Microemulsions as a New Working Medium in Physical Chemistry An Integrated Practical Approach Julio Casado, Carmen Izquierdo, and Santiago Fuentes Departamento de Quimica Fisica, Facultad de Quimica, Universidad, E-37008 Salamanca, Espaha Maria Luisa Moya Departamento de Quimica Fisica, Facultad de Quimica, Universidad, E-41012 Sevilla, Espaiia The term microemulsion, first used in 1959 (11, designates auuarentlv homoeeneous mixtures of water and oil with large amounts of detergent. Microemulsions are formed spontaneously. Unlike macroemulsions (droplet size: 200500 nm) and miniemulsions (droplet s u e 100400 nm), microemulsions are thermodynamically stable. Also, they are optically transparent because the droplets of oil and water are too small (10-200 nm) to scatter visible light (2). Over the past 10 years, increasing basic and applied research (3.4) on the structure, dynamics, and interactions of microemulsions has given rise to "a novel Chemistry" (3).Points of signif~cantpractical interest include the following.
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1. Microemulsion droolets can be considered as micmreadors in whlrh certain chemical reactions can be roncentrated in a very small region w i t h the following rrsults. These reactions may proceed at rates that differ hy
large factors from the rates observed in conventional media ( 4 4 ) . Regulation of droplet size can allow control of the grawth of particles or polymers formed by such reactions (7,8). 2. Microemulsions can be more suitable than conventional media for the study of reactions of biological interest be-
cause they approximate more closely the conjunction of hydrophilidipophilic conditions present in cells. In particular, many enzymes can be solubilized as monomers rather than enzyme clusters in the polar cores of waterin-oil (wlo) microemulsions. 3. Microemulsions are also of considerable interest in focd and nutrition science (2, 9). 4.
Microemulsions have recently been used to c a m out separations and purifications (10,Il).
In spite of their growing importance, microemulsions are not yet considered, even cursorily, in most textbooks of general or experimental physical chemistry In this article we describe a series ofthree exueriments ~ l a n n e dto familiarize students of physical chemistry with the preparation of mieroemulsions and the dependence of their oraoerties on their com~osition . . thew bas~cstructural parameters (core radius and ngpgation number, and their grnvimetric detprminntion theoretical and practical 86pCCLF of thcir influence on rsaction kinetics The microemulsions used in these experiments are AOTI decanelwater mixtures where AOT is aerosol OT, that is, bis(2-ethylhexy1)sodiurn sulfosuccinate. These microemulsions are the ones most commonly used in academic research, and they do not require a fourth component as cosurfactant. Hexane or heptane can be used instead of 446
Journal of Chemical Education
decane. We used AOT from Fluka without further purification. Apart from standard chemistry laboratory equipment, the only apparatus the experiments require is a conventional UV-vis spectrophotometer. Three Laboratory Experiments Preparation of Micmemulsions The first task is to nreoare microemulsions of various compositions, that is, dith'different values of the following mole ratio for one or more concentrations of the AOT in decane
where n. is the number of moles of x. To this end. suitable of AOT and decane are accurately weighed and mixed to eive solutions with molal concentrations of AOT in decanein the range 0.1-0.4 m, that is, 0.1-0.4 mol of AOT per kg of decane. Once the AOTIdecane solutions have been prepared, the appropriate weights of water are added to give values of w between 6 and 40. The students'attentionshouldbe drawn to the spontaneous formation of the microemulsion phase and-once the mixture has been homogenized by vigorous shakine-to ., its stabilitv and transuarencv. The microemulsions prepared as described above are charactrrized by the molal concentration of AOT in the initial AOT~dccanesolution and b.y the value of w. Because they have been made up by weight, their percentage composition by weight is readily calculated and will be used below. Because the critical micellar concentration of AOT is of the order of 103-104mol dm3 (121, much lower than the concentrations used in these exueriments, all the surfactant can be considered as located a t the waterldecane interface. Asuitable and simple structural model that can be presented is that of a monodisperse population of spherical water droplets separated from the organic phase by a monolaver of AOT (Fie. la). The radius r~ of the droulets is the sum of the radiGs oofthe aqueous ;ore (r,) and the length of the surfactant tails ( I ) . Determination of StructuralParameters A Simple GravimetricMethod
The first part of this experiment is to determine the mass M of a known volume VT ( 2 0 4 0 cm3) of each of the microemulsions prepared a s above. For each microemulsion this determination should be camed out five times using a balance with a precision of 0.0001 g. The coefficient of variation of the five replicate measurements should not exceed 0.1%. Temperature should be constant throughout.
