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Breaking of multiple emulsions under osmotic pressure and the effect of W1/O relation.

RESUMEN

Se estudio el comportamiento de emulsiones multiples de bajo contenido de fase dispersa y relaciones diferentes de W1/O, con la fase interna acuosa (W1) como fase concentrada, para producir diferentes gradientes de presion osmotica. El comportamiento fue seguido por conductimetria, medidas reologicas y microscopia. La microscopia muestra un aumento del volumen de la fase dispersa, hasta alcanzar un maximo, para luego disminuir por rompimiento de las gotas. Las medidas reologicas muestran un aumento inicial de la viscosidad, hasta llegar a un valor maximo, para luego disminuir hasta alcanzar el valor de la viscosidad inicial. Tambien se observo que a medida que la relacion W1/O es menor, el rompimiento de las emulsiones multiples es mas lento.

SUMMARY

Multiple emulsions of low dispersed phase, in which the internal phase (W1) was the concentrated phase, were made and their behavior under different osmotic pressure gradients and W1/ O relations were studied. The behavior was followed by conductivity measurements, rheological measurements and microscopy. The microscopy results indicate a volumetric increase of the dispersed phase until it reaches a maximum, and then a decrease of the dispersed phase due to the breaking of the emulsion. The rheological measurements showed an initial increase in viscosity until it reached a maximum value, and finally a decrease towards the initial viscosity value. It was also observed that for smaller W1/O relations the breaking of the emulsion was slower.

RESUMO

Estudou-se o comportamento de emulsoes multiplas de baixo conteudo de fase dispersa e relacoes diferentes de W1/O, com a fase interna aquosa (W1) como fase concentrada, para produzir diferentes gradientes de pressao osmotica. O comportamento foi seguido por conductimetria, medidas reologicas e microscopia. A microscopia mostra um aumento do volume da fase dispersa, ate alcanfar um maximo, para logo diminuir por rompimento das gotas. As medidas reologicas mostram um aumento inicial da viscosidade, ate chegar a um valor maximo, para logo diminuir ate alcanqar o valor da viscosidade inicial. Tambem se observou que a medida que a relacado W1/O e menor, o rompimento das emulsoes multiplas e mais lento.

KEYWORDS / Multiple Emulsions / Osmotic Gradient Controlled Release / Emulsion Rheology /

Introduction

Multiple emulsions consist of small droplets that have, in turn, smaller droplets inside them. The outer droplets are dispersed in a continuous phase. The inmiscible phase that separates the small internal droplets from the continuous external phase can act as a liquid membrane. For this reason, multiple emulsions are also known as liquid surfactant membranes or liquid emulsion membranes (Cardenas, 1993). There are two types of multiple emulsions, water/oil/water (W1/O/W2) or oil/water/oil (O1/W/O2). In the first case the membrane is liquid oil and in the second case it is water of an aqueous solution. The most common multiple emulsions are of the W1/O/W2 type. The existence of a liquid membrane makes multiple emulsions strong candidates for industrial applications and for pharmacy and medical uses (Schugerl et al., 1985; Chaudhuri and Pyle, 1992; Owusu et al. 1992; Chakravarti et al. 1995; Kim et al., 1995; Nakhare and Vyas, 1995; Nakano et al., 1996; Juang et al., 1997; Lin and Long, 1997).

Multiple emulsions nave been extensively studied due to their potential applications in separation operations and in controlled release. Their instability has been for a long time a problem in industrial and pharmaceutical extensive applications. Few industrial applications are known, such as treatment of effluents in a textile plant in Austria (Draxler and Marr, 1986). From the point of view of separation, the multiple emulsion has to be stable enough to permit separation of the components from the stream of interest, but it has to be sufficiently unstable to coalesce the emulsion and to break the membrane so as to recover the separated components; a compromise must be kept between two antagonistic characteristics, that is, to be stable and unstable at the same time. From a release point of view, the instability of the multiple emulsion has to be controlled so that the component to be released, does it at the proper time, or is stable enough as to permit the release by diffusion through the membrane. Two mechanisms are important in this type of application, the transfer of the component through the liquid membrane and/or the breaking of the droplets at the proper time. Of course, the two mechanisms can occur simultaneously.

