Article pubs.acs.org/Langmuir
Influence of Propylene Glycol on Aqueous Silica Dispersions and Particle-Stabilized Emulsions Bernard P. Binks,* Paul D. I. Fletcher, and Michael A. Thompson Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K.
Russell P. Elliott Stiefel, a GSK company, 20 T.W. Alexander Drive, RTP, North Carolina 27709, United States S Supporting Information *
ABSTRACT: We have studied the influence of adding propylene glycol to both aqueous dispersions of fumed silica nanoparticles and emulsions of paraffin liquid and water stabilized by the same particles. In the absence of oil, aerating mixtures of aqueous propylene glycol and particles yields either stable dispersions, aqueous foams, climbing particle films, or liquid marbles depending on the glycol content and particle hydrophobicity. The presence of glycol in water promotes particles to behave as if they are more hydrophilic. Calculations of their contact angle at the air−aqueous propylene glycol surface are in agreement with these findings. In the presence of oil, particle-stabilized emulsions invert from water-in-oil to oilin-water upon increasing either the inherent hydrophilicity of the particles or the glycol content in the aqueous phase. Stable multiple emulsions occur around phase inversion in systems of low glycol content, and completely stable, waterless oil-in-propylene glycol emulsions can also be prepared. Accounting for the surface energies at the respective interfaces allows estimation of the contact angle at the oil−polar phase interface; reasonable agreement between measured and calculated phase inversion conditions is found assuming no glycol adsorption on particle surfaces.
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INTRODUCTION Immiscible mixtures of oil and water may be made kinetically stable by addition of an emulsifier to form emulsions in which drops of one of the liquids become dispersed in the continuous phase of the other liquid.1 Stable emulsions occur in a wide range of industries including the food, personal care, cosmetic, oilfield chemical, and pharmaceutical sectors. Certain pharmaceutical emulsions often incorporating paraffin oil are administered either as creams to the skin or injected directly and can contain high concentrations of polar glycol (or diol) species such as propane-1,2-diol. It is believed that such diols improve the solubility of the active ingredient and facilitate their transport across the skin surface.2−4 Propylene glycol has many other applications industrially including as a humectant, as a moisturizer, as a carrier in fragrance oils, and as a nontoxic antifreeze. Although a few reports exist on the stabilization of emulsions of oil and nonaqueous polar liquids,5−7 little is documented on emulsions containing water−diol mixtures as the polar phase. We recently investigated the effects of the addition of various diols on the properties of emulsions of paraffin liquid and water stabilized by nonionic surfactant.8 Addition of diol promotes emulsion phase inversion from water-in-oil (w/o) to oil-in-water (o/w), that is it increases the hydrophilicity of the system. It was suggested that the diol acts as a cosurfactant adsorbing at the oil−water interface swelling © 2013 American Chemical Society
the headgroup region of the surfactant. The purpose of the work described here is to understand the influence of adding propane-1,2-diol to emulsions of the same oil but stabilized by nanoparticles of silica. It is now well documented that stable emulsions or foams can be prepared from mixtures of solid particles, water, and either oil or air, respectively.9−12 The particles, partially wetted by both bulk phases, are irreversibly adsorbed giving rise to drops or bubbles, which are resistant to both coalescence and disproportionation. The ability of particles to stabilize emulsions of oil and water depends, inter alia, on their wettability at the interface.13 This is quantified for spherical particles through the three-phase contact angle θ (measured through the aqueous phase). For equal volumes of oil and water, hydrophilic particles of θ < 90° stabilize o/w emulsions, whereas hydrophobic particles of θ > 90° stabilize w/o emulsions. The change in free energy accompanying desorption of a spherical particle from the oil−water interface to either bulk phase is given by14,15 ΔE = πr 2γow(1 ± cos θ )2
(1)
Received: March 8, 2013 Revised: April 18, 2013 Published: April 30, 2013 5723
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in which r is the particle radius, γow is the bare oil−water interfacial tension, and the plus sign refers to desorption into oil, whereas the minus sign refers to that into water. At fixed particle size and interfacial tension, ΔE is maximum at θ = 90° because this situation corresponds to the maximum area of interface obliterated by placing the particle at it. Systems in which ΔE is large (several hundred kT where k is the Boltzmann constant and T is the absolute temperature) exhibit contact angles of intermediate values (not close to 0 or 180°) and produce the most stable emulsions to coalescence. Conversely, particles of very low or very high θ are not well held at the interface (ΔE very low) and give rise to emulsions of low coalescence stability.15−17 Because stabilization of emulsions composed of polar liquids other than water requires quite exotic surfactants or polymers,7 we decided to investigate whether such emulsions could be stabilized using silica particles in which the hydrophobicity (akin to the surfactant hydrophile−lipophile balance, HLB, number) can be systematically varied. Such particles are excellent stabilizers of many kinds of oil−water emulsions with an optimum particle hydrophobicity being required depending on the oil type. We investigate miscible mixtures of water and propane-1,2-diol both in the absence and presence of an oil for a range of silica particles of different inherent hydrophobicity. Rationalization of the data is discussed in terms of the influence of propane-1,2-diol on the contact angles of the particles at the air−polar phase or oil−polar phase interfaces.
