Tuning Surface Tension and Aggregate Shape via a Novel Redox

Jan 27, 2006 - This desorption of surfactant molecules gives rise to the oxidation-induced surface tension change up to 15 mN/m. Although this new mol...
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Langmuir 2006, 22, 2028-2033

Tuning Surface Tension and Aggregate Shape via a Novel Redox Active Fluorocarbon-Hydrocarbon Hybrid Surfactant Nihal Aydogan* and Nihan Aldis Department of Chemical Engineering, Hacettepe UniVersity, 06800 Ankara, Turkey ReceiVed October 16, 2005. In Final Form: December 13, 2005 This paper reports the surface and bulk properties of a newly designed redox active hybrid surfactant Fc(CH2)11N+(C2H5)2(CH2)2(CF2)5CF3 I- or FcFHUB, where Fc is ferrocene. This new surfactant displays strong surface tension lowering ability (31 mN/m) and low critical micelle concentration (0.03 mM in 100 mM Li2SO4). The minimum area per surfactant molecule at the interface is determined as 121 Å2/molecule. The electrochemical oxidation of ferrocene (Fc) to ferrocenium cationic (Fc+) leads to reversible changes in the surface and bulk properties of this surfactant. Following the oxidation, desorption of surfactant molecules from the surface of the solution takes place. This desorption of surfactant molecules gives rise to the oxidation-induced surface tension change up to 15 mN/m. Although this new molecule shows salt-insensitive behavior in its reduced form, the oxidized form of the surfactant shows slight sensitivity to the electrolyte concentration. The molecular structure of FcFHUB allows the formation of large aggregates in the form of coils at a temperature of 33 °C. When the temperature rises to 50 °C, the aggregates are determined to be in the vesicle structure. The oxidation of Fc to Fc+ disrupts large aggregates to the smaller aggregates at low temperatures. The oxidation of surfactant molecules at high temperature leads to disruption of the aggregates to monomers.

Introduction Redox active surfactants have been investigated by many researchers because of their properties, which cannot be obtained from classical surfactants.1-5 Ferrocene is one of the redox active groups which is used frequently to give redox active character to the surfactant molecule.2-5 The properties of ferrocenyl surfactant with a single hydrocarbon chain FTMA ((11-ferrocenylundecyl)trimethylammonium bromide) and ferrocenyl surfactant with double hydrocarbon chain BFDMA (bis(11ferrocenylundecyl)dimethylammonium bromide) have been studied in detail.6-9 The redox active surfactant FTMA leads to large and reversible changes in the surface tension of aqueous solutions.6,7 Oxidation of the ferrocene group chemically or electrochemically gives rise to an increase in the surface tension of the surfactant solution from 49 to 72 mN/m at its critical micellization concentration (cmc). Reduction of the ferrocenium group results in the reduction of surface tension to its original value.8 This reversible change in surface tension permits electrochemical control of a variety of surfactant-induced phenomena, including convection at the surface of aqueous solutions.10 In addition to these experimental studies, detailed investigation of the molecular level contribution of the presence of the ferrocene group within the surfactant structure to the redox active behavior of this molecule has been performed using the molecular thermodynamic approach.8 It has been revealed from this study that oxidation of the ferocene group within the surfactant molecule gives rise to an increase * Corresponding author. E-mail: [email protected]. (1) Hayashita, T.; Kurosawa, T.; Miyata, T.; Tanaka, K.; Igawa, M. Colloid Polym. Sci. 1994, 272, 1611. (2) Rosslee, C.; Abbott, N. Curr. Opin. Colloid Interface Sci. 2000, 5, 81-87. (3) Saji, T.; Hoshino, K.; Aoyagui, S. J. Chem. Soc., Chem. Commun. 1985, 865-866. (4) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450-456. (5) Takei, T.; Sakai, H.; Yukishige, K.; Yoshino, N.; Abe, M. Colloids Surf., A 2001, 183-184, 757-765. (6) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209-4212. (7) Gallardo B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 41164124. (8) Aydogan, N., Gallardo, B. S., Abbott, N. L. Langmuir 1999, 15, 722-730. (9) Yoshino, N., Shoji, H.; Kondo, Y.; Kakizawa, Y.; Sakai, H.; Abe, M. J. Jpn. Oil. Chem. Soc. 1996, 45, 55-61. (10) Bennet, D. E.; Gallardo, B. S.; Abbot, N. L. J. Am. Chem. Soc. 1996, 118, 6499-6505.

