Langmuir 1997, 13, 3915-3920
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Articles Microstructure and Diffusion Properties of Dodecane Ionic Microemulsions Containing Cobalticenium Ion as the Electrochemical Probe J. Santhanalakshmi* and G. Vijayalakshmi Department of Physical Chemistry, University of Madras, Guindy Campus, Madras 600 025, India Received February 20, 1996. In Final Form: April 9, 1997X The electrochemical properties of cobalticenium hexafluorophosphate in water and in oil in water (O/W), dodecane-SDS-n-pentanol-water, microemulsions (ME) are studied through cyclic voltammetry, conductivity, and viscosity measurements at 300 K. The one-electron, reversible redox process of the cobalticenium (CP2CO)+ a cobaltocene (CP2CO)0 couple exhibit shifts in reaction potential E1/2 ) 1/2(Epc + Epa) values, due to the partitioning effect on the probe caused by water soluble CP2CO+ and oil soluble CP2CO species. E1/2 values shift toward more positive values from its aqueous formal potential -1.086 ( 0.012 V with the increase in droplet volume fraction (φd) of the ME. Microstructural and dynamic properties of microemulsions, such as droplet diffusion coefficients, surfactant aggregation number, fraction of the free ions, and number density of droplets are evaluated from the electrochemical data. Compositional variations with the electrochemical properties showed O/W droplet to bicontinuous phase transitions at φd ) 0.4, which is the percolation threshold. Viscosity and conductivity results also coincide with this value. These results indicate CP2CO+ ion to be a successful electrochemical probe in microemulsions on par with ferrocene.
Introduction Microstructure, dynamics, and transport properties of microemulsions (ME) are fundamentally important to correlate microemulsion syntheses with industrial applications. The microheterogeneity and the macrohomogeneity of the microemulsions provide attractive potential applications, such as model membrane systems, in catalysis, solubilization media, etc.1-3 Studies on the microstructure and dynamics have been made through spectroscopic, light scattering, small angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and electron microscopy methods.4-7 However, some of these properties may be evaluated from electrochemical measurements using rotating disk voltammetry (RDV), electrical conductivity, and cyclic voltammetry (CV) involving an electroactive compound (EC) and an appropriate electrode.8-13 The values of the formal potential of the EC in MEs and in the aqueous medium can be used to X
Abstract published in Advance ACS Abstracts, June 15, 1997.
(1) Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 1, 169. (2) Mackay, R. A. Adv. Colloid. Interface Sci. 1981, 15, 131. (3) Luisi, P. L.; Giomini, M.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 229. (4) Nicoli, D. F.; DeBuzzaccarini, F.; Romsted, L. S.; Bunton, C. A. Chem. Phys. Lett. 1981, 80, 422. (5) Lindman, B.; Ahinas, T.; Soderman, O.; Walderhaug, H.; Rapacki, K.; Stilbs, P. Faraday. Discuss. Chem. Soc. 1983, 76, 317. (6) Santhanalakshmi, J.; Parameswari, A. Indian J. Chem. 1992, 31A, 630. (7) Eastoe. J. Langmuir 1992, 8, 1503. (8) Santhanalakshmi, J.; Anandhi, K. J. Colloid Interface Sci. 1995, 176, 226. (9) Mackay, R. A. In Microemulsions; Robb, I. D., Ed.; Plenum: New York, 1982; p 207. (10) Georges, J.; Desmettre, S. Electrochim. Acta 1984, 29, 521. (11) Iwunze, M. O.; Sucheta, A.; Rusling, J. Anal. Chem. 1990, 62, 644. (12) Mackay, R. A.; Myers, S. A.; Bodalbhai, L.; Brajter Toth, A. Anal. Chem. 1990, 62, 1084. (13) Mandal, A. B.; Nair, B. U. J. Chem. Soc., Faraday Trans. 1991, 87, 133.
