© Copyright 2001 American Chemical Society
SEPTEMBER 18, 2001 VOLUME 17, NUMBER 19
Letters Comparison of the Surface Activity and Bulk Aggregation of Ferrocenyl Surfactants with Cationic and Anionic Headgroups Nihal Aydogan and Nicholas L. Abbott* Department of Chemical Engineering, University of WisconsinsMadison, 1415 Engineering Drive, Madison, Wisconsin 53706 Received February 2, 2001. In Final Form: June 22, 2001 We report the synthesis and aqueous solution properties of a novel redox-active surfactant, Fc(CH2)11SO3-Na+, where Fc is ferrocene. Electrochemical oxidation of Fc to the ferrocenium cation (Fc+) reversibly transforms the anionic surfactant (Fc(CH2)11SO3-) into a zwitterionic species (Fc+(CH2)11SO3-). When dissolved at concentrations between 0.15 and 0.4 mM in aqueous solutions containing 10 mM Li2SO4, oxidation is accompanied by the reversible transformation of a solution of monomeric species of Fc(CH2)11SO3- into vesicle-like (Dh ) 70 ( 8 nm, Rg/Rh ) 1.0 ( 0.3) surfactant assemblies of Fc+(CH2)11SO3-. In contrast, at concentrations above 0.4 mM, oxidation reversibly transforms a solution of globular (Dh ) 6 ( 2 nm) micelles into vesicle-like assemblies of Fc+(CH2)11SO3-. These various oxidation-induced changes in aggregation are reflected in the interfacial properties of this surfactant system, which are substantially different from those of ferrocenyl surfactants with cationic headgroups (Fc(CH2)11N+(CH3)3).
This paper reports the synthesis of a novel redox-active surfactant, sodium (11-ferrocenylundecyl) sulfonate (Fc(CH2)11SO3-Na+, where Fc is ferrocene). Electrochemical oxidation transforms this anionic surfactant into a zwitterionic species (Fc+(CH2)11SO3-). Whereas past studies by us and others have demonstrated that oxidation of a ferrocenyl surfactant with a cationic headgroup (Fc(CH2)nN+(CH3)3Br- where n ) 8, 11, or 15) can lead to the disassembly of micellar aggregates (Dh ) 6 ( 2 nm, where Dh is the hydrodynamic diameter) to monomeric species within bulk solution,1-5 here we report the * To whom correspondence should be addressed. Fax: 608-2625434. E-mail:
[email protected]. (1) Saji, T.; Hoshino, K.; Aoyagui, S. J. Chem. Soc., Chem. Commun. 1985, 865. (2) Saji, T.; Ishii, Y. J. Electrochem. Soc. 1989, 136, 2953. (3) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209. (4) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116. (5) Aydogan, N.; Gallardo, B. S.; Abbott, N. L. Langmuir 1999, 15, 722.
oxidation-induced assembly of a ferrocenyl surfactant with an anionic headgroup into vesicle-like aggregates with hydrodynamic diameters of 70 ( 8 nm. Differences in the aggregation behaviors of the cationic and anionic ferrocenyl surfactants in bulk solution are reflected in the interfacial properties of these two surfactant systems. The surfactant system described in this paper is founded on our recent observations3-5 that electrochemical control of the oxidation state of redox-active surfactants with the structure Fc(CH2)nN+(CH3)3Br-, where n ) 8, 11, and 15, can lead to large and reversible changes in the surface tensions of aqueous solutions. We have demonstrated the use of these surfactants to achieve spatial and temporal control of properties of aqueous surfactant solutions.6,7 These past studies, when combined with a molecular thermodynamic model of Gibbs monolayers of redox-active surfactants, led us to identify the central role that (6) Bennett, D. E.; Gallardo, B. S.; Abbott, N. L. J. Am. Chem. Soc. 1996, 118, 6499. (7) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57.
