J. Phys. Chem. 1983, 87, 4737-4743
the X-ray measurements and many helpful discussions. Registry No. I (n = 0), 74997-33-4; I (n = l),77069-42-2; I (n = 2), 77069-43-3; I (n = 3), 77069-44-4; I (n = 4), 77069-45-5; I (n = 5), 77069-46-6; I (n = 6), 77069-47-7; I (n = 7), 77069-48-8; I (n = 8), 77069-49-9; I (n = 9), 77069-50-2; I (n = lo), 77069-51-3; I (n = ll), 77069-52-4; I (n = 13), 77069-53-5; I (n = 15), 77069-54-6; I1 (n = 0), 74997-31-2; I1 (n = 6), 87100-37-6; I1 (n = 9), 87100-38-7;I1 (n = lo), 87100-39-8;I1 (n = ll),87100i45-6;
4737
I11 (n = O), 74997-32-3; I11 (n = 9), 87100-40-1; I11 ( n = lo), 87100-41-2; I11 (n = l l ) , 87100-42-3; IV (n = 0), 83038-71-5; IV (Iz = IO), 87100-43-4; IV (n = 11), 87100-44-5; C ~ ~ H , , - P - C ~ H ~ CO-p-C6H4-COOR (R = cholestanyl), 87100-46-7; CllH23-pC6H4-CO-p-C6H4-COOR(R = 8-sitosteryl),87100-47-8; CllH23p-CsH4-CO-p-C6H,-COOR (R = stigmasteryl), 87100-35-4; C11H23-p-C6H4-CO-p-C6H4-cooR (R = ergosteryl),87100-36-5; cholesteryl myristate, 1989-52-2;cholesteryl benzoate, 604-32-0; cholesteryl 4-benzoylbenzoate,74997-33-4.
Oil-in-Water Microemulslon Globules as Carriers of Lipophilic Substances across Liquid Membranes A. Xenakis and C. Tondre' Laboratoire de Chimie Physique Organique, E.R.A. C.N.R.S.222, Universit6 de Nancy I, B.P. 239, 54506 Vandoeuvre-les-Nancy Cedex, France (Received: November 17, 1982; I n Final Form: h4arch 28, 1983)
The carrier properties of microemulsion droplets were investigated by using biphasic systems of the Winsor I type (that is, constituted of an oil phase floating on the top of an oil-in-water (o/w) microemulsion phase). The systems investigated were constituted of sodium dodecyl sulfate (SDS)/l-pentanol/n-dodecanelwater (or brine). The microemulsion was used as a liquid membrane between two oil phases (a "source" phase and a "receiving" phase) and the rate of transfer of neutral arenes (pyrene, perylene, and anthracene), practically insoluble in the water continuous phase of the microemulsion, was determined from UV spectrophotometric measurements. The influence of different parameters on the transported solutes was studied: initial concentration of solute in the source phase, composition of the microemulsion, salt concentration. The results are shown to be consistent with a model in which the diffusion of droplets is coupled with a fast solubilization-desolubilization process and other possible mechanisms are critically examined. Some information is obtained concerning the thermodynamics of solubilization-desolubilizationof neutral arenes in microemulsion droplets. The results also allow one to get an insight into the structural organization of the microemulsion investigated: when the oil content of the microemulsion phase becomes higher than 12% in weight, a dramatic increase of the transfer rate of solute is observed which has been attributed to the percolation of oil droplets preceding the formation of bicontinuous phases.
Introduction The tremendously increasing interest which is developing in the literature concerning microemulsion systems arises essentially from their numerous potential applications in various branches of modern science or technology. As examples, the use of microemulsions in the following fields can be mentioned: tertiary oil recovery,1 chemical energy production from water cleavage,2metal recovering from liquid-liquid e ~ t r a c t i o ndevelopment ,~ of potential blood substitutes4 (this list is far from exhaustive). Among these applications are those related to the possibility of using such systems to carry lipophilic substances through an aqueous medium or inversely to carry hydrophilic substances across a lipoidic medium. We have recently given a short preliminary report5 on results concerning this aspect of the physical chemical properties of microemulsions which had not received much attention so far. In this paper we intend to investigate in more detail the kinetics of the transport of lipophilic substances (neutral arenes) by oil-in-watermicroemuhion globules and (1) V. K. Bansal and D. 0. Shah in 'Micellization, Solubilization and Microemulsions", Vol. 1, K. L. Mittal, Ed., Plenum Press, New York, 1977. (2) J. Kiwi and M. Gritzel, J. Am. Chem. SOC.,100, 6314 (1978). (3) P. Fourre and D. Bauer, C. R. Hebd. Seances Acad. Sci., Ser. B , 292, 1077 (1981). (4) G . Mathis and J.-J. Delpuech, French Patent 8022875, 1980; G. Mathis, DSc. Thesis, University of Nancy I, Nancy, France, 1982. (5) C. Tondre and A. Xenakis, Colloid Polym. Sci., 260, 232 (1982).
the mechanism involved in this process. The method used was mainly suggested to us by the numerous studies dealing with the transport of metal ions across liquid membranes, using antibiotics or macrocyclic carriers,- but a number of experimental difficulties had to be overcome before successful experiments could be performed. The results obtained have some implications regarding the thermodynamics of solubilization-desolubilization of neutral arene in (outside of) a microemulsion droplet and they are, as expected, very much dependent on the structural organization of the system investigated.
