oil interface

oil interface

Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 78–82 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Phys...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 399 (2012) 78–82

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Spontaneous emulsification at the water/oil interface Jesús Santana-Solano a,∗ , Carla M. Quezada b , Sandra Ozuna-Chacón a , José Luis Arauz-Lara b a b

Cinvestav Unidad Monterrey, Parque de Investigación e Innovación Tecnológica (PIIT), Apodaca, Nuevo León 66629, Mexico Instituto de Física “Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosí, Alvaro Obregón 64, 78000 San Luis Potosí, S.L.P., Mexico

a r t i c l e

i n f o

Article history: Received 22 December 2011 Received in revised form 16 February 2012 Accepted 21 February 2012 Available online 3 March 2012 Keywords: Spontaneous emulsification Water droplets Droplet growth Water–oil interface

a b s t r a c t Emulsification processes usually require the application of external energy to enlarge the interfacial area between both media. We study here a case where practically no external energy is required to form an emulsion, i.e., it is produced spontaneously when both liquids are put in contact in the presence of one surfactant species. We report the observation and measurements of the kinetics of the spontaneous formation and growth of water droplets at the water/oil + lipophilic surfactant interface. The droplets can reach sizes up to several microns, and their growth is found to obey a power law for a range of the surfactant concentration. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Emulsions are systems of great scientific and technological interest, exhibiting a vast phenomenology [1–3]. Although the literature addressing different aspects of the production and stability of such systems is abundant, there are still many other aspects to be understood. Emulsions are unstable systems due to the high interfacial tension between both liquids. Nevertheless, these systems can be prevented to phase separate or slow down that process, by addition of one or more surfactant species which self-assemble at the interface providing a stabilizing barrier against coalescence. The production of emulsions in most cases requires the application of mechanical external energy to enlarge the interfacial area between both liquids. Most of that applied energy during emulsification is expended in shearing the liquids and, therefore, lost due to the viscous resistance, which is high in many cases of interest. Thus, in the commercial production of emulsions the amount and the way of supplying energy are important issues to consider. Remarkable, however, there are systems where practically no external energy is required to form emulsions, i.e., they are produced spontaneously when both liquids are put in contact in the presence of at least one surfactant species. These phenomenon, referred to as spontaneous emulsification, has been reported to occur in many specific systems, for instance at interfaces such as steel-slag at high temperatures [4], crude oil–water [5], dodecane–water [6,7], to mention a few. For further discussion on other systems where spontaneous emulsification has been observed, see reviews in the literature

∗ Corresponding author. Tel.: +52 8111561740; fax: +52 8111561741. E-mail address: [email protected] (J. Santana-Solano). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.02.032

[3,8,9]. Although there is an abundant work reported on spontaneous emulsification, the phenomenon is still not well understood, mainly since spontaneous emulsification seems to occur through different mechanisms in different systems. Among the mechanisms discussed in the literature, the most favored are those due to interfacial turbulence, negative interfacial tension, diffusion and stranding, formation and swelling of water/surfactant aggregates, phase inversion, etc. [2,3,8,9]. Spontaneous emulsification is thus an intriguing and fascinating phenomenon whose understanding is of much scientific interest. However, due to the importance of emulsion processes in many industries such as foods, cosmetics, petroleum, paints, etc, a full understanding of the different manifestations of spontaneous emulsification is important in order to incorporate them in industrial processes to reduce the energetic costs. In a previous work [6], we reported the observation of spontaneous emulsification of water in oil (w/o) at an interface water–dodecane with the aid of a lipophilic surfactant. In that system, the aqueous phase consisted of water droplets in the range 10–100 ␮m, immersed in a continuous oil phase consisting of dodecane (purity >99%, Sigma) containing sorbitan monooleate (Span 80, purity >99%, Sigma). After few minutes of sample preparation, we observed the appearance of small water droplets on the spherical interface of the initial water droplets. The small water droplets were observed to be uniform in size (initially below 1 ␮m), forming crystalline arrays covering the total surface of the mother water droplet. Most interestingly, the small water droplets grew with time (reaching sizes of few microns) enlarging in this way the water–oil interfacial area without the application of external energy. Naturally, the growth of the daughter water droplets is at the expense of the water of the mother droplet which was observed

