Colloid and Polymer Science

, Volume 294, Issue 1, pp 1–12 | Cite as

Preparation of non-aqueous Pickering emulsions using anisotropic block copolymer nanoparticles

  • S. L. Rizzelli
  • E. R. Jones
  • K. L. Thompson
  • S. P. Armes
Invited Article – Award Contribution

Abstract

In this work, we show that amphiphilic diblock copolymer worms prepared via alcoholic RAFT dispersion polymerization can be used to stabilize non-aqueous Pickering emulsions. A previously reported synthesis protocol based on polymerization-induced self-assembly (PISA) was modified to enable the preparation of poly(2-(dimethylamino)ethyl methacrylate)-poly(benzyl methacrylate) (PDMA-PBzMA) worm-like particles directly in methanol at relatively high solids. A dilute dispersion of these highly anisotropic nanoparticles was then homogenized with sunflower oil to produce sunflower oil-in-methanol emulsions. The mean droplet diameter ranged from 9 to 104 μm, depending on the nanoparticle concentration and the stirring rate used for homogenization. The sunflower oil content was increased systematically, with stable emulsions being obtained up to a volume fraction of 0.60. In all cases, the sunflower oil droplets gradually increase in size on ageing for up to 4 days. However, stable emulsions were obtained after this time period, with no further change in the mean droplet diameter for at least 2 months on standing at ambient temperature. Turbidimetry studies of the continuous phase after sedimentation of the relatively dense emulsion droplets indicated that the initial adsorption efficiency of the PDMA-PBzMA worms is very high, but this is reduced significantly as the droplet diameter gradually increases during ageing. There is a concomitant increase in fractional surface coverage over the same time period, suggesting that the increase in droplet diameter is the result of limited coalescence, rather than an Ostwald ripening mechanism.

Keywords

RAFT polymerization Self-assembly Pickering emulsions Non-aqueous emulsions Block copolymers Nanoparticles 

Introduction

A Pickering emulsion comprises either oil or water droplets that are stabilized by solid particles adsorbed at the oil/water interface. This type of emulsion was first reported by Ramsden over a century ago [1] but Pickering’s subsequent studies received more credit and led to today’s nomenclature [2]. Various classes of solid particles have been employed as Pickering emulsifiers, including silica [3, 4, 5, 6, 7], inorganic clays [8, 9, 10, 11, 12], and organic polymer latexes [13, 14, 15, 16, 17, 18]. Either oil-in-water (o/w) or water-in-oil (w/o) Pickering emulsions can be obtained, with the emulsion type mainly depending on the contact angle (θ) between the particles and the interface. Thus, hydrophilic particles possess contact angles less than 90° and reside in the aqueous phase, favoring the stabilization of o/w emulsions. In contrast, hydrophobic particles are characterized by contact angles greater than 90° and hence are preferentially located within the oil phase, resulting in the formation of w/o emulsions.

The main driving force for the interfacial adsorption of particles is the reduction in surface area. The energy required to detach spherical particles adsorbed at the interface is dictated by the contact angle and the particle radius [19, 20]. In many cases, larger particles can be considered to be essentially irreversibly adsorbed, since the detachment energy is several orders of magnitude greater than the particle thermal energy [19, 20]. Since the adsorbed particle layer prevents droplet coalescence by providing a strong steric barrier, Pickering emulsions tend to be far more stable than surfactant-stabilized emulsions [21]. Moreover, Pickering emulsions also offer several other advantages, such as reduced foaming during homogenization, more reproducible formulations, and lower toxicity [20, 21].

Although far less commonly reported than w/o or o/w emulsions, there are various examples of non-aqueous emulsions in the literature [22, 23, 24, 25, 26]. Such systems require a pair of immiscible solvents [27]. In principle, non-aqueous emulsions could be utilized for water-sensitive reactions or polymerizations [26, 28, 29], for reactions that need to be conducted above the normal boiling point of water [30] or for specific applications where the presence of water is problematic [31, 32].

