Recent Developments in Phase Inversion Emulsification

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Industrial & Engineering Chemistry Research

Recent Developments in Phase Inversion Emulsification

Ankit Kumara, Shigeng Lib, Chieh-Min Chengb, Daeyeon Leea,* *Corresponding Author a

Department of Chemical and Biomolecular Engineering, University of Pennsylvania,

Philadelphia, USA E-mail: [email protected] Tel: +1-215-573-4521 b

Manufacturing & Materials Technology Area, Toner Development & Manufacturing Group,

Xerox Corporation, Webster, NY 14580

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ACS Paragon Plus Environment


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Abstract Keywords Introduction Theoretical models of emulsion morphology and phase inversion Bancroft’s rule Hydrophile-lipophilie balance (HLB) Phase inversion temperature (PIT) Winsor R-ratio Hydophilic-liphophilic deviation (HLD) Techniques to Monitor and Characterize Phase Inversion Emulsification Conductivity Viscosity Measurement Light scattering Infrared (IR) spectroscopy Transitional phase inversion emulsification Transitional phase inversion via phase inversion temperature (PIT) method Transitional phase inversion emulsification via competitive adsorption of surfactants Transitional phase inversion of polymer-stabilized emulsions Light-triggered transitional phase inversion Phase Inversion of Particle-Stabilized Emulsions Effect of particle wettability on phase inversion of Pickering emulsions Varying particle wettability by surfactant adsorption Effect of suspension composition and concentration on phase inversion Phase inversion using stimuli-responsive particles Phase inversion of Pickering air-water emulsions Phase inversion of shape-changing amphiphilic particle-stabilized emulsions Flow-Induced Phase Inversion Phase inversion using static mixers Thixotropy-induced phase inversion Phase inversion in microfluidic devices Phase inversion using membrane emulsification Summary & Outlook Acknowledgement References Figures

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Abstract

Emulsions are multiphasic fluid systems in which liquid droplets are dispersed in another immiscible liquid. The main components of an emulsion are the two liquid phases, typically oil and water, and the emulsifier, which stabilizes the interface between the two liquid phases. Emulsifiers can be a variety of molecules such as polymers, amphiphilic surfactants, and proteins, and they can also be colloidal particles. Emulsion phase inversion is the process of inter-conversion between two types of simple emulsions: water-in-oil and oil-in-water emulsions. Phase inversion can be induced by shifting the emulsifier affinity from one phase to the other, which is called transitional phase inversion. It can also be triggered by a change in the water-tooil ratio (WOR) of the emulsion, which leads to a process known as catastrophic phase inversion. With recent advances in the stabilization of emulsions using colloidal particles and stimuliresponsive surfactants, numerous novel emulsion systems that undergo emulsion phase inversion based on various mechanisms have been developed. In this review, we highlight the most recent developments in the field of emulsion phase inversion focusing on transitional phase inversion, inversion of particle-stabilized emulsions as well as flow- and shear-induced phase inversion. We also discuss and compare state-of-the-art analytical methods that have been used to detect and understand the emulsion phase inversion process. Our coverage spans from the early concepts of Bancroft’s rule, through the concept of semi-quantitative hydrophilic-lipophilic balance, and to the more recent theoretical models used to predict and control phase inversion phenomena. We conclude this review by presenting outlook on the future directions and outstanding problems that warrant future investigations to fully understand the mechanism of emulsion phase inversion at the single droplet level. 3



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Keywords Phase inversion emulsification, catastrophic phase inversion, transitional phase inversion, stimuli responsive surfactants, shear-induced phase inversion.

Introduction Emulsions are mixtures of two immiscible liquid phases, typically water- and oil-based phases, in which one phase is dispersed in the other in the form of droplets. A large number of products in pharmaceutical, food, agricultural and cosmetics industries are based on emulsions. For numerous applications, it is necessary to generate emulsions with small droplets (< 1 µm) and narrow size distribution. Generating such an emulsion using the conventional process of directly agitating a macroscopically phase separated binary mixture requires a huge amount of energy because dividing small droplets into even smaller droplets involves overcoming a huge Laplace pressure. The use of surfactants can reduce the Laplace pressure, but in that case, a large surfactant concentration would be required for the generation of industrially relevant micro- and nano-emulsions, which may not be desirable. Thus, low-energy methods that can be used to form these types of emulsions including nano-emulsions with a fraction of energy required by direct emulsification methods are highly desirable. Phase inversion emulsification (PIE) is one such low-energy emulsification technique. In PIE, the physicochemical properties of an emulsion are changed to invert the two phases: that is, from oil-in-water (O/W) to water-in-oil (W/O) emulsions, or vice versa.1-4 This process can be utilized in the generation of not only nano-emulsions with narrow size distribution,5, 6 but also emulsions with high-internal volume fractions, also known as high internal phase emulsions (HIPEs).7,

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Because PIE enables the generation of emulsions that are difficult to make using the conventional methods, it is practiced widely in industry as a method to generate various emulsions. PIE has predominantly been classified into two categories- transitional and catastrophic. Transitional phase inversion is initiated by controlling the strength of interactions between the surfactant and fluid phases. It has been shown, for example, that the interaction between water and the hydrophilic part of a non-ionic surfactant becomes more favorable with decreasing temperature.9 As a consequence, the adsorbed surfactant monolayer has a concave tendency towards oil at lower temperatures, and a concave tendency towards aqueous phase at higher temperatures, passing through a tendency to form flat monolayers at intermediate temperatures. This trend leads to the formation of O/W emulsions at lower temperatures and W/O emulsions at higher temperatures.9, 10 The temperature at which this curvature inversion occurs is called the phase inversion temperature (PIT).11-14 In addition to temperature, other external stimuli of chemical and physical nature can also change the affinity of the surfactant toward water- and oilphases.15-17 In contrast to transitional phase inversion, the system formulation can be changed to invert an emulsion system through changing of the water-to-oil ratio (WOR) of the system18-20 This mechanism is known as catastrophic phase inversion. The critical volume ratio that induces phase inversion is called the emulsion inversion point (EIP), which depends upon the system formulation as well as the conditions of emulsification, such as stirring intensity, location of impeller and rate of addition of dispersed phase.21-23 The EIP has also been shown to depend on the interfacial tension between the fluids. Lower interfacial tension requires higher volume fraction of the dispersed phase for phase inversion, in effect making emulsion inversion more 5



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challenging.24,

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In both of these processes, the inversion of emulsions is a complex

phenomenon, which is often perceived as an uncontrollable instability because until recently, very limited information about the phase inversion mechanism and factors affecting it were understood.26 In addition to these two commonly studied PIE mechanisms involving surfactant-stabilized emulsions, other types of phase inversion systems have also gained recent interest. These include oil-water mixture systems that are stabilized by colloidal particles, also known as Pickering emulsions.8, 27-30 Recent studies have shown that these particle-stabilized emulsions can undergo phase inversion and also exhibit novel double inversion behaviors.31-34

While phase inversion

emulsification typically is performed under vigorous mixing, a different approach to induce phase inversion of emulsions is by using shear and extensional flows.35,

36

Flow- or shear-

induced phase inversions are especially interesting because of the possibility of directly observing the phase inversion process and also the scalability of such a process. Formulation-composition maps, reporting the nature and properties of emulsions as a function of composition (e.g., water-to-oil ratio) and formulation (e.g., hydrophile-lipophile balance (HLB) or the equivalent alkane carbon number of oil), are useful in studying phase inversion systems.37, 38

