Emulsions with structured continuous phases☆
Introduction
According to the conventional and most simple definition, emulsions are dispersions of one liquid (internal phase) into another (the continuous or external phase), both liquids being mutually immiscible. High specific surface areas resulting from the dispersion process are not energetically favoured, and therefore emulsions are thermodynamically unstable and supposed to break, i.e. to completely separate into two phases, after a finite amount of time. Since emulsions are non-equilibrium systems, their properties are highly dependent on the way they are prepared. Much scientific and technological effort has been dedicated to the retardation of emulsion breaking, i.e. to increase the kinetic stability. But it is not only important to expand the lifetime of emulsions but also to control their properties, particularly the droplet size and the polydispersity. Moreover, structuring of both the dispersed and continuous phase has become recently an issue for the production of specialty materials.
The continuous phase plays a fundamental role in the preparation, stabilization and characteristics of emulsions. This can be understood by taking into account the major destabilization processes occurring in emulsions, particularly creaming, flocculation, and coalescence. Accordingly, and increase in viscosity of the continuous phase reduce the creaming rate, and therefore this is a common strategy to counter emulsion breaking. Coalescence and flocculation are very much dependent on interfacial phenomena, but changes in the droplet collision rate (inversely proportional to the viscosity of the continuous phase) and in the interaction potentials (electrostatic, van der Waals) could be induced by varying the properties of the continuous phase.
Besides, the nature of the continuous phase evidently affects the solution behaviour of the surface active emulsifiers and vice versa. The molecular properties of surfactants (particularly the hydrophile-lipophile balance), the surfactant–solvent interactions, and the structures formed by the self-aggregated surfactant molecules may determine the size and the polydispersity of emulsion droplets, and even how such droplets are spatially distributed. Moreover, the presence of nanometric self-organizing structures within the continuous phase separating droplets with sizes in the order of microns might be interesting for the possible applications of emulsions as scaffolds for materials with dual nano-microstructure.
In this review, we intend to highlight some recent scientific contributions on the role of the continuous phase in the properties of emulsions and related applications.
In many emulsification methods, several phase regions within the phase diagram are crossed depending on the emulsification path [1]. A typical example is the spontaneous formation of highly concentrated emulsions by a temperature change in systems such as that depicted in Fig. 1a. The region adjacent to the two-phase domain in which emulsions are formed has a strong effect on the final properties. This is particularly true in the case of nano-emulsions. When using the Phase Inversion Temperature (PIT) method to produce nano-emulsions, it has been found that the size of the nano-emulsion droplets is governed by the surfactant self-organized structure (bicontinuous or lamellar) present at the inversion point [2•], [3]; in fact, the main requirement for the formation of nano-emulsions with the minimum droplet size is apparently to achieve a complete solubilization of the oil phase in a bicontinuous microemulsion, independently of whether the initial phase equilibrium is single or multiphase [4].
It has also been reported that in the so called Emulsion Inversion Point or Phase Inversion Composition method to produce nano-emulsions at constant temperature with a low energy input and small droplet sizes, it is necessary to cross a direct cubic liquid crystal phase (with all oil dissolved in a single phase) along the emulsification path, and it is also crucial to remain in this phase long enough during the emulsification process to incorporate all of the oil into the liquid crystal [5], i.e. the liquid crystal zone should be wide and extend to high water content.
The role of lyotropic liquid crystals in emulsion stabilization is known since the first report on the subject by Friberg et al. [6] and this role has been also addressed in recent papers [7], [8]. Emulsions with a liquid crystal phase are formed in the regions of the phase diagram in which an isotropic phase coexists with a liquid crystal phase. Therefore, lamellar, hexagonal or cubic phases can be present as the continuous phase [9] depending on the emulsification path, as shown in Fig. 1b. In contrast to conventional emulsions, liquid crystals films surround emulsion droplets and prevent their coalescence. Recently, biocompatible, highly stable emulsions using a lamellar phase as the whole dispersing medium were prepared in the system glycerol trioleate/sodium oleate/water [10]. Long-term stable water-in-liquid crystal emulsions were also prepared by dispersing water in reverse hexagonal phases formed by monoolein-based systems [11].