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determined by C, H, N analysis. In this work, a series of particles ranging from 14% SiOH (most hydrophobic) to 100% SiOH (most hydrophilic) were used. All other chemicals were AnalaR grade.
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METHODS
Surface and Interfacial Tensions. The surface tensions (against air) of water, propylene glycol, paraffin liquid, and aqueous solutions of propylene glycol in addition to the interfacial tension (against paraffin liquid) of aqueous solutions of propylene glycol were measured using a Krüss K10 digital tensiometer using the du Noüy ring method at 20 °C. The balance and ring were checked before use by measuring the surface tension of undecane. The appropriate correction factors were applied according to ref 19. The cleaning procedure consisted of rinsing the glass vessel in alcoholic KOH solution and then Milli-Q water. The ring was heated to glowing in a blue Bunsen flame. Prior to measurement of the oil−aqueous propylene glycol tension, the two fluid phases were mutually saturated. Mixtures of Air, Silica Particles, and Aqueous Propylene Glycol. The immersion time of silica powder in aqueous propylene glycol solutions was determined by carefully placing 25 mg of powder evenly on the surface of 10 cm3 of solution contained in a glass vessel of diameter 2.7 cm at room temperature. The time taken for all of the powder to enter the liquid was measured (if wetting occurred).20 This was determined for silica powders of different wettability in water− propylene glycol mixtures. The same samples were subsequently used to investigate the materials formed from aqueous propylene glycol− silica−air mixtures. This was achieved by hand shaking the vessels for 30 s and observing whether a dispersion, a foam, or a climbing film formed. In addition, liquid marbles21 (macroscopic drops coated with particles in air) were formed by rolling a 50 μL drop of aqueous propylene glycol on a bed of hydrophobic silica powder (14% SiOH) in a Petri dish. Preparation of Particle-Stabilized Emulsions. Five milliliters of oil, 5 ml of polar phase, and the required mass of silica particles were emulsified in glass vessels (diameter 2.5 cm, length 7.5 cm) thermostatted at 25 °C. The polar phase was either water, propylene glycol, or an aqueous solution of propylene glycol and contained 4 mM NaCl to increase the conductivity. Emulsions were prepared using the powdered particle method.22 In this method, fumed silica particles were added as a powder on top of the most dense liquid phase (aqueous) followed by the least dense phase (oil). Emulsification was achieved with an IKA Ultra-Turrax homogenizer fitted with a dispersing head of diameter 18 mm operating at 13 000 rpm for 5 min. This method removes the possibility that the initial location of the particles may influence the subsequent emulsion properties and so particles dictate the behavior due solely to their inherent wettability. Seven series of emulsions were prepared all containing equal volumes of oil and polar phases. In four series differing in the water/propylene glycol volume percentage (50:0, 25:25, 20:30, and 0:50, respectively), the effect of particle wettability (via % SiOH) was investigated. In the three other series in which the particle wettability was fixed (at 23, 37, or 42% SiOH), the effect of water/propylene glycol volume percentage was investigated. All emulsions contained 1 wt % silica particles. Characterization of Emulsions. The continuous phase of an emulsion was inferred by observing whether a drop of emulsion dispersed or remained when added to either pure oil or pure water. Water continuous emulsions disperse in water and remain as drops in oil, whereas oil continuous emulsions remain as drops in water but disperse in oil. A Jenway 3540 conductivity meter using Pt/Pt black electrodes was used to determine the conductivity of emulsions. Conductivity measurements were made immediately after emulsification. Low conductivity values were indicative of oil continuous emulsions, whereas relatively high conductivity values were associated with aqueous continuous emulsions. Emulsions were stored at room temperature (21 ± 2 °C) in the vessels used during homogenization. The stability of aqueous continuous emulsions to creaming and coalescence was assessed by monitoring the change with time of the aqueous−emulsion boundary and oil−emulsion interface, respectively.
