Tuning Surface Tension and Aggregate Shape via a Novel RedoxActive 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 tensionlowering ability (31 mN/m) and low critical micelle concentration (0.03 mM in 100 mM Li2SO4). The minimum areaper 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 desorptionof surfactant molecules gives rise to the oxidation-induced surface tension change up to 15 mN/m. Although this newmolecule shows salt-insensitive behavior in its reduced form, the oxidized form of the surfactant shows slight sensitivityto the electrolyte concentration. The molecular structure of FcFHUB allows the formation of large aggregates in theform of coils at a temperature of 33°C. When the temperature rises to 50°C, the aggregates are determined to bein 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.

IntroductionRedox active surfactants have been investigated by many

researchers because of their properties, which cannot be obtainedfrom classical surfactants.1-5 Ferrocene is one of the redox activegroups which is used frequently to give redox active characterto the surfactant molecule.2-5 The properties of ferrocenylsurfactant with a single hydrocarbon chain FTMA ((11-ferro-cenylundecyl)trimethylammonium bromide) and ferrocenyl sur-factant with double hydrocarbon chain BFDMA (bis(11-ferrocenylundecyl)dimethylammonium bromide) have beenstudied in detail.6-9

The redox active surfactant FTMA leads to large and reversiblechanges in the surface tension of aqueous solutions.6,7Oxidationof the ferrocene group chemically or electrochemically givesrise to an increase in the surface tension of the surfactant solutionfrom 49 to 72 mN/m at its critical micellization concentration(cmc). Reduction of the ferrocenium group results in the reductionof surface tension to its original value.8 This reversible changein surface tension permits electrochemical control of a varietyof surfactant-induced phenomena, including convection at thesurface of aqueous solutions.10 In addition to these experimentalstudies, detailed investigation of the molecular level contributionof the presence of the ferrocene group within the surfactantstructure to the redox active behavior of this molecule has beenperformed using the molecular thermodynamic approach.8 It hasbeen revealed from this study that oxidation of the ferocenegroup within the surfactant molecule gives rise to an increase

of the electrostatic interaction between surfactant moleculesadsorbed to the interface as well as a decrease in the hydro-phobicity of the surfactant molecule. These changes in the balanceof forces are used to explain the reasons behind the oxidationinduced desorption of surfactant molecules from the interface.11

Moreover it is considered that the ferrocenyl moiety of thesurfactant is able to take the molecular orientations dependingon the redox states. The conformational changes of the ferrocene-modified surfactants affect the micelle formation in the surfactantsolution as well as the interfacial properties.12,13These observa-tions underline the redox active character of the surfactants andexplain the usefulness of them for active control of the interfacialand the bulk properties of the aqueous solutions.12

Another ferrocene-containing surfactant studied in the literatureis the BFDMA, which has double alkyl chains.9,13 Because thismolecule bears two ferrocene groups in addition to twohydrophobic chains, its hydrophobicity is large compare to FTMAwhich leads to lower cmc. In the studies, where BFDMA isutilized, formation of vesicles is observed. The formation ofaggregates in the form of vesicles is attributed to the rodlikestructure of the BFDMA molecule.9,13Recently, it has been shownthat the redox active state of this lipid can be used to control celltransfection.14

Fluorinated or hybrid surfactants are the other types ofsurfactants that find attention in the literature.15,16 These kindsof surfactants are able to lower the surface tension of water muchmore effectively and more efficiently than their hydrocarboncounterparts.Their criticalmicellizationconcentrationsareusually3-4 orders of magnitude lower than that of the correspondinghydrocarbon surfactants.15,16 The fluorocarbon chain is morerigid than a hydrocarbon chain because of the bulky fluorineatoms.17 The aggregates of the fluorocarbon-containing surfac-

* Corresponding author. E-mail: anihal@hacettepe.edu.tr.(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.Langmuir1995, 11, 4209-4212.(7) Gallardo B. S.; Metcalfe, K. L.; Abbott, N. L.Langmuir1996, 12, 4116-

4124.(8) Aydogan, N., Gallardo, B. S., Abbott, N. L.Langmuir1999, 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.

(11) Gallardo, B. S.; Gupta, V. K.; Eagerton, F. D.; Jong, L. I.; Craig, V. S.;Shah, R. R.; Abbott, N. L.Science1999, 283, 57-60.