S0743-7463(96)00149-7 CCC: $14.00
probe the microstructure of the microemulsion through the estimated values of diffusion coefficient (D) and partitioning constant of the oxidized (KO) and reduced (KR) forms of the EC in the different phases of the ME. Cd(II) ion,14 methyl viologen, ferricyanide, and ferrocene1,2 are a few of the ECs that are successfully utilized to know the droplet size (RH) and number density of the dispersed droplets (Nd). The variations of these parameters with the microemulsion composition are helpful in phase studies in particular to the L1, L2 droplets and bicontinuous phase distinctions.15,16 In view of the partitioning, number, pathway of electron transfer, stability, and solubility of the probe in the different phases, a ferrocene π complex of iron has been used on par with other ECs like Cd(II), Fe(CN)63-, and [Co(en)3](ClO4)3 to probe the micellar and microemulsion media. dc polarography, differential pulse, square wave voltammetry, and cyclic voltammetric studies of ferrocene containing anionic, cationic, and nonionic micellar solutions,10,17 oil in water (O/W), and bicontinuous microemulsions11,18 stand well established. In this paper we report the results of an attempt on the utility of cobalticenium ion in lieu of ferricenium ion as an EC to probe the microstructure, dynamics, and transport properties of a representative O/W microemulsion, i.e., dodecane/SDS/n-pentanol/water system containing various compositions of oil and surfactant. the bis(π-cyclopentadienyl)cobalt(II) complex (cobaltocene) may readily undergo electrochemical one-electron oxidation and reduction to produce cobalticenium cation (CP2CO)+ and cobaltocene anion (CP2CO)-. (14) Novodoff, J.; Rosano, H. L.; Hoyer, H. W. J. Colloid Interface Sci. 1972, 38, 424. (15) Myers, S. A.; Brajter-Toth, A.; Mackay, R. A. Submitted for publication in J. Phys. Chem. (16) Shinoda, K.; Araki, M.; Sadaghiani, A.; Lindman, B. J. Phys. Chem. 1991, 95, 989. (17) Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1982, 136, 353. (18) Georger, J.; Berthod, A. J. Electroanal. Chem. 1984, 175, 143.
© 1997 American Chemical Society
3916 Langmuir, Vol. 13, No. 15, 1997
Santhanalakshmi and Vijayalakshmi
Electrochemical studies of cobalticenium ion and its reduced counterparts are well studied in different solvents.19-22 The unipositive cobalticenium ion (CP2CO)+ being isoelectronic with ferrocene gives a well-defined polarographic wave at the potential of -1.16 V versus SCE at the dropping mercury electrode. In the CP2CO+/0 redox couple, (CP2CO)+ is water soluble while (CP2CO)0 is oil soluble resembling Fc0/+ redox couple in which FC0 is oil soluble and FC+ is water soluble. On the basis of these characteristics of the CP2CO+/0 redox system, the electrochemical properties of CP2CO+/0 in the microemulsion medium and in water are compared and correlated with the microemulsion properties using cyclic voltammetry, viscosity, and electrical conductivity results. Theory Mackay and co-workers have been the first to apply electrochemical methods to obtain information about the microstructure of microemulsion.9 The reduction and oxidation of the redox couple CP2CO+/0 in a microheterogeneous system can be described and subsequently used to derive a relation between the reaction potential and the formal potential similar to the described procedure2 as follows
[CP2CO+]w y\ z [CP2CO0]w -e +e
KO Vv
Vv KR
[CP2CO+]o y\ z [CP2CO0]o -e +e
where [CP2CO+]w, [CP2CO+]o, [CP2CO0]o, and [CP2CO0]w are the oxidized and reduced forms of the probe in water and in the nonpolar oil phase of the ME. KO and KR are the partitioning constants of the oxidized and reduced forms of the EC.
