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Effects of the Density Difference between Water and Oil on Stabilization of Powdered Oil-in-Water Emulsions Ryo Murakami,*,†,‡ Hiroshi Moriyama,† Tatsuyuki Noguchi,†,‡ Masahiro Yamamoto,†,‡ and Bernard P. Binks*,§ †

Department of Chemistry of Functional Molecules, Konan University, 8-9-1 Okamoto, Kobe 658-8501, Japan CREST, Japan Science and Technology Agency, Kobe 658-8501, Japan § Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. ‡

ABSTRACT: A powdered oil-in-water (o/w) emulsion is a dispersed system in which the continuous phase of a particle-stabilized o/w emulsion is dispersed in air by encapsulating the water globules with hydrophobic particles. During their preparation, oil droplets in water globules are forced to move because of high-shear mixing, leading to creaming of the oil droplets and possible wetting of the hydrophobic particles with the oil droplets, which induces destabilization. To prepare powdered o/w emulsions efficiently, the extent of creaming of the oil droplets has to be suppressed. We describe how to achieve this by mixing two oils of different densities and preparing powdered o/w emulsions from oil mixtures exhibiting a decreasing density difference with water. As the extent of creaming is reduced, enhanced stabilization of the powdered emulsions occurs.

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INTRODUCTION Dispersed systems in which at least one phase is dispersed in another are represented by emulsions, foams, dispersions, and aerosols. Among them, emulsions of oil and water and foams of air and water are not thermodynamically stable, but they can be kinetically stabilized with the aid of molecular or polymeric surfactants. The surface-active molecules form adsorbed layers around the dispersed phase, imparting stability against the coalescence of droplets or bubbles. However, it is well known that colloidal particles also act as an excellent stabilizing agent for such fluid dispersed systems.1−5 Other fluid dispersed systems consisting of liquid drops dispersed in air (L/A) are also stabilized using colloidal particles. To the best of our knowledge, L/A materials have not been prepared using molecular or polymeric surfactants. Examples of L/A materials are liquid marbles6,7 and dry liquids.8 Liquid marbles are millimeter-sized liquid drops covered by liquid-repellent particles, and the dry liquids consist of micrometer-sized liquid drops coated with liquid-repellent particles. The dry liquids usually contain a large amount of liquid (>95 wt %) but behave as a free-flowing powder.8−12 A range of liquids can be used to prepare dry liquids, including water,8−11 oil,13 and ionic liquid.14 We recently demonstrated that, by analogy to the stabilization mechanism of dry water, globules of water containing oil droplets that are stabilized by partially hydrophobic silica particles can be encapsulated by very hydrophobic silica particles when aerated, producing an oil-in-water-in-air (o/w/a) material as shown in Figure 1.15 The o/w/a materials were named powdered o/w emulsions because they contain © 2013 American Chemical Society

Figure 1. Schematic representation of oil-in-water-in-air material (powdered o/w emulsion). Encapsulation of an o/w emulsion stabilized by partially hydrophobic particles is achieved by using very hydrophobic particles upon aeration.

micrometer-sized liquid drops of oil and globules of water but display free-flowing behavior. They could be used, for example, as a carrier of ingredients (nonpolar or polar) to the skin surface but also as a safer way to store and transport harmful materials. It is imperative to stabilize the precursor o/w emulsions by colloidal particles rather than using low-molarReceived: November 8, 2013 Revised: December 20, 2013 Published: December 20, 2013 496

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sedimentation−redispersion cycles in pure water. Oil-in-water emulsions were prepared by emulsifiying 5 mL of an oil mixture of n-dodecane and silicone oil with 5 mL of an aqueous dispersion of partially hydrophobic silica using an Ultra Turrax T25 homogenizer (1 cm head) at 24 000 rpm for 2 min at room temperature. A blender (LM-PLUS, Osaka Chemical Co. Ltd.) with a blade and a small glass container (75 mL nominal capacity) was used to prepare powdered o/ w emulsions. A mixture of an o/w emulsion (10 mL) and pure water (15 mL) was aerated with the blender for 15 s at 6000 rpm after gently placing the very hydrophobic particles on the surface of the liquid. At least three samples of powdered emulsion were prepared for each composition. Characterization. The densities of mixtures of the two oils at different volume fractions of the silicone oil were measured using a digital density meter with an oscillating U-tube sensor (DMA 4100 M, Anton Paar) at 25 °C. Optical micrographs of samples placed on a glass slide with a depression were taken with an Olympus BX51 transmission/reflection microscope fitted with a CMOS camera (Moticam 2000, Shimazu). By measuring the diameter of ca. 100 oil droplets in both o/w emulsions and in powdered o/w emulsions and ca. 100 water globules in a powdered o/w emulsions (Motic Image Plus 2.2S, Shimazu), their number-average diameters were determined. For each average diameter, the coefficient of variation (CV) defined by the average diameter divided by the standard deviation was also determined. The angle of repose of powdered o/w emulsions was measured by passing the material through a funnel (top inner diameter 70 mm, stem length 75 mm, stem inner diameter 6 mm) and loading it onto a circular base (diameter 45 mm) made of acrylic resin. The conically shaped powdered o/w emulsions that formed were imaged with a digital camera (CX2, Ricoh).

