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W emulsions
Advances in Colloid and Interface Science 123 – 126 (2006) 295 – 302 www.elsevier.com/locate/cis
On the formation and stability of high internal phase O/W emulsions Jerzy Kizling, Bengt Kronberg ⁎, Jan Christer Eriksson Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Available online 18 July 2006
Abstract High internal phase o/w emulsions have been investigated with respect to stability. A series of aliphatic hydrocarbons were used as the oil component. By matching the refractive index of both phases, transparent, concentrated emulsions were produced and these emulsions were found to have the highest long-term stability. The long-term stability of transparent emulsions is attributed to a minimum in free energy at the equilibrium thickness, which, in turn, is related to a reduced attraction over the thin aqueous lamellae. Another factor that contributes to the stability is the absence of the destabilizing mechanisms commonly encountered for ordinary emulsions and foams. © 2006 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Repulsive forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Attractive forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Preparation and assessment of samples . . . . . . . . . . . . . . . 4. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Optical properties of the hydrophilic phase . . . . . . . . . . . . . 4.2. Influence of composition on the optical transmittance of emulsions 4.3. Stability of concentrated o/w emulsions . . . . . . . . . . . . . . . 4.4. Formation and thickness of the aqueous film . . . . . . . . . . . . 4.5. Properties of alkanes and the thickness of the aqueous film . . . . 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A particular kind of emulsions is the so-called highly concentrated emulsions, where the internal phase volume fraction is in the range 0.8 to 0.99. In these emulsions the tightly packed liquid droplets are deformed forming a structure similar to that of foams justifying an alternative name viz. biliquid foams or
⁎ Corresponding author. E-mail address: [email protected] (B. Kronberg). 0001-8686/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2006.05.006
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aphrons [1,2]. These emulsions are also known under the names like high internal phase ratio emulsions (HIP) [3–6] or gel emulsions [7–9]. Like ordinary emulsions they can be either of w/o or o/w type [1,2,10]. As mentioned above, these emulsions have a structure resembling that of foams. Thus, for an o/w emulsion the oil droplets are separated by a planar thin film made up of an aqueous phase. NMR diffusion measurements confirm an aqueous phase continuity and a hydrocarbon component that is distributed in a discontinuous fashion [11,12]. One interesting feature of the highly concentrated emulsions is that they form at very low
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surfactant concentrations, typically less than 1%. Despite this low surfactant concentration, the emulsions resemble stiff gels. The stiffness is attributed to its internal structure being similar to that of foams with large air/liquid ratio. Recently the highly concentrated emulsions have become the subject of a comprehensive research. Several detailed studies of concentrated emulsions have been reported [13–15]. The relationships between factors like composition, internal structure, rheology and appearance on the one hand and stability on the other hand have been considered [16–20]. Fig. 1 shows the region in the Gibbs three-component phase triangle, where the highly concentrated emulsions occur. The lines in the diagram have previously been discussed as follows [20]. Let us start with a stable o/w concentrated emulsion, i.e. an emulsion inside the shaded area in Fig. 1. When adding oil, the line AA in the figure is approached. This is the line where the water lamellae become so thin that they are no longer able to separate the oil droplets. On the other hand, if more aqueous phase (surfactant+ water at a constant ratio) is added, then we reach a system that is no longer a concentrated emulsion. Either the oil droplets are separated by the aqueous phase to such an extent that the emulsion becomes fluid, or there will be a separation of the concentrated emulsion in equilibrium with the excess aqueous phase. Hence, there must be a line (be it fuzzy) BB that separates the concentrated o/w emulsions from ordinary emulsions. If we instead remove water from the aqueous phase from the concentrated emulsion, we will eventually hit the line CC in Fig. 1. This is the limit where the lamellae are too thin to separate the oil domains. Hence, there is simply not sufficient material to form lamellae in the concentrated o/w emulsion. Finally, if we remove the surfactant from the concentrated o/w emulsion we will hit line DD where there is no sufficient surfactant molecules to stabilize the lamellas. We, therefore, conclude that there are natural boundaries for the formation of high internal phase emulsions. There are two requirements for the stability of these emulsions. The first requirement is that there should be no propensity for the
Fig. 1. Natural limits of high internal phase o/w emulsions, see text.
continuous phase to form a discontinuous phase. Hence, for an o/w emulsion there should be no reason for forming water droplets in the oil phase. This is normally achieved by choosing the surfactants such that the system is far below the phase inversion temperature, PIT, of the system. Conversely, for concentrated w/o emulsions the system should be far above the PIT. The second requirement for the formation of stable concentrated emulsions is that there should be a repulsive force in the thin film separating the emulsion droplets. This repulsion is normally described as the disjoining pressure. The causes for this repulsive force might stem from an electrostatic repulsion from the charges of the surfactants and/or a steric repulsion from the head-groups (or tails) of non-ionic surfactants. Other causes might be undulation and peristaltic movement of the film. In general, one might say that is can be described as an Osmotic equilibrium of the fluid in the film and the fluid in the Plateau borders. This repulsive force in the film is counteracted by an attractive force acting across the film which stems from van der Waals forces. The purpose of this paper is to further investigate high internal phase emulsions with respect to the conditions for stability. In our previous publications [20 21] we have shown that by regulating the refractive index of the discontinuous phase one can minimize the attraction between the water droplets in w/o high internal phase emulsions, creating extremely stable systems. In this paper, we report that the same holds true for high internal phase o/w emulsions, if the continuous phase is regulated with respect to the refractive index. 2. Theoretical background 2.1. Repulsive forces The stability of the high internal phase emulsions depends indeed on the stability of the aqueous film of the continuous phase. The equilibrium thickness of the film is a result of the balance between attractive and repulsive forces acting across and within the film. Stabilizing mechanisms for liquid films have been discussed by Israelachvili and Wennerström and are proposed to be composed of three parts: undulation, peristaltic and protrusion forces [22]. All three mechanisms require a flexible and liquid-like film. Since the films in our systems are not likely to be flexible, especially for the higher alkanes, we prefer to consider the film stability in terms of hydration of the polar groups of the surfactants. It should be noted, though, that this paper does not deal with investigation of the repulsive forces and the conclusions are independent of the choice of explanation model for the repulsive forces. Each surfactant head group binds water. This hydration causes a lowering of the activity of the water in the films compared to that in the Plateau borders. As a result of this there is an osmotic force creating a driving force for the water to be transported from the Plateau borders to the films. One point of view is that the films spontaneously swell water until the chemical potential of water is the same in the films as in the Plateau borders.