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to decrease continuously with time. This spontaneous process was observed to occur for a range of surfactant concentrations, whose amount determined the kinetics of the process and other specific features. In order to gain a basic understanding of this spontaneous emulsification processes, one would require a more extensive study of various aspects involved such as the role of the interface curvature, the rate of emulsification, the quantity of surfactant and its nature, etc. In this work we address one of these issues by studying the kinetics of spontaneous emulsification in the systems described above [6], i.e., at the water–dodecane–Span 80 interface. However, in order to minimize the effects of the interface’s curvature on the rate of surface enlargement, we used here a custom designed sample cell which allows us to control better the interfacial’s curvature and the total initial area. Since the interface curvature of this geometry is lower, we can characterize more accurately the emulsification process. Thus, we report measurements of the size increase of the emulsified small water droplets at the interface water–dodecane at different concentrations of Span 80. As it is shown below, for the range of surfactant concentration studied, the growth rate of the small droplets follows a power law. This behavior resembles the slow growth of domains in the phase separation process in binary mixtures of liquids [1]. 2. Experimental 2.1. Materials Materials used in this work are dodecane, water and Span 80. Stock solutions of Span 80 in dodecane, at different concentrations, are prepared and storage for later use. The oil solution is put in contact with pure water using a sample cell schematically illustrated in Fig. 1(a).

Fig. 1. Schematic representation of the transversal view of the sample cell (a), here g represents the direction of gravity, and an optical microscope image of the structure at the water–oil interface with 0.5% (w/w) of Span 80, 3 h after sample preparation (b).

2.2. Methods 2.2.1. Sample preparation Samples are prepared following a two-day procedure. The cell is fabricated using a stainless steel bar, dimensions 7.5 cm × 2 cm × 0.2 cm. Two concentric, contiguous, cylindrical chambers are drilled across the bar, one of diameter 1.2 mm and height 1.85 mm, and the other of diameter 4 mm and height 0.15 mm. The sample cell is reusable, and it is cleaned by two cycles in a ultrasonic cleaner, in the first one the cell is immersed in ethanol and in the second one in a water solution of detergent (Micro, Cole Parmer), followed by repeated rinses with fresh water and drying with napkins (Kimwipes, Kimberly-Clark) and nitrogen. Before the liquids are loaded, the top surface is sealed using a circular 0.17 mm thick glass cover slip, which serves as an optical window, and epoxy resin. In order to facilitate filling the chambers, the sample cell is turned upside down. The narrower chamber is filled with pure water (resistivity 18.2 M cm, Mili-Q). Then, the oily phase is gently added filling the second chamber, creating in this way the water–oil interface. A cover slip is also used to seal the second chamber, by gently sliding it over the surface of the bar, previously polished to facilitate this process avoiding turbulence and the formation of bubbles. The cell is then turned over to have the water on top of the oily phase as shown in Fig. 1(a). The volume of water used is ≈0.5 ␮l, which is enough to fill part of the top chamber and a convex meniscus of water protruding into the oil with a height in the range 200–250 ␮m. Thus, the interface is a spherical cap of a sphere of diameter ≈2 mm. The volume of oil is 1.4 ␮l, and it fills the larger chamber and wets partially the lower part of the top chamber as shown in Fig. 1(a). The distance between the bottom glass surface and the lower point of the interface is 100–150 ␮m. All samples were prepared and studied at the temperature of 23 ◦ C.