Cubic liquid crystals are those with the highest viscosity among lyotropic phases in amphiphilic systems, showing a yield stress and a high elastic modulus (> 104 Pa), therefore they are expected to be good emulsion stabilizers. There are two classes of cubic liquid crystals: bicontinuous and micellar. The bicontinuous cubic phases consist of multiple connected bilayers separating two distinguishable and continuous domains of the same solvent. On the other hand, micellar cubic phases are formed by discrete micellar aggregates packed in a cubic array. Direct micellar cubic phases (denoted as I1) are formed by micelles having a hydrophobic core, and are usually found in highly hydrophilic surfactant systems. It has been found that a considerable amount of oil (up to 90 wt.%) can be incorporated in the I1 + Oil two-phase region in transparent or translucent gel-like emulsions having an I1 cubic phase as the continuous phase [12•]. These emulsions are quite stable, since coalescence and creaming is prevented by the extremely high viscosity of the external phase. The translucent gels (see Fig. 1), which are formed at a certain water/surfactant weight ratio, can be diluted with water to form normal liquid-liquid emulsions with a small droplet size (1–10 μm). The translucency comes from the similarity between the refractive index of the oil and the cubic phase. In order to prepare a cubic-phase-based emulsion the cubic phase should be melted to enable adequate mixing during emulsification process. As the mixture is agitated, the oil incorporates gradually into the surfactant phase, and finally a gel is obtained by cooling the sample. The emulsions are polydisperse and some of the droplets are polyhedral (see Fig. 2), since the volume fraction of the internal phase (hydrocarbon) exceeds the maximum value for a packing of spheres, 0.74. Transparent emulsions based in the cubic phase were also reported for ionic surfactants [13]. It is important to point out that the nature of the dispersed oil and its solubilisation site (which can be the micellar core or the interfacial surfactant layer) affect the melting temperature of the cubic phase and hence the emulsion preparation process [14]. In any case, the enthalpy associated with melting (order-disorder transition) is small [15], as expected for lyotropic liquid crystals.
Although less common than their counterparts (due to packing constraints), reverse micellar cubic phases (denoted as I2), which consists of micellar aggregates with a hydrophilic core, are also found in a few highly hydrophobic amphiphilic systems. Water-in-liquid crystal highly concentrated emulsions can be prepared as well based on the use of the reverse micellar cubic phase. In a silicon oil-poly(dimethylsiloxane)poly(oxyethylene) binary system in which an I2 is present [16•], water was continuously added beyond the solubilization capacity of the I2 phase, and it was possible to incorporate about 90 wt.% of water in viscous emulsions. The emulsions remain stable even after centrifugation for long time. The emulsions also show thermal stability and can be diluted with silicon oil to form normal emulsions with a small droplet size (1–10 μm).
Similarly to I1-based emulsions, to prepare a reverse cubic phase based emulsion, the I2 phase should be melted to enable proper mixing during the emulsification process. Water incorporates gradually into the surfactant phase, and finally a gel is obtained by cooling the sample. The gels look turbid over the entire range of compositions in which they can be produced, because of the difference in the values of refractive indices of the cubic phase and water [16•]. However, by adding a refractive index matching additive, such as glycerol, transparent emulsions can be obtained [17].
The stress modulus of cubic-phase-based emulsions (in the two-phase region) tends to decrease with the volume fraction of the dispersed phase (ϕd) [15]. This rheological behaviour is opposite to that usually found in two-liquid concentrated emulsions, in which stress moduli and viscosity increases with ϕd [18]. If it is considered that the structural units of the cubic phase forms an elastic network, the elastic modulus would be proportional to the number of such units in the sample, then the viscosity will decrease with the cubic phase fraction in the system. Namely, the viscosity of the cubic-phase-based emulsions is mainly determined by the structured cubic phase [15].
Cubic-phase-based concentrated emulsions have been already used as templates for the one-step synthesis of dual meso-macro porous silica materials, which feature both pore accessibility and high specific surface area [19]. For the preparation of those materials, hydrolysis-condensation reactions of alkylsiloxanes are carried out within the continuous phase of the emulsions. The original oil droplets and surfactant molecules are removed by calcination, leaving behind a siliceous meso-macroporous structure.