Experimental Section Chemicals. The origins of the chemicals used were the following: n-dodecane, 1-pentanol, pyrene, perylene, anthracene from Fluka (purum or puriss); sodium dodecyl sulfate (SDS) from Serva (Heidelberg, W.G.). All these chemicals were used as supplied. Preparation and Characterization of Biphasic Systems. The biphasic systems used in this study were obtained from compositions chosen in the pseudoternary diagram represented in Figure 1, where the components are SDS/l-pentanol/n-dodecane/HzO. The weight ratio be.s
(6) R. Ashton and L. K. Steinrauf, J. Mol. Biol., 49, 547 (1970). (7) K. H. Wong, K. Yagi, and J. Smid, J.Membr. Biol., 18,379 (1974). (8) C. F. Reusch and E. L. Cussler, AIChE J.,19, 736 (1973). (9) J. D. Lamb, J. J. Christensen, S. R. Izatt, K. Bedke, M. S. Astin, and R. M. Izatt, J. Am. Chem. SOC.102, 3399 (1980).
0022-3654/83/ 2087-4737$01.50/0 0 1983 American Chemical Society
4730
The Journal of phvsical Chemistry, Vol. 87, No. 23, 1983
Xenakis and Tondre
TABLE I : Composition (in wt % ) of the Separated Phases of the Different Biphasic Systems" pi(o/w microemulsion) ps(oil phase) d , g, system c m q , CP H,O NaCl n-C,, C,OH SDS H,O NaCl n-C,, C,OH SDS C 0.975 13.06 78.60 6.82 7.31 7.27 0.16 90.24 9.42 0.18 B 0.963 23.89 74.74 8.63 8.56 8.07 0.24 89.26 10.40 0.10 F 0.959 32.90 72.35 9.52 9.63 8.50 0.28 87.71 11.40 0.61 G 0.952 39.79 69.42 11.31 10.42 8.84 0.37 87.02 12.03 0.58 A 0.946 37.64 67.91 12.25 10.78 9.06 0.35 86.58 12.51 0.56 E 0.938 32.31 62.97 13.05 14.41 9.57 0.40 89.46 9.58 0.56 D 0.926 21.73 57.25 17.54 15.14 10.07 0.43 87.84 11.49 0.24 A1 0.948 30.19 66.93 0.07 12.45 11.34 9.21 0.34 87.86 11.65 0.15 A3 0.943 22.60 65.43 0.21 13.30 12.05 9.01 0.39 88.95 10.47 0.19 A6 0.938 16.42 63.54 0.39 15.98 11.36 8.73 0.39 87.68 11.66 0.27 A10 0.929 10.98 61.21 0.61 18.21 11.30 8.46 0.38 0.02 87.62 11.74 0.24 a pi and ps are respectively the inferior and superior phases. Note that the numbers do not reflect the actual accuracy but the need of adjusting the mass balance. The values of viscosity ( q ) and density ( d ) are given for the microemulsion phases. 1-PENTANOL
j d" I
WATEY
Flgure 1. Initial mixtures in the pseudoternary diagram SDS ('/&lpentanol (2/s)/ndodecane/water (plane A ) are represented by open circles. The separated phases are tentatlvely represented by the fflled circles; the oil-rich phases are close to the plane d',whereas the oil-in-water microemulsion phases are on the hatched demixlng surface close to plane i7'.
tween 1,pentan01 and SDS was equal to 2 in the initial mixture (this is no longer true in the separated phases). The corresponding phase diagram has been the object of previous investigations (see ref 10 and references therein) and for this reason the phase separation lines were known. The mixtures corresponding to the points indicated in Figure 1 separate out more or less rapidly (from 1 day to 2 weeks approximately) in two perfectly clear phases, whose compositions are given in Table I. The upper phase is mainly constituted of n-dodecane, whereas the lower phase is an oil-in-water microemulsion, the oil content of which increases when increasing the initial amount of tensioactive agents. The compositions of the separated phases were calculated from a combination of different analyses, the volume and density of each phase being known: the Karl Fisher method (Metrohm E 457) gave the water content in the oil phase; gas chromatography (Intenmat IGC 15 equipped with an Autolab digital integrator) gave the 1-pentanol and n-dodecane content in the microemulsion phase; and the SDS content in this last phase was obtained from potentiometric measurements." The relative accuracy of the determinations is estimated to be *5% for the 1-pentanol and n-dodecane and better than fl% for water and SDS. Note that the amount of each component was determined in only one of the two phases, the total mass balance being (10) C. Tondre and R. h a , J. Dispersion Sci. Technol., 1, 179 (1980). (XI) 2 Xenakis and C. Tondre, J . Colloid Interface Sci., in press.