EXPERIMENTAL SECTION
MATERIALS Water was purified by passing through an Elgastat Prima reverse osmosis unit followed by a Millipore Milli-Q reagent water system. Its surface tension was 72.2 mN m−1 at 20 °C, in good agreement with the accepted literature value. The resistivity of the Milli-Q water was consistently around 18 MΩ cm and its natural pH was 5.6 due to dissolved carbon dioxide. Propane-1,2-diol (propylene glycol) from Dow Corning (98%, racemic mixture) was used as received. Paraffin liquid oil (Total, grade 783LP) was columned over neutral alumina to remove polar impurities. It is a mixture of heavier alkanes (C12−C20) and has a density of 0.86 g cm−3 at 25 °C. Fumed silica particles with different hydrophobicities were provided by Wacker-Chemie (Germany).18 The hydrophilic silica particles, possessing surface silanol groups (SiOH) and with a surface area of 200 m2 g−1, from which the others are derived are produced by hydrolysis of silicon tetrachloride in an oxygen−hydrogen flame at high temperature. In the flame process, molecules of SiO2 collide and coalesce to give smooth and approximately spherical primary particles of 10−30 nm in diameter. These primary particles collide and may fuse at lower temperatures to form stable aggregates of 100−500 nm in diameter. Hydrophobization is achieved by reacting hydrophilic silica with dichlorodimethylsilane (DCDMS) in the presence of molar amounts of water followed by drying at 300 °C for 1 h. This reaction results in the formation of dimethylsiloxy groups on the particle surface without significantly altering the particle diameter. The silanol content was determined by acid−base titration with sodium hydroxide and the relative content of silanol groups after surface modification was determined by dividing the silanol content of the modified silica by that of the unmodified silica (100% SiOH). The carbon content was 5724
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Figure 1. (a) Immersion time of fumed silica powders of different hydrophobicity (given) in propylene glycol−water mixtures at room temperature. Arrows indicate times >24 h. (b) Immersion time of powder into a 50% v/v mixture of propylene glycol and water as a function of % SiOH on silica particles. For oil continuous emulsions, the stability to sedimentation and coalescence was assessed by monitoring the change with time of the oil−emulsion boundary and aqueous−emulsion interface, respectively. The fractions of aqueous ( faq) and oil (foil) phase resolved from an emulsion are determined from the height of aqueous phase resolved (hw), height of oil resolved (ho), volume fraction of aqueous phase (ϕw) and oil phase (ϕo), and the total height of the emulsion plus water and oil resolved (ht). Photographs of the vessels were taken with a Sony Cyber-shot DSC-S950 digital camera. Optical Microscopy. Emulsions were observed with optical microscopy using an Olympus BX51 M microscope. The emulsion was observed raw or diluted in its continuous phase in a dimple cell covered by a coverslip. Digital micrographs were taken with an Olympus DP70 digital camera and edited using Adobe Photoshop 5.0. The mean droplet diameter of an emulsion was calculated from at least one hundred individual drop diameter measurements within 5 min of emulsification. Optical microscopy was also used to distinguish between simple (w/o or o/w) and multiple (w/o/w or o/w/o) emulsions. Freeze Fracture Cryo-Scanning Electron Microscopy of Emulsions. A drop of emulsion was plunge-frozen in slushed liquid nitrogen and transferred to a Gatan Alto-2500 cryo-transfer unit. It was subsequently fractured, warmed briefly to −95 °C, recooled, platinum sputtered, and transferred to a cold stage in a Hitachi S4500 fieldemission scanning electron microscope (SEM). The sample was maintained at −160 °C and imaged at an accelerating voltage of 12 kV.
Digital micrographs were collected using Quartz PCI (v. 4.20) software.
