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1 This article was downloaded by: [informa internal users] On: 6 December 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Encyclopedia of Surface and Colloid Science Publication details, including instructions for authors and subscription information: Jean-Louis Salager a ; Johnny Bullón a ; Aldo Pizzino a ; Marianna Rondón-González ab ; Laura Tolosa a a Laboratory of Formulations, Interfaces, Rheology and Processes, University of the Andes, Merida, Venezuela b Total Petrochemicals, Lacq, France Online publication date: 13 October 2010 To cite this Section Salager, Jean-Louis, Bullón, Johnny, Pizzino, Aldo, Rondón-González, Marianna and Tolosa, Laura(2010) '', Encyclopedia of Surface and Colloid Science, 1: 1, 1 16 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Jean-Louis Salager Johnny Bullón Aldo Pizzino Marianna Rondón-González* Laura Tolosa Laboratory of Formulations, Interfaces, Rheology and Processes, University of the Andes, Merida, Venezuela *Now with Total Petrochemicals, Lacq, France INTRODUCTION Abstract The formulation and formation of emulsions imply a very large number of variables; hence, attaining the sought properties is usually long and tedious because of the large number of experiments to be carried out. The current state-of-the-art of physicochemical formulation may be thoroughly used to considerably reduce the number of required experiments and to indicate to the formulator the relation between the kind of properties to expect and the generalized formulation expression. After these first-order formulation factors are dealt with, several second-order factors are shown to be available to adjust the emulsion properties upward or downward in order to satisfy the specifications, either directly or by means of a multistep process. Emulsions are systems in which two immiscible liquids (referred to as oil and water) are temporarily mixed in some homogeneous way at some microscale to macroscale to produce the dispersion of one phase in the other. In general, it is called an emulsion only if the coalescence of the drops is inhibited during some time by some process involving surfactants or particles. This entry only deals with surfactant-stabilized emulsions. The emulsion desirable properties (type, drop size, and distribution, rheology, and persistence, often called stability) depend on the application. In most cases, the formulator challenge is to attain the sought-after product through 1) selecting the right substances (surfactant, oil, aqueous phase) and their right proportions, then 2) mixing them and making an emulsion through a process which could be quite complex. The optimization of the formulation and emulsification procedure may be made relatively simpler if the different aspects are segregated and treated separately, as shown in the following. FORMULATION CONCEPTS APPLIED TO EMULSIONS Early Concepts: HLB, PIT, Winsor s R A surfactant has a dual affinity for the aqueous (polar) and oil (nonpolar) phases, and the emulsion properties depend on that, i.e., strongly on the surfactant nature (because it Encyclopedia of Surface and Colloid Science DOI: /E-ESCS Copyright 2010 by Taylor & Francis. All rights reserved. can change considerably) and in a lesser measure on the oil and aqueous phase nature, and other variable such as temperature. The characteristics of the components, particularly, the surfactant, have been represented by various parameters introduced over the years. About 50 years ago, the hydrophilic lipophilic balance (HLB) number [1] was introduced as an empirical estimation of the dual tendency of the surfactant, essentially related to the proportions of its hydrophilic and lipophilic parts and, to some extent, to the oil type. This approach is rather inaccurate, but it is extremely simple and is still useful as a first approximation and to compare similar cases. However, it may be completely erroneous in some instances like comparing cases with different surfactant or oil families. Davies [2] intent to separate the HLB number into group contributions made it easier to use in practice, but it did not really improve the accuracy, which is sometimes worse than ±2 units. Winsor s R ratio approach is based on the interactions of the surfactant molecule adsorbed at interface with neighboring oil and water molecules (labeled Aco and Acw, respectively, in Fig. 1). [3] It allows to carry out some qualitative reasoning and to relate the R ratio of interactions at interface with the phase behavior at equilibrium as indicated in Fig. 1. There are only four different cases, hence much less than the number of actual variables in practice. R<1 and R>1 correspond to the so-called Winsor I and Winsor II biphasic systems, respectively, which exhibit an aqueous and an oil phase microemulsion in equilibrium with an excess oil and aqueous phase, respectively. R=1 is associated with the three-phase behavior so-called Manila Typesetting Company 11/06/ :10AM

3 Fig. 1 Winsor s R ratio and its relation with the phase behavior of surfactant oil water systems. Winsor III, in which a bicontinuous microemulsion, containing essentially all the surfactant, is in equilibrium with two (oil and aqueous) excess phases. [4] If the amount of surfactant is high enough, the microemulsion embodies the whole system volume and a so-called Winsor IV singlephase behavior is observed. Hence, when a variable like the aqueous phase salinity or the length of surfactant tail changes (in some monotonous way), a so-called formulation scan results in a continuous change in the ratio of interactions. When R changes from R<1 to R>1, a phase behavior transition takes place and is noted WIàWIIIàWII or vice versa. Fig. 2 illustrates the evolution of the systems during a phase behavior transition versus some formulation changes. [4] As far as emulsions are concerned, the Winsor R ratio allows to accurately interpret rules of thumb such as Bancroft rule and Langmuir wedge theory. However, the R ratio cannot be translated into numbers to be handled quantitatively. Shinoda s phase inversion temperature (PIT) [5] approach is based on an experimentally attainable information, i.e., the temperature at which the emulsion made with a polyethoxylated surfactant inverts from oil/water (O/W) to water/oil (W/O). This temperature essentially corresponds to the swapping of the dominant affinity of the surfactant from water to oil phase, which is essentially related to the phase behavior. In other words, the PIT corresponds to Winsor R=1 