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2.2. Attractive forces The free energy per unit area, W, between two identical planar surfaces, due to van der Waals attraction, is given by [23] X¼−
A 12kg2
ð1Þ
where A is the Hamaker constant and h is the separation distance. For two identical planar surfaces interacting across a thin film the Hamaker constant is [23]: ew − eo 2 ðn2w − n2o Þ2 A¼a þb ð2Þ ew þ eo ðn2w þ n2o Þ3=2 where ε is the dielectric constant, n is the refractive index and a and b are constants. The sub-indices refer to the water film and the oil phase. Eq. (2) reveals that the Hamaker constant consists of two terms. The first term depends on the dielectric properties and the second is expressed in terms of the optical properties, the refractive index, of the interacting media. Hence, the attraction over the thin film may be manipulated by modifying the refractive index of the film or of the oil phase. When the refractive indices of the interacting media are matched, i.e. when no = nw, the second term in Eq. (2) vanishes, reducing the Hamaker constant to a minimum. This second term contributes with as much as 38% for the interaction between dodecane across a water film according to Israelachvili [23]. Hence, the stability is expected to increase considerably when this term is vanished through matching of the refractive index of the two phases. Fig. 2 shows the results from stability studies of highly concentrated o/w emulsions plotted versus the difference in refractive index of the oil and aqueous phase [21]. The figure clearly reveals that the stability has a pronounced maximum when the refractive indices are matched. The stability of these emulsions is increased from hours to many years by matching the refractive indices of the phases. Besides, by matching the refractive indices the emulsions
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naturally become transparent. In our case where o/w emulsions are formed, the surfactant is soluble in the aqueous phase. Hence, by varying the surfactant concentration the refractive index can be regulated. This implies that the surfactant/water ratio must be constant in order to keep the refractive index the same as the oil. Therefore transparent emulsions can only occur along a straight line reaching from the oil corner to a fixed surfactant water ratio on the surfactant–water line as shown later in Figs. 6 and 7. It is interesting to note that the appearance of a transparent emulsion resembles an ordinary homogeneous phase at the macroscopic level. Nevertheless, in spite of the apparent homogeneity this kind of system is a two-phase system. It should be emphasized that when the composition of the continuous phase is correctly selected already at the very moment of its formation the transparent emulsion attains a quasi-equilibrium state. In other words, the foam-like network of the continuous phase does not undergo any structural changes after its formation. It is interesting to note that a slightly opaque concentrated emulsion, containing an excess of water, can be turned into a transparent emulsion by centrifugation. In this process, the excess water is removed from the emulsion network as has previously been described [24]. From the published results, it is concluded that opaque concentrated emulsions exhibit lower stability compared to transparent emulsions. The present paper singles out possible factors that make highly transparent emulsions extremely long-term stable (years). We present a study on the formation and stability of concentrated o/w transparent emulsions with a homologous series of aliphatic hydrocarbons as the oil phase ranges from hexane to hexadecane. The aqueous phase consists of NH4NO3 and surfactant or, in some cases, only surfactant and water. In addition it is found that transparent emulsions can be formed by replacing the salt with glycerol or ethylene glycol as both these compounds have fairly high refractive indices and are soluble in water but not in the alkanes. The surfactant used in this study is C12E10, decyl ethylene glycol mono n-dodecyl ether, which has a HLB value of 14.7 and a critical micelle concentration of 1.25 × 10− 2 wt.%. Hence, the surfactant is clearly water-soluble and consequently o/w emulsions will form according to Bancroft's rule. 3. Experimental 3.1. Materials
Fig. 2. Stability (in days) of w/o high internal phase emulsions showing maximum stability when the refractive index difference Δn, between the oil and aqueous phase is zero. (Reproduced with permission from Ref. [21]).
The surfactant C12E10, decyl ethylene glycol mono n-dodecyl ether, was purchased from Fluka. The water additives were ammonium nitrate (pro analysi) obtained from Merck, glycerol (99.5%) and ethylene glycol obtained from BDH. The alkanes were n-hexane (extra pure), heptane (pro analysi), octane (pro analysi), decane (for synthesis), dodecane (for synthesis) and nhexadecane (for synthesis) all obtained from Merck. All the chemicals were used as delivered. The solutions were prepared in double-distilled deionized water produced by a Milli-Q system from Millipore.
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3.2. Preparation and assessment of samples The samples were prepared in test tubes (volume of 14 ml and length of 10 cm) tightly sealed with screw caps. To about 0.1 g of the hydrophilic phase, i.e. a mixture of the surfactant and salt solution or glycerol solution, approximately 0.5 g of the hydrocarbon was added. The content of the test tube was vigorously stirred using a heavy-duty vortex shaker until it became homogeneous. Occasionally the sample had to be heated to promote thorough homogenization. The rest of the hydrocarbon was gradually added; after each addition, the emulsion was thoroughly mixed. The samples were thereafter centrifuged in a table centrifuge (about 1750 g) in order to remove air bubbles. The refractive index was determined using a Pulfrich refractometer. The transparency (transmittance) of the investigated emulsions was measured using a Perkin–Elmer UV/VIS Spectrometer (model Lambda 18). The measurements were made at the wavelength of 450 nm. All the experimental work was carried out at room temperature. 4. Results and discussion 4.1. Optical properties of the hydrophilic phase Like ordinary emulsions, the highly concentrated and transparent emulsions are formed upon mixing two mutually insoluble phases i.e. a hydrophilic and a hydrophobic phase. In this study, o/w emulsions were investigated which according to Bancroft's rule calls for water-soluble surfactants. For this reason, the hydrophilic phases were aqueous solutions of the highly water-soluble non-ionic surfactant C12E10 together with the appropriate inorganic salt, viz., NH4NO3. The organic phases were aliphatic hydrocarbons. This choice of surfactant and organic phase guarantees that the system is far below its phase inversion temperature, PIT, resulting in o/w emulsions. As already mentioned, an essential prerequisite for the formation of transparent emulsions is the matching of the refractive indices of the two phases. As the organic phases were pure hydrocarbons with rather high refractive indices, matching the re-
Fig. 3. Refractive indices of the aqueous phase, based on 5 M NH4NO3 solution, as a function of the surfactant concentration (wt.%). As indicated the refractive indices of the studied hydrocarbons fall within the concentration limits of the studied solutions.
Fig. 4. Dependence of light transmittance of concentrated emulsions as a function of the ratio of the refractive index of the heptane and water phase. The figure shows that a slight deviation in optical matching strongly affects the transparency. The system consists of heptane, C12E10 and 5 M NH4NO3 aqueous solution.
fractive indices was achieved by modifying the optical properties of the aqueous phase. Fig. 3 shows how the optical properties of 5 M NH4NO3 solutions with refractive index of 1.3854 can be modified by adding C12E10 which has a refractive index of 1.4582. As can be seen from the diagram, owing to the rather high refractive index of the surfactant, the refractive index of the aqueous salt solution can be varied between that of heptane and hexadecane. Thus, for each hydrocarbon a corresponding hydrophilic phase with the same optical properties can be found. Nevertheless, as seen from the figure, a relatively very large amount of surfactant has to be added to the NH4NO3 solution to reach the refractive index level of hexadecane. 4.2. Influence of composition on the optical transmittance of emulsions Next, we discuss the sensitivity of the transmittance on small variations in the composition of the aqueous phase. The crucial importance of the matching of the hydrophilic and hydrophobic phases for the high optical transmittance of the studied emulsions is most strikingly evident from Figs. 4 and 5. A series of concentrated emulsions based on heptane and hydrophilic phases with different ratios of the 5 M NH4NO3 solution to the surfactant (C12E10) were prepared. (The amount of the surfactant was varied with a starting point for which the hydrophilic phase based on this salt solution forms a transparent emulsion.) The refractive index for each of the hydrophilic phases in the series was measured. The transmittance of the emulsions was measured and the results plotted versus the ratio of refractive index of the heptane to that of the hydrophilic phase as shown in Fig. 4. The diagram shows that even a tiny change in the refractive index of the hydrophilic phase is sufficient to cause a considerable effect on the optical properties of the studied emulsions. As seen in the figure the transmittance goes to zero if the nH/nw ratio changes as little as 1%. Obviously, the maximum transmittance coincides with the optical matching of the two phases. Fig. 4 shows that the maximum transmittance is about 97% with reference to that of pure heptane. The reason it
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Fig. 5. Light transmittance of concentrated emulsions where the aqueous phase is based only on C12E10 solution in water and the oil phase is hexane.