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RESULTS AND DISCUSSION In the first part, we discuss the immersion of fumed silica powders of varying wettability into water−propylene glycol mixtures and, from simple theory, estimate the contact angles of the liquid on particles in air. The materials formed upon aerating these mixtures are then described in terms of the wettability of the particles in situ. In the second part, the properties of emulsions of paraffin liquid and aqueous propylene glycol stabilized by silica particles is discussed in detail. Transitional phase inversion as a function of particle hydrophobicity is described for different water/propylene glycol ratios, and emulsion inversion as a function of propylene glycol content is demonstrated for selected particle hydrophobicities. A model used to calculate the contact angle of particles at the oil−aqueous phase interface is summarized allowing us to predict, for a given propylene glycol content, the hydrophobicity of particles required for phase inversion, which is then compared to the experimental findings. a. Materials Formed in Water−Propylene Glycol− Silica−Air Systems. An indication of the wettability of a 5725
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= 50.7 mN m−1 at 20 °C for the contributions to the surface tension of water from the literature.25 Likewise, we take γpgd = 26.4 mN m−1 and γpgp = 9.0 mN m−1 for those of propylene glycol.26 Because the dispersion contribution varies less than 5 mN m−1 between water and propylene glycol, we assume a monotonic variation of γpgd with propylene glycol content in the aqueous phase and calculate γpgp from this and the liquid− air surface tension γla. (If γpgd and γpgp are calculated using values of γla and the tension of the liquid−oil interface, γlo, given in Table 1 the difference in the calculated contact angles
powder, which does not require a contact angle measurement comes from the immersion test in which one measures the time for a powder to float on the surface of a liquid before sinking into it. The submersion of a powder involves 3 stages − adhesion, immersion, and spreading − in which the solid− vapor surfaces are replaced by solid−liquid interfaces. For a powder of cube side length equal to 1 cm, the total work for the overall dispersion process, Wd, is equal to −6γlacosθ, where γla is the tension of the liquid−air surface and θ is the contact angle between the solid and liquid phases measured into the liquid. Powders for which θ < 90°, Wd is negative and the process is spontaneous. Powders for which θ > 90°, Wd is positive and work must be done to bring about complete immersion. The immersion time for a fixed mass of silica powder on the surface of the polar liquid is plotted in part a of Figure 1 as a function of the propylene glycol content in water for different particle hydrophobicities (given as % SiOH). For nearly all particles, the time decreases progressively on adding the diol to water, that is wetting is enhanced. As can be seen from part b of Figure 1 for a 50:50 volume ratio of propylene glycol and water, an increase in the hydrophilicity of the particles (or % SiOH) also enhances their wetting by the liquid. To rationalize the above, we briefly describe a model used to calculate the contact angle θ at the air−polar liquid surface as a function of the % SiOH on silica surfaces and the composition of the polar phase. It assumes that no adsorption of propylene glycol occurs on silica surfaces. For a solid particle (s) located at the air (a)−polar liquid (l) interface, the three interfacial tensions are related to θ (measured in the polar liquid phase) by the Young equation γ − γsl cos θ = sa γla (2)
Table 1. Calculated Contact Angles θ at the Air−Aqueous Propylene Glycol−Silica Surface at 20 °C as a Function of Propylene Glycol Content in Water for Surfaces of Different Hydrophobicity (Given as % SiOH)a θ/deg 10% SiOH
30% SiOH
50% SiOH
γlo/ mN m−1
0 10 20 30 40 50 60 80 100
72.2 63.5 56.8 51.4 47.2 44.7 43.6 41.1 35.4
88.0 81.9 76.0 70.1 64.6 60.6 58.4 53.2 39.6
70.8 63.2 55.8 47.9 40.1 34.0 30.6 20.9 0.0
56.6 46.8 36.3 22.6 0.0 0.0 0.0 0.0 0.0
44.5 35.8 30.0 26.6 24.2 21.4 18.9 15.9 12.1
Measured liquid−air surface tensions, γla, and liquid−paraffin liquid (oil) interfacial tensions, γlo, are also given (±0.1 mN m−1) at 20 °C. Surface tension of paraffin liquid−air = 29.9 ± 0.1 mN m−1.