In 2004, two research groups led by Paunov and Velev reported the formation of colloidosomes, Pickering emulsions, and foams stabilized using “polymeric microrods” [33, 34], rather than conventional spherical particles. These microrods were prepared from epoxy-type photoresist SU-8 using the liquid-liquid dispersion technique and possessed relatively large dimensions (mean rod length = 23.5 μm; mean rod width = 0.6 μm). More recently, we have described the use of much smaller hydrophilic diblock copolymer worms as Pickering emulsifiers for the preparation of o/w [35] or w/o [36] emulsions. In principle, such highly anisotropic particles should be more strongly adsorbed than precursor spherical particles (i.e., whose mean diameter is comparable to the mean worm width). Other research teams have also reported that anisotropic particles are highly effective Pickering emulsifiers [37, 38]. For example, Madivala et al. found that the emulsion droplet stability depended strongly on the particle aspect ratio when using elongated hematite or polystyrene latex particles [39]. Similarly, a recent study by Kalashnikova et al. reported the use of cellulose nanorods to form Pickering emulsions. Interestingly, it was found that too high an aspect ratio enabled these particles to adsorb simultaneously onto multiple droplets, rather than stabilizing individual droplets [40].

The recent development of living radical polymerization techniques [41, 42, 43, 44, 45, 46] has provided a facile route for the production of diblock copolymer nanoparticles based on polymerization-induced self-assembly (PISA) [47, 48, 49, 50, 51, 52, 53, 54]. In particular, the ability to form well-defined amphiphilic diblock copolymers via reversible addition-fragmentation chain transfer (RAFT) polymerization under appropriate reaction conditions allows the in situ formation of copolymer nanoparticles with either spherical, worm-like, or vesicular morphologies [52, 55, 56, 57, 58, 59]. Moreover, this approach has proven to be highly versatile, with all three copolymer morphologies being reported as pure phases in either water [52, 60, 61], alcohol [55, 58, 62, 63, 64, 65], or n-alkanes [59, 66, 67]. Aqueous emulsion polymerization has been extensively researched by Charleux and coworkers [50, 68, 69, 70, 71, 72, 73, 74]. However, RAFT dispersion polymerization formulations are arguably rather more versatile [58, 59, 61, 62, 64, 65, 75, 76, 77, 78, 79, 80]. Of particular relevance to the present study, Thompson et al. have recently shown that block copolymer worms prepared via RAFT PISA are effective stabilizers for non-aqueous emulsions comprising ethylene glycol droplets within various n-alkanes [81]. Herein, we utilize a RAFT PISA formulation to conveniently prepare diblock copolymer worms directly in methanol. These flexible, highly anisotropic nanoparticles are then examined as putative Pickering emulsifiers for the preparation of new non-aqueous emulsions composed of sunflower oil droplets dispersed in a methanolic continuous phase. Thus, this new non-aqueous emulsion formulation is complementary to that reported by Thompson et al. [81].

Materials and methods

Ethanol was obtained from VWR Chemicals (UK) and n-hexane was purchased from Fisher Scientific (UK). All other reagents were purchased from Sigma-Aldrich (UK) and were used as received unless otherwise noted. Either 4,4′-azobis(4-cyanovaleric acid) (ACVA) or 2,2′-azobis(isobutyronitrile) (AIBN) were used as initiators. Benzyl methacrylate (BzMA) (96 %; Sigma-Aldrich) was passed through an inhibitor removal column prior to use. 4-Cyano-4-(2-phenyl-ethanesulfanylthiocarbonyl) sulfanylpentanoic acid (PETTC) was prepared in-house as reported previously [79].

Synthesis of poly(2-(dimethylamino)ethyl methacrylate) (PDMA) macro-CTA agent

A round-bottomed flask was charged with 2-(dimethylamino)ethyl methacrylate (DMA; 40.0 g, 254 mmol), PETTC (2.156 g, 6.36 mmol; target DP = 40), ACVA (178 mg, 0.636 mmol; PETTC/ACVA molar ratio = 10) and THF (40.0 g). The sealed reaction vessel was purged with nitrogen and placed in a pre-heated oil bath at 66 °C for 6 h. The resulting crude PDMA (monomer conversion = 77 %; Mn = 6500 g mol−1, Mw/Mn = 1.22) was purified by precipitation into excess petroleum ether. The mean degree of polymerization (DP) of this PDMA macro-CTA was calculated to be 43 using 1H NMR spectroscopy by comparing the integrated signals corresponding to the aromatic protons at 7.2–7.4 ppm with those assigned to the methacrylic polymer backbone at 0.4–2.5 ppm.

