phase, with generation of the MF shell occurring on heating. The same team subsequently prepared novel double-shell inorganic–inorganic microcapsules comprising a MF inner shell and an outer shell of CaCO3 nanoparticles. In particular, it was demonstrated that the rate of release from these double-shell microcapsules was lower than that observed for the respective single shells [33]. Herein we report the adsorption of a commercial cationic polyelectrolyte, Magnafloc, onto Laponite nanoparticles to produce an effective Pickering emulsifier for sunflower oil droplets at pH 4. We show that novel inorganic/organic hybrid microcapsules can be formed by the in situ polymerisation of melamine formaldehyde onto the surface of the Pickering emulsions. Moreover, such hybrid double-shell microcapsules can be generated either in the presence or absence of poly(acrylamide-co-sodium acrylate) (see Fig. 1). 2. Experimental section 2.1. Materials Sunflower oil, n-dodecane, benzyl benzoate and isopropyl myristate were all purchased from Aldrich and used as received. MF precondensate (supplied as an 70% w/w aqueous solution; a. k.a. ‘Beetle resin’) with a formaldehyde-to-melamine molar ratio of 0.20 was kindly supplied by British Industrial Plastics Ltd. (Birmingham, UK) and was used without further purification. Poly(acrylamide-co-sodium acrylate) (MW = 2 105 g mol 1) was supplied by Polysciences (Pennsylvania, USA) and used without further purification. Laponite RD was obtained from Rockwood Additives Ltd., UK. Magnafloc (50% w/w poly(dimethyla mine-co-epichlorohydrin-co-ethylenediamine)) was kindly supplied by Vesuvius (Chesterfield, UK). Ethanol was purchased from Fisher (UK) and used as received. Deionised water was used in all experiments and all solution pH adjustments using HCl/NaOH were monitored using a pH meter. 2.2. Adsorption of Magnafloc onto Laponite Magnafloc (0–2.0 g, 50% w/w) was dissolved in water (48–50 mL). This copolymer solution was then added to a 1.0% w/w aqueous dispersion of Laponite (50.0 g) and the resulting
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
73
Fig. 1. Reaction scheme for the formation of sunflower oil-loaded inorganic–organic double-shell hybrid microcapsules. Route A involves the deposition of a cross-linked melamine formaldehyde (MF) shell onto Magnafloc/Laponite stabilised sunflower oil-in-water Pickering emulsion droplets in the presence of poly(acrylamide-co-sodium acrylate) at pH 4 via a two-step protocol. Route B involves the deposition of a cross-linked melamine formaldehyde (MF) shell onto Magnafloc/Laponite stabilised sunflower oil-in-water Pickering emulsion droplets in the absence of poly(acrylamide-co-sodium acrylate) at pH 4 in a one-pot protocol. MF polymerisation was achieved by heating at 65 °C for 4 h in each case.
mixture was stirred for 12 h at 20 °C so as to allow electrostatic adsorption of the cationic Magnafloc chains onto the anionic Laponite particles at pH 10. 2.3. Preparation of Magnafloc/Laponite 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 dispersion of Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc). The solution pH was adjusted from pH 3 to pH 12 using 0.1 M HCl or NaOH as required. Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool.
2.4. Preparation of MF/Laponite hybrid microcapsules in the presence of poly(acrylamide-co-sodium acrylate) Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of an aqueous suspension of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4). Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool. This Pickering emulsion precursor was then added to an aqueous solution of MF precondensate (2.50 g) and poly(acrylamide-co-sodium acrylate) (0.58 g) in water (70 mL) that had been stirred for 105 min at pH 4 (adjusted using HCl) at room temperature. This suspension was heated to 65 °C and stirred for 4 h. The resulting suspension of
74
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
microcapsules was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. The same protocol was repeated with the Magnafloc/Laponite particles being replaced with PEI/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.25% w/w poly(ethylene imine), pH 9) as reported previously [13]. In this case emulsification was conducted at pH 9, followed by lowering the solution pH to pH 4. 2.5. Preparation of MF/Laponite hybrid microcapsules in the absence of poly(acrylamide-co-sodium acrylate) Sunflower oil (5.0 mL) was added to a 14 mL sample vial, followed by the addition of an aqueous dispersion of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4). Emulsification was achieved by stirring at 12,000 rpm for 2.0 min at 20 °C using an IKA Ultra-Turrax T-18 homogeniser equipped with a 10 mm dispersing tool. This emulsion was added to an aqueous solution of MF precondensate (2.50 g) in water (70 mL) at pH 4 (adjusted with HCl) and this solution was then heated to 65 °C and stirred for 4 h. The resulting microcapsule suspension was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation using aqueous 1 M NaOH. The same protocol was also conducted using PEI/Laponite particles in place of the Magnafloc/Laponite particles (5.0 ml, 0.50% w/w Laponite, 0.25% w/w poly(ethylene imine), pH 9) as reported previously [13]. In this case the emulsification was conducted at pH 9, followed by lowering the pH to pH 4. 