(12) Saji, T.; Hoshino, K.; Aoyagui, S.J. Am. Chem. Soc.1985, 107, 6865-6866.

(13) Kakizawa, Y.; Sakai, H.; Nishiyama, K.; Abe, M.Langmuir1996, 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.Biomaterials1998, 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.

2028 Langmuir2006,22, 2028-2033

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

tants have structures with less surface curvature (i.e. vesicle,lamellar, and threadlike micelles), thereby having a largermolecular volume of the aggregate compared to that for thecorresponding hydrocarbon-containing surfactants in aqueoussolution.18 In addition, surfactants having the hybrid dimericsurfactant structure (C8FC4-2-C12) forms aggregates at a 0.2 mMsurfactant concentration which is a lower value than those ofhydrogenated surfactants with the same chain length.19 Thesestudies underline the good surface and interfacial tension loweringability of hybrid surfactants. Beside the strong surface tensionlowering action, the hybrid type surfactants can exhibit a thermalresistance, 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-).21This new hybrid surfactant has properties that were reachedby combining a fluorocarbon chain and the hydrocarbon chainin a way to give better performance (i.e., low critical micellizationconcentration, low limiting surface tension, salt insensitivity,and antifoaming character).21,22

The surface and bulk properties of a surfactant molecule whichbears a ferrocenyl group as well as a fluorocarbon chain havenot been reported yet. In this paper, properties of a new surfactantmolecule FcFHUB (Fc(CH2)11N+(C2H5)2(CH2)2(CF2)5CF3 I-)whose chemical structure has been modified by replacing thehydroxyl group of FHUB with the redox-active ferrocene groupas seen from Figure 1 have been investigated. This new modifiedsurfactant combines the features of FHUB molecule such as lowcritical micellization concentration, low limiting surface tension,salt insensitivity, and low foaming ability with the properties ofthe redox active surfactants such as reversible control of surfacetension and aggregation state. In addition, the comparison of theproperties of this new ferrocenyl surfactant with previouslyreported molecules are also given to bring more understandingto the behavior of ferrocenyl surfactants.

Materials and Methods

1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8-iodooctane, diethylamine,and trimethylamine were purchased from Across (Belgium). Theacetone, acetonitrile, hexane, diethyl ether, lithium sulfate, ceriumsulfaten-hydrate, and mercury(II) bromide were purchased fromSigma (Germany). The new redox active surfactant (CF3(CF2)5-CH2CH2N+(CH3CH2)2(CH2)11Fc I- or FcFHUB) was synthesizedin our laboratory. (11-Bromoundecanoyl)ferrocene was synthesizedas described before.6 The obtained (11-bromoundecyl)ferrocene issolved in 20 mL of ethanol in a flask. Then 9 mL of diethylaminewas added into the flask. The reaction mixture was stirred at roomtemperature for 48 h. Following the reaction, the volatile substanceswere evaporated. Equal molar 1,1,1,2,2,3,3,4,4,5,5,6,6-tridecafluoro-8-iodo-octane (0.8 mL) was added slowly into ethanol solution of11-ferroceneundecane diethylamine over 30 min. The reactionmixture was stirred and refluxed for 96 h. After the solvent wasevaporated, the surfactant FcFHUB was extracted with diethyl etherand further purified by recrystalization. Chemical characterizationof FcFHUB has been performed through1H NMR and elementalanalysis.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 eachexperiment using water from a water purification system (Human,Korea). The equilibrium surface tensions were measured in electrolytesolutions of Li2SO4 using a tensiometer (Kruss, Germany) in thereduced and oxidized states with the Wilhelmy plate method.Preparation of aqueous Fc+FHUB (oxidized form) solutions wasmade by adding a sufficient amount of cerium sulfate to the solutionof FcFHUB (reduced form). All the surface tension measurementswere repeated at least twice. Surface tension measurements wereperformed at 25, 30, and 40°C in 100 mM Li2SO4 and 10 mMLi2SO4 electrolytes at pH 2. All the glassware was cleaned in piranhasolution (18 M H2SO4, 30% H2O2, 70:30 (v/v)).Warning: piranhasolution should be handled with extreme caution; in some circum-stances (most probably when it has been mixed with significantquantities of an oxidizable organic material), it has detonatedunexpectedly.