KO ) [CP2CO+]w/[CP2CO+]o KR ) [CP2CO0]w/[CP2CO0]o [CP2CO+] ) [CP2CO+]w + [CP2CO+]o [CP2CO0] ) [CP2CO0]w + [CP2CO0]o
E1/2 ) E′0(aq) +
( )
DR RT ln nF DO
1/2
+
[
]
(1 + KR) RT ln nF KR
(2)
DR * DO when the oxidized and reduced forms of the probe are in different phases. E′0(aq) of CP2CO+/0 ) -1.086 ( 0.012 V.24 Cobalticenium ion due to its (i) very little oil solubility, (ii) diffusion-controlled electrochemical charge transfer, and (iii) negligible adsorption on the droplet interface and electrode surface, is suitable as a probe in the droplet microemulsions. Also, when the characteristics of the MEs are not affected by CP2CO+ in cyclic voltammetry, the Randles-Sevcik equation may be considered for the diffusion coefficient measurements25 where n is the number of electrons involved in the electron
ip ) 0.4463nFAC0(nF/RT)1/2D1/2ν1/2
(3)
transfer, F is the Faraday constant 96 500 C equivalent-1, A is the area of the electrode in cm2, C is the concentration of the probe in mol/cm3, D is the diffusion coefficient in cm2/s, ν is the scan rate, mV s-1, T is the temperature (K), and R is the gas constant (J/(K, mol)). Area of the working electrode is determined by controlled coulometry (CC) in a solution of 1 × 10-3 M ferricyanide in 0.1 M KCl, using the diffusion coefficient value DO ) 7.6 × 10-6 cm2/s. The DR and DO values are determined from the ip values of the respective anodic and cathodic waves of the voltammograms. Hence with E1/2, DR, and DO values, eq 2 furnishes the values of KR. In this method the apparent diffusion coefficient of the oxidant DO can be expressed as DO ) Dowfw + Domfm, where Dow and Dom refer to the oxidant diffusion coefficients in water and oil phases of the microemulsion and fw and fm are the fractions of the probe in the water and the oil phases, fw + fm ) 1. DO in terms of KO, Dow, and Dom can be written as KO ) (Dom - DO)/(DO - Dow). Dow of CP2CO+ ) 6.0 × 10-6 cm2/s is determined from the slope of CC plot of Q versus t1/2 with probe concentration equal to that used in E1/2 measurements. Hence, Dom values which also represent the diffusion properties of the oil droplets in O/W microemulsion are obtained. The diffusing droplet consists of surfactant, cosurfactant, and oil molecules, which diffuse or ride along with the diffusing droplet as a whole. Thus the Dom values of the Ec probe solubilized into the oil droplet are assumed to represent the diffusion coefficient of the droplet as a whole. Experimental Section
The aqueous phase formal potential E′0(aq) and the measured diffusion coefficients of the EC in the oxidized (DO) and reduced (DR) forms are related to the half-wave potential E1/2 ) (EPa + EPc)/2 through KR and KO values according to the relation2,10,17,23
The one-electron electrochemical redox couple of the EC, CP2CO+/0, represents the limiting case where CP2CO+ is water soluble and the other CP2CO0 is oil soluble; since CP2CO+ is water soluble, KO is .1 and eq 1 may be rewritten as
Materials. Sodium dodecyl sulfate (SDS) supplied by BDH with 99% purity was purified further, adopting an earlier procedure.26 n-Dodecane (Fluka), NaCl (AnalaR, BDH), 1-pentanol 99% pure (Sigma), and cobalticenium hexafluorophosphate (Aldrich) were used as received. Water used was doubly distilled in an all-glass assembly and deionized using an activated charcoal column. Microemulsion Preparation. Microemulsions were prepared corresponding to the compositions given in Table 1 by adding weighed amounts of water, surfactant oil, and 1-pentanol in sequence with constant stirring until transparency was obtained. For CV experiments, MEs were prepared in 0.01 M NaCl. EC probe in ME was dissolved with stirring and then allowed to equilibrate at 300 K. After equilibration, the D values of the EC probe were the same as those measured at various long time intervals. In all CV measurements, the freshly prepared MEs were deoxygenated by bubbling AnalaR grade N2 for at
(19) Hsiung, H. S.; Brown, G. H. J. Electrochem. Soc. 1963, 110, 1085. (20) Geiger, W. E. Jr. J. Am. Chem. Soc. 1974, 96, 2632. (21) Wilkinson, G. J. Am. Chem. Soc. 1952, 74, 6148. (22) Wilkinson, G. J. Am. Chem. Soc. 1952, 74, 6146. (23) Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1983, 145, 109.