Synthesis of poly(2-(dimethylamino)ethyl methacrylate)-poly(benzyl methacrylate) (PDMA-PBzMA) diblock copolymer particles via dispersion polymerization in methanol

In a typical RAFT dispersion polymerization synthesis conducted at 15 % w/w total solids, BzMA (2.0 g, 11.4 mmol), PDMA43 macro-CTA (0.85 g, 0.119 mmol; target DP = 95) and AIBN (3.9 mg, 0.024 mmol; macro-CTA/AIBN molar ratio = 5) were dissolved in methanol (16.16 g). The reaction mixture was sealed in a round-bottomed flask, purged with nitrogen gas for 15 min, and then placed in a preheated oil bath at 64 °C for 24 h. The final monomer conversion was determined by 1H NMR analysis by comparing the integral due to the two benzylic protons assigned to the PBzMA block at 4.9 ppm to that of the BzMA monomer vinyl signals at 5.2 and 5.4 ppm.

Copolymer characterization

Diblock copolymer molecular weight distributions were assessed using gel permeation chromatography (GPC). The GPC set-up comprised two 5 μm (30 cm) “Mixed C” columns and a WellChrom K-2301 refractive index detector operating at 950 ± 30 nm. THF eluent contained 2.0 % v/v triethylamine and 0.05 % w/v butylhydroxytoluene (BHT) was used at a flow rate of 1.0 ml min−1. A series of ten near-monodisperse linear poly(methyl methacrylate) standards (Mp ranging from 1280 to 330,000 g mol−1) were purchased from Polymer Laboratories (Church Stretton, UK) and employed for calibration using the above refractive index detector.

1H NMR spectra were acquired on a Bruker 400 MHz spectrometer in either CDCl3 or CD2Cl2. All chemical shifts are reported in ppm (δ). DLS measurements were conducted using a Malvern Instruments Zetasizer Nano series instrument equipped with a 4 mW He-Ne laser operating at 633 nm, an avalanche photodiode with high quantum efficiency, and an ALV/LSE-5003 multiple tau digital correlator electronics system.

TEM studies were conducted using a Philips CM 100 instrument operating at 100 kV equipped with a Gatan 1 k CCD camera. Solutions were diluted with methanol at 20 °C to generate 0.20 % w/w dispersions. Copper/palladium TEM grids (Agar Scientific, UK) were surface-coated in-house to yield a thin film of amorphous carbon. The grids were then plasma glow-discharged for 30 s to create a hydrophilic surface. Each methanolic diblock copolymer dispersion (0.20 % w/w, 10 μl) was placed onto a freshly glow-discharged grid for 1 min and then blotted with filter paper to remove excess solution. To stain the deposited nanoparticles, 10 μl of a 0.75 % w/w aqueous solution of uranyl formate was placed on the sample-loaded grid via micropipet for 20 s and then carefully blotted to remove excess stain. Each grid was then carefully dried using a vacuum hose.

Preparation of Pickering emulsions

Sunflower oil (5.0 ml) was homogenized with 5.0 ml of a 0.01–2.65 % w/w methanol copolymer dispersion for 2 min at 20 °C using a IKA Ultra-Turrax T-18 homogenizer equipped with a 10 mm dispersing tool operating at between 3500 and 13,500 rpm. The homogenizer was washed thoroughly using methanol between samples to ensure that there was no cross-contamination.

Optical microscopy

Optical microscopy images of Pickering emulsion droplets were recorded using a Motic DMBA300 digital biological microscope equipped with a built-in camera and analyzed using Motic Images Plus 2.0 ML software.

Laser diffraction

Each emulsion was sized in methanol using a Malvern Mastersizer 2000 instrument equipped with a small volume Hydro 2000SM sample dispersion unit (ca. 50 ml), a He-Ne laser operating at 633 nm and a solid-state blue laser operating at 466 nm. The stirring rate was adjusted to 500 rpm in order to avoid droplet coalescence. After each measurement, the cell was rinsed once with doubly distilled water, followed by rinsing with first ethanol and then methanol. The glass walls of the cell were carefully wiped with lens cleaning tissue to avoid cross-contamination and the laser was aligned centrally to the detector prior to data acquisition.