mass surfactant molecules. Only with the use of a specially designed branched copolymer surfactant in combination with very hydrophobic particles could powdered o/w emulsions be stabilized because using an ionic surfactant (sodium dodecyl sulfate) or a nonionic one (Brij 30) failed to prepare the powdery materials.16 For successful preparation, the inner partially hydrophobic particles of the required wettability for the oil−water interfaces are irreversibly adsorbed and do not affect the very hydrophobic particles at the water−air interface required for the encapsulation of outer water globules. In preparing the powdered o/w emulsions, we showed that it was crucial to decrease the extent of coalescence of oil droplets in o/w emulsions.15 In aerating the o/w emulsions in the presence of very hydrophobic particles, drops of oil and water are subjected to substantial shear, stretching their interfaces and leaving parts devoid of particles. If the velocity of oil droplet movement is fast enough, the drops may coalesce with each other and also collide with the newly created surfaces between water and air, leading to wetting of the very hydrophobic particles by oil and subsequent destabilization. The velocity (v) of an isolated, rigid oil droplet of diameter d in water under gravitational force g is given by the Stokes equation: v = d2Δρg/ 18η, where Δρ is the density difference between water and oil and η is the viscosity of the water phase.17 We have shown that the powdered o/w emulsions are progressively stabilized by reducing the creaming of oil drops (hence reducing v), effected by decreasing d or increasing η.15 The variation of Δρ is then the only uninvestigated parameter in understanding the stabilization mechanism of powdered emulsions. One of the ways to control Δρ is to use oils of different densities. However, the wettability of the particles used for the stabilization of o/w emulsions depends on the type of oil. Because the energy of adsorption of particles to an oil−water interface is significantly affected by the wettability, the change in the wettability would lead to incoherent oil droplet sizes, which is clearly undesirable in a study aiming to clarify the effect of changing Δρ in emulsions. In this study, we have selected two types of oil with different densities (n-dodecane and a silicone oil) but for which the wettability of partially hydrophobic particle surfaces at the oil−water interface is similar.18 By mixing the two oils in different ratios, we are able to control Δρ but can keep the average oil droplet size in precursor o/w emulsions virtually constant. As a result, the effects of varying the density difference between water and oil on the stabilization of powdered emulsions have been investigated.

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RESULTS AND DISCUSSION Precursor o/w Emulsions. To control the density difference between water and oil (Δρ), n-dodecane and a silicone oil were mixed. The density of silicone oil used is 0.9660 g cm−3, and that of n-dodecane is 0.7488 g cm−3 at 25 °C. By using the measured density of water at 25 °C of 0.9970 g cm−3, the density difference between water and the oil mixtures was calculated from measured values of the density of oil mixtures. Values of Δρ are shown in Figure 2, where it can

EXPERIMENTAL SECTION

Materials. Two types of fumed silica particles supplied by WackerChemie (Germany) were selected for optimum adsorption at oil− water and water−air interfaces. The partially hydrophobic grade possessed 37% residual SiOH groups on its surface, and the very hydrophobic grade (HDK H18) possessed 20% SiOH, with both being prepared by the reaction of hydrophilic silica with dichlorodimethylsilane.8 The primary particle diameter in both cases is 20−30 nm, but particles are partially fused in the preparation process, resulting in branched particle aggregates with a diameter of about 200 nm. nDodecane (Reagent-Plus, ≥99% Sigma-Aldrich) was passed through an alumina column twice prior to use. A poly(dimethylsiloxane) (PDMS) silicone oil with a viscosity of 100 cSt (Sigma-Aldrich) and ethanol (99.5%, Sigma-Aldrich Japan) were used as received. Water was taken from a Direct-Q ultrapure water system (Millipore). Methods. Preparation of Aqueous Particle Dispersion, o/w Emulsions, and Powdered o/w Emulsions. Partially hydrophobic silica powder was dispersed in water to a concentration of 1 wt% with the aid of a small quantity of ethanol that was removed by repeated

Figure 2. Density difference between water and oil mixtures (Δρ) as a function of the volume percent of silicone oil in mixtures with dodecane measured at 25 °C.