(24) Strehlow, H. Z. Elektrochem. 1952, 56, 827. (25) Bard, A. J.; Faulkner, L. R. In Electrochemical methods; John Wiley and Sons, Inc.: New York, 1980; p 218. (26) Blackely, D. C.; Burger, W. P. H. In Emulsion Polymerization; Pirima, I., Gorden, J. K., Eds.; ACS symposium series No. 24; American Chemical Society: Washington, DC, 1976; p 162.
E1/2 ) E′o(aq) +
( )
DR RT ln nF DO
1/2
+
[
]
KO(1 + KR) RT ln nF KR(1 + KO) (1)
Microstructure of Ionic Microemulsions
Langmuir, Vol. 13, No. 15, 1997 3917
Table 1. Phase Compositions and Relative Viscosity (η/η0) Data on Dodecane (Oil)-SDS-1-Pentanol (Cosurfactant)-Water Microemulsions at 300 K (Od ) Droplet Volume Fractions) η0 ) 1 × 10-2 cP sample no.
watera
oil
wt % SDS
cosurfactant
φd
η/η0
1 2 3 4 5 6 7 8 9 10
91.23 89.49 90.69 89.00 88.02 76.00 56.12 45.53 64.00 52.02
0.38 0.50 1.00 1.00 1.98 4.00 4.01 5.01 6.00 8.00
1.72 1.68 1.68 3.35 3.32 6.66 13.27 16.60 10.00 13.32
6.68 8.34 6.65 6.66 6.68 13.34 26.60 33.27 20.00 26.66
0.19 0.20 0.35 0.40 0.50 0.54 0.65 0.70 0.60 0.68
1.20 1.30 1.40 1.88 2.66 5.56 6.70 9.20 6.50 8.96
a
0.01 M NaCl solution.
least 3 h through a three-way stopcock that permitted pure N2 gas to flow over the solution after the deoxygenation process and as well as during the electrochemical measurements thereby preventing the O2 redissolution in the system. A prebubbler containing water was used to minimize compositional changes. Electrochemical Measurements. Cyclic Voltammetry. Cyclic voltammograms were obtained with a Princeton Applied Research 362 scanning potentiostat coupled with PAR universal programmer and a three-compartment all-glass double-walled electrolytic cell in which the desired temperature was maintained through water circulation. A saturated calomel electrode (SCE) and platinum wire served as a reference and auxiliary electrode. Before each measurement, a glassy carbon (GC) working electrode was polished with 5 µm diamond paste on a polishing wheel and subsequently the surface of the electrode was cleaned by ultrasonification in distilled water for 5 min before use. The reversible one-electron redox process in the ME and in water is found from the separation of anodic and cathodic peaks which lie around 58-64 mV and also from the ipa/ipc values, which lie around 1.0. Diffusion coefficient value DO of the probe was calculated from the slope of CC plot of Q vs t1/2 using the same concentration of EC and the potential steps of CV of MEs. All the D values reported lie within (0.005 accuracy. When CC was performed for the DR determination, attempts to obtain cobaltocene were unsuccessful. On continued electrolysis, the cathodic wave of cobalticenium ion disappeared indicating the complete reduction of the ion. The complexity arises due to the destruction of cobaltocene due to its oxidation by water. IR compensations