Dynamic light scattering was used to determine the hydrodynamicradius of aggregates formed by FcFHUB and Fc+FHUB at differenttemperatures and concentrations. For this purpose Zetasizer NanoSeries, Zen 1600 (Malvern, England), was used where the averagedecay rate was obtained from the measured autocorrelation functionusing the method of cumulants employing a quadratic fit. Themagnitude of the scattering vector was given byq ) (4πn/λ) sin-(θ/2), wheren is the refractive index of the solvent,λ is the wavelengthof light, andθ is the scattering angle. Measurements were made atan angle of 173°. The samples were prepared by using an electrolytesolution which is prefiltered.

Static light scattering experiments were performed using a CGS-3goniometer (Malvern, England) in which measurements at anglesfrom 30 to 150° with 5-deg increments were carried out. Theseexperiments have been used to determine the radius of gyration andform factor, which can be employed to evaluate and compare theshape of the aggregates with the selected structures using ALV-STAT software.

Results and Discussion

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

(18) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T.J. Phys. Chem. B1999, 103, 9237-9246.

(19) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M.Langmuir1996, 12,5768-5772.

(20) Abe, M.Curr. Opin. Colloid Interface Sci.1999, 4, 354-356.(21) Aydogan, N.; Aldis, N.; Guvenir, O.Langmuir2003, 19, 10726-10731.(22) Calik, P.; Erdinc, B. I., Ileri, N.; Aydogan, N.; Argun, M.Langmuir2005,

21, 8613-8619.(23) Aydogan, N.; Abbott, N. L.J. Colloid Interface Sci.2001, 242, 411-418.

Figure 1. Molecular structure of the redox active hybrid surfactantFcFHUB.

Redox ActiVe and Temperature SensitiVe Surfactant Langmuir, Vol. 22, No. 5, 20062029

(FcFHUB), as expected. The limiting surface tension of thesolution containing Fc+FHUB is measured as 32 mN/m. Anotherimportant observation obtained from Figure 2 is that the oxidationof the surfactant molecule of solution containing 0.02 mMFcFHUB results in the increase of surface tension from 32 to 47mN/m (a 15 mN/m increase), which is similar to the surfacetension change obtained from ferrocenyl surfactant with an anionicheadgroup.24

To understand the structure performance relation of this newredox active hybrid surfactant, the interfacial properties ofFcFHUB are compared with several other surfactants, whichhave certain structural similarities as summarized in Table 1.Both FcFHUB and FTMA contain a ferrocene group at the otherend of the hydrocarbon chain which has 11 carbons. However,FcFHUB bears a fluorocarbon chain which is expected to resultin an increase in the hydrophobic driving force for adsorptionof the surfactant (ghyd, hydrophobic contribution to the free energyof the monolayer) and a decrease in the cmc.21,25The hydrophobicfree energy contribution to the free energy of the monolayer ofthe reduced form of FcFHUB is calculated as-42.5 kT by usingthe formulations in predictingghyd of classical surfactants25 andthe hydrophobic free energy contribution of the ferrocene groupthat has been used in the past study.8 This number is larger thanthe hydrophobic free energies of FTMA (-26 kT) and FHUB(-34.4 kT). This high hydrophobicity of FcFHUB leads to lowercmc compared to FHUB (0.45 mM) and FTMA (0.1 mM) asseen from Table 1. On the other hand, a dimeric surfactant whichcontains two ferrocene groups at the other end of the hydrocarbonchains which is called BFDMA has a hydrophobic free energycontribution (-52 kT) which is larger than that of the FcFHUB,and consequently, it has a cmc (0.0001 mM) lower than the cmcof the FcFHUB in its reduced state (see Table 1). Oxidation of

FcFHUB and BFDMA molecules gives rise to an increase in theelectrostatic interaction between molecules (which is a positivecontribution to the Gibbs free energy of the monolayer) as wellas the lowering of the hydrophobic driving force (-2.7 kT forFcFHUB and-5.40 kT for BFDMA).8 However, theghyd ofFc+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 fluorocarbonand hydrocarbon segments are also important, which have thepotential to affect the interfacial and bulk properties of a surfactantsolution. It is known from previous studies that the ferrocenegroup (Fc) within FTMA molecule prefers to stay close to theair-water interface which affects the configuration of surfactantmolecule.7,8Although, FTMA behaves like bolaform surfactantssuchasHTAB(ω-hydroxyundecyltrimethylammoniumbromide),the minimum area per molecule of FTMA (85( 5 Å2) moleculewithin the interface is larger than the minimum area per moleculeHTAB (68 ( 5 Å2).7,23This difference in the area per moleculesis explained simply by the presence of the ferrocene group.23