be seen that they decrease linearly with increasing volume percent of the silicone oil. Δρ can be controlled from 0.031 to 0.248 g cm−3 by selecting the appropriate oil mixture. All of the emulsions prepared by mixing n-dodecane−silicone oil mixtures with an aqueous dispersion of partially hydrophobic silica particles could be dispersed in water but remained as a drop when placed on n-dodecane, indicating they are of the o/w type. Optical micrographs of the emulsions with different 497

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Δρ values are shown in Figure 3. Most of the oil droplets are spherical when Δρ is 0.248 g cm−3 (corresponding to pure n-

mimic silica particle surfaces) reported advancing contact angles measured through water of 122° for n-dodecane and 123° for PDMS of viscosity 50 cSt, which is very similar to that used in this study. The o/w emulsions were stable with respect to the coalescence of oil droplets for at least 1 month, but creamed to different extents depending on the composition. We have measured the time dependence of the stability of the emulsions to creaming, quantified by f w equal to the volume of the resolved water phase divided by the volume of the water phase before emulsification. If all of the water is resolved, then f w = 1. Values of f w for all of the emulsions increase with time but become almost constant after 100 min for an oil mixture with Δρ = 0.248 g cm−3, increasing to 4000 min for the mixture with Δρ = 0.033 g cm−3. In Figure 5, the time required for 50 and

Figure 5. Time required for 50% (⧫) and 80% (▲) creaming relative to the maximum creaming for o/w emulsions vs Δρ.

Figure 3. Optical micrographs of o/w emulsions prepared using oil mixtures with different Δρ values of (a) 0.248, (b) 0.194, (c) 0.143, (d) 0.089, and (e) 0.033 g cm−3. Scale bar refers to 200 μm on each.

80% creaming relative to the maximum creaming is shown for emulsions with different Δρ values. As predicted by the Stokes equation, the rate of creaming of o/w emulsions decreases as the magnitude of Δρ decreases. It is thus predicted that the powdered o/w emulsions should be increasingly stabilized using emulsions of decreasing Δρ, in which the coalescence between oil droplets and the coalescence of oil droplets with air−water surfaces (of water globules) should be suppressed. Powdered o/w Emulsions. By aerating o/w emulsions in the presence of very hydrophobic silica particles, powdery materials were formed. An optical micrograph of the powdery material prepared using an o/w emulsion with Δρ = 0.033 g cm−3 is shown in Figure 6a, where water globules containing dispersed oil drops are clumped together. The powders were subsequently dispersed into a volume of n-dodecane and observed using microscopy in order to deduce the structure of the original powdery material.9,15 It has been shown that if powdered o/w emulsions are formed then the continuous phase (i.e., air) can be replaced by oil, leading to the formation of particle-stabilized multiple oil-in-water-in-oil (o/w/o) emulsions.15 Figure 6b−f shows optical micrographs of the powdery materials dispersed in n-dodecane made from emulsions with different Δρ values, showing large water globules of size ranging from several tens of micrometers to several hundreds of micrometers (up to about 700 μm) and oil droplets of size ranging from a few micrometers to tens of micrometers (up to about 20 μm). It seems that the number of oil droplets in water globules increases with decreasing Δρ. For all of the as-prepared powdered o/w emulsions, we did not observe any separation of oil or water up to 6 months after preparation if stored in a sealed plastic vessel, implying that a

dodecane), but oil droplets with nonspherical shapes are progressively observed with decreasing values of Δρ, in which an increase in oil viscosity upon increasing the silicone oil content hinders the shape relaxation to spherical droplets. By analyzing the micrographs, the number-average oil droplet diameter and the average coefficient of variation (CV) were estimated, as shown in Figure 4. The constant value of the oil droplet diameter with Δρ is probably due to the similar wettability of the particles for the two oils against water. In fact, an earlier study18 on the contact angles of water under oil on planar glass substrates modified with dimethyldichlorosilane (to

Figure 4. Dependence of the average diameter (solid symbols) and average CV (open symbols) for oil droplets in o/w emulsions on Δρ. 498

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Figure 8. Dependence of the average diameter (solid symbols) and average CV (open symbols) for oil droplets in water globules in powdered o/w emulsions on Δρ. The dotted line represents the average oil droplet diameter in precursor o/w emulsions.

than that of the original precursor o/w emulsion (20 μm), it is suggested that relatively large oil droplets coalesce more frequently with air−water surfaces than relatively small ones during aeration; we measure only the diameter of oil droplets that survive after aeration. Upon decreasing Δρ, the oil droplet diameter increases and approaches that in the original o/w emulsions, suggesting a reduction in the extent of coalescence of oil droplets. The powder flowability can be assessed in terms of the angle of repose, which is the internal angle between the surface of the powder and the horizontal surface when a granular material is poured onto a substrate.19 Some digital photographs of the powdered o/w emulsions piled on a cylindrical base are shown in Figure 9a,b. It has been shown that powdered o/w emulsions behave like dry water in terms of the flowability of the powder if the coalescence of oil droplets is insignificant.15 The angle of repose for powdered o/w emulsions prepared using emulsions of different Δρ values is shown in Figure 9c. The repose angle