of the electrostatic interaction between surfactant molecules adsorbed to the interface as well as a decrease in the hydrophobicity of the surfactant molecule. These changes in the balance of forces are used to explain the reasons behind the oxidation induced desorption of surfactant molecules from the interface.11 Moreover it is considered that the ferrocenyl moiety of the surfactant is able to take the molecular orientations depending on the redox states. The conformational changes of the ferrocenemodified surfactants affect the micelle formation in the surfactant solution as well as the interfacial properties.12,13 These observations underline the redox active character of the surfactants and explain the usefulness of them for active control of the interfacial and the bulk properties of the aqueous solutions.12 Another ferrocene-containing surfactant studied in the literature is the BFDMA, which has double alkyl chains.9,13 Because this molecule bears two ferrocene groups in addition to two hydrophobic chains, its hydrophobicity is large compare to FTMA which leads to lower cmc. In the studies, where BFDMA is utilized, formation of vesicles is observed. The formation of aggregates in the form of vesicles is attributed to the rodlike structure of the BFDMA molecule.9,13 Recently, it has been shown that the redox active state of this lipid can be used to control cell transfection.14 Fluorinated or hybrid surfactants are the other types of surfactants that find attention in the literature.15,16 These kinds of surfactants are able to lower the surface tension of water much more effectively and more efficiently than their hydrocarbon counterparts. Their critical micellization concentrations are usually 3-4 orders of magnitude lower than that of the corresponding hydrocarbon surfactants.15,16 The fluorocarbon chain is more rigid than a hydrocarbon chain because of the bulky fluorine atoms.17 The aggregates of the fluorocarbon-containing surfac(11) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.; Shah, R. R.; Abbott, N. L. Science 1999, 283, 57-60. (12) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. Soc. 1985, 107, 68656866. (13) Kakizawa, Y.; Sakai, H.; Nishiyama, K.; Abe, M. Langmuir 1996, 12, 921-924. (14) Abbott, N. L.; Jewell, C. M.; Hays, M. E.; Kondo, Y.; Lynn, D. M. J. Am. Chem. Soc. 2005, 127, 11576-11577. (15) Riess, J. G.; Krafft, M. P. Biomaterials 1998, 19, 1529-1539. (16) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591-7599. (17) Krafft, M. P. AdV. Drug DeliVery ReV. 2001, 47, 209-229.

10.1021/la052786q CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006

Redox ActiVe and Temperature SensitiVe Surfactant

Langmuir, Vol. 22, No. 5, 2006 2029

Materials and Methods

Figure 1. Molecular structure of the redox active hybrid surfactant FcFHUB.

tants have structures with less surface curvature (i.e. vesicle, lamellar, and threadlike micelles), thereby having a larger molecular volume of the aggregate compared to that for the corresponding hydrocarbon-containing surfactants in aqueous solution.18 In addition, surfactants having the hybrid dimeric surfactant structure (C8FC4-2-C12) forms aggregates at a 0.2 mM surfactant concentration which is a lower value than those of hydrogenated surfactants with the same chain length.19 These studies underline the good surface and interfacial tension lowering ability of hybrid surfactants. Beside the strong surface tension lowering action, the hybrid type surfactants can exhibit a thermal resistance, chemical resistance, and lubricating action.20 Our past study has demonstrated the usefulness of hybrid (fluorocarbon-hydrocarbon) unsymmetrical bolaform surfactant, which is called FHUB (OH(CH2)11N+(C2H5)2(CH2)2(CF2)5CF3 I-).21 This new hybrid surfactant has properties that were reached by combining a fluorocarbon chain and the hydrocarbon chain in a way to give better performance (i.e., low critical micellization concentration, low limiting surface tension, salt insensitivity, and antifoaming character).21,22 The surface and bulk properties of a surfactant molecule which bears a ferrocenyl group as well as a fluorocarbon chain have not been reported yet. In this paper, properties of a new surfactant molecule FcFHUB (Fc(CH2)11N+(C2H5)2(CH2)2(CF2)5CF3 I-) whose chemical structure has been modified by replacing the hydroxyl group of FHUB with the redox-active ferrocene group as seen from Figure 1 have been investigated. This new modified surfactant combines the features of FHUB molecule such as low critical micellization concentration, low limiting surface tension, salt insensitivity, and low foaming ability with the properties of the redox active surfactants such as reversible control of surface tension and aggregation state. In addition, the comparison of the properties of this new ferrocenyl surfactant with previously reported molecules are also given to bring more understanding to the behavior of ferrocenyl surfactants. (18) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237-9246. (19) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1996, 12, 5768-5772. (20) Abe, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 354-356. (21) Aydogan, N.; Aldis, N.; Guvenir, O. Langmuir 2003, 19, 10726-10731. (22) Calik, P.; Erdinc, B. I., Ileri, N.; Aydogan, N.; Argun, M. Langmuir 2005, 21, 8613-8619. (23) Aydogan, N.; Abbott, N. L. J. Colloid Interface Sci. 2001, 242, 411-418.

