ARTICLE pubs.acs.org/Langmuir
High Internal Phase Emulsions: Catastrophic Phase Inversion, Stability, and Triggered Destabilization Timothy S. Dunstan, Paul D. I. Fletcher,* and Saeed Mashinchi Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, United Kingdom
bS Supporting Information ABSTRACT: We have investigated the formation, drop sizes, and stability of emulsions prepared by hand shaking in a closed vessel in which the emulsion is in contact with a single type of surface during its formation. The emulsions undergo catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) as the oil volume fraction is increased. We find that the oil volume fraction required for catastrophic inversion exhibits a linear correlation with the oilwatersolid surface contact angle. W/o high internal phase emulsions (HIPEs) prepared in this way contain water drops of diameters in the range 10100 μm; emulsion drop size depends on the surfactant concentration and method of preparation. W/o HIPEs with large water drops show water separation but w/o HIPEs with small water drops are stable with respect to water separation for more than 100 days. The destabilization of the w/o HIPEs can be triggered by either evaporation of the oil continuous phase or by contact the emulsion with a solid surface of the “wrong” wettability.
1. INTRODUCTION An emulsion consists of a thermodynamically unstable dispersion of liquid drops in a continuous phase of a second liquid which is immiscible with the first. They can be made kinetically stable by addition of suitable stabilizers, surfactants, polymers, or solid particles, which adsorb at the drop surfaces. For emulsions containing oil and water as the immiscible liquids, the common emulsion types are either oil-in-water (o/w) or water-in-oil (w/o). Phase inversion between o/w and w/o can be achieved by either transitional or catastrophic inversion. In transitional inversion, the oil/water volume ratio is held constant (normally at equal volumes of oil and water) and the hydrophiliclipophilic balance (HLB) of the stabilizer (or parameter such as electrolyte concentration or temperature which affects the surfactant HLB) is varied. Hydrophilic stabilizers produce o/w, whereas hydrophobic stabilizers produce w/o emulsions. In catastrophic inversion, the stabilizer and all variables affecting its HLB are held constant and only the volume ratio of oil and water is varied. Low volume fractions of water tend to produce oil-continuous emulsions and vice versa. In this study, we focus on w/o emulsions which contain a high volume fraction of water drops, in excess of volume fraction = 0.74, corresponding to close packing of monodisperse spheres. In these w/o high internal phase emulsions (HIPEs), the water drops are deformed to polyhedra which are separated by thin oil films producing an overall liquid-in-liquid foam type structure.112 These w/o HIPEs have several advantages in formulations for many practical applications. First, the high drop volume fraction causes the emulsions to behave as gels and this suppresses instabilities due to sedimentation or creaming of the drops. Second, w/o HIPEs contain a lower content of organic solvent than their r 2011 American Chemical Society
non-HIPE alternatives and are thus more environmentally and economically acceptable. Third, it has been shown recently that chemically incompatible aqueous species can be compartmentalized in separate water drops of mixed w/o HIPEs.13 This opens the possibility to produce “smart” liquid formulations for which triggered emulsion destabilization and reaction of the water drop contents can be used to produce a desirable additional functionality in the product, for example, foam generated by chemical reaction of the water drop contents following destabilization and drop mixing. Lastly, w/o HIPEs for which only the oil continuous phase is in contact with external container surfaces are advantageous for several practical applications. Examples include emulsion explosives which consist of a HIPE of aqueous ammonium nitrate drops dispersed in a fuel oil continuous phase. The oil as continuous phase aids the emulsions ability to resist the ingress of water when injected into wet blasting holes in quarries.14 Second, using w/o HIPEs also offers a means to suppress corrosion problems in metal containers since the presence of the oil continuous phases avoids contact of the corrosive water drops with metal container surfaces and thereby reduces corrosion.15 In this work, we have investigated the formation and stability of w/o HIPEs when they are prepared by simple hand shaking of mixtures of oil, water, and stabilizer in closed containers of different wall materials and address following questions: (1) How does the emulsion type and catastrophic inversion depend on the wettability of the container surface by the oil and aqueous phases? In addition to being a simple, low Received: October 20, 2011 Revised: November 30, 2011 Published: November 30, 2011 339
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the emulsion, i.e. does the point of catastrophic phase inversion correlate with the contact angle formed by the oilwater interface with the solid surface of the vessel wall? Although this possible effect is not generally invoked in discussions of the mechanism of catastrophic phase inversion, early work by Davies and Rideal demonstrated a strong influence of the wettabilities of the contacting surfaces on emulsion type.37 They examined emulsification occurring when oil and water was pumped between rotating plates formed from a range of materials and found that the oil volume fraction required for catastrophic phase inversion depended on the rotating plate material. In agreement with this early finding, recent studies of emulsion formation in microfluidic devices have shown that the walls of the microfluidic channels must have the appropriate wettability to produce the desired emulsion drop type.3841 Although data is limited and not systematic, the available evidence suggests the transition from the formation of water drops in oil to oil drops in water occurs when the oilwater contact angle with the channel wall is around 90°. In the context of whether or not oilwatersolid contact angle partially determines the point of catastrophic phase inversion, it is worth noting that hysteresis between advancing and receding contact angles could potentially account for the hysteresis observed in catastrophic phase inversion. This paper is organized as follows. Following the experimental section, systematic measurements of how the oil volume fraction at catastrophic phase inversion depends on the oilwatersolid contact angle for a wide range of hand-shaken emulsion systems are described. Second, emulsion drop sizes and stability of selected w/o HIPE systems are discussed. Finally, we show how selected w/o HIPEs can undergo triggered destabilization by either (i) evaporation of the oil continuous phase or (ii) by contact with a solid surface of the “wrong” wettability.