does not reach 100%, despite the matching of the refractive indices, is attributed to a possible difference in composition of the aqueous phase in the Plateau borders and in the aqueous films separating the oil droplets. We speculate that the composition in the Plateau borders should be slightly different from that in the lamellas in order for a long-term stability to occur. This is further elaborated below. As mentioned earlier, the high refractive index of the surfactant (C12E10) facilitates the formation of transparent emulsions using only a solution of the surfactant in water as the hydrophilic phase, i.e. salts are not needed. The influence of the concentration of the surfactant in salt-free water on the optical properties of the concentrated emulsions was tested. A series of emulsions with the surfactant content in the water ranging from 27 to 35 wt.% were prepared. Here the emulsions were based on hexane. The results are presented in Fig. 5 and show that the transmittance increased steeply with the amount of the surfactant in the hydrophilic phase and reached a maximum when its content in the system satisfied the condition for the optical matching of both phases, which was found to be at 33.0 ± 0.1 wt.% of the surfactant in water. For this system, the maximum transmittance was as high as 99% with reference to that of pure hexane.
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should be transparent.) Consequently, upon addition of the hydrocarbon the transparent emulsion composition varies along the straight line as shown in Fig. 6. Point a, on the diagram denotes onset of emulsion formation and point b, the limit with respect to the incorporation of the hydrocarbon into the cells of the threedimensional foam structure formed by the hydrophilic phase. The amount of the hydrocarbon in the studied transparent emulsions varies between about 82 and 97.5 wt.%. Beyond the upper limit, it is impossible to form a homogeneous emulsion since there always will be an excess of alkane. Fig. 7 shows a diagram where the water phase is 5 M NH4NO3, which illustrates the effect of replacing 2 M NH4NO3 solution by a 5 M one. The refractive index is now n = 1.3854, i.e. higher than in the previous case and as a result the amount of the surfactant necessary to achieve optical matching is smaller in this case. For octane, for instance, the surfactant quantity is about 3 times lower, while for decane only about 2 times but for dodecane almost unchanged. The quantity of the hydrocarbon that could be incorporated into the network is almost the same as in the previous case. We note in passing that not only the non-ionic surfactant is able to form stable, optically isotropic emulsions; The experimenal results showed that also anionic and cationic surfactants such as, sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB) form such phases and the stability of these concentrated emulsions was excellent as well. These latter results will be published in a forthcoming paper. As remarked before, both glycerol and ethylene glycol can be used instead of salt in order to modify the optical properties of the hydrophilic phase. In other words, it is immaterial if the modifier is an electrolyte or a non-electrolyte but it is essential that it must not exhibit any solubility in the oil phase. Finally, it is worth noticing that depending on the hydrocarbon and salt contents, the amount of surfactant, contained in the hydrophilic phase, varied between ca. 4 and 50 wt.%, while the total surfactant concentration in the emulsion normally is less than 1%. It is our experience that the appearance of the studied concentrated emulsions is very sensitive to deviation of the
4.3. Stability of concentrated o/w emulsions Fig. 6 shows the hydrocarbon-rich corner of the triangle diagram for a system consisting of C12E10 and 2 M NH4NO3 (n = 1.3491) solution and the studied aliphatic hydrocarbons at room temperature. We note with interest that no homogeneous phase, typically encountered in ternary systems based on nonionic surfactants and hydrocarbons, can be found in this region of the phase diagram. The reason for this lies in the insolubility of the surfactant in aliphatic hydrocarbons. As indicated in Fig. 6 for each hydrocarbon used, there exists a unique mixture of the salt solution and the surfactant, which together with the hydrocarbon produces an optically isotropic system, viz., a transparent mixture. Once the formation of the transparent emulsion is initiated, more hydrocarbons can be added. (Note that no other parameters except the amount of the hydrocarbon can be changed, if the emulsions
Fig. 6. Hydrocarbon content in the transparent emulsions when the aqueous phase is based on 2 M NH4NO3 solution. Each hydrocarbon requires a fixed ratio of the surfactant to salt solution in order to match the refractive index. Hence, the emulsion composition varies along straight lines. The capacity limits to incorporate the hydrocarbons into the network of the continuous phase are denoted by a and b, and these are only slightly dependent on the type of hydrocarbon. The numbers refer to the ratio of salt solution to surfactant.
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processes. Hence, it is reasonable to assume that the formation of new oil/water interfaces in the form of aqueous films will occur as long as there are micelles in the system. Thus, we can assume that the surfactant at the oil/water interface is in equilibrium with the aqueous solution at the cmc of the surfactant. We note that higher hydrocarbons require more surfactant, or alternatively, more added salt in order to create stable concentrated emulsions. This can be understood by considering the thickness of the aqueous films. The thickness of the aqueous film can be estimated as follows. The material balance of the surfactant can be expressed as Fig. 7. Hydrocarbon content in the transparent emulsions when the aqueous phase is based on 5 M NH4NO3 solution. The capacity to introduce the hydrocarbons is the same as in the Fig. 6. The numbers refer to the ratio of salt solution to surfactant.