Emulsions with a bicontinuous cubic liquid crystal (designated as V2) as external (continuous) phase have also been reported [20•]. They were created by adjusting the temperature of a surfactant (diethylene glycol dodecyl ether, abbreviated C12E2) aqueous sample into a miscibility gap in which there is coexistence of a bicontinuous cubic phase and a liquid phase. Interestingly, short living, triangle shaped droplets, containing surfactant monomer solution and dispersed in the V2 phase, were observed upon abrupt heating in a diffusive interfacial transport (DIT) device. The shape is unusual as it costs energy to create the additional surface area associated with a pyramid-shaped droplet. The growth of the droplets is restricted by the very viscous cubic-phase; the triangular appearance presumably reflects the [111] projection of the cubic unit cell.
Emulsions made of droplets suspended in thermotropic liquid crystals are systems that have recently attracted much interest, because they lead to new colloidal structures and offer novel ways to control stability [21], [22••], [23], [24]. Since most studies have taken an approach out of the scope of this review, the mentioned systems will be referred only very briefly here.
In contrast to isotropic binary mixtures, systems in which thermotropic liquid crystals coexist with an isotropic phase do not lead to a full phase separation but to a self-ordering of colloidal particles [22••]. When droplets are suspended in a nematic host, formed by rigid, rod-like organic molecules having long range orientational order, the properties of the suspension are controlled by the topological defects and distortions of the liquid crystal [22••], leading to the formation of highly ordered arrays of uniformly sized droplets that are extremely stable (see Fig. 3a). Such phenomenon has been studied theoretically as well [23]. When the continuous phase is a nematic liquid crystal, the defects prevent the droplets from approaching too closely and coalescing, namely, the emulsions are topologically stabilized [24] and can be obtained without addition of any surfactant.
One important variable in the above-mentioned emulsions is the anchoring energy, coming from local interactions between the liquid crystal molecules and the interfaces of both droplets and the boundaries of the sample. In these systems the dispersion process is more costly energetically than in isotropic fluids due to the contribution of the elastic distortions surrounding the particles.
Water-in-cholesteric liquid crystal emulsions have also been reported [22••], [25•]. Nucleation of numerous point defects was observed in the cholesteric planar texture immediately after termination of shear to the emulsion, leading to the formation of a regular defect array. Under steady shear flow, several distinct periodic structures appear, characterized primarily by the hexagonal defect array (see Fig. 3b) [26•].
Light-induced structural transformations of the topological defects in isotropic droplet/liquid-crystal emulsions are also possible [27•], which provide new possibilities of manipulation of colloidal superstructures by external stimuli.
It is well known that the viscosifying effect of non-adsorbing polymers (synthetic polymers or naturally occurring macromolecules like gums) may influence emulsion stability by decreasing the rate of creaming and coalescence [28]. In some cases, the flow properties can also be controlled by the changing the viscosity of the continuous medium [28]. Amphiphilic-associating polyelectrolytes offer the opportunity to combine both electrosteric and viscosifying stabilization mechanisms, since they form a strong gel network structure and a protective layer [29]. For example, oil droplets were dispersed and kept stable in the strong gel structure of hydrophobically–hydrophilically modified hydroxyethylcellulose (HHM-HEC) [30]. The stability of the emulsion using HHM-HEC is based on both protective colloidal effects and associative thickening caused by alkyl chains in HHM-HEC. The stability is particularly accomplished in a region where three-dimensional network structure shows dominantly elastic properties.
Several food products can be categorised as emulsion-filled gels. As a consequence, gels containing emulsion droplets have been subject of extensive study by food scientists [31], [32]. Emulsion droplets may either decrease or increase gel stiffness. The interactions between oil droplet and gel matrix depend on the surface properties of the oil droplet, in particular the nature of the stabilising agent. Droplets may be either bound (active) or unbound (inactive) to the gel matrix, which affects oil droplet release [31].
One food grade system which has been extensively investigated is casein, an important component of dairy products. Gels may be produced from casein-stabilized emulsions by a variety of treatments, including heating, acidification and high-pressure processing [33•], [34]. Aerated emulsions gels of good stability can be formulated by incorporating air bubbles into gradually flocculating caseinate-stabilized emulsions [33]. Recently, Diffusing Wave Spectroscopy (DWS) was used to study the transitions in structure of sodium caseinate stabilized emulsions [35]. The emulsion droplets in an emulsion without extra stabilizer formed a continuous network upon acidification, while the droplets in emulsions with an excess of stabilizer formed a network of oil droplets at neutral pH. Upon acidification of the latter one, the initial network of oil droplets fell apart, and eventually a network of sodium caseinate, in which the oil droplets were embedded, was formed.