between the two approaches is ±4°, well within the uncertainties of experimental values of θ). As argued earlier,27 the dispersion and polar components of the solid surface energy, γsa, are taken as linearly related to the percentage of silanol groups on particle surfaces. For the most hydrophilic surface (100% SiOH), we choose γsd = 42.0 mN m−1 and γsp = 34.0 mN m−1, the values found experimentally for a clean glass surface with a range of probe liquids.28 For the most hydrophobic surface (0% SiOH), we choose γsd = 22.0 mN m−1 and γsp = 0.9 mN m−1, which are for a high molecular weight poly(dimethylsiloxane) fluid28 whose structure is chemically similar to that of the coating produced by DCDMS. Table 1 shows the calculated contact angles using eq 2−4 at the air−aqueous propylene glycol−silica surface as a function of propylene glycol content in water and for selected surfaces of different hydrophobicity. In line with experiment, θ decreases with propylene glycol content and with increasing particle hydrophilicity. The same samples resulting from the immersion tests were then used to investigate the materials formed on aerating the mixtures by hand shaking in a controlled manner. For air-fumed silica−water systems, we reported earlier that upon increasing the particle hydrophobicity (decreasing % SiOH) stable aqueous dispersions were replaced by foaming dispersions (air-in-water foams) finally leading to phase inversion producing a powder called dry water (water-in-air).29 The appearance of the vessels in the case of particles possessing 51% SiOH is given in part a of Figure 2 for different propylene glycol/water ratios. In pure water (A), unstable foam bubbles coalesce with the liquid surface releasing particles that exert a sizable surface pressure causing a climbing film to coat the
(3)
The interfacial tension between two phases is then expressed in terms of these two components for each phase and becomes γsl = γsa + γla − 2 γsdγld − 2 γspγlp
γla/ mN m−1
a
Although the surface tension between air and polar liquid, γla, can be measured accurately, there are no direct methods for measuring γsa and γsl. It would be useful to use eq 2 to predict values for the contact angle if it were possible to estimate these two interfacial tensions from some other source of data. This requires the use of combining rules that allow any interfacial tension to be predicted from surface tension components and the determination of such components from solid surfaces. A common approach is that of expressing any surface tension γ as a sum of components due to dispersion forces (γd) and polar forces (γp):23
γ = γd + γ p
propane-1,2-diol/ % v/v
(4)
for the solid−polar phase interface. In an attempt to relate components more closely to the chemical nature of the phases, van Oss et al.24 suggested that the polar component could be better described in terms of acid−base interactions. These are the electron-acceptor and the electron-donor surface tension parameters and are complementary in nature. In our experience, when such components are used to predict quantities such as contact angle, the values are very close to those obtained by the simple Owens and Wendt method described here. To determine γsl (eq 4) in cases where the liquid varies from water to propylene glycol, we take γwd = 21.5 mN m−1 and γwp 5726
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Figure 2. (a) Photograph of vessels taken 1 min after shaking mixtures of air, aqueous propylene glycol, and 25 mg of 51% SiOH fumed silica powder at room temperature; (A) represents a climbing film (powder remains on liquid surface), (B) represents a stable foam with climbing film, (C) is for a stable foam, and (D) refers to an aqueous dispersion; (b) liquid marbles of propylene glycol stabilized by 14% SiOH particles. Scale bar = 1 cm.
vessel sides.30 Upon increasing the propylene glycol content, because particles are rendered more hydrophilic in situ (lower θ), the progression of behavior is a foam + climbing film (B) to a stable foam (C) to a stable dispersion (D). For very hydrophobic particles (14% SiOH), liquid marbles (drops in air) of neat propylene glycol can be stabilized by adsorption of particles, part b of Figure 2, which are resistant to coalescence and very stable to evaporation (right photo). A schematic showing the location of the different materials formed in water−propylene glycol mixtures for all of the silica particles is given in Figure 3, in which we designate a stable foam as one
propylene glycol/water ratios and those formed as a function of the propylene glycol/water ratio for selected particle hydrophobicities. It was reported earlier that propylene glycol, miscible in all proportions with water, has a solubility in paraffin oil at 20 °C of only 0.3 wt %, that is it partitions virtually exclusively to the aqueous phase.8 In the absence of propylene glycol, emulsions invert from w/o to o/w upon increasing the hydrophilicity of the particles (or % SiOH). This transitional phase inversion is readily detected via conductivity measurements alongside the classical drop test as seen in part a of Figure 4. It occurs around 55% SiOH. The appearance of the
Figure 3. Schematic of the materials formed after aerating mixtures of aqueous propylene glycol and fumed silica particles of different hydrophobicity.