composition of the hydrophilic phase from the one that produces a transparent emulsion with a given hydrocarbon, as shown in Figs. 4 and 5. Hence, on both sides of the maximum transmittance, the concentrated emulsions are turbid. In case when the turbidity is caused by a deficiency of the surfactant (excess of water) the turbidity could be removed by centrifugation of the emulsion [24]. Thus, a slightly turbid emulsion after centrifugation in a gravitational field of 1750 g became transparent, presumably due to the removal of excess water from the emulsion, in which the forces across the aqueous lamellae then reach a minimum. The constituent in excess was collected at the bottom of the centrifuge tube leaving a stable transparent emulsion. Certainly, the equilibrium composition and thickness of the film was in these circumstances constrained by the centrifugal force applied to the emulsion. Thus, not surprisingly, when separated phases were left in contact the turbidity started spreading throughout the body of the once transparent emulsion. As already mentioned in the Introduction, one can safely assume that polar groups of the surfactants are hydrated (otherwise they would not be hydrophilic, as in our case). This hydration causes significant lowering of the activity of the water in the film compared to that in the Plateau border. Hence, there will be an osmotic force creating a driving force for the water from the Plateau borders to the films. As mentioned, our point of view is that the film spontaneously swells water until the chemical potential of water is the same in the film as in the Plateau borders. The exchange of surfactant with NH4NO3 provides the possibility to vary the thickness of the aqueous film. Surprisingly an increase in the salt content increases the thickness without disturbing the stability of the film. There is, however, a minimum amount of surfactant required in order to form a film. We note with interest that the stability of the transparent emulsions does not change when the surfactant is exchanged for salt, keeping the refractive index constant. 4.4. Formation and thickness of the aqueous film It is well known that surfactant adsorption at a water/oil interface is akin to micellization of the surfactant and that there is a very small, if any, difference in the free energy of the two
Ntot ¼ Ns þ Nb ;
ð3Þ
where Ntot is the total number of surfactant molecules in the system, Ns is the number of surfactant molecules at the surface and Nb is the number of surfactant molecules remaining in the solution at equilibrium. At the water/oil interface each surfactant molecule occupies an area of a2 (expressed in Å2/molecule, or cm2/molecule) [20] and hence a2 Ns ¼ A;
ð4Þ
where A is the total surface area of the films, each with a thickness of h. The total film volume is Vtot ¼
Ah 2
ð5Þ
The factor 2 appears since there are two faces of the films, each covered with surfactant. Combining the equations above gives, Ns ¼
2Vtot a2 h
ð6Þ
Since Ntot = VtotCtot and Nb = Vtot cmc, we have Ctot ¼
2 þ cmc a2 h
ð7Þ
And hence h¼
2 1 a2 Ctot − cmc
ð8Þ
Since the total surfactant concentration is much larger than the cmc we have hc
2 1 a2 Ctot
ð9Þ
We are, in fact, by using Eq. (9) assuming that all the surfactant present in the system is adsorbed at the o/w interface. The above relation shows that the calculated average film thickness is directly proportional to the inverse of the total concentration of surfactant in the system. The latter quantity is, on the other hand, dependent on the type of oil (alkane) used (i.e. the refractive index of the oil). Below we relate the film thickness to one property of the alkanes, which we choose to be the cohesive energy density of the alkanes, i.e. a property that reflects the energy required to
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create a film separating two droplets of the alkane. The cohesive energy density was obtained from Ref. [25]. It is directly proportional to the surface tension and hence the work of cohesion, which is twice the surface tension. 4.5. Properties of alkanes and the thickness of the aqueous film Figs. 8 and 9 show the relation between the cohesive energy density of the alkanes and the calculated film thickness of the aqueous film. On closer inspection of these figures it is evident that the thickness of the aqueous films decreases rapidly with the length of the alkane chain length. It is also evident from the figures that replacing surfactant by NH4NO3 inevitably results in thicker film as mentioned earlier. The effect of salt is especially pronounced for lower alkanes. Nevertheless, the thickness of the films of 2 M NH4NO3 solution converges with those of 5 M solution as the alkane chain length increases, as can be seen in the figures. They converge to a thickness of 3.4 nm, which is a limiting thickness for the aqueous film formed in the transparent emulsion containing water, C12E10 and decane [23]. It appears that in this transparent emulsion the content of water is just enough to completely hydrate the polar groups of the surfactant assuming two water molecules for each ethylene oxide unit [26]. The reason for convergence is the appreciable increase in the refractive index with the length of the alkane chain. Thus for 5 M NH4NO3 solution (n = 1.3854) only 4.22 wt.% of the surfactant in the aqueous phase is sufficient to obtain a matching of the refractive index with that of heptane (n = 1.3877). From the above it follows that for the higher alkanes the surfactant will be the dominating factor in the matching of the refractive index of the hydrophilic phase. As an example, hexadecane requires as much as 51 wt.% of the surfactant in the aqueous phase and hence the salt only contributes to a minor degree. Hence, the contribution of NH4NO3 to the optical properties becomes insignificant for the higher chain length alkanes. Nevertheless, these results indicate that NH4NO3 in transparent emulsions does not act as an elec-
Fig. 8. Thicknesses of the lamellas of the continuous phase versus the cohesive energy (expressed per unit area) needed to create a film in the hydrocarbons. The transparent emulsions are based on 2 M NH4NO3 water solution.
Fig. 9. Thicknesses of the lamellas of the continuous phase versus the cohesive energy (expressed per unit area) needed to create a film in the hydrocarbons. The transparent emulsions are based on 5 M NH4NO3 water solution.
trolyte but only contributes to modifying the optical properties of the hydrophilic phase. In fact, this observation implies that the thickness of the aqueous film depends on the amount of the surfactant even though the conditions for optical matching are satisfied by adding the salt. Since both the refractive index and cohesive energy density are only properties of oil, the curves predicted in Figs. 8 and 9 are self-consistent in the sense that the input values are only taken from bulk values. This is so because in the choice of system the aqueous part is designed to match the oil used in the emulsions. 5. Summary We have made transparent o/w emulsions with long-term stability, which is achieved when the refractive index of the oil and water phases is matched. It was found that the refractive index of the hydrophilic phase could be made to match that of the hydrophobic phase by the addition of surfactant or surfactant and salt. Thus, higher hydrocarbons, with larger refractive index require a larger amount of surfactant. The results show that the aqueous film thickness depends on the cohesive energy density of the alkanes but it can be varied if the surfactant is replaced with salt keeping the refractive index unchanged. It was also found that not only salt, but also some organic substances that are watersoluble and have high refractive indices, can replace part of the surfactant. Finally, it is shown that assuming that all surfactant molecules adsorb at the oil/water interface, the thickness of the aqueous film, separating the oil domains, can be estimated. The higher the alkane the thinner the film. On the other hand, exchanging the surfactant with salt increases the film thickness. Finally, an additional factor that promotes the stability is the closed rigid network of the continuous phase meaning that the transparent emulsions are not affected by the same destabilizing mechanisms as commonly operative in the ordinary emulsions.
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References [1] Sebba F. Foams and Biliquid Foams — Aphrons. New York: John Wiley and Sons; 1987. [2] Lye GJ, Stuckey DC. Colloids Surf, A Physicochem Eng Asp 1998;131:119. [3] Lissant KJ. J Colloid Interface Sci 1966;22:462. [4] Williams JM. Langmuir 1991;7:1370. [5] Pal R. Colloid Polym Sci 1999;277:583. [6] Reynolds PA, Gilbert EP, White JW. J Phys Chem, B 2000;104:7012. [7] Kunieda H, Solans C, Shida N, Para JL. Colloids Surf 1987;24:225. [8] Kunieda H, Fukui Y, Uchiyama H, Solans C. Langmuir 1996;12:2136. [9] Ravey JC, Stébé MJ, Sauvage S. Colloids and Surfaces A, Physicochemical and Engineering Aspects 1994;91:237. [10] Aronson MP, Petko MF. J Colloid Interface Sci 1993;159:134. [11] Ti-Quang L. Private communication; 1995. Stockholm. [12] Hoffmann H. Adv Colloid Interface Sci 1990;32:123. [13] Chen HH, Ruckenstein E. J Colloid Interface Sci 1990;138:437. [14] Chen HH, Ruckenstein E. J Colloid Interface Sci 1991;145:260.
[15] Babak VG, Stébé MJ. J Dispers Sci Technol 2002;23:1. [16] Ruckenstein E, Ebert G, Platz G. J Colloid Interface Sci 1989;133:432. [17] Kunieda H, Cherian A, Pons JR, Solans C. In: Esumi K, editor. Structure — Performance Relationships in Surfactant. New York: Dekker; 1997. p. 359. [18] Princen HM. J Colloid Interface Sci 1985;105:150. [19] Princen HM. Langmuir 1986;2:519. [20] Kizling J, Kronberg B. Colloids Surf 1990;50:131. [21] Kizling J, Kronberg B. Adv Colloid Interface Sci 2001;89–90:395. [22] Israelachvili JN, Wennerström H. J Phys Chem 1992;96:520. [23] Israelachvili JN. Intermolecular and Surface Forces. 2nd ed. London: Academic Press; 1991. [24] Sonneville-Aubrun O, Bergeron V, Gulik-Krzywicki T, Jönsson B, Wennerström H, Linder P, Cabane B. Langmuir 2000;16:1566. [25] Barton AFM. Handbook of Solubility Parameters and other Cohesion Parameters 2nd ed. Boca Raton, FL: CRS Press; 1991. [26] Results to be published.