2.6. Preparation of MF latex in the presence of poly(acrylamideco-sodium acrylate) An aqueous solution of MF precondensate (2.50 g) and poly (acrylamide-co-sodium acrylate) (0.58 g) in water (70 mL) was stirred for 105 min at pH 4 (adjusted using HCl) at 20 °C. This suspension was heated to 65 °C and stirred for 4 h. The resulting milky suspension was cooled to 20 °C, and the solution pH was raised to 10 to quench the MF polymerisation by adding 1 M NaOH. 2.7. Preparation of MF latex in the absence of poly(acrylamideco-sodium acrylate) An aqueous solution of MF precondensate (2.50 g) was diluted in water (70 mL), heated to 65 °C and stirred for 4 h at pH 4 (adjusted using HCl). The resulting milky suspension was cooled to 20 °C, and its pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.8. Preparation of MF latex in the presence of Laponite nanoparticles An aqueous solution of MF precondensate (2.50 g) was dispersed in water (70 mL) and added to a 0.5% w/w aqueous dispersion of Laponite (10.0 g) stirred for 105 min at pH 4 (adjusted using HCl) at room temperature. This suspension was heated to 65 °C and stirred for 4 h. The resulting milky suspension was cooled to 20 °C, and its pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.9. Preparation of MF latex in the presence of Magnafloc/Laponite particles An aqueous solution of MF precondensate (2.50 g) was dispersed in water (70 mL) followed by the addition of an aqueous dispersion of as-prepared Magnafloc/Laponite particles (5.0 mL, 0.50% w/w Laponite, 0.05% w/w Magnafloc). The solution was stirred for 105 min at pH 4 (adjusted using HCl) at 20 °C. This suspension was heated to 65 °C and stirred for 4 h. The resulting
milky suspension was cooled to 20 °C, and the pH was raised to 10 to quench the MF polymerisation by adding aqueous 1 M NaOH. 2.10. Ethanol challenge Pickering emulsions and MF/Laponite hybrid microcapsules (synthesised in the presence or absence of poly(acrylamideco-sodium acrylate)) were diluted in turn using ethanol (50 mL) to obtain a final concentration of 2.0% w/w oil in ethanol and the sealed vial was shaken vigorously to break the emulsion. 2.11. Dynamic light scattering The intensity-average hydrodynamic diameter was obtained by DLS using a Malvern Zetasizer Nano ZS instrument. Dilute aqueous dispersions (0.01% w/v) were analysed using disposable plastic cuvettes and the data were averaged over three consecutive runs. The deionised water used to dilute each dispersion was ultrafiltered through a 0.20 lm membrane prior to use to remove extraneous dust. 2.12. Aqueous electrophoresis Zeta potentials were determined at pH 10 for both pristine Laponite and Magnafloc/Laponite particles prepared at various Magnafloc/Laponite mass ratios using a Malvern Zetasizer Nano ZS instrument. Zeta potential vs. pH curves were also determined for pristine Laponite and Magnafloc, as well as Magnafloc/Laponite particles (5.0 mL, prepared using 0.50% w/w Laponite and 0.05% w/w Magnafloc). Zeta potential vs. pH curves were also determined for Laponite, Magnafloc and Magnafloc/Laponite particles, as well as MF latex prepared in the presence of poly(acrylamideco-sodium acrylate). The solution pH was varied between pH 3 and pH 12 in the presence of 1 mM KCl, using either dilute NaOH or HCl for pH adjustment as required. 2.13. Thermogravimetric analysis Analyses were conducted using a Perkin–Elmer Pyris-1 TGA instrument. Prior to analysis, the Magnafloc/Laponite particles were purified by centrifugation at 20,000 rpm for 45 min, carefully replacing the supernatant with mildly alkaline water (pH 10) each time followed by redispersion of the sedimented particles with the aid of an ultrasonic bath. This centrifugation–redispersion cycle was repeated four times to ensure that no excess non-adsorbed Magnafloc remained in the aqueous continuous phase. The dried Magnafloc/Laponite particles were heated up to 800 °C in air at a heating rate of 20 °C min 1. The amount of Magnafloc adsorbed onto the Laponite particles was calculated from the weight loss observed between 200 °C and 650 °C. 2.14. Conductivity measurements Conductivities of emulsions were recorded immediately after their preparation using a digital conductivity meter (Hanna model Primo 5). High conductivities (>10 lS cm 1) indicated an aqueous continuous phase, i.e. formation of an oil-in-water emulsion. These 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 in each liquid was assessed by visual inspection. Relatively rapid dispersion into water was taken as confirmation that the continuous phase of the emulsion was indeed water.