used to get the quantity present in the other phase. This may introduce a large error when a component is present in a small quantity (case of SDS in the oil phase). The values of densities (Table I) were obtained at 15 "C with the aid of a A. Paar digital densimeter (DMA 10). Some experimentshave been performed with an aqueous solution of NaCl in place of pure water: in this instance the notation Al, A3, A6, A10 in Table I refers to systems whose initial composition is the same as A except that water was replaced by aqueous solutions of NaCl with concentrations of 1, 3, 6, or 10 g L-l, respectively. The corresponding pseudoternary diagrams are of course different from the one represented in Figure 1: when the salt concentration is increased, the monophasic region progressively disconnects from the line joining the water apex to the tensioactive agents apex.I2 The concentration of NaCl in the microemulsion phase was obtained from potentiometric titration of C1- by a standard AgN03 solution using an Ag/Ag+ electrode combined with a Pt/Hg/HgSO, saturated/K2S04saturated reference electrode. Viscosity measurements were performed at 15 "C by using an automatic Ubbelhode flow viscosimeter (Schott). Transport Experiments. The transport cell used for these experiments was very comparable in its principle to the U-shaped tubes previously described by different authors studying the transport of solutes at oil-water int e r f a ~ e s . ~ ~The J ~ Joil-in-water ~ microemulsion phase is put in the bottom of the U-tube and both arms are then carefully filled with the oil phase in thermodynamic equilibrium with the microemulsion. Good stirring was ensured in the three compartments either by a bar magnet (bottom compartment) or by peristaltic pumps (arms of U-tube). The whole device was carefully thermostated (15 "C). The solute to be transported was introduced into the source compartment (S) and the detection of solute attaining the receiver compartment (R) was carried out by measuring continuously the optical density of the phase R. A Unicam SP 1800 UV-visible spectrometer was used for this purpose with a microflow cell. The number of solute molecules crossing the interface with time was then determined from calibration curves. A schematic drawing of the whole setup is shown in Figure 2. The characteristics of the cell used were the following: the microemulsion compartment M was a cylinder of diameter 5 cm and thickness 1.3 cm and the compartments S and R were cylinders of diameter 2 cm (12) A. Graciaa, DSc. Thesis, Universit6 de Pau et des Pays de 1'Adour, Pau, France, 1978. (13) W. I. Higuchi, A,-H. Ghanem, and A. B. Bikhazi, Fed. Proc., Fed. Am. SOC. Exp. Biol., 29, 1327 (1970). (14)H. L. Rosano, P. Duby, and J. H. Schulman, J. Phys. Chem., 65, 1704 (1961).
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4739
Oil-In-Water Microemulsion Globules as Carriers Thermostated tmnspor t pens taltic
--
,--+-
microflow cell
7
Pump
pensidtic Pump
Figure 2. Schematic representationof the setup used for the transport experiments. S, R, and M designate respectively the source, receiver, and membrane compartments. The hatched areas represent the oil (oil-rich continuous phase in S and R, dispersed phase in M).
TABLE 11: Transfer Rate (in lo-' mol h-') of Lipophilic Solutesu pyrene anthraperylene cene 1o3cPi, system system (system (system M A B A) A) 0.1 0.18 0.125 0.25 0.147 0.14 0.15 0.34 0.166 0.33 0.196 0.18 0.25 0.54 0.53 0.39 0.85 0.4 0.43 0.39 0.5 0.90 0.48 0.76 1.0 1.53 1.23 0.92 1.25 3.36 1.5 2.07 0.98 4.22 2.0 3.51 5.0 6.14 3.01 10.0 8.66 Cross section of interface = 3.14 cm2. Cp' is the initial concentration of solute in compartment S. 130
Figure 3. Plots of the number of moles of pyrene crossing 1 cm2 of the second interface vs. time in microemulsionsystem A. Initial pyrene concentrations Cp' in the source compartment S are indicated in mol L-'.
and height 5 cm. The volumes of solutions introduced into compartments M, S, and R were respectively 26,22, and 26 cm3. The bar magnet in compartment M had a rotation speed of 120 rpm, that is, close to the highest tolerable speed for keeping a very clean interface between the two phases. The peristaltic pumps ensured a good agitation close to the interface in compartments S and R. The rotation speed of these pumps had practically no influence on the transfer rate of solutes.
