Reaction kinetics as a probe for the structuring of microemulsions

Colloids and Surfaces, 35 (1989) 237-249 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

237

R e a c t i o n K i n e t i c s as a P r o b e for the S t r u c t u r i n g o f Microemulsions C. MINERO 1,E. PRAMAURO1and E. PELIZZETTI2 IDipartimento di Chimica Analitica, University of Turin, Via P. Giuria 5, 10125 Torino (Italy) 2Istituto Chimica Fisica Applicata, University of Parma, 43100 Parma (Italy)

(Received 31 December 1987; accepted 23 December 1988) ABSTRACT An electrontransferreactionbetween two polycharged anions isstudiedin the presence of SDS/ 1-butanol/toluene/brinemicroemulsions. A simple three-pseudophase equilibrium model is able to predictthe observed ratesin regions of the phase diagram where the homogeneous solutionis structured.The comparison of kinetic data and the predictionof the model gives information on the hydration of the surfactantand of the cosurfactant,and revealsclearlyiffree-waterdomains are present in solution.

INTRODUCTION Microemulsions are t r a n s p a r e n t isotropic thermodynamically stable dispersions of two, or more, immiscible liquids obtained with an emulsifier mixture, i.e. a surfactant and often a cosurfactant [1-4 ]. In the single phase composition range a variety of structures can exist, such as direct micelles ( O / W ) , reverse micelles ( W / O ) , molecular solutions or bicontinous phases. However, the detailed structure of microemulsion systems is still not precisely defined. Experimental investigations are currently being undertaken using physicochemical methods, such as density and heat capacity measurements [5,6], light scattering [ 7,8 ], neutron scattering [9,10 ], conductivity [ 11,12 ], nuclear magnetic resonance [ 13-15 ], fluorescence probes [ 16 ] and polarographic methods [17]. T h e phase diagrams for a lot of quaternary systems are reported [2-4]. Some theoretical approaches based on the analysis of microscopic forces between structures and molecules [18-22], or on t h e r m o d y n a m i c treatments [23,24] can rationalize the phase diagrams and the supposed structure of microemulsions. T h e large interest in m a n y practical applications of microemulsions, from oil recovery [25,26 ] to microemulsion polymerization [27 ], has stimulated the studies on the reactivity in these systems [28,29 ]. In a preceding paper [30] we started the analysis of reactivities in micro0166-6622/89/$03.50

© 1989 Elsevier Science Publishers B.V.

238 emulsions with the aim of understanding the dependence of the reaction rate and equilibria on the microemulsion composition and on the hydrophobicity of reactants. A simple thermodynamic approach based on the three-pseudophase approximation and on the proper definition of partition coefficients of the reactants, was able to predict the reactivity of two benzenediols of different hydrophobicity with an anionic oxidant. The proposed three-pseudophase model was based mainly on the supposition about the existence of supramolecular structures in microemulsions and on some hypotheses about the partition of microemulsion components in the pseudophases. The kinetic data and the upper demixing line in the pseudoternary phase diagram at constant surfactant/cosurfactant ratio were well fitted by taking into account the hydration of the surfactant in the interfacial region. These hypotheses are further tested in the present work. An electron transfer reaction between two polycharged anions (IrCl~- and CoW120~o- ) was carried out in the same microemulsion medium of the preceding paper, i.e. SDS/1-butanol/toluene/water + 0.09 M NaC1 + 0.01 M HC1. The charge of the surfactant and the charge and low hydrophobicity of reagents ensure that the reaction would take place in the water pseudophase, and thus that the reaction rates would be sensitive to the "effective volume" of the aqueous region. The reactants play the role of local probes in the microemulsion medium and thus it will be possible to infer some information about the structure of the microemulsion in the surfactant-rich or in the oil-rich regions (Xwat~r< 0.4). EXPERIMENTAL Materials Sodium hexachloroiridate (IV) was supplied by Alfa. Sodium dodecyl sulphate (SDS) was purchased from Merck and used as received.Toluene and 1butanol were used as received from Carlo Erba. All reagents were the purest available. Sodium hexachloroiridate (III) was prepared [31] from hot filtered ethanolic solution of the oxidized product (1 g of Na2IrC16" 6H20 in 60 ml of absolute ethanol) by reduction with sodium nitrite (250 mg NaNO2 in 15 ml of absolute ethanol and 2 ml of water). The crude crystalline green powder was then collected on a 0.5/~m Durapore membrane (Millipore), washed with hot anhydrous ethanol, and air-dried. The visible absorption spectra of Na3IrC16- 2H20 and that of the corresponding reoxidized solution coincided with those reported [32 ]. Potassium hydrogen 12-tungstocobaltoate (II) was synthesized by following literature methods [33 ]. The UV-VIS absorption spectra of KsH [Co (II) 04 (W03) 12]" 17H20 (MW = 3408) are in agreement with the data reported [34]: e6~' =215, e42o=65 and e~ss= 130 M -1 cm -t.