The mechanism of swelling of the emulsion droplets until their breakup occurs when the internal droplets nave a more concentrated solution than the continuous external phase. This creates an osmotic pressure difference, which makes the solvent go from the external phase to the internal phase and thus the droplet grows and can eventually break. When this happens, the rheological properties change due to the change in the dispersed/continuous phase ratio (Matsumoto and Kohda, 1980). In this study, the quantity of the membrane phase or membrane thickness, the concentration of salt of the internal droplets and the agitation speed of the multiple emulsion were changed, and their effect on the breaking of the multiple emulsion analyzed.

Experimental

The multiple emulsions prepared were of the W1/O/W2 type.

Materials

For the oily phase, heptane (JT Baker) was used as received. As surfactants, SPAN 80 (Sorbitan monooleate, ICI Americas) was used as the lipophilic surfactant for the preparation of the primary W1/O emulsion and TWEEN 80 (Sorbitan monooleate with poliethylene oxide, ICI Americas) was used as the hydrophilic surfactant for the external phase. As the electrolyte, NaCl (Riedel de Haen) was used as received. Distilled water was used.

Multiple emulsion preparation

The multiple emulsions were prepared following the two-step procedure, i.e., a primary W1/O simple emulsion was prepared and then this emulsion was poured into the continuous phase under agitation.

Several primary emulsions were prepared with NaCl solutions of different concentrations (0.5, 2.5 and 10% in weight) in heptane with 1% weight of SPAN 80. The SPAN 80, being lipophilic, stabilizes water in oil emulsions following Bancroft's rule, which states that the dispersed phase is the one where the surfactant has higher affinity. The mixing was done with an Ultra Turax mixer, at 10000rpm during lmin to obtain small droplets. The volumetric relations of water to oil phase were 30/70, 50/50 and 60/40.

Following, 150ml of the primary emulsion was poured into 350ml of water (W2) containing 1% of TWEEN 80, so as to stabilize the W1/ O emulsion in the external W2 phase. This was done under continuous agitation using a four-propeller agitatot driven by an IKA EUROSTP CV S1 agitator motor at 500rpm. The vessel used was a 1000ml beaker with 4 baffles. The dimensions of the baffles and the propeller were taken from those proposed by Bart et al. (1995). Experiments were also made varying the agitation speed: 275, 350, 500 and 750rpm. All the experiments were done at room temperature, 23 [+ o -] 1[grados]C.

To follow the behavior of the multiple emulsion, conductivity measurements were made with a Taccusel CD 6N conductimeter. The rheological behavior was followed with a rheomat-30 Contraves using cylindrical cells. The emulsions were observed with a standard 18 Zeiss microscope and an inverted light Olimpus LSM microscope.

Results and Discussion

Concentration effects

To follow the breaking of the multiple emulsions, conductivity measurements were made. The initial concentration of salt (NaCl) in the inner droplets was varied, being the external phase pure water with a non-ionic surfactant of extremely low conductivity (Tween 80). Hence, the conductivity of the external phase was in the order of [micron]S while the internal salt solution was in the order of mS. As shown in Figure 1, the breaking percentage of the multiple emulsion was faster for the emulsion with 10% of salt in the internal phase, followed by the emulsion of 2.5%, and 0.5% for the slowest breaking. All of the emulsions had the same W1/O ratio of 30/70 and were prepared under similar conditions.

