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Langmuir 1997, 13, 2410-2413
Notes Determination of the Critical Micelle Concentration of the Methoxy Polyethylene Glycol Based Macromonomer and Partition Coefficient of a New Electrochemical Probe Using a Cyclic Voltammetric Technique Bhaskar Geetha and Asit Baran Mandal* Chemical Laboratory, Physical and Inorganic Chemistry Division, Central Leather Research Institute, Adyar, Madras 600 020, India
the method of the well-known Schiff and Baumann reaction where acryloyl chloride was reacting with methoxy ether of PEG in nonaqueous medium. The macromonomer 1 is found to form micelles in water due to the presence of poly(oxyethylene) (nine oxyethylene units) group as the head, and the hydrophobic acryloyl and methoxy groups as tail. Details regarding synthesis, purification, characterization, micelle formation and liquid crystalline properties of the macromonomer 1 are described in our recent publication.4
Received November 17, 1995. In Final Form: December 19, 1996
Introduction Polymeric surfactants in the rapidly growing areas of organized assemblies have been the subject of wide interest1 to both biologists and chemists. Polymeric surfactants consist of hydrophilic and hydrophobic blocks linked by covalent bond and are therefore able to form stable micelles/aggregates and prevent the free exchange of the monomer between the micellar and bulk phase. Moreover, they have the beneficial properties of stable and uniform particles with the fluidities of micelles, microemulsions, etc. Block or graft copolymers are best suited as polymeric surfactants. However, the latter provides a more facile synthetic route. A number of macromonomers have so far been synthesized to be useful in designing a variety of well-defined graft copolymeric surfactants. Recently, surface active monomers, capable of forming aggregates at a certain concentration, known as critical micelle concentration (cmc), are gaining considerable importance. Such macromonomers find application in the systems like emulsions polymerization2 with uniform particle size distribution. Furthermore, macromonomers are also capable of altering the nature of the surface of the systems wherein they are employed. It has been found that the micellization of the macromonomers accelerates the thermochemical polymerization.3 The surface active macromonomer 1 based on acrylic ester of methoxy polyethylene glycol (PEG) synthesized by us is unique in the sense that it has special properties such as solubility in a variety of solvents, nontoxicity, biodegradability, commercial availability, a high degree of crystallinity, a lower glass transition temperature, etc. The synthesis of the macromonomer 1 is carried out by * To whom correspondence should be addressed: fax, +91-444911589; phone, +91-44-4911386/4911108; e-mail,
[email protected]. (1) (a) Fendler, J. H. Membrane Kinetic Chemistry; Wiley Interscience: New York, 1982. (b) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (c) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153-161; 1980, 13, 7-13. (d) Gratzel, M. Acc. Chem. Res. 1981, 14, 376-384. (e) Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1983, 104, 1752-1759. (f) Gregoriadeg, G. Drug Carriers in Biology and Medicines Academic Press: London, 1979. (2) (a) Chen, S. A.; Chang, H. S. J. Polym. Sci., Polym. Chem. Ed. 1985, A23, 2615-2630. (b) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 4279-4280. (3) (a) Ito, K.; Tanaka, K.; Tanaka, H.; Imai, G.; Kawaguchi, S.; Itsuno, S. Macromolecules 1991, 24, 2348-2354. (b) Ito, K.; Hashimura, K.; Itsuno, S. Macromolecules 1991, 24, 3977-3981. (c) Ito, K.; Tomi, Y.; Kawaguchi, S. Macromolecules 1992, 25, 1534-1538.
