1.5-Order Differential Electroanalysis on Triton X-100 Microemulsion

The diffusion coefficients of probes in Triton X-100 microemulsion microenvironment were measured. The phase inversion from water-in-oil to oil-in-wat...
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Langmuir 2002, 18, 4047-4053

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1.5-Order Differential Electroanalysis on Triton X-100 Microemulsion Chunsheng Mo Department of Chemistry, Zhanjiang Normal College, Zhanjiang 524048, People’s Republic of China Received November 13, 2001. In Final Form: March 19, 2002 By use of trace amounts of the electroactive probes and low supporting electrolyte concentrations, 1.5order differential electroanalysis is used as an effective method for investigating the physicochemical properties of aqueous Triton X-100 solutions. The diffusion coefficients of probes in Triton X-100 microemulsion microenvironment were measured. The phase inversion from water-in-oil to oil-in-water microemulsions in the Triton X-100/C6H4(CH3)2/H2O system over the entire single-phase microemulsion region was examined by 1.5-order differential electroanalysis. The results were found to be in agreement with the results of conductivity measurements.

Introduction Electrochemical techniques, such as polarography, rotation disk voltammetry (RDV), chronocoulometry (CC), cyclic voltammetry (CV), and so on, popular tools for colloid and surface chemists, have been successfully applied in the study of physicochemical properties of aqueous surfactant solutions.1-22 In our recent articles,23-26 we employed the cyclic voltammetric method to investigate properties of aqueous surfactant micelle solutions and to identify microstructure and structural transition of microdroplets in microemulsion systems using an oil-soluble ferrocene and a water-soluble potassium ferricyanide as the electroactive probes. In addition to being less sophis(1) Rusling, J. F. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O. M., Eds.; Plenum: New York, 1994; No. 26, pp 49104. (2) Texter, J.; Horch, F. R.; Qutubuddin, S.; Dayalan, E. J. Colloid Interface Sci. 1990, 135, 263. (3) Mandal, A. B.; Nair, B. U.; Ramaswamy, D. Langmuir 1988, 4, 736. (4) Zana, R.; Mackay, R. A. Langmuir 1986, 2, 109. (5) Rusling, J. F.; Hu, N.; Zhang, H.; Howe, D.; Miaw, C.-L.; Couture, E. In Electrochemistry in Colloid and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH: New York, 1992; pp 303-318. (6) Rusling, J. F. Electrochemistry in micelles, microemulsions, and related organized media. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1994; Vol. 19, pp 1-88. (7) Miyagishi, S. Langmuir 1998, 14, 7091. (8) Rusling, J. F. Langmuir 1997, 13, 3693. (9) Santhanalakshmi, J.; Anandhi, K. J. Colloid Interface Sci. 1995, 176, 226. (10) Nassar, A.-E. F.; Willis, W. S.; Rusling, J. F. Anal. Chem. 1995, 67, 2386. (11) Rusling, J. F.; Nassar, A.-E. F. J. Am. Chem. Soc. 1993, 115, 11891. (12) Geetha, B.; Mandal, A. B. Langmuir 1995, 11, 1464. (13) Onuoha, A. C.; Rusling, J. F. Langmuir 1995, 11, 3296. (14) Rusling, J. F. Langmuir 1996, 12, 3067. (15) Mandal, A. B. Langmuir 1997, 13, 2410. (16) Zhang, S. P.; Rusling, J. F. J. Colloid Interface Sci. 1996, 182, 558. (17) Vijayalakshmi, G. Langmuir 1997, 13, 3915. (18) Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1982, 136, 353. (19) Mandal, A. B. Langmuir 1993, 9, 1932. (20) Mandal, A. B.; Nair, B. U. J. Chem. Soc., Faraday Trans. 1991, 87, 133. (21) Georges, J.,; Berthod, A. J. Electroanal. Chem. 1984, 175, 143. (22) Herrero, R.; Barriada, J. L.; Lopez-Fonseca, J. M.; Moncelli, J. R.; Sastre de Vicente, M. E. Langmuir 2000, 16, 5148. (23) Mo, C. S.; Huang, Z. Z.; Zhong, M. H. Chin. Chem. Lett. 1995, 6, 415. (24) Mo, C. S.; Kochurova, N. N. Mendeleev Comm. 1999, 6, 243. (25) Mo, C. S. Chin. Chem. Lett. 2000, 11, 271. (26) Mo, C. S.; Zhong, M. H.; Zhong, Q. J. Electroanal. Chem. 2000, 493, 100.