2.2.2. Confocal microscopy The sample cell is placed on the stage of an inverted optical microscope for observation, we use a confocal microscope (Leica TCS SP5) operated in the transmitted light mode, and an oil immersion 20× objective (numerical aperture of 0.70) in all experiments. Images of the sample, with a spatial resolution of 1024 × 1024 pixels, are taken at intervals of few minutes during several hours. The images are analyzed using a custom home made algorithm implemented in Image Pro Plus. 3. Results and discussion 3.1. Emulsion formation and measurement of droplet size As mentioned above, we report the observation and quantification of the spontaneous emulsification of water in oil at the interface between the two media. Fig. 1(b) shows a typical image of the water–oil interface of the systems studied here, this particular one corresponding to a system where the surfactant concentration is 0.5% (w/w). The picture shows the bottom of the meniscus 3 h after sample preparation. As one can see, attached to the interface there is a cluster of small round objects of diameter ≈2 ␮m, forming a crystalline structure at the center of the cluster and a less ordered distribution at the edge. One can also appreciate the curvature of the interface, the center of the cluster is clear and brighter because it is in the focal plane of the microscope optics, whereas toward the edge the picture is darker since that area is out of focus. The objects observed are small water droplets, spontaneously emulsified at the interface [6], whose initial size is below the optical resolution and they grow until they reach a diameter 20–30 ␮m before they detach by gravity from the interface. This process, i.e., the formation and

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Fig. 2. Images of a small area of the water/oil interface in a system with 1.0% (w/w) of Span 80. Images were taken at time t = 2.2, 3.3, 4.0, 6.0, 10.0, and 24.8 h, after sample preparation, (a)–(f) respectively.

growth of the water droplets, is observed to occur essentially in the same manner for a range of surfactant concentrations, whose variation determines mainly the kinetics of the process. In this work, we report results for systems with different surfactant concentrations, from 0.25 to 2% (w/w). As a reference, let us mention that in the samples where the surfactant concentration was below 0.025% (w/w) no emulsification was observed after one week, which is in agreement with previous work [6] and meaning that the curvature of the interface does not affect the onset of spontaneous emulsification. In systems with low surfactant concentration, such as that in Fig. 1(b), nothing is observed, either at the interface or in the bulk of the oil phase, until approximately 1 h after sample preparation. At this time, droplets of size ≈500 nm are observed attached to the interface, protruding into the oil phase. This early stage being

reported here, as mentioned above, is not the actual initial stage of water emulsification, the initial size of the produced water droplets is likely to be well below the spatial scale of our observations. We report the appearance and growth of water droplets when their size is already within the resolution of optical microscopy. At this early time, the droplets are scattered across the interface, i.e., no cluster is observed. As time goes on, the droplets grow and slide toward the bottom of the meniscus while keeping attached to the interface. In samples where the interface is nearly flat, the droplets are observed to fall performing Brownian motion, whereas they fall more straight when the interface is more curved. As the number and size of droplets at the bottom grow, they form a compact and ordered structure at the center as shown in Fig. 1(b). At the border, the droplets are still wandering, searching for a place in

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a(c)

4

1

0.5

3

R(t) (µm)

0

0

1

c (% w/w)

2

2

1

0

5

10

t (h)

15

20

25

Fig. 3. Time evolution of the water droplets radius for surfactant concentrations of 2.0, 1.0, 0.5, and 0.25% (w/w), from top to bottom, respectively. Symbols represent experimental values and solid lines calculated curves using Eq. (1). Inset shows the fitting parameter a(c) for the different Span 80 concentrations (symbols) and the power law fitting (solid line) as explained in the text.