case, with R<1 at temperature below the PIT and R>1 above it. Because it is an experimental approach, the PIT takes into account all variables, which is quite an improvement with respect to earlier concepts like the HLB number. However, there are serious limitations to its general use, e.g., the liquid water temperature range and its applicability mostly limited to polyethoxylated surfactants whose hydrophilic group is dehydrated as temperature increases. formulation variables for many systems, and thus extended the PIT approach. The balance of affinity was written as a formula that is a generalized expression of the surfactant affinity difference (SAD), i.e., the variation of chemical potential of a surfactant molecule when it transfers from water to oil (Dµ). [6] This expression was related to thermodynamic concepts, such as the surfactant partition coefficient between excess phases in three-phase systems, which can be experimentally measured. It is generally expressed in a dimensionless way with respect to a reference in a situation in which the surfactant exhibits exactly the same standard chemical potential in the oil and aqueous phases, at ambient temperature and pressure. The generalized formulation expression is a linear summation of energetic contributions that involves all variables describing the nature of the components as well as temperature and pressure. In the simplest cases, the expression is similar to the correlation for three-phase behavior and minimum tension, [7,8] and may be written as the dimensionless hydrophilic lipophilic deviation from the optimum formulation for three-phase behavior, which is taken as the reference state [9]. This deviation, so called HLD, may be expressed as follows (see Eqs. 1 and 2) HLD=(SAD-SAD ref )/RT=lnS-kACN-f(A)+s-a T DT for ionic surfactant HLD=(SAD-SAD ref )/RT=bS-kACN-f(A)+b+c T DT for nonionic surfactant where S is the salinity of the aqueous phase in wt.% NaCl; ACN is the alkane carbon number, i.e., the number of carbon atoms in an n-alkane oil; s or b are the ionic and nonionic characteristic parameters of the surfactant. These parameters increase linearly with the surfactant tail length [10] and decrease with the number of ethylene oxide groups for polyethoxylated nonionics. [8] f(a) is a function that renders both the type (through m A ) and the concentration (C A ) of the (alcohol) cosurfactant in f(a)=m A C A. DT is the temperature difference with respect to a reference, usually ambient temperature. These expressions apply to simple surfactant oil brine systems but may be extended (1) (2) Generalized Formulation Expression In the 1970s, the enhanced oil recovery research promoted many experimental studies, which may be said to have numerically quantified Winsor s R concept in terms of all Fig. 2 Phase behavior transition along some formulation changes Manila Typesetting Company 11/06/ :10AM

4 to different oils and aqueous phase electrolytes, change in ph, mixtures of surfactants, and oils as in most real world cases. [11 18] In the past 20 years, the characteristic parameters for surfactants, oil and water phases, as well as the temperature and pressure effects were determined in a large number of different systems. They may be predicted in many instances, in practice, and may also be experimentally determined when no estimate is available. [19] Mixing rules for surfactants and oils were found to be simple (e.g., linear), in some cases, and depart from ideal behavior in others. The non-ideality of the mixing rules was also quantified and interpreted through the partitioning and segregation of species, hence showing that mixtures could be either a problem or a solution. [19,20] The generality of HLD formulation expression results in a direct relationship with the phase behavior, i.e., HLD<0 and HLD>0 correspond to R<1 and Winsor I and R>1 and Winsor II, respectively. HLD=0 is associated with Winsor III three-phase behavior transition. [9] This corresponds to R=1, and it is an extremely important circumstance because it is a reference and a boundary, as far as the properties of equilibrated and non-equilibrated systems are concerned, independent of the actual variable values such as salinity, oil nature, temperature, etc. HLD determines the physicochemical balance (if HLD=0) or unbalance (if HLD ¹ 0), whatever the actual values of the formulation variables. This is noteworthy because it means that two systems may be considered to be in equivalent physicochemical states if they have the same HLD value, even though they do not exhibit necessarily the same salinity, same surfactant type, same temperature, etc. Actually, none of the variables may coincide, although the two systems are in similar physicochemical states and may be compared as far as performance is concerned. QUALITATIVE GENERAL PHENOMENOLOGY: EMULSION PROPERTIES VERSUS FORMULATION AND COMPOSITION (FIRST-ORDER FACTORS) Early systematic studies to relate the formulation with the emulsion properties [21] indicated that reproducible results were attained when the system were equilibrated prior to emulsification, so that no mass transfer was taking place at the time of interfacial area generation during emulsification. A few studies with non-equilibrated systems indicated that the time-scale of equilibration could change the results, with a shorter pseudo-equilibration time-scale, as HLD is close to zero. [22] When the systems are not preequilibrated prior to emulsification, a reproducible result is usually attained when the surfactant and co-surfactant species are introduced in the phase where they will partition at equilibrium. Emulsion Properties The type of emulsion is essentially related to the natural interfacial curvature that results in O/W or W/O simple morphology and is related to formulation according to the century old Bancroft s rule. Drop size and distribution depend on physical properties like the interfacial tension and surfactant adsorption. [23] Emulsions are not stable systems, and sooner or later, they will separate into their phases. However, they may be quite persistent, and the expression kinetics stability (or often just stability ) is used to describe their lifetime scale, which is frequently expressed as the time required for some fraction, e.g., 50% or 75%, of one of the phases, usually the originally dispersed one, to separate. Rheological properties of emulsions are essentially dues to physical factors and effects like the external phase viscosity, the internal phase volume, drop interactions, and eventual deformation