ticated and generally less time-consuming, electrochemical measurements are useful for characterizing organized surfactant solutions. But, we feel, a limitation of these methods lies in the fact that the concentration of an electroactive probe and/of the supporting electrolyte in solutions should be moderately high because the internal resistance of solution is high in the presence of surfactant and hydrocarbon. It is a matter of general experience that an electroactive probe is either is oil soluble or water soluble or is an amphiphilic compound. Adding sufficient electroactive probe and the supporting electrolyte into a surfactant solution must cause some changes in properties of dispersion, which makes parameters obtained from electrochemical measurements not be true to the original in a sense. To overcome these shortcomings, we attempt to use a novel techniquesdifferential electroanalysiss for characterizing surfactant solutions. The advantage of this approach over general voltammetric methods lies in its high sensitivity and resolving power to the change of current produced on the working electrode when a chemical reaction occurs. To examine the utility of this approach, a nonionic surfactant, Triton X-100, was chosen as our system because reports on the microstructure of Triton X-100/C6H4(CH3)2/H2O microemulsions are somehow lacking. It is the purpose of this work to demonstrate that 1.5-order differential electroanalysis can be used to investigate physicochemical properties of organized surfactant solutions at very low electroactive probe concentrations and low supporting electrolyte concentrations. Background Theory In cyclic voltammetry, the peak current for a reversible system is described by the Randles-Sevcik equation27

i ) (4.463 × 10-4)nFAc(nF/RT)1/2D1/2υ1/2

(1)

where n is the number of electrons involved in oxidation or reduction, F is the Faraday constant, A is the area of the electrode, c is the concentration of electroactive probe, R is the gas constant, T is the absolute temperature, D is the diffusion coefficient of the electroactive probe, υ is the scan rate, and i is the peak current. It follows from eq 1 that i linearly increases with υ1/2 at a given electrode (27) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry; Ellis Horwood Limited: Chichester, 1985; p 183.

10.1021/la015680d CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

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surface area and a constant probe concentration, a plot of i against υ1/2 is linear, and the diffusion coefficient D can be calculated from the slope of this straight line. The semidifferential electroanalytical method was first introduced in 1975 by Goto and Ishii28 based on the semiintegral electroanalysis method. It measures the semidifferential of current against the electrode potential. In the case of the reversible electrode reaction, the following relationship between the electrode potential, E, and the semiintegral of current, m, applies for a planar electrode and a ramp signal29

m(t) ) (mc/2){1 - tanh[(nF/2RT)(E - E1/2)]} (2) Differentiating eq 2 with respect to time, t, we obtain an expression for the semidifferential of current, e(t)

e(t) ) (n2F2AυD1/2c/4RT)[sech2[(nF/2RT)(E - E1/2)] (3) where E1/2 is the half-step potential. Equation 3 represents a symmetrical peak, the peak height is 2

2

1/2

ep ) (n F AυD c/4RT)

(4)

and the potential corresponding to the peak is simply

Ep ) E1/2

Figure 1. The electrochemical behavior of ferrocene in o/w microemulsion at 25 °C. Sample solution: 0.0020 g of Fc + 1 g of C6H4(CH3)2 + 4 g of Triton X-100 + 6.7 g of H2O (in 0.02 mol dm-3 KCl): (a) 1.5-order differential of current vs electrode potential curves, scan rate 40, 60, and 80 mV s-1; (b) cyclic voltammogram, scan rate 20, 40, 60, 80, and 120 mV s-1.