10.1021/la010178e CCC: $20.00 © 2001 American Chemical Society Published on Web 08/24/2001
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Figure 1. Molecular structure of (A) Fc(CH2)11SO3- and (B) Fc+(CH2)11SO3-.
oxidation-induced changes in the bulk aggregation properties of these surfactants play in strategies aimed at achieving active control of interfacial properties of surfactant systems.5 For example, whereas oxidation of Fc(CH2)11N+(CH3)3 within an aqueous solution at a concentration below its critical micelle concentration (cmc) (0.1 mM) results in desorption of the oxidized surfactant (Fc+(CH2)11N+(CH3)3) from the surface of the solution, at concentrations above the cmc of Fc(CH2)11N+(CH3)3, oxidation of Fc(CH2)11N+(CH3)3 leads to the adsorption of the oxidized surfactant to the surface of the solution. This qualitative difference (oxidation-induced adsorption versus oxidation-induced desorption) in the response of the system to oxidation of Fc(CH2)11N+(CH3)3 was demonstrated to reflect the change in chemical potential of the surfactant within the bulk of the solution. At concentrations above the cmc of Fc(CH2)11N+(CH3)3, oxidation is accompanied by the disassembly of micelles (increase in cratic contribution to chemical potential), whereas below the cmc of Fc(CH2)11N+(CH3)3, there is no change in the state of aggregation with oxidation. These results, when combined, led us to hypothesize that the design of redoxactive surfactants that possess bulk aggregation behaviors that differ substantially from those of Fc(CH2)11N+(CH3)3 would also offer interfacial responses to oxidation that differ substantially from those measured in our past studies. This paper reports an experimental study that tests the above-described proposition by synthesis of a ferrocenyl surfactant that possesses an anionic (sulfonate) headgroup. This choice of surfactant is based, in part, on our recent observation that irreversible chemical oxidation of N+(CH3)3(CH2)8SS(CH2)8N+(CH3)3 (a bolaform cationic surfactant) to N+(CH3)3(CH2)8SO3- (a zwitterionic surfactant) promotes the self-association of this type of surfactant into aggregates that have large hydrodynamic diameters (∼100 nm, as determined by quasi-static light scattering).8 Here, we test the hypothesis that reversible electrochemical oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3- will promote the reversible aggregation of this surfactant in solution and lead to types of aggregates that differ substantially from those in our past studies with Fc(CH2)11N+(CH3)3. Because oxidation-induced aggregation will decrease the cratic contribution to the chemical potential of the surfactant in bulk solution, we hypothesized that oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3(Figure 1) would lead to an increase in the surface tension of aqueous solutions of these surfactants. The surfactant Fc(CH2)11SO3- was synthesized by modification of a procedure reported previously.9,10 The (8) Jong, L. I.; Abbott, N. L. Langmuir 2000, 16, 5553.
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Figure 2. Cyclic voltammogram of 0.2 mM Fc(CH2)11SO3measured in an aqueous solution of 100 mM Li2SO4, 37 °C, pH 5. The scan rate was 10 mV/s. The surface area of the platinum working electrode was approximately 1 cm2.