energy method of emulsion preparation, the hand-shaking method used here has the advantage that the emulsion contacts only a single type of solid surface during its formation. (2) How does the emulsion drop size depend on the preparation method? (3) How does the emulsion stability depend on the stabilizer and drop size? (4) Can emulsion destabilization be triggered by either (i) evaporation of the continuous oil phases or (ii) contacting the emulsion with a surface which is wettable by the emulsion drops? The background literature relating to this study is found in several diverse fields and key points are briefly summarized here. Previous work has established the relationship between HIPE formation and the equilibrium phase behavior of the corresponding mixtures of oil + water + surfactant.5 At equilibrium, such mixtures generally form either a two-phase Winsor I system (consisting of an o/w microemulsion plus an excess water phase), a three phase Winsor III (consisting of a bicontinuous microemulsion plus excess oil and water phases), or a two-phase Winsor II system (consisting of a w/o microemulsion phase plus an excess water phase). Which Winsor system forms at equilibrium is determined by the surfactant HLB and prevailing conditions (e.g., electrolyte content, temperature, etc.) and is rationalized in terms of the “preferred” curvature of the adsorbed surfactant monolayer at the oilwater interface. Systems with a positive (corresponding to hydrophilic headgroups on the external surface) “preferred” curvature form Winsor I systems, whereas negative curvature produces a Winsor II. The point of microemulsion phase inversion, which occurs in the Winsor III range, corresponds to zero “preferred” monolayer curvature and produces a minimum in the value of the oilwater tension. Emulsification of the equilibrium, multiphase Winsor systems produces thermodynamically unstable emulsions which may or may not be kinetically stable. For equal volumes of oil and waterrich phases, it is well established that a Winsor I system always produces an o/w emulsion whereas Winsor II gives w/o; that is, the point of microemulsion phase inversion also corresponds to the point of emulsion transitional phase inversion.1623 Emulsification of three-phase Winsor III systems can produce a variety of nonsimple emulsions but are generally very unstable as a consequence of the very low oilwater tension. Stable w/o HIPEs are found for surfactants and prevailing conditions which show Winsor II equilibrium phase behavior.5 Catastrophic phase inversion of emulsions has been found to depend on the surfactant HLB and prevailing conditions, in-line with the patterns of behavior described above for transitional phase inversion. In addition, the point of catastrophic phase inversion has been shown the exhibit hysteresis; that is, it depends on whether the inversion is driven by the addition of oil or by the addition of water. It is also influenced by the method and extent of energy input during emulsification and, under some circumstances, the mechanism is thought to involve the formation of multiple emulsions (oil-in-water-in oil or water-in-oil-in-water). The behavior of surfactant-containing systems prepared by standard emulsification methods is detailed in refs 2433; studies of systems (with or without surfactants) in turbulent flow in pipes are described in refs 3436. In this paper, we investigate whether emulsion type is dependent on the wettability of the vessel walls by the liquid phases of
2. EXPERIMENTAL SECTION 2.1. 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 71.9 mN m1 at 25 °C, in good agreement with literature. The oils n-pentane (Fisher, >99%), n-hexane (Fisher, 99%), n-heptane (Fisher, 99%), and n-octane (Alfa Aeser, 99%) were columned over neutral aluminum oxide 90 (Merck) to remove polar impurities. n-Butane (99%) was supplied by Sigma. A series of nonionic surfactants consisting of oleyl hydrophobic chains bonded to polyglcerol units are denoted here with the abbreviation OnGm. The average molecular structures were determined from the manufacturers’ information on the average number of glycerol units per molecule and the measured saponification number to derive the number of oleyl chains. O2G2 (trade name Emulsogen OG, Clariant), O2.2G1 (glycerol dioleate, Clariant), O1.9G3 (trade name Mazol PGO 31K, BASF), O1.4G1 (trade name Cithrol GMO 0041, Croda) and O0.9G2 (trade name DGMO-90 V, Nikkol) were used as supplied. A similar series of surfactants consisting of iso-stearate units coupled with polyglycerol units were denoted here as ISnGm. IS3.2G2 (Clariant) was used as supplied. Alkyl ethoxylates of the general structure H(CH2)n (OCH2CH2)mOH are denoted here with the abbreviation CnEm. The samples used (“Brij” series) contained a distribution around the mean number of ethoxy units. C12E4 and C16E2 (Fluka), C18E2 (Aldrich), C18E10 (ICI), and C18E20 (Sigma) were used as supplied. The polymeric surfactants Hypermer B210 (Croda), Hypermer B206 (Uniqema), Hypermer B246 (Uniqema), Hypermer KD9 (Croda), Hypermer LP1 (Uniqema), and Atlox 4913 (Croda) were used as supplied. The ionic surfactants sodium dodecyl sulfate (SDS, Sigma-Aldrich, approximately 95%), cetyltrimethylammonium bromide (CTAB, ACROS, >99%), and 340
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benzyldimethylhexadecylammonium chloride (Fluka, >97%) were used as supplied. The salts sodium molybdate dihydrate (Sigma, 99+%) and sodium tetraborate decahydrate (Sigma, 99.5%) were used as supplied. 