The minimum area per FcFHUB molecule at the interface isdetermined to be 121( 5 Å2/molecule in the presence of 100mM Li2SO4 by using the Gibbs adsorption equation,26,27whichis larger than the area of FHUB (88( 5 Å2) molecules at theinterface. This result suggests that the ferrocene group of FcFHUBassociates with the aqueous subphase, thereby forcing FcFHUBinto looped configuration. Oxidation of Fc to Fc+ leads to anincrease in the electrostatic contribution to the free energy aswell as a decrease in the hydrophobic driving force for adsorption.8

The minimum area of Fc+FHUB (135( 5 Å2) is calculated tobe larger than the minimum area per molecule of the reducedstate. Increased electrostatic repulsion of the Fc+FHUB explainsthe increase at the area per molecule at the interface. The areaper molecule calculated for Fc+FHUB is also larger than the areaper molecule calculated for FcFHUB and FHUB, suggesting thisnew redox active and hybrid surfactant (hydrocarbon part) adoptsreverse U configuration at the air-water interface. The result ofelectrostatic repulsion between molecules of Fc+FHUB drivesits desorption from the surface of the solution, so the limitingsurface tension increases from 32 to 47 mN/m upon oxidationat a 0.03 mM surfactant concentration.

Effect of Electrolyte Concentration on the InterfacialProperties.One of the interesting properties of the unsymmetricalbolaform surfactants is their salt-insensitive interfacial behavior.23

This behavior is related to the dominant contribution to thelowering of the surface tension.23 Therefore, we test unsym-metrical bolaform character of the reduced and the oxidized formsof FcFHUB by changing electrolyte concentrations (see Table2). As can be seen from Table 2, a decrease in the electrolyteconcentrations from 100 to 10 mM Li2SO4 causes a slight changein the cmc of FcFHUB, whereas the cmc’s of DTAB and HTAB

(24) Aydogan, N.; Abbott, N. L.Langmuir2002, 18, 7826-7830.(25) Tanford, C.The Hydrophobic Effect: Formation of Micelles and Biological

Membranes, 2nd ed.; John Wiley and Sons: New York, 1980.

(26) Ito, A.; Sakai, H.; Kondo, Y.; Kamogawa, K.; Kondo, Y.; Yoshino, N.;Uchiyama, H.; Harwell, J. H.; Abe, M.Langmuir2000, 16, 9991-9995.

(27) Rosen, M. J.Surfactants and Interfacial Phenomena, 2nd ed.; John Wileyand Sons: New York, 1989.

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 21.000 48 68( 5 -16.0 28 23FTMA 0.1 49 85( 5 -26.0 8BFDMA 0.0001 43 52 9FHUB 0.450 25 88( 5 -34.4 38 21FcFHUB (100 mM Li2SO4, 30°C) 0.02( 0.005 31( 2 121( 5 -42.5 28Fc+FHUB (100 mM Li2SO4, 30°C) 0.13( 0.005 32( 2 135( 5 -39.8

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

Figure 2. Surface tension of the aqueous solutions of FcFHUB at30 °C in (b) reduced form and (O) oxidized form, in 100 mMLi2SO4 electrolyte.