Results The ideal way of characterizing the carrying properties of oil-in-watermicroemulsions would have been to prepare directly a definite isotropic (monophasic) system and then use it as a liquid membrane in which the transport of lipophilic solutes would be ensured by the microglobules constituting the dispersed phase. Unfortunately it' is not possible to choose any oil-in-water microemulsion and then superpose an oil phase because not only the solute to be transported will migrate from the oil compartment S to the oil compartment R but also part of the oil will dissolve into the microemulsion.1° This is why we were led to find biphasic systems of the Winsor I type,15thus demixing in one microemulsion rich in water and one oil-rich phase lying on top of it. When the phases were in thermodynamic equilibrium, the small concentrations of solutes added were assumed not to perturb the established equilibrium. Blank experiments with water (or water saturated with 1-pentanol) in place of the microemulsion did not reveal any transport of lipophilic solutes after 1 day. The transport of pyrene with different initial concentrations of pyrene C,' in the source compartment S was studied in systems A and B (cf. Table I). Typical plots (15)P. A. Winsor, Trans. Faraday SOC.,44, 376 (1948).
120
110
10
0
3
6
9
12
15
t( h o w5)
Figure 4. Evolution of the number of moles of pyrene in each compartment with time (microemulsion system A with initial pyrene concentration Cp' = 5.32 X M). The curves are experimental ones. When a steady-state situation is reached, there is a linear evolution in R and a plateau in M.
of the number of moles of pyrene transported vs. time through 1cm2of the second interface are shown in Figure 3. These plots show an initial curvature, as observed in many other transport studies and attributed to the time required to reach a steady state. The corresponding rates of transfer as given in the second column of Table I1 were obtained from the slopes of the linear part of the plots after establishment of a steady state, that is, when the membrane has reached an equilibrium situation. This is clearly demonstrated by Figure 4 which corresponds to a set of two experiments in which the change of pyrene concentration with time was measured successively in compartment S and in compartment R. The number of moles of pyrene in compartment M was obtained from the difference of the total number of moles initially in S and the measured number of moles both in S and in R vs. time. Different microemulsion systems referred to as A-G in Table I have been used in order to investigate the influence of the volume fraction of oil on the flux of transported
4740
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983
Flgwe 5. Plot of the flux of pyrene (initial concentration Cp' = 1.5 X M) vs. the volume fraction of dodecane in the microemulsion phases (left scale): (X) systems without salt, (0)systems with salt. Also represented are the vlscoslties of systems wlthout salt (0)(right scale). The letters refer to the system denomination as in Table I .
pyrene. Figure 5 shows that the flux is roughly proportional to the volume fraction of dodecane up to a value of ca. 15% and then increases dramatically. Also represented in the same figure are the viscosities of the microemulsion phases. Some experiments have been conducted in the presence of salt in order to try to see how the screening of the electrostatic potential developed by the oil droplets influences the transport process. Unfortunately the main influence due to the presence of salt in the microemulsion phase is the incorporation of an increasing amount of oil. On the other hand, the oil phase remains practically unaffected (see Table I). This situation results in a large increase of the flux with the salt concentration very likely to be related to the increasing amount of oil in the dispersed phase of the microemulsion as shown by Figure 5. The influence of the bulkiness of the transported solute was also examined and the results obtained with system A for the transport of perylene and anthracene are compared in Table I1 to that relative to pyrene. The differences observed for comparable initial conditions are rather weak. The partition coefficients k of the different solutes between the two phases have !,en determined spectrophotometrically. For this purpose the solute was added to the initial mixture before the phase separation was achieved. The values obtained in system A were 3.7, 2.55, and 4.46, respectively, for pyrene, perylene, and anthracene, whereas in system B the value for pyrene was 4.58.
Discussion Solute transport at liquid-liquid interfaces has been the subject of numerous papers and the main question common to all these studies is the following: what is the rate-controlling step? The answer is expected to depend very much on the kind of setup used to perform the transport experiments because the transfer of reactants from one phase to another will strongly depend on the hydrodynamics of the system.16 Of particular importance is the thickness 1 of the so-called diffusion layers. In the case of the experiments on which we report here good mechanical stirring of the membrane compartment M is (16) R. H. Guy, T. R. Aquino, and D. H. Honda, J. Phys. Chem., 86, 280 (1982).