S0743-7463(95)01047-X CCC: $14.00
In view of the current interest of the micellar characteristics of the macromonomer in designing polymeric surfactants, it is therefore necessary to investigate and explore various micellar properties of the above macromonomer. We have determined the cmc, hydration, thermodynamics, and various physicochemical parameters for surfactants, synthetic and vegetable tanning materials, collagens, and peptide micellar systems using various experimental techniques.5 Recently, we have developed a novel electrochemical technique,6 viz., cyclic voltammetry (CV), for the determination of the cmc of various surfactants. In doing so, we have also developed some concepts6 for the solubilization (partition coefficient) and location of the EC probe in polar and nonpolar regions of the micelles and microemulsion systems. In all these (4) Geetha, B.; Mandal, A. B.; Ramasami, T. Macromolecules 1993, 26, 4083-4088. (5) (a) Mandal, A. B.; Ray, S.; Moulik, S. P. Indian J. Chem. 1980, 19A, 620-625. (b) Mandal, A. B.; Moulik, S. P. ACS Proceedings: Solution Behavior of Surfactants-Theoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 1, pp 521-541. (c) Mandal, A. B.; Mukherjee, D.; Ramaswamy, D. Leather Sci. 1981, 28, 283-288. (d) Mandal, A. B.; Ramaswamy, D.; Das, D. K.; Santappa, M. Colloid Polym. Sci. 1982, 260, 702-707. (e) Mandal, A. B.; Kanthimathi, M.; Govindaraju, K.; Ramaswamy, D. J. Soc. Leather Technol. Chem. 1983, 67, 147-158. (f) Mandal, A. B. J. Surf. Sci. Technol. 1985, 1, 93-98. (g) Mandal, A. B. J. Am. Oil Chem. Soc. 1987, 64, 1202-1207. (h) Mandal, A. B.; Ramaswamy, D. J. Surf. Sci. Technol. 1987, 3, 31-38. (i) Mandal, A. B.; Ramesh, D. V.; Dhar, S. C. Eur. J. Biochem. 1987, 169, 617-628. (j) Mandal, A. B.; Jayakumar, R. J. Chem. Soc., Chem. Commun. 1993, 237-238. (k) Jayakumar, R.; Mandal, A. B.; Manoharan, P. T. J. Chem. Soc., Chem. Commun. 1993, 853-855. (l) Mandal, A. B.; Dhathathreyan, A.; Jayakumar, R.; Ramasami, T. J. Chem. Soc., Faraday Trans. 1993, 89, 3075-3079. (m) Mandal, A. B.; Jayakumar, R. J. Chem. Soc., Faraday Trans. 1994, 90, 161-164. (n) Jayakumar, R.; Jeevan, R. G.; Mandal, A. B.; Manoharan, P. T. J. Chem. Soc., Faraday Trans. 1994, 90, 2725-2730. (6) (a) Mandal, A. B.; Nair, B. U.; Ramaswamy, D. Langmuir 1988, 4, 736-739. (b) Mandal, A. B.; Nair, B. U.; Ramaswamy, D. Bull. Electrochem. 1988, 4, 565-569. (c) Mandal, A. B.; Nair, B. U. J. Chem. Soc., Faraday Trans. 1991, 87, 133-136. (d) Mandal, A. B.; Nair, B. U. Advances in Measurement and Control of Colloidal Processes; Williams, R. A., Jaeger, N. C. De, Eds.; Butterworth Scientific Publications: London, 1991; Chapter 2, pp 136-149. (e) Mandal, A. B.; Nair, B. U. J. Phys. Chem. 1991, 95, 9008-9013. (f) Mandal, A. B. Langmuir 1993, 9, 1932-1933. (g) Zana, R.; Mackay, R. A. Langmuir 1986, 2, 109.
© 1997 American Chemical Society
Notes
Langmuir, Vol. 13, No. 8, 1997 2411
electrochemical investigations,6 we have used K4Fe(CN)6, [Co(en)3](ClO4)3, and ferrocene as redox-active EC probe. In this paper, we report the results for the determination of cmc of the macromonomer 1 and partition coefficient of a new EC probe, viz., Co(sep)Cl3, 2, in nonmicellar and micellar states, respectively, by using CV technique.