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
75
A single droplet (ca. 100 lL) of an o/w Pickering emulsion was placed on a microscope slide and digital images were recorded using a Motic DMBA300 digital biological microscope equipped with a built-in camera and Motic Images Plus 2.0 ML software. The same protocol was utilised to image the MF latex particles and the MF/Laponite hybrid microcapsules.
CaCO3/MF hybrid double-shell microcapsules [33]. Thus the MF precondensate is added to an aqueous solution containing a water-soluble polymeric stabiliser, poly(acrylamide-co-sodium acrylate), to produce a partially cross-linked precursor that is subsequently deposited onto the Magnafloc/Laponite-stabilised Pickering emulsion droplets. In contrast, Route B depicts a facile onepot route in which the MF precondensate is added to the Pickering emulsions without any initial cross-linking step.
2.16. Laser diffraction particle size analysis of emulsion droplets
3.1. Magnafloc adsorption on Laponite
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 solidstate blue laser operating at 466 nm was used to size the Pickering emulsions. 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 analysed five times, and the data were averaged. A typical acquisition time was 2 min per sample after alignment and background measurements. The raw data was analysed using Malvern software. The mean droplet diameter was taken to be the mean volumeaverage diameter (D4/3), which is mathematically expressed as D4/3 = RD4i Ni/RD3i 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 ethanol, followed by three rinses with deionised water. The glass walls of the cell were carefully wiped with lens cleaning tissue to avoid crosscontamination, and the laser was aligned centrally on the detector.
Prior to MF deposition, the electrostatic adsorption of Magnafloc onto Laponite required systematic investigation to establish the optimum Magnafloc/Laponite mass ratio for the optimum Pickering emulsifier performance. Accordingly, the concentration of Magnafloc added to a 0.50% w/w aqueous dispersion of Laponite particles at pH 10 was varied from zero up to 1.0% w/w (i.e. up to a Magnafloc/Laponite mass ratio of 2.0). The aqueous dispersion of Laponite used in this study had a mean hydrodynamic diameter of 65 nm (see Fig. S1) and a specific surface area of 353 m2 g 1, as determined by BET analysis. As shown in Fig. 2, the initial negative zeta potential of the pristine Laponite particles is gradually reduced with increasing adsorption of the cationic Magnafloc; an isoelectric point is observed at a Magnafloc/Laponite mass ratio of around 0.20 and surface charge reversal occurs thereafter. The actual adsorbed amount of Magnafloc determined by thermogravimetry is consistent with the observed electrophoretic behaviour and suggests Langmuir-type adsorption.
2.15. Optical microscopy
2.17. Scanning electron microscopy SEM images were obtained using a FEI inspect F FEG instrument operating at 20 kV. All samples were sputter-coated with a thin overlayer of gold prior to inspection to prevent sample charging problems. 2.18. BET surface area analysis The specific surface area of Laponite-RD 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 and a linear fivepoint adsorption isotherm was constructed.
3.2. Magnafloc/Laponite stabilised Pickering emulsions As in the case of the PEI/Laponite particles [13], physical adsorption of Magnafloc onto Laponite particles enabled stable Pickering emulsions to be obtained for a range of oils, with systematic variation of the Magnafloc/Laponite mass ratio allowing emulsifier performance to be tuned for a given oil (see Fig. S3). In the case of sunflower oil-in-water emulsions, Magnafloc/Laponite particles prepared using a relatively low level of Magnafloc (0.50% w/w Laponite, 0.05% w/w Magnafloc; i.e. well below the plateau region of the adsorption isotherm, see Fig. 2), conferred optimal Pickering emulsifier performance. Moreover, an acidic reaction solution (e.g. pH 4) is required for the MF synthesis, so the precursor Pickering emulsion must be stable at this pH in order to prepare the inorganic/organic hybrid double-shell microcapsules. Magnafloc/Laponite particles prepared using a Magnafloc/Laponite mass ratio of 0.10 can be used to stabilise Pick-
3. Results and discussion Fig. 1 shows the two synthetic routes used to produce inorganic–organic hybrid microcapsules via deposition of an MF overlayer onto Laponite-stabilised Pickering emulsions. Magnafloc is a commercial water-soluble copolymer comprising secondary amine repeat units: it is adsorbed onto Laponite particles to facilitate the stabilisation of sunflower oil-in-water Pickering emulsions. Recently, we reported that a similar cationic polyelectrolyte, poly (ethylene imine) or PEI, adsorbs onto Laponite and modifies its surface wettability, producing an effective PEI/Laponite Pickering emulsifier. Systematic variation of the PEI/Laponite mass ratio allowed fine-tuning of the Pickering emulsifier performance, enabling stable oil-in-water emulsions to be produced for a range of model oils [13]. However, attempts to coat such PEI/Laponite Pickering emulsions with MF using either of the two methods summarised in Fig. 1 invariably led to aggregation of the emulsion droplets, and a non-contiguous MF coating (see Fig. S2). Route A in Fig. 1 is based on a previously reported protocol used to prepare
Fig. 2. Effect of varying the target Magnafloc/Laponite mass ratio on: (a) the zeta potential ( ) recorded at pH 10 for the resulting Magnafloc/Laponite particles and (b) the adsorbed mass of Magnafloc per unit surface area of Laponite ( ). In each case the Laponite concentration was fixed at 0.50% w/w.