239 Solutions of potassium 12-tungstocobaltoate (III), later referred to as compound B, were prepared by oxidation of a 1-10 -2 M solution in 0.02 M H2SO4 with solid Pb02, and then filtering on a 0.2/~m Millipore filter. The resulting yellow solution was stable for at least one month and gave specific absorption e625~0, ~42o=920, emax ~88 = 1230 M -1 cm -1, in good agreement with the literature data [34,35].

Microemulsion preparation The pseudoternary phase diagram of the quaternary system SDS/1-butanol (at constant mass ratio = 1: 2)/toluene/water + 0.09 M NaC1 + 0.01 M HCI was determined experimentally and was found to agree with that reported in the preceding paper, except for the upper demixing line which had shifted a little toward the emulsifier mixture/oil side. The actual phase diagram is in good agreement with those previously reported in the literature [36,37 ]. The difference can probably be attributed to the purity of the SDS used. Microemulsions of different compositions were prepared by weighing the appropriate amount of stock solution of emulsifier mixture (25% SDS + 50% 1-butanol + 25% 0.09 M NaCI/0.01 M HC1/water) with toluene, and then diluting the resulting transparent homogeneous solution with 0.09 M NaC1/0.01 M HCl/water (later referred to as solution C). A series of microemulsions characterized by the SDS/1-butanol/toluene ratio {dilution line 0 = 0.33: 0.67: 0.0, dilution line 1 = 0.3: 0.6: 0.1, dilution line 2 = 0.25: 0.5: 0.25, dilution line 3 = 0.2: 0.4: 0.4, dilution line 4 - 0.15: 0.3: 0.55, dilution line 5-- 0.11: 0.22: 0.67) were then used in kinetic experiments.

Apparatus The UV-VIS absorption spectra were recorded on a Cary 219 Varian spectrophotometer. Kinetic experiments were carried out on a HI-TECH stopped flow spectrophotometer equipped with an Apple II Europlus personal computer and a home-made program for fast data acquisition and calculation of the observed rate constant. The reaction progress was followed at the wavelength of maximum absorbance of IrCl~- (487 nm, e = 3950 M - 1 c m - 1) [ 32 ]. The two solutions used for the rapid mixing were always of the same composition, except for the reactants, in order to avoid bubble formation and incomplete mixing due to different viscosities. The initial concentration of reactants were: [IrCl~- ] = 1-10 -5 m and [B ] -- 0.8-1.5-10 -4 m. Good pseudo-first-order kinetics were observed for all microemulsion compositions. The experiments were all performed at 25.0 +_0.1 °C.

240 RESULTS AND DISCUSSION The pure exponential decay of the absorbance of a reactant as a function of time under pseudo-first-order conditions suggests that reagents are exchanged between the pseudophases at a rate much higher than that of the reaction itself. Under this hypothesis the observed second-order constant, kobs ( M - 1 s-1 ), for a bimolecular reaction that takes place in f pseudophases is: [Af] [Bf] kob, = ~f 0f'kr [Atot] [Btot]

(1)

where [Rf] and [Rtot] are the local and analytical molar concentrations of the reactants R, kf are the specific second-order rate constants in the pseudophases, and 0f the pseudophase volume fractions. When three pseudophases are considered and the partition coefficients of reagents are defined in terms of molar fractions, and when, for experimental convenience, weight fractions are used, Eqn (2) is obtained [30]: , t A B , A B (kw/Xw) + (k~/Xi)Pwi Pw~ Si2 + (ko/Xo)Pwo Pwo So2 kexp(l+PA S~+pAwoSo) (l+pBw~Si+PBoSo)