The phase inversion line can either be a ‘standard inversion line’, which determines emulsion

type that would result from stirring a pre-equilibrated surfactant/oil/water (S/O/W) system under constant formulation and composition, or it can be a dynamic inversion line if phase inversion is induced by changing the formulation/composition of the system. If dynamic inversion is induced by solely changing system formulation while keeping the composition (water-to-oil ratio) constant (i.e. transitional PIE), the system passes through the ‘optimum formulation’ point where the surfactant affinity for the oil and aqueous phases are balanced. Although a few exceptions 6


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have been reported,39 transitional phase inversion, in general, occurs at the same point on the formulation-composition map irrespective of the direction it is approached. In contrast, if composition of the system is changed to induce catastrophic phase inversion, the system exhibits hysteresis, such that the composition of phase inversion depends on the dynamics (such as stirring speed)40 and properties of the system (such as surfactant concentration).38 In this review article, we highlight the recent advances in understanding various phase inversion emulsification phenomena. We discuss the fundamentals of PIE, describing the main parameters that determine the emulsion type and the mechanism of phase inversion based on the emulsion formulation. A concise overview of the common techniques for studying and monitoring phase inversion is provided. This is followed by a detailed discussion of the recent advances in the field of transitional phase inversion. In addition, we highlight novel phase inversion systems such as the phase inversion of particle-stabilized emulsions and shear- and flow-induced phase inversion, which have not been extensively described in recent reviews on phase inversion of emulsions.41 We refer the readers interested in catastrophic phase inversion to seminal studies focusing on this important topic.22, 42-55

Theoretical models of emulsion morphology and phase inversion In this section, we discuss some of the fundamental concepts that are critical in understanding phase inversion emulsification, focusing on the models that have been developed to describe emulsion types and how such models have progressively evolved to explain phase inversion phenomena. One of the very first models that were developed to describe phase inversion was proposed by Ostwald56, 57 who suggested that the internal-phase volume fraction of an emulsion cannot exceed the close-packing volume fraction of hard spheres ( ), which is 74 vol% for 7



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monodisperse spheres, but can be higher for polydisperse systems. The type of emulsion is fixed above ( ), and below (1- ), however there exists ambivalence in the intermediate range. A major drawback of this model is that the role played by system formulation, composition as well as processing conditions is not considered. A statistical mechanical model supporting the idea of an ambivalence region was proposed by Ross and Kornbrekke.58 According to this model, the chance of finding an O/W morphology increases gradually as the water volume fraction ( ) increases from very low to very high. This model is, however, susceptible to statistical mechanical errors in the calculation of the partition function. Additionally, effects of physico-chemical factors such as stirring intensity, addition rate of dispersed phase are difficult to account for. Thermodynamic approaches that take advantage of the analogy between phase separation and emulsion formation have been used to describe the nature of phase inversion processes. In particular, catastrophe theory, which has been used to describe dynamical systems that exhibit sudden changes in their behaviors, has proven to be a useful tool in describing the phase inversion of emulsions. In this approach, a free energy (G(x)) of a system is defined as a polynomial function of a system state variable, x (x, for example, can represent the average density of the outer phase) and control variables such as temperature, pressure, composition or formulation (control variables are variables that can be manipulated). The 4th order polynomial leading to the cusp catastrophe theory18,

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was later extended to the 6th order butterfly

catastrophe to explain the phase inversion of a ternary mixture composed of surfactant, oil and water.60, 61 Depending on the values of the control variables (b and d, representing formulation and composition respectively in Figure 1), the system can exist as either O/W or W/O emulsion as seen in the locus of free energy extrema (Figure 1(a)). In the region where an emulsion can 8


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exist as either O/W or W/O for a given pair of b and d, the type of the emulsion is determined by its history. For example, as illustrated in Figure 1(a), when the mixture is moved along the path ABDE, the system in the ambivalent region (between B & D) is O/W if it starts at A and W/O if it starts at E. At the edge of the ambivalent region (i.e., at D (or B’)), the system suddenly jumps to the emulsion type which is possible at D’ (or B). This sudden jump is catastrophic phase inversion. Also, since the jump occurs at different points (D or B’), depending on the direction of the path, catastrophic inversion exhibits a hysteresis and is therefore irreversible. Transitional phase inversion occurs when the system WOR is kept constant but the system formulation is changed past the optimum formulation. Such a path is shown in Figure 1(b). When the control variable b, representing the formulation of the mixture, is changed while d is kept constant (around 0), the external phase continuously changes its nature from O (x < 0) to W(x > 0). This change is continuous, at equilibrium and reversible and therefore occurs without any hysteresis. Therefore, the reversibility as well as hysteresis behavior of the inversion mechanisms can be explained based on the butterfly catastrophe theory. 62 Vaessen and coworkers argued that while a thermodynamic treatment is useful for modeling transitional phase inversion, catastrophic phase inversion is better understood with a kinetic approach.47 They reported a predictive model for catastrophic phase inversion with a qualitative description of hysteresis, based on the kinetics of droplet breakup and coalescence.48 Bancroft’s rule. Transitional phase inversion involves the reversal of affinity of the surfactant for the two fluid phases. An early attempt to relate surfactant behaviors to emulsion morphology was made by Bancroft.63 The so-called Bancroft’s rule qualitatively states that the phase (oil or aqueous) in which the surfactant is more soluble becomes the continuous phase in the resulting emulsion. The preference of a surfactant towards the water phase bends the interface such that 9



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water-surfactant layer has higher surface area, and is convex, thereby making water as the continuous phase. The idea of spontaneous curvature of the interface based on surfactant affinity differences emerged from Bancroft’s classic paper, and was later extended by Harkins and Langmuir in the form of the ‘oriented wedge theory’.64-66 Hydrophilic surfactant molecules, for example, will have strong interaction with the water phase, and the size of the head-groups will be larger due to hydration. This will limit the packing of the surfactant molecules in the aqueous phase inducing the interface to bend in a convex shape towards water to fit the bulged headgroups. However, considering the enormous size of typical emulsion droplets as compared to that of molecular surfactants, the determination of the type of emulsion generated solely by the packing parameter of surfactants is difficult to justify. Also, the effect of phase volume ratio is not considered. Hydrophile-lipophilie balance (HLB). The quantification of interaction energies between a fluid phase and a surfactant was first tackled by Griffin67 using the HLB concept. The HLB of a surfactant is a measure of the balance between the hydrophilic and lipophilic nature of the surfactant molecules. It presents a guideline for emulsifier selection and was first issued by Atlas Powder Company in 1948.68 HLB is defined as

= ∗ 100

(1)

where Mphil is the molecular weight of the hydrophilic part of the molecule, and Mtotal is the molecular weight of the surfactant. This scale was arbitrarily defined between 0-20, with more hydrophilic surfactants given higher numbers, which correspond to R 1. When R ≈ 1, both water and oil phase interactions are balanced and surfactant-poor organic (top) and aqueous phases (bottom) are in equilibrium with a middle micro-emulsion phase (Type 3). Another (rarer) possibility is a single phase homogeneous mixture containing solubilized oil and aqueous phases in the form of a bicontinuous/lamellar phase, depending on composition of the system (Type 4).78 The R-ratio is not sufficient to distinguish between Type 3 and Type 4 systems; however when magnitudes of interaction energies are high, Type 4 results, whereas Type 3 results in cases when interaction energies are low. 12


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Hydophilic-liphophilic deviation (HLD). The concepts of HLB, PIT and Winsor R-ratio were unified by Salager and co-workers into the so-called hydrophilic-lipophilic deviation(HLD).79 HLD is a numerical expression of the deviation from optimum formulation; that is HLD is a concept similar to hydrophile-lipophile balance (HLB) and takes into account not only the surfactant but also all the other formulation variables.80 According to the definition, HLD=0 is defined as the point of ‘optimum formulation’ at which the free energy of transfer of surfactant from the oil to the aqueous phase is zero. In this reference state (HLD=0), the interfacial tension of the system is at its minimum, and the system spontaneous curvature is zero. HLD=0 is equivalent to the case of R = 1 or to T = PIT.81, 82 Any changes in the system properties, such as temperature, type of surfactant and oil, the addition of salt or co-surfactants, will thus lead to a departure from the optimum formulation. HLD can be expressed as a function of formulation variables using: # = $ − &'( − ) *+( + - . + /0*1 − 23 04 − 251 .