lasting for more than 1 week and an unstable foam as one which collapses in less than 5 min. The actual data from which we designate the boundaries (taken as midway between two different point types) is given in Figure S1 of the Supporting Information. Using the data in Table 1, it can be seen that stable foams occur in systems in which θ is in the range 35−70° and require particles of increased inherent hydrophobicity upon increasing the propylene glycol content in water. b. Emulsions Formed in Water−Propylene Glycol− Silica−Paraffin Liquid Systems. i. Experimental Findings. At constant particle concentration (1 wt %) and fixed oil/polar phase volume ratio (1:1), we have investigated emulsions formed as a function of the particle hydrophobicity for selected
Figure 4. (a) Conductivity and type of emulsions prepared from equal volumes of paraffin liquid and water containing 1 wt % silica particles as a function of particle hydrophobicity, (b) appearance of vessels after 6 months containing emulsions in (a) with % SiOH given beneath.
emulsions 6 months after preparation can be seen in part b of Figure 4. For relatively hydrophobic particles (≤42% SiOH) much water remains nonemulsified below the w/o emulsion. The most stable emulsion to coalescence and sedimentation occurs with 51% SiOH particles just prior to phase inversion. The water-continuous emulsions after inversion are stable to coalescence but cream with time; no stable emulsion forms 5727
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Figure 5. (a) Optical microscopy images of emulsions formed in paraffin liquid−water systems for silica particles possessing different % SiOH (given). Emulsions are w/o for 14 and 51% SiOH and either w/o/w or o/w above this. Scale bars = 100 μm. (b) Mean drop diameter vs particle hydrophobicity of emulsions in (a). The values refer to oil globules in w/o/w emulsions.
As observed in a variety of emulsions stabilized by such particles,15,22 the mean drop diameter passes through a minimum value around phase inversion, as seen in part b of Figure 5, alongside a maximum stability to coalescence and either sedimentation (w/o) or creaming (w/o/w). As an example of the influence of propylene glycol on emulsion behavior, we have selected the system containing propylene glycol and no water. Figure 6 shows the appearance of the systems as a function of the particle hydrophilicity (with the conductivity variation being given in Figure S2 of the Supporting Information). Phase inversion of the emulsions
with the most hydrophilic particles (100% SiOH) however. Optical microscopy images given in part a of Figure 5 reveal that simple emulsions of w/o initially invert to multiple emulsions of water-in-oil-in-water (w/o/w) and finally to simple emulsions of o/w upon increasing the silanol content on particle surfaces. The appearance of stable multiple emulsions around inversion with fumed silica particles has been reported previously in the case of oils of commercial purity, for example silicones,31 as in the case here with paraffin liquid. It may be due to oils of different type, for example chain length, favoring both emulsion types with particles of the same hydrophobicity. 5728
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Figure 6. Appearance of vessels after 6 months containing emulsions prepared from 50 vol % paraffin liquid and 50 vol % propylene glycol containing 1 wt % silica particles of different % SiOH content given beneath.
Figure 7. (a) Mean drop diameter vs particle hydrophobicity of emulsions formed in paraffin liquid (50 vol %) and propylene glycol (50 vol %) systems stabilized by 1 wt % silica particles. (b) Optical microscopy images of emulsions in (a). Emulsions are glycol-in-oil for 14% SiOH and oil-inglycol above this. Scale bars = 100 μm.