2.2. Methods. Three different vessels were used for emulsion preparation: a glass cylinder (height 33 cm, internal diameter 5 cm), a tin aerosol can (height 20 cm, internal diameter 5 cm), and a high density polyethylene (HDPE) bottle (height 17 cm, internal diameter 6.5 cm). The critical oil volume fraction required for catastrophic phase inversion was determined by addition of the required volumes of aqueous and oil phases to the vessel in separately prepared samples. The aqueous phases contained the required concentrations of ionic surfactants whereas nonionic surfactants were initially dissolved in the oil phase. For each emulsion sample, the total volume of liquid was normally constant at 300 mL. The vessels were sealed and vigorously hand shaken five times to produce the emulsion; one hand shake corresponded to vigorous motion from shoulder to waist height and back. It was checked using repeated experiments with two different researchers that this procedure gave emulsions with properties which were reproducible within the estimated uncertainties. The type of emulsion formed (w/o or o/w) was determined using drop tests in which drops of the emulsion were added to the aqueous or oil phase. For a w/o emulsion, the emulsion drop dispersed in the oil phase whereas it remained as a separate drop when added to the water phase. The converse was observed for an o/w emulsion. For a series of emulsions prepared with increasing oil volume fraction, it was observed that increasing oil volume fraction produced a catastrophic phase inversion from o/w to w/o at a critical oil volume fraction ϕ*. For each system, the measurements were repeated with different oil volume fractions until ϕ* was obtained within an accuracy of about (0.02. The stability of emulsions was determined by preparing the emulsion sample in the glass measuring cylinder and recording the heights of the water-emulsion, emulsion-oil and emulsion-air interfaces over time. To determine oil evaporation triggered destabilization of the emulsions, the emulsions were prepared in glass vessels. Ten milliliter emulsion samples initially containing 0.34 volume fraction of pentane were transferred to open glass sample tubes (25 mm diameter, 75 mm height) containing a magnetic stirrer. The magnetically stirred samples were contained within a perspex box through which air (thermostatted at 25 °C) was blown at a constant rate. The sample mass loss over time was recorded. This mass loss was equated with the mass of pentane lost by evaporation since separate experiments showed that the rate of water loss under these conditions was small enough to be neglected. Literature data for the rates of pentane and water mass loss by evaporation show that pentane evaporates approximately 50 times faster than water at 25 °C.42 Within a few minutes, the samples showed resolved pentane at their upper surfaces due to sedimentation of the water drops. Following evaporation of this resolved pentane, the samples contained uniformly white, opaque emulsions for which continued evaporation of pentane continuous phase causes the mass fraction of the water drops to increase with concomitant increased droplet deformation and thinning of the oil films between the drops. At a (sample dependent) critical water drop weight fraction, water phase resolution was seen at the emulsion surface by visual observation. Allowing the pentane to evaporate fully eventually produced a fully separated, single aqueous phase. Emulsion drop sizes were determined by optical microscopy using a Nikon Labophot microscope with a Qimaging digital camera connected to a PC. Number average mean drop diameters were determined from the images using Image Pro Plus 5.1 software. All drop size distributions were monomodal. Emulsion samples were withdrawn from the vessel using a glass Pasteur pipet, diluted 25 fold with heptane (for w/o) and a drop placed in a dimpled glass microscope slide which was then covered with a coverslip to avoid rapid oil evaporation. Problems due to emulsion instability during imaging or arising from contact of the emulsion with the “wrong” type of surface were minimized by obtaining
Figure 1. Upper plot: advancing contact angle θ (measured through water drop) versus surfactant HLB for aqueous solutions of OnGm (0.35 wt %) and CnEm (2.0 wt %) on tin surfaces under pentane. The solid line is guide for the eye. Lower plot: correlation between pentane volume fraction at phase inversion and the contact angle for the two classes of nonionic surfactant. The solid line shows the correlation according to eq 1. the images as fast as possible (within 1 min or so). Results for systems which were observed to be unstable during imaging were discarded. Equilibrated aqueous and oil phases were used to measure the contact angles of drops of the aqueous phase under oil on the different solid surfaces. Ten milliliters of the aqueous phase and 10 mL of the oil phase were magnetically stirred overnight in tightly stoppered vessels held in a thermostat bath at 25 °C. The level of stirring was adjusted to be insufficient to cause emulsification but rapid enough to enable the equilibrium distributions of all species between the oil and water phases. To measure the contact angle, a transparent glass cell containing a 1 1 cm2 piece of glass, tin, or HDPE surface was half filled with the equilibrated oil phase. Glass pieces were cleaned with 1.8 M KOH in ethanol, rinsed in pure water, and air-dried prior to use. Tin and HDPE pieces were cleaned with acetone, rinsed in pure water, and air-dried. The cleaning was checked using measurement of the contact angle of a drop of pure water under air. The cell was mounted within a Kruss contact angle goniometer (Type G-1). A small water drop of volume approximately 5 μL was then formed on the surface using a 50 μL syringe and the contact angles measured over time for both the left and right-hand sides of the drop until no further change was observed (typically a few minutes). Final, averaged contact angles were reproducible to (5°. All contact angles quoted here refer to the angle measured through the water phase and correspond to the static, advancing angles. 341
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Figure 2. Correlation of pentane volume fraction at phase inversion with advancing contact angle θ (measured through water drop) for different classes of nonionic surfactants on tin surfaces. The data set includes systems with different surfactant concentrations (0.15 wt %) with or without molybdate and borate salts (typically 0.4 wt %) and low concentrations of perfume oils (0.25 wt %). The solid diagonal line shows the locus of catastrophic phase inversion (according to eq 1) from o/w (below the line) to w/o above the line. The dashed diagonal lines indicate (20%. Except where noted, all measurements were made at room temperature of 21 ( 2 °C.
3. RESULTS AND DISCUSSION
Figure 3. Advancing contact angle θ (measured through water drop) for aqueous solutions of O2G2 mixtures with SDS (upper plot) and CTAB (lower plot). The total mixed surfactant concentration (with respect to the total emulsion volume) was 10 mM.