(3)

In this linear equation, $ depends on the chemical nature of the surfactant hydrophobic group, EON is the number of ethylene oxide groups per molecule of the surfactant, ACN is the number of carbon atoms in the linear alkane (oil) phase, S is the salinity in the aqueous phase in wt% of NaCl, /0*1 is a parameter which depends on the nature of surfactant structure and T is the temperature in Celsius. This linear equation can be used to calculate the PIT of the system by determining the temperature for which the system HLD=0. The advantage of HLD over other variables is that it includes the numerical contributions of all formulation variables and in turn allows the calculation of combined effects when more than one variable is changed simultaneously. Further details on HLD can be found in the following references: 15, 26, 83-86. The phase behavior of various ternary systems can also be illustrated in the 13



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form of a phase diagram, which shows the interdependence of emulsion morphology and type on multiple formulation variables. Detailed discussions on phase diagrams can be found in other articles.26, 37, 87 Techniques to Monitor and Characterize Phase Inversion Emulsification This section covers some of the most common phenomena that accompany PIE and methods with which these processes can be detected. Visual observation (microscopy) of PIE is difficult because the events at inversion point occur very rapidly and also emulsions tend to be highly opaque, making it extremely challenging to directly observe phase inversion. Therefore, indirect methods are required for detection, observation, and visualization of the phase inversion events. Conductivity. The most common and conventional method to monitor and detect phase inversion is by measuring the electrical conductivity of the emulsions. Ion conductivity of emulsions depends on the emulsion type as well as composition. To enable conductivity measurements, electrolytes such as sodium chloride are added to the aqueous phase to enhance its conductivity. When an O/W emulsion inverts to a W/O emulsion, the new continuous phase is not conductive, making it possible to detect phase inversion point via the detection of a rapid increase in electrical resistance. Because equipment required for conductivity measurements are inexpensive compared to other instruments that are based on light scattering or spectroscopy, this method quickly became very popular for detection of phase inversion. Mathematical models have been developed for the conductivity of emulsions.88 These models, without adjustable parameters, can predict the conductivity of emulsions based on the volume fractions and conductivity of the constituent phases, especially for the nonionic amphiphile/oil/water systems. Although very convenient to use, conductivity-based methods have some limitations. These methods require the addition of electrolytes in the system. Electrolytes have been reported to 14


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change interfacial tension between the oil and aqueous phases, and increase the stability of emulsion droplets against both coalescence and Ostwald ripening.89-92 This change in emulsion and emulsifier properties due to the presence of electrolytes, at concentrations as low as 0.02M, significantly undermines the reliability of this method. In addition, conductivity measurements detect phase inversion point with limited accuracy. While conductivity of O/W and W/O emulsions is distinctly different, the exact cut-off conductivity for morphology change is somewhat arbitrary. Some researchers utilize the composition corresponding to the mid-point of the high and low conductivity as the point of phase inversion, whereas others do the same operation on a logarithmic scale. Apart from the detection of the phase inversion ‘zone’, conductivity measurements do not provide any information on the morphology of droplets. For example, presence of multiple emulsions, especially at high internal phase volume fraction is not possible to detect simply by monitoring the conductivity of the emulsion. By a similar reasoning, presence of O/W/O multiple emulsions is not detectable by conductivity alone because there is hardly any conductivity difference between W/O and O/W/O emulsions. While the simplicity of the technique makes conductivity-based methods one of the most popular ways to detect phase inversion of emulsions, complementary methods are necessary to provide additional useful information regarding phase inversion of emulsions. Viscosity measurement. Conductivity measurements, due to the limitations described above, are almost always used in conjunction with other complementary techniques. One such technique is the measurement of viscosity of an emulsion during phase inversion. Emulsion viscosity depends on the volume fraction of the dispersed phase as well as the formulation of the emulsion system (quantified by HLD, which was discussed earlier). For catastrophic phase inversion, a sharp increase in emulsion viscosity is expected near the phase inversion point due to a change in the 15



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dispersed phase content.51 The point of transitional phase inversion, at which the free energy of transfer of the surfactant from oil to water phase is zero, is called the optimum formulation point (also HLD=0). As the emulsion composition approaches the optimum formulation point from either side, the interfacial tension decreases by several orders of magnitude.16, 93-102 Lowered interfacial tension induces a decrease in the average drop size. However, when the formulation becomes extremely close to HLD=0, further lowering of interfacial tension does not produce any significant reduction in drop size. In addition to lowering of interfacial tension, there is also a very steep fall in the stability of the emulsion close to HLD=0.98, 103-105 As a result of lowered stability, droplets undergo rapid coalescence and become larger drops. This leads to an increase in drop size in the close vicinity of HLD=0. The combination of the two opposite trends results in the occurrence of two maxima in viscosity around HLD=0 and a minimum at HLD=0, as seen in Figure 2(a,b).106, 107 In situ rheometric measurements have been used to detect phase inversion in catastrophic and transitional systems. The torque produced by an impeller in an emulsion can be detected by a rheometer attached to the impeller. Different impeller geometries, such as helical ribbon and Utype impellers have been used based on the range of viscosities that need to be detected. For a Utype impeller (also known as anchor impeller), shown in Figure2(c), a Couette analogy method has been reported. This method converts the relationship between torque and rotational speed to that between the ‘absolute viscosity’ and shear rate. This enables the in situ measurement of the viscosity of the system and the detection of a sudden change in viscosity, typically an indication of phase inversion. This method has been successfully used for rheologically complex systems such as mayonnaise and salad dressing, over a wide range of shear rates, with an error range within 5%. 108-112 16


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Viscosity data from rheomixer monitoring can provide insights into the phase inversion process, when the conductivity data is limited. One such case is when the external phase is the nonconducting oil phase (e.g., in the B- region in Figure 3). In this study, Kerosene cut (equivalent alkane carbon number (EACN)= 9.6) is used as the oil phase, 1 wt% NaCl solution is used as the aqueous phase with hydrophilic Tween 80 (HLB=15) as the surfactant.51 Dynamic catastrophic phase inversion is carried out by the addition of aqueous phase; the system is moved from B- to A- region (refer to formulation-composition map in Figure 3). Sudden change in viscosity (Figure 4) corresponds to catastrophic phase inversion. In the special case of catastrophic phase inversion, where droplet size is also a function of the rate of dispersed phase addition113, additional information about droplet size and distribution can be inferred from the height of viscosity maxima before inversion point. This is a clear advantage over conductivity-only data, which does not capture such nuances. Light Scattering. When emulsions are irradiated with electro-magnetic waves, the intensity of reflected/scattered radiation depends on the size of dispersed droplets as well as the angle of detection. Thus, scattering of light can provide crucial droplet-size information close to inversion point. Conventionally, laser light scattering has been used in the detection of particle sizes. However, the concentration of particles in the suspensions has to be reduced to prevent multiple scattering and to enable reliable characterization. This requirement poses a limitation on the in situ observation of emulsion systems, which generally are highly concentrated. Detection of backscattered light, however, overcomes this challenge. In this technique, no additives are necessary, and the method works equally well for both O/W and W/O emulsions, as opposed to conductivity measurements which is not suitable for characterizing W/O emulsions. Scattering can be modeled based on Rayleigh’s model114 when scatterer size d < λ/20. For most emulsions, 17