Volume Fraction 8 Taken up by Solute and Basic Parametersa and Literature Values at nand r, Determined by Different Techniques for AOTlOrganic PhaseWater Microemulsions
Organic Phase
n
rw(A)
ref
Small Angle Neutron Scattering Figure 1. (a)Schematic illustration of an AOTcoated water droplet in an AOTIdecanelwater microemulsion. (b) Dependence of the droplet core radius r, on the ratio o = kt$nAoTfor a molal concentration of AOT in decane of 0.2 m. (Similar results are found for AOT concentrations of 0.1 m and 0.4 m.)
~Decane,
34.4
~Hexane
33.1
n-Heptane
20
35.8
n-Octane
15
34.4
Viscosity Assuming negligible penetration of the organic phase in the interface (131, we define 8, the volume fraction occupied by the solute (water + AOT) in the bulk organic phase, by
Cyclohexane Toluene
8
Chlorobenzene
114
18.6
112
18.7
108
19.3
16
Sedimentation/Uilracentrifugation
lsooctane where VDis the volume of decane in the known total volume VT . VDis given as a function of M by
11.11
21.35
17
'%lute (water + AOT) with aggregation number n and aqueous core radius h of droplets formed in micmemulsions (AOTldecanelwater) of diferent mmpaslion at 298 K ( C m = molal concentration of AOT in decane)
where NAis Avogadro's number; and Vd is the total volume of the droplet (including the surfactant tails of length I), given by where WDis the concentration of decane in the microemulsion on a weight basis; and SDis the known density of decane. Thus, the measurement of the mass M of the known volume of microemulsion VT also allows e to be calculated. The table lists values of 9 determined for microemulsions with 0.1 and 0.2 m concentrations of AOT in decane for several values of w. The value of 9 allows us to calculate the basic parameters of the simple structural model described above: the core radius r, and the aggregation number n (the number of AOT molecules per water droplet). Calculating the Basic Parameters To show how, we first defme NAOTand Nd as the total numbers of AOT molecules and water droplets in the volume of microemulsion VT. Then the aggregation number is given by
Now in our spherical droplet model, r, is related to V, the volume of the droplet core, by
Because the number of water molecules per droplet is nw, eq 5 can be rewritten in terms of S,, the volume of a water molecule in the droplet.
Thus, from eqs 3-6, we get
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447
which can be rearranged to give n explicitly as
'
On the right-hand side of eq 8, S, and 1 are still unknown. However, under the working conditions of these experiments (w 2 6) the density of water in the droplet core can be assumed to be unity, as in bulk water, making S, = N&8 x lo3) dm3; and 1 has been estimated by Day et al. (141 as 1.03 nm. Hence eq 8 allows calculation of n, and r, can then be obtained from eq 6. The table lists values of n and r, calculated as above, together with values obtained by other methods: small angle neutron scattering (151, viscosity measurements (161, and sedimentation~ultracentrifugation(17). If the measurements have been carried out properly, the students' attention may be drawn to the good agreement found between the results of the simple gravimetric method used in this experiment and those of the more elaborate techniques. Supporting the Hypothesis with Empirical Obseruations As expected, both the aggregation number and r, depend on w. The linear dependence of r, on w (Fig. lb) has a ready explanation in terms of our model because, fmm eq 6, molecules of water per droplet w= molecules of Am per droplet
The surfaee number density of surfactant molecules a t the interface (a,)is defmed by n = msurface area
-- n
4N;
tions are4 x 10 M for the persulfate solution and between 4 x 10 M and 12 x 10 M for the iodide solution. Iodide and persulfate micmemulsions that are otherwise of the same com~ositionare brouzht to the workine temperature of the ihermostated spe&rophotometer cuvette, where the reaction is initiated by mixing and vigorously stirring 2 cm3 of the iodide microemulsion with 0.8 cm3 of the persulfate microemulsion. Absorbance-time data are thereafter recorded a t 355 nm, a wavelength of peak absorbance by Ii ions produced in the reaction.
(10)
Reaction Order The general rate equation for this kind of reaction is =k[~~0~1"[17*
(13)
With iodide in excess of persulfate, as in the working conditions of this experiment, eq 13 reduces to
The hypothesis that the reaction is first-order with respect to persulfate (i.e., a = 1)may therefore be tested by determining whether t h e absorbance-time data fit Guggenheim's equation for pseudo-first-order kinetics (e.g., see ref 20). In b4 = constant - kobst (15) where AA is the change in absorbance over a fured time interval At. In the working conditions described, the goodness of fit between the experimental AA-time data and eq 15 (Fig. 2) confirms that the reaction is firstorder with respect to persulfate, and the slope of the regression line is the value of the correspondingfirst-order pseudoconstant, kOb.. The results of a series of these experiments with differing concentrations of iodide in the range 4-12 x 10.' M show that the reaction is also frat-order with respect to iodide (Fig. 3). Thus, as in water (18,19), the experimental rate equation is
Thus, eq 9 yields Reaction Rate and Droplet Size To show how the size of the water droulets of the microemulsion affects the reaction rate, experiments are carried out for different values of w at the same mold concentrawhich is Thus, if a, and S, are assumed not to vary significantly with w, r, depends linearly on w. Put another way, given the assumptions of the model (including the assumption that S, is constant), the hypothesis that a. does not vary significantlywith w is supported by the empirical observation that r, depends linearly on w. Kinetics of Reactions in Microemulsions
The reaction chosen for the experiment is the oxidation of iodide ions by peroxide sulfate (called persulfate below) for the following reasons.