3.1. Catastrophic Phase Inversion and Correlation with Contact Angle. We first investigated how the oil volume fraction
at catastrophic phase inversion depended on surfactant structure for different nonionic surfactants for emulsification within tin vessels. Results for OnGm and CnEm surfactants of different hydrophilelipophile balance (HLB) are shown in Figure 1. For the different surfactants the HLB values were calculated as the wt% of hydrophilic groups in the molecule according to the scale devised by Griffin.43,44 As seen in Figure 1 (upper plot), hydrophobic surfactants (low HLB) show high water drop contact angles on tin under oil. In Figure 1 (lower plot), the hydrophobic surfactants (with low HLB and high contact angles) also show catastrophic phase inversion from o/w to w/o at low pentane volume fractions. For a wide range of systems including variable concentrations of different nonionic surfactants, electrolytes and perfume oils, we find an approximate correlation between the pentane volume fraction at phase inversion and the contact angle as seen in Figure 2. To test whether the catastrophic phase inversion behavior was sensitive to the nature of the oil phase, the value of ϕ* was measured for pentane, hexane, heptane, and a butanepentane mixture with average chain length of 4.8. The emulsions all contained 0.35 wt % O2G2, 0.08 wt % sodium molybdate and 0.37 wt % sodium borate and were prepared in glass cylinders. (The salts were included in these emulsions as they are commonly added as corrosion inhibitors in related formulations used in many practical applications.) For the different oils, the value of ϕ* was found to be constant at 0.29 ( 0.03. The correlation between phase inversion oil fraction and contact angle was tested further by using three different solid surfaces (tin, glass, and HDPE) and stabilizer consisting of a nonionic surfactant mixed with an anionic or cationic surfactant.
Figure 4. Correlation of pentane volume fraction at phase inversion with advancing contact angle θ (measured through water drop) for aqueous solutions of O2G2 mixtures with either SDS or CTAB on different solid surfaces. The total mixed surfactant concentration (with respect to the total emulsion volume) was 10 mM. The solid diagonal line shows the locus of catastrophic phase inversion according to eq 1 from o/w (below the line) to w/o above the line. The dashed diagonal lines indicate (20%.
Although the three solids show similar contact angles, the water drop contact angle decreases strongly with addition of the anionic 342
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Table 1. Literature Values of the SolidOilWater Contact Angle (measured through the water) for Equilibrated Oil and Water Phases under Conditions Corresponding to the Point of Microemulsion Phase Inversion (taken to be the point at which the oilwater tension is minimum)a solid polyester
oil triolein
surfactant C12E5
variable used to produce
contact
microemulsion phase inversion
angle/deg
temperature
squalane
polyester
85 (ns) C16OXS
NaCl concentration
squalane
140 (ns)
n-decane
glass
triolein
>170 (ns) C12E3/SDS
Mole ratio of C12E3:SDS
quartz
85 (ns)
squalane
90 (ns)
n-hexadecane
95 (ns)
n-decane n-decane
AOT
NaCl addition
hydrophobized glass pyrex
85 (ns) 50 (ns) 140 (ns)
n-hexadecane polyester
47
105 (ns)
n-hexadecane n-decane triolein
85 (ns)
ref
90 (ns) 118 (adv)
48
70 (adv) 90/10 Isopar M/heavy aromatic naptha oil
63/37 monoethanolamine salt of dodecyl
NaCl addition
68b (adv and rec)
46
68b (adv and rec)
o-xylene sulfonate/t-amyl alcohol
68b (adv and rec)
Teflon a
These conditions correspond to the point of transitional phase inversion of the corresponding emulsions containing 0.5 volume fraction of oil and water. b These contact angles refer to solid(water or oil)microemulsion phase angle. ns = not specified whether angle is advanced or receded. adv = advanced contact angle. rec = receded contact angle.
SDS but remains approximately constant with addition of the cationic species CTAB (Figure 3). As detailed in ref 45, the wateroil-solid contact angle depends on the balance of the solidoil, solidwater, and oilwater interfacial tensions according to Young’s equation. Differential surfactant adsorption to these three interfaces can cause the contact angle to either increase, decrease, or remain approximately constant with increasing surfactant concentration. Figure 4 shows that the correlation of ϕ* with θ for these systems is remarkably similar to that shown in Figure 2 and strongly supports the conclusion that the approximate correlation between oil volume fraction at catastrophic phase inversion and water drop contact angle is general. As discussed in the Introduction, the type of emulsion formed with different surfactants generally follows considerations of their HLB which, in turn, reflects the equilibrium microemulsion phase behavior and “preferred” monolayer curvature. For the nonionic surfactant data set of Figure 2, it could be argued that low HLB surfactants favor both high contact angles and the formation of w/o emulsions and that the correlation between contact angle and ϕ* is somewhat incidental. It could further be argued (entirely reasonably) that the differences in contact angles measured for the three different solid surfaces are too small to provide credible evidence for a correlation between ϕ* and θ for an individual surfactant system. However, for the mixed ionic/nonionic surfactant systems, both the anionic and cationic surfactants are hydrophilic (high HLB) surfactants but only the anionic species changes the contact angle. Hence, if the emulsion type was determined by the HLB of the surfactant mixture, both the anionic and cationic mixtures with nonionic would be expected to behave similarly. The large differences in measured ϕ* seen for the anionic and cationic mixtures therefore strongly supports the conclusion that emulsion catastrophic phase inversion correlates with contact angle rather than following solely HLB considerations.