(2)

where kexp is the observed second-order rate constant in m -1 s -~ units; k~ = kf. Sf ; Sr and Xr are the pseudophase densities and weight fractions, respectively; PwRfare the partition coefficients for transfer of R from water to the pseudophase f; subscripts f, w, i, o mean generic, aqueous, interfacial and oil pseudophases, respectively; Si= (Ej ? ~ j ) i / ( ~ j n j ) w and So= (~j n j ) o / ( ~ j n j ) w where n is the number ofj molecules in the pseudophase. The quantities S~ and So, and the pseudophase weight fractions Xf are calculated by considering the partition of microemulsion components in the pseudophases. The number of molecules of a j species in a pseudophase is assumed to be related to that of the other constituents through proportionality constants or solvation numbers. In the preceding paper a detailed discussion of these hypotheses is reported. The state of water in W/O microemulsions has been the subject of many investigations. All these studies conclusively showed that at low-water content all the water is bound to the surfactant head groups, to counterions and alcohol [38-40]. In line with these findings, the upper demixing line (emulsifier mixture/oil side) of a pseudoternary phase diagram, at constant weight ratio H of cosurfactant/surfactant, is in the present model the locus of points where the number of water molecules in the aqueous pseudophase is zero. This condition leads to Eqn (3): K X c = (1--Xoil)

-

-

(K+I)

(3)

241

where Xc and Xoi I a r e the experimental weight fractions of solution C and of the oil, respectively. K=(a2+c~3"~4) M W w / [ ( H + I ) MWsDs]; ~2 is the number of water molecules per molecule of SDS, ~3 is the ratio of water to 1butanol molecules, and ~4 that of 1-butanol to SDS molecules in the interface. For the quaternary system under study the solvation numbers ~2, ~ and a4 estimated by using Eqn (3) were quite reasonable [30]. In particular it was found that both experimental kinetic data and the upper demixing line were well fitted if it is assumed that the interfacial pseudophase was constituted by the surfactant, almost all the cosurfactant (~4"-7.5 for H = 2 ), and by about 5-7 molecules of water per molecule of surfactant [41 ]. For the pseudoternary phase diagram determined experimentally in this work a value of ~2 = 4-5 was found. ~3 was evaluated taking the value of the solubility of water in 1-butanol. Equation (2) can predict the experimental kinetic behavior as a function of the microemulsion composition (Xc, Xs, Xoil). Here subscripts refer to microemulsion constituents, i.e. to solution C, to emulsifier mixture 1-butanol/ SD S = 2 w/w, and to oil, respectively. If part of the total water content (in the following called the water of hydration) is in the interfacial pseudophase, Xw will be less than Xc and a reaction between very hydrophilic ions will be strongly dependent on the "effective volume" of the aqueous pseudophase. Figure 1 gives the relative rate, predicted by Eqn (2), versus Xc for single phase microemulsions which have an oil content ranging from 0 (dilution line 0) to that

6o

4o

2O

J

1..................... 1

.8

.6

.4

.2 Xc

Fig. 1. Relative rate in the microemulsion calculated with Eqn (2) as a function of the fraction of added aqueous solution. (a) pAi =P~Si =0; pAo =P~o =0. (b) PwAi=P~i =0.1; PAo =PwSo=0. (c) pAi =PBw i=0.5; pAo =PwBo =0. The dashed areas correspond to the regions of the pseudoternary phase diagram in between the dilution lines 0 and 5; values of ~ used in the calculation are fitted on the upper demixing line and are reported in the text.

242 corresponding to the dilution line 5. The values of hydrophilic-reagent partition coefficients have a marked effect. They are related to the usual binding constant K~f ( M - 1) by: P~f = 55.5 (Kwf+v) R