'Kinetica for this reaction in water are well-known (18.19). .Changes in pH, a quantity that is daeult to det&ne.in microemulsions, have little effect on the reaction. The chemicals involved are innocuous
Experimental The ex~erimentis b e m bv nreuarine micmemulsions as in the brevious expe&nen&, but'with&ueous iodide or persulfate solutions instead of water. Suitable concentra448
Joumal of Chemical Education
Figure 2. Guggenheim plot for the reaction of I- with S& in an AOTIdecanelwater microemulsion. Reaction conditions: molar conin water, 0.1 M and 4 x lo4 M, respeccentration of I- and s,~$tivelv: molal concentration of 0.1 m AOT in decane: o = 15:
thesamewayas homogeneouslydistributedions; theplausibility ofthis will be discussed further below. Ionic Strength Assuming total ionization of AOT (23,24), the mntribution of the surfactant to p* is
Figure 5 shows that log k depends linearly on the value of p& thus calculated. This is predicted by substituting p & ~for p in the Guggenheim-Giintleberg equation, In ( [I-] 1 M )
Figure 3. Determinationof reaction order with respect to iadide forthe reaction of I- with s50;-in AOTIdecanelwater microemulsion.Working conditions similar to those specified for Figure 2. tion of AOT in decane. Typical results are presented in Figure 4 and show the following. .For a given surfactant wncentration, the reaction rate increases as o decreases, that is, as the size of the water dmp-
lets decreases (see Fig. lb). 'For a given value of w the reaction rate is independent of
surfactant concentration. The last statement supports the hypothesis that the reaction occurs exclusively in the aqueous core of the droplets (21, 22); The influence of the microemulsion that is reflected by the dependence ofk on w (Fig. 4) may be hypothesized as possibly due, among other things, to a saline effect of the ionized surfactant. This contributes Na+ions to the aqueous core and envelops this core with a layer of surfactant heads bearing negatively charged S O ; groups. Some care must be taken with the exact meaning of this hypothesis because it is not obvious how one can calculate the contribution of the "fixed" -SO3 groups to the ionic strength of the medium (the aqueous core) and hence to the activity of the reagent ions. We shall nevertheless investigate the hypothesis in terms of an "ionic strength" p* to which "fixed" ions contribute in
Figure 4. Dependence of k, the rate constant of the reaction of Iwith s,~: in an AOTIdecanelwater microemulsion on the ratio o = ~,,JnAoTat 298 K. Molal concentration of 0.2 m AOT in decane. The same kvalues are obtained with molal concentration of 0.1 m AOT in decane.
because the second term on the righehand side of eq 18is practically negligible in comparison with the third a t high ionic strength (25,261. Thus, it appears that p* is a "valid" ionic strength for the aqueous core, in the sense that it allows prediction of kinetic behavior via the same equations as the standard ionic strength for bulk aqueous media, p. This is probably because the interface is a highly dynamic structure--far from being a rigid wall separating aqueous and organic phases. The S O ? groups "fixed" to the surfactant envelope are in fact continually entering and withdrawing from the inner regions of the aqueous core, which becomes strikingly apparent when the percolation phenomenon gives rise to a sharp change in the conductivity of the microemulsion (27). Comparisons with ConventionalAqueous Media The correlation of p*data (Fig. 5) can in general only enable prediction of reaction rates in the same type of medium. (In particular, it does not predict the reaction rate in conventional aqueous media.) Likewise, the GuggenheimGiintleberg equation does not allow reaction rates in microemulsions to be predicted on the basis of data for the conventional ionic strength and reaction rates in conventional aqueous media. It does nevertheless seem reasonable to compare the rate ohserved in conventional mehum with the value obtained by extra~olatina - the rnrrelation of Figure 5 to p l m = 0, a point a t which there is no micmemulsion (k = 4.7 x 103 molY1 dm3 s-' 1.