The breaking percentage was calculated as the conductivity measured in a given time (Gt) divided by the conductivity the external phase would have if all the salt were liberated (G):

(Gt/G) * 100 = breaking percentage

The results were as expected, since the osmotic pressure increases with the difference in salt concentration. Thus, with 10% of salt in W1 an osmotic pressure of 83.2atm is obtained, and with 0.5% the osmotic pressure is 4.0atm (Dow Chemical, 2002). Most of the breaking of the multiple emulsion occurs in the first 30min for the 10% and 2.5% emulsions, due to the greater concentration gradient between the inner and outer water phases. As salt is released, breaking diminishes because of the reduction in the concentration gradient. At 360min a plateau is almost reached at a breaking percentage of 57% for the solutions with 2.5% and 10% initial salt concentration. For the 0.5% solution, the plateau is reached at almost 50%. Near 50% of breaking, the concentration of salt in the inner and outer phases could be nearly equal as two effects are involved. One is the liberation of salt to the outer phase (W2), which increases its concentration and decreases the concentration gradient between Wl and W2. The second effect is the transport of water into the emulsion droplets, which in turn reduces the concentration of the inner phase and reduces the swelling (breaking) of the droplets. When calculations are made of the final concentration of salt in the internal (Wl) aqueous phase, the results show a concentration of 0.75g/1 vs 0.50g/1 measured for W2 for the 0.5% curve shown in Figure 1, which still has a breaking tendency that shows that the equilibrium has not been reached. It should be noted that for the 0.5% curve the initial concentration of salt in the inner droplets is of 5g/l and zero for the external phase. These calculations were made measuring the final salt concentration by conductimetry and the final external volume phase (W2), and performing a mass balance of the salt.

[FIGURA 1 OMITIR]

For all the above discussion to be valid, there must be very little or no diffusion of NaCl through the oil membrane. The NaCl is not soluble in oil. Furthermore, studies by Wen and Papadopoulos (2001) show that when there is an osmotic pressure difference between aqueous phases separated by a membrane composed of oil and surfactant, there is transport of water but not of salt.

The results obtained indicate that the larger the osmotic pressure, the faster the water transport through the membrane into the inner water droplets (W1). Similar results were obtained by Wen and Papadopoulos (2001) in a different system, not in multiple emulsions, by Matsumoto and Kohda (1980) using glucose as inner water solute and measuring viscosity changes in the multiple emulsions instead of simpler conductivity measurements used in this work, and by Kinugasa et al. (1989) in multiple emulsions with nickel nitrate and sulphuric acid in the W1 phase and water in the W2 phase.

W1/O relation effects (membrane thickness)

Three W1/O relations were studied: 60/40, 50/50 and 30/ 70. These are related to the thickness of the oil layer of the droplets. In all the experiments the volume of the W1/ O primary emulsion was maintained constant, as well as the external water phase (W2). The behavior of these systems is shown in Figure 2, where it is clear that the multiple emulsion with the thicker membrane (W1/O= 30/70) is the one that breaks the least in the same time period. The thinner membrane is the one that breaks the most (W1/O= 60/ 40). The same behavior is observed for all the salt concentrations of the internal water phase, but with a steeper slope for the more concentrated solutions. When the W1/O emulsions are prepared, a volume of the W1 phase is dispersed in the oily phase and, as the relation between water and oil increases, the volume of oil that surrounds the volume of water (W1) decreases, so the membrane becomes thinner. If the membrane thickness for the three W1/O emulsions prepared and dispersed in the external water phase (W2) is calculated, assuming an average droplet diameter of 80 [micro]m (from microscopic photographs), three different membrane thicknesses are obtained (Table I).

[FIGURA 2 OMITIR]

The slopes of the initial curves (dB/dt) show the breaking velocity of the emulsions. Table I shows the values of the slopes of the membranes shown in Figure 2 and, as expected, the steeper slope is the one for the thinnest membrane and the lesser slope is for the thickest. This is because when the membrane is thicker, the drop can grow more and the rate of breaking is smaller. It is worth noting that actually there are different droplet sizes and distributions for the emulsions prepared. This means that the surface area of the droplets changes from emulsion to emulsion and so does the water transfer, which is the reason why the water transfer is inversely proportional to membrane thickness in the experiments performed, but when compared between the different emulsions, there is no direct proportionality between them. In any case, all the emulsions were prepared under similar conditions, so they are expected to have similar droplet sizes and distribution. Also it is important to note that the membrane thickness can vary from droplet to droplet and this, added to differences in droplet sizes, results in some breaking faster than others, but the overall behavior can be taken as an average of the bulk properties for these experiments.