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however, this assumption is not valid; thus, the Mie scattering model needs to be used.115 According to Mie’s model, the scattering intensity varies as an inverse of square root of the particle diameter for anisotropic scatterers. Near the phase inversion temperature point, two competing factors are in play. The low interfacial tension favors the breakup of existing droplets and the formation of smaller droplets, whereas the high coalescence rate favors increase in droplet sizes. Because of these competing factors, as shown qualitatively in Figure 5 (a,b,c), the average droplet size reaches minima on both sides of PIT and shows a maximum at the phase inversion point.88 116, 117 Figure 5(d) shows the schematic of an on-line light scattering instrument, which can be used to expose an emulsion to near infra-red light (λ=850nm) and detect the backscattered or reflected light from the emulsion sample. When conductivity measurements are employed in detection of PIT, it is somewhat ambiguous to clearly pinpoint the phase inversion point because conductivity varies over a range of temperatures. The backscattering signal, in contrast, goes through a minimum at the inversion point as seen in Figure 5(e). Hence, backscattering data points to a distinct inversion point. Another distinct advantage of the light-scattering technique is the detection of multiple emulsions. As shown in Figure 6(a) there is a steep rise in backscattered intensity at close to inversion point (fw=0.25), which is speculated to be due to the release of inner water droplets from the oil droplets of the W/O/W emulsions just prior to phase inversion.116, 117 Figure 6(b) also shows that at a high fraction of water phase (fw= 0.9), the backscattering intensity increases which is believed to be due to the formation of O/W/O multiple. This is not possible to detect using only conductivity measurements.

18


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Infrared (IR) Spectroscopy. Scattering and absorbance of IR have long been used to characterize colloidal suspensions and emulsions. 118-120 The main advantage of using IR spectroscopy in the investigation of PIE is that in addition to information about the physical nature of the emulsion (size, morphology etc.), chemical information can be obtained, which is extremely valuable in understanding the events in real-time during phase inversion process. IR spectroscopy has been utilized in recent studies to confirm phase inversion boundary in model transitional phase inversion systems121 and also in catastrophic phase inversion system.122 In addition to the aforementioned common methods for detection and characterization of phase inversion point, neutron scattering has also been employed for the same purpose. We refer our readers to recent articles for detailed information on the application of small angle neutron scattering (SANS) for detection of phase inversion. 123, 124

Transitional Phase Inversion Emulsification Transitional phase inversion emulsification is induced by changing the interaction between the emulsifiers and the two fluid phases. Such interactions can be varied by changing the temperature when non-ionic surfactants are used.

For systems that are prone to thermal

degradation, competitive adsorption of two surfactants with opposite emulsification tendencies has been shown to induce transitional phase inversion. More recently, transitional phase inversion using stimuli-responsive polymeric surfactants have been reported. Changes in the pH, ionic strength, temperature as well as light irradiation can cause drastic changes in the conformation of polymers leading to transitional phase inversion of emulsions. Because polymer surfactants can be designed and synthesized with a wide range of monomers, they offer new opportunities in inducing transitional PIE. Each of these approaches is briefly summarized below. 19



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Transitional phase inversion via phase inversion temperature (PIT) method. It was demonstrated by Shinoda and co-workers that increasing system temperature above a critical temperature, known as the phase inversion temperature (PIT) could lead to inversion from O/W to W/O emulsions, stabilized with non-ionic surfactants.12 PIT is also called the ‘HLB temperature’ at which the hydrophilic and lipophilic natures of the surfactant are exactly balanced. PIT was shown to be dependent on system composition, concentration and nature of surfactants, water-oil ratio and effective alkane carbon number of the oil phase. One of the main applications of PIT method is in the generation of nanoemulsions that are composed of droplets that are smaller than 100 nm.6, 125

Nanoemulsions are especially useful in drug delivery, food and personal care

applications because of their transparency and high stability.126,

127

At close to the phase

inversion temperature (PIT) conditions, the interfacial tension is extremely low, and hence formation of smaller droplets is relatively less energy-intensive. A significant number of studies have focused on the stability as well as the droplet size of emulsions generated via the PIT method. The concentration of surfactants plays a central role in determining the stability as well as the size of resulting emulsions generated via PIT method.87, 128

In general, the droplet size decreases when the surfactant concentration is increased, because

of the increase in the interfacial area and reduction in the interfacial tension of the system. However, the stability of emulsions produced by this method decreases with increasing surfactant concentration due to enhanced Ostwald ripening through the diffusion of solubilized oil from smaller to larger droplets. Figure 7(a) shows the variation of droplet size with surfactant concentration, comparing the experimental data (square dots) with a theoretical prediction (solid line), under the assumption of spherical droplets and that all surfactant molecules are at the interface. Interestingly, the theoretical estimation of droplet size tends to always be smaller than 20


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the experimental size. 87 This result indicates that not all of the surfactant molecules reside at the interface. Higher surfactant concentrations lead to smaller discrepancy between theoretical and experimental radii because higher surfactant concentration can lead to more effective emulsification and generate emulsions with smaller droplets and a narrower distribution with less energy input. 114, 115, 129 PIT is considered invariant with surfactant concentration when the surfactant is monodistributed. But interestingly for systems employing commercially available long-chain alcoholic surfactants, which are composed of polydistributed alkyl chains with varying ethylene oxide (EO) content, the PIT shows a marked decrease with increasing surfactant concentration. Figure 7(b) shows the case of Brij30 (polyoxyethylene laury ether), where increasing surfactant concentration leads to a significant decrease in PIT.87 This behavior is attributed to the dependence of surfactant partitioning on the EO content of the surfactant.128 Chains with low EO content, preferentially partition into the oil phase. Hence, increase in total concentration of a polydisperse surfactant leads to accumulation of chains with low EO in the oil phase, thereby reducing the PIT. In addition, as the solubility of the oil phase in the aqueous phase increases, the HLB temperature decreases further. These effects lead to a decrease in the PIT of the system. Transitional phase inversion emulsification via competitive adsorption of surfactants. In addition to external stimuli such as temperature, pH and salt, competitive adsorption of the surfactants to the oil-water interface of an emulsion can also lead to PIE. By adding surfactant that favors the opposite emulsion type, phase inversion of an emulsion can be induced. For example, when hydrophobic Span 80 was added to hydrophilic surfactant (Tween 80)-stabilized O/W emulsions phase inversion was observed.

130

A schematic of the proposed mechanism is shown in Figure

8(a). Competitive adsorption between Span 80 and Tween 80 determines the composition of 21



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adsorption layer at the interface. Inversion is reported to proceed via the formation of W/O/W multiple emulsions. Coalescence of the multiple emulsion droplets is expected to lead to W/O emulsions. In an analogous study, the addition of a hydrophilic surfactant to a W/O emulsion induced phase inversion into an O/W emulsion.131 A similar (but opposite) mechanism, as schematically illustrated in Figure 8(b), shows formation of mixed micelles leading to O/W/O multiple emulsions before phase inversion. A report describing phase inversion from W/O to O/W emulsions upon the addition of hydrophilic surfactants claimed that owing to nonreversibility of the process, the mechanism of phase inversion may be more similar to catastrophic phase inversion than transitional phase inversion.132 Transitional phase inversion of polymer-stabilized emulsions. Polymers, which can modify the curvature of the oil-water interface, in response to external stimuli have also shown to induce transitional phase inversion in emulsion systems. Changes in the conformation of polymers in response to changes in the environmental conditions likely play crucial roles in inducing transitional PIE. In a recent report, diblock copolymers of polystyrene (PS) and random copolymers of styrene and 2-(dimethylamino)ethyl methacrylate (DMAEMA) were used to demonstrate thermo-responsiveness.133

