Article pubs.acs.org/Langmuir
Particle-Stabilized Powdered Water-in-Oil Emulsions Bernard P. Binks* and Andrew T. Tyowua Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. ABSTRACT: The preparation of powdered water-in-oil (w/o) emulsions by gentle aeration of w/o emulsions stabilized by hydrophobic fumed silica particles in the presence of oleophobic fluorinated clay particles is reported for an alkane and a triglyceride oil. The resultant powders consist of water drops dispersed in oil globules themselves dispersed in air (w/o/a). They contain ∼80 wt % of the precursor w/o emulsion and were stable to phase separation for over 1 year but release oil and water when sheared on a substrate. Above a certain ratio of w/o emulsion:fluorinated clay particles, the powdered emulsions partially invert to an emulsion paste, composed of air bubbles and water droplets dispersed in oil. The tap density and angle of repose of the powdered emulsions were measured and compared with those of the corresponding powdered oils making up the continuous phase of the precursor emulsions. The contact angles of water droplets under oil on glass slides spin coated with silica particles and oil drops and w/o emulsion droplets in air on compressed disks of fluorinated clay particles are consistent with the stabilization of w/o emulsions and powdered emulsions, respectively.
■
liquid. This has been illustrated recently13 for different oils using a range of particle types. Another novel material that has been prepared by exploiting the surface-active character of small solid particles is powdered emulsions,15,16 although in the former reference a mixture of particles and surface-active polymer was used. Unlike powdered water or powdered oil materials which are a dispersion of particle-coated liquid droplets in air, a (w/a or o/a), powdered emulsions are a dispersion of emulsion globules in air, e.g. o/w/ a, requiring two types of particle with different wettability for coating the inner droplet interfaces (say o/w) and the outer globule interfaces (say w/a). To obtain a powdered emulsion, the inner emulsion droplets must to be stable to coalescence while the bulk continuous liquid phase is expected to have low affinity for the surfaces of the particles coating the emulsion globules. The first condition can be achieved by using surfactants or solid particles, but suitable small solid particles have been preferably used where powdered o/w emulsions were prepared.15,16 It may be that powdered water-in-oil, w/o, emulsions may similarly be prepared, but to the best of our knowledge they are yet to be reported. In this case, the second condition can be fulfilled by using oleophobic or omniphobic solid particles. Using two different particle types also of different shape, we describe the preparation and properties of powdered w/o emulsions. This is guided by our recent ability to prepare powdered oils (no water).13,14
INTRODUCTION The realization that certain small solid particles are surfaceactive at fluid interfaces has led to the creation of novel materials which would have been impossible to prepare in their absence. For example, liquid marbles1 and powdered (or dry) liquid materials2 have been prepared by taking advantage of the surface-active nature of certain particles at liquid−air interfaces. Liquid marbles are millimeter-sized particle-coated droplets while powdered liquid materials are a dispersion of smaller particle-coated liquid droplets in air. Such solid particles have little affinity for the liquid phase and protrude more into the air phase upon adsorption. The affinity (or particle wettability) is quantified in terms of the three-phase contact angle θ.3 For spherical particles at an air−water interface, θ is required to be above 90° and the particles are classified as hydrophobic.3 This is also the case with an air−oil interface where the particles are in this case oleophobic.4 Certain omniphobic particles5 have low affinity for both water and oils and exhibit θ in excess of 90° with them and can also be good candidates for the preparation of these materials. Superhydrophobic solid particles (θ ≥ 150° for water), superoleophobic solid particles (θ ≥ 150° for oils), and superomniphobic solid particles (θ ≥ 150° for both water and oils) can equally be used. Successful preparation of aqueous liquid marbles has been reported in many papers.1,6−11 However, this is limited in the case of oil liquid marbles mainly due to the fact that many particles have high affinity for oil and are completely wetted by them. The particle affinity for oils can be reduced via surface chemical modification using fluorinated groups, allowing the preparation of oil liquid marbles to be achieved.4,12−14 Because liquid marbles can be conceived as an element of powdered liquid materials, the preparation of a powdered liquid material from a given particle type should be possible if it forms a liquid marble with the © XXXX American Chemical Society
Received: January 15, 2016 Revised: March 11, 2016
A
DOI: 10.1021/acs.langmuir.6b00140 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
■
were then homogenized using an IKA T25 Digital Ultra-Turrax homogenizer with a homogenization element of inner diameter 0.8 cm at 12 000 rpm for 4 min. The emulsions exhibited very low conductivity values (ca. 0.01 μS cm−1) and dispersed readily in oil (but not in water), indicating that they are oil continuous. The density of an emulsion was determined by weighing a known volume (200 μL). Stability and Optical Microscopy of Emulsions. The stability of the emulsions to sedimentation and coalescence was assessed by noting the fraction of oil fo and water f w released, respectively, with time. fo is the ratio of the volume of oil released within a given time to the initial volume of oil used. Similarly, f w is the ratio of the volume of water released within a given time to the initial volume of water used. The microscopic structure of the emulsions was investigated using optical microscopy. A drop of an emulsion was diluted with a drop of oil that forms its continuous phase. The diluted emulsion was then viewed on a dimple glass slide (Fisher Scientific) using an Olympus BX-51 optical microscope fitted with a DP50 digital camera using Image-Pro Plus 6.0 software (Media Cybernetics). Photographs of glass vials containing the emulsions and other photographs were taken with a Canon Power Shot SX230 HS camera. Preparation of Powdered Water-in-Oil Emulsions. Emulsions of w/o containing 1 wt %) remained stable to sedimentation and coalescence for several weeks but were gelled and did not flow freely. As a result, emulsions stabilized by 1 wt % of particles were used for the preparation of powdered emulsions. In some cases, the water phase was colored with ∼0.2 mg of fluorescein dye (Sigma) prior to emulsion preparation to aid visualization of water droplets in the emulsions and to aid visualization of the oil emulsion globules in powdered emulsions. A screw cap glass vial (i.d. 2.5 cm, height 7.2 cm) was used to prepare the emulsions containing silica particles (1 wt %) and ϕw = 0.3 by homogenization at 12 000 rpm for 3 min with a homogenization element of inner diameter 1.3 cm. To prepare the powdered water-in-squalane and water-in-olive oil emulsions, the oil released from the w/o emulsions due to sedimentation was removed and the residual emulsions were used in which ϕw ≈ 0.4. The powdered emulsions were prepared by the sequential method described previously13,14 using the PF-12 Eight Pearl 300S-Al fluorinated sericite particles. The required mass (∼0.25 g) of the w/o emulsion was placed on the surface of the required mass (∼0.5 g) of the particles in a screw cap glass vial (i.d. 1.8 cm, height 7.2 cm) and hand shaken gently for 3 min. The process was repeated until the critical emulsion:particle ratio was attained. This is the ratio of the mass of emulsion to the mass of particles above which a powdered emulsion inverts to an emulsion-paste (i.e. air-in-oil + water-in-oil paste). The powdered emulsions reported hereafter are those at the critical emulsion:particle ratio. Using the same procedure, powdered squalane and olive oil were also prepared. The powdered oils were prepared at the critical oil:particle