equilibrium surface tensions of aqueous solutions of Fc(CH2)11SO3- and Fc+(CH2)11SO3- were measured using a FTÅ200 pendant bubble tensiometer (First Ten Angstroms, Portsmouth, VA). Dynamic surface tensions were measured using a maximum bubble pressure apparatus (Sensadyne) at a bubble rate of 1 Hz. All surface tension measurements were repeated at least twice. Because the Krafft temperature of Fc(CH2)11SO3- in water was measured to be 32 °C, all surface tension measurements were performed at 37 °C (10 mM Li2SO4, pH 5). Oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3- was performed electrochemically using a platinum flag working electrode. The cyclic voltammogram of 0.2 mM Fc(CH2)11SO3- in 0.1 M Li2SO4 (Figure 2) shows one oxidation peak at +150 mV (vs SCE) and one reduction peak at +80 mV (vs SCE). Both peaks are within the range of reported redox couples for ferrocene derivatives11 and indicate that the electrochemistry is reversible. We performed static and dynamic light scattering measurements of the bulk solutions using a Dawn DSP laser photometer (Wyatt Technology Inc., Santa Barbara, CA) and Coulter N4 photon correlation spectrometer (Beckman Coulter Inc., Fullerton CA). The autocorrelation curves were analyzed by Contin,12 and the angular dependence of the intensity of scattered light was analyzed using standard methods.13 Samples for light scattering measurements were prepared using aqueous solutions filtered through a 0.22 µm filter (Millipore, (9) Bromoundecyl ferrocene was synthesized by using a previously reported procedure (Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450.). Surfactant Fc(CH2)11SO3-Na+ was prepared by reaction of 11-bromoundecyl ferrocene with Na2SO3 (ref 10). The surfactant was purified by hot filtration in boiling ethanol and repeated crystallization from ethyl acetate/acetone solutions (Fisher, Pittsburgh, PA) until the surface tensions of aqueous solutions did not change upon further crystallization. The surfactant was characterized as follows. NMR (DMSO) δ: 4 (9 H, Hferrocene), 2.8 (2 H, CH2SO3), 2.3 (2 H, ferroceneCH2), 2.7 (2H, CH2CH2SO3), 2.5 (2H, CH2CH2CH2SO3), 1.3 (14 H, CH2CH2CH2). Mass spectroscopy: MW of Fc(CH2)11SO3- was measured as 419.1 (calculated 419). Elemental analysis (measured): C, 0.546; H, 0.071; Na, 0.053; S, 0.075; Fe, 0.121. Calculated for pure compound: C, 0.57; H, 0.070; Na, 0.052; S, 0.072; Fe, 0.127. A comparison of calculated and measured compositions reveals the presence of the H2O and NaHSO4 in the sample. The ratio of C/Fe is similar for both calculated and measured compositions. We conclude that the sample contains 15% water and less than 5% NaHSO4 (ref 10). By including the H2O and NaHSO4, we calculated the following composition: C, 0.550; H, 0.070; Na, 0.054; S, 0.075; Fe, 0.124. Further purification of the surfactant does not change the properties reported in this study. (10) Gilbert, E. E. Sulfonation and Related Reactions; Interscience: New York, 1965. (11) Emilia, M.; Silva, N. P. R. A.; Pombeiro, A. J. L.; Dasilva, J. J. R. F.; Herrmann, R.; Deus, N.; Bozak, R. E. J. Organomet. Chem. 1994, 480, 81. (12) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213.
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Langmuir, Vol. 17, No. 19, 2001 5705 Table 1. Hydrodynamic Diameters of Aggregates of Ferrocenyl Surfactants (0.6 mM) within Aqueous Solutions (10 mM Li2SO4)a surfactant
Dh (nm)
Fc(CH2)11SO3Fc+(CH2)11SO3Fc(CH2)11N+(CH3)3b Fc+(CH2)11N+(CH3)3b
6(2 70 ( 8 6(2 does not aggregate