(5)

Differentiating the semidifferential of current, e(t), with respect to time, t, one can obtain a relationship of e′, the 1.5-order derivation of current, as a function of the electrode potential, E30

e′(t) ) (n3F3Aυ2D1/2c/4R2T2) sech2(x) tanh(x) (6) where x ) (nF/2RT)(E - E1/2). The curve of e′ vs E is comprised of a maximum peak and a minimum peak of current; they are fairly symmetrical in the case of the reversible electrode reaction. Two important parameters for the 1.5-order derivative electroanalysis are Enp′, the negative peak potential, and ep′, the peak height at the Enp′:

Enp′ ) E1/2 - 1.32(RT/nF)

(7)

ep′ ) (0.77n3F3Aυ2D1/2c/4R2T2)

(8)

It follows from eqs 7 and 8 that the negative peak potential Enp′ is independent of the scan rate υ and the concentration c, which can be used to carry a qualitative analysis, and the peak height ep′ is directly proportional to the concentration c, the area of the electrode A, and the scan rate υ2, which is the theoretical basis of the quantitative analysis. A plot of ep′ against υ2 should give a straight line; the diffusion coefficient D can also be calculated from the slope of this line. Experimental Section Chemicals and Reagents. The nonionic surfactant, p-tC8H17C6H4O(C2H4O)nH (Triton X-100, abbreviated as TX 100), was obtained from Shanghai Chemical Reagent Company and was used without further purification. Triton X-100 is a polydisperse preparation of [p-1,1,3,3-tetramethylbutyl)phenyl] poly(28) Goto, M.; Ishii, D. J. Electroanal. Chem. 1975, 61, 361. (29) Dalrymple-Alford, P.; Goto, M.; Oldham, K. B. J. Electroanal. Chem. 1977, 85, 1. (30) Goto, M.; Hirano, T.; Ishii, D. Bull. Chem. Soc. Jpn. 1978, 51, 470.

(oxyethylene) with an average of 9.5 oxyethylene units per molecule. Xylene [C6H4(CH3)2] (A.R. grade) was purchased from Serva Chemicals. Ferrocene (Fc) and potassium ferrocyanide [K4Fe(CN)6] were used as the electroactive probes. Potassium chloride (A.R. grade) was used as supplied. Water used was doubly distilled and deionized. Apparatus and Procedure. 1.5-order differential electroanalysis and cyclic voltammetric measurements on TX100 microemulsion solutions were performed using an electrochemical analyzer (model XJP-821(B), Jiangsu Electrochemical Instruments Factory) equipped with a 3036 X-Y recorder (Sichuan Instruments Factory). A glassy carbon working electrode, a saturated calomel reference electrode (SCE) (all experimental potentials are referred to this electrode), and a platinum counter electrode were used. The glassy carbon electrode was polished using 0.05 µm aluminum oxide slurries and then washed carefully with distilled water. Finally, the electrode was ultrasonicated in triply distilled water for 15 min to dislodge retained aluminum oxide particles on the electrode surface. The working electrode area was determined using cyclic voltammetry experiments on a reversible system (4 mM K4Fe(CN)6 in 1 M KCl). By use of the diffusion coefficient31 D ) 6.3 × 10-10 m2 s-1, the electrode area was A ) 3.3 × 10-6 m2. The potential was scanned between -0.6 and +0.8 V, and the sweep rate range used was 10-120 mV s-1 in this work. The microemulsion conductivity, κ, was measured by means of a DDS-11A conductivity meter (Rex Instruments Factory, Shanghai) equipped with a DJS-1 platinum conductance electrode coated with platinum black. A dip-type cell was used with a cell constant 1.009 cm-1, The uncertainty in the conductivity measurement was within (0.5%. The experimental temperature was maintained at 25.0 ( 0.1 °C using a thermostat.

Results and Discussions The Electrochemical Behavior of Probes in TX 100 Microemulsions. 1.5-order differential electroanalyses on TX 100 microemulsions have been carried out with different water content. Figure 1a shows typical ep′ vs E curves for a given system, with the corresponding cyclic voltammogram, i.e., i vs E curves (Figure 1b), being (31) Stackelberg, M. V.; Pilgram, M.; Toome, V. Z. Electrochem. 1953, 57, 342.