Figure 6. Optical micrographs of powdered o/w emulsions prepared using oil mixtures of different densities. (a) Powdered o/w emulsion as is with Δρ = 0.033 g cm−3. (b−f) Powdered o/w emulsions dispersed in dodecane for Δρ = (b) 0.248, (c) 0.194, (d) 0.143, (e) 0.089, and (f) 0.033 g cm−3. Scale bar refers to 500 μm on each.

significant coalescence of oil droplets and water globules does not occur. The number-average water globule diameter and the average CV values are shown in Figure 7. The diameters are more or

Figure 7. Dependence of the average diameter (solid symbols) and average CV (open symbols) for water globules in powdered o/w emulsions on Δρ.

less constant for all of the Δρ values (190 to 240 μm), implying that the size of water globules is simply determined by the aeration speed and most likely by the concentration of very hydrophobic particles. It should be noted, however, that the oil droplet diameter in the water globules is quite dependent on the Δρ value, as seen in Figure 8. When Δρ is 0.248 g cm−3, the oil droplet diameter is smallest (around 8 μm), with the average CV value being relatively large. Because this diameter is smaller

Figure 9. Digital photographs of powdered o/w emulsions prepared using o/w emulsions with Δρ = (a) 0.248 and (b) 0.033 g cm−3, which were piled on a cylindrical base showing different angles of repose. The diameter of the cylindrical base is 45 mm. (c) Angle of repose of powdered emulsions as a function of Δρ. The dotted line shows the angle of repose for dry water (no oil). 499

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decreases and approaches that for dry water (44 ± 1°) upon decreasing the value of Δρ. It is known that the angle of repose of granular materials is increased by adding oil, which forms a thin layer around particles and bridges them.20 A decrease in the angle of repose with decreasing Δρ suggests that coalescence between oil droplets and air−water surfaces is suppressed by decreasing the density difference between oil and water, leading to less wetting of the very hydrophobic particles with the oil phase.

(11) Saleh, K.; Forny, L.; Guigon, P.; Pezron, I. Dry water: from physico-chemical aspects to process-related parameters. Chem. Eng. Res. Des. 2011, 89, 537−544. (12) Carter, B. O.; Wang, W.; Adams, D. J.; Cooper, A. I. Gas storage in “dry water” and “dry gel” clathrates. Langmuir 2010, 26, 3186− 3193. (13) Murakami, R.; Bismarck, A. Particle-stabilized materials: dry oils and (polymerized) non-aqueous foams. Adv. Funct. Mater. 2010, 20, 732−737. (14) Shirato, K.; Satoh, M. ‘‘Dry ionic liquid’’ as a newcomer to ‘‘dry matter’’. Soft Matter 2011, 7, 7191−7193. (15) Murakami, R.; Moriyama, H.; Yamamoto, M.; Binks, B. P.; Rocher, A. Particle stabilization of oil-in-water-in-air materials: powdered emulsions. Adv. Mater. 2012, 24, 767−771. (16) 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. (17) Morrison, I. D.; Ross, S. In Colloidal Dispersions: Suspensions, Emulsions, and Foams; John Wiley & Sons: New York, 2002; Chapter 3. (18) Binks, B. P.; Lumsdon, S. Effects of oil type and aqueous phase composition on oil-water mixtures containing particles of intermediate hydrophobicity. Phys. Chem. Chem. Phys. 2000, 2, 2959−2967. (19) Schulze, D. Powders and Bulk Solids; Springer: Berlin, 2008. (20) Hornabaker, D. J.; Albert, R.; Albert, I.; Barabasi, A.-L.; Schiffer, P. What keeps sandcastles standing? Nature 1997, 387, 765.

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CONCLUSIONS The reduction of the oil droplet velocity in water globules during the aeration of o/w emulsions is achieved by decreasing the density difference between water and oil. By doing so, oilin-water-in-air materials (powdered o/w emulsions) are progressively stabilized. The decrease in the density difference leads to the suppression of coalescence between oil droplets and between oil droplets and air−water surfaces. In optimizing the stabilization of powdered o/w emulsions, the oil droplet velocity in water during aeration must be reduced by controlling four parameters: the oil droplet diameter, the water phase viscosity, the aeration speed, and the density difference between water and oil.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank JSPS KAKENHI (Grant-in-Aid for Young Scientists (B), 12018977) and the Cosmetology Research Foundation for funding and Wacker-Chemie (Burghausen) for the fumed silica particles.

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REFERENCES

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