1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8-iodooctane, diethylamine, and trimethylamine were purchased from Across (Belgium). The acetone, acetonitrile, hexane, diethyl ether, lithium sulfate, cerium sulfate n-hydrate, and mercury(II) bromide were purchased from Sigma (Germany). The new redox active surfactant (CF3(CF2)5CH2CH2N+(CH3CH2)2(CH2)11Fc I- or FcFHUB) was synthesized in our laboratory. (11-Bromoundecanoyl)ferrocene was synthesized as described before.6 The obtained (11-bromoundecyl)ferrocene is solved in 20 mL of ethanol in a flask. Then 9 mL of diethylamine was added into the flask. The reaction mixture was stirred at room temperature for 48 h. Following the reaction, the volatile substances were evaporated. Equal molar 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluoro8-iodo-octane (0.8 mL) was added slowly into ethanol solution of 11-ferroceneundecane diethylamine over 30 min. The reaction mixture was stirred and refluxed for 96 h. After the solvent was evaporated, the surfactant FcFHUB was extracted with diethyl ether and further purified by recrystalization. Chemical characterization of FcFHUB has been performed through 1H NMR and elemental analysis. 1H NMR (DMSO): δ 4.2 (9H, Fc), 3.4 (4H, CH3CH2N+), 3.2 (2H, N+CH2CH2CF2), 3.0 (2H, N+CH2CH2), 2.5 (2H, CH2CF2), 1.6 (2H, N+CH2CH2), 1.4 (6H, CH3CH2N), 1.2 (14H, CH2). Anal. Calcd: C, 0.51; H, 0.052; N, 0.016. Found: C, 0.5; H, 0.052; N, 0.016. Aqueous surfactant solutions were prepared freshly for each experiment using water from a water purification system (Human, Korea). The equilibrium surface tensions were measured in electrolyte solutions of Li2SO4 using a tensiometer (Kruss, Germany) in the reduced and oxidized states with the Wilhelmy plate method. Preparation of aqueous Fc+FHUB (oxidized form) solutions was made by adding a sufficient amount of cerium sulfate to the solution of FcFHUB (reduced form). All the surface tension measurements were repeated at least twice. Surface tension measurements were performed at 25, 30, and 40 °C in 100 mM Li2SO4 and 10 mM Li2SO4 electrolytes at pH 2. All the glassware was cleaned in piranha solution (18 M H2SO4, 30% H2O2, 70:30 (v/v)). Warning: piranha solution should be handled with extreme caution; in some circumstances (most probably when it has been mixed with significant quantities of an oxidizable organic material), it has detonated unexpectedly. Dynamic light scattering was used to determine the hydrodynamic radius of aggregates formed by FcFHUB and Fc+FHUB at different temperatures and concentrations. For this purpose Zetasizer Nano Series, Zen 1600 (Malvern, England), was used where the average decay rate was obtained from the measured autocorrelation function using the method of cumulants employing a quadratic fit. The magnitude of the scattering vector was given by q ) (4πn/λ) sin(θ/2), where n is the refractive index of the solvent, λ is the wavelength of light, and θ is the scattering angle. Measurements were made at an angle of 173°. The samples were prepared by using an electrolyte solution which is prefiltered. Static light scattering experiments were performed using a CGS-3 goniometer (Malvern, England) in which measurements at angles from 30 to 150° with 5-deg increments were carried out. These experiments have been used to determine the radius of gyration and form factor, which can be employed to evaluate and compare the shape of the aggregates with the selected structures using ALVSTAT software.