2030 Langmuir, Vol. 22, No. 5, 2006 Aydogan and Aldis

change 2.37 and 1.27 times as a result of the decrease in theelectrolyte concentration from 100 to 10 mM LiBr.28 Therefore,we propose that FcFHUB molecules behave like an unsymmetricalbolaform surfactant. Like insignificant changes in the cmc ofFcFHUB with added electrolyte concentration in the reducedstate, the surface properties of FcFHUB change slightly with theaddition of electrolyte to the solution. In the oxidized state, anincrease in the electrolyte concentration from 10 to 100 mMLi2SO4 reduces the cmc of Fc+FHUB from 0.2 to 0.10 mM,which is related to the differences in the electrostatic contributionof the standard free energy. In parallel, the minimum area permolecule of the oxidized form of Fc+FHUB increases slightlywith decreasing electrolyte concentration (Table 2). In thepresence of 100 mM Li2SO4, the minimum area per Fc+FHUBmolecule is determined to be 135 Å2/molecule by using Gibbsadsorption equation, which is larger than the minimum area ofFcFHUB. As mentioned before, oxidation of Fc to Fc+ leads toan increase in the electrostatic contribution because oxidation ofFc to Fc+ doubles the ionic charge on each surfactant moleculethat adsorbed to the surface of the solution.8 Therefore, theelectrostatic repulsions between surfactant molecules in theoxidized state are higher than the reduced state. The minimumareas per charge of Fc+FHUB are calculated as 67.5 and 77 Å2

in the presence of 100 and 10 mM Li2SO4, respectively. As aresult, we determined that Fc+FHUB is more sensitive to thechanges in electrolyte concentration than that of FcFHUB.

Effect of Temperature on the Interfacial Behavior ofFcFHUB. Measurements of the surface tensions of aqueoussolutions of FcFHUB and Fc+FHUB in 100 mM LiSO4electrolyteare repeated at 42°C in order to investigate the effect of thetemperature (Figure 3). The limiting surface tensions of FcFHUBand Fc+FHUB at that temperature are measured as 30 mN/m.The cmc’s of FcFHUB and Fc+FHUB are determined to be 0.03and 0.065 mM, respectively. We make three observationscomparing the surface tensions of FcFHUB and Fc+FHUBmeasured at these temperatures (30 and 42°C). First of all, thecmc’s and limiting surface tension of FcFHUB are similar atboth temperatures. Second, the cmc and surface tensions of Fc+-FHUB decrease by increasing temperature. This unexpectedbehavior of Fc+FHUB can be explained by the presence of thefluorocarbon chain and the ferrocene group in the structure. Inliterature, it has been suggested that the organization of the watermolecule around the surfactant molecules and also around themicelles could be affected by the change in temperature.29 Thetwo-case model of hydration is a good example for explainingthe temperature effect on the behavior of the surfactant molecule.29

Briefly, in this model, it is postulated that there is hydrophilichydration around the surfactant monomers, being in the form ofmicelles and two types of hydration around the surfactantmolecules existing in the monomeric form, that is, hydrophobic

hydration around the alkyl chain of the surfactant monomer andhydrophilic hydration around its polar headgroup.29

However, the reduced form of the surfactant is not affectedsignificantly by the change in temperature; while the oxidizedform shows more temperature sensitivity, we suspect that it shouldbe related to the partitioning of the ferrocenium moiety betweenwater and oil as well as the decrease in the hydration of thismoiety with temperature. The hydrophobicity of ferrocene andferrocenium is calculated from the partitioning of these groupsbetween water and oil (interior of DTAB micelles) usingelectrochemical methods.30Unfortunately, the temperature effecton the partioning of the ferrocenium group is not investigatedin detail in order to obtain a clear indication of the effect oftemperature on the partitioning, and this point needs furtherinvestigation.

The surface tension difference obtained by changing theoxidation state of the surfactant molecule is observed to be higherat 30°C than that of 40°C. As mentioned above, oxidation ofFc to Fc+ leads to an increase in the surface tension of aqueoussolutions from 32 to 47 mN/m (∆γlim ) 15 mN/m) at 30°C andan increase from 31 to 41 mN/m (∆γlim ) 10 mN/m) at 42°C.Changing the difference of surface tension (∆γlim) with tem-perature between reduced and oxidized state depends on thesurface tension of surfactant in oxidized form. Therefore, wemeasured the surface tension of Fc+FHUB at 25, 30, and 42°C.As shown in Figure 4, the increase in temperature leads to adecrease in the surface tension of solution containing Fc+FHUB.Therefore, the differences of the surface tension between reducedand oxidized forms of surfactant decrease with increasingtemperature. In parallel, the surface tension differences betweenthe reduced and the oxidized states of FTMA have beendetermined as 20 mN/m at 25°C and 10 mN/m at 30°C.7,31We

(28) The electrolyte used in this study (Li2SO4) is a 1:2 type electrolyte, whereasLiBr is a 1:1 type electrolyte. A change in the concentration from 100 to 10 mMin the case of Li2SO4 is more effective than that of LiBr.