under shear. [21] Emulsions with drop in the 1 50 µm range with internal phase content over 60% exhibits non-newtonian behavior of the shearthinning type and are often viscoelastic when they contain a large proportion of submicron droplets. Monodispersed distributions tend to exhibit a high viscosity, while very polydispersed, particularly bimodal distributions, result in a reduced viscosity which is quite useful for some applications. [24] Turbidity or opacity have to do with the drop proportion, drop size, and refractive indexes of the liquid, as well as the radiation wavelength. [21] The higher reflectivity with daylight takes place for a drop size around the micrometer. [25] Emulsion Properties versus Generalized Formulation (Unidimensional Scan) The variation of formulation variables for systems containing similar amounts (i.e., from 30% to 70%) of oil and water results in systematic patterns when the formulation is expressed as HLD. These studies were carried out more than 20 years ago according to the so-called unidimensional formulation scan method, in which a single formulation variable is monotonously changed so that a phase behavior transition takes place in the WIàWIIIàWII direction or vice versa as in Fig. 2. [8] The general qualitative property trends along a unidimensional formulation scan are essentially determined by what happens at the formulation that corresponds to HLD=0 in the scan. Whatever the formulation variable used to change the balance of affinity, in particular, the temperature for polyethoxylated nonionics, the surfactant oil-water equilibrated system exhibits at HLD=0 the following two characteristics (see Fig. 3a graph): 1. a minimum of interfacial tension, sometimes ultra low, which is why it was originally referred to as optimum formulation by researchers on enhanced oil recovery. [26 28] Manila Typesetting Company 11/06/ :10AM

5 Fig. 3 a) Interfacial tension, phase behavior, and emulsion morphology; b) stability; c) drop size 9; d) viscosity and light scattering (reflectance) versus generalized formulation HLD. 2. a three-phase behavior (bicontinuous microemulsion in equilibrium with both oil and aqueous excess phases) and eventually a single-phase behavior if the surfactant concentration is high enough. [29] As far as the emulsified system is concerned, HLD=0 is associated with a crucial feature in each of the five properties as shown in Fig. 3 as follows. First is a change in emulsion type from O/W to W/O or vice versa (Fig. 3a), which is referred to as morphology inversion. This is generally determined by a drastic change in conductivity as well as other properties like viscosity and optical reflectance, whose variations might be more difficult to interpret. [29 34] Second, a very deep minimum in the emulsion stability exactly at HLD=0 (Fig. 3b). [29,30,35,36] The actual value of the phase separation time at HLD=0 is similar to the case of a system containing equal oil and water phases without surfactant. [37] This clearly shows that at optimum formulation the surfactant is trapped in the microemulsion and that it is not available to adsorb at the interface formed during the stirring process. Other more complex explanations have been proposed. [38,39] When the formulation departs considerably from HLD=0, then it may be said that the surfactant is either too hydrophilic or too lipophilic. In such case, it is not likely to adsorb at interface but rather to partition into one of the bulk phases, and thus it does not protect the emulsion drops against coalescence. As a consequence, the emulsion stability curve versus formulation typically exhibits two maxima first noted a long time ago, [40] one on the negative HLD side and one on the positive side, typically somewhere in the HLD=2 4 range in absolute value (see Fig. 3.b). The exact position of the maxima and the extension of the highstability range essentially depend on second-order factors to be discussed later. Third, the drop size attained along a formulation scan, when a constant emulsification procedure (i.e., same stirring equipment, energy, and duration) is applied, results from the competition of two opposite phenomena: the drop breakup, which depends on the tension and is quicker and easier close to HLD=0 formulation, and the coalescence of formed drops before the surfactant can adsorb efficiently, which is much faster at HLD=0 formulation since the surfactant does not migrate to the created interface. The two opposite effects generate a minimum of drop size at some distance of HLD=0 and on both sides of it (see Fig. 3c plot), where the tension is low enough to result in small droplets, which are not too likely to coalesce instantly. [41] The minima in drop size are much closer to HLD=0 than the stability maxima, typically at about 1 HLD unit or less. However, their occurrence and characteristics depend on second-order factors susceptible to alter the tension and stability curves, as well as on the stirring energy. [42] The increasing drop-size tendency, indicated in Fig. 3c region to be close to HLD=0, is the consequence of the very low stability that quickly increases coalesce and increases the size if it could be measured. However, this could not result in larger droplets available, in practice, because the emulsion vanishes in seconds. Fourth, the viscosity of an emulsion submitted to shear is in local minimum at HLD=0 formulation, where the tension is ultra low and the drops are easy to elongate in the direction of flow (Fig. 3d, lower plot). The effect is similar in laminar flow through porous media and in turbulent flow cases. Since there are two drop-size minima on both sides of HLD=0, hence, there are often two concomitant viscosity maxima (see Fig. 3d, lower plot) because of the general association between smaller drops and higher viscosity. [43] Fifth, optical effects are associated with the morphology as well as some physical properties such as refractive index, light wavelength, and drop proportion and size. [21] The transmittance obscuration and diffusion, particularly, the backscattering, increase with the drop proportion, but does not vary monotonously with the size. A maximum backscattering (reflectance) is attained for droplets about µm in size. [25] As a consequence, and for the case of drops larger than 1 µm, the variation of size associated with the formulation, i.e., the presence of a drop-size minima on both sides of HLD=0 induces a complex variation of the backscattering (Fig. 3d, upper plot). [44] Combined Effects of Formulation and Composition (F C Map) Up to now, only the effect of the nature of the components, temperature, and pressure, i.e., intensive formulation variables, have been considered. Since the chemical potential driving force also depends on concentrations, extensive variables, labeled here as composition variables, such as surfactant concentration and water-to-oil ratio (WOR), should be taken into account somehow Manila Typesetting Company 11/06/ :10AM