76
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
Fig. 3. Zeta potential vs. pH curves recorded for aqueous dispersions of melamine formaldehyde (MF) particles prepared: (i) in the absence of poly(acrylamide-cosodium acrylate) (d); (ii) in the presence of poly(acrylamide-co-sodium acrylate) ( ); (iii) in the presence of Laponite alone ( ); (iv) in the presence of Magnafloc/ Laponite particles (Magnafloc/Laponite mass ratio = 0.10) ( ).
ering emulsions with a volume-average droplet diameter of 54 ± 24 lm that exhibit good long-term stability at pH 10. On adjusting the pH from pH 10 to pH 4, these emulsion droplets remained essentially unchanged in size and their long-term stability was not compromised (see Fig. S4). Thus MF deposition onto this colloidal template to produce inorganic–organic hybrid microcapsules could now be investigated. 3.3. Melamine formaldehyde interaction with Laponite Route A involves stabilisation of the MF precondensate using poly(acrylamide-co-sodium acrylate) prior to polymerisation, see
Fig. 1. This interaction is likely to be merely electrostatic in nature, although we cannot rule out the possibility of amide bond formation between the anionic carboxylate groups on the copolymer and the amine groups of the precondensate. In control experiments, MF latex particles were synthesised both in the presence and absence of poly(acrylamide-co-sodium acrylate) in order to examine the influence of this stabiliser. Fig. 3 shows the corresponding aqueous electrophoretic data in the form of two zeta potential vs. pH curves. MF particles prepared in the absence of poly(acrylamide-cosodium acrylate) are cationic at pH 4, and exhibit an isoelectric point at around pH 8.5. In contrast, MF particles prepared in the presence of poly(acrylamide-co-sodium acrylate) possess negative zeta potentials at pH 4, indicating electrostatic adsorption of this anionic stabiliser. Furthermore, dynamic light scattering studies of the latter MF particles indicate a hydrodynamic diameter of around 400 nm, whereas the former MF particles were much larger and more polydisperse (10 ± 5 lm diameter, as judged by laser diffraction). This significant difference in particle size is confirmed by optical microscopy studies, see Fig. 4A and B. Such observations suggest that the copolymer confers steric stabilisation. In contrast, Route B in Fig. 1 does not involve this poly(acrylamide-co-sodium acrylate) stabiliser. The surface of the Magnafloc/Laponite particles prepared using a relatively low adsorbed amount of Magnafloc has both anionic character and secondary amine functionality. The former feature allows electrostatic adsorption of the nascent cationic MF nuclei, while the latter most likely enables in situ chemical grafting to occur. To test this hypothesis, an MF polymerisation was conducted in the presence of bare Laponite particles and the zeta potential data obtained for the resulting MF particles are shown in Fig. 3. The addition of Laponite resulted in weakly cationic particles at pH 4, with an isoelectric point being observed at pH 5.5. Moreover, at pH 4 the electrophoretic mobility is characterised
Fig. 4. Optical microscopy images recorded for melamine formaldehyde (MF) particles prepared in the presence of: (A) no additive; (B) poly(acrylamide-co-sodium acrylate) stabiliser; (C) Laponite; and (D) Magnafloc/Laponite particles (Magnafloc/Laponite mass ratio = 0.10).