These diblock copolymers have thermo-responsive

repeat units that reversibly control the type of emulsions that can be stabilized. Temperature can modify the hydrophobicity/hydrophilicity of the DMAEMA units by disrupting or favoring the strength of hydrogen bonds between DMAEMA repeat units and water molecules. Also, since monomer units (DMAEMA) are ionizable, pH can be used as a second stimulus to change the nature of the emulsifier, and therefore, the emulsion system. Simple O/W emulsion droplets of 10µm, which existed far from the phase inversion point, can be converted into W/O emulsions

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by changing the solution pH. In addition, the existence of multiple emulsions in certain intermediate steps of phase inversion was also reported. Light-triggered transitional phase inversion. In addition to more conventional thermodynamic stimuli such as pH and temperature, light also has been used as a stimulus to induce phase inversion of emulsions.134 A number of photosensitive polymeric surfactants have been explored135 but the effect of light-triggered structural changes in the surfactant molecules is relatively small. This is due to the opacity of emulsions containing light-responsive surfactants. To ensure that the system stays optically transparent, the concentration of photochromes needs to be controlled. This upper limit on photo-responsive surfactant concentration compromises the emulsion stability. This limitation, coupled with requirement for very specific photo-responsive chemicals, remains a challenge in this field. This limitation has been overcome by mixing a conventional non-ionic surfactant with a small amount photo-responsive polymer (PRP) or by mixing two polyelectrolyte-based surfactants, one of which exhibited photosensitivity.136 Very low concentrations of photo-chromes (10-4 – 10-3M) have been reported to be sufficient to provide appropriate sensitivity and reversibility to light stimulus. In the mixture, a non-ionic surfactant provides stability to the emulsion, whereas a small concentration of PRP, azobenzenemodified poly(acrylate) imparts the photo-responsive properties. There have been prior studies focused on photo-responsiveness137 but most of the systems studied were based on photodestructive polymers, which responded to UV irradiation by emulsion phase separation, rather than phase inversion. In the system studied by Porcar and coworkers the azobenzene system provides long-term reversibility, where the emulsion type can be switched merely by changing the wavelength of incident radiation. It is known that polymers with azobenzene moieties in their backbone undergo reversible coil-globule transition upon irradiation in aqueous solutions.13, 13823



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At constant temperature, the properties of PRP can be switched using light of different

wavelengths (365 and 436nm), to modify the properties of the emulsions. As can be seen in Figure 9, the inversion of emulsions can be reversibly triggered multiple times, by switching the irradiation wavelength. In another study on triggered phase inversion of emulsions stabilized with photo-responsive emulsifiers, CdSe quantum dots (QDs) functionalized with a photocleavable ligand was used to stabilize W/O emulsions.142 Irradiation of emulsion with 365nm light was shown to cause PIE due to wettability change of QD. Light-induced PIE, was thus achieved in a 10-minute timeframe with potential applications in the encapsulation and triggered release of materials. Particle-stabilized emulsions, in fact, represent unique emulsions systems that undergo PIE. We highlight some of the recent developments in this area in the next section.

Phase Inversion of Particle-Stabilized Emulsions In the early 1900s, Pickering143 and Ramsden144 reported that finely divided solid colloidal particles can be used for the stabilization of emulsions. Pickering emulsions prepared using particles such as silica145-148, latex149-152, carbon, metal oxides, metal sulfates and clays153-155, have been used for numerous applications. Several methods have been used to induce phase inversion of these Pickering emulsions.

Initially, these efforts focused on changing the

wettability of particles by in situ modification with surfactants. Subsequently, stimuli-responsive particles that would change their wettability in response to changes in pH and temperature were used to induce phase inversion of Pickering emulsions. More recently, advances in synthesis of complex colloidal particles with high degree of control on their shape and surface properties have enabled the phase inversion emulsification using shape-changing amphiphilic colloids. For 24


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those who are interested in the fundamentals of Pickering emulsions and the factors that influence their behavior, we would like to refer the readers to references 156, 157. Effect of particle wettability on phase inversion of Pickering emulsions. Finkle et.al.158 proposed a relationship between the particle wettability and the kind of emulsions that those particles can efficiently stabilize, which is similar to the Bancroft rule for molecular surfactants. According to their argument, hydrophilic particles prefer to stay suspended in aqueous environments and therefore they prefer water phase to be the continuous phase, giving rise to O/W emulsions, and vice versa. The wettability of particles is quantified as their interfacial contact angle, which is the angle that the particle makes with the oil-water interface, through the aqueous phase (Figure 10).159 A more hydrophobic particle, for example, will tend to stay submerged predominantly in the oil phase and therefore make an obtuse contact angle, while a hydrophilic particle will lead to an acute contact angle (Figure 10). The link between particle wettability and the type of generated emulsion was demonstrated by Binks and coworkers.33 They established that the change in particle wettability by surfactant adsorption can cause emulsion inversion. The wettability of particles is analogous to the HLB of molecular surfactants. Therefore, tuning the wettability of particular surfactants, should in principle, lead transitional phase inversion.8, 160-162 Varying particle wettability by surfactant adsorption. Particle wettability can be altered by the adsorption of surfactants, which has been shown to cause phase inversion.33, 163-166 Figure 11(a) shows the relationship between the surface wettability of calcite crystal particles and the behavior of emulsions composed of water and decane. As stearic acid is added, the calcite crystal, which was initially hydrophilic, becomes hydrophobic, as can be seen from the contact angle increase. As the contact angle passes 90o, phase inversion of emulsion is reported.

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At this point, it is essential to discuss the competitive nature of adsorption between colloidal particles and surfactants. In emulsions stabilized by mixtures of particles and surfactants, the adsorption of surfactants is very fast at high surfactant concentrations and does not leave any room for the particles to adsorb to the interface.167 As a result, such a system leads to solely surfactant-laden interfaces. In fact, if the concentration of a surfactant is increased in a particlestabilized system, it can lead to the desorption of particles from the interface. Particle wettability (quantified by three-phase contact angles) as well as nature of surfactant (O/W vs. W/O stabilizing) have been shown to affect the competition as well.

168-170

Hence, it is expected that

such competition plays a crucial role in the phase inversion process. Based on the hypothesized correlation between HLB and particle wettability, the reversal of hydrophilicity can be understood as an effective change in the HLB of the particulate surfactants. Similar to surfactants with low HLB numbers, particles which prefer to remain in the oil phase form W/O emulsions. In addition to change in wettability of particulate surfactants by the adsorption of surfactants on the surface, a second reversal of wettability and a second phase inversion has also been reported.171 Charge reversal as well as first phase inversion of emulsions stabilized by negatively charged silica nanoparticles was observed on increasing concentration of a di-chain cationic surfactant. Upon further increasing the surfactant concentration a second phase inversion was observed. It was proposed that at higher cationic surfactant concentration, a bilayer is formed on the particle surface, exposing the head-groups to the continuous phase. (Figure 12).172 The exact cause of second inversion has not been fully elucidated, however it is conjectured that the second inversion could be caused by interfacial adsorption of the excess surfactants in solution after the particles are completely saturated.