6 Fig. 4 Generalized F C maps with typical emulsion properties: a) type; b) stability; c) drop size; and d) viscosity. The previous results are quite general and valid whenever the WOR is close to unity, say from 30% to 70% of one of the phases. When one of the phase proportions is too low, e.g., less than 20%, it turns out that it may be difficult to disperse the high-volume phase into the smallervolume phase, and the phase in higher volume tends to be the external phase. The way the phases are mixed becomes crucial to favor one or the other type of dispersion independently of the formulation, [45] and the global or local WOR value is the key to establish the emulsion morphology in some processes and with particular devices. [46] When the formulation and composition effects are in conflict, e.g., the formulation would favor an O/W emulsion but there is only 10% of W which cannot be made the external phase, then double or multiple emulsions are formed with droplets in drops. The droplets in drops inner emulsion is usually the one favored by the formulation, while the outer emulsion, i.e., drops in continuous phase, is the one imposed by the W/O composition. The resulting multiple emulsion morphologies are indicated as w/o/w and o/w/o in Fig. 4a. These results may be summed up in a bidimensional formulation composition (F C) map, first introduced with the PIT, [47] then systematized, [48] in which the formulation refers to any variable modification able to change HLD or any combination of them, and the composition to the water/oil proportions, generally the water fraction as in Fig 4 plots. Six regions are found in this map (Fig. 4a), depending on the formulation and composition. [48,49] A is in the center part, where oil and water proportions are similar, and B and C are located on the left and right, respectively, where there is much more oil than water in the former and more water than oil in the latter. In the upper and lower parts, HLD is positive and negative and the letter indicating the region is associated with (+) and ( ) sign, respectively. The so-called standard inversion line is the frontier that separates the two regions with different kinds of morphology, i.e., water external or oil external emulsions. In the central A region, the morphology only depends on the formulation, i.e., on the HLD sign, and the emulsions are referred to as normal ones because their interface curvature corresponds to the formulation requirement. In the B region the external phase is oil, whatever the formulation, and there is a simple W/O emulsion in the B + region (as in A + ), where HLD>0. In the so-called abnormal B - region, a multiple emulsion of the o/w/o type is often found, where o represent oil droplets inside W drops which are dispersed in the O oil external phase. In the C region where there is a large excess of water, the emulsions are water external, with an O/W normal morphology in the C - zone, and an abnormal one of the multiple w/o/w type in the C + zone. This mapping is quite general and qualitatively the same whatever the formulation variable used to produce the scan. [48,50 56] The central horizontal branch of the inversion frontier closely follows the HLD=0 line in the formulation scale and the three-phase behavior region in the map. Departure from a completely horizontal central segment is due to phenomena that alter the actual interfacial formulation when the WOR changes such as the partitioning of the species present in a surfactant mixture, [57,58] which is out of the present scope. In most practical cases, the inversion line crosses the three-phase behavior zone as indicated in Fig. 4a plot, i.e., slightly slanted even when an isomerically pure surfactant is used. [59] The lateral almost vertical branches of the standard inversion frontier may be shifted with the change of some other variables. An increase in oil and aqueous phase viscosities tend to shift the A + /C + and the B /A frontiers to the left and right sides, respectively. [46,48,60] An increase in surfactant concentration tends to widen the A zone, i.e., to shift both vertical branches outward. [61] Conversely, the A region tends to shrink as the stirring rate used to emulsify the systems is increased. [62] Since the bidimensional F C map has always the same aspect, with the width of the A region as the only change, this later may be associated to any other variable effect to produce a very general phenomenology which accounts for the effect of score of variables in a threedimensional space, which is described elsewhere. [63,64] The previously discussed relationship between formulation and emulsion properties is valid in the central A region and may be extended to the normal C and B + regions, provided that the property changes due to a variation in WOR are taken into account, for instance, an increase in viscosity with an internal phase content rise. In the abnormal regions, the presence of multiple emulsions implies that the same surfactant has to stabilize two opposite curvatures, which is not possible since the curvature is associated with HLD. [65,66] Hence, in such regions, the most outer emulsion Manila Typesetting Company 11/06/ :10AM