The rheological behavior is shown in Figure 3, where all the emulsions with a given initial concentration of salt in the inner water phase (W1) behaved in a similar fashion. Initially, there is an increase in the viscosity of the emulsion. This can be explained by an increase in the volume of the droplets, which translates into an increase of the volume of the dispersed phase. The same type of behavior was observed by Matsumoto and Kohda (1980), although they did not attribute it to thickness of oil layers (Matsumoto et al., 1980). They used optical microscopy to determine this, and it is difficult to see the membrane thickness with an optical microscope. They do not explain the relationship between the increase in viscosity and the W1/O ratio.

Normally, an increase in the size of the droplets of an emulsion tends to reduce its viscosity, but in this case the most important effect is the reduction in volume of the external continuous phase. As this volume decreases, viscosity increases. This effect is expected to be much more pronounced in high internal phase type emulsions, where small changes in the external phase volume can cause a very large increase in viscosity (Salager, 2000).

Figure 3 shows the behavior at low internal concentrations of salt (0.5%) in which the consecutive breaking of the multiple emulsion is observed. This breaking can be correlated to the time when the maxima are reached. In this case, the first emulsion breaks at near 30min, the second at 60min and the third at 90min. As the internal salt concentration is increased, the effect of the membrane thickness becomes less pronounced. At 10% salt concentration, the breaking speed becomes so fast that the differences in the effect of membrane thickness are by far smaller. In this case the membranes of W1/O= 60/40 and 50/50 break at the same time. The 30/70 membrane breaks a little afterwards.

Once the maxima are reached, there is a gradual decrease in the viscosity of the emulsion, which could be taken as a gradual return to the initial volumes of the dispersed and continuous phases, because the breaking of the droplets becomes more important than their growth. Even if these phases do not reach their exact initial values, the effect of the viscosity of the external phase becomes as important as in the beginning of the experiment. This explains why the viscosity tends to return to its initial value.

For comparison purposes, the behavior of a multiple emulsion with no osmotic pressure (0% NaCl) is also shown in Figure 3. In this case there is no swelling of the droplets (no maximum) and the viscosity diminishes continuously due to the breaking of droplets by mechanical effects, which causes a different droplet distribution compared to the initial one. This also shows that there is no measurable effect of the surfactant concentration on the osmotic pressure.

The rheological behavior of the multiple emulsions was as that of a Newtonian fluid at the beginning of the experiment. As the internal phase increased, a weak shear thinning behavior was observed, to finally return to the initial Newtonian fluid. As with emulsions with a low dispersed phase in an aqueous external phase, the Newtonian behavior is expected. This is shown in Figure 4, where the shear stress is plotted against the shear rate. At initial time (t0) the relation is linear, as expected for a Newtonian fluid, but at intermediate times (t2 ... t5) there is no linear relationship and the fluid shows a shear thinning behavior typical of non Newtonian fluids, due to the increase in internal (W1/O) phase volume. Finally, at longer times (t8, t9) the emulsion has broken, the external phase is a major fraction of the volume, and the multiple emulsion regains the linear relationship of the Newtonian behavior of a high external aqueous phase emulsion. The initial internal fraction was always 0.3. The calculated maximum fraction reached 0.48 for W1/0= 30/70, which is the thicker membrane. The fraction of the internal phase was calculated using the Thomas equation (Poletto and Joseph, 1995), which is valid for internal fractions as high as 0.6. The equation is

[eta]r = 1 + 2.5[PHI] + 10.05[[PHI].sup.2] + 000273exp(16.6[PHI])

where [PHI] is the internal fraction of the emulsion and [eta]r is the relative viscosity.