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Effect of suspension composition and concentration on phase inversion. Transitional phase inversion of particle-stabilized emulsions has also been achieved by changing the ratio of hydrophilic/hydrophobic particles used for emulsion stabilization.145 For example, toluene-inwater emulsions were stabilized by two kinds of fumed silica particles with opposite wettability. In addition to using either hydrophilic or hydrophobic particles, mixtures of particles in different ratios were used to study the change of “effective HLB” of the particulate surfactants. It was shown that twice the mass of hydrophilic silica was required to invert the system as compared to the mass of hydrophobic silica. The reason for this difference is due to differences in the extent of adsorption of these two particles from the respective bulk phases to the O/W interface. It was also shown that the phase inversion point depends on the initial location of the hydrophilic and hydrophobic particles. If both particles are dispersed in the oil phase at the beginning, phase inversion from O/W to W/O occurs on much less addition of hydrophobic particles. The exact reason for this observation is unknown but it is speculated that presence of hydrophilic particles in the hydrophobic-oil mixture, either disrupts the microstructure or the hydrophilic particles get incorporated into the existing microstructure in the oil phase, leading to the resulting emulsion to be closer to phase inversion boundary. Instead of changing the ratio of hydrophilic-to-hydrophobic particles, it was reported that merely changing particle concentration could also lead to phase inversion.173 Hydrophilicity of silica nanoparticles is attributed to the presence of surface silanol (-SiOH) groups. It was proposed that increasing the concentration of particles in the oil phase promotes inter-particle hydrogen bond formation leading to a reduction in the population of free silanol groups, thereby causing a wettability reversal of particles. Such a concentration induced wettability reversal was

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not observed in the aqueous phase because the probability of hydrogen bond formation with water is higher than inter-particle hydrogen bonds. Phase inversion using stimuli-responsive particles. pH and ionic strength responsive polystyrene nanoparticles were shown to stabilize either type of emulsions (O/W and W/O) depending on the solution conditions. Precise control was reported with polystyrene particles coated with ionizable carboxylic acid surface groups.174 Transitional phase inversion was caused by changing the pH and ionic strength of the aqueous phase independently, showing that both of these factors are important in controlling the surface properties of the particles. When pH of the aqueous phase was increased from 2 to 10, emulsions inverted from W/O to O/W. The reason for this inversion is attributed to the increase in the degree of ionization of the acid groups with increasing pH, making them more hydrophilic. It was noted that a relatively high pH value (pH=10), as seen in Figure 13, was required for phase inversion compared with the pH value for 50% ionization of a short-chain carboxylic acid in bulk water (pKa=4.8). It was conjectured that dissociation of the acid groups is energetically less favorable when they are present at solid-liquid interfaces than when they are in the bulk fluid phase because at solid surfaces, the proximity of similarly charged groups generates a surface potential175-177. Therefore, a higher pH in the bulk is required to induce the ionization of the surface acid groups. In addition to pH, the importance of ionic strength was demonstrated by changing salt concentration in the aqueous phase from 0.01M to 2M. At a high pH value, when the acid groups are ionized and particles are hydrophilic, O/W emulsions are formed, but they invert to W/O on decreasing the salt concentration to 0.6M. It was proposed that the surface potential on the particles is reduced due to solvation of the charged groups when salt concentration is high enough. But the reduction of salt concentration increases

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the surface potential, making it more difficult for the acid groups to deprotonate, and hence rendering the system more hydrophobic, impeding phase inversion. In a related study, the nature and polarity of the oil-phase and the oil-phase volume fraction were reported to be crucial factors in determining whether the pH stimulus would cause phase inversion or just rapid de-emulsification in polystyrene particle-based Pickering emulsions.178 It was shown that low polarity oils lead to pH-responsive O/W emulsions, whereas relatively polar oils lead to pH-responsive W/O emulsions. Lowering the solution pH led to rapid deemulsification in both cases. However, oils of intermediate polarity were reported to undergo a transitional phase inversion as the solution pH was lowered. A schematic representation of such pH-induced transitional phase inversion is shown in Figure 14. In addition it was reported that transitional phase inversion was observed only in a narrow volume fraction range (around 0.50 with respect to the oil phase). Utilization of phase inversion emulsification in biocatalysis using photo-responsive particlestabilized Pickering emulsions was demonstrated recently.179 NIR/visible light tuned interfacially active nanoparticles were utilized to stabilize water-in-oil emulsions. The aqueous phase was loaded with enantio-selective biocatalytic active bacteria. Its activity was studied to demonstrate enhanced catalytic performance with relieved substrate inhibition effect with the use of Pickering emulsions. Phase inversion without requirement of any chemical auxiliaries or high temperature variation was shown to be advantageous in recovery of biocatalysts. Phase inversion of Pickering air-water emulsions. Recently, phase inversion of particlestabilized air-water systems proceeding via progressive change in the surface hydrophobicity of stabilizing particles and change in the air-to-water ratio was demonstrated.180 Transitional phase inversion was studied by using a series of fumed silica particles of varying hydrophilicity from 29



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~100% SiOH to 66%), aqueous dispersions were formed without any bubble or foam formation. Intermediate hydrophilicity results in stable air-in-water foams, and the amount of entrapped air increases with decreasing hydrophilicity. This can be seen in the increasing amount of foam as hydrophilicity is decreased (from right to left in Figure 15(a)). The water phase at the bottom progressively becomes clearer with decreasing hydrophilicity, showing that an increasing number of particles are getting attached to the surface of air bubbles. For particles with extremely low hydrophilicity (SiOH 0.6, air-in-water foams are formed whereas for φW 3 nanoparticles and sodium dodecyl sulphate. Colloids Surf., A 2008, 329, 67-74. (172) Atkin, R.; Craig, V.; Wanless, E.; Biggs, S., Mechanism of cationic surfactant adsorption at the solid–aqueous interface. Adv. Colloid Interface Sci. 2003, 103, 219-304. (173) Binks, B. P.; Philip, J.; Rodrigues, J. A., Inversion of silica-stabilized emulsions induced by particle concentration. Langmuir 2005, 21, 3296-3302. (174) Binks, B. P.; Rodrigues, J. A., Inversion of emulsions stabilized solely by ionizable nanoparticles. Angew. Chem. 2005, 117, 445-448. (175) Stone-Masui, J.; Watillon, A., Characterization of surface charge on polystyrene latices. J. Colloid Interface Sci. 1975, 52, 479-503. (176) Schulz, S.; Gisler, T.; Borkovec, M.; Sticher, H., Surface charge on functionalized latex spheres in aqueous colloidal suspensions. J. Colloid Interface Sci. 1994, 164, 88-98. (177) Aveyard, R.; Binks, B.; Carr, N.; Cross, A., Stability of insoluble monolayers and ionization of Langmuir-Blodgett multilayers of octadecanoic acid. Thin Solid Films 1990, 188, 361-373. (178) Read, E.; Fujii, S.; Amalvy, J.; Randall, D.; Armes, S., Effect of varying the oil phase on the behavior of pH-responsive latex-based emulsifiers: Demulsification versus transitional phase inversion. Langmuir 2004, 20, 7422-7429.