7 is unstable (because of the unfavorable formulation), whereas the most inner one is as stable as the one with a more balanced WOR in the corresponding A region. The change in properties with the WOR has a consequence on the drop-size map, because in the high-viscosity normal region close to the vertical branch of the inversion line, a particularly efficient emulsification takes place at low-velocity stirring, with a pseudo-plastic or viscoelastic behavior able to concentrate the shear on the droplets. [67 69] This very favorable situation is not only a flow mechanics curiosity but also a way to make small drop emulsions with high-viscosity oils, sometimes at a very large scale. [70] The four maps displayed in Fig. 4 are very general [49] and qualitatively express the variations of the emulsion properties according to the first-order effects of HLD generalized formulation and WOR composition. These maps extend the results shown in Fig. 3 to the regions containing a large excess of one of the phases. The position of the horizontal branch in the central part is essentially matching the HLD=0 line when the proper formulation scale is used, i.e., when an eventual correction to substitute the overall formulation by the interfacial one is carried out. [58] As mentioned previously, the lateral vertical branches of the standard inversion line may be shifted outward or inward by a change in any of the other variables, e.g., the surfactant concentration, emulsification energy, or phase viscosity ratio. Since each A/B/C + and - regions is associated with specific emulsion characteristics as indicated in Fig. 4, e.g., high stability or small drop size, once the inversion frontier is known, it is quite easy to know where some emulsion property is likely to happen. In Fig 4c, there is no indication of large drop size close to HLD=0 since this is not really a general size property because of very unstable emulsions. In practice, the change in a variable susceptible to laterally shift the vertical branches is the best way to increase or limit the area where some property occurs. There is another way to shift the vertical branches of the inversion line, which is the use of a dynamic process and the associated memory features to be discussed next. on the bidimensional map is moving vertically upward or downward, as indicated in Fig. 5 HLD WOR map, and at some time, it can hit and cross the horizontal branch of the standard inversion frontier. This event is called a dynamic inversion process, since it is due to a variation of conditions with time. [60] When such a change in formulation of the emulsion under constant stirring produces an emulsion morphology inversion, it is called a transitional inversion. This label comes from the fact that in such change the underlying phenomenon is a WIàWIIIàWII phase-behavior transition (or vice versa) in which the microemulsion middle phase present at HLD=0 becomes the water or the oil phase in the two-phase systems found in the WI and WII extremes of the scan in Fig. 2. [32,60,75 78] As a consequence, this is not really an emulsion inversion in which the curvature of the interface swaps, but rather the disappearance of one of the excess phase by solubilization in the microemulsion surfactant-rich phase, which always constitutes the external phase of the emulsion. [78] Another way to produce a dynamic inversion is to change the WOR of the emulsion along the arrows in Fig. 6, while keeping it stirred, until its morphology swaps. [78,79] This second type is referred to as catastrophic inversion because it exhibits some features such as hysteresis and irreversibility, which may be modeled by catastrophe theory. [80, 81] In this case, the curvature of the interface is able to suddenly reverse when the change is from a simple O/W to a simple W/O morphology or vice versa. [78,79] Such a change takes place usually with some delay in both directions, thus generating hysteresis zones (located around the standard inversion vertical branches) in which the emulsion type depends on the previous history. [78] In some cases, the inversion does not take place directly from one to the other simple morphologies (O/W or W/O) but undergoes through a multiple emulsion o/w/o or w/ O/W. [82 84] A multiple emulsion is actually a structure in which both morphologies coexist; hence, the change may Dynamic Emulsion Inversion The emulsion inversion frontier which has been discussed in the previous section is the limit between the regions where one morphology or another is attained upon stirring a pre-equilibrated system. This is the best condition to correlate the formulation with resulting properties. However, sometimes, the stirred system is not at equilibrium and a mass transfer could take place during the emulsification or after it as time elapses. This is the case in complex phenomena that take place in the so-called apparent equilibration process and spontaneous emulsification, [22,71 74] which are out of the scope of this entry. In other instances, the formulation is continuously modified by changing an appropriate variable, particularly, the temperature. In such case, the system representative point Fig. 5 Phase behavior at equilibrium and emulsion transitional inversion process induced by a change in formulation or temperature in a nonionic system Manila Typesetting Company 11/06/ :10AM

8 Fig. 6 Catastrophic inversion process induced by a change in WOR. be considered as being not sudden but progressive with one of the emulsions prevailing in one direction while the other in the opposite direction. When the direction of change is from normal to abnormal morphology, the occurrence of a multiple emulsion is not likely to happen. On the contrary, it is almost always the case when the change is from abnormal to normal morphology. In such a process with an intermediate multiple morphology, the inner normal emulsion, i.e., the droplets in drops, tends to prevail by the formation of more and more droplets which do not coalesce but swell the external drops and make them enter in contact and coalesce. These processes are difficult to control because the actual mechanism and results are quite dependent on the operational conditions. [82,85 88] By the way, a long-time stirring of an abnormal emulsion produce the same effect, because the inner emulsion is generally much more stable than the outer one and thus tend to [86, 89,90] prevail when the system is submitted to perturbations. Emulsion inversion processes may be even more complex in the presence of mixtures of surfactants or surfactants and cosurfactants that are able to migrate as time elapses and produce changes in interfacial formulation. This is the case of the so-called emulsification along a dilution path, e.g., by the addition of water to an oil phase containing a high concentration of surfactant(s) and cosurfactant(s). [91,92] These transfers result in a spontaneous emulsification, [73,74,93] a generic label to describe several different out-of-equilibrium phenomena, used in practice because they lead to very small droplet emulsions at a very low expense of energy, although they are not very well understood yet. in Fig. 7, left plot. However, the actual exact location (in formulation terms) and the actual value of the property (i.e., the persistence time), whose position could change as indicated by arrows in Fig. 7, depend on other issues which will next be referred to as second-order factors. Nevertheless, it is worth remarking