occurs at a much lower % SiOH (between 14 and 23%) than in the case with pure water, that is replacement by propylene
glycol renders otherwise hydrophobic particles more hydrophilic. Again, emulsification of all of the water is incomplete for 5729
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images in part b of Figure 9). Using cryo-SEM, the fractal nature of the fumed silica particles at the surface of an aqueous drop in oil can be visualized (bottom left image of part b of Figure 9). Interestingly, the water-continuous emulsion closest to inversion is a stable multiple emulsion of the aqueous-in-oilin-aqueous type (bottom right image of part b of Figure 9). A similar study was conducted in the case of particles possessing 37% and 23% SiOH on their surfaces and will be summarized below. In quaternary systems of silica particles, paraffin liquid, propylene glycol, and water, the results of the studies mentioned above are summarized in Figure 10 in which the glycol content in the emulsion is plotted against the inherent hydrophobicity of the particles. Here, fd represents the fraction of dispersed phase (aqueous or oil), which separates compared to its initial content. Addition of glycol leads to a reduction in the hydrophilicity of particles required to invert emulsions from aqueous-in-oil to oil-in-aqueous. As a result, the region in which aqueous-in-oil emulsions can be stabilized is significantly reduced. Stable multiple emulsions of the aqueous-in-oil-inaqueous type occur near phase inversion for low contents of propylene glycol. The most unstable emulsions to coalescence ( fd > 0.1) are found at high glycol content with relatively hydrophilic particles (>50% SiOH). However, oil-in-glycol emulsions completely stable to coalescence ( fd = 0) can be formed at high levels of glycol, including systems without water, with particles of intermediate hydrophobicity. ii. Theoretical Approach. We seek to understand the main influence of propylene glycol in these emulsions in terms of the apparent contact angles of the particles at the paraffin liquid− polar phase interface. For a solid particle (s) located at the oil (o)−polar phase (l) interface, the three interfacial tensions are related to the contact angle θ (measured in the polar phase) by the equivalent Young eq 2, in which γsa becomes γso and γla becomes γ lo. The interfacial tension between aqueous propylene glycol and oil, γlo, has been measured (Table 1) and values of γsl for silica surfaces of different SiOH content were calculated earlier. To use the modified eq 2 to predict values for the contact angle, we need to calculate γso. Using combining rules and the methodology discussed earlier in relation to eq 3, the interfacial tension for the solid−oil and polar liquid−oil interfaces respectively become
particles forming glycol-in-oil emulsions. For oil-in-glycol emulsions, the stability to coalescence and creaming over 6 months is very high near phase inversion but by 61% SiOH both processes are significant until, at and above 88% SiOH, complete phase separation (CPS) occurs immediately after emulsion formation. The average drop diameter is minimum at phase inversion (part a of Figure 7) with particularly small oil drops of around 5 μm being stabilized by particles possessing 23% SiOH. Optical micrographs given in part b of Figure 7 reveal the nonspherical nature of these drops arising from the jamming of particles at interfaces preventing shape relaxation. This long-term stability afforded by adsorbed particles is in sharp contrast to emulsions stabilized by low molecular weight surfactants.8 There, even with 10 wt % surfactant, no stable emulsion is possible for propylene glycol mole fractions (in mixtures with water) above 0.4. The same investigation was repeated in systems containing a 20:30 or a 25:25 ratio of propylene glycol/water and will be summarized later. At fixed particle hydrophobicity (42% SiOH), the effect of increasing the propylene glycol content in water on particlestabilized emulsions has also been investigated. The conductivity variation shown in Figure 8 indicates phase inversion
Figure 8. Conductivity and type of emulsions prepared from equal volumes of paraffin liquid and aqueous propylene glycol containing 1 wt % silica particles possessing 42% SiOH as a function of glycol content in the emulsion.
γso = γsa + γoa − 2 γsdγod − 2 γspγop
(5)
γlo = γoa + γla − 2 γodγld − 2 γopγlp
from oil-continuous to aqueous-continuous emulsions around 4 vol % propylene glycol in the emulsion (8 vol % in water). The reason for the decrease in conductivity above inversion is the decreased dissociation of NaCl in propylene glycol compared with water. The aqueous-in-oil emulsions do not sediment over a 6 month period but show some coalescence, whereas the oilin-aqueous emulsions including the emulsion made of neat propylene glycol exhibit no coalescence (Figure S3 of the Supporting Information). Their stability to creaming increases moving away from inversion. We thus observe a different trend in emulsion stability when varying the composition of these emulsions compared with varying the particle hydrophobicity. The variation in mean drop diameter with propylene glycol content is given in part a of Figure 9 where it can be seen that the smallest drops (aqueous-in-oil at low glycol content and oilin-aqueous at high glycol content) give rise to emulsions with high stability to gravity-induced separation as expected. Optical microscopy reveals spherical drops in all emulsions (first 4
(6)
γod
For the determination of γso (eq 5), estimates of and γo are required. The appropriate form of eq 3 for the oil−air surface tension is γoa = γod + γop with that for the polar phase−oil tension γlo given by eq 6. From experiment, we know values of both these tensions (Table 1). As a result, solution of the simultaneous eqs 3 and 6 allows us to calculate γod and γop (and hence γso for surfaces of different SiOH content). The values are 29.9 and 0 mN m−1, respectively. As an example, the calculated contact angle θ of a particle at the oil−polar phase interface is plotted as a function of the % SiOH on particle surfaces in part a of Figure 11 for systems containing oil and 50 vol % propylene glycol in water (left-hand ordinate, filled line). It decreases as expected with an increase in particle hydrophilicity and equals 90° for particles possessing 13% SiOH. Assuming emulsions are of the opposite type below and above 90° (Introduction), this denotes the calculated % 5730
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Figure 9. (a) Mean drop diameter vs. propylene glycol content in emulsions of paraffin liquid and aqueous propylene glycol stabilized by 1 wt % of 42% SiOH silica particles. (b) Upper four − optical microscope images of emulsions described in (a); scale bars = 100 μm. Lower two − cryo-SEM images of emulsions containing 2.5 (aqueous-in-oil) and 5 (aqueous-in-oil-in-aqueous) vol % propylene glycol in the emulsion.