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Figure 2. The plots of (a) ep′ vs υ2, and (b) i vs υ1/2. Sample solution: as Figure 1.

given for comparison. It is clear that the ep′ vs E curves have the analytical advantage over the ordinary i vs E curves. At a relative low supporting electrolyte concentration (0.02 mol dm-3 KCl) and at low probe concentrations, the values of 1.5-order differential of current, ep′, are more sensitive than those of cyclic voltammetric measurements. The plots in Figure 2 are straight lines passing through the origin. Similar results can also be obtained for the K4Fe(CN)6 probe. This fact indicates that the electron transport properties of Fc+/Fc and [Fe(CN)6]3-/[Fe(CN)6]4electrode reactions in the microemulsion medium are diffusion controlled. Although diffusion measurements have become a widely used technique for characterizing surfactant aggregate in solutions, still there is a controversy between “slow” and “fast” theory diffusion. Hussam et al.32 used quasi-elastic light scattering (QELS) and two electrochemical techniques, cyclic voltammetry (CV) and rotating disk voltammetry(RDV), to determine the diffusion coefficient of microemulsion droplet in the CTAB/ 1-butanol/n-octane/water/NaBr system; the differences in diffusion coefficient observed between electrochemical and light scattering measurements were thought to be due to the different modes of diffusion probed by these techniques. The electrochemical techniques yield values of the selfdiffusion coefficients; i.e., diffusion occurs over macroscopic distances larger than the extension of any aggregate in the solution, the diffusion time is very long, whereas light scattering techniques yield mutual (collective) diffusion coefficients. Mandal et al.33 used 0.1 mol dm-3 KBr and KCl in aqueous CTAB and cetylpyridinium chloride (CPC) solutions to determine the self-diffusion coefficient of micelles Dm0 in order to avoid the controversy between “slow” and “fast” theory of the diffusion. However, in the absence of intermicellar interactions, mutual and selfdiffusion coefficients are indistinguishable.34 In the present investigation, we obtained the self-diffusion coefficient as has been observed by Mandal in CV and Fourier transform pulsed gradient spin echo NMR experiments.35,19 Phase Behavior of TX 100/C6H4(CH3)2/H2O ThreeComponent System. The phase behavior of the TX 100/ C6H4(CH3)2/H2O three-component system was represented in a ternary phase diagram. The ternary phase diagram was constructed adopting a simple titration technique: A mixture of TX 100 and C6H4(CH3)2 was first prepared by combining the required mass of two components. In (32) Chokshi, K.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989, 129, 315. (33) Mandal, A. B.; Nair, B. U. J. Phys. Chem. 1991, 95, 9008. (34) Phillies, G. D. J. J. Colloid Interface Sci. 1982, 86, 226. (35) Mandal, A. B.; Geetha, B. Langmuir 1995, 11, 1464.

Figure 3. Phase diagram of TX 100/C6H4(CH3)2/H2O threecomponent system at 25 °C.

this mixture, the initial oil (xylene) content, R, is fixed but may be changed. Water is the titration component; the phase boundary was noted by observing the transition from turbidity to transparency or from transparency to turbidity. By repeating this experimental procedure for other values of R, the boundaries of the microemulsion domain were determined. The content of each component in solutions was derived from precise mass measurements. Figure 3 shows the phase behavior of a TX 100/C6H4(CH3)2/ H2O system at 25 °C. The region marked “µE” is the onephase microemulsion. Winsor is a two-phase region; an excess oil phase is observed in equilibrium with the microemulsion when the initial oil content is increased. It is evident from Figure 3 that a continuous stable singlephase microemulsion region can be observed over the water content range 0-100% when the initial oil content is lower than 40%. This single-phase channel is nicely suited for the study of the microstructure and structural transition of the microemulsion. The Diffusion Coefficient of Probes and Microstructures of the Microemulsion. With a microemulsion system involving an electroactive probe completely solubilized in the microemulsion droplet, the diffusion coefficient D in eq 8 would correspond to the microemulsion droplets diffusion coefficient since the probe diffuses with the microemulsion droplets. In the case where the probe has a difference in solubility between different regions of a microheterogeneous system, however, D in eq 8 is an apparent diffusion coefficient Dapp. Dapp can be assumed to be an average of the diffusion coefficients of the probe in the aqueous phase and in the organic solvent, which constitutes the microdroplets at high water content. It is generally believed that the water-in-oil microemulsion

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Figure 4. The apparent diffusion coefficient of ferrocene as a function of water content (0.02 mol dm-3 KCl) in a single-phase microemulsion region. The initial composition of system is 0.002 g of ferrocene (Fc) + 1 g of C6H4(CH3)2 + 4 g of TX 100 (R ) 0.20).