Results and Discussion Interfacial Behavior of FcFHUB. Figure 2 shows the concentration dependence of the equilibrium surface tension of the oxidized and reduced forms of the aqueous FcFHUB solution which contains 100 mM Li2SO4 electrolyte at 30 °C. It is revealed from the figure that the reduced form of the surfactant starts to form aggregates at surfactant concentration as low as 0.02 mM, where the surface tension is measured as 31 mN/m. The cmc of the oxidized surfactant (Fc+FHUB) is determined to be 0.13 mM, which is higher than the cmc of the reduced surfactant

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Aydogan and Aldis

Table 1. Comparison of the Interfacial and Bulk Properties of FcFHUB with Those of Other Surfactants surfactanta

CMC (mM)

γlim (mN/m)

Alim (Å2)

ghyd (kT)

TKrafft (°C)

ref no.

HTAB FTMA BFDMA FHUB FcFHUB (100 mM Li2SO4, 30 °C) Fc+FHUB (100 mM Li2SO4, 30 °C)

21.000 0.1 0.0001 0.450 0.02 ( 0.005 0.13 ( 0.005

48 49 43 25 31 ( 2 32 ( 2

68 ( 5 85 ( 5

-16.0 -26.0 52 -34.4 -42.5 -39.8

28

23 8 9 21

88 ( 5 121 ( 5 135 ( 5

38 28

a

HTAB, ω-hydroxyundecyltrimethyle ammonium bromide; FTMA, (11-ferrocenylundecyl) trimethylammonium bromide; BFDMA, bis(11ferrocenylundecyl)dimethylammonium bromide.

Figure 2. Surface tension of the aqueous solutions of FcFHUB at 30 °C in (b) reduced form and (O) oxidized form, in 100 mM Li2SO4 electrolyte.

(FcFHUB), as expected. The limiting surface tension of the solution containing Fc+FHUB is measured as 32 mN/m. Another important observation obtained from Figure 2 is that the oxidation of the surfactant molecule of solution containing 0.02 mM FcFHUB results in the increase of surface tension from 32 to 47 mN/m (a 15 mN/m increase), which is similar to the surface tension change obtained from ferrocenyl surfactant with an anionic headgroup.24 To understand the structure performance relation of this new redox active hybrid surfactant, the interfacial properties of FcFHUB are compared with several other surfactants, which have certain structural similarities as summarized in Table 1. Both FcFHUB and FTMA contain a ferrocene group at the other end of the hydrocarbon chain which has 11 carbons. However, FcFHUB bears a fluorocarbon chain which is expected to result in an increase in the hydrophobic driving force for adsorption of the surfactant (ghyd, hydrophobic contribution to the free energy of the monolayer) and a decrease in the cmc.21,25 The hydrophobic free energy contribution to the free energy of the monolayer of the reduced form of FcFHUB is calculated as -42.5 kT by using the formulations in predicting ghyd of classical surfactants25 and the hydrophobic free energy contribution of the ferrocene group that has been used in the past study.8 This number is larger than the hydrophobic free energies of FTMA (-26 kT) and FHUB (-34.4 kT). This high hydrophobicity of FcFHUB leads to lower cmc compared to FHUB (0.45 mM) and FTMA (0.1 mM) as seen from Table 1. On the other hand, a dimeric surfactant which contains two ferrocene groups at the other end of the hydrocarbon chains which is called BFDMA has a hydrophobic free energy contribution (-52 kT) which is larger than that of the FcFHUB, and consequently, it has a cmc (0.0001 mM) lower than the cmc of the FcFHUB in its reduced state (see Table 1). Oxidation of (24) Aydogan, N.; Abbott, N. L. Langmuir 2002, 18, 7826-7830. (25) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; John Wiley and Sons: New York, 1980.