(4)

where v is the specific molar volume of the interface. When both reactants have very low partition coefficients and are thus completely in the aqueous pseudophase, the increase of the reaction rate is due to the dependence on the reciprocal of the aqueous pseudophase weight fraction. Divergence to infinity is obtained for Xw-~0, or, in other words, where the microemulsion demixes. If the water of hydration is not accounted for, the rate diverges at Xc-,0. The choice of the test reaction was based on the above discussion and on the following considerations: (i) a marked change in the observed rate as a function of the microemulsion composition is desirable in order to minimize the experimental uncertainty. This condition forces us to choose a reaction that takes place in the aqueous pseudophase, and, as a consequence, a reaction between two polycharged anions with very poor hydrophobicity; (ii) the reaction should have k~ as low as possible, in order to be able to follow the kinetics even when a marked increase is observed. Thus we tested a number of electron transfer reactions between negatively charged ions which, according to the Marcus theory [42 ] and the literature data [43 ], should satisfy the mentioned prerequisites. The reactions of all the redox couples between IrC12-/3-, Mo (CN)s3-/4-, COW12 0450/6-, Os (CN) 63-/4- and Fe (CN)~-/4- were checked and the most promising one was found to be the reaction between hexachloroiridate (III) and 12-tungstocobaltoate (III): IrCl~- + [Co(m)O4(WO3)12]x--,IrCl~ - + [Co(toO4 (W03)12 ] (x--l)--

(5)

This reaction has already been investigated in the literature [35,44]. The experimental rate constant in solution C is kc = 5.24 + 0.28-103 M - 1 S--1 and is the lowest one between the reaction of redox couples mentioned. This value is consistent with the reported small difference in the redox potential of the reagents [43 ], and with the self-exchange rate of the 12-tungstocobaltoate and hexachloroiridate couple [44 ]. The dependence of reaction (5) on the added NaC1 at pH 2 (HC1) is shown in Fig. 2. The data is surprisingly well fitted by a conventional Bronsted-Scatchard equation [45 ] (correlation coefficient -- 0.998 ), giving values of the ionic charge product ZAZB= 3.32 _+0.10 and mean ionic diameter a = 4.0 + 0.2 ,h,. Considering that, at the high ionic strength used and at the actual pH, hydrogen and cation association can easily occur, these values are consistent with the dimension and the effective charge of the polycharged anions. The validity of the Bronsted-Scatchard equation may be doubtful in these conditions. Yet, for

243

0

f

e"

lO ~J / /

o

I

I

I

I

I

I

1

2

3

4

5

6

|

[NaCl]

M

Fig. 2. Relative rate constant ki/hc of reaction (5) as a function of the salt concentration at pH 2.0. The absolute value of kc is reported in the text. The breakpoint indicated by s is the solubility of NaC1 in water. The dashed curve is fitted to experimental data by using a Brensted-Scatchard equation (see text). 1.2 J<~ 1.0

• .... •__

l .....

•...

.8 .6

I

I

I

1

2

3 %w/w

I

!

4 5 I-butanol

I

~

6

Fig. 3. Relative rate constant kbut/k C of reaction (5) as a function of the 1-butanol content. The absolute value of kc is reported in the text. the purpose of the pr e s ent discussion, only an equation t h a t describes t he dependence of reaction (5) on the added salt c o n c e n t r a t i o n is needed. T h e addition to the solution C of 1-butanol slightly inhibits t he reaction rate. T h e second-order rate c ons t ant s are given in Fig. 3 as a function of t he but anol content. T h e observed decrease is comparable with t h a t not ed for other reactions in the presence of alcohols [46]. T h e experimental rate cons t a nt s as a function of the microemulsion composition are reported in Fig. 4, where the dilution lines correspond to a fixed ratio between S D S / 1 - b u t a n o l / t o l u e n e (see experi m ent al section). It can be n o t ed t h a t at low water c o n t e n t t her e is an increase in the reaction rates with respect to the aqueous solution, which is d e p e n d e n t on t he oil content. For dilution lines with high oil c o n t e n t the rate diverges asymptotically when t he

244 3

4

ol211P

:: j / / s

100

,

, : i

;:ii

, , :