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
by a single signal (see Fig. S5), which lies between that of bare Laponite and MF latex prepared in the absence of Laponite. This intermediate mobility indicates that an electrostatic interaction between the cationic MF latex and the anionic Laponite particles leading to the formation of inorganic–organic hybrid particles. Furthermore, the optical microscopy image shown in Fig. 4C indicates ill-defined non-spherical aggregates of MF particles prepared in the presence of Laponite. Since this particle morphology differs significantly from the spherical MF latexes shown in Fig. 4A and B, this supports the hypothesis of an electrostatic interaction between the cationic MF and the anionic Laponite. When MF was synthesised in the presence of Magnafloc/Laponite particles (see Figs. 3 and 4D), the zeta potential of the resulting hybrid particles was 16.2 mV at pH 10. This value is close to that obtained for the Magnafloc/Laponite particles (see Fig. 2), indicating their adsorption onto the surface of the MF particles. On lowering the solution pH, an isoelectric point is observed at pH 6.5, which is similar to the electrophoretic behaviour of the Magnafloc/Laponite particles alone (see Fig. S6). The optical microscopy image shown in Fig. 4D suggests that the particle size of the MF/Magnafloc/Laponite aggregates is comparable to those formed in the presence of bare Laponite alone. This is confirmed by laser diffraction studies which indicate volume-average diameters of 35 ± 13 lm and 30 ± 11 lm, respectively. SEM images for the four types of particles shown in Fig. 4 are provided in the supporting information, see Fig. S7. Previously, we reported that emulsion droplets stabilised by the PEI/Laponite particles do not promote efficient MF deposition [13]. In the light of the present study, we suggest that this is because the PEI/Laponite mass ratio employed corresponded to almost monolayer coverage, thus there are relatively few anionic sites remaining on the Laponite surface available for electrostatic adsorption of the cationic MF particles. In contrast, utilising a Magnafloc/Laponite mass ratio that corresponds to submonolayer coverage enables a strong interaction between the Laponite and the MF components.
77
3.4. Formation of inorganic/organic hybrid microcapsules Having established experimental evidence for a favourable electrostatic interaction between MF and the Magnafloc/Laponite particles, inorganic–organic hybrid double-shell microcapsules were prepared by the two routes summarised in Fig. 1. Fig. 5 shows the particle size distributions obtained using laser diffraction for a Magnafloc/Laponite Pickering emulsion (prepared at pH 4) and MF/Laponite hybrid microcapsules prepared in the presence and absence of the poly(acrylamide-co-sodium acrylate) stabiliser. The volume-average diameter of the MF/Laponite hybrid microcapsules produced by either Route A or Route B is comparable to that of the precursor Pickering emulsion. However, the presence of the polymeric stabiliser leads to a somewhat broader particle size distribution. Moreover, there is a significant difference in the appearance of the underlying aqueous phase after the oil droplets are allowed to cream on standing, as indicated by the digital photo images shown in Fig. 5. The aqueous phase obtained for microcapsules prepared in the presence of poly(acrylamide-co-sodium acrylate) via Route A is milky-white, which suggests the formation of sterically-stabilised MF latex particles. In contrast, the corresponding aqueous phase obtained for microcapsules prepared in the absence of poly(acrylamide-co-sodium acrylate) via Route B is transparent, suggesting that all of the MF has been deposited onto the Pickering emulsion droplets. This is presumably because secondary nucleation is suppressed in the absence of poly (acrylamide-co-sodium acrylate). Furthermore [13], since the Magnafloc is employed at submonolayer coverage when coating the Laponite particles (see Fig. 2), there are little or no non-adsorbed Magnafloc chains remaining in the aqueous continuous phase, which further reduces the probability of MF particle nucleation occurring in solution. To test this hypothesis, the underlying milky-white aqueous phase formed when preparing microcapsules in the presence of poly(acrylamide-co-sodium acrylate) was isolated and compared to that of MF latex particles synthesised in the presence and absence of poly(acrylamide-co-sodium acrylate) in control experiments (see Fig. 3). Dynamic light scattering studies of the MF particles formed in the presence of poly(acrylamideco-sodium acrylate) and the MF particles present in the underlying milky aqueous phase indicated similar hydrodynamic diameters of around 400 nm. Optical microscopy and SEM images of these two types of MF particles are also comparable (see Fig. S8). Finally, the electrophoretic footprint of the MF particles present in the underlying milky aqueous phase is very similar to that of the reference MF latex prepared in the presence of poly(acrylamide-cosodium acrylate), see Fig. S9. Thus it seems that the aqueous supernatant isolated after formation of microcapsules in the presence of poly(acrylamide-co-sodium acrylate) is indeed contaminated with excess MF latex particles formed via secondary nucleation. Nevertheless, Routes A and B both lead to the formation of inorganic/ organic hybrid microcapsules that exhibit good long-term stability due to a combination of both steric and electrostatic stabilisation conferred on the microcapsules by the dense MF shell and the presence of the anionic copolymer stabiliser. 3.5. Alcohol challenge of inorganic/organic hybrid microcapsules
Fig. 5. Mastersizer droplet size distributions and corresponding digital photographs recorded for: (i) an uncoated Magnafloc/Laponite-stabilised Pickering emulsion; (ii) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamideco-sodium acrylate); (iii) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate). Conditions used for preparation of the Pickering emulsifier particles: 0.50% w/w Laponite, 0.05% w/w Magnafloc, pH 4.