concentration (mM) >0.4 >0.15 >0.1 0.4 mM). By using static light scattering, we measured the radius of gyration of aggregates of Fc+(CH2)11SO3- to be 38 ( 8 nm.13 Past studies have shown by experiment and theory15 that the ratio Rg/Rh varies with the geometry of the aggregate: solid spheres (Rg/Rh ∼ 0.78),15 thin-shelled vesicles (Rg/Rh ∼ 1.0), random coil (Rg/Rh ∼ 1.5-1.7),16 and rods (Rg/Rh ∼ 2).17 We calculate Rg/Rh to be 1.0 ( 0.3, consistent with the presence of vesicular aggregates. Past studies of mixtures of anionic and cationic surfactants have also reported formation of vesicular structures.18,19 The above-described changes in the state of aggregation of the surfactant (driven by oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3-) manifest themselves in a variety of changes in the interfacial properties of this surfactant system. We structure our observations using the three regimes of bulk solution behavior: (I) c < 0.15 mM, (II) 0.15 mM < c < 0.4 mM, and (III) c > 0.4 mM (Figure 3A). First, at low surfactant concentrations (c < 0.15 mM), where neither Fc(CH2)11SO3- nor Fc+(CH2)11SO3- aggregate in bulk solution, changes in interfacial properties are driven largely by oxidation-induced changes in the interactions between molecules within the Gibbs monolayer at the surface of the solution. Oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3- results in a decrease in the electrostatic contribution to the free energy of formation of the monolayer. This decrease in the electrostatic contribution to free energy of formation is compensated by a decrease in the hydrophobic driving force for adsorption.5 Because these two effects largely compensate each other, the excess concentration of surfactant at the surface of the solution changes little upon oxidation (4.7 ( 0.2 to 4.9 ( 0.2 µmol/m2 at 0.08 mM). The influence of the slight increase in the excess concentration of surfactant (15) Brown, W. Y. N. Light Scattering: Principles and Development; Clarendon Press: Oxford, 1996. (16) Herzog, B.; Huber, K.; Rennie, A. R. J. Colloid Interface Sci. 1994, 164, 370. (17) Vandesande, W.; Persoons, A. J. Phys. Chem. 1985, 89, 404. (18) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 1371. (19) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267.
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on the surface pressure is modest (∼2 mN/m) because the electrostatic contribution to the surface pressure is also diminished upon oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3-. Second, at intermediate concentrations of surfactant (0.15 mM < c < 0.4 mM), where oxidation of Fc(CH2)11SO3to Fc+(CH2)11SO3- leads to the assembly of monomers of Fc(CH2)11SO3- into aggregates of Fc+(CH2)11SO3- with hydrodynamic sizes of 70 ( 8 nm, oxidation leads to larger changes in surface tension than are observed at lower concentrations (c < 0.15 mM). Whereas the chemical potential of Fc+(CH2)11SO3- is pinned at a value corresponding to its critical aggregation concentration (0.15 mM), Fc(CH2)11SO3- does not aggregate in this same window of concentrations and thus the excess surface concentration of Fc(CH2)11SO3- increases with bulk concentration. This increase in the excess surface concentration of Fc(CH2)11SO3- causes the surface tensions of the reduced surfactant to lie substantially below those of the oxidized surfactant. We note that the limiting surface tension of Fc(CH2)11SO3- is 40 mN/m, a value that is ∼9 mN/m below the limiting surface tensions of Fc(CH2)11N+(CH3)3 that are reported in our past studies.3-5 We also note that the limiting excess surface concentration of Fc(CH2)11SO3- (4.7 ( 0.2 µmol/m2) is substantially higher than that measured in our past studies of Fc(CH2)11N+(CH3)3 (2.2 ( 0.2 µmol/m2; see Figure 3B).3-5 We do not yet understand the factors that cause these differences in the limiting excess surface concentrations and surface tensions of aqueous solutions of Fc(CH2)11N+(CH3)3 and Fc(CH2)11SO3-. Third, at concentrations above 0.4 mM, both Fc(CH2)11SO3- and Fc+(CH2)11SO3- form aggregates and thus the chemical potential of both surfactants are very weak functions of the bulk concentration. This behavior in the bulk solution gives rise to interfacial properties that depend weakly on the bulk concentrations. The change in surface tension (∼15 mN/m) that is measured in this regime largely reflects the different critical aggregation concentrations