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

CMC (mM) γlim (mN/m) Alim (Å2)

surfactant 10 mM Li2SO4 100 mM Li2SO4 10 mM Li2SO4 100 mM Li2SO4 10 mM Li2SO4 100 mM Li2SO4

FcFHUB (red.) 0.04 0.03 33 31 132( 5 121( 5FcFHUB (ox.) 0.2 0.10 32 32 154( 5 135( 5FHUBa,21 0.45 25 88( 5HTABa,23 28 22 48 48 65( 5DTABa,23 8 3 37 36 41( 5

a In LiBr electrolyte.

Figure 3. Surface tension of the aqueous solutions of FcFHUB at42 °C in (b) reduced form and (O) oxidized form, in 100 mMLi2SO4 electrolyte.

Redox ActiVe and Temperature SensitiVe Surfactant Langmuir, Vol. 22, No. 5, 20062031

suggest, if the Krafft temperature of FcFHUB did not preventthe measure of surface tensions at low temperatures, such as 25°C, an oxidation induced change in the surface tension of FcFHUBwould be more effective than that of FTMA.

Aggregation Behavior of FcFHUB.The bulk properties offerrocene containing surfactants are reported to be different dueto their redox active behavior.9 In addition, the aggregates formedby the fluorocarbon hydrocarbon hybrid surfactants are largeaggregates with low curvature.18 We hypothesized that thepresence of the more rigid hydrophobic part (fluorocarbon chain)and redox active ferrocenyl group within the surfactant changethe aggregation properties of this new surfactant FcFHUB. Inparallel to our expectations, theZ-average radius of FcFHUB(see Figure 5) is measured as 183( 10 nm at a concentrationclose to its cmc (0.05 mM). TheZ-average radius of FcFHUBshows a decline as a result of the increase at the surfactantconcentrations. This kind of behavior has been observed fromwormlike aggregates in the literature.32 The geometry of theaggregates formed by FcFHUB is evaluated by using static anddynamic light scattering data to determine the particle scatteringfunction as well as from the ratios of radius of gyration tohydrodynamic radius (F ) Rg/Rh).32,33

In summary, the angular dependence of the reduced scatteringintensity (Rθ) often contains further information on the particleshape. In general,Rθ can be given in the following form for amonodisperse system

wherePθ is the particle scattering function (characteristic for theparticle shape) andSθ is the structure factor (determined by theparticle-particle interactions). More detailed discussions on lightscattering theory are available in several books.32 The experi-mentalPθ functions can be determined by extrapolatingKc/Rθto zero particle concentration (whenSθ ) 1). The particlescattering function of hard sphere and coillike aggregates couldbe expressed as a function of radius of gyration and scatteringvector (q) as given below.33

For spherical aggregates,

where

For coils,

where

In our evaluation, the theoretical curves ofPθ were calculatedusing the radius of gyration (Rg) determined from the Guineranalysis.32 Figure 6 shows the shape analysis of the aggregatesformed by 0.27 mM FcFHUB solution at 33°C. From thisanalysis, it is seen that the calculatedPθ of FcFHUB is in goodagreement with thePθ of coillike aggregates. Moreover, the ratioof the radius of gyration to hydrodynamic radius (see Table 3)is determined as 1.5, which is also an indication of coillikeaggregates. From these two findings, we concluded that FcFHUBforms large coillike aggregates at 33°C.

The change in the temperature is expected to affect theaggregates formed by the hybrid surfactant FcFHUB. The solution

(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. ColloidInterface 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; ClarendonPress: Oxford, U.K., 1996.

(33) Gilayni, T.; Varga, I.; Meszaros, R.; Filipcsei, G.; Zrinyi, M.Phys. Chem.Chem. Phys. 2000, 2, 1973-1977.

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

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

Figure 6. Comparison of the scattering function (Pθ) obtained fromstatic light scattering measurements of FcFHUB in 100 mM Li2SO4at 33°C (9) with the theoretical ones calculated for different particleshapes.