Determination of Pickering emulsifier adsorption efficiency via turbidimetry

UV spectra were recorded at 20 °C for the PDMA43-PBzMA94 worms in methanol using a Shimadzu UV-1800 instrument operating between 400 and 800 nm. A linear calibration plot of absorbance versus concentration at an arbitrary wavelength of 430 nm with known concentrations of copolymer dispersed in methanol was constructed in order to determine the nanoparticle adsorption efficiency at the oil-ethanol interface. This was assessed by analysis of the (upper) methanol continuous phase after sedimentation of the relatively dense sunflower oil droplets had occurred on standing for 24 h (or longer) at 20 °C. The remaining non-adsorbed worms were detected and thus the fraction of adsorbed worms was calculated by difference.

Results and discussion

PDMA-PBzMA diblock copolymer nanoparticles were synthesized by RAFT alcoholic dispersion polymerization of benzyl methacrylate (BzMA) in methanol at 64 °C, see Scheme 1. A similar ethanolic PISA formulation has been recently reported [58, 64]. Unfortunately, ethanol is miscible with most oils of interest. Hence methanol was selected as the continuous phase since this more polar solvent was more likely to allow the preparation of non-aqueous emulsions. As for the earlier ethanolic PISA formulation reported by our group [64], if the PDMA stabilizer block was sufficiently short, a range of nanoparticle morphologies could be produced by simply varying the mean degree of polymerization of the core-forming PBzMA block, see Fig. 1.
Scheme 1

Synthesis of a poly(2-(dimethylamino)ethyl methacrylate) (PDMA) macro-CTA and subsequent chain extension with benzyl methacrylate (BzMA) via RAFT alcoholic dispersion polymerization at 64 °C to produce sterically stabilized PDMA-PBzMA diblock copolymer worm-like nanoparticles via polymerization-induced self-assembly (PISA)

Fig. 1

TEM images obtained for PDMA43-PBzMAx diblock copolymer nanoparticles prepared at 15 % w/w solids via RAFT dispersion polymerization of BzMA in methanol at 64 °C using a PDMA43 macro-CTA, AIBN initiator, and a (macro-CTA)/(AIBN) molar ratio of 5.0. Varying the DP of the core-forming PBzMA results in either a spheres (x = 60), b worms (x = 94), or c vesicles (x = 200). d Higher magnification image of b, indicating a mean worm width of 20 nm

For this study, worm-like micelles was the desired copolymer morphology. Such highly anisotropic nanoparticles can be obtained by targeting a mean PBzMA DP of 95. Approximately 99 % BzMA conversion was achieved within 24 h as judged by 1H NMR spectroscopy, which suggested a mean DP of 94 for the core-forming PBzMA block. THF GPC indicated a mean number-average molecular weight of 12,800 g mol−1 and a polydispersity (Mw/Mn) of 1.08. The representative TEM images shown in Fig. 1b, d confirm a well-defined worm morphology, with a mean worm width of 20 nm. However, these worms exhibit considerable polydispersity in length, ranging from less than 1 μm up to around 5 μm, as estimated from TEM images. This is typical for such PISA syntheses, since the worms are formed via random sphere-sphere fusion events during the in situ RAFT polymerization. A sphere-equivalent hydrodynamic diameter of 609 nm (polydispersity = 0.50) was determined at 25 °C using dynamic light scattering (DLS). (N.B. DLS measurements are based on the Stokes-Einstein equation, which assumes a spherical particle morphology; hence, the data obtained for such highly anisotropic worms should be treated with caution.) THF GPC data recorded for the final diblock copolymer indicates a relatively high blocking efficiency for the PDMA macro-CTA and a narrow molecular weight distribution, which are consistent with a well-controlled RAFT polymerization. Various oils were evaluated for homogenization with the methanolic copolymer dispersion, see Table 1. A range of n-alkanes were evaluated in addition to sunflower oil. However, for n-octane, n-dodecane, n-tetradecane, or n-hexadecane, the resulting emulsions were only stable for a few hours, if formed at all. In contrast, Pickering emulsions with good long-term stability could be consistently obtained using sunflower oil. Thus, only this latter oil was selected for further studies.
Table 1

Attempted Pickering emulsification of various oils using a methanolic dispersion containing 0.66 % w/w PDMA43-PBzMA94 diblock copolymer worms. Homogenization conditions: 13,500 rpm for 2 min at 20 °C using a sunflower oil volume fraction of 0.50