Gelled oil-in-water emulsions have also attracted interest as encapsulation devices in cosmetic and drug delivery. Gellation also helps the use of emulsions as templates for the preparation of porous materials [36], [37], [38]. Gelled emulsions are often produced using a two-step process in which first oil is dispersed in an aqueous polymer solution that is then gelled to trap the oil droplets in the gel matrix. This gelation effect has been achieved in several ways: ionic crosslinking of polyelectrolytes, using associating polypeptides and carbohydrates, or via thermogelation of block copolymer systems. In many applications these materials appear in the form of spherical particles that are either used separately (e.g., gel capsules as drug carriers) or as components in liquid or gelatinous formulations. Recently, several studies have shown that insoluble gel particles can also be formed from mixtures of oppositely charged surfactants and polyelectrolytes [36], [37], [38], [39].In this case surfactant aggregates act as physical crosslinks between the polyelectrolyte chains. These materials present an attractive alternative for the encapsulation of oils in surfactant-based products. Associative phase separation occurs when oppositely charged surfactants and polyelectrolytes are mixed in near-stoichiometric proportions. Lapitsky and Kaler [40•] investigated the performance of gel particles as materials for the encapsulation and release of oil (see Fig. 4). The experimental data and model analysis indicate that the release rate is determined by the effective diffusivity and solubility of the oil in the aqueous gel matrix, both of which depend on the presence of surfactant in the receiving solution, and the swelling of the gel particle.
Oil-in-water emulsions showing reversible, temperature-induced gelation have been prepared using temperature-responsive polymers, such as poly(NIPAM-co-PEGMa)(N-isopropylacrylamide and poly(ethyleneglycol) methacrylate [41]. The stability of the emulsions arises primarily from steric stabilisation afforded by the adsorbed copolymer. The mechanism suggested for emulsion gelation involves thermally induced flocculation due to the collapse of adsorbed poly(NIPAM-PEGMa) layer at temperatures greater than the lower critical solution temperature (LCST). The collapse of the adsorbed layer presumably results in a rigid interface which opposes coalescence of flocculated droplets. Addition of surfactant to such kind of emulsions appears to decrease the number density of effective chains that bind neighbouring droplets together in the gelled state. This is believed to be the result of increased electrostatic repulsion [42]. Small Angle Neutron Scattering (SANS) measurements [43] indicated that in the poly(NIPAM-PEGMa) emulsion the interfacial polymer chains condensed to give a relatively thick polymer layer at the oil-water interface. The gelled emulsion appears to consist of oil droplets with an encapsulating layer of collapsed polymer to which sticky microgel particles are adsorbed. The latter act as “glue” between coated droplets in the emulsion gel.
Very stable and fully reversible High Internal Phase Ratio Emulsions (HIPRE) have been produced by self assembly of protein-coated monodisperse oil droplets [44••]. The method consists of cross-linking of the interfacial protein layer by thermal, enzymatic or chemical processes. Due to such cross-linking, coalescence is completely suppressed. Thus water can be removed to virtually nil content, leaving the HIPRE in the form of a protein-percolating network of surfaces dispersed in an oil matrix, corresponding to an oil volume fraction as high as 99.9% (see Fig. 5) This method shows to be fully reversible, allowing a full re-hydration of the HIPRE back to the original emulsion template, thus recovering the original particle size distribution [45••].
In some food emulsions, it has been found that the stability of the dispersed phase was dictated by the concerted role of both interfacial fat crystals and a solid fat crystal network. Due to the presence of a well-defined network structure, emulsions consisting of wax-crystallized in situ were more stable than emulsions with crystals added prior to emulsification [46].