the cluster. We quantify the spontaneous emulsification process, at various surfactant concentrations, by measuring the increase in size of the water droplets once they are clustered in a crystalline array at the bottom of the meniscus. In this way, we measure the evolution of a statistically significant number of droplets at the same time, i.e., those which are in focus. Furthermore, for each surfactant concentration the experiment is repeated at least ten times. In order to illustrate the phenomenon studied here, Fig. 2 shows a set of images taken at different times, in a system where the concentration of Span 80 is 1.0% (w/w). Fig. 2(a)–(f) were taken at times t = 2.2, 3.3, 4.0, 6.0, 10.0, and 24.8 h after sample preparation, respectively. For clarity, we show only a small fraction of the area in the field of view with the droplets in focus. As one can see, the water droplets grow with time uniformly, preserving the ordered structure, and for this system they grow from a diameter of ≈2 ␮m to ≈10 ␮m, in a time period of about 22 h. The size distribution and the location of the droplets in the field of view, are determined by fitting a circle around the dark ring of each droplet as it is shown in Fig. 2(c). In this figure, there are black circles superimposed on some droplets. The diameter of the circles are equal to the measured mean droplets’ diameter. The droplets’ size polydispersity can also be appreciated here. Some droplets are a bit smaller and others a bit larger than average, but most of them are fitted well by the average circle. The measured size polydispersity is less than 10% in all cases reported here. 3.2. Growth of the droplets as a function of surfactant concentration Fig. 3 shows curves of the time evolution of the measured average radius R(t) of the water droplets attached to the water/oil interface. Different curves correspond to results from systems with different concentrations, 2.0, 1.0, 0.5, and 0.25% (w/w) of Span 80, from top to bottom, respectively. For each surfactant concentration, measurements of R(t) were carried out in different sample preparations, the average values are shown in Fig. 3. As one can see, the results are quite reproducible, qualitatively and quantitatively, as demonstrated by the low dispersion of the curves. This is

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remarkable, since the physical phenomenon being observed is not a single process but the result of a combination of them, among which the main role is being played by the spontaneous assembly of surfactant molecules, leading to the spontaneous formation and growth of droplets of one liquid into the other. One can also see, that the effect of varying the amount of surfactant is a change in the kinetics of the process, being faster for higher concentrations of Span 80, but the general phenomena is essentially the same. Fig. 3 shows that, for all surfactant concentrations, R(t) increases faster at early times but the rate of growth decreases as time increases. In fact, an analysis of the data shows that R(t) is well fitted to a power law curve in all cases, i.e., R(t) ∼ a(c)tb(c) , where c is the surfactant percentage concentration (w/w), with b(c) < 1. Power law fits to the averaged experimental curves of R(t), considering the data taken up to 12 h after sample preparation, lead to values of the parameter b(c) almost independent on the surfactant concentration. The values obtained are b = 0.51, 0.58, 0.58, and 0.62, for c = 0.25, 0.5, 1.0, and 2.0%, respectively. Since these values are similar, we fixed b = 0.58 and recalculated the parameter a(c). The inset in Fig. 3 shows the parameter a(c) (symbols) which fills well to the curve a(c) = (3/4)c(0.4) (solid line). Combining both results, we find that the growth of the water droplets observed in our experiments follows Eq. (1). The solid lines in Fig. 3 are the corresponding curves for R(t) calculated using Eq. (1). As one can see here, the time evolution of the water droplets’ size is in general well reproduced by this equation. For the lower surfactant concentrations the predictions of Eq. (1) are a bit off the experimental data. However, adjusting the value of the surfactant concentration only by few percent one can achieve an excellent agreement. R(t) =

3 0.4 0.58 c t . 4

(1)

As explained in Section 3.1, the droplets in the field of view are produced scattered on the water/oil interface. When the droplets reach a threshold size, they slide down along the interface toward the bottom by gravity. The structure observed in Fig. 1(b) is then formed by the deposition of droplets at the lower part of the interface, where they keep growing with time. Such a process can occur provided the droplets, once produced, they keep attached to the interface and connected to the water phase in order to grow. The interface is then a permeable membrane allowing the flow of water from the aqueous phase to the droplets, and at the same time it serves as a fluid glue holding the water droplets attached to the interface. Thus, as we pointed out before, the water/oil interface is not a single layer of surfactant but a film made up of surfactant, water, and oil. Under this conditions the interfacial tension must be very low and the Laplace pressure inside the droplets vanishing small. A simple assumption one can do now, is to consider that the flow of water toward the water droplets is constant, i.e., R2 dR/dt = const., leading to the solution R(t) ∼ t1/3 . This predicts a much more slower process than that observed in the experiments. Assuming, on the other hand, that the flow is proportional to contact area between the interface and the droplet, which in turn one can assume to be proportional to the surface area of the droplet, i.e., R2 , leads to the solution R(t) ∼ t. According to Eq. (1), the actual growth of the droplets is something in between these two extremes. In fact, assuming the flow ∼R4/3 leads us to a solution close to the experimental one. 4. Conclusions Eq. (1) establishes a general behavior for the spontaneous enlargement of interfacial surface between water and dodecane containing Span 80. Unlike other spontaneous emulsification