Figure 5. The apparent diffusion coefficient of K4Fe(CN)6 as a function of water content (0.02 mol dm-3 KCl). The initial composition is 1 g of C6H4(CH3)2 + 4 g of TX 100. CK4Fe(CN)6 ) 4 × 10-3 mol dm-3 (in 0.02 mol dm-3 KCl).

can be formed at low water content and the oil is continuous. With increasing water-to-oil ratio, there must be some kind of structural transition by which the waterin-oil microemulsion inverts into an oil-in-water (o/w) microemulsion. As long as there is no phase separation and the system remains isotropic, the inversion may be a gradual change through a bicontinuous structure. If it were true, there would be three types of microstructure in the TX 100/C6H4(CH3)2/H2O microemulsions: water droplets in oil, oil droplets in water, and a bicontinuous microemulsion. It is possible to identify three different droplet-type structures by comparing the diffusion behavior of electroactive probes in the microemulsion microenvironment. 1.5-order differential electroanalysis was made in the single-phase microemulsion region at different water contents. The changes in the apparent diffusion coefficient of the probes with water content are illustrated in Figures 4 and 5. As can be seen in Figure 4, the apparent diffusion coefficient of Fc decreases with increasing water content over the entire single-phase microemulsion region. At

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water content lower than 33.5%, this decrease is gradual; an abrupt decrease in the diffusion coefficient is observed in the range from 33.5 to 47.5%, and a gently sloping curve is also observed at water content above 47.5%. Similar behaviors are also observed in Figure 5 for potassium ferrocyanide used as the electroactive probe; however, the diffusion coefficients of K4Fe(CN)6 in microemulsions increase with increasing water content. Corresponding inflection points are 34.5 and 48.4% water content, respectively. The difference between the diffusion behavior of ferrocene and potassium ferrocyanide over the same microemulsion region can be explained by the solubility in water and oil. Fc was expected to probe the oil environment because of its limited water solubility. At low water content, a water-in-oil microemulsion is formed, and the oil is the medium. In this case, the diffusion coefficient of Fc was found to be relatively high. In contrast to Fc, the diffusion coefficient of K4Fe(CN)6 in an oil medium corresponds to the diffusion coefficients of water-in-oil microemulsion droplets because K4Fe(CN)6 diffuses with aqueous phase. The diffusion coefficients of both ferrocene and K4Fe(CN)6 with water content ranging from 5.66 to 33.5% (34.5% for K4Fe(CN)6) change slowly. This fact indicates that the microenvironment of microemulsions remains unchanged. A similar behavior was observed in this microemulsion at a high water content (above 47.5% for Fc and 48.4% for K4Fe(CN)6). In the latter case, the oil microdroplets were dispersed in a water medium, and the diffusion coefficient of Fc can be considered as that of oil-in-water microemulsion droplets. However, a dramatic change in the diffusion coefficients of both Fc and K4Fe(CN)6 was observed at water content in the range from 33.5 to 48.4%. This fact is indicative of a change in the microenvironment of microemulsions. In other words, neither water-in-oil nor oil-in-water microemulsions exist in this region. We can suggest that a bicontinuous microstructure was formed,36 in which both aqueous and oil solutions are local continuous phases. As already mentioned, in the case where a probe is distributed sparingly between different regions of a microheterogeneous system, D in eq 8 is an apparent diffusion coefficient Dapp, which depends on the probe equilibria with microstructures of droplets. Rusling et al. have developed several models to describe the dependence of the electrochemically measured apparent diffusion coefficient (Dapp) on concentration (cx) of a micellebound electroactive probe based on multisite binding equilibria.37 Mackay and co-workers employed these models in examination of the effect of probe partitioning on electrochemical formal potentials and differences between diffusion coefficients of hydrophobic probes in microheterogeneous solutions.38,39 According to Rusling’s two-state model of micelles, Dapp can be expressed by eq 9

Dapp ) fwDw + fmDm

(9)

where fm is the fraction of electroactive solute in the micelles and Dm and Dw are the diffusion coefficients of the micelle and the free solute in water, respectively. The fraction of free solute in the water is fw ) 1 - fm. In the (36) Guering, P.; Lindman, B. Langmuir 1985, 1, 464. (37) Rusling, J. F.; Shi, C. N.; Kumosinski, T. F. Anal. Chem. 1988, 60, 1260. (38) Myers, S. A.; Mackay, R. A.; Brajter-Toth, A. Anal. Chem. 1993, 65, 3447. (39) Mackay, R. A.; Myers, S. A.; Bodalbhai, L.; Brajter-Toth, A. Anal. Chem. 1990, 62, 1084.