ratio as reported previously.13,14 This the ratio of the mass of oil to the mass of particles at which a powdered oil inverts to an oil foam (a/o). Optical Microscopy of Powdered Emulsions, Powdered Oils, Emulsion-Paste, and Oil Foams. The internal structure of the powdered emulsions, powdered oils, and the emulsion-paste as well as the oil foams were studied using optical microscopy. For the powders, a small amount was spread on a dimple glass slide (Fisher Scientific) and viewed using the above microscope. In some cases, the powders were sheared on a glass slide with an index finger before viewing. To improve visualization of the water droplets in the powdered emulsions, 0.5 cm3 of an emulsion was diluted with 2 cm3 of oil and then dispersed in small volume of perfluorononane (Sigma-Aldrich, ∼97% pure) chosen due to its immiscibility with the hydrocarbon oils. For the emulsion-paste and oil foams, a small sample was smeared on a dimple glass slide and viewed using the same microscope. Tap Density and Angle of Repose of Powdered Emulsions and Powdered Oils. The required mass (0.5 g) of powdered emulsion/oil was tapped in small glass vials to a constant height. From the height and vial diameter, the volume of powder was estimated, enabling the
EXPERIMENTAL SECTION
Materials. Two different solid particle types, namely quasispherical hydrophobic 51% SiOH fumed silica and platelet omniphobic PF-12 Eight Pearl 300S-Al fluorinated sericite (clay) particles, were used. The fumed silica particles (primary diameter 20− 30 nm) were from Wacker-Chemie (Burghausen) and were obtained by treating hydrophilic 100% SiOH fumed silica particles with a given amount of dichlorodimethylsilane.17 The sericite particles (primary size several μm) were from Daito Kasei Kogyo Co. Ltd. (Japan). The particles were obtained by treating hydrophilic sericite particles with a given amount of perfluoroalkyl phosphate diethanolamine salt.14 A scanning electron microscope (SEM) was used to view the dried powdered particles. The dry powdered particles were applied to a sticky carbon disc and coated with a thermally evaporated carbon film (5−15 nm thick) using an Edwards high vacuum coating unit. Excess particles were removed with compressed air. Micrographs were taken with a Zeiss EVO 60 SEM at a voltage of 20 kV and a probe current of 70 pA. SEM images of the particles are shown in Figure 1.
Figure 1. SEM images of (a) powdered hydrophobic 51% SiOH silica particles and (b) powdered oleophobic PF-12 Eight Pearl 300S-Al fluorinated sericite particles. The oils used were nonpolar squalane (Aldrich, 99% pure) and a more polar triglyceride, olive oil (Sigma, highly refined, low acidity). They were passed twice through basic alumina before use. Milli-Q water was used of surface tension and resistivity equal to 72 mN m−1 and 0.18 MΩ cm at 25 °C, respectively. Methods. Preparation of Water-in-Oil Emulsions. Water-insqualane and water-in-olive oil emulsions were prepared by the powdered particle method in screw cap glass vials (i.d. 1.8 cm, h. 7.2 cm). The hydrophobic 51% SiOH fumed silica particles were used to stabilize the emulsions. The emulsions contain a volume fraction of water ϕw = 0.3 and were prepared in the presence of different concentrations of particles (up to 4 wt %). The particle concentration is based on the total volume (5 cm3) of the liquid phases in the emulsions. The required volume of water was placed in the glass vial followed by addition of the required mass of the particles and then the required volume of the oil. The oils progressively wetted the particles (which were not wetted by water) and entered them. The mixtures B
DOI: 10.1021/acs.langmuir.6b00140 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 2. (Upper) Photograph of vials containing (i) water-in-squalane and (ii) water-in-olive oil emulsions (ϕw = 0.3) stabilized by different concentrations (given, wt %) of hydrophobic 51% SiOH fumed silica particles 3 weeks after preparation. (Lower) Corresponding optical micrographs of (i) water-in-squalane and (ii) water-in-olive oil emulsions at selected particle concentrations (wt %).