FcFHUB and BFDMA molecules gives rise to an increase in the electrostatic interaction between molecules (which is a positive contribution to the Gibbs free energy of the monolayer) as well as the lowering of the hydrophobic driving force (-2.7 kT for FcFHUB and -5.40 kT for BFDMA).8 However, the ghyd of Fc+FHUB is still high to encourage the formation of aggregates. In addition to the hydrophobicity of a surfactant molecule, configurational constraints and mutual phobicity of fluorocarbon and hydrocarbon segments are also important, which have the potential to affect the interfacial and bulk properties of a surfactant solution. It is known from previous studies that the ferrocene group (Fc) within FTMA molecule prefers to stay close to the air-water interface which affects the configuration of surfactant molecule.7,8 Although, FTMA behaves like bolaform surfactants such as HTAB (ω-hydroxyundecyltrimethylammonium bromide), the minimum area per molecule of FTMA (85 ( 5 Å2) molecule within the interface is larger than the minimum area per molecule HTAB (68 ( 5 Å2).7,23 This difference in the area per molecules is explained simply by the presence of the ferrocene group.23 The minimum area per FcFHUB molecule at the interface is determined to be 121 ( 5 Å2/molecule in the presence of 100 mM Li2SO4 by using the Gibbs adsorption equation,26,27 which is larger than the area of FHUB (88 ( 5 Å2) molecules at the interface. This result suggests that the ferrocene group of FcFHUB associates with the aqueous subphase, thereby forcing FcFHUB into looped configuration. Oxidation of Fc to Fc+ leads to an increase in the electrostatic contribution to the free energy as well as a decrease in the hydrophobic driving force for adsorption.8 The minimum area of Fc+FHUB (135 ( 5 Å2) is calculated to be larger than the minimum area per molecule of the reduced state. Increased electrostatic repulsion of the Fc+FHUB explains the increase at the area per molecule at the interface. The area per molecule calculated for Fc+FHUB is also larger than the area per molecule calculated for FcFHUB and FHUB, suggesting this new redox active and hybrid surfactant (hydrocarbon part) adopts reverse U configuration at the air-water interface. The result of electrostatic repulsion between molecules of Fc+FHUB drives its desorption from the surface of the solution, so the limiting surface tension increases from 32 to 47 mN/m upon oxidation at a 0.03 mM surfactant concentration. Effect of Electrolyte Concentration on the Interfacial Properties. One of the interesting properties of the unsymmetrical bolaform surfactants is their salt-insensitive interfacial behavior.23 This behavior is related to the dominant contribution to the lowering of the surface tension.23 Therefore, we test unsymmetrical bolaform character of the reduced and the oxidized forms of FcFHUB by changing electrolyte concentrations (see Table 2). As can be seen from Table 2, a decrease in the electrolyte concentrations from 100 to 10 mM Li2SO4 causes a slight change in the cmc of FcFHUB, whereas the cmc’s of DTAB and HTAB (26) Ito, A.; Sakai, H.; Kondo, Y.; Kamogawa, K.; Kondo, Y.; Yoshino, N.; Uchiyama, H.; Harwell, J. H.; Abe, M. Langmuir 2000, 16, 9991-9995. (27) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989.

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Langmuir, Vol. 22, No. 5, 2006 2031

Table 2. Effect of the Electrolyte Concentration on the Surface Activity of FcFHUB and Classical Ionic and Unsymmetrical Bolaform Surfactants (T ) 30 °C) Alim (Å2)

γlim (mN/m)

CMC (mM) surfactant

10 mM Li2SO4

100 mM Li2SO4

10 mM Li2SO4

100 mM Li2SO4

10 mM Li2SO4

100 mM Li2SO4

FcFHUB (red.) FcFHUB (ox.) FHUBa,21 HTABa,23 DTABa,23

0.04 0.2

0.03 0.10 0.45 22 3

33 32

31 32 25 48 36

132 ( 5 154 ( 5

121 ( 5 135 ( 5 88 ( 5 65 ( 5 41 ( 5

a

28 8

48 37

In LiBr electrolyte.