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(179) Chen, Z.; Zhou, L.; Bing, W.; Zhang, Z.; Li, Z.; Ren, J.; Qu, X., Light Controlled Reversible Inversion of Nanophosphor-stabilized Pickering Emulsions for Biphasic Enantioselective Biocatalysis. JACS 2014, 136, 7498-7504. (180) Binks, B. P.; Murakami, R., Phase inversion of particle-stabilized materials from foams to dry water. Nat. Mater. 2006, 5, 865-869. (181) Schutte, D., Franz-Theo, S., Brunner, H. Predominantly aqueous compositions in a fluffy powdery form approximating powdered solids behavior and process for forming same. US Patent US3393155 A, 1968. (182) S. Hasenzahl, A. G., E. Walzer, A.Braunagel, Dry water for the skin. SÖFW-J 2005, 131, 2-8. (183) Aussillous, P.; Quéré, D., Liquid marbles. Nature 2001, 411, 924-927. (184) Brooks, B. W.; Richmond, H. N., Phase inversion in non-ionic surfactant—oil—water systems—I. The effect of transitional inversion on emulsion drop sizes. Chem. Eng. Sci. 1994, 49, 1053-1064. (185) Fang, J.; Lee, D., Micromixing efficiency in static mixer. Chem. Eng. Sci. 2001, 56, 37973802. (186) Akay, G.; Leslie, F., Microstructure orientation in liquid crystalline and glass fibre reinforced polymer melts. Advances in Rheology 1984, 3, 496-502. (187) Leslie, F., Hamel flow of certain anisotropic fluids. J. Fluid Mech. 1964, 18, 595-601. (188) Peterfi, T., Die abhebung der Befruchtungsmembran bei seeigeleiern. Wilhelm Roux'Archiv für Entwicklungsmechanik der Organismen 1927, 112, 660-695. (189) Kawashimà, Y.; Hino, T.; Takeuchi, H.; Niwa, T.; Horibe, K., Rheological study of w/o/w emulsion by a cone-and-plate viscometer: Negative thixotropy and shear-induced phase inversion. Int. J. Pharm. 1991, 72, 65-77. (190) Bremond, N.; Thiam, A. R.; Bibette, J., Decompressing emulsion droplets favors coalescence. Phys. Rev. Lett. 2008, 100, 024501. (191) Bremond, N.; Doméjean, H.; Bibette, J., Propagation of drop coalescence in a twodimensional emulsion: A route towards phase inversion. Phys. Rev. Lett. 2011, 106, 214502. (192) Joscelyne, S. M.; Trägårdh, G., Membrane emulsification—a literature review. J. Membr. Sci. 2000, 169, 107-117. (193) Suzuki, K.; Hayakawa, K.; Hagura, Y., Preparation of High Concentration O/W and W/O Emulsions by the Membrane Phase Inversion Emulsification Using PTFE Membranes. Food Sci. Technol. Res. 1999, 5, 234-238. (194) Kawashima, Y.; Hino, T.; Takeuchi, H.; Niwa, T.; Horibe, K., Shear-induced phase inversion and size control of water/oil/water emulsion droplets with porous membrane. J. Colloid Interface Sci. 1991, 145, 512-523. (195) Paez, D. A.; Bardesi, O. E. A.; Gallegos, M. C.; Martinez, B. F. J.; Moreno, M. E.; Navarro, D. F. J.; Partal, L. P.; Romero, P. E. Process for continuous preparation of submicronic bitumen emulsions. European Patent 2213704 A1, 2010. (196) Allouche, J.; Tyrode, E.; Sadtler, V.; Choplin, L.; Salager, J.-L., Simultaneous conductivity and viscosity measurements as a technique to track emulsion inversion by the phase-inversiontemperature method. Langmuir 2004, 20, 2134-2140. (197) Binks, B.; Lumsdon, S., Catastrophic phase inversion of water-in-oil emulsions stabilized by hydrophobic silica. Langmuir 2000, 16, 2539-2547.

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Figures

Figure 1. Locus of free energy extrema based on the butterfly catastrophe theory as a function of the state variable, x, and control variables, b and d. State variable, for example, can be the average density of the continuous phase relative to the average of oil and water densities. Hence, x > 0 signifies water continuous (O/W) emulsions. Control variable b represents the system formulation (positive means O/W and vice versa), whereas d represents the system water-oil ratio (WOR). (a) Representation of catastrophic phase inversion (CPI) of a system starting as an O/W emulsion at A and moving to a W/O emulsion at E, passing through a sudden drop from D to D’ where catastrophic inversion occurs. Reverse CPI occurs at B’, thereby explaining hysteresis and irreversibility of CPI. (b) Transitional phase inversion path shown as the system formulation, b, is varied while d is kept constant. Change of outer phase density (‘x’) occurs gradually as the system inverts via the microemulsion phase, which occurs reversibly following the same path without hysteresis. Reproduced with permission from Ref. 62. Copyright 1998 Marcel Dekker, New York.

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(a)

(b)

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(c)

Figure 2. (a) Variation of viscosity with temperature (oC) for two compositions (grey diamonds: fw=0.5; black filled triangles: fw=0.35). Phase inversion temperature indicated by arrow; (b) three-phase behavior (winsor type III) indicated by shaded region and two local maxima in viscosity as well as two local minima in droplet sizes shown on either side of the three-phase zone. (c) Schematic of the rheomixer device used for conducting the experiments. Reproduced with permission from reference 196. Copyright 2004 American Chemical Society.

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Figure 3. Generalized formulation-composition map showing different regions as well as the emulsion types in those regions delimited by the stair-like zigzagged standard inversion frontier (shown with a broken line). Formulation represents a variable that can alter the balance of interactions of the surfactant toward the two phases. Systems whose formulation-compositions are represented on one side of the inversion frontier (eg. A-,C-,C+) are water-continuous while the other side is oil-continuous; Reproduced with permission from reference 51. Copyright 2005 American Chemical Society.

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Figure 4. Change in viscosity during the dynamic B- to A phase inversion process for two different rates of addition of the dispersed phase. Reproduced with permission from reference 51. Copyright 2005 American Chemical Society.

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(a)

(b)

(c) % Transmission % Backscattering

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Conductivity (mS/cm)

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o

Temperature ( C)

(d)

(e)

Figure 5. (a) Qualitative variation of droplet size as a function of temperature showing two minima on either side of PIT; (b) Variation of reflectance with particle size; (c) Expected qualitative variation of reflectance with temperature showing maximum reflectance on either side of PIT and minimum at PIT; (d) Schematic representation of ‘Turbiscan-on-line’ equipment showing circular cross-section of the glass tube through which the emulsion is pumped through while it is irradiated with a laser beam (λ=850nm) and the two detectors measure the transmitted and backscattered light; (e) Variation of backscattered and transmitted light as well as conductivity as a function of temperature showing transitional phase inversion for (S/O/W) 5 wt.% Tech-C12E4/n-decane/NaCl 10−2M aqueous solution, fw = 0.6. (c) Reproduced with permission from reference 116. Copyright 2009 Elsevier.

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W/O/W O/W/O

(b)

(a)

Figure 6. Comparison between light-backscattering (filled circes) and conductivity (empty squares) data for catastrophic inversion systems. (a) System: 1 wt % Span 20 (HLB 8.6)/1 wt % NaCl brine/petroleum ether. Catastrophic phase inversion from O/W to W/O by addition of oil phase, with intermediate W/O/W formation; (c) System: 1 wt % Tween 80 (HLB 15)/1 wt % NaCl brine/petroleum ether. Catastrophic phase inversion from W/O to O/W by addition of water phase, with intermediate O/W/O formation. Reproduced with permission from reference 117. Copyright 2007 American Chemical Society.

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(a)

(b)

Figure 7. System: water(W)/C12E4/isohexadecane(O) (a) Conductivity as a function of temperature at different concentrations of surfactant, S, and constant oil concentration (20 wt %). NaCl is added to the aqueous phase in order to enhance conductivity; (b) Change in nanoemulsion droplet (r) as a function a function of C12E4 concentration at 25 °C keeping oil concentration fixed at 20wt%. Solid line represents theoretical particle radii obtained by geometrical considerations, assuming that emulsion droplets are spherical and that all surfactant molecules are at the interface. Reproduced with permission from reference 87. Copyright 2004 American Chemical Society.