■
tap density to be calculated. A known mass (1 g) of the powdered emulsions and powdered oils was allowed to pass through a Pyrex glass funnel (i.d. wide end 3.6 cm, narrow end 0.6 cm) onto a planar circular glass substrate (diameter 5.4 cm). The tip of the funnel was 1.5 cm from the glass. The angle of repose α that the powdered emulsions and the powdered oils make with the glass was measured from photographs using a digital protractor. The angle was also calculated from the height and width of the powder piles using α = tan−1(height/ half width). For both, an average of two separate angles is reported. Measurement of Oil−Water−Solid and Air−Emulsion−Solid Three-Phase Contact Angles. The oil−water−solid contact angles (related to w/o emulsions) were inferred from the contact angle θ that a water droplet under oil makes with particle-coated glass slides. To obtain a particle-coated glass slide, 0.8 g of 51% SiOH silica particles was dispersed in 10 cm3 of ethanol using a Branson ultrasonic digital sonifier 450. Sonication was at 30% amplitude for 2 min with constant cooling in an ice bath. The required volume (500 μL) of the particle dispersion was then used to spin coat hydrophilic microscope glass slides (3 cm × 3 cm; Fisher Scientific) using a Cookson Electronics P6700 Spin Coater at 800 rpm for 45 s. The particle-coated glass slides were dried at ambient conditions for 24 h in Petri dishes. For contact angle measurement, the required volume of liquid was used to form a sessile droplet on the particle-coated glass slides. This was done in air for both water and oil drops. For the oil−water−particle contact angles, a water droplet was formed on the substrate in a transparent glass cuvette (inner dimensions 3 cm × 3 cm × 3 cm, OG Hëllma) containing the required oil (3 cm3). The volume of water used was either 20 μL (advancing θ) or 10 μL (receding θ). Contact angles were obtained by analyzing a profile of the sessile droplet using a Krüss DSA Mk 10 instrument. The result reported is an average of two separate measurements. For the air−emulsion−solid interface (related to powdered emulsions), contact angles were measured on compressed disks of the PF-12 Eight Pearl 300S-Al fluorinated particles. Approximately 0.2 g of the particles were compressed under a pressure of 9 × 108 N m−2 in two steel dies (diameter 13 mm) using a hydraulic press (Research and Industrial Co., UK). A 20 μL emulsion droplet (from the emulsion used in the preparation of its powdered counterpart) was placed on the compressed particle disk.
RESULTS AND DISCUSSION Water-in-Oil Emulsions Stabilized by Hydrophobic Silica. Photographs of the water-in-squalane and water-in-olive oil emulsions (ϕw = 0.3) are shown in Figure 2 along with their optical micrographs. The water-in-squalane emulsions gelled at ≥2 wt % of particles while those of water-in-olive oil did so at ≥3 wt % of particles. The gelled emulsions did not flow even when the glass vial containing them is inverted (Figure 2). The control emulsions (containing no particles) exhibited complete phase separation within 10 min after preparation. Overall, the stability of the emulsions to sedimentation and coalescence increased as the particle concentration increases as shown in Figure 3 and its inset. The water-in-olive oil emulsions were slightly more stable to sedimentation and coalescence than the water-in-squalane emulsions. The average droplet diameter of the emulsions (Figure 4), measured by averaging the diameter of 200 droplets, initially decreases with increasing particle concentration before becoming constant at relatively high particle concentrations (>2 wt %). This is similar to what has been observed in many particle-stabilized emulsion systems and has been linked to drop coalescence (relatively low particle concentration) and drop breakage (relatively high particle concentration) during the emulsification process.18 Powdered Water-in-Oil Emulsions and EmulsionPaste-Containing Fluorinated Sericite. The above w/o emulsions are then aerated in the presence of powdered fluorinated sericite particles in a gentle, low shear manner. This serves to break up the original continuous oil phase into globules which become coated with sericite particles, leaving inner water drops undisturbed. Up to a certain emulsion:particle mass ratio, powdered emulsions are formed which are freeflowing, Figure 5(a). Above this ratio, an emulsion-paste forms which is sticky and does not flow, Figures 5(b) and 6. As C
DOI: 10.1021/acs.langmuir.6b00140 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
Figure 5. Photographs of (a) powdered water-in-squalane emulsion containing 78 wt % of the emulsion (with respect to the total mass of the powder) and (b) an emulsion-paste obtained from the dry powdered emulsion in (a) above a critical emulsion:particle ratio containing 80 wt % of the emulsion (with respect to the total mass of the emulsion-paste). The photographs were obtained 2 months after preparation.
Figure 3. Fraction of oil fo (measure of sedimentation) released from water-in-squalane (filled circles) and water-in-olive oil (unfilled circles) emulsions described in Figure 2 versus particle concentration 3 weeks after preparation. Inset: Fraction of water f w (measure of coalescence) released from the same emulsions.
Figure 6. Photograph of emulsion-paste stabilized by 0.26 g of PF-12 Eight Pearl 300S-Al fluorinated sericite particles containing (a) 1.15 g of water-in-olive oil and (b) 1.00 g of water-in-squalane emulsion taken 24 h after preparation.