AOT
Figure 5. Effect of p& (seetext) on the rate wnstant of the reaction - AOTIdecanelwater microemuision at 298 K. of I- with ~ ~ 0in; an Molal concentration of 0.2 m AOTin decane. Volume 71 Number 5 May 1994
449
Series of experiments in which the reaction was carried out in conventional aqueous medium with the same concentrations of reagent; and a t the same temperature gave the value k = 3.0 x lo3 mol-' dm3 s-'. The microemulsion structure therefore appears to have an effect on the reaction rate in addition to the effect mediated by its influence on the ionic streneth of the medium. The neeativelv ~ the anionic reage& in thk charged heads of t h e - ~ orepel droulet core. thus increasine their local concentration and he&e the reaction rate.
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Extension An extension of the work desaihed above would be to cany out experiments at several temperatures between 285 and 303 Kin order to calculate activation Darameters. (Arrhenius' equation is followed over this range.) The obsewed increase in activation energy with increasing w is readily explained as due to the increased height of the potential barrier a s solvation increases with o. Conclusion The experiments suggested here are just a few of the possible illustrations of the new horizons o ~ e n e dto the chemist by the advent of microemulsions as reaction medium. In the comina sears, these media will undoubtedly mow in importance in both theoretical and applied fields. It is therefore highly desirable that undergraduates studying physical chemistry become acquaintedwith the associated concepts and experimental techniques.
a
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Acknowledament The authors thank the Spanish Comisidn Interministerial de Ciencia y Tecnologia (CICYT) for financial support
450
Journal of Chemical Education
under Grants AL.19010389 and the Commission of the European Communities for fmancial support under Contract No. 93CVVF1-610-0. Literature Cited 1. Shulman, J. H.; Stoeckenius, W;Rinee, L. M.J. Phw. Chem 1959,63,1677. 2. Damodaran, S. I" Ad". in Food and Nutrition I(r~(rarch;IGnaeus, J. E., Ed.; Aeademic: San Diego, 199%Val. 34. 3. Pabinsm,B. H. Cham. Bz 1990,26,542. . k J. Am. Chem. Soc 1972,%, 4. Fendler,J H.; Fendler, E. J.:Medaty, R. T.: Woods, V 7288. 5. la) O'Canor, C. J.; Fendler, E. J.: Fendler. J. H. J.Am. Chem. Soe, lW3,95,6a). (bl IbM 1974,96,370. J.; C1ark.B. J. Chrm. Soc. Chem. Commun. 1983,659. 6. B1andamer.M. J.;B~ug~ugs, M. A.:Puvas,J. InSfruefum, Eyzamics and EquflLriumPmperties 7. L6pez Qintela, ofColloi&l Systems; Bloor, D. M.; WynJones, E.. Eds.: Kluwer Academic Dordreeht, 1990. 8. Lianoa, P.; Thomas, J. K J. Colloid2nferfueSd. 1997, 227, 505. 9. El-Nokdy, M. A.: Cornell, D. Eda.; MicmamuSions and Emulsions in Food+ ACS Smpoaium Series No. 448, Am. Chem. Sac: Washington. DC,1991. 10. Fletcher, P 0. I.: Parmtt, J, J. Chem. Soc, Fomdoy k m . II988,84,1131. 11. GoWen, K. E.; Hatt0n.T. A . S q . Sci. lkhnol. 1987,22,831. 12. Peplasse, J.;Boned, C. J Phys. Chem. 1985,89,370. 13. Izqluerdo, C.: MoyB, M. L.: Usem, J. L.;Casado, J Momtsh Cham. 1992,123,383. J.V.J.ChemSocFomdnyI)avL1. 14. Dsy,R.A.;Pabinson.B.H.;Clarke,J.H.:Doherry. 11819. 75.132. 15. Pabinson, B. H.; Toprakdoglu. C.; Dore, J. C.; Chiem, P J. Chem. Sae. Fernday ks.1 1984,80,13. 18. Wone. M.: Thomas. J. K: Nowak. T J. Am. Cham Soc 1977.99.4730,
21. MgV6, M.L.:Izquierdo, C.;casado, J. J ~ h y s c h e I991,95,6001. ~. 22. Izquierdo, C.; Casado, J.; Radflguez, A,; MoyA, M. L.1°C J Chem. Kind. 1992.24, 19. 28. h i s , E. S.;Pottr, J. E., JI J A m . Cham. Soc 1941,63,2883. 24. Ahmed, M. G.;Uddin,F.;Jam, J.A.:Pmk,J. J. Sci. I n d R e s 1919.22, 128. 25. Dun", M. H.; Kozak, J. J. J. Chem. Phya. 1982, 76,984. 26. Brunh.H.:Nkan.S:Holzwalt.J. F FrcdavDiscws. Chem Soc 1982.74. 129.