Fig. 6 shows optical microscopy images obtained for Magnafloc/ Laponite stabilised Pickering emulsions and MF/Laponite hybrid microcapsules prepared in the presence and absence of poly (acrylamide-co-sodium acrylate), both before and after a challenge with excess ethanol. Ethanol is miscible with both sunflower oil and the aqueous continuous phase, hence addition of excess ethanol can be used to assess the strength and permeability of the microcapsules after complete removal of both phases. Both optical microscopy and scanning electron microscopy were used to exam-
78
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
Fig. 6. Optical microscopy images recorded for: (A) a Magnafloc/Laponite-stabilised Pickering emulsion; (B) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamide-co-sodium acrylate); (C) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate); (D) a Magnafloc/Laponitestabilised Pickering emulsion after an ethanol challenge; (E) an MF/Laponite hybrid microcapsule prepared in the presence of poly(acrylamide-co-sodium acrylate) after an ethanol challenge; (F) an MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate) after an ethanol challenge. In each case the final sunflower oil concentration after ethanol addition was 2.0% w/w.
ine the microcapsule integrity after this ethanol challenge. Prior to ethanol addition, the precursor Pickering emulsion comprised spherical, polydisperse droplets and exhibited good long-term stability towards coalescence (Fig. 6A). The MF/Laponite hybrid microcapsules prepared in the presence of poly (acrylamide-co-sodium acrylate) are buckled and collapsed, resulting in loss of their original spherical morphology (Fig. 6B). In contrast, the MF/Laponite hybrid microcapsules prepared in the absence of poly(acrylamide-co-sodium acrylate) retain their spherical morphology and are much less prone to deformation (Fig. 6C). Addition of excess ethanol breaks the precursor Pickering emulsion and also removes the encapsulated oil from within the hybrid microcapsules. In the former case, no remnants of the original morphology can be detected by optical microscopy (Fig. 6D). In contrast, MF/Laponite hybrid microcapsules are much more robust and largely survive the ethanol challenge (see Fig. 6E and F). However, it is difficult to judge whether these ethanol-washed microcapsules contain any residual oil or merely comprise empty hollow shells. Fig. 7 shows representative SEM images obtained for MF/Laponite hybrid microcapsules prepared in the presence and absence of poly(acrylamide-sodium acrylate) following an ethanol challenge in each case. Fig. 7A and B depict MF/Laponite
hybrid microcapsules prepared using poly(acrylamide-co-sodium acrylate). Under the ultrahigh vacuum (UHV) conditions required for SEM studies, these microcapsules appear to be quite flexible: following the ethanol challenge they collapse without rupturing, which suggests that the poly(acrylamide-co-sodium acrylate) may act as a plasticiser for the walls of the microcapsules. This would also account for the partially buckled appearance of these spherical microcapsules when viewed by optical microscopy, see Fig. 6B. Furthermore, the presence of excess MF latex particles is confirmed, not only in the background, but also on the surface of the microcapsules. In contrast, Fig. 7C and D show MF/Laponite hybrid microcapsules prepared in the absence of poly (acrylamide-co-sodium acrylate). These microcapsules are much more rigid than those prepared in the presence of poly (acrylamide-co-sodium acrylate): they remain free-standing even under UHV conditions and retain their original spherical morphology. There is no evidence for any MF latex and the shell walls appear to be relatively smooth. However, many of these microcapsules have ruptured, revealing their hollow interior. This observation suggests that they may be rather more brittle than microcapsules prepared in the absence of poly(acrylamideco-sodium acrylate). Thus, although this water-soluble stabiliser
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
79
Fig. 7. Scanning electron microscopy images recorded after an ethanol challenge followed by drying in air at 20 °C for: (A and B) MF/Laponite hybrid microcapsules prepared in the presence of poly(acrylamide-co-sodium acrylate); (C and D) MF/Laponite hybrid microcapsule prepared in the absence of poly(acrylamide-co-sodium acrylate).