Figure 4. Average droplet diameter of water-in-squalane (○) and water-in-olive oil (●) emulsions versus concentration of hydrophobic 51% SiOH silica particles.
evidenced by the optical micrographs of these two materials (Figure 7), the powdered emulsions (w/o/a) are composed of particle-coated oil-in-air globules which are larger than 200 μm just like the corresponding powdered oils (o/a) shown in Figure 8. We verified that the average diameter of the water droplets in powdered emulsions is comparable to that in the precursor w/o emulsion. The emulsion-paste, however, is seen to contain smaller (90° for relatively spherical particles.3 In the case of the powdered w/o emulsion here, one fluid interface is water−oil covered with silica particles while the other interface is oil−air covered with clay particles. Because many of the solid particles used are usually very small (nanometer to micrometer size) and nonspherical, these angles are often inferred from the apparent contact angle which droplets of the liquid make with a solid substrate composed of the particles.4,13 This technique has also been employed here. The advancing and receding contact angles of the oils, water, and w/o emulsion droplets on glass slides composed of 51% SiOH silica or on disks of compressed powdered PF-12 Eight Pearl 300S-Al particles were measured and are given in Table 2. The contact angles of the oil droplets in air on hydrophobic silica-coated slides were less than 10°, indicative of their oleophilic character.20 For water droplets in air, the angle was ≫90°, indicative of their hydrophobic nature.3 For water droplets under the oils, the angles were around 145−155°, consistent with the stabilization of water-inoil emulsions. Relevant to the oil−air surface, for droplets of the oils in air on compressed disks of PF-12 Eight Pearl 300S-Al particles, the advancing (>100°) and receding (85−92°) angles indicate oleophobic surfaces and they are consistent with those reported earlier13,14 and deemed necessary for dry powder formation. We also measured the contact angle of a drop of water-in-oil emulsion stabilized by silica particles in air on the clay disk in an attempt to mimic the composite bulk material (w/o/a). These angles (measured through the emulsion) are also high and similar to those of the oils alone as expected, since oil is the continuous phase.
bubbles in oil (Figure 8). The powdered emulsions are stable to phase separation for over 1 year and did not release oil or water in this time scale. The powdered water-in-squalane and waterin-olive oil emulsions inverted to the emulsion-paste at an emulsion:particle ratio of 4.0 and 4.6, respectively. These values are higher than those for the powdered oils where catastrophic phase inversion from the powders to the foams occurred at an oil:particle ratio of 2.1 and 3.5 for squalane and olive oil, respectively. This is understandable because, for a fixed mass of w/o emulsion, there is less oil present compared with the same mass of neat oil. Apart from the emulsion-paste containing 1.15 g of water-in-olive oil emulsion which released 3.5% of water (with respect to the mass of water in the encapsulated emulsion) after 24 h, that containing 1 g of water-in-squalane emulsion remained stable to phase separation for over 1 year. Upon shearing the powdered emulsions on a glass substrate, optical microscopy revealed the presence of water-rich and oilrich regions as shown in Figure 7. The size of the water-rich region is smaller than that of a typical water droplet in the precursor emulsion, indicating that shearing breaks both the oil globules and water droplets in the powdered emulsions. This is in contrast to the shearing of powdered oils where only oil-rich regions were observed as shown in Figure 8. Tap Density and Angle of Repose of Powders. The angle of repose was measured and calculated from images of the piles of the powders on a planar glass substrate. The liquid