that a clear discrimination of the first-order factors as the formulator priority and the use of a generalized formulation framework is determinant, in practice, to decide where to look for a solution, and how to make comparisons and optimize a process by modifying second-order factors. The paramount importance of such an approach is illustrated in Fig. 8, left plot, which shows the stability of two different emulsions as a function of salinity. The formulator first thinking may be to carry out the comparison at the same salinity so that the conditions are equal or similar. If so, if the fixed salinity is taken as indicated by dashed lines, for instance, it is clear that depending on the salinity, emulsion (A) is going to appear to be more or less stable than emulsion (B), and this is a wrong conclusion. On the contrary, if the two stability curves are represented versus a HLD scale (graduated in lns as in Fig. 8, right plot), it is clear that emulsion A is more stable than emulsion B in the regions where it matters, i.e., at some distance from HLD=0 where both are stable. This representation also allows making comparisons and further discussing some precise differences. For instance, it may be said that both kinds of emulsions are more stable in the high HLD case, but that the range of stability of the B case is quite wider for the O/W type than for the O/W one, particularly, farther from HLD=0, i.e., at low salinity. QUANTITATIVE TRENDS AND FEATURES EMULSION PROPERTY ADJUSTMENT (SECOND-ORDER FACTORS) On top of the general phenomenology dealing with firstorder factors, there are some additional effects that would alter some phenomena and mechanisms and would produce a quantitative change in some property in one direction or the other. Sometimes two opposite effects would compete, hence with the attainment of a minimum or maximum in some property. Final Comment on First-Order Factors Before moving to the second category of factors, it is worth remarking that the trends, indicated as the effect of a formulation unidimensional scan or of an F C bidimensional scan, are general in a qualitative way. For example, they indicate that there is a zone with a specific property, e.g., maximum stability on both sides of HLD=0, as indicated Fig. 7 General trend on a qualitative basis (left) and specific quantitative changes in stability (center and right) according to particular second-order effects Manila Typesetting Company 11/06/ :10AM

9 Fig. 8 Emulsion stability comparison according to a formulation variable and to the generalized formulation HLD. Basic Phenomena Related to Emulsion Properties The emulsion persistence is related to different phenomena which are influencing the rate of coalescence of drops and depend on the presence of adsorbed surfactant molecules at interface. This adsorption inhibits the coalescence in different ways through various mechanisms, namely: 1) the physical strength and elastic behavior of the interfacial film with respect to drop drop collision; 2) the repulsion forces that prevent the drops to approach under the effect of attractive forces; 3) the fluid properties of the interdrop film of continuous phase, particularly, its rheology; 4) some geometric considerations such as the drop size and distribution, and the internal phase ratio; 5) the temperature, which has an effect on a large number of phenomena and, on top of it, may have a strong formulation influence one way or the other depending on the type of surfactant. These phenomena are related to intensive properties that mostly depend on the nature of the components, not on the size of the system or the concentrations. Hence they may be altered by changing the conditions or adding some products as discussed in the next section. On the other side there are phenomena which depends on extensive properties like the WOR or the species concentrations, particularly, of surfactant, with a direct effect on the emulsion morphology and drop size, and hence on other properties. Surfactant concentration affects properties in different ways. Below the critical micelle concentration (CMC), an increase in surfactant concentration decreases the interfacial tension and thus tends to decrease the drop size. Above the CMC, the equilibrium interfacial tension does not change, but the increase in surfactant concentration could rise the adsorption rate of surfactant molecules from the bulk phase to the interface, thus reducing the dynamic tension and consequently the drop size, and finally increasing the emulsion stability. In general, an increase of surfactant concentration decreases rapidly the drop size of the emulsion until a minimal size is reached, which becomes independent of the surfactant concentration. [94] The electrolyte concentration is also an extensive variable to be taken into account. It will be seen later that an increase in salinity tends to reduce the repulsion of head group between adjacent ionic surfactant molecules, e.g., dodecyl sulfate, hence it will allow for more adsorption density, and a more compact interfacial film tends to increase the stability. However, it will also decrease the electrical repulsion between approaching drops, with the opposite effect. Finally the Laplace s law about the difference in pressure between the sides of a curved interface results in the so-called Oswald ripening decay, particularly, in very small droplet emulsions. This effect is an osmotic drive transfer of matter between a small drop and a neighboring large one, because of the gradient of internal pressure. [95] It is particularly significant with nanoemulsions, since the Laplace pressure is inversely proportional to the drop size. Of course the first way to reduce this effect is to have an almost monodispersed drop-size distribution, but this is of course associated to a higher viscosity. Since the transfer takes place through a film of external phase located between neighboring drops, a low solubility of the internal phase in the external one would reduce the effect. A classical trick to offset the osmotic pressure transfer is to use a mixture of substances in the internal phase, one which is much less soluble in the external phase that the other. As a consequence of a difference in transfer velocity, a gradient in the concentration of one specie is quickly established and results in a counter effect that cancels out the osmotic pressure driving force. [96] Effects of Second-Order Factors Which May Be Manipulated by the Formulator to Adjust the Emulsion Properties Surfactant film at interface The