[FIGURE 4 OMITTED]

For the other membranes of W1/O= 60/40 and 50/50, the viscosity did not reach the values obtained with the membrane of 30/70. This can be explained by the possibility of the 30/70 membranes to swell much more before breaking in comparison with the other two, because of a thicker membrane.

The breaking of the multiple emulsions is thus easier for the 60/40 formulation, followed by the 50/50 and finally by the 30/70. As explained above, the release is mainly through the breaking mechanism, and then the control of the timing could be achieved by varying the thickness of the membranes. This could be useful in the dosing of drugs in the case of pharmaceutical applications, in the releasing of catalysts or reactants in the chemical industry, or in the design of processes where the drops do not break at certain residence time intervals. Moreover, a combination of different multiple emulsions with membranes of different thickness could be used to control the breaking times.

Agitation effects

Different agitation speeds were compared to observe their influence on the breaking of the emulsions. The results (Figure 5) showed the following pattern: At the slowest speed, the breaking is the least, and as the speed is increased, the breaking of the emulsion increases until a limit tends to be reached. This can be explained by the fact that as the power applied into the emulsion is increased, there are more collisions between the droplets and between the droplets and the impeller and baffles. But there is always a limit reached in the breaking of the droplets, i.e., there is a minimum size of droplet, no matter what the speed (energy) introduced into the emulsion is. This limit is given when the energy is no longer useful in breaking the droplets, due to their small size, and is dissipated in form of heat. In this case, at 500 and 650rpm, the maximal viscosity is reached at the same time.

[FIGURE 5 OMITTED]

Microscopy

Finally, under the microscope it was observed that nearly all the droplets increased their size and broke in a stepwise manner, by layers; first the inner droplets nearer to the surface of the oily droplet, then the next ones and so on. Only a few droplets, which contained only one inner droplet, broke in one step. This observation corroborates the behavior obtained with the rheological and conductivity measurements.

Conclusions

It was found that the membranes with a lower W1/O relationship and lower inner salt concentrations broke in a longer period of time. The fact that the membranes broke in an inverse relation to the W1/O (membrane thickness) value could be used in the design of controlled dosage of compounds or in preventing breaking of droplets. Another parameter to take into account in the multiple emulsion breaking is the agitation speed. At low speeds of agitation the breaking diminishes, and at high speeds there is a limit at which an increase in the speed makes very little difference in the breaking speed of the multiple emulsion.
TABLE I

INITIAL SLOPES OF BREAKING PERCENTAGE vs TIME
FOR DIFFERENT MEMBRANE THICKNESS.
INNER PHASE NaCl CONCENTRATION 2.5%

W1/O                                        60/40    50/50    30/70

Slope (dB/dt)                                1.46      0.93     0.54
Thickness (Fun)                              6.26      8.25    13.22


ACKNOWLEDGEMENTS

The authors thank M Briceno for her valuable comments, Nilo Morillo for his help in the experimental work, and H Medina, C Colasante and N Joshi for their help with the microscope. The CDCHT of the Universidad de los Andes financed the project through grants 1-617-98-02-F and 1-653-99-02-B.

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Received: 05/08/2003. Modified: 08/07/2003. Accepted: 08/18/2003

Antonio Cardenas R. Chemical Engineer and M.Sc in Chemical Engineering, Universidad de los Andes (ULA), Venezuela. Doctor, Universidad Montpellier II, Francia. Head, Laboratory of Mixing, Separation and Industrial Synthesis, School of Chemical Engineering, ULA. Address: Escuela de Ingenieria Quimica, Universidad de los Andes, Merida Merida 5101 ,Venezuela. e-mail: antonioc@ula.ve

Elizabeth Castro. Chemical Engineer, ULA. Ph.D. student, University of Arizona, USA.
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Date:Sep 1, 2003
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