Oil phase

Emulsion

Comments

Sunflower Oil

Yes

Stable emulsion

n-Octane

No

Initial emulsion, but demulsified after 2–3 h

n-Dodecane

No

Complete phase separation

n-Tetradecane

No

Complete phase separation

n-Hexadecane

No

Complete phase separation

Isopropyl myristate

No

Miscible with methanol

Scheme 2 depicts the schematic representation of the formation of a PDMA43-PBzMA94 worm-stabilized sunflower oil-in-methanol Pickering emulsion. Figure 2a shows how the sunflower oil diameter varies with PDMA43-PBzMA94 worm concentration; these laser diffraction measurements were recorded immediately after homogenization. Figure 2b shows the same trend, but in this case measurements were recorded on 4-day-old emulsions. Optical microscopy images (see Figure S1) support the data sets shown in Fig. 2a, b: lower particle concentrations produce larger sunflower oil droplets. This relationship has been reported for many other Pickering emulsions [35]. This is because there are more nanoparticles available to coat and stabilize the oil droplet surface at higher copolymer concentrations, thus enabling the formation of smaller droplets. At a copolymer concentration of 0.66 % w/w, the freshly made emulsions had a volume-average droplet diameter of 9 ± 6 μm, as judged by laser diffraction. However, this mean diameter increased up to 39 ± 21 μm on standing at 20 °C for 4 days. Following this discovery, fresh emulsions were prepared and the evolution in the droplet diameter was monitored daily. After 4 days, no further increase in droplet diameter was observed and the resulting relatively coarse emulsions remained stable for at least 2 months on storage at 20 °C; optical microscopy images recorded over a period of 7 days are shown in Fig. 2c.
Scheme 2

Homogenization of 0.66 % w/w PDMA43-PBzMA94 diblock copolymer worms in methanol with sunflower oil at 13,500 rpm at 20 °C for 2 min produces stable sunflower oil-in-methanol Pickering emulsions

Fig. 2

a Mean droplet diameter versus copolymer concentration for the PDMA43-PBzMA94 worms for the freshly prepared emulsion and b the same emulsions after 4 days standing at 20 °C, as determined using laser diffraction. In both cases, the error bars represent the standard deviation of each mean volume-average droplet diameter, rather than the experimental error. c Optical microscopy images recorded for the fresh and aged emulsions prepared at 0.66 % w/w worm concentration measured over a period of 7 days. The 200 μm scale bar in the first image applies to all images

The sunflower oil volume fraction was systematically varied between 0.10 and 0.90 to assess the efficiency of emulsification. Stable emulsions were formed up to an oil volume fraction of 0.60; using higher volume fractions of sunflower oil did not produce stable emulsions. Figure 3 shows the optical microscopy images obtained for a series of emulsions prepared using 0.66 % w/w PDMA43-PBzMA94 worms at various sunflower oil volume fractions.
Fig. 3

Optical microscopy images recorded for sunflower oil-in-methanol Pickering emulsions prepared using 0.66 % w/w PDMA43-PBzMA94 worms at sunflower oil volume fractions of between 0.10 and 0.60. The 200 μm scale bar shown in the first image applies to all images

Next, we investigated whether the shear rate affected the mean droplet diameter. Six emulsions were prepared at 20 °C using equal volumes of sunflower oil and methanol via homogenization for 2 min using stirring speeds ranging between 3500 and 24,000 rpm. Figure 4 shows that the droplet diameter is significantly reduced at higher stirring speeds. This was expected, as greater shear creates a higher droplet surface area.
Fig. 4

a Mean laser diffraction droplet diameter versus stirring speed for sunflower oil-in-methanol emulsions prepared with 0.66 % w/w PDMA43-PBzMA94 worms using equal volumes of methanol and sunflower oil. The error bars represent the standard deviation of each mean volume-average droplet diameter, rather than the experimental error. b Optical microscopy images recorded for homogenization conducted at stirring speeds of between 3500 and 24,000 rpm. The 200 μm scale bar in the first image applies to all images