Xenakis and Tondre
expected to reduce to a minimum the thickness of the unstirred layer on both interfaces, so that the effective membrane has a thickness 21 = L. As previously pointed out the rotation speed of the peristaltic pumps had no influence on the transfer rate of solutes, which indicates that there is not need to consider an unstirred layer on the oil side of the interfaces. In a study dealing with the migration of salts through nonaqueous liquid membranes, Rosano et al.14 attributed the rate-controlling step to the crossing of the interface and not to the diffusion through the variable concentration layers. This kind of interpretation is a much debated question'" (see also discussion following the preceding paper14). On the other hand, a model in which diffusion is coupled with a chemical reaction takes very well into account the transport of metal ion by macrocyclic carriers"+ when some assumptions are made regarding whether the diffusion in the unstirred layer or the chemical reaction is the fastest. Theoretical Interpretation. We have considered the transport of lipophilic solutes by an oil-in-water microemulsion droplet in a way similar to that developed to interpret the facilitated transport of ions by macrocyclic ~arriers.'-~J~ Following a scheme analogous to that described by Reusch and Cusslers and more recently by Lamb et al.,9the following steps can be postulated (with P referring to the lipophilic solute, the circle to the microemulsion droplet, and the indices to the cell compartments): interface 1 p(S)
(a)
P(continuous phase of M)
membrane P(continuous phase of M)
+0
k
@
diffusion of @ across the membrane k
@
kl
0+ P(continuous
(C)
phase of M)
interface 2 P(continuous phase of M)
e p(R)
(e)
The last step is followed by the back-diffusion of the empty droplet across M. Steps a and e are characterized by the partition coefficient k of the solute between the oil phase and the continuous aqueous phase of the microemulsion (and not the microemulsion phase itself). This partition coefficient is thus different from, and must not be confused with kp. Steps b and d represent the solubilization-desolubilization process characterized by rate constants kl and k2 and an equilibrium constant K = kl/k2. Three species of concentration Ci have thus to be considered inside the membrane, that is, P, 0, and @. The transport of each species across the membrane, at steady state, is governed by an equation of mass conservation of the form Di(d2Ci/dX2)= Ri
(1)
where X is the coordinate in the direction of solute transport, Di is the diffusion coefficient of species i, and Ri is the rate of depletion of i by reactions b (or d). These equations are subject to boundary conditions (17) B. J. R. Scholtens and B. H. Bijsterbosch, Colloid Polym. Sci., 258, 1197 (1980). (18) E. L. Cussler, 'Multicomponent Diffusion", Elsevier, New York, 1976.
The Journal of Physical Chemistry, Vol. 87,
Oll-In-Water Microemulsion Globules as Carriers
CpM= kCpo for X = 0
(2)
CpM= kCpL for X = L
(3)
I
where CpM designates the solute concentration in the continuous phase of the membrane and Cpo and CpLare the concentrations outside the membrane, respectively, in the source and receiving oil phases. Assuming that the equilibrium between the species involved is always established inside the membrane (or, in other words, that step c is much slower than all other steps), it follows that the concentration of solute-containing droplets can be expressed as CaM= KCpMCD/(l KCpM) (4)
J
No. 23, 1983 4741
I
+
with CD being the total droplet concentration. The flux of transported solute can be obtained by using a reasoning similar to that previously developed by WardlQ and Cussler18 (note that the diffusion of free lipophilic solute through the membrane can be ignored as indicated by the blank experiments):
F=-
DKkCD L
Cp"
1
+ KkCp"
(5)
where D is the diffusion coefficient of a microemulsion droplet, which is assumed to be independent of whether or not it contains a solute molecule. To obtain this expression, CpLwas set equal to zero at the beginning of the steady state (in good agreement with Figure 4). Differing from comparable studies using macrocyclic carriers, Cpo(the concentration of solute in compartment S a t the beginning of the steady state) cannot be taken as the initial concentration Cpi of solute in compartment S because the concentration inside the membrane at steady state is far from being negligible. Cpocan be obtained from the partition coefficient of solute k p , which has been measured in the biphasic systems: 1 Cp" = Cp' 1 VM 1+kp (Vs + VR) where VM, Vs, and VR are respectively the volumes of membrane, source, and receiver compartments. A convenient way of checking the validity of eq 5 to describe correctly the experimental results was suggested by Cussler: l8 it consists of plotting the reciprocal flux of solute vs. the reciprocal of the solute concentration Cp", which should give a straight line with slope L / (DKkCD) and intercept L/DCD. Such plots are shown in Figure 6 for the results obtained with pyrene, perylene, and anthracene. A good linear dependence is observed for pyrene and perylene (for the last one the solubility limit did not allow us to investigate higher concentrations). It is not as nice for anthracene, but the results obtained in system A can be considered in good agreement with the predictions that the intercept should be independent of the solute used ( L is completely determined by the geometry of the transport cell, the mechanical stirring, and the biphasic system used, whereas D and CDare characteristics of the microemulsion alone; all these conditions were indeed fulfilled for the results obtained in system A). Thermodynamics of Solubilization-Desolubilization Process. It is interesting to notice that the product Kk can be easily obtained by combining the values of the slope and the intercept which can be determined from the straight lines above. If one assigns, for the three sets of (19)W.J. Ward, AIChE J.,16, 405 (1970).