change 2.37 and 1.27 times as a result of the decrease in the electrolyte concentration from 100 to 10 mM LiBr.28 Therefore, we propose that FcFHUB molecules behave like an unsymmetrical bolaform surfactant. Like insignificant changes in the cmc of FcFHUB with added electrolyte concentration in the reduced state, the surface properties of FcFHUB change slightly with the addition of electrolyte to the solution. In the oxidized state, an increase in the electrolyte concentration from 10 to 100 mM Li2SO4 reduces the cmc of Fc+FHUB from 0.2 to 0.10 mM, which is related to the differences in the electrostatic contribution of the standard free energy. In parallel, the minimum area per molecule of the oxidized form of Fc+FHUB increases slightly with decreasing electrolyte concentration (Table 2). In the presence of 100 mM Li2SO4, the minimum area per Fc+FHUB molecule is determined to be 135 Å2/molecule by using Gibbs adsorption equation, which is larger than the minimum area of FcFHUB. As mentioned before, oxidation of Fc to Fc+ leads to an increase in the electrostatic contribution because oxidation of Fc to Fc+ doubles the ionic charge on each surfactant molecule that adsorbed to the surface of the solution.8 Therefore, the electrostatic repulsions between surfactant molecules in the oxidized state are higher than the reduced state. The minimum areas per charge of Fc+FHUB are calculated as 67.5 and 77 Å2 in the presence of 100 and 10 mM Li2SO4, respectively. As a result, we determined that Fc+FHUB is more sensitive to the changes in electrolyte concentration than that of FcFHUB. Effect of Temperature on the Interfacial Behavior of FcFHUB. Measurements of the surface tensions of aqueous solutions of FcFHUB and Fc+FHUB in 100 mM LiSO4 electrolyte are repeated at 42 °C in order to investigate the effect of the temperature (Figure 3). The limiting surface tensions of FcFHUB and Fc+FHUB at that temperature are measured as 30 mN/m. The cmc’s of FcFHUB and Fc+FHUB are determined to be 0.03 and 0.065 mM, respectively. We make three observations comparing the surface tensions of FcFHUB and Fc+FHUB measured at these temperatures (30 and 42 °C). First of all, the cmc’s and limiting surface tension of FcFHUB are similar at both temperatures. Second, the cmc and surface tensions of Fc+FHUB decrease by increasing temperature. This unexpected behavior of Fc+FHUB can be explained by the presence of the fluorocarbon chain and the ferrocene group in the structure. In literature, it has been suggested that the organization of the water molecule around the surfactant molecules and also around the micelles could be affected by the change in temperature.29 The two-case model of hydration is a good example for explaining the temperature effect on the behavior of the surfactant molecule.29 Briefly, in this model, it is postulated that there is hydrophilic hydration around the surfactant monomers, being in the form of micelles and two types of hydration around the surfactant molecules existing in the monomeric form, that is, hydrophobic (28) The electrolyte used in this study (Li2SO4) is a 1:2 type electrolyte, whereas LiBr is a 1:1 type electrolyte. A change in the concentration from 100 to 10 mM in the case of Li2SO4 is more effective than that of LiBr.

hydration around the alkyl chain of the surfactant monomer and hydrophilic hydration around its polar headgroup.29 However, the reduced form of the surfactant is not affected significantly by the change in temperature; while the oxidized form shows more temperature sensitivity, we suspect that it should be related to the partitioning of the ferrocenium moiety between water and oil as well as the decrease in the hydration of this moiety with temperature. The hydrophobicity of ferrocene and ferrocenium is calculated from the partitioning of these groups between water and oil (interior of DTAB micelles) using electrochemical methods.30 Unfortunately, the temperature effect on the partioning of the ferrocenium group is not investigated in detail in order to obtain a clear indication of the effect of temperature on the partitioning, and this point needs further investigation. The surface tension difference obtained by changing the oxidation state of the surfactant molecule is observed to be higher at 30 °C than that of 40 °C. As mentioned above, oxidation of Fc to Fc+ leads to an increase in the surface tension of aqueous solutions from 32 to 47 mN/m (∆γlim ) 15 mN/m) at 30 °C and an increase from 31 to 41 mN/m (∆γlim ) 10 mN/m) at 42 °C. Changing the difference of surface tension (∆γlim) with temperature between reduced and oxidized state depends on the surface tension of surfactant in oxidized form. Therefore, we measured the surface tension of Fc+FHUB at 25, 30, and 42 °C. As shown in Figure 4, the increase in temperature leads to a decrease in the surface tension of solution containing Fc+FHUB. Therefore, the differences of the surface tension between reduced and oxidized forms of surfactant decrease with increasing temperature. In parallel, the surface tension differences between the reduced and the oxidized states of FTMA have been determined as 20 mN/m at 25 °C and 10 mN/m at 30 °C.7,31 We

Figure 3. Surface tension of the aqueous solutions of FcFHUB at 42 °C in (b) reduced form and (O) oxidized form, in 100 mM Li2SO4 electrolyte.

2032 Langmuir, Vol. 22, No. 5, 2006

Figure 4. Effect of the temperature on the surface activity of Fc+FHUB solutions, in 100 mM Li2SO4 electrolyte for (b) 42, (O) 30, and (9) 25 °C.