2

i

'-i

50

i

i

0.8

i

i

0.6

i

i

I

OA

I

02

i

o

Xc

Fig. 4. Experimental (points) and calculated (dashed and dotted lines) rate constants in a microemulsion for dilution lines 0-5 versus the aqueous weight fraction Xc. ( © ) Dilution line 0; ([~) dilution line 1; ( A ) dilution line 2; ( 0 ) dilution line 3; (m) dilution line 4; (&) dilution line 5. The fit curves are calculatedwith the followingparameters in Eqn (2): P~ = 0.15, P w oA B B 0, Pwi =0.02, Pwo =0, where A=IrCI~- and B=CoW120~. The values reported in the text for the degree of dissociation of the surfactant and for solvation numbers were used. upper demixing line is approached. In the absence of oil (line 0) the increase is quite low and it is comparable with the increment due to the ionic strength (see Fig. 2). The rate constants measured along dilution lines 1 and 2 have a behavior intermediate between line 0 and lines 3-5. The experimental rate constant trend is quite similar to that expected for low partitioning of reactants in the interfacial and oil pseudophases (compare with Fig. 1 ). Yet, the rate of reaction (5) is dependent on the ionic strength and the fit of data with Eqn (2) requires some further investigation. Since an ionic surfactant is used and some water is immobilized on it, the pseudophase modelling of the microemulsion requires that the effective ionic strength in the aqueous pseudophase should differ from t h a t of solution C. The calculation of the effective ionic strength (/~) is a difficult task, since the system is very far from a dilute limit. Yet, an approximate value of the ionic strength in the aqueous pseudophase can be estimated by considering the volume of the aqueous pseudophase from the number of "free" water molecules and calculating the local concentration of the salt and of the counterions dissociated from the surfactant. The surfactant degree of dissociation fl is treated as an additional constant parameter in Eqn (2) under the hypothesis that the alcohol and ionic strength contributions of fl balance together. W h e n the microemulsion composition is changed, the actual aqueous pseudophase ionic strength is calculated, and the value of k~ (/~) is estimated by using the Bronsted-Scatchard equation obtained from the fit of data in Fig. 2.

245

Equation (2), even when ionic strength contribution is taken into account, is not able to predict the observed kinetic behaviour for all microemulsion compositions if the ~ values are taken as constant. Since dilution lines 3-5 show the trend predicted by the model, the fit was made only on these ones. The fitting curves are shown in Fig. 4. The value ofP~i = 0.15 ( A = I r C I ~ - ) is comparable with that of the hexachloroiridate (IV) evaluated in the preceding paper, while the partition coefficient of the 12-tungstocobaltoate (III) anion (referred to as B) is found to be slightly less (PBi =0.02). This value is consistent with the anion charge and its very low hydrophobicity, i.e. its large solubility in water [33]. The simultaneous fitting of the upper demixing line and of the kinetic data on dilution lines 3-5 require that the degree of dissociation of the surfactant will be quite high, i.e. fl= 0.55, but still reasonable if almost all the alcohol lies in the surfactant palisade of the interface (c~4= 7.5 ). In fact, the value of fl for micelles increases when alcohol is added [47-49]. The good fit of data on dilution line 3-5, where according to literature suggestions [49] the microemulsion is structured, and the agreement with previously reported considerations [30] on the experiments with 1,2-benzendiols support the proposed model and confirm the crucial role of the water of hydration. By considering the conductivity data reported in Fig. 5 and the fit showed in Fig. 4, the pseudoternary phase diagram can be divided into three regions, corresponding to the microemulsion components at the corners. (a) High oil content single-phase domain between dilution line 3 and the oil

I/x 2 /

.., ~,~

'% It .5

Xc

Fig. 5. Experimental conductivity (10 4 ~2- ~ c m - ~) normalized with respect to the aqueous phase weight fraction, versus the aqueous weight fraction Xc. Symbols refer to dilution lines as in Fig. 4.

246

corner. Experimental investigations using laser light scattering [8,37,50 ], conductometry (Fig. 5 ), and refractive index measurements [ 51 ] suggest that microemulsions are formed by water globules dispersed in an oil matrix (W/O microemulsions). The scattering intensities begin to increase rapidly when the microemulsion water content exceeds a value corresponding to a water/ionicsurfactant molar ratio of about 10. In this work it was found that O~2-bO/3"OQ---~12.