Experimental Section Synthesis and Purification of Co(sep)Cl3. The compound [Co(sep)]Cl3, 2, was prepared by modifying the published procedure.7 A 20-g portion of tris(ethylenediamine)cobalt chloride was dissolved in 80 mL of 37% (w/v) formaldehyde solution. Then dry ammonia was bubbled through the resulting solution. The temperature of the system was maintained at room temperature by cooling the reaction vessel. The excess ammonia was removed by aeration after completion of the reaction. The resulting mixture was filtered and the pH of this filtrate adjusted to ∼3 with HCl. Following rotary evaporation and removal of the hexamethylenetetramine side product, a crude product was obtained. The pure compound of Co(sep)Cl3, 2, was obtained by recrystallization from aqueous HCl solution. For details regarding synthesis and purity of 2, refer to an earlier report.8 The structural details of the macromonomer and Co(sep)Cl3 have also been given in our recent publications.4,9 Cyclic Voltammetric Technique. In the electrochemical measurements, the solutions were deoxygenated by bubbling argon for at least 10 min. The measurements were performed using a cyclic voltammeter (Princeton Applied Research Model 173 with a PARC Model 175 universal programmer) and a threenecked electrolytic cell. A saturated calomel reference (SCE) electrode was employed in this study, and all potentials that are quoted are with reference to the SCE. The working electrode was Pt with a Pt counter electrode. Regarding experimental details for the determination of the cmc, partition coefficient, and self-diffusion coefficient of the micelles by CV technique, refer to our previous publications.6
Results and Discussion It would be mentioned that the EC probe Co(sep)Cl3, 2, acts as reversible and gives a suitable oxidation reduction cycle (ORC) in the pH range of 4.5-8.0. As the macromonomer 1 under cmc investigation shows pH of 6.06.5 in the presence of EC probe in aqueous medium, therefore, the probe Co(sep)Cl3, 2, is extremely useful in studying micellar characteristics of the macromonomer. However, in the presence of extremely high concentrations of macromonomer 1, the above probe does not give any suitable ORC as the resulting system shows pH < 4. We have discussed this in the recent paper9 where selfdiffusion coefficient measurement was restricted at extremely higher concentrations of the macromonomer. (7) Farraudi, G. J.; Endicott, J. F. Inorg. Chim. Acta 1979, 37, 219225. (8) Govindaraju, K. Ph.D. Thesis, Madras University, India, 1986; Chapter 2. (9) Geetha, B.; Mandal, A. B. Langmuir 1995, 11, 1464-1467.
Figure 1. Cyclic voltammogram of Co(sep)Cl3 solution (0.8 mmol dm-3, constant) in various concentrations of macromonomer 1 at 25 °C: (a) below cmc of macromonomer (curve numbers 0-4) 0, 2 × 10-5, 4 × 10-5, 6 × 10-5, and 10 × 10-5 mol dm-3 of macromonomer, respectively; (b) above cmc of macromonomer (curve numbers 5-7), 12 × 10-5, 16 × 10-5, and 20 × 10-5 mol dm-3 of macromonomer, respectively. [KCl] ) 0.1 mol dm-3 (constant); sweep rate ) 100 mV/s. Area of the Pt working electrode ) 0.0918 cm2.
In CV, the peak current, ip (µA) for a redox-active reversible system at 25 ˚C is given by the following equation:
ip ) (2.687 × 105)n3/2AD1/2Cv1/2
(1)
where n is the number of electrons involved in oxidation or reduction, A is the area of the electrode (cm2), D is the diffusion coefficient of the EC species (cm2 s-1), C is the concentration of the EC species in the solution (mol dm-3), and v is the sweep rate (V s-1). With micellar systems involving an EC probe completely solubilized in the micelles, the D in eq 1 would correspond to the micelle diffusion coefficient Dm, since the probe diffuses with the micelle, whereas C would still be the probe concentration. Figure 1 shows some typical cyclic voltammetric curves for Co(sep)Cl3 in the absence and presence of various concentrations of macromonomer solutions (below and above its cmc). The diffusion-controlled nature of the processes has been examined in the absence and presence of macromonomers at concentrations below and above cmc (cf. Figure 2). Although the [probe]/[micelle] molar concentration ratio was greater than 1 at the highest employed probe concentration, it does not affect the absolute cmc values of the macromonomer 1. This is at par with our earlier observations.6 The cmc value determined by this CV method is in good agreement with our recent spectroscopic techniques4 (see Table 1). It can be seen in Table 1 that the slight low cmc value of the macromonomer 1 obtained by the CV method compared to spectroscopic techniques4 may be due to the salt effect by the supporting electrolyte. Figure 3 shows the changes in ip and E1/2 upon increasing macromonomer concentration Cs at constant probe concentration C. The half-wave potential, E1/2 of Co(sep)Cl3 probe in aqueous solution at 25 °C is -0.502 V. E1/2 becomes increasingly negative, and ip shows a very large increase as Cs increases, and these are valid up to the cmc of the macromonomer: beyond cmc a decrease of both ip and -ve E1/2 with the increase of Cs were observed (cf. Figure 3). We have used
2412 Langmuir, Vol. 13, No. 8, 1997
Notes
Table 1. Critical Micelle Concentration (cmc) of 1 and Partition Coefficient of the Electrochemical Probe 2 Between Macromonomer and Water in Bulk (Kb) and Micellar (Km) Phases at 25 °Ca K/mol-1 dm3
cmc/mol dm-3 solvent H2O-0.1 M KCl a
surfactant macromonomer
this work 1.0 ×
10-4
lit. value 1.2 ×
10-4
ref.