is not essential for the production of hybrid microcapsules, its inclusion appears to result in more durable microcapsule walls, which may be important for certain microencapsulation applications. 4. Conclusions The formation of novel inorganic–organic hybrid microcapsules based on coating Magnafloc/Laponite stabilised Pickering emulsions with a melamine–formaldehyde (MF) overlayer is described. Empirically, it was found that Magnafloc/Laponite particles prepared using a relatively low amount of Magnafloc (10% by mass based on Laponite, i.e. well below the ‘knee’ of the adsorption isotherm) conferred optimal Pickering emulsifier performance. Moreover, these particles stabilised Pickering emulsions at pH 4, which is the solution acidity required for MF deposition. Inorganic– organic hybrid microcapsules were prepared in the presence and absence of poly(acrylamide-co-sodium acrylate). The presence of this water-soluble polymeric stabiliser led to robust microcapsules that can survive both an alcohol challenge and also the UHV conditions required for scanning electron microscopy studies. However, this copolymer also facilitates the formation of excess MF latex in the aqueous continuous phase, since it can electrostatically adsorb onto these particles and act as a steric stabiliser. Provided that the Magnafloc is employed at submonolayer coverage, the precipitating nascent cationic MF nuclei can adsorb onto the partiallycoated anionic Laponite particles, which removes the requirement for the poly(acrylamide-co-sodium acrylate). In the absence of this copolymer, the secondary amine groups on the Magnafloc chains most likely react with the MF, leading to chemical grafting. Thus secondary nucleation of MF latex particles can be avoided, which
allows more efficient MF deposition at the surface of the emulsion droplets. However, the resulting MF shells appear to be more brittle, as SEM studies reveal that extensive rupture of the microcapsules occurs after addition of excess alcohol. However, such embrittlement may actually be beneficial for ‘consumer-activated’ fragrance release in certain laundry products. Thus we conclude that microcapsule preparation via Route B appears to offer a number of advantages over Route A. Future work will focus on establishing the advantages (if any) of encapsulation of various hydrophobic actives within these new hybrid inorganic/organic microcapsules using Route B. Acknowledgments We thank P & G Technical Center (Newcastle-upon-Tyne, UK) for an Industrial EPSRC CASE studentship to support MW and also for permission to publish this work. The three reviewers of this manuscript are thanked for their helpful comments.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2015.08.044. References [1] W. Ramsden, Separation of solids in the surface-layers of solutions and ’Suspensions’ (observations on surface-membranes, bubbles, emulsions, and mechanical coagulation). Preliminary Account, Proc. Royal Soc. London 72 (1903) 156–164. [2] S.U. Pickering, Emulsions, J. Chem. Soc. 91 (1907) 2001–2021.
80
M. Williams et al. / Journal of Colloid and Interface Science 460 (2015) 71–80
[3] A.D. Dinsmore, M.F. Hsu, M.G. Nikolaides, M. Marquez, A.R. Bausch, D.A. Weitz, Colloidosomes: selectively permeable capsules composed of colloidal particles, Science 298 (2002) 1006–1009. [4] S. Laib, A.F. Routh, Fabrication of colloidosomes at low temperature for the encapsulation of thermally sensitive compounds, J. Colloid Interface Sci. 317 (2008) 121–129. [5] O.J. Cayre, P.F. Noble, V.N. Paunov, Fabrication of novel colloidosome microcapsules with gelled aqueous cores, J. Mater. Chem. 14 (2004) 3351– 3355. [6] K.L. Thompson, S.P. Armes, J.R. Howse, S. Ebbens, I. Ahmad, J.H. Zaidi, D.W. York, J.A. Burdis, Covalently cross-linked colloidosomes, Macromolecules 43 (2010) 10466–10474. [7] A. Walsh, K.L. Thompson, S.P. Armes, D.W. York, Polyamine-functional sterically stabilized latexes for covalently cross-linkable colloidosomes, Langmuir 26 (2010) 18039–18048. [8] L.A. Fielding, S.P. Armes, Preparation of Pickering emulsions and colloidosomes using either a glycerol-functionalised silica sol or core–shell polymer/silica nanocomposite particles, J. Mater. Chem. 22 (2012) 11235–11244. [9] Q. Yuan, O.J. Cayre, S. Fujii, S.P. Armes, R.A. Williams, S. Biggs, Responsive core– shell latex particles as colloidosome microcapsule membranes, Langmuir 26 (2010) 18408–18414. [10] H.N. Yow, A.F. Routh, Release profiles of encapsulated actives from colloidosomes sintered for various durations, Langmuir 25 (2009) 159–166. [11] K.L. Thompson, S.P. Armes, From well-defined macromonomers to stericallystabilised latexes to covalently cross-linkable colloidosomes: exerting control over multiple length scales, Chem. Commun. 