The correlation between pentane volume fraction at phase inversion and contact angle (the solid diagonal lines in Figures 2 and 4) can be expressed in equation form as j ¼ 1 ð1=180Þθ
ð1Þ
where ϕ* is the pentane volume fraction at catastrophic phase inversion and θ is the contact angle measured in degrees. The solid diagonal lines in Figures 2 and 4 correspond to eq 1; the parallel dashed diagonal lines correspond to an uncertainty of (0.2 in the values of ϕ* and “bracket” the experimental data. Overall, it is concluded that the volume fraction of oil required to catastrophically phase invert emulsion prepared by hand shaking from o/w to w/o is predicted by eq 1 within an uncertainty of 20% or so. This deviation of (20% is somewhat larger than the experimental uncertainties in the measured pentane volume fractions at inversion ((0.02) and the contact angles ((5°). Although further work is required, we speculate here that the rather high deviation may be related to (arbitrarily chosen) use of the advancing contact angles as opposed to the receding or average angles. The linear correlation represented by eq 1 approximately captures the behavior of the range of systems studied here which include nonionic surfactants and mixtures of ionic and nonionic surfactants. Is the correlation represented by eq 1 obeyed universally for all oil + water + surfactant systems? Some further assessment of this point is made by noting that eq 1 also predicts that transitional phase inversion, for systems with oil volume fraction of 0.5, should occur when θ = 90°. As noted in the Introduction, the point of transitional emulsion phase inversion corresponds to the point of microemulsion phase inversion which, in turn, corresponds to the point at which the oilwater tension passes through a minimum value. It therefore follows that if the linear 343
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correlation of eq 1 is universally valid, that the oilwatersolid contact angle for all solids should be 90° at the point of microemulsion phase inversion when the oilwater tension is minimum. The limited range of values of contact angles at microemulsion phase inversion drawn from the literature4648 are summarized in Table 1. It can be seen that the contact angle is equal to 90° (within 15°) for polyester solid surfaces with either nonionic or nonionic/anionic surfactant mixtures as used here. Systems with a single anionic surfactant deviate from the predicted value of 90°. The contact angles of Reed and Healy46 refer to either the solid-oil-microemulsion phase or solidwater microemulsion phase contact angle for systems containing a mixture of anionic surfactant and nonionic cosurfactant. These angles correspond to the required solidoilwater contact angle for either Winsor I or II systems approaching microemulsion phase inversion but are potentially different for the Winsor III systems at microemulsion phase inversion, they are not solid-oil water angles but solid(water or oil)microemulsion phase angles. Although not directly relevant to the argument presented here, Reed and Healy’s contact angles are included since they show that the measured angles are constant for different solid surfaces. Overall, the very limited data available supports the validity of the linear correlation represented by eq 1 for systems containing nonionic or ionic/nonionic surfactant mixtures with different solid surfaces but not for systems with a single ionic surfactant. We speculate that the different behavior of systems not containing nonionic system may be related to the fact that nonionic surfactant monomers generally show significant partitioning to oils whereas ionic surfactant monomers generally show very low or negligible partitioning to oil from water. As noted above, the contact angle is determined by the balance of solidoil, solidwater and oilwater tensions. Nonionic surfactants, with significant monomer concentrations in both the oil and water phases, will change the tensions of all three interfaces. Ionic surfactants, which generally have virtually zero monomer concentrations in the oil, are probably unable to significantly affect the solidoil interfacial tension and hence may show different contact angle behavior. Overall, we conclude that the linear correlation between ϕ* and θ appears to be general for systems containing different solids and nonionic surfactants and their mixtures with ionics but may not be applicable to systems containing ionic surfactants alone. Further work to test the correlation for a wider range of systems would be useful, particularly to check whether anomalous behavior might be the result of lack of proper phase equilibration or irreproducible contact angles due to measurement difficulties associated with the ultralow oilwater interfacial tensions. In addition, it is currently unclear which of the advancing or receding contact angles is appropriate and whether possible contact angle hysteresis correlates with hysteresis in the measurement of ϕ*. Despite these caveats, the correlation of eq 1 provides a useful tool to predict emulsion type, both for product formulations based on HIPEs and for emulsion formation in microfluidic devices (for which limited available data support the correlation presented here). 3.2. Emulsion Drop Sizes and Stability. In general, the stability of the w/o HIPEs is expected to depend on the nature of the surfactant and additional factors including the emulsion drop size and the oil volume fraction which, in turn, may be affected by the method of preparation and the evolution of the emulsion structure over time. This complexity makes it difficult to unambiguously determine the effects of different single variables on the
Figure 5. Variation of advancing contact angle θ measured through the water drop on tin (upper plot) and pentane volume fraction at phase inversion (lower plot) with surfactant concentration for different OnGm surfactants with or without electrolyte (0.37 wt % sodium borate and 0.08 wt % sodium molybdate).