On the formation and stability of high internal phase O/W emulsions Jerzy Kizling, Bengt Kronberg ⁎, Jan Christer Eriksson Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden Available online 18 July 2006
Abstract High internal phase o/w emulsions have been investigated with respect to stability. A series of aliphatic hydrocarbons were used as the oil component. By matching the refractive index of both phases, transparent, concentrated emulsions were produced and these emulsions were found to have the highest long-term stability. The long-term stability of transparent emulsions is attributed to a minimum in free energy at the equilibrium thickness, which, in turn, is related to a reduced attraction over the thin aqueous lamellae. Another factor that contributes to the stability is the absence of the destabilizing mechanisms commonly encountered for ordinary emulsions and foams. © 2006 Elsevier B.V. All rights reserved.
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theoretical background . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Repulsive forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Attractive forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Preparation and assessment of samples . . . . . . . . . . . . . . . 4. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Optical properties of the hydrophilic phase . . . . . . . . . . . . . 4.2. Influence of composition on the optical transmittance of emulsions 4.3. Stability of concentrated o/w emulsions . . . . . . . . . . . . . . . 4.4. Formation and thickness of the aqueous film . . . . . . . . . . . . 4.5. Properties of alkanes and the thickness of the aqueous film . . . . 5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A particular kind of emulsions is the so-called highly concentrated emulsions, where the internal phase volume fraction is in the range 0.8 to 0.99. In these emulsions the tightly packed liquid droplets are deformed forming a structure similar to that of foams justifying an alternative name viz. biliquid foams or
⁎ Corresponding author. E-mail address: [email protected] (B. Kronberg). 0001-8686/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cis.2006.05.006
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aphrons [1,2]. These emulsions are also known under the names like high internal phase ratio emulsions (HIP) [3–6] or gel emulsions [7–9]. Like ordinary emulsions they can be either of w/o or o/w type [1,2,10]. As mentioned above, these emulsions have a structure resembling that of foams. Thus, for an o/w emulsion the oil droplets are separated by a planar thin film made up of an aqueous phase. NMR diffusion measurements confirm an aqueous phase continuity and a hydrocarbon component that is distributed in a discontinuous fashion [11,12]. One interesting feature of the highly concentrated emulsions is that they form at very low
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surfactant concentrations, typically less than 1%. Despite this low surfactant concentration, the emulsions resemble stiff gels. The stiffness is attributed to its internal structure being similar to that of foams with large air/liquid ratio. Recently the highly concentrated emulsions have become the subject of a comprehensive research. Several detailed studies of concentrated emulsions have been reported [13–15]. The relationships between factors like composition, internal structure, rheology and appearance on the one hand and stability on the other hand have been considered [16–20]. Fig. 1 shows the region in the Gibbs three-component phase triangle, where the highly concentrated emulsions occur. The lines in the diagram have previously been discussed as follows [20]. Let us start with a stable o/w concentrated emulsion, i.e. an emulsion inside the shaded area in Fig. 1. When adding oil, the line AA in the figure is approached. This is the line where the water lamellae become so thin that they are no longer able to separate the oil droplets. On the other hand, if more aqueous phase (surfactant+ water at a constant ratio) is added, then we reach a system that is no longer a concentrated emulsion. Either the oil droplets are separated by the aqueous phase to such an extent that the emulsion becomes fluid, or there will be a separation of the concentrated emulsion in equilibrium with the excess aqueous phase. Hence, there must be a line (be it fuzzy) BB that separates the concentrated o/w emulsions from ordinary emulsions. If we instead remove water from the aqueous phase from the concentrated emulsion, we will eventually hit the line CC in Fig. 1. This is the limit where the lamellae are too thin to separate the oil domains. Hence, there is simply not sufficient material to form lamellae in the concentrated o/w emulsion. Finally, if we remove the surfactant from the concentrated o/w emulsion we will hit line DD where there is no sufficient surfactant molecules to stabilize the lamellas. We, therefore, conclude that there are natural boundaries for the formation of high internal phase emulsions. There are two requirements for the stability of these emulsions. The first requirement is that there should be no propensity for the
Fig. 1. Natural limits of high internal phase o/w emulsions, see text.
continuous phase to form a discontinuous phase. Hence, for an o/w emulsion there should be no reason for forming water droplets in the oil phase. This is normally achieved by choosing the surfactants such that the system is far below the phase inversion temperature, PIT, of the system. Conversely, for concentrated w/o emulsions the system should be far above the PIT. The second requirement for the formation of stable concentrated emulsions is that there should be a repulsive force in the thin film separating the emulsion droplets. This repulsion is normally described as the disjoining pressure. The causes for this repulsive force might stem from an electrostatic repulsion from the charges of the surfactants and/or a steric repulsion from the head-groups (or tails) of non-ionic surfactants. Other causes might be undulation and peristaltic movement of the film. In general, one might say that is can be described as an Osmotic equilibrium of the fluid in the film and the fluid in the Plateau borders. This repulsive force in the film is counteracted by an attractive force acting across the film which stems from van der Waals forces. The purpose of this paper is to further investigate high internal phase emulsions with respect to the conditions for stability. In our previous publications [20 21] we have shown that by regulating the refractive index of the discontinuous phase one can minimize the attraction between the water droplets in w/o high internal phase emulsions, creating extremely stable systems. In this paper, we report that the same holds true for high internal phase o/w emulsions, if the continuous phase is regulated with respect to the refractive index. 2. Theoretical background 2.1. Repulsive forces The stability of the high internal phase emulsions depends indeed on the stability of the aqueous film of the continuous phase. The equilibrium thickness of the film is a result of the balance between attractive and repulsive forces acting across and within the film. Stabilizing mechanisms for liquid films have been discussed by Israelachvili and Wennerström and are proposed to be composed of three parts: undulation, peristaltic and protrusion forces [22]. All three mechanisms require a flexible and liquid-like film. Since the films in our systems are not likely to be flexible, especially for the higher alkanes, we prefer to consider the film stability in terms of hydration of the polar groups of the surfactants. It should be noted, though, that this paper does not deal with investigation of the repulsive forces and the conclusions are independent of the choice of explanation model for the repulsive forces. Each surfactant head group binds water. This hydration causes a lowering of the activity of the water in the films compared to that in the Plateau borders. As a result of this there is an osmotic force creating a driving force for the water to be transported from the Plateau borders to the films. One point of view is that the films spontaneously swell water until the chemical potential of water is the same in the films as in the Plateau borders.