Systems such as those that are highlighted in the previous sections have also been investigated from a completely different point of view which considers the continuous phase either as template or precursor for the fabrication of microstructured materials with different degrees or order. This may be considered as a specific type of emulsion templating, a technique for the preparation of well-defined porous polymers and inorganic materials, essentially consisting in the formation of a HIPRE, followed by a reaction induced phase separation (e.g. by cross-linking as outlined for proteins) to lock in the structure of the “external” phase [47]. In the applications herewith discussed, it is shown that the effect of the dispersion of water on the continuous phase is critical and is the fundamental parameter that allows controlling the homogeneity and regularity of the resulting templated structure. The possibility to utilize emulsions with self-organizing structures within the continuous phase separating the droplets as a template has been investigated only in the last decade and the comprehension of the real potential offered by block copolymers is even more recent [48], [49]. The interest in block copolymers arises from their unique solution properties, as the immiscibility between different blocks and the competing thermodynamic effects give rise to their self-assembly in a series of ordered morphologies in the nanoscale that for amphiphilic macromolecules in a block selective solvent corresponds to well-defined micelles [50], [51]. The plausible process of formation of well-defined porous structures is shown in Fig. 6. When water is added to a good organic solvent (for both the blocks) the well extended amphiphilic diblock copolymer macromolecules undergo phase inversion, with the hydrophobic blocks precipitating out from the aqueous medium and the hydrophilic component blocks remaining fully extended. The occurrence of interactions between micelles due to the progressive evaporation of the organic solvent forces a disorder-order transition that usually corresponds to the formation of a hexagonal superstructure. Subsequently, further evaporation results in a matrix with hexagonal morphology with the hydrophilic blocks still extended, which after complete drying collapse on the hydrophobic blocks leaving behind well-defined and well-structured pores.
The use of kinetically stable water-in-oil emulsions by diverse diblock copolymers, often polystyrene-based [52••], [53], [54], [55] but also with poly(methyl methacrylate) [56] allowed to point out the parameters controlling the whole process of emulsification and film formation, with the sizes of the water swollen micelles obviously depending on the amount of water added (usually less than 5 wt.%) and also on the molecular weight, i.e. the length, of the hydrophobic segment. In contrast, a change in the length of the hydrophilic block does not directly correlate to a diameter variation. Similarly, a strong dependence of the regularity of self-assembled water droplets into the close-packed structure on the hydrophobic block length is generally observed: the larger the block the more regular the resulting porous films are. In the case of copolymers with an ionisable block, also the pH value of the aqueous medium influences the micelle dimension as well as the pore size [52••].
Even though water swollen micelles have sub-micrometric sizes, the final pore diameters in regular patterns were observed to be up to 20 times bigger [53], i.e. in the order of tens of micrometers. This may certainly be explained in terms of partial merging of micelles, without forgetting that in some cases, during the process of film-formation, the droplets may grow in size due to the humidity of the systems in which the operation is carried out. In this context, it is also worth mentioning that small amounts of water may be introduced in block copolymer solutions during casting [48], [49], [57], [58], [59], [60]. The latent heat of evaporation of the solvent induces water vapour condensation into droplets that grow with time, thus giving rise to an inverse emulsion. By this strategy that we consider as a variation within the general method shown in Fig. 6a, excellent micrometer-size honeycomb structures were obtained, e.g. from polystyrene-b-oligothiophene (Fig. 6b). Recently, Hayakawa and colleagues were also able to prove the chemical heterogeneity of an emulsion templated porous structure by transmission electron microscopy and time-of-flight secondary ion mass spectrometry imaging, showing different composition on the film surface that clearly corresponds to the structured phase morphology of the block copolymer [61], [62••].
Section snippets
Conclusions
A variety of soft materials and complex fluids, such as liquid crystals (lyotropic and thermotropic) and gels can be used as a continuous phase in emulsions. The structuration of the continuous phase opens new possibilities for the control of emulsion properties and performance and for their use as scaffolds in the preparation of advanced materials with different orders of length scale. Much research is still to be done on the mechanism of emulsion formation and evolution during the templating
Acknowledgements
C.R.A. and M.L. acknowledge the Ministerio de Ciencia y Tecnología (Spain) for support through the Ramon y Cajal program.
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Major recent advances. The use of structured continuous phases in emulsions allows stability and morphology control. The recent fabrication of well-defined porous materials using emulsion templates and the structural control of nematic emulsions by external fields are promising from both academic and technological points of view.
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of special interest.
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of outstanding interest.