emulsion stability with respect to drop sedimentation/creaming, flocculation, Ostwald ripening, and coalescence. In this work, we have determined mean initial water drop diameters and overall rates of oil and water separation (corresponding to drop sedimentation and coalescence, respectively) for selected systems and use the results to attempt to determine which factors are dominant in controlling the overall emulsion stability. For the stability measurements, the w/o emulsion were prepared with a constant pentane volume fractions of 0.34, larger than ϕ*. Figure 5 shows how contact angle θ and ϕ* vary with OnGm concentration for emulsions prepared in tin vessels. Both θ and ϕ* change only slightly at low surfactant concentrations (0.1 to 0.51.0 wt %) before reaching concentration-independent plateau values. It appears likely that the surfactant concentrations required to reach the plateau values correspond to the critical aggregation concentrations required to achieve maximum surfactant adsorption at the oilwater interface. Figure 6 shows an optical micrograph of a typical emulsion sample immediately after preparation. In order to resolve individual drops and to avoid complications due to the rapid evaporation of pentane, the emulsions were diluted 25-fold with heptane prior to imaging. Figure 6 also shows the variation of the mean initial drop size with surfactant concentration for three OnGm surfactants. The O2G2 and O1.4G1 drop sizes decrease with increasing surfactants 344
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are slower. The variation in initial oil resolution rates for O1.4G1 with surfactant concentration (Figure 7b) is smaller, in line with the fact that the drop size variation is less. For the O2.2G1 emulsions, which show surfactant concentration independent drop sizes, both the extent and rate of oil phase resolution is less than the other two surfactants. The origin of this different pattern of behavior for O2.2G1 is unclear at present. Over 10100 days, O2G2 emulsions show significant water phase resolution resulting to water drop coalescence. The time taken for the first appearance of resolved water increases with O2G2 concentration; this effect may be either due to the increased surfactant concentration or because the emulsions with high surfactant also have smaller water drops. Emulsions containing 0.2 wt % O2.2G1 show immediate water resolution; this is probably due to there being insufficient surfactant present to achieve the maximum possible adsorption at the oilwater interfaces of the emulsion drops. All higher concentrations of both O2.2G1 and O1.4G1 show no water phase resolution. Having sedimented to form w/o HIPEs containing oil volume fractions of less than 0.19, all the systems possessing low initial water drop sizes are remarkably stable with respect to water phase separation. A further experiment was made to determine whether drop size or surfactant concentration is the main determinant of stability with respect to water phase resolution. For emulsification by hand shaking in a closed vessel, it was observed that the same amount of hand shaking in systems with reduced total liquid volume (i.e., increased empty headspace volume of the closed vessel) produced smaller initial emulsion drop sizes for emulsions of identical composition. This method was used to make a series of emulsions of identical composition but different initial drop sizes. The time taken for observable water phase resolution was then measured for this series of samples. The results are shown in Figure 8 where it can be seen that the time at which water resolution is first observed is strongly dependent on the initial drop size at fixed emulsion composition. Also shown in Figure 8, the water resolution times for emulsions with different initial drop sizes produced by changing surfactant concentration show a very similar dependence on drop size. For the systems studied here, it is concluded that the emulsion stability with respect to water resolution is mainly determined by water drop size, however this is achieved. Having determined that achieving small drops is crucial to obtain emulsions with good long-term stability, we have investigated alternative low-energy input methods to produce small emulsion drop sizes. Figure 9 shows how the water drop size is decreased by addition of 12 mm diameter steel spheres in a tin vessel during hand shaking. This is presumably due to the emulsion experiencing increased shear as a result of the motion of the ball bearings. It was also observed that ϕ* decreased with the addition of the steel balls, but it remains unclear whether this is a consequence of the changing water drop size or the fact that the contact angle may be different for the tin walls of the vessel and the steel surfaces of the ball bearings. Qualitatively similar behavior was observed for the addition of glass spheres and glass or steel sphere of different sizes. 3.3. Triggered Emulsion Destabilization. As discussed in the Introduction, w/o HIPEs provide a design of liquid formulation which can be used to compartmentalize chemically reactive aqueous reagents (for example, acid + base, oxidizing + reducing agent or other combinations) in water drops separated by oil films. For many applications, it would be advantageous to be able to trigger the breakdown of the emulsion structure to cause the compartmentalized
Figure 6. Typical optical micrograph showing the initial appearance of an emulsion containing 0.35 wt % O2G2 (after dilution with heptane) used to derive mean drop diameters. The lower plot shows the initial mean emulsion water drop diameter as a function of surfactant concentration for three OnGm surfactants. The emulsions (300 mL volume) all contained 34 vol % pentane, 0.08 sodium molybdate, and 0.37 wt % sodium borate. They were emulsified by five hand shakes in 500 mL glass measuring cylinders. The solid lines are guides for the eye.
(as commonly observed for emulsion systems), whereas O2.2G1 behaves unusually in that the drop sizes are virtually independent of surfactant concentration. Immediately following preparation, the emulsion samples appear uniformly white and opaque. Over time, upper clear oil phase resolves due to the sedimentation of the more dense water emulsion drops. At (mostly) longer times, lower clear water phase may resolve due to water drop coalescence. The plots of oil and water resolution as a function of time for the different concentrations of the three OnGm systems are shown in Figure 7. The plots of oil resolution as a function of time mostly show biphasic behavior. The O2G2 and O1.4G1 systems show an initial rise in 0.011 days to a plateau at about 4050% resolution of the total oil present which corresponds to a residual oil volume fraction in the emulsion of about 0.19 (equal to (10.45) 0.34 initial oil volume fraction). This oil volume fraction corresponds to a HIPE in which the close packing and deformation of the water drops then resists and slows further oil resolution. Over 10100 days incubation, these HIPEs show further, slow oil resolution either associated with water phase resolution (for O2G2) or in its absence (for O1.4G1). For individual drops, the rate of sedimentation is proportional to the square of the drop radius and hence the time scale for oil resolution is expected to depend on drop size. This effect can be seen for O2G2 drops in Figure 7a where low surfactant concentrations (big water drops) show fast initial oil resolution and high concentrations (small drops) 345
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Figure 7. (a) Oil (upper plot) and water (lower plot) resolution as a function of time for emulsions (290 mL volume) containing O2G2 (at wt% concentrations shown in key), 34 vol % pentane, 0.08 sodium molybdate, and 0.37 wt % sodium borate. They were emulsified by five hand shakes in 500 mL glass measuring cylinders. (b) Oil and water resolution as a function of time for emulsions (290 mL volume) containing O2.2G1 (at wt % concentrations shown in legend), 34 vol % pentane, 0.08 sodium molybdate, and 0.37 wt % sodium borate. They were emulsified by five hand shakes in 500 mL glass measuring cylinders. (c) Oil (upper plot) and water (lower plot) resolution as a function of time for emulsions (290 mL volume) containing O1.4G1 (at wt % concentrations shown in key), 34 vol % pentane, 0.08 sodium molybdate, and 0.37 wt % sodium borate. They were emulsified by five hand shakes in 500 mL glass measuring cylinders.