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2.2. Attractive forces The free energy per unit area, W, between two identical planar surfaces, due to van der Waals attraction, is given by [23] X¼−
A 12kg2
ð1Þ
where A is the Hamaker constant and h is the separation distance. For two identical planar surfaces interacting across a thin film the Hamaker constant is [23]: ew − eo 2 ðn2w − n2o Þ2 A¼a þb ð2Þ ew þ eo ðn2w þ n2o Þ3=2 where ε is the dielectric constant, n is the refractive index and a and b are constants. The sub-indices refer to the water film and the oil phase. Eq. (2) reveals that the Hamaker constant consists of two terms. The first term depends on the dielectric properties and the second is expressed in terms of the optical properties, the refractive index, of the interacting media. Hence, the attraction over the thin film may be manipulated by modifying the refractive index of the film or of the oil phase. When the refractive indices of the interacting media are matched, i.e. when no = nw, the second term in Eq. (2) vanishes, reducing the Hamaker constant to a minimum. This second term contributes with as much as 38% for the interaction between dodecane across a water film according to Israelachvili [23]. Hence, the stability is expected to increase considerably when this term is vanished through matching of the refractive index of the two phases. Fig. 2 shows the results from stability studies of highly concentrated o/w emulsions plotted versus the difference in refractive index of the oil and aqueous phase [21]. The figure clearly reveals that the stability has a pronounced maximum when the refractive indices are matched. The stability of these emulsions is increased from hours to many years by matching the refractive indices of the phases. Besides, by matching the refractive indices the emulsions
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naturally become transparent. In our case where o/w emulsions are formed, the surfactant is soluble in the aqueous phase. Hence, by varying the surfactant concentration the refractive index can be regulated. This implies that the surfactant/water ratio must be constant in order to keep the refractive index the same as the oil. Therefore transparent emulsions can only occur along a straight line reaching from the oil corner to a fixed surfactant water ratio on the surfactant–water line as shown later in Figs. 6 and 7. It is interesting to note that the appearance of a transparent emulsion resembles an ordinary homogeneous phase at the macroscopic level. Nevertheless, in spite of the apparent homogeneity this kind of system is a two-phase system. It should be emphasized that when the composition of the continuous phase is correctly selected already at the very moment of its formation the transparent emulsion attains a quasi-equilibrium state. In other words, the foam-like network of the continuous phase does not undergo any structural changes after its formation. It is interesting to note that a slightly opaque concentrated emulsion, containing an excess of water, can be turned into a transparent emulsion by centrifugation. In this process, the excess water is removed from the emulsion network as has previously been described [24]. From the published results, it is concluded that opaque concentrated emulsions exhibit lower stability compared to transparent emulsions. The present paper singles out possible factors that make highly transparent emulsions extremely long-term stable (years). We present a study on the formation and stability of concentrated o/w transparent emulsions with a homologous series of aliphatic hydrocarbons as the oil phase ranges from hexane to hexadecane. The aqueous phase consists of NH4NO3 and surfactant or, in some cases, only surfactant and water. In addition it is found that transparent emulsions can be formed by replacing the salt with glycerol or ethylene glycol as both these compounds have fairly high refractive indices and are soluble in water but not in the alkanes. The surfactant used in this study is C12E10, decyl ethylene glycol mono n-dodecyl ether, which has a HLB value of 14.7 and a critical micelle concentration of 1.25 × 10− 2 wt.%. Hence, the surfactant is clearly water-soluble and consequently o/w emulsions will form according to Bancroft's rule. 3. Experimental 3.1. Materials
Fig. 2. Stability (in days) of w/o high internal phase emulsions showing maximum stability when the refractive index difference Δn, between the oil and aqueous phase is zero. (Reproduced with permission from Ref. [21]).
The surfactant C12E10, decyl ethylene glycol mono n-dodecyl ether, was purchased from Fluka. The water additives were ammonium nitrate (pro analysi) obtained from Merck, glycerol (99.5%) and ethylene glycol obtained from BDH. The alkanes were n-hexane (extra pure), heptane (pro analysi), octane (pro analysi), decane (for synthesis), dodecane (for synthesis) and nhexadecane (for synthesis) all obtained from Merck. All the chemicals were used as delivered. The solutions were prepared in double-distilled deionized water produced by a Milli-Q system from Millipore.
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3.2. Preparation and assessment of samples The samples were prepared in test tubes (volume of 14 ml and length of 10 cm) tightly sealed with screw caps. To about 0.1 g of the hydrophilic phase, i.e. a mixture of the surfactant and salt solution or glycerol solution, approximately 0.5 g of the hydrocarbon was added. The content of the test tube was vigorously stirred using a heavy-duty vortex shaker until it became homogeneous. Occasionally the sample had to be heated to promote thorough homogenization. The rest of the hydrocarbon was gradually added; after each addition, the emulsion was thoroughly mixed. The samples were thereafter centrifuged in a table centrifuge (about 1750 g) in order to remove air bubbles. The refractive index was determined using a Pulfrich refractometer. The transparency (transmittance) of the investigated emulsions was measured using a Perkin–Elmer UV/VIS Spectrometer (model Lambda 18). The measurements were made at the wavelength of 450 nm. All the experimental work was carried out at room temperature. 4. Results and discussion 4.1. Optical properties of the hydrophilic phase Like ordinary emulsions, the highly concentrated and transparent emulsions are formed upon mixing two mutually insoluble phases i.e. a hydrophilic and a hydrophobic phase. In this study, o/w emulsions were investigated which according to Bancroft's rule calls for water-soluble surfactants. For this reason, the hydrophilic phases were aqueous solutions of the highly water-soluble non-ionic surfactant C12E10 together with the appropriate inorganic salt, viz., NH4NO3. The organic phases were aliphatic hydrocarbons. This choice of surfactant and organic phase guarantees that the system is far below its phase inversion temperature, PIT, resulting in o/w emulsions. As already mentioned, an essential prerequisite for the formation of transparent emulsions is the matching of the refractive indices of the two phases. As the organic phases were pure hydrocarbons with rather high refractive indices, matching the re-
Fig. 3. Refractive indices of the aqueous phase, based on 5 M NH4NO3 solution, as a function of the surfactant concentration (wt.%). As indicated the refractive indices of the studied hydrocarbons fall within the concentration limits of the studied solutions.
Fig. 4. Dependence of light transmittance of concentrated emulsions as a function of the ratio of the refractive index of the heptane and water phase. The figure shows that a slight deviation in optical matching strongly affects the transparency. The system consists of heptane, C12E10 and 5 M NH4NO3 aqueous solution.
fractive indices was achieved by modifying the optical properties of the aqueous phase. Fig. 3 shows how the optical properties of 5 M NH4NO3 solutions with refractive index of 1.3854 can be modified by adding C12E10 which has a refractive index of 1.4582. As can be seen from the diagram, owing to the rather high refractive index of the surfactant, the refractive index of the aqueous salt solution can be varied between that of heptane and hexadecane. Thus, for each hydrocarbon a corresponding hydrophilic phase with the same optical properties can be found. Nevertheless, as seen from the figure, a relatively very large amount of surfactant has to be added to the NH4NO3 solution to reach the refractive index level of hexadecane. 4.2. Influence of composition on the optical transmittance of emulsions Next, we discuss the sensitivity of the transmittance on small variations in the composition of the aqueous phase. The crucial importance of the matching of the hydrophilic and hydrophobic phases for the high optical transmittance of the studied emulsions is most strikingly evident from Figs. 4 and 5. A series of concentrated emulsions based on heptane and hydrophilic phases with different ratios of the 5 M NH4NO3 solution to the surfactant (C12E10) were prepared. (The amount of the surfactant was varied with a starting point for which the hydrophilic phase based on this salt solution forms a transparent emulsion.) The refractive index for each of the hydrophilic phases in the series was measured. The transmittance of the emulsions was measured and the results plotted versus the ratio of refractive index of the heptane to that of the hydrophilic phase as shown in Fig. 4. The diagram shows that even a tiny change in the refractive index of the hydrophilic phase is sufficient to cause a considerable effect on the optical properties of the studied emulsions. As seen in the figure the transmittance goes to zero if the nH/nw ratio changes as little as 1%. Obviously, the maximum transmittance coincides with the optical matching of the two phases. Fig. 4 shows that the maximum transmittance is about 97% with reference to that of pure heptane. The reason it
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Fig. 5. Light transmittance of concentrated emulsions where the aqueous phase is based only on C12E10 solution in water and the oil phase is hexane.