Figure 6, Plots of the reciprocal flux of solutes vs. the reciprocal of solute concentratlon at steady state in source compartment: pyrene (0),perylene (X), and anthracene (0) in microemulsion system A; pyrene (A) in mlcroemulslon system B.
results in system A, the same value for the intercept, the values of the product K k obtained for perylene, pyrene, and anthracene are respectively 312,260, and 212 M-l with an estimated error of i 5 0 M-l. For pyrene in system B a linear least-squares procedure gives-a value of 214 M-l, in good agreement with the value in system A (note that for pyrene in system A a value of 216 M-' is similarly obtained if the experimental point corresponding to the largest value of l / C p o ,which is less accurate, is omitted). The equilibrium constant K could in principle be obtained if a measure of the partition coefficient k was possible. There is unfortunately no way to directly measure this coefficient because the composition of the continuous phase of the microemulsion is not known. A tentative estimation from the solubility values of arenes in water and in hydrocarbon solventsz0 has been nevertheless performed.21 These calculations, which neglect the possible influence of l-pentanol on the solubility data, led to estimated K values of 3.9 X lo8,3.2 X lo', and 1.35 X lo7 M-l, respectively, for perylene, pyrene, and anthracene. As the forward rate constant k l characterizing the entrance of the solute inside a microemulsion droplet is very likely diffusion controlled,20the preceding values are expected to parallel the values of l / k z ,the reciprocal of the exit rate constant. The reason for the surprisingly small effect of the nature of the transported solute on the observed flux (see Table 11) is due to the fact that the change of the exit rate of the solutes from a microemulsion droplet is practically compensated by the change of the partition coefficient, so that there is no drastic change of the product K k . Discussion of Alternative Models. The simple model proposed seems to reasonably describe our results. The linear dependences observed in Figure 6 can be considered as an a posteriori justification of the simple assumption that only one solute molecule enters into a microemulsion droplet. This suggests that the size of the droplets is such that their concentration was always higher than the solute concentration inside the membrane. If it were not the case, a distribution of solute molecules among the droplets would have occurred (a Poisson's distribution is commonly admitted in that instance). It is only recently that a ~~~~~
(20) M. Almgren, F. Grieser, and J. K. Thomas, J . Am. Chem. SOC., 101,279 (1979). (21)C. Tondre and A. Xenakis, "Proceedings of the Internatior.". Symposium on Surfactants in Solution, Lund, Sweden, 1982",K. ', Mittal, Ed., in press.
4742
The Journal of Physical Chemistty, Vol. 87,No. 23, 1983
fluorescence method, initially proposed by Atik et al.,22has been applied to the determination of the size of oil-in-water microemulsion globules23similar to those investigated in this work; globule radii ranging from 36 to 40 A have thus been obtained when the volume fractions of dodecane are varied between 0.06 and 0.09. Although the volume fraction was higher in the experiments reported in Figure 6, the preceding values give an order of magnitude of the droplet radius. A t volume fraction 0.15 the droplet concentration would be of the order of 2 x M if the radius of the oil core alone of the droplet is taken around 30 A (the oil core radius was determined to be 26 A for a system containing 9% dodecaneZ3). The solute concentration inside the membrane was always lower than this estimation. A modification to the present model could have been to consider the migration of the transported solute directly from the oil phase into an oil droplet without solubilization in water, and then a possible transfer by collisions between droplets. It was recently shown that such a process can be ruled out in the case of lipid vesicles, the migration of pyrene taking place mainly via the aqueous phase.24 Owing to the dynamic nature of micro emulsion^^^ the situation could have been different, but collisions between droplets remain very unlikely because of the electrostatic repulsions. The other alternative model suggested by the work of Rosano et al.I4 (rate of transfer governed by the transfer across the interface and no consideration of a carriermediated transport) would lead to a flux at the beginning of the steady state proportional to Cpo whereas a plot of F vs. Cp" shows a strong curvature. This would also lead to a zero intercept when plotting the reciprocal flux as done above. This is definitely in contradiction with the experimental observations. Thickness of the Diffusion Layer. An estimation of the diffusion layer thickness can tentatively be performed in order to check the self-consistency of the proposed model. This can be done if we have an estimation of the droplet concentration as well as of the diffusion coefficient of droplets. Indeed, knowledge of these values, in addition to that of the Kk product previously determined, allows one to calculate L from slope s of the plots shown in Figure 6:
-
L = SDKkCD
(7)