In the first strategy, the emulsion destabilization is triggered by evaporation of the pentane continuous phase. As described in the Experimental Section, the emulsions were prepared in glass vessels. Ten milliliter emulsion samples initially containing 0.34 volume fraction of pentane were transferred to open glass sample tubes (25 mm diameter, 75 mm height) containing a magnetic stirrer. The magnetically stirred samples were contained within a perspex box through which air (thermostatted at 25 °C) was blown at a constant rate. The sample mass decreased over time. In principle, both water and oil are lost from the emulsions by evaporation; however, the measured mass loss was equated with the mass of pentane lost by evaporation since separate experiments showed that the rate of water loss under these conditions was negligible. Initially, the emulsion samples showed resolved pentane at their upper surfaces due to sedimentation of the water drops. Following evaporation of this resolved pentane, the samples contained uniformly white, opaque emulsions for which continued evaporation of pentane continuous phase causes the mass fraction of the water drops to increase with concomitant increased droplet deformation and thinning of the oil films between the drops. At a (sample dependent) critical water drop mass fraction, water phase resolution was seen at the emulsion surface by visual observation. Allowing the pentane to evaporate fully produced a single aqueous phase. The measured value of the critical water weight fraction at which water separation is first observed provides a measure of the ability of the emulsion system to resist water drop coalescence under this “evaporative
Figure 8. Time at which water separation is first observed as a function of the initial mean water drop diameter for emulsions containing 34 vol % pentane, 0.35 wt % O2G2, 0.08 sodium molybdate, and 0.37 wt % sodium borate. Different drop sizes were obtained by either varying the headspace above the emulsion in tin or glass vessels during hand shaking or varying the surfactant concentration (glass vessels). The solid line is a guide for the eye.
reagents to meet and react and thereby provide additional “smart” product functionality. With this in mind, we have investigated two possible strategies to achieve the triggered destabilization of some of the w/o HIPEs used here. 346
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Figure 9. Effect of addition of steel ball bearings during hand shaking emulsification in tin vessels on the mean initial drop diameter (upper plot) and pentane volume fraction at phase inversion. The emulsions, of total volume 300 mL, contained 0.34 volume fraction pentane and 0.35 wt % O2G2. Figure 11. Images of a w/o HIPE containing 0.9 volume fraction of water and 0.1 volume fraction of pentane stabilized with 5 mM O2G2 by hand shaking in an HDPE vessel. The emulsion water drops are maintained intact when poured into pentane from the HDPE vessel (upper image, right-hand sample) but coalesced into a separate water phase when added via a glass Pasteur pipet (middle image, left-hand sample). The final appearance of both sample tubes is shown in the lower image.
ultracentifugation compression of emulsions to assess their resistance to drop coalescence.4951 Using repeated experiments, the value of the critical water weight fraction by evaporative compression was found to be reproducible to within 0.02. Figure 10 shows the variation of the critical water drop weight percent required to trigger water phase resolution as function of surfactant concentration for three OnGm stabilized systems. The curve for each surfactant shows a maximum, and the curve shapes are tentatively interpreted as follows. At low surfactant concentration (below the critical aggregation surfactant concentration required to produce microemulsion aggregates at equilibrium), the surfactant adsorption at the oilwater interfaces of the emulsion drops is below the maximum possible and the emulsion drops are relatively unstable. At high surfactant concentrations, in excess of that required to achieve monomer concentrations equal to the critical aggregation concentrations plus that required to achieve maximum adsorption onto the emulsion droplets, the excess surfactant is likely to be present in the form of nanometersized w/o microemulsion aggregates in the oil phase. It has been shown that increased concentrations of microemulsion drops
Figure 10. Water concentration at which drop coalescence is first observed under evaporative compression versus surfactant concentration. The emulsions (290 mL volume) all contained 34 vol % pentane, 0.08 sodium molybdate, and 0.37 wt % sodium borate. They were emulsified by five hand shakes in 500 mL glass measuring cylinders.