Kb
Km
4, 9
4840 ( 66
3024 ( 83
Diffusion coefficient of the probe, Dp ) 1.9 × 10-6 cm2 s-1.
between macromonomer and water, K, was readily calculated using the equation
(D - Dm) ∆C2K2 + (D - Dm1/2Dp1/2) 2∆CK + D ) 0 (2)
Figure 2. Plot of peak current, ip, vs square root of sweep rate, v1/2, in absence and presence of macromonomer environment at 25 °C: curve numbers 1-3, 0, 2 × 10-5, and 20 × 10-5 mol dm-3 of macromonomer, respectively. [Co(sep)Cl3] ) 0.8 mmol dm-3 (constant); [KCl] ) 0.1 mol dm-3 (constant). Area of the Pt working electrode ) 0.0918 cm2.
Figure 3. Plot of ip and E1/2 vs concentration of macromonomer at 25 °C. [Co(sep)Cl3] ) 0.8 mmol dm-3 (constant); [KCl] ) 0.1 mol dm-3 (constant). Curve numbers 1 and 2 are for larger (area ) 0.0918 cm2) and smaller (area ) 0.0428 cm2) sizes Pt working electrodes, respectively. Curve numbers correspond to ordinate scale numbers. Sweep rate ) 100 mV/s.
two different sizes of Pt working electrodes in order to double check the cmc determination (curves 1 and 2 of Figure 3). The cmc values obtained by both ip and E1/2 profiles are same. The partition coefficient of the probe
where D is the experimental value of the apparent diffusion coefficient of the partitioned probe in the micellar system, Dp the probe diffusion coefficient in the bulk and ∆C ) Cs - cmc. Regarding details of eq 2, we refer to earlier works.6 We have already found6 that K is a function of composition as well as surfactant concentration and decreases as Cs increases at constant C. Because of this reason, the K values at below and above cmc regions, namely, Kb and Km, were estimated. The calculated K values for the macromonomer are given in Table 1. It may be possible that E1/2 is also being affected by surfactant adsorption on the electrode and might be expected that the adsorption increases with increasing negative potential. However, above cmc, the negative E1/2 value decreases with increasing Cs indicating that the adsorption is less of a problem in the case of macromonomer micelles. Regarding details of surfactant adsorption, we have discussed these in previous publications.6 The number of electron transfer (n) and the peak to peak separation for the Co3+ probe in macromonomer micellar environments are 1 and 59-63 mV, respectively. Therefore, in conclusion, 2 is an excellent reversible EC probe for the determination of the cmc of the investigated macromonomer. We wish to mention here that the cmc of the macromonomer obtained by CV method is also in good agreement with the Fourier transform pulsed gradient spin-echo (FT-PGSE) technique.9 Recently, we have determined10 the exact shape, size, hydration, correlation times, and thermodynamic and various physicochemical parameters of macromonomer micelles. Such studies10 indicated that the macromonomer in aqueous solutions is spherical and ellipsoidal in nature depending on the temperature and concentration of the macromonomer. However, at the cmc of the macromonomer at 25 °C it is spherical even in the presence of 0.1 M KCl. The hydrodynamic radius, Rhm, of the whole of macromonomer micelles was estimated by the FT-PGSE technique on the basis of the number of protons and it was found to be 22.4 Å9, which is in good agreement with our transport studies.10 The Rhm value from the selfdiffusion coefficient data of the poly(oxyethylene) (POE) group in macromonomer micelles (well above cmc) determined by the FT-PGSE technique9 is found to be 13 Å, suggesting that the POE group is in the meander conformation in macromonomer micelles. Therefore, it is clearly evident from the above results that the hydrophillic group (size) contribution is approximately 1.6 times higher than that of hydrophobic group for the above macromonomer micelles. In fact, our investigated macromonomer is highly water soluble and it binds approximately six water molecules around the oxygen center of each unit of oxyethylene chains.10 Electrochemical diffusivity measurements in micellar systems have been reported by (10) Geetha, B.; Mandal, A. B. J. Chem. Phys. 1996, 105, 96499656.