Taking D as cm2s-l, that is, within the range of values measured on comparable systems,26CDof the order of 2 x 10+ mol cm-3 for system A as discussed above, and for py-rene s = 4.93 x lo4 s cm-I and K k = 2.6 x IO5 mol-' cm3 gives L 26 X cm. This value, which must not be taken for more than a rough estimation, compares very well with the value of 30 X cm experimentally estimated by Rosano et al.14 for a similar experimental device. In addition, it is also in reasonable agreement with calculations of Danesi et al.27giving a thickness of diffusion films from 30 x to 80 x low4cm (depending on rpm) in liquid-liquid metal extraction processes involving precisely strong surface-active extractants. This point strengthens (22) S. Atik, M. Nam,and L. Singer, Chem. Phys. Lett., 67,75 (1979). (23) P. Lianos, J. Lang, C. Strazielle, and R. Zana, J . Phys. Chem., 86, 1019 (1982). (24) M. Almgren, Chem. Phys. Lett., 71, 539 (1980). (25) J. Lang, A. Djavanbakht, and R. Zana in 'Microemulsions", I. D. Robb, Ed., Plenum Press, New York, 1982, p 233. (26) A. Graciaa, J. Lachaise, P. Chabrat, L. Letamendia, J. Rouch, and C. Vaucamps, J . Phys. Lett., 39, L-235 (1978). (27) P. R. Danesi, G. F. Vandegrift, E. P. Horwitz, and R. Chiarizia, J . Phys. Chem.. 84, 3582 (1980).
Xenakis and Tondre
our degree of confidence in the model proposed above. Percolation of Oil Droplets. Up to now we have mainly discussed experimental results which were obtained with the same system, so that the hydrodynamics of the liquids as well as the structure of the membrane phase (microemulsion) were fixed. This is no longer the case when the composition of the microemulsion phase is changed and we have thus to be very careful when trying to interpret the behavior of the flux observed in Figure 5 when increasing the volume fraction of dodecane &, Equation 4 predicts a linear dependence of the flux with the droplet concentration CDprovided that the other quantities do not change. In fact a linear dependence with $d is observed as long as $d I 0.15, but CDwill be proportional to & only if the size of the droplets does not change. It seems that there is at least one point to be mentioned which goes in this direction. The compositions of the microemulsion phases given in Table I allow one to calculate the ratio of the number of moles of dodecane to the total number of moles of tensioactive agents, which to a first approximation (neglecting their concentration in the continuous phase) constitute the droplet membrane. For microemulsions for which there is no salt present, this ratio is not far from being constant (except for system D) with an average value around 0.4. This fact can be taken as an indication that, as long as the globules are well-defined (probably spheri d B ) entities, their size does not change too much. This statement can be easily verified by examining the opposite situation: if all the hydrocarbon is assumed to constitute the droplet core and the surface area per tensioactive agents a t the membrane surface is fixed,23simple calculations show that the hydrocarbon to tensioactive agents mole ratio necessarily increases if the size of the droplets increases. Such calculations have been performed by using an average surface area of 25 A2 taking into account the fact that, according to values in Table I, there are approximately 3.5 pentanol molecules (surface area 20 A2 (ref 23)) for 1SDS molecule (surface area 40 A2 (ref 23)). These calculations lead to a hydrocarbon to tensioactive agents mole ratio of -0.4 when the droplet radius is 45 A taking 12 A for the membrane thickness (a value of 14 A, which is a sort of average between the lengths of SDS and pentanol molecules, has been experimentally obtained when there are two pentanol molecules for one SDS;23the "penetration" of oil inside the palisade layer of tensioactive agents is expected to be larger if there are more pentanol molecules, thus reducing the effective thickness of the membrane). These values are again in satisfactory agreement with the previous estimations of the droplet concentration. If this discussion seems to justify the linear dependence of the flux with CD,on the other hand, it is very difficult to tell something about the behavior of L and D when increasing the volume fraction of oil. D is expected to follow a Stokes-Einstein relation generalized to interacting particles26and L may also change with the viscosity of the system (viscosity values are given in Table I), but any estimation of their respective variations with & would be purely speculative and it will not be attempted here. The linear dependence observed in Figure 5 when & is smaller than 0.15 may thus be purely fortuitous. A more important point is the drastic increase of the flux of solute which is observed when '$d becomes higher than 0.15. We first observed this phenomenon precisely when attempting to modify the interparticle interactions by a salt addition. The drastic change observed is very likely to be attributed to the increased incorporation of oil in the microemulsion phase compared to a similar system in the
Oil-In-Water Microemulsion Globules as Carriers