content, tap density, and angle of repose of the powdered oils and powdered emulsions are given in Table 1, along with the tap density and angle of repose of the PF-12 Eight Pearl 300S-Al particles stabilizing the powdered materials. The tap densities of the powdered oils are slightly less than those of the liquid oils as expected. Those of the powdered emulsions are the same as that of the PF-12 Eight Pearl 300S-Al particles even though two Table 1. Oil/Emulsion Content, Tap Density, and Angle of Repose of Powdered Oils and Powdered Emulsionsa angle of repose (deg) powder powdered squalane powdered olive oil powdered water-insqualane powdered water-inolive oil PF-12 Eight Pearl 300S-Al particles
oil or emulsion content (wt %)
tap density (g cm−3)
measured
calculated
67 75 77
0.80 0.88 0.78
43 ± 3 42 ± 1 45 ± 6
37 ± 1 33 ± 1 32 ± 2
79
0.78
43 ± 4
30 ± 1
−
0.78
56 ± 1
48 ± 1
■
CONCLUSIONS We describe a method for preparing a powdered w/o emulsion (w/o/a) containing inner water drops within oil globules
a
The tap density and angle of repose of the PF-12 Eight Pearl 300S-Al particles are also given. E
DOI: 10.1021/acs.langmuir.6b00140 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
(7) Arbatan, T.; Li, L.; Tian, J.; Shen, W. Liquid marbles as microbioreactors for rapid blood typing. Adv. Healthcare Mater. 2012, 1, 80− 83. (8) McHale, G.; Newton, M. I. Liquid marbles: topical context within soft matter and recent progress. Soft Matter 2015, 11, 2530−2546. (9) Bormashenko, E.; Bormashenko, Y.; Pogreb, R.; Gendelman, O. Janus droplets: Liquid marbles coated with dielectric/semiconductor particles. Langmuir 2011, 27, 7−10. (10) Zhao, Y.; Xu, Z.; Niu, H.; Wang, X.; Lin, T. Magnetic liquid marbles: Toward lab in a droplet. Adv. Funct. Mater. 2015, 25, 437− 444. (11) Fujii, S.; Suzaki, M.; Armes, S. P.; Dupin, D.; Hamasaki, S.; Aono, K.; Nakamura, Y. Liquid marbles prepared from pH-responsive sterically stabilised latex particles. Langmuir 2011, 27, 8067−8074. (12) Gao, L.; McCarthy, T. J. Ionic liquid marbles. Langmuir 2007, 23, 10445−10447. (13) Binks, B. P.; Johnston, S. K.; Sekine, T.; Tyowua, A. T. Particles at oil−air surfaces: Powdered oil, liquid oil marbles and oil foam. ACS Appl. Mater. Interfaces 2015, 7, 14328−14337. (14) Binks, B. P.; Sekine, T.; Tyowua, A. T. Dry oil powders and oil foams stabilised by fluorinated clay platelet particles. Soft Matter 2014, 10, 578−589. (15) Carter, B. O.; Weaver, J. V. M.; Wang, W.; Spiller, D. G.; Adams, D. J.; Cooper, A. I. Microencapsulation using an oil-in-water-in-air ‘dry water emulsion’. Chem. Commun. 2011, 47, 8253−8255. (16) Murakami, R.; Moriyama, H.; Yamamoto, M.; Binks, B. P.; Rocher, A. Particle stabilisation of oil-in-water-in-air materials: Powdered emulsions. Adv. Mater. 2012, 24, 767−771. (17) Fletcher, P. D. I.; Holt, B. L. Controlled silanization of silica nanoparticles to stabilize foams, climbing films, and liquid marbles. Langmuir 2011, 27, 12869−12876. (18) Destribats, M.; Lapeyre, V.; Sellier, E.; Leal-Calderon, F.; Schmitt, V.; Ravaine, V. Water-in-oil emulsions stabilized by waterdispersible poly(N-isopropylacrylamide) microgels: Understanding anti-Finkle behavior. Langmuir 2011, 27, 14096−14107. (19) Lumay, G.; Boschini, F.; Traina, K.; Bontempi, S.; Remy, J. C.; Cloots, R.; Vandewalle, N. Measuring the flow properties of powders and grains. Powder Technol. 2012, 224, 19−27. (20) Binks, B. P.; Rocher, A. Stabilisation of liquid-air surfaces by particles of low surface energy. Phys. Chem. Chem. Phys. 2010, 12, 9169−9171.
Table 2. Advancing and Receding Contact Angles (Both Liquid−Air and Water−Oil) of Liquids and Emulsion Drops on Glass Slides Spin-Coated with 51% SiOH Silica Particles and on Compressed Particle Disks of PF-12 Eight Pearl 300S-Al Particles contact angle (±1°) solid substrate glass slides spin-coated with 51% SiOH silica particles
disks of PF-12 Eight Pearl 300S-Al particles
liquid
advancing receding
squalane−air