compression” of the drops. Stable emulsions show a high critical weight fraction of water drops signifying very low volume fractions of oil continuous phase with correspondingly thin oil films separating the drops when they first coalesce. Less stable emulsions show water drop coalescence at lower water weight fractions corresponding to higher oil volume fractions and thicker oil films at drop coalescence. This “evaporative compression” experiment is similar in concept to the use of either osmotic or 347
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Langmuir present in the continuous phase of an emulsion causes depletion flocculation and adhesion between the emulsion drops.52,53 The presence of microemulsion drops in the emulsion continuous phase has two possible consequences. First, the additional volume of the w/o microemulsion drops in the oil phase means that the oil phase volume fraction is actually larger (and the water phase fraction is correspondingly smaller) than estimated using the mass fraction of the pentane alone. Hence, the apparent decrease in critical water weight fraction at surfactant concentrations above the maximum may be explained by this effect. Second, depletion-driven adhesion between emulsion drops will act to thin the oil films between the drops leading them to coalesce at lower critical water weight fractions. Potentially, the data of Figure 10 could be used to derive the critical oil film thickness between the emulsion drops when coalescence first occurs. However, such a full quantitative analysis has not been attempted here since it would require extensive additional measurements (including the critical aggregation concentrations, adsorption isotherms, microemulsion phase behavior, and structure) and is complicated by the polydisperse nature of the emulsion. In line with the limited experiment aims of this work, the results demonstrate the viability of evaporation-triggered destabilization of these w/o HIPEs. The second strategy to achieve triggered destabilization, termed “surface destabilization”, is based on the observations that the emulsion type is determined by the solid-oilwater contact angle and is therefore sensitive to the nature of the solid surface with which it is in contact. The basic idea is to form a w/o emulsion in a vessel of a solid material which favors the formation of a w/o HIPE and then expose it to a second solid surface which favors the formation of an o/w. Does exposure to the second surface favoring the “wrong” emulsion type act to destabilize the w/o emulsion? We prepared a w/o HIPE containing 0.9 volume fraction water and 0.1 volume fraction pentane stabilized with 5 mM O2G2 by hand shaking in an HDPE vessel. For this system, the water drop contact angles are 144° for HDPE and 138° for glass. The corresponding pentane volume fractions at phase inversion are 0.08 for HDPE and 0.13 for glass. Hence, it is predicted that the prepared w/o HIPE emulsion containing 0.1 volume fraction pentane is the “correct” type for HDPE surfaces but the “wrong” type for glass surfaces. As seen in Figure 11 (and the video file in the Supporting Information), the water drops of the w/o HIPE maintain their integrity when poured directly from the HDPE vessel into pentane diluting solvent. However, when a glass Pasteur pipet is used to transfer the same emulsion to the pentane, the emulsion drops are rapidly coalesced within the Pasteur pipet and bulk water is ejected into the pentane. The glass surface of the Pasteur pipet acts to trigger the destabilization of the emulsion water drops. This surface triggered destabilization was not observed with emulsions containing pentane volume fractions outside the critical range of 0.08 to 0.13. W/o HIPEs with pentane volume fractions of 0.2, 0.25, and 0.3, for which w/o is predicted to be the favored emulsion type for both HDPE and glass, all gave diluted w/o emulsions when ejected into pentane from either the HDPE vessel directly or transferred via a glass Pasteur pipet.
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correlation with the solidwateroil contact angle within approximately 20%. This correlation is found for nonionic surfactants of different HLB, mixtures of nonionic with either anionic or cationic surfactants, and different solid surfaces. Second, the stability of w/o HIPEs prepared by this method is very sensitive to the water droplet size; large water drop emulsions are relatively unstable but emulsions with small water drops are extremely stable with respect to separation of the water dispersed phase. For emulsions prepared by hand shaking, smaller water drops (and concomitantly more stable emulsions) can be produced by either increasing the headspace volume in the vessel during hand shaking or by including steel or glass spheres. Finally, we have shown that the emulsions can be destabilized in a triggered manner, either by evaporation of the oil continuous phase or by exposure of the emulsion to a solid surface of unfavorable wettability.
’ ASSOCIATED CONTENT
bS
Supporting Information. Video file. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Mr. Adrian Arksey of the University of Hull for making the measurements shown in Figure 9. ’ REFERENCES (1) Solans, C.; Dominguez, J. G.; Parra, J. L.; Heuser, J.; Friberg, S. E. Colloid Polym. Sci. 1988, 266, 570. (2) Aronson, M. P.; Petko, M. F. J. Colloid Interface Sci. 1993, 159, 134. (3) Pons, R.; Erra, P.; Solans, C.; Ravey, J.-C.; Stebe, M.-J. J. Phys. Chem. 1993, 97, 12320. (4) Solans, C.; Pons, R.; Zhu, S.; Davis, H. T.; Evans, D. F.; Nakamura, K.; Kunieda, H. Langmuir 1993, 9, 1479. (5) Kunieda, H.; Fukui, Y.; Uchiyama, H.; Solans, C. Langmuir 1996, 12, 2136. (6) Cameron, N. R.; Sherrington, D. C. Adv. Polym. Sci. 1996, 126, 163. (7) Hakansson, B.; Pons, R.; Soderman, O. Langmuir 1999, 15, 988. (8) Babak, V. G. J. Dispersion Sci. Technol. 2002, 23, 1. (9) Solans, C.; Esquena, J.; Azemar, N. Curr. Opin. Colloid Interface Sci. 2003, 8, 156. (10) Saiki, Y.; Horn, R. G.; Prestidge, C. A. J. Colloid Interface Sci. 2008, 320, 569. (11) Ikem, V. O.; Menner, A.; Bismarck, A. Angew. Chem., Int. Ed. 2008, 47, 8277. (12) Mudeme, S.; Masalova, I.; Haldenwang, R. Chem. Eng. Process. 2010, 49, 468. (13) Dunstan, T. S.; Fletcher, P. D. I. Langmuir 2011, 27, 3409. (14) Mudeme, S.; Masalova, I.; Haldenwang, R. Chem. Eng. Process. 2010, 49, 468. (15) Foss, M.; Gulbrandsen, E.; Sjoeblom, J. Corrosion 2008, 64, 905. (16) Baldauf, L. M.; Schechter, R. S.; Wade, W. H.; Graciaa, A. J. Colloid Interface Sci. 1982, 85, 187. (17) Anton, R. E.; Salager, J.-L. J. Colloid Interface Sci. 1986, 111, 54. (18) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X.; Lu, J. R. In Emulsions: A Fundamental and Practical Approach; Sjoblom, J., Ed.; Kluwer: Amsterdam, 1992; p 97. (19) Binks, B. P. Langmuir 1993, 9, 25.
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