Aydogan and Aldis

Figure 6. Comparison of the scattering function (Pθ) obtained from static light scattering measurements of FcFHUB in 100 mM Li2SO4 at 33 °C (9) with the theoretical ones calculated for different particle shapes.

In summary, the angular dependence of the reduced scattering intensity (Rθ) often contains further information on the particle shape. In general, Rθ can be given in the following form for a monodisperse system

Rθ ) KcMPθSθ

Figure 5. Effect of the FcFHUB concentration on the aggregate size (100 mM Li2SO4, 33 °C at pH 2).

suggest, if the Krafft temperature of FcFHUB did not prevent the measure of surface tensions at low temperatures, such as 25 °C, an oxidation induced change in the surface tension of FcFHUB would be more effective than that of FTMA. Aggregation Behavior of FcFHUB. The bulk properties of ferrocene containing surfactants are reported to be different due to their redox active behavior.9 In addition, the aggregates formed by the fluorocarbon hydrocarbon hybrid surfactants are large aggregates with low curvature.18 We hypothesized that the presence of the more rigid hydrophobic part (fluorocarbon chain) and redox active ferrocenyl group within the surfactant change the aggregation properties of this new surfactant FcFHUB. In parallel to our expectations, the Z-average radius of FcFHUB (see Figure 5) is measured as 183 ( 10 nm at a concentration close to its cmc (0.05 mM). The Z-average radius of FcFHUB shows a decline as a result of the increase at the surfactant concentrations. This kind of behavior has been observed from wormlike aggregates in the literature.32 The geometry of the aggregates formed by FcFHUB is evaluated by using static and dynamic light scattering data to determine the particle scattering function as well as from the ratios of radius of gyration to hydrodynamic radius (F ) Rg/Rh).32,33 (29) Zielinski, R.; Ikeda, S.; Nomura, H.; Kato, S. J. Colloid Interface Sci. 1989, 129, 175-184. (30) Calvaruso, G.; Cavasino, F. P.; Sbriziolo, C.; Liveri, L. T. J. Colloid Interface Sci. 1994, 164, 35-39. (31) Sakai, H.; Imamura, H.; Kondo, Y.; Yoshino, N.; Abe, M. Colloids Surf., A 2004, 232, 221-228. (32) Brown, W. Y. N. Light Scattering Principles and DeVelopment; Clarendon Press: Oxford, U.K., 1996. (33) Gilayni, T.; Varga, I.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. Phys. Chem. Chem. Phys. 2000, 2, 1973-1977.

where Pθ is the particle scattering function (characteristic for the particle shape) and Sθ is the structure factor (determined by the particle-particle interactions). More detailed discussions on light scattering theory are available in several books.32 The experimental Pθ functions can be determined by extrapolating Kc/Rθ to zero particle concentration (when Sθ ) 1). The particle scattering function of hard sphere and coillike aggregates could be expressed as a function of radius of gyration and scattering vector (q) as given below.33 For spherical aggregates,

Pθ(X) )

(

3 {sin(X) - X cos(X)} X3

)

where

X)q

x2012R

g

For coils,

Pθ(X) )

(

2 {exp(-X2) + X2 - 1} 4 X

)

where

X ) qRg In our evaluation, the theoretical curves of Pθ were calculated using the radius of gyration (Rg) determined from the Guiner analysis.32 Figure 6 shows the shape analysis of the aggregates formed by 0.27 mM FcFHUB solution at 33 °C. From this analysis, it is seen that the calculated Pθ of FcFHUB is in good agreement with the Pθ of coillike aggregates. Moreover, the ratio of the radius of gyration to hydrodynamic radius (see Table 3) is determined as 1.5, which is also an indication of coillike aggregates. From these two findings, we concluded that FcFHUB forms large coillike aggregates at 33 °C. The change in the temperature is expected to affect the aggregates formed by the hybrid surfactant FcFHUB. The solution

Redox ActiVe and Temperature SensitiVe Surfactant

Langmuir, Vol. 22, No. 5, 2006 2033

Figure 7. Comparison of the scattering function (Pθ) obtained from static light scattering measurements of FcFHUB in 100 mM Li2SO4 at 50 °C (9) with the theoretical ones calculated for different particle shapes. Table 3. Effect of Temperature and Oxidation States of FcFHUB on the Radius of Gyration and Hydrodynamic Radius surfactant