presence of an adsorbed surfactant film at interface results in some barrier that prevents the approaching drops to coalesce instantly as it happens when two pure liquids are used. To be effective in stabilizing the emulsion, the surfactant layer should exhibit a good coverage of the surface, forming a rigid elastic layer that resists collision and stretching. The interfacial coverage depends on the equilibrium concentration in the bulk, but in all cases it is attained at or below the CMC. Since the surfactant concentration should be larger than CMC, lowering the CMC by changing the surfactant or altering the formulation (e.g., by increasing salinity for ionics or temperature for nonionics) may help. A better condensed adsorbed layer may be attained by increasing the density, e.g., with linear tails instead of branched ones, with no or less electrical charge between adjacent hydrophilic heads. A higher density is also attained by selecting a surfactant with strong lateral interactions, either between the head or tail groups, eventually with interaction through overlapping to induce elasticity and to boost the Gibbs-Marangoni effect. If the surfactant is able to produce a mesophase, this is also a way to generate some wrapping around the drops and protect Manila Typesetting Company 11/06/ :10AM

10 them against collision effectiveness. These molecular arrangements may be extremely well organized like in liquid crystals, or more or less isotropic as in asphaltene gel precipitation close to the interface. Generally, an increase in the surfactant molecular size on both sides of the interface results in a thicker interfacial layer, which also results in a gain in the resistance of the film through a repulsion of some kind. Larger surfactants, surfactant mixtures with hydrophilic and lipophilic components, polyethoxylated surfactant mixture of oligomers, lipophilic linker additives and extended surfactants are all candidates to contribute this way. [97,98] The size of the surfactant influences also the emulsion properties. A low molecular weight surfactant migrates quickly to the interface, allowing a rapid stabilization of interfaces during the emulsification process. However, short surfactant molecules adsorb in a low-energy way at interface, producing less stable emulsions than long chains and polymeric surfactants. It was reported that in some cases the droplet size of an O/W emulsion is inversely related to the length of the alkyl tail of the surfactant. [99] When a mixture of surfactants is used, a change in surfactant concentration can alter the formulation at interface because of the partitioning of different species, and this could directly influence the drop size, stability, and viscosity of emulsions. Similarly, a change in purity, from isomerically pure to commercial mixture could result in a change of the PIT with surfactant concentration. [100,101] Amphiphilic polymers could have a complementary role when they are used combined with surfactants. Short chain surfactants adsorb rapidly at the interface during emulsification, hence avoiding instant coalescence and favoring a small drop size. On the other hand, polymers adsorb later providing a long-term stability. In some cases, some synergy occurs, and surfactants and polymers which do not stabilize emulsion on their own, could do it if they are used together. [102] The presence of very short amphiphiles at interface like alcohols, tends to pull apart surfactant molecules and tend to reduce their adsorption density, even at relatively high concentration in the bulk. As a consequence, the surfactant stabilizing effects are weakened. For instance, the addition of a few percents of sec-butanol, an alcohol which has some intermediate hydrophilic lipophilic tendency hence no significant formulation shift, produces a considerable decrease in stability of any kind of emulsion. It is often used as an additive to speed up stability experimentation, to test some formulation effects at a much shorter timescale, i.e., a few days instead of a few months. Repulsions The approach of neighboring drops is driven by different kinds of mechanisms like gravity settling or other body forces (electrical field) as well as Brownian motion. Then, when the drops are at some distance the issue is a matter of balance between the attractive Van der Waals forces which depends on the drop nature and size, and the repulsive ones which depend on the adsorbed surfactant at their surface. The increase in repulsive force is a direct and very efficient way to boost the emulsion stability, in particular, if it prevents the drops to be close enough for the interdrop film to break. This may be produced by an electrostatic repulsion due to the adsorption of ions at or close to the interface, e.g., the use of ionic surfactants. This result in a diffuse layer more or less extended, which produces a repulsion at a large or a short distance whenever in the absence or in the presence of electrolyte. This has an important effect when the continuous phase is water, i.e., for the stabilization of O/W emulsions. A stronger surface charge and a lower electrolyte concentration tend to increase stability. Things are not straightforward anyway, because in presence of an ionic surfactant the adsorbed molecules repeal each other and the interface density is not that good. A small concentration of electrolyte produces a screen between the head groups and the adsorbed density tends to increase, hence resulting in a higher charge density at interface. But, if a high electrolyte concentration is added, then the electrical double layer becomes thinner and the interdrop repulsion decreases. [103] This means that there is an optimum electrolyte content to boost the ionic emulsion stability; in general, it is quite low, but non-zero. It is worth noting too that the surface charge may be attained with nonionic adsorbed surfactants, typically a negative 60 mv zeta potential because of the preferential adsorption of OH - ions in the polyethylenoxide chains. The effect of the surface potential is clearly illustrated in the so-called DLVO theory, which has been developed for colloid particles, but is qualitatively valid for macroemulsions, [22] whose stability also depends on the resulting balance of attractive and repulsive forces. In some cases, there is a so-called secondary minimum in the potential and the drops may flocculate, i.e., stick together with a weak attraction that does not promote coalescence. This flocculation results in transient emulsion properties like thyxotropy and may be reversed by some shearing, thus it is not considered as an instability. Steric repulsion forces become quite effective when there is a physical contact between the adsorbed layers at the interface of approaching drops. Thick layers are particularly efficient if they prevent the interfaces to come close enough for the attractive forces to be significant. [104] Such thick layers may be attained with large size surfactants, e.g., extended surfactants or mixture of hydrophilic and lipophilic kinds, or gemini or polymeric types, or flocculated species as asphaltenes that stabilize W/O emulsions. It is worth remarking that the concept of thickness of the adsorbed layer is relative, and that the same 40 Å layer may be thick or thin depending on the size of the drop. For O/W emulsions, highly ethoxylated nonionics would Manila Typesetting Company 11/06/ :10AM