SiOH at phase inversion. The free energy change upon desorption of a single particle, ΔE, calculated using eq 1 is also plotted in part a of Figure 11 (right-hand ordinate, dotted line). A distinct maximum occurs at 13% SiOH implying maximum emulsion stability at this condition. A comparison between the
measured and calculated emulsion phase inversion points derived in this way for all of the systems investigated is shown in part b of Figure 11. Overall, the agreement is reasonable within the uncertainties over a range of particle wettability and similar to that observed for silica-particle stabilized emulsions of 5731
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Figure 10. Emulsion type and coalescence stability as a function of propylene glycol content and % SiOH on particles (1 wt %) in paraffin liquid− propylene glycol−water−silica emulsions. Solid lines indicate the boundaries between the different emulsions, dotted line indicates most hydrophobic particles, dashed line indicates waterless emulsions. Within 6 months: fd = 0 (unfilled squares), fd < 0.1 (gray squares), fd > 0.1 (black squares).
water and a range of polar oils.22 However, emulsions prepared with the powdered particle method at 1 wt % particles (open points) invert at a consistently higher SiOH content (more hydrophilic) than calculated. The relatively high concentration combined with the absence of predispersion in one of the phases results in particle aggregation believed to enhance particle hydrophobicity demonstrated previously.22 As a result, two new emulsion series were prepared for different particle hydrophobicities at a lower particle concentration of 0.1 wt %, one containing water and no glycol, the other containing 50% glycol in water. The results are given by the filled points in part b of Figure 11, where the improvement in agreement between measured and calculated inversion only occurs in emulsions without glycol. Possible reasons for the mis-match in glycolcontaining emulsions are not accounting fully for all of the interactions between the different phases (oil, polar phase, and solid) and the possibility that glycol may adsorb to some extent on particle surfaces for certain systems. Both of these ideas require further investigation in the future.
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CONCLUSIONS
The effect of adding propylene glycol to aqueous dispersions of silica nanoparticles and paraffin liquid−water emulsions stabilized by the same particles has been investigated. Without oil, aerated mixtures of aqueous propylene glycol and particles yield stable dispersions, aqueous foams, climbing particle films, or liquid marbles depending on the glycol content and particle hydrophobicity. Particles behave as if they are more hydrophilic in the presence of glycol. In the presence of oil, particlestabilized emulsions invert from aqueous-in-oil to oil-inaqueous on increasing either the hydrophilicity of the particles or the glycol content in the system, and stable waterless oil-inglycol emulsions can be prepared. Using calculated contact angles at the oil−polar phase interface, reasonable agreement is found between measured and calculated phase inversion conditions.
Figure 11. (a) Calculated oil−polar phase contact angle (filled line, left-hand ordinate) and detachment energy (dashed line, right-hand ordinate) of silica nanoparticles in paraffin liquid and 50 vol % propylene glycol in water systems as a function of % SiOH on particle surfaces. Particle radius = 10 nm. (b) Comparison of calculated and measured % SiOH at phase inversion for emulsions containing paraffin liquid, water, aqueous propylene glycol, or neat propylene glycol. Filled and open points represent emulsions prepared with 0.1 or 1.0 wt % particles, respectively.
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ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S3 contain additional data relating to Figures 3, 6, and 8, respectively. This information is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank G. S. K. (U.K.) for a studentship to MAT, WackerChemie (Burghausen) for the fumed silica powders and Miss K. Adom, Mr. L. Kang, Mr. J. Muir, and Mrs. P. Underwood for collecting some of the data presented. We also thank Mr. R. Lees of Intertek MSG (Redcar) for carrying out the cryo-SEM measurements.
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