does not reach 100%, despite the matching of the refractive indices, is attributed to a possible difference in composition of the aqueous phase in the Plateau borders and in the aqueous films separating the oil droplets. We speculate that the composition in the Plateau borders should be slightly different from that in the lamellas in order for a long-term stability to occur. This is further elaborated below. As mentioned earlier, the high refractive index of the surfactant (C12E10) facilitates the formation of transparent emulsions using only a solution of the surfactant in water as the hydrophilic phase, i.e. salts are not needed. The influence of the concentration of the surfactant in salt-free water on the optical properties of the concentrated emulsions was tested. A series of emulsions with the surfactant content in the water ranging from 27 to 35 wt.% were prepared. Here the emulsions were based on hexane. The results are presented in Fig. 5 and show that the transmittance increased steeply with the amount of the surfactant in the hydrophilic phase and reached a maximum when its content in the system satisfied the condition for the optical matching of both phases, which was found to be at 33.0 ± 0.1 wt.% of the surfactant in water. For this system, the maximum transmittance was as high as 99% with reference to that of pure hexane.
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should be transparent.) Consequently, upon addition of the hydrocarbon the transparent emulsion composition varies along the straight line as shown in Fig. 6. Point a, on the diagram denotes onset of emulsion formation and point b, the limit with respect to the incorporation of the hydrocarbon into the cells of the threedimensional foam structure formed by the hydrophilic phase. The amount of the hydrocarbon in the studied transparent emulsions varies between about 82 and 97.5 wt.%. Beyond the upper limit, it is impossible to form a homogeneous emulsion since there always will be an excess of alkane. Fig. 7 shows a diagram where the water phase is 5 M NH4NO3, which illustrates the effect of replacing 2 M NH4NO3 solution by a 5 M one. The refractive index is now n = 1.3854, i.e. higher than in the previous case and as a result the amount of the surfactant necessary to achieve optical matching is smaller in this case. For octane, for instance, the surfactant quantity is about 3 times lower, while for decane only about 2 times but for dodecane almost unchanged. The quantity of the hydrocarbon that could be incorporated into the network is almost the same as in the previous case. We note in passing that not only the non-ionic surfactant is able to form stable, optically isotropic emulsions; The experimenal results showed that also anionic and cationic surfactants such as, sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTAB) form such phases and the stability of these concentrated emulsions was excellent as well. These latter results will be published in a forthcoming paper. As remarked before, both glycerol and ethylene glycol can be used instead of salt in order to modify the optical properties of the hydrophilic phase. In other words, it is immaterial if the modifier is an electrolyte or a non-electrolyte but it is essential that it must not exhibit any solubility in the oil phase. Finally, it is worth noticing that depending on the hydrocarbon and salt contents, the amount of surfactant, contained in the hydrophilic phase, varied between ca. 4 and 50 wt.%, while the total surfactant concentration in the emulsion normally is less than 1%. It is our experience that the appearance of the studied concentrated emulsions is very sensitive to deviation of the
4.3. Stability of concentrated o/w emulsions Fig. 6 shows the hydrocarbon-rich corner of the triangle diagram for a system consisting of C12E10 and 2 M NH4NO3 (n = 1.3491) solution and the studied aliphatic hydrocarbons at room temperature. We note with interest that no homogeneous phase, typically encountered in ternary systems based on nonionic surfactants and hydrocarbons, can be found in this region of the phase diagram. The reason for this lies in the insolubility of the surfactant in aliphatic hydrocarbons. As indicated in Fig. 6 for each hydrocarbon used, there exists a unique mixture of the salt solution and the surfactant, which together with the hydrocarbon produces an optically isotropic system, viz., a transparent mixture. Once the formation of the transparent emulsion is initiated, more hydrocarbons can be added. (Note that no other parameters except the amount of the hydrocarbon can be changed, if the emulsions
Fig. 6. Hydrocarbon content in the transparent emulsions when the aqueous phase is based on 2 M NH4NO3 solution. Each hydrocarbon requires a fixed ratio of the surfactant to salt solution in order to match the refractive index. Hence, the emulsion composition varies along straight lines. The capacity limits to incorporate the hydrocarbons into the network of the continuous phase are denoted by a and b, and these are only slightly dependent on the type of hydrocarbon. The numbers refer to the ratio of salt solution to surfactant.
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processes. Hence, it is reasonable to assume that the formation of new oil/water interfaces in the form of aqueous films will occur as long as there are micelles in the system. Thus, we can assume that the surfactant at the oil/water interface is in equilibrium with the aqueous solution at the cmc of the surfactant. We note that higher hydrocarbons require more surfactant, or alternatively, more added salt in order to create stable concentrated emulsions. This can be understood by considering the thickness of the aqueous films. The thickness of the aqueous film can be estimated as follows. The material balance of the surfactant can be expressed as Fig. 7. Hydrocarbon content in the transparent emulsions when the aqueous phase is based on 5 M NH4NO3 solution. The capacity to introduce the hydrocarbons is the same as in the Fig. 6. The numbers refer to the ratio of salt solution to surfactant.