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phenomena reported in the literature, the one reported here appears to be a concatenation of different processes leading to the spontaneous formation and growth of droplets of water inside the interface between water and oil. We do not provide a direct evidence on the initial stage of droplets formation, we observe the droplets only when their size is already within the optical resolution. Nevertheless, our observations suggest the self-assembly of surfactant at the interface in a more elaborated structure than a simple single layer, probably a multilayer [10–12]. Such structure provides the medium for the spontaneous formation and growth of water domains. Such domains are maintained attached to the interface and connected to the water phase, but at the same time they are independent entities which move along the interface. The phenomenon described here is also a continuous production of interfacial area without the need to supply external energy. Eq. (1) allows us to calculate the rate of interfacial area production by means of droplets growth. Finally, let us note that the phenomenon is not specific of the tertiary system considered in this work, but its occurrence is more general. We have also observed spontaneous emulsification when using different oils (olive and motor oil) and when mixtures of water and glycerol are used as the aqueous phase, only the process is slower due to the increase in viscosity. A further investigation of other important issues, for instance the role of the surfactant properties, relative solubilities of oil and the aqueous phase, etc., is needed in order to gain a better understanding of this spontaneous emulsification phenomenon and to consider it in applications and to attempt a more basic (molecular) description. Such investigations remain as prospectives of the present work whose main objective is summarized in Eq. (1).

Acknowledgments We acknowledge technical support from M.A. Ramírez-Saito, and financial support from Consejo Nacional de Ciencia y Tecnología (Conacyt), Mexico, grants 84076 and 82835. References [1] P. Kumar, K.L. Mittal, Handbook of Microemulsion Science and Technology, Marcel Dekker, New York, 1999. [2] J. Sjöblom, Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York, 2001. [3] F. Leal-Calderon, V. Schmitt, J. Bibette, Emulsion Science, second ed., Springer, New York, 2007. [4] Y. Chung, A.W. Cramb, Direct observation of spontaneous emulsification and associated interfacial phenomena at the slag-steel interface, Phil. Trans. R. Soc. Lond. A 356 (1998) 981–993. [5] A. Yeung, T. Dabros, J. Czarnecki, J. Masliyah, On the interfacial properties of micrometre-sized water droplets in crude oil, Proc. R. Soc. Lond. A 455 (1999) 3709–3723. [6] H. González-Ochoa, J.L. Arauz-Lara, Spontaneous two-dimensional spherical colloidal structures, Langmuir 23 (2007) 5289–5291. [7] S. Tsukahara, Y. Shishino, T. Fujiwara, Microscope measurements for the transient formation of W/O emulsions of sodium bis(2-ethylhexyl) sulfosuccinate in the dodecane/water interfacial region, Langmuir 27 (2011) 7392–7399. [8] J.C. López-Montilla, P.E. Herrera-Morales, S. Pandey, D.O. Shah, Spontaneous emulsification: mechanisms, physicochemical aspects, modeling, and applications, J. Disp. Sci. Technol. 23 (2002) 219–268. [9] C.A. Miller, Spontaneous emulsification: recent developments with emphasis on self-emulsification, in: J. Sjöblom (Ed.), Emulsions and Emulsion Stability, vol. 132, second ed., Taylor and Francis, Boca Raton, FL, 2006, pp. 107–126. [10] Y.A. Shchipunov, P. Schmiedel, Phase behavior of lecithin at the oil/water interface, Langmuir 12 (1996) 6443–6445. [11] S. Pautot, B.J. Frisken, J.-X. Cheng, X.S. Xie, D.A. Weitz, Spontaneous formation of lipid structures at oil/water/lipid interfaces, Langmuir 19 (2003) 10281–10287. [12] H. González-Ochoa, L. Ibarra-Bracamontes, J.L. Arauz-Lara, Two-stage coalescence in double emulsions, Langmuir 19 (2003) 7837–7840.