46 (2010) 5274–5276. [12] A.R. Bausch, M.J. Bowick, A. Cacciuto, A.D. Dinsmore, M.F. Hsu, D.R. Nelson, M. G. Nikolaides, A. Travesset, D.A. Weitz, Grain boundary scars and spherical crystallography, Science 299 (2003) 1716–1718. [13] M. Williams, S.P. Armes, D.W. York, Clay-based colloidosomes, Langmuir 28 (2012) 1142–1148. [14] T. Chen, P.J. Colver, S.A.F. Bon, Organic–inorganic hybrid hollow spberes prepared from TiO2-stabilized pickering emulsion polymerization, Adv. Mater. 19 (2007) 2286–2289. [15] W. Chen, X. Liu, Y. Liu, H.-I. Kim, Synthesis of microcapsules with polystyrene/ ZnO hybrid shell by Pickering emulsion polymerization, Colloid Polym. Sci. 288 (2010) 1393–1399. [16] H. Liu, C. Wang, Q. Gao, X. Liu, Z. Tong, Magnetic hydrogels with supracolloidal structures prepared by suspension polymerization stabilized by Fe2O3 nanoparticles, Acta Biomater. 6 (2010) 275–281. [17] Q. Gao, C. Wang, H. Liu, C. Wang, X. Liu, Z. Tong, Suspension polymerization based on inverse Pickering emulsion droplets for thermo-sensitive hybrid
[18]
[19]
[20] [21] [22]
[23]
[24]
[25] [26] [27] [28] [29]
[30]
[31] [32]
[33]
microcapsules with tunable supracolloidal structures, Polymer 50 (2009) 2587–2594. E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Mohwald, Novel hollow polymer shells by colloid-templated assembly of polyelectrolytes, Angew. Chem.-Int. Ed. 37 (1998) 2202–2205. A.P.R. Johnston, C. Cortez, A.S. Angelatos, F. Caruso, Layer-by-layer engineered capsules and their applications, Curr. Opin. Colloid Interface Sci. 11 (2006) 203–209. C.S. Peyratout, L. Dahne, Tailor-made polyelectrolyte microcapsules: from multilayers to smart containers, Angew. Chem.-Int. Ed. 43 (2004) 3762–3783. J. Li, H.D.H. Stover, Pickering emulsion templated layer-by-layer assembly for making microcapsules, Langmuir 26 (2010) 15554–15560. X. Wang, W. Zhou, J. Cao, W. Liu, S. Zhu, Preparation of core–shell CaCO3 capsules via Pickering emulsion templates, J. Colloid Interface Sci. 372 (2012) 24–31. H. Strohm, P. Lobmann, Porous TiO2 hollow spheres by liquid phase deposition on polystyrene latex-stabilised Pickering emulsions, J. Mater. Chem. 14 (2004) 2667–2673. J. Li, A.P. Hitchcock, H.D.H. Stover, Pickering emulsion templated interfacial atom transfer radical polymerization for microencapsulation, Langmuir 26 (2010) 17926–17935. S. Luzius, Process for the Encapsulation of Dispersible Materials, US 3594328 A, 1971. P.L. Foris, R.W. Brown, P.S. Phillips Jr., In situ Polymerization, Microencapsulation, US 4100103 A, 1978. Y. Lvov, A.A. Antipov, A. Mamedov, H. Mohwald, G.B. Sukhorukov, Urease encapsulation in nanoorganized microshells, Nano Lett. 1 (2001) 125–128. G. Sun, Z. Zhang, Mechanical properties of melamine–formaldehyde microcapsules, J. Microencapsul. 18 (2001) 593–602. X.X. Zhang, X.M. Tao, K.L. Yick, X.C. Wang, Structure and thermal stability of microencapsulated phase-change materials, Colloid Polym. Sci. 282 (2004) 330–336. X.X. Zhang, Y.F. Fan, X.M. Tao, K.L. Yick, Fabrication and properties of microcapsules and nanocapsules containing n-octadecane, Mater. Chem. Phys. 88 (2004) 300–307. K. Hong, S. Park, Melamine resin microcapsules containing fragrant oil: synthesis and characterization, Mater. Chem. Phys. 58 (1999) 128–131. Y. Long, D. York, Z. Zhang, J.A. Preece, Microcapsules with low content of formaldehyde: preparation and characterization, J. Mater. Chem. 19 (2009) 6882–6887. Y. Long, B. Vincent, D. York, Z. Zhang, J.A. Preece, Organic–inorganic double shell composite microcapsules, Chem. Commun. 46 (2010) 1718–1720.