The fractional surface coverage, Cw, for the worms adsorbed onto the sunflower oil droplets was calculated by dividing the total surface area of the adsorbed worms by the total surface area of the droplets to afford Eq. (1), as reported previously by Kalashnikova and coworkers [82].
$$ C\mathrm{w}=\frac{m_{\mathrm{p}}D}{6{\rho}_{\mathrm{p}}{h}_{\mathrm{p}}V\mathrm{d}} $$
(1)
The mean droplet diameter, D, was determined by laser diffraction, mp is the nanoparticle mass, ρp is the nanoparticle density (1.15 g cm−3 for the PBzMA core-forming block, as determined by helium pycnometry) and Vd is the total volume of the oil droplet phase (which is fixed at 5.0 ml in these experiments). In this case hp represents the mean worm thickness of 20 nm, as estimated from TEM images. The fractional surface coverage, Cw, was calculated for the various worm-stabilized emulsions and these data are summarized in Table 2. These Cw values are typically less than unity, but in two cases they exceed unity. This is interpreted as evidence for (partial) bilayer formation, as previously reported by Kalashnikova et al. for similarly anisotropic cellulosic nanocrystals [40, 82]. In each case, the adsorption efficiency of the particles was determined 24 h after initial emulsification using turbidimetry at an arbitrary fixed wavelength of 430 nm, see Fig. 5. This time period was sufficient to ensure complete sedimentation of the relatively dense sunflower oil droplets, leaving only non-adsorbed worms in the methanolic continuous phase. The adsorption efficiency indicates the proportion of the initial worms that actually become adsorbed at the droplet surface. This efficiency is reduced from 99 to 82 % on lowering the worm concentration. The mean droplet diameter increased significantly for up to 4 days after homogenization, before attaining a constant value. Turbidimetry studies were repeated 7 days after homogenization (i.e., long after the droplet diameter had stabilized); these data showed the worm adsorption efficiency had decreased. However, no significant change in worm adsorption efficiency was observed thereafter, while laser diffraction studies of emulsions aged for several months at 20 °C confirmed their long-term droplet stability.
Table 2

Effect of varying the PDMA43-PBzMA94 worm concentration on the mean droplet diameter, fractional surface coverage (Cw) and the adsorption efficiency of the worms on the sunflower oil droplets

Concentration

Initial emulsion after 24 h

Aged emulsion after 7 days

Aged emulsions after 2 months

Mean laser diffraction droplet diameter (μm)

Cw

Pickering emulsion adsorption efficiency (%)

Mean laser diffraction droplet diameter (μm)

Cw

Pickering emulsion adsorption efficiency (%)

Mean laser diffraction droplet diameter (μm)

1.32 % w/w

9 ± 5

0.85

99

37 ± 30

2.12

60

40 ± 32

0.66 % w/w

9 ± 6

0.42

98

39 ± 21

1.25

67

39 ± 16

0.33 % w/w

14 ± 7

0.33

97

43 ± 18

0.82

80

44 ± 17

0.04 % w/w

48 ± 28

0.13

91

77 ± 31

0.19

85

79 ± 29

0.02 % w/w

53 ± 28

0.06

82

116 ± 60

0.11

68

104 ± 67

Fig. 5

Visible absorption spectra recorded for methanolic dispersions of PDMA43-PBzMA94 worms at various concentrations between 400 and 800 nm. An arbitrary wavelength of 430 nm was used to construct a linear calibration plot (see inset), which was used to determine the concentration of free copolymer worms present in the methanol continuous phase, after emulsification and subsequent sedimentation of the relatively dense sunflower oil droplets

We postulate the following mechanism to account for our experimental observations. After initial homogenization, the surface of the droplets is only partially covered by the worms, which have a relatively high adsorption efficiency. Thus, the droplets are able to undergo limited coalescence, which lowers the total interfacial area and hence increases the fractional surface coverage, Cw, of the sunflower oil droplets, see Fig. 6. Coalescence no longer occurs when the droplet surface is sufficiently coated by the worms. Indeed, the fractional surface coverages calculated for the 7-day aged emulsions are significantly greater than the corresponding initial Cw values, see Table 2. This suggests that the worm fractional surface coverage gradually increases as the emulsion ages. There is a concomitant reduction in the worm adsorption efficiency, indicating desorption of some of the worms from the droplet surface into the continuous phase, see Fig. 6.
Fig. 6

Proposed mechanism for the observed increase in mean droplet diameter for PDMA43-PBzMA94 worm-stabilized sunflower oil-in-methanol Pickering emulsions. The initial droplets formed immediately after emulsification are relatively small, with a patchy coating of worms adsorbed at the methanol-sunflower oil interface with relatively high efficiencies (82–99 %). On ageing for approximately 4 days, some of the initial droplets undergo limited coalescence to form appreciably larger droplets, with a rather higher fractional surface coverage of adsorbed worms and a significant fraction of non-adsorbed worms now residing in the methanolic continuous phase. This coarser emulsion remained stable for at least several months