T (°C)

Rg (nm)

Rh (nm)

F ) Rg/Rh

FcFHUBa

33 50 30 50

248 ( 10 192 ( 10 160 ( 15 Non

161 ( 10 195 ( 10 180 ( 15 Non

1.54 0.99 0.88

Fc+FHUBa a

Figure 8. Schematic representation of aggregate size and shape by changing oxidation state and temperature: (a) Large aggregates with coillike structures, (b) aggregates upon heating (vesicle like aggregates), (c) aggregates formed from oxidized surfactants (vesicle like aggregates), and (d) oxidized surfactant at high temperatures (no aggregates).

oxidation of FcFHUB disrupts large coil type aggregates into the vesicles at low temperature. At high temperature, the oxidation of molecules results in the disruption of vesicles to monomers. In other words, aggregates formed by Fc+FHUB disappear upon heating (Figure 8).

Conclusions

In 100 mM Li2SO4.

of FcFHUB is heated to 50 °C, and aggregate size and shape analysis is performed at that temperature. The hydrodynamic radius of aggregates is determined as 195 ( 10 nm, which is higher than the size of aggregates present at 33 °C. The radius of gyration, on the other hand, is determined as 192 ( 10 nm, which results in the F values close to unity. Vesicles with a thin monolayer represent F values close to unity, and as the thickness of the monolayer increases, it gets closer to the value of hard spheres. We consider this difference in the F values (1.5 vs 1) as an indication of the change in aggregate shape upon heating. Meanwhile, the Pθ values of FcFHUB are evaluated and compared with the Pθ of the spherical, coillike, and rodlike aggregates at 50 °C (see Figure 7). This comparison also supports our proposition that the aggregates formed by FcFHUB at high temperature do not have coillike structure but they form vesicles. In addition, we evaluated the temperature effect on the aggregation behavior of oxidized surfactant (Fc+FHUB). The aggregate size of Fc+FHUB is measured as 195 nm at 25 °C and decreases slightly to 180 nm with an increase in temperature to 30 °C. Then, the aggregates of Fc+FHUB disappear above 30 °C. It was reported that 11-BFDMA molecules formed smaller aggregates in the oxidized state than in the reduced state,31 which is explained by the decrease at the hydrophobic driving force for self-association of the surfactants.8 The molecular structure of Fc+FHUB makes the formation of small aggregates harder unlike BFDMA because of the more rigid structure of the fluorocarbon chain. The change in the aggregate size of FcFHUB by changing the temperature and the oxidation state of the surfactant molecule can be summarized as shown in Figure 8. As discussed above, FcFHUB forms large aggregate molecules gathered in coillike structures at low temperature. The aggregates of FcFHUB transform to vesicles by increasing temperature. Moreover, the

This paper reports the properties of the new type of surfactant with fluorocarbon and hydrocarbon chains as well as a redox active ferrocene group. This new surfactant exhibits a strong surface tension lowering ability and low critical micelle concentration even at the high area per molecule compared to classical ionic surfactants. Moreover, electrochemical oxidation of FcFHUB to Fc+FHUB causes a large and reversible change in the surface tension of aqueous solutions of FcFHUB. The surface activity of FcFHUB is not affected by the change in the electrolyte concentration like unsymmetrical bolaform surfactants. However, in the oxidized form, increasing the electrolyte concentration changes the surface properties of FcFHUB slightly because of the difference in the electrostatic contribution to the standard free energy. While the surface properties of FcFHUB in the reduced state do not change with temperature, the cmc’s and surface tensions of the oxidized form of Fc+FHUB decrease with increasing temperature. This unexpected behavior is related to the presence of a ferrocenium cation and a more hydrophobic fluorocarbon chain. The presence of the more rigid hydrophobic part (fluorocarbon chain) and the redox active part (ferrocenyl group) prevented the formation of small aggregates so that the reduced form of FcFHUB formed larger aggregates at low surfactant concentrations. The size and shape of aggregates change with increasing temperature in the reduced state. The aggregate shape has been changed with oxidation. Moreover, when the temperature rises, the aggregates of Fc+FHUB disappeared. This study demonstrates, for the first time in the literature to our knowledge, the use of temperature and redox reaction to control the surface tension and the bulk properties of the surfactant solution. Acknowledgment. This study was supported by the grant 03K120570 from the State Planning Organization of Turkey. LA052786Q