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Table 1. Results of 1.5-Order Differential Electroanalysis on Ferrocene in an Oil-in-Water Microemulsiona H2O (%)

TX 100 (%)

xylene (%)

cFc/mol m-3

1011Dapp/m2 s-1

103Kd

1011Do/m2 s-1

Rh/Å

48.98 50.50 51.92 53.70 55.36 57.27 59.35 62.41 65.04 68.75 71.43 73.68 76.19 78.26 80.00 82.14 85.29 88.10

40.82 39.60 38.46 37.04 35.71 34.19 32.52 30.08 27.97 25.00 22.86 21.05 19.05 17.39 16.00 14.29 11.76 9.52

10.20 9.90 9.62 9.26 8.93 8.54 8.13 7.51 6.99 6.25 5.71 5.27 4.76 4.35 4.00 3.57 2.95 2.38

1.077 1.046 1.016 0.979 0.945 0.905 0.861 0.798 0.742 0.664 0.608 0.561 0.508 0.464 0.427 0.382 0.315 0.255

4.924 4.552 3.968 3.900 3.616 3.794 3.660 3.155 2.844 2.305 2.238 2.006 1.783 1.381 1.349 1.158 0.866 0.636

4.84 4.84 4.84 4.84 4.85 4.85 4.85 4.86 4.86 4.87 4.88 4.88 4.89 4.90 4.91 4.92 4.95 4.99

4.622 4.249 3.662 3.594 3.308 3.487 3.352 2.844 2.531 1.989 1.921 1.688 1.463 1.059 1.026 0.833 0.537 0.303

53.1 57.7 67.0 68.3 74.2 70.4 73.26 86.3 96.9 123.4 127.7 145.3 167.7 231.8 239.2 294.4 456.6 808.6

a

The initial composition of system is 0.002 g of Fc + 1 g of C6H5(CH3)2 + 4 g of TX 100.

case of an oil-in-water microemulsion, for an oil-soluble electroactive probe (such as ferrocene or its derivatives), one may express the partition coefficient of the probe in a water-continuous medium and in microemulsion droplets as Kd

Kd ) cw/cO

(10)

where cw and cO are the concentrations in moles of the probe in the water and oil phases of the microemulsion, respectively. A relationship relating the apparent diffusion coefficient Dapp to the partition coefficient Kd was obtained by Georges and Desmettre40

Dapp ) (DO + DwKd)/(1 + Kd)

(11)

DO ) Dapp(1 + Kd) - DwKd

(12)

i.e.

The DO so obtained gives the direct measure of the diffusion coefficient of the oil droplet of the microemulsion and can be utilized to estimate the hydrodynamic radius Rh of the droplet via the Stokes-Einstein equation, DO ) kT/6πηRh, where k, η, and T are the Boltzmann constant, the viscosity of the medium, and the absolute temperature, respectively. Considering the partition equilibrium of Fc between the water and oil phases of the microemulsion, we calculated the diffusion coefficients of oil-in-water microdroplets, DO, from the measured apparent diffusion coefficients Dapp by using eq 12. In computing the Kd values, 1 × 10-5 mol dm-3 for the solubility of Fc in water was adopted.39,41 The diffusion coefficient of Fc in water was estimated using the Stokes-Einstein relationship Dw ) kT/6πηRh. Assuming the molecule of Fc to be spherical with a radius of 0.365 nm42 and with η ) 0.8903 × 10-3 Pa s (25 °C), we estimated a diffusion coefficient Dw ) 6.722 × 10-10 m2 s-1. The values of Fc concentration and percentage of components, Kd, Dapp, DO, and Rh with changes in composition of the o/w microemulsion are tabulated in Table 1. In oil-in-water microemulsions, one may expect an increase in size of the microdroplet or a decrease of its diffusion coefficient when the water content was decreased (40) Georges, J.; Desmettre, S. Electrochim Acta 1984, 29, 521. (41) Peter, Y.; Theodore, K. J. Electrochem. Soc. 1976, 123, 1334. (42) Armstrong, N. R.; Quinn, R. K.; Vanderborgh, N. E. Anal. Chem. 1974, 46, 1759.