In principle, an alternative mechanism for the observed increase in emulsion size could be Ostwald ripening. According to Weidner and coworkers, the solubility of sunflower oil in methanol is approximately 0.5–1.0 % w/w at 20 °C [83]. Thus, Ostwald ripening might occur for the present Pickering emulsion formulation via gradual diffusion of the sparingly soluble sunflower oil from smaller to larger droplets. However, this explanation does not appear to be consistent with the experimental observations. Interfacial adsorption of the worms is expected to be strong and essentially irreversible. Thus, if such sunflower oil diffusion occurred, both an increase in mean droplet diameter and a reduction in Cw would be expected. In practice, only the former change is observed—the worm surface coverage actually increases as the emulsion coarsens on ageing. In summary, we suggest that the increase in emulsion droplet dimensions over time is most likely the result of a limited coalescence mechanism. However, one reviewer of this manuscript has suggested that our experimental observations may be consistent with Ostwald ripening, provided that the worms released after preferential dissolution of the smaller droplets are partially readsorbed onto the growing larger droplets. It seems that further studies are warranted to clarify the true situation, but unfortunately, this is beyond the scope of the present study.

The spontaneous formation of methanol-in-sunflower oil-in-methanol Pickering double emulsions was also observed, see Fig. 7. The presence of such double emulsions was confirmed by optical microscopy studies of freshly prepared emulsions at all worm concentrations used in this work (0.02 % w/w to 2.65 % w/w). However, when the aged emulsions were re-examined after 7 days (i.e., after limited coalescence had occurred), double emulsions were only observed for emulsions prepared at the higher worm concentrations (0.66, 1.35, and 2.65 % w/w). In these three cases, a significant proportion of double emulsion droplets were still present, indicating that such double emulsions are stable beyond the period of droplet coalescence, see Fig. 7. The precise mechanism of this double emulsion formation is not understood at the present time, but clearly warrants further studies.
Fig. 7

Optical microscopy images indicating the presence of methanol-in-sunflower oil-in-methanol double emulsions within a sunflower oil-in-methanol emulsion prepared by homogenizing a methanolic dispersion of 0.66 % w/w PDMA43-PBzMA94 worms with an equal volume of sunflower oil for 2 min at a stirring speed of 13,500 rpm at 20 °C. a immediately after homogenization and b after ageing for 7 days

Conclusions

Sunflower oil-in-methanol Pickering emulsions can be prepared using PDMA43-PBzMA94 diblock copolymer worms as Pickering emulsifiers. Increasing the particle concentration allows stabilization of smaller droplets (<10 μm at 1.32 % w/w) but emulsions formed at particle concentrations as low as 0.02 % w/w also remained  stable (average droplet diameter 53 μm). Systematically varying the stirring speed during homogenization produced emulsions with adjustable diameters up to a sunflower oil volume fraction of 0.60. Turbidimetry studies were employed to assess Pickering adsorption efficiency and an appreciable increase in mean droplet diameter was observed on ageing at ambient temperature. On closer inspection, no further increase in droplet diameter occurred after around 4 days. At all worm concentrations investigated, Pickering adsorption efficiencies were lower for 7-day-old emulsions than for the initial emulsion, while the worm fractional surface coverage increased significantly on this time scale. After this ageing period, the droplet diameter remained essentially unchanged for at least 2 months. Based on these experimental observations, we suggest that this increase in droplet diameter is the result of limited coalescence. Methanol-in-sunflower oil-in-methanol double emulsion droplets were also observed for this non-aqueous Pickering emulsion formulation and this observation warrants further studies.

Notes

Acknowledgments

We thank DSM Advanced Surfaces (Geleen, The Netherlands) and EPSRC for a CASE PhD studentship for ERJ. SPA thanks EPSRC (platform grant EP/J007846/1) for post-doctoral support of KLT and also acknowledges partial project funding from Stichting Innovatie Alliantie (Foundation Innovation Alliance).

Supplementary material

396_2015_3785_MOESM1_ESM.docx (1.2 mb)
ESM 1(DOCX 1.21 MB)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • S. L. Rizzelli
    • 1
  • E. R. Jones
    • 1
  • K. L. Thompson
    • 1
  • S. P. Armes
    • 1
  1. 1.Department of Chemistry, Dainton BuildingUniversity of SheffieldSheffieldUK

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