Rθ ) KcMPθSθ

Pθ(X) ) ( 3

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

X ) qx2012

Rg

Pθ(X) ) ( 2

X4{exp(-X2) + X2 - 1})X ) qRg

2032 Langmuir, Vol. 22, No. 5, 2006 Aydogan and Aldis

of FcFHUB is heated to 50°C, and aggregate size and shapeanalysis is performed at that temperature. The hydrodynamicradius of aggregates is determined as 195( 10 nm, which ishigher than the size of aggregates present at 33°C. The radiusof gyration, on the other hand, is determined as 192( 10 nm,which results in theF values close to unity. Vesicles with a thinmonolayer representF values close to unity, and as the thicknessof the monolayer increases, it gets closer to the value of hardspheres. We consider this difference in theF values (1.5 vs 1)as an indication of the change in aggregate shape upon heating.Meanwhile, thePθ values of FcFHUB are evaluated and comparedwith thePθ of the spherical, coillike, and rodlike aggregates at50 °C (see Figure 7). This comparison also supports ourproposition that the aggregates formed by FcFHUB at hightemperature do not have coillike structure but they form vesicles.

In addition, we evaluated the temperature effect on the ag-gregation behavior of oxidized surfactant (Fc+FHUB). The ag-gregate size of Fc+FHUB is measured as 195 nm at 25°C anddecreases slightly to 180 nm with an increase in temperature to30 °C. Then, the aggregates of Fc+FHUB disappear above 30°C. It was reported that 11-BFDMA molecules formed smalleraggregates in the oxidized state than in the reduced state,31whichis explained by the decrease at the hydrophobic driving force forself-association of the surfactants.8 The molecular structure ofFc+FHUB makes the formation of small aggregates harder unlikeBFDMA because of the more rigid structure of the fluorocarbonchain.

The change in the aggregate size of FcFHUB by changing thetemperature and the oxidation state of the surfactant moleculecan be summarized as shown in Figure 8. As discussed above,FcFHUB forms large aggregate molecules gathered in coillikestructures at low temperature. The aggregates of FcFHUBtransform to vesicles by increasing temperature. Moreover, the

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

ConclusionsThis paper reports the properties of the new type of surfactant

with fluorocarbon and hydrocarbon chains as well as a redoxactive ferrocene group. This new surfactant exhibits a strongsurface tension lowering ability and low critical micelle con-centration even at the high area per molecule compared to classicalionic surfactants. Moreover, electrochemical oxidation of FcF-HUB to Fc+FHUB causes a large and reversible change in thesurface tension of aqueous solutions of FcFHUB. The surfaceactivity of FcFHUB is not affected by the change in the electrolyteconcentration like unsymmetrical bolaform surfactants. However,in the oxidized form, increasing the electrolyte concentrationchanges the surface properties of FcFHUB slightly because ofthe difference in the electrostatic contribution to the standardfree energy. While the surface properties of FcFHUB in thereduced state do not change with temperature, the cmc’s andsurface tensions of the oxidized form of Fc+FHUB decreasewith increasing temperature. This unexpected behavior is relatedto the presence of a ferrocenium cation and a more hydrophobicfluorocarbon chain. The presence of the more rigid hydrophobicpart (fluorocarbon chain) and the redox active part (ferrocenylgroup) prevented the formation of small aggregates so that thereduced form of FcFHUB formed larger aggregates at low sur-factant concentrations. The size and shape of aggregates changewith increasing temperature in the reduced state. The aggregateshape has been changed with oxidation. Moreover, when thetemperature rises, the aggregates of Fc+FHUB disappeared. Thisstudy demonstrates, for the first time in the literature to ourknowledge, the use of temperature and redox reaction to controlthe surface tension and the bulk properties of the surfactantsolution.

Acknowledgment. This study was supported by the grant03K120570 from the State Planning Organization of Turkey.

LA052786Q

Figure 7. Comparison of the scattering function (Pθ) obtained fromstatic light scattering measurements of FcFHUB in 100 mM Li2SO4at 50°C (9) with the theoretical ones calculated for different particleshapes.

Table 3. Effect of Temperature and Oxidation States ofFcFHUB on the Radius of Gyration and Hydrodynamic Radius

surfactant T (°C) Rg (nm) Rh (nm) F ) Rg/Rh

FcFHUBa 33 248( 10 161( 10 1.5450 192( 10 195( 10 0.99

Fc+FHUBa 30 160( 15 180( 15 0.8850 Non Non

a In 100 mM Li2SO4.

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

Redox ActiVe and Temperature SensitiVe Surfactant Langmuir, Vol. 22, No. 5, 20062033