because of the growth of the microdroplet. However, an opposite trend is observed from Table 1; DO values show an increasing trend with decreasing water content. This phenomenon is similar to the observations made by Mandal39 and Santhanalakshmi9 in CTAB/1-butanol/ hexadecane/water and SDS/vinyltoluene/1-pentanol/water microemulsions, respectively. Decrease in water content leads to an increase in the diffusion coefficients of hydrophobic probes. This parallels directly the increase in the self-diffusion coefficients of hydrocarbons measured by NMR with similar changes in composition.43 In explanation they considered that if with a decrease in percent water in microemulsion a transition from a water continuous to a bicontinuous hydrocarbon/water structure occurs, an increase in the diffusion coefficient of ferrocene can be expected and is observed. In a water continuous microemulsion region, however, increase in the diffusion coefficient of ferrocene with decreasing water content is due to an increase in the inherent mobility of Fc because of its preferential partitioning into the oil phase. In addition, the number of the probes in an oil droplet varies with water content as well as the droplet size, and an increase in the number can also give large DO. The physicochemical properties of aqueous TX 100 solution have been the subject of extensive research using various techniques. Mandal et al.44 had determined successfully the exact shape and size of the aqueous TX 100 micelles as a function of temperature and in the presence of various additive environments by using conductivity and transport studies. They had found that the shape of the TX 100 micelles is spherical at 15 °C, whereas at higher temperature it is oblate in shape. Mandal et al.44,45,19 have also found that the hydrodynamic radius of pure TX 100 micelles is around 40 Å when it is spherical, obtained by transport and cyclic voltammetric studies. However, our results showed that the hydrodynamic radius of the TX 100 (see Table 1) increases from 53 to 808 Å when the water content increases from 49% to 88% in the present microemulsion (ternary) system. This observation is similar to the observations made by Mandal et al.46 and Tamamushi et al.47 in decaglycerol dioleate (DGD) (nonionic surfactant)/heptane/water and (43) Lindman, B.; Kamenka, N.; Kathopoulis, T. M.; Brun, B.; Nilsson, P. G. J. Phys. Chem. 1980, 84, 2485. (44) Mandal, A. B.; Ray, S.; Biswas, A. M.; Moulik, S. P. J. Phys. Chem. 1980, 84, 856. (45) Mandal, A. B.; Geetha, B. J. Chem. Phys. 1996, 105, 9649. (46) Mandal, A. B.; Nair, B. U.; Ramaswamy, D. Colloid Polym. Sci. 1988, 266, 575.

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AOT/isooctane/water ternary systems, respectively, which is due to the formation of “swollen micelles” in the presence of high water content. It should be pointed out that Rh in Table 1 is only the apparent hydrodynamic radius of oil-in-water microdroplets. We assume that the probe itself does not influence the size of microemulsion droplet and that the results are not affected by the number of probes per oil droplet. In the case of low probe concentrations, this condition has been shown to hold for microemulsions.48,49 At high probe concentrations, however, such is generally not the case. Conductivity and Structure of Microemulsion. Conductivity is a structure-sensitive property and is frequently used to investigate structure and structural changes in macro- and microemulsions. The determination of the microstructrue of microemulsions using conductivity data was based on the percolation theory. For conductorinsulator composite materials, the effective conductivity, κ, is zero as long as the conductor volume fraction Φ is smaller than a critical value, Φc, called the percolation threshold, because there is no connection between the disperse conducting particles. Suddenly nonzero values occur when Φ becomes slightly greater than Φc and then increases with Φ, owing to the formation of an “infinite cluster” of conducting particles. In the vicinity of Φc, the dependency of κ on Φ can be demonstrated in the case of three-dimensional systems, by the following power law50

κ(Φ) ∝ (Φc - Φ)8/5

Figure 6. Variations of microemulsion electrical conductivity, κ, versus Φw, the water volume fraction, along the km ) 1/2, the surfactant-to-cosurfactant mass ratio, Rh/s ) 25/75, the mass ratio of oil-to-(surfactant + cosurfactant), experimental path in the system water/sodium dodecyl sulfate/1-pentanol/ n-dodecane (T ) 25 °C).