Notes
Rusling,11 Mandal et al.,6,9 and others.12 All these studies have involved the diffusion of EC probe in isotropic micellar solutions. Recently, the diffusional anisotropy of the EC probe in thin lyotropic liquid crystal films has been elaborately studied by Murray et al.13 The probe diffusion within the hydrophobic regions of the micelles (Dm) is expected to be much slower than that in the aqueous region(Dw), and accordingly the partition coefficient values are reflected. However, in our present case the anisotropy behavior has been ruled out because the existing macromonomer is highly water soluble, and accordingly the solubility of the EC probe is approximately 1.6 times higher in bulk phase than that in micellar phase (cf. Table 1, where the value of Kb is 1.6 times higher than that of Km). We have recently found14 on the basis of the rate of the spin-lattice relaxation per single proton of the macromonomer that the core of the macromonomer micelles is rigid compared to its surface. Therefore, the low (11) Rusling, J. F. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 28-41, and references therein. (12) (a) Dayalan, E.; Qutubuddin, S.; Texter, J. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH: New York, 1992; Chapter 10. (b) Verrall, R. E.; Milioto, S.; Giraudeau, A.;Zana, R. Langmuir 1989, 5, 1242-1249. (c)Avranas, A.; Sazou, D. J. Colloid Interface Sci. 1994, 164, 309-317. (13) (a) Postlethwaite, T. A.; Samulski, E. T.; Murray, R. W. Langmuir 1994, 10, 2064-2067. (b) Chen, C; Postlethwaite, T. A.; Hutchison, J. E.; Samulski, E. T.; Murray, R. W. J. Phys. Chem. 1995, 99, 88048811. (14) (a) Geetha, B.; Mandal, A. B. Chem. Phys. Lett. 1997, in press. (b) Geetha, B.; Mandal, A. B. Submitted for publication in Langmuir. Molecular dynamics of the macromonomer micelles in absence and presence of sodium dodecyl sulfate micelles as a function of their mole compositions have been investigated using 1H NMR spin-lattice relaxation time measurements at 300 and 400 MHz field frequency dispersions.
Langmuir, Vol. 13, No. 8, 1997 2413
solubility of the present EC probe in the micellar phase of the investigated macromonomer compared to bulk phase is justified.6e The use of [Co(sep)]Cl3, reported in our recent publication9 and present investigation, as an EC probe with the surface active macromonomer compound is new. However, in view of its water solubility, its utility as a micellar probe is limited at higher concentration. The use of CV to determine cmc is neither new nor novel, but it is one of the supplementary techniques for the determination of the cmc. Furthermore, CV is an excellent tool to determine the extent of solubility of the EC probe in micelles, and accordingly partition coefficients have been estimated. The investigated macromonomer forms secondary aggregated structures14 in aqueous solutions. The second cmc value of this macromonomer at 25 °C is ∼0.12 M in water, which is more prone to undergo polymerization. In fact, the neat macromonomer or its solution of high concentration in water form liquid crystalline behavior.4 However, we have not yet studied elctrochemically by using a high concentration of the macromonomer solutions in water. The present studies will be useful in quantifying the extent of partitioning behavior during polymerization processes. Acknowledgment. We are thankful to Dr. K. V. Raghavan, former Director, Central Leather Research Institute, Madras, for his keen interest and encouragement in this work. Stimulating discussion with Dr. T. Ramasami, Director, CLRI is highly appreciated. The support of the Research Council is gratefully acknowledged. LA951047E