composition of the hydrophilic phase from the one that produces a transparent emulsion with a given hydrocarbon, as shown in Figs. 4 and 5. Hence, on both sides of the maximum transmittance, the concentrated emulsions are turbid. In case when the turbidity is caused by a deficiency of the surfactant (excess of water) the turbidity could be removed by centrifugation of the emulsion [24]. Thus, a slightly turbid emulsion after centrifugation in a gravitational field of 1750 g became transparent, presumably due to the removal of excess water from the emulsion, in which the forces across the aqueous lamellae then reach a minimum. The constituent in excess was collected at the bottom of the centrifuge tube leaving a stable transparent emulsion. Certainly, the equilibrium composition and thickness of the film was in these circumstances constrained by the centrifugal force applied to the emulsion. Thus, not surprisingly, when separated phases were left in contact the turbidity started spreading throughout the body of the once transparent emulsion. As already mentioned in the Introduction, one can safely assume that polar groups of the surfactants are hydrated (otherwise they would not be hydrophilic, as in our case). This hydration causes significant lowering of the activity of the water in the film compared to that in the Plateau border. Hence, there will be an osmotic force creating a driving force for the water from the Plateau borders to the films. As mentioned, our point of view is that the film spontaneously swells water until the chemical potential of water is the same in the film as in the Plateau borders. The exchange of surfactant with NH4NO3 provides the possibility to vary the thickness of the aqueous film. Surprisingly an increase in the salt content increases the thickness without disturbing the stability of the film. There is, however, a minimum amount of surfactant required in order to form a film. We note with interest that the stability of the transparent emulsions does not change when the surfactant is exchanged for salt, keeping the refractive index constant. 4.4. Formation and thickness of the aqueous film It is well known that surfactant adsorption at a water/oil interface is akin to micellization of the surfactant and that there is a very small, if any, difference in the free energy of the two
Ntot ¼ Ns þ Nb ;
ð3Þ
where Ntot is the total number of surfactant molecules in the system, Ns is the number of surfactant molecules at the surface and Nb is the number of surfactant molecules remaining in the solution at equilibrium. At the water/oil interface each surfactant molecule occupies an area of a2 (expressed in Å2/molecule, or cm2/molecule) [20] and hence a2 Ns ¼ A;
ð4Þ
where A is the total surface area of the films, each with a thickness of h. The total film volume is Vtot ¼
Ah 2
ð5Þ
The factor 2 appears since there are two faces of the films, each covered with surfactant. Combining the equations above gives, Ns ¼
2Vtot a2 h
ð6Þ
Since Ntot = VtotCtot and Nb = Vtot cmc, we have Ctot ¼
2 þ cmc a2 h
ð7Þ
And hence h¼
2 1 a2 Ctot − cmc
ð8Þ
Since the total surfactant concentration is much larger than the cmc we have hc
2 1 a2 Ctot
ð9Þ
We are, in fact, by using Eq. (9) assuming that all the surfactant present in the system is adsorbed at the o/w interface. The above relation shows that the calculated average film thickness is directly proportional to the inverse of the total concentration of surfactant in the system. The latter quantity is, on the other hand, dependent on the type of oil (alkane) used (i.e. the refractive index of the oil). Below we relate the film thickness to one property of the alkanes, which we choose to be the cohesive energy density of the alkanes, i.e. a property that reflects the energy required to
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create a film separating two droplets of the alkane. The cohesive energy density was obtained from Ref. [25]. It is directly proportional to the surface tension and hence the work of cohesion, which is twice the surface tension. 4.5. Properties of alkanes and the thickness of the aqueous film Figs. 8 and 9 show the relation between the cohesive energy density of the alkanes and the calculated film thickness of the aqueous film. On closer inspection of these figures it is evident that the thickness of the aqueous films decreases rapidly with the length of the alkane chain length. It is also evident from the figures that replacing surfactant by NH4NO3 inevitably results in thicker film as mentioned earlier. The effect of salt is especially pronounced for lower alkanes. Nevertheless, the thickness of the films of 2 M NH4NO3 solution converges with those of 5 M solution as the alkane chain length increases, as can be seen in the figures. They converge to a thickness of 3.4 nm, which is a limiting thickness for the aqueous film formed in the transparent emulsion containing water, C12E10 and decane [23]. It appears that in this transparent emulsion the content of water is just enough to completely hydrate the polar groups of the surfactant assuming two water molecules for each ethylene oxide unit [26]. The reason for convergence is the appreciable increase in the refractive index with the length of the alkane chain. Thus for 5 M NH4NO3 solution (n = 1.3854) only 4.22 wt.% of the surfactant in the aqueous phase is sufficient to obtain a matching of the refractive index with that of heptane (n = 1.3877). From the above it follows that for the higher alkanes the surfactant will be the dominating factor in the matching of the refractive index of the hydrophilic phase. As an example, hexadecane requires as much as 51 wt.% of the surfactant in the aqueous phase and hence the salt only contributes to a minor degree. Hence, the contribution of NH4NO3 to the optical properties becomes insignificant for the higher chain length alkanes. Nevertheless, these results indicate that NH4NO3 in transparent emulsions does not act as an elec-
Fig. 8. Thicknesses of the lamellas of the continuous phase versus the cohesive energy (expressed per unit area) needed to create a film in the hydrocarbons. The transparent emulsions are based on 2 M NH4NO3 water solution.
Fig. 9. Thicknesses of the lamellas of the continuous phase versus the cohesive energy (expressed per unit area) needed to create a film in the hydrocarbons. The transparent emulsions are based on 5 M NH4NO3 water solution.
trolyte but only contributes to modifying the optical properties of the hydrophilic phase. In fact, this observation implies that the thickness of the aqueous film depends on the amount of the surfactant even though the conditions for optical matching are satisfied by adding the salt. Since both the refractive index and cohesive energy density are only properties of oil, the curves predicted in Figs. 8 and 9 are self-consistent in the sense that the input values are only taken from bulk values. This is so because in the choice of system the aqueous part is designed to match the oil used in the emulsions. 5. Summary We have made transparent o/w emulsions with long-term stability, which is achieved when the refractive index of the oil and water phases is matched. It was found that the refractive index of the hydrophilic phase could be made to match that of the hydrophobic phase by the addition of surfactant or surfactant and salt. Thus, higher hydrocarbons, with larger refractive index require a larger amount of surfactant. The results show that the aqueous film thickness depends on the cohesive energy density of the alkanes but it can be varied if the surfactant is replaced with salt keeping the refractive index unchanged. It was also found that not only salt, but also some organic substances that are watersoluble and have high refractive indices, can replace part of the surfactant. Finally, it is shown that assuming that all surfactant molecules adsorb at the oil/water interface, the thickness of the aqueous film, separating the oil domains, can be estimated. The higher the alkane the thinner the film. On the other hand, exchanging the surfactant with salt increases the film thickness. Finally, an additional factor that promotes the stability is the closed rigid network of the continuous phase meaning that the transparent emulsions are not affected by the same destabilizing mechanisms as commonly operative in the ordinary emulsions.
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References [1] Sebba F. Foams and Biliquid Foams — Aphrons. New York: John Wiley and Sons; 1987. [2] Lye GJ, Stuckey DC. Colloids Surf, A Physicochem Eng Asp 1998;131:119. [3] Lissant KJ. J Colloid Interface Sci 1966;22:462. [4] Williams JM. Langmuir 1991;7:1370. [5] Pal R. Colloid Polym Sci 1999;277:583. [6] Reynolds PA, Gilbert EP, White JW. J Phys Chem, B 2000;104:7012. [7] Kunieda H, Solans C, Shida N, Para JL. Colloids Surf 1987;24:225. [8] Kunieda H, Fukui Y, Uchiyama H, Solans C. Langmuir 1996;12:2136. [9] Ravey JC, Stébé MJ, Sauvage S. Colloids and Surfaces A, Physicochemical and Engineering Aspects 1994;91:237. [10] Aronson MP, Petko MF. J Colloid Interface Sci 1993;159:134. [11] Ti-Quang L. Private communication; 1995. Stockholm. [12] Hoffmann H. Adv Colloid Interface Sci 1990;32:123. [13] Chen HH, Ruckenstein E. J Colloid Interface Sci 1990;138:437. [14] Chen HH, Ruckenstein E. J Colloid Interface Sci 1991;145:260.
[15] Babak VG, Stébé MJ. J Dispers Sci Technol 2002;23:1. [16] Ruckenstein E, Ebert G, Platz G. J Colloid Interface Sci 1989;133:432. [17] Kunieda H, Cherian A, Pons JR, Solans C. In: Esumi K, editor. Structure — Performance Relationships in Surfactant. New York: Dekker; 1997. p. 359. [18] Princen HM. J Colloid Interface Sci 1985;105:150. [19] Princen HM. Langmuir 1986;2:519. [20] Kizling J, Kronberg B. Colloids Surf 1990;50:131. [21] Kizling J, Kronberg B. Adv Colloid Interface Sci 2001;89–90:395. [22] Israelachvili JN, Wennerström H. J Phys Chem 1992;96:520. [23] Israelachvili JN. Intermolecular and Surface Forces. 2nd ed. London: Academic Press; 1991. [24] Sonneville-Aubrun O, Bergeron V, Gulik-Krzywicki T, Jönsson B, Wennerström H, Linder P, Cabane B. Langmuir 2000;16:1566. [25] Barton AFM. Handbook of Solubility Parameters and other Cohesion Parameters 2nd ed. Boca Raton, FL: CRS Press; 1991. [26] Results to be published.