of these two states of the surfactant. We note that the concentration-independent interfacial properties in this regime of behavior do contrast with our past studies of cationic ferrocenyl surfactants. This difference is due to the fact that Fc+(CH2)11N+(CH3)3 does not aggregate in bulk solution (over the range of concentrations that we have studied; see Figure 3B). We conclude this paper by making two additional comments regarding the properties of aqueous solutions of Fc(CH2)11SO3-. First, because the oxidation of Fc(CH2)11SO3- to Fc+(CH2)11SO3- transforms an anionic surfactant into a zwitterionic one, we predicted that the critical aggregation concentrations of these two states of the surfactant (0.4 and 0.15 mM, respectively) could be separated further by decreasing the ionic strength below the value corresponding to 10 mM Li2SO4. This prediction was verified by measurement of the critical aggregation concentrations of Fc(CH2)11SO3- and Fc+(CH2)11SO3- in 1 mM Li2SO4 (0.15 and 0.5 mM, respectively). Second, we have demonstrated the chemical stability of both Fc(CH2)11SO3- and Fc+(CH2)11SO3- within aqueous solutions by the reversible transformation of these species with concurrent measurement of surface tension (Figure 4). In summary, this paper reports the bulk solution and interfacial properties of aqueous solutions of Fc(CH2)11SO3and Fc+(CH2)11SO3-, where Fc+(CH2)11SO3- is obtained by electrochemical oxidation of Fc(CH2)11SO3-. In contrast to past studies of the bulk and interfacial properties of cationic ferrocenyl surfactants (Fc(CH2)11N+(CH3)3), the surfactant described here exhibits substantially different
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Figure 4. Dynamic surface tensions of an aqueous solution of 0.6 mM Fc(CH2)11SO3-/Fc+(CH2)11SO3- (at 10 mM Li2SO4, 37 °C, pH 5) measured during the repeated cycling of the surfactant between oxidation states. The high values of surface tension correspond to a solution of Fc+(CH2)11SO3-, and the low values correspond to a solution of Fc(CH2)11SO3-.
(and potentially useful) properties. At low concentrations (c < 0.4 mM), we observe the oxidation of Fc(CH2)11SO3to Fc+(CH2)11SO3- to lead to the spontaneous transformation of monomeric species into large vesicle-like aggregates (Dh ) 70 ( 8 nm and Rg/Rh ) 1.0 ( 0.3). At high concentrations (c > 0.4 mM), globular micellar aggregates are transformed into vesicle-like aggregates. In contrast, past studies of Fc(CH2)11N+(CH3)3 have reported the oxidation of Fc(CH2)11N+(CH3)3 to Fc+(CH2)11N+(CH3)3 to be accompanied by the disassembly of globular micelles to monomers. The bulk solution behavior of Fc(CH2)11SO3and Fc+(CH2)11SO3- leads to an oxidation-induced, concentration-independent change in the surface tension of ∼15 mN/m for concentrations of surfactant above 0.4 mM. In contrast, the interfacial properties of Fc(CH2)11N+(CH3)3 are strongly concentration dependent because Fc+(CH2)11N+(CH3)3 does not aggregate in solution (c < 30 mM). Finally, we note that the limiting surface tension of Fc(CH2)11SO3- was measured to be ∼9 mN/m lower than the limiting surface tension of aqueous solutions of Fc(CH2)11N+(CH3)3. Acknowledgment. This work was supported in part by the Camille and Henry Dreyfus Foundation (TeacherScholar Award), the David and Lucile Packard Foundation, the donors of the Petroleum Research Fund (ACSPRF35409-AC7), and the National Science Foundation (CTS-9911863). Nihal Aydogan acknowledges partial support from the Ministry of Education of Turkey. Appendix The Gibbs adsorption equation for a ferrocenyl can be written as3,4
Γ)-
dγ 1 mkT d ln c
(1)
where Γ is the excess surface concentration, C is the concentration of surfactant in bulk solution, and m is a numerical prefactor that depends on the concentration of added electrolyte.20 We calculate the prefactor to be 1.012 for a solution containing 0.4 mM Fc(CH2)11SO3- and 10 mM Li2SO4.4 LA010178E (20) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989.