absence of salt, and not to a change of the diffusion coefficient. This increased incorporation of oil (whether it is caused by a salt addition or not) would lead to a structural change of the microemulsion which no longer exists in the form of isolated droplets. The value of 0.15 corresponds in fact almost exactly to the known percolation threshold for hard spheres.28 Contrary to the reverse systems (water-in-oil microemulsions) for which some experimental evidence exists,2g3onot much is known concerning the percolation of oil droplets in a continuous aqueous phase. Nevertheless, Bennet et al.31have made some theoretical predictions of a symmetrical behavior between oil or water continuous phases. Their expectations concerning the viscosity behavior seem to be confirmed by the experiment: a peak in viscosity would occur at the oil percolation threshold. The experimental conditions used by these authors were similar to ours in the sense that the microemulsions whose viscosities were measured were one phase of a biphasic system. We have measured the viscosity of the microemulsion phases and found a maximum which correlates very well with the abrupt change of the flux. It is interesting to note that the transport experiments performed with system A (see Table I), for which the microemulsion phase is very slightly above the assumed percolation threshold, are still consistent with a droplet model. This could be an answer to the alternative model of percolation as recently pictured by Cazabat et al.: 32 open structures or connected droplets. If the droplets still have their entity in system A, the abrupt change of the flux suggests that open structures are probably more realistic to describe the situation in systems E and D (continuouschannels are then likely to be formed from one interface to the other one). This conclusion is supported by the sudden increase of 1-pentanol in the microemulsion phase of systems E and D (see values in Table I), the solubility of pentanol being much higher in a continuous oil phase as indicated by the composition of the superior phases cps. One may wonder why the percolation occurs when the oil cores of the droplets fill 15% of the space whereas a (28) M. Lagues, R. Ober, and C. Taupin, J. Phys. Lett., 39, L-487 (1978). (29) M. Lagues, J. Phys. Lett., 40, L-331 (1979). (30) M. Lagues and C. Sauterey, J. Phys. Chem., 84, 3503 (1980). (31) K. E.Bennett, J. C. Hatfield, H. T. Davis, C. W. Macosko, and L. E.Scriven in 'Microemulsions", I. D. Robb, Ed., Plenum Press, New York, 1982, p 65. (32) A. M. Cazabat, D. Chatenay, P. Guering, D. Langevin, J. Meunier, 0. Sorba. J. Lane. R. Zana. and M. Paillette. "Proceedines of the International Symposium on Surfactants in Solution, Lund, Sweden, 1982", K. L. Mittal, Ed., in press.
The Journal of Physical Chemistry, Vol. 87, No. 23, 1983 4743
hard sphere should also include the droplet membrane. In fact, in the case of reverse systems (water-in-oil microemulsions) it has been shown from conductivity measurementsmthat the percolation can occur for hard-sphere concentrations varying from 0.1 to 0.26 (in volume) depending on the type of microemulsion. The part played by the interactions, the polydispersity of droplets, or their nonsphericity was invoked as possible explanations. The importance of the dynamic effects, first examined by Lagues,= was also discussed recently by Cazabat et These authors underline the fact that geometrical percolation of hard spheres always occurs when they occupy a fraction -0.14 of the total volume but the physical properties of the medium will change only if the droplets can exchange matter during the lifetime of a cluster. Of course, the interactions are completely different in direct systems and the repulsive forces could explain the high value of the droplet volume fraction for which the percolation occurs (the exact value cannot be calculated without knowing the composition of the continuous phase), although exchange processes between the droplet membrane and the continuous phase are known to occur.25
Conclusion The results presented here show that the droplets constituting the dispersed phase of an oil-in-water microemulsion can be used as carriers of lipophilic solutes across an aqueous environment. A mechanism coupling the diffusion of the microemulsion droplets with a fast solubilization-desolubilization of the transported solute is consistent with all the present observations. Some information has been obtained concerning the thermodynamics of the solubilization-desolubilization process of different arene molecules in a microemulsion droplet. The results also allow one to get an insight into the structural organization of the microemulsion phases investigated. In particular, experimental evidence of the percolation of oil droplets in a continuous aqueous phase is obtained. A brief account of preliminaryresults on similar experiments using water-in-oil microemulsion systems to transport hydrophilic solute has recently been given elsewhere.22 It is hoped that such experiments will help in developing the potential applications of these systems. Acknowledgment. We acknowledge the valuable technical assistance of Mr. J. L. Vasseur, who fabricated the transport cell used in this work. Registry No. Sodium dodecyl sulfate, 151-21-3;1-pentanol, 71-41-0; n-dodecane,112-40-3; pyrene, 129-00-0;perylene, 198-55-0; anthracene, 120-12-7.