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(a)

(b) Figure 8. (a) Schematic of the proposed mechanism of phase inversion from O/W to W/O on addition of hydrophobic Span 80 surfactant to O/W emulsion system. Reproduced with permission from reference 130. Copyright 2014 Elsevier; (b) Schematic illustration of W/O to O/W/O to O/W phase inversion due to addition of lipophilic Span80 to W/O emulsion stabilized by Tween 80. Reproduced with permission from reference 131. Copyright 2014 Elsevier

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Figure 9. System: 20 gL-1 C12E4 (tetra ethylene glycol mono dodecyl ether surfactant), 3gL-1 PRP (an azobenzene-modified poly(acrylate) amphiphilic photo-responsive polymer) and equal volumes of 0.3M NaNO3 and n-dodecane. A reversible wavelength sweep generating direct, unstable and inverse emulsions by alternately irradiating the sample with 365 and 436nm light is studied using change in ionic conductivity of the emulsion system as a function of time. Temperature is held constant. Arrows indicate the starting time of the wavelength switch. Reproduced with permission from reference 134. Copyright 2014 Wiley VCH.

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Figure 10. Schematic of a solid spherical particle at an oil−water interface interfacial energies and the contact angle measured into the water phase. Reproduced with permission from reference 159. Copyright 2002 American Chemical Society.

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Figure 11. System: Equal volumes (15 cm3) of oil (containing stearic acid) and water (containing calcium carbonate particles, 2.5 wt %). Variations of contact angle (through water) of a water drop on a calcite crystal under decane containing different concentrations of stearic acid as well as the emulsion stability (expressed as the volume of emulsion remaining after 6 h) as a function of stearic acid concentration in decane. Volume of emulsion remaining after 6 h is plotted with o/w emulsions shown above the zero level and w/o emulsions below it. Data from reference 166;. Reproduced with permission from reference 197. Copyright 2000 American Chemical Society.

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Figure 12. The proposed model for the adsorption of cationic surfactants onto negatively charged silica particles first leading to charge neutralization and then followed by charge reversal. Reproduced with permission from reference 172. Copyright 2003 Elsevier.

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Figure 13. Transitional phase inversion of Pickering emulsions stabilized by pH-responsive latex particles by changing solution pH. System: 2wt% carboxy-coated PS latex particles (200nm) stabilizing hexadecane and 1M aq. NaCl solution. Above pH=9.5, emulsions are W/O and O/W above 9.5. Schematic in the inset depicts position of a particle at the interface with changing pH. Reproduced with permission from reference 174. Copyright 2005 Wiley VCH.

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Neutral steric stabilizer

Cationic steric stabilizer

Figure 14. Schematic representation of pH-induced transitional phase inversion. Particles are sterically stabilized with diblock copolymer. Polarity of oil phase was varied to observe that transitional phase inversion can only be observed with oils which have intermediate polarity (for example, less than 1-undecanol and more than dodecane). For example with methyl myristate or cineole as the oil phase. Reproduced with permission from reference 178. Copyright 2004 American Chemical Society.

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(a)

(b)

(d)

(c)

(e)

Figure 15. (a) Photograph of vessels containing 2 wt% (relative to water) of DCDMS-coated silica particles with Φw=0.056 two weeks after mixing and aeration, for particles of different wettabilities. Numbers on the top indicate the (%)SiOH content on particle surfaces (lower means more hydrophobic); Inversion occurs first from water-in-air powders to foam on left dashed line, and from foam to aqueous dispersions across the second dashed; (b) Free-flowing powder flowing down through a glass funnel, made by aerating 5 g of silica particles possessing 20% SiOH and 95 g of water (Φw =0.056); (c) Foam extruded through a serrated metal nozzle prepared by aerating 5 g of silica particles possessing 32% SiOH and 95 g of water (Φw =0.056); (d) Photograph of vessels containing 0.114 wt/vol% (relative to total volume) of DCDMS-coated silica particles possessing 14% SiOH, two weeks after mixing and aeration, for different initial Φw (given). The mixtures change (left to right) from water-in-air powders to air-in-water foams via a soufflé-like material with increasing Φw (e) Soufflé-like material formed at Φw for =0.2;

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Scale bar=1 cm. Scale bars=1 cm. Reproduced with permission from reference 180. Copyright 2006 Nature Publishing Group.

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(a)

(b)

Figure 16. (a) (Top) Macroscopic and (bottom) fluorescence microscopy images of emulsions made with aqueous phases of pH between 2.2-11, deionized water (water-in-oil emulsion), showing change from W/O emulsions at pH=2.2 and O/W emulsions at pH=11. Φw = 0.5 in all cases, and the oil phase contains 0.01 wt% Nile Red. Scale bar = 500 µm; (b) (Top) Macroscopic and (bottom) fluorescence microscopy images of emulsions inverted from Emulsion-2 and Emulsion-3 in (a) by addition of ∼20 µL of 1 M NaOH and 1 N HCl, respectively. Scale bar = 500 µm. Reproduced with permission from reference 28. Copyright 2014 American Chemical Society.

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(a)

(b)

(c)

(d)

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(e) Figure 17. (a) Geometry of a typical multiple expansion-contraction static mixer (MECSM) unit showing two flow dividers and a capillary. Pressure across each element can be measured and samples at the inlet and outlet to the unit can be taken; (b) Simplified schematic illustration of

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the emulsification equipment. Water from the container (A) is delivered [using a pump (B)] into the mixing vessel (C) which is fitted with two impellers (D) attached to a torque meter (E). W/O emulsion from the mixing vessel (C) is transferred to the reservoir of the piston pump (F) driven with a variable speed motor. After passing through the multiple expansion contraction static mixer (MECSM) the polymer solution may be added to emulsion using the mixing vessel (G); (c) The effect of extension rate on PIE behaviour of Epikote 828 resin in which the oil phase consists of 90% Epikote 828 and 10% surfactant blend with CN/CA = 5 (Dobanol 91 2.5/Aerosol OT) when Φw = 0.3, T, = 45°C and N = 3 or N = 6. Variation of emulsion conductivity, K, with Z; (d) Variation of emulsion particle size distribution with extensional rate; (e) Illustration of the proposed mechanism of flow-induced phase-inversion emulsification based on flow visualization of polymerizable high-internal-phase emulsions. Reproduced with permission from reference 36. Copyright 1998 Elsevier.

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Figure 18. Schematic of proposed mechanism for phase inversion of w/o/w multiple emulsion droplets into w/o emulsions using shear provided by a cone-plate viscometer. Reproduced with permission from reference 189. Copyright 2014 Elsevier.

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(a)

(b)

Figure 19. (a) Sequence of images at short time intervals showing the destabilization of a pair of droplets passing through a symmetrical coalescence chamber: collision, relaxation, separation, and fusion. The channel width expands from 36 μm to 72 μm. From top to bottom, the time is 0, 3, 5, 7, 7.1, and 10ms; (b) Formation of two facing nipples in the contact area prior to coalescence induced by separating the droplets. The scale bar is 20 μm. Reproduced with permission from reference 190. Copyright 2014 American Physical Society

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(a)

(c)

(b)

Figure 20. (a) Fabrication (top) and destruction (bottom) of a concentrated two-dimensional emulsion in a microfluidic device; (b) Probability of coalescence Pc during the propagation as a function of the angle between the approaching drops. (Continuous line is the 4th-order polynomial fit. (inset) Number of the angles that lead to coalescence (filled circle) among the angles encountered during many coalescence propagations (open circles); (c) Probability that phase inversion occurs via the coalescence of four drops (Pi) having radii R and aR as a function of the distance D between the two largest drops for a=1 (straight line) and a=0.5 (dashed line. (inset) Maximum probability of phase inversion as a function of the drop size ratio. Reproduced with permission from reference 191. Copyright 2011 American Physical Society. 70


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Determina;on of emulsion type


Oil-‐in-‐Water

Water-‐in-‐Oil

Light-‐induced Transi;onal PIE

Phase Inversion Emulsiﬁca;on (PIE)

Flow-‐induced PIE PIE of Pickering emulsions


Recent Developments in Phase Inversion Emulsification

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