These findings correspond to the hypothesis followed in the formulation of the three-pseudophase model, i.e. that water is effectively bound to the surfactant. The increase in the scattered intensity reflects the formation of water domains in the oil. The kinetic data on dilution lines 3-5 confirm this idea. Since the observed rate is higher than in homogeneous solution, the reactants have to be locally much more concentrated than in the whole solution, or, equivalently, are held in a narrow water domain. In the regions in between oil and brine corners it was found, using an N M R technique [52], that hydrocarbon self-diffusion is rapid and comparable to that of water, indicating that the structure, on the time scale of the self-diffusion measurement, is bicontinuous, i.e. there are no closed water or oil droplets. In the present model the structuring of the solution is defined as the presence of different homogeneous spatial regions over the time scale of the reaction. Thus the model is not able to distinguish between reversed micelles or bicontinuous phases. Since the model does not take into account the geometrical configuration of the structure, it works quite well in fitting the kinetic data in the region of supposed bicontinuous structures. (b) High water content region. The decrease of the observed rate for the reaction of IrCl~- and 1,2-benzenediols suggests that in the microemulsion there are micelles or swollen micelles. The same conclusion is supported by conductivity data [49]. The initial increase in the conductance (see Fig. 5) reflects the progressive ionization of the ionic surfactant molecule and the formation of dissociated micelles, to a value where, because of the fall of the dielectric constant due to the increasing amount of 1-butanol, the conductance reaches a maximum. (c) High emulsifier mixture content region. Data for solubilization of shortchain alcohols in SDS micelles [53 ] and conductivity measurements [49 ] suggest that the alcohol causes a breakdown in the micellar aggregates, and that the micelles are completely converted into dissociated monomers. The kinetic data on dilution lines 0-2 strongly support this hypothesis. The enhancement in the specific rate constant along dilution line 0 is of the order of that observed at higher ionic strength reported in Fig. 2, and is consistent with the idea that the surfactant behaves like a 1 : 1 polyelectrolyte. Nevertheless, the observed increase of rate constants along the dilution line 0 is slightly higher than that measured at the solubility limit for NaC1 in water. This fact forces one to think that, even in a very limited extent, the solution is

247 still structured, and that the failure of the model can be ascribed to incorrect values of c~. In other words the c~values obtained by fitting the upper demixing line are no longer valid. Since only a limited increase in the intensity of the scattered light is reported in analogous systems along dilution lines in approaching the the emulsifier mixture corner [8], we conclude that the "molecular dispersion" or the "normal solution" consists of small hydrated surfactant n-mers (n = 2-10 ), solvated by and dispersed in a matrix of alcohol. Yet, the kinetic data on the oxidation ofbenzenediols [ 30 ] were fitted by the model also along dilution lines 0-2. The disagreement with the actual experiment can be overcome by assuming that more hydrophobic probes induce a local organization around them of the surfactant and of related water and alcohol molecules, or prefer the small n-mers as solubilization sites. From the kinetic point of view, the hydrophobic probes can prove the structuring of the solution which is induced by themselves. However, this is not the case for reagents in reaction (5), which probe only the extent of the free-water domains. Finally it can be noted that, in Fig. 4, by increasing the oil content (dilution lines 1 and 2 ) there is no sudden change in the kinetic behaviour, and so in the change of the microstructure discussed for dilution line 0 and that of line 3 (W/O microemulsion ). The influence of the oil content on the structuring has already been stressed and multiple equilibria involved in the association process of microemulsion constituents were supposed [8 ]. By following these suggestions and the previous discussion it can be postulated that at low oil content the surfactant is solubilized in the alcohol, and that, increasing the oil content, the surfactant (which is insoluble in oil) is gradually pulled out from the alcohol-oil matrix. In this way, as pointed out in the literature [4], the interface is progressively formed, leading to a gradual change in the microscopic structure as the oil content increases. CONCLUSIONS The proposed three-pseudophase model is able to predict quantitatively the rate of reactions in the microemulsion, in regions of the phase diagram where the solution is structured. The partition coefficients for reagents, the hydration number of the surfactant, and also its mean degree of dissociation agree quite well with reported or estimated values obtained using other techniques. Some explicit hypotheses about the partition of microemulsion components between the pseudophases are introduced in order to simplify the calculations, although more complete three-pseudophase models are reported in the literature [24]. The comparison between kinetic data for a proper reaction and the prediction of the model, makes a clear distinction between microemulsion compositions which lead to structured and non-structured systems, and allows us to obtain some information on the hydration of the surfactant and of the cosur-

248 factant without assuming any microscopical geometrical configuration of the solutionl ACKNOWLEDGEMENTS

We are grateful to CNR, EniRicerche and European Research Standardization Group under Contract DAJA 45-85-C-0023 for support of this work.

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