(13)

As Φ further increases, the power law is no longer valid and the conductivity increases according to the following linear law:

κ(Φ) ∝ (Φ - Φc)

(14)

In their experiments, which consisted of measuring conductivity (κ) of microemulsion against different water volume fraction (Φw) using a system of water/sodium dodecyl sulfate/1-pentanol/n-dodecane, Clausse and coworkers51 demonstrated that, as the water content increased, κ, the microemulsion electrical conductivity, varied according to four successive models. As shown in Figure 6, the initial nonlinear increase of κ, which was governable by the eq 13, revealed the existence of a percolation phenomenon that could be ascribed to inverse microdroplet aggregation. The next linear increase, which was described by eq 14, could be interpreted as the consequence of the formation of aqueous microdomains resulting from the partial fusion of clustered inverse microdroplets. It was obvious that a water-in-oil type microemulsion formed in this low water content gap. The third section of curve, nonlinear increase of κ, revealed that the medium underwent further structural modifications and became bicontinuous, owing to progressive growth and interconnection of the aqueous microdomains. The final decrease of κ with increasing water content corresponded to the existence of water-continuous microemulsion-type media. In other words, an oil-in-water type (47) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980, 258, 174. (48) Mackay, R. In Microemulsions; Robb, I., Eds.; Plenum: New York, 1982; p 207. (49) Mackay, R.; Dixit, N.; Agarwal, R.; Seiders, P. J. Dispersion Sci. Technol. 1983, 4, 397. (50) Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Nature 1979, 281, 61. (51) Clausse, M.; Zradba, A.; Nicolas-Morgantini, L. In Microemulsion Systems; Rosano, H. L., Clausse, M., Eds.; Marcel Dekker, Inc.: New York, 1987; pp 387-425.

Figure 7. Electric conductivity κ as a function of water content Φ in a single-phase microemulsion region (25 °C). Sample solution: as in Figure 4 (without ferrocene).

microemulsion formed at high water content. The decrease of κ merely resulted from the fact that the continuous aqueous phase was progressively diluted with water. We also measured the electrical conductivity of several microemulsion samples. Taking a sample for which the composition of the system changes along line a-d noted in Figure 3. Figure 7 demonstrates typical experimental results. The electrical conductivity κ plotted against water content exhibits features characteristic of percolate conduction. At Φ < Φb, the conductivity of microemulsions, κ, linearly and steeply increases up to κ ) κb, which indicates the formation of an “infinite cluster” of the water-in-oil microdroplets. At high water content, for example, at Φ > Φm, the value of κ, after arriving the maximum value κm, decreases with increasing water content. This obvious decrease in the electric conductivity κ results from dilution with the added water, which decreases the concentration of the dispersion phase. Evidently, an oil-in-water microemulsion was formed in this region of high water content. However, in the region of moderate water content at Φb < Φ < Φm, the conductivity

Electroanalysis of Triton X-100 Microemulsion

curve exhibits an abnormal behavior, κ nonlinearly increases up to a maximum. This feature of conductivity curve was often used to identify the occurrence of a bicontinuous microemulsion. The conductivity curve in Figure 7 clearly illustrates the occurrence of the three regions: water-in-oil (48% water), and bicontinuous (3448% water) microemulsions. Thus, the results obtained by the two electrochemical methods, e.g., 1.5-order differential electroanalysis and the electrical conductivity measurements, are in agreement, and the three microstructures of TX 100 microemulsion, i.e., water-in-oil, oilin-water, and bicontinuous microemulsion, are indicated in Figure 3. In summary, the preliminary results reported in this paper show that 1.5-order differential electroanalysis can

Langmuir, Vol. 18, No. 10, 2002 4053

be used to investigate the diffusion of particles in organized surfactant solutions at very low electroactive species concentrations and low supporting electrolyte concentrations. Compared with other electroanalytical methods, this differential electroanalytical technique has the merits of high sensitivity and high resolvability. It is reasonable to expect that the method can also be extended to many microheterogeneous systems, including micelles, macroand micremulsions, vesicles, and polymer films. Acknowledgment. This work was supported by a grant from the Natural Science Foundation (0120023) of Jiangxi Province, People’s Republic of China. LA015680D