11 10 produce hydrophilic chains that overlap one another and result in a gel like thick layer on the water side of the interface. With W/O emulsions, it is the surfactant hydrophobic tail that should exhibit a strong steric repulsion, i.e., it should be bulky, branched, or exhibit multiple chains. Mixing effects generally improve stability because of entropic considerations, not only bulky hindrance. Nature tends to make use of polymeric surfactants that lay flat on the interface, rather than perpendicular to it. Such species often overlap and pile up on top of each other, thus producing a very high interfacial viscosity effect that is quite similar to an encapsulation with a plastic wrap. This often considerably increases the stability against coalescence, although it does not prevent flocculation to take place. Film drainage Before two droplets coalesce, they must first approach each other, hence the interdrop film has to drain. Anything that reduces this drainage rate would slow down the coalescence rate. The increase in the external phase viscosity does that and tends to improve the emulsion stability. However, when a liquid-liquid system is stirred, the general tendency, independently of the formulation and composition effects, is that the internal phase is the more viscous one. [105] This has some consequence on the formulator clever strategy. If the external phase is going to be made more viscous to increase the stability of the emulsion, the more effective way to do it is to first produce the emulsion with the original phases, and to add later the polymer to the external phase. The film drainage rate can also be reduced by some dynamic phenomena like the streaming potential and the interfacial rheology, which are quite complex to handle in practice. Geometric factors One of the principal stabilization mechanism has to do with the repulsion forces which are due to the presence of adsorbed surfactant at interface, and as a consequence depend on the drop surface area, i.e., they are proportional to the square of the drop size. On the other hand the drop drop attractive forces vary with the volume of the drop, hence as the third power of its size. This means that when the drop size is decreased by 10 times, the repulsive forces decrease 100 times and the attractive forces 1000 times. This size reduction tends to favor the repulsive forces over the attractive forces, and accordingly it is likely to reduce the coalescence rate. This is not the only reason why a smaller drop size tends to increase the emulsion kinetic stability, since it also results in a slower Stokes law settling velocity and increases the entropic disorder. The internal phase volume proportion is also an important geometric parameter. The hexagonal compact packing is attained at 74% of occupation of the space. It means that monodispersed drops are often in contact at about 65 70% of internal phase or less, hence they coalesce instantly unless there is a barrier. In most cases the coalescence of unstable abnormal emulsions starts when the internal phase ratio approaches 30 40%. On the contrary, in normal emulsions exhibiting a high stability of the interdrop film, the drops may be deformed in a foam-like morphology containing, in some cases, more than 90% of internal phase such as in mayonnaise. As far as the emulsification process is concerned the WOR has an importance, and the general trends are complex (see Fig. 4c map). [69] Temperature complex influence As seen previously, the temperature is a primordial variable in formulation. Its increase turns ionic surfactants more hydrophilic and nonionics more lipophilic. But temperature is also influencing second-order effects, because it alters many physical properties. An increase in temperature generally decreases the interfacial tension and the phase viscosity, which both result in an easier emulsification, hence often a smaller drop size and related properties. An increase in temperature could, when approaching the cloud point from below (HLD<0) make the polyethoxylated surfactant less hydrophilic, and drive it to interfaces hence increasing its adsorption and even precipitating it as a coacervate. The same phenomena can take place on the other side (HLD>0) when the temperature decreases. This is what happens in water-in-crude oil emulsions stabilized by asphaltenes that aggregate close to interface when temperature is lowered during production. On the other hand a temperature rise tends to decrease the emulsion stability, because it alters some phenomena and mechanisms. First, it increases the molecular motion and thus decreases the adsorption at interface, independently of the fact that the HLD is also linked to adsorption. Since the temperature also decreases the viscosity of the interdrop film, the drainage turns out to be quicker. Aside, an increase in temperature that moves the formulation far away from HLD=0 would result in a surfactant which is more soluble in a bulk phase (i.e., more unbalanced) hence it would decrease its adsorption, and thus its stabilization performance. This alteration could happens at HLD>2 units (or HLD<-2) or 6 or 8 units, depending on the case. Temperature variations produce opposite formulation effects on ionic and nonionic surfactants; hence, they may be opposed and cancel out. Indeed, the combination of ionic and nonionic surfactants has been found to offset the temperature dependence and to allow the formulation of systems insensitive to temperature. [106,107] Process variables A stirring energy increase is highly efficient to reduce drop size in poor yield conditions (low surfactant concentration, Manila Typesetting Company 11/06/ :10AM