Interfacial and Bulk Properties of the NewFluorocarbon-Hydrocarbon Hybrid Unsymmetrical

Bolaform Surfactant

Nihal Aydogan,* Nihan Aldis, and Ozge Guvenir

Department of Chemical Engineering, Hacettepe University Beytepe Campus,Ankara, Turkey 06542

Received July 3, 2003. In Final Form: September 19, 2003

We report the interfacial properties of a new fluorocarbon-hydrocarbon hybrid surfactant [OH(CH2)11-N+(C2H4)2(CH2)2(CF2)5CF3 I-, or FHUB], which has a fluorocarbon chain as well as a ω-hydroxyalkylchain. This new hybrid surfactant has properties that are different than classical hydrocarbon- andfluorocarbon-containing surfactants. This difference is created by combining a fluorocarbon chain and thehydrocarbon chain in a way to give the best performance (i.e., low critical micellization concentration, lowlimiting surface tension, salt insensitivity). The unsymmetrical bolaform character of FHUB gives rise tothe salt-insensitive interfacial behavior of this surfactant. The presence of the fluorocarbon chain reducesthe critical micelle concentrations of this new hybrid surfactant to a lower value (0.5 mM in 100 mM LiBr)compared to classical ionic and unsymmetrical bolaform surfactants such as dodecyltrimethylammoniumbromide (2.5 mM in 100 mM LiBr) and (11-hydroxyundecyl)trimethylammonium bromide (21 mM in 100mM LiBr). Moreover, the presence of the more rigid hydrophobic part (fluorocarbon chain) affects theaggregation properties of FHUB. The FHUB molecules start to form aggregates with less curvature at alow surfactant concentration. The formation of small aggregates, which have been reported to form byunsymmetrical bolaform surfactants, is prevented. As a result, the limiting surface tension of FHUB isobtained as a low value. The aggregate size of FHUB shows an increase first, goes through a maximum,and decreases with an increase at the surfactant concentration. The solubilization of water-insoluble dyewithin the aggregates is also determined to be a function of the surfactant concentration.

IntroductionRecently, unsymmetrical bolaform surfactants have

been introduced for active1-4 and passive control5-7 ofsurface and bulk properties of aqueous solutions. It isdemonstrated that the presence of a hydrophilic group atthe ω position within the surfactant structure affects themolecular forces acting on the system, which results indifferent bulk and surface properties of this class ofsurfactants compared to classical ionic surfactants.5 Interms of active control of the bulk and solution properties,the electrochemical control of the oxidation state of theredox-active ferrocene group within the structure ofFc(CH2)11N+(CH3)3Br-, which has been shown to behavelike the unsymmetrical bolaform surfactant, leads to alarge and reversible change in the surface tension andaggregation state of the aqueous solution.1,2 The principalconclusions to emerge from past experimental and theo-retical studies of these types of surfactants are (i)surfactants tend to occupy larger areas at the interfacethan classical ionic surfactants such as dodecyltrimethyl-ammonium bromide (DTAB) and sodium dodecyl sulfate(SDS), (ii) they adopt a looped configuration at theinterface, (iii) their limiting surface tensions are relativelyhigh, (iv) their critical micelle concentration (cmc) values

are higher than those of the classical ionic surfactants,which have the similar hydrophobic driving force foradsorption, (v) the dominant contribution of surfacetension lowering is the configurational term, not theelectrostatic, as expected from an ionic surfactant, and(vi) theyare less sensitive tovariations in the ionic strengthof the aqueous subphase compared to classical ionicsurfactants.1-8

The high limiting surface tension and cmc of unsym-metrical bolaform surfactants limit their applicabilitywhen they are compared with classical ionic surfactants.The behavior of unsymmetrical bolaform surfactants canbe optimized with the evaluation of the molecular levelforces related with the surface and bulk behaviors of thesesurfactants. This evaluation of the molecular forces actingon the system using a molecular thermodynamic modelreveals that the looped configuration of the unsymmetricalbolaform surfactant inhibits the adsorption of surfactantto the interface.5,9 To achieve same degree of surfacetension reduction with classical ionic surfactants, largerdriving forces (larger concentrations) are needed. More-over, it has been reported recently that unsymmetricalbolaform surfactants such as (11-hydroxyundecyl)tri-methylammonium bromide (HTAB) form smaller ag-gregates with a smaller aggregation number compared tothat of DTAB.10 The formation of smaller aggregates,before significant surface tension reduction is achieved,leads to a high limiting surface tension of the unsym-metrical bolaform surfactants. It is hypothesized that,

* To whom correspondence should be addressed. E-mail: anihal@hacettepe.edu.tr. Fax: +90-312-299 21 24.

(1) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11,4209.

(2) 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.

(3) Aydogan, N.; Abbott, N. Langmuir 2001, 17, 5703-5706.(4) Aydogan, N.; Abbott, N. Langmuir 2002, 18, 7826-7830.(5) Aydogan, N.; Abbott, N. L. J. Colloid Interface Sci. 2001, 242,

411-418.(6) Davey, T. W.; Ducker, W. A.; Hayman, A. R.; Simpson, J. Langmuir

1998, 14, 3210.(7) Merger, F. M.; Chow, J. F. J. Am. Chem. Soc. 1983, 105, 5501.

(8) Abid, S. K.; Hamid, S. M.; Sherrington, D. C. J. Colloid InterfaceSci. 1987, 120, 245.

(9) Aydogan, N.; Gallardo, B. S.; Abbott, N. L. Langmuir 1999, 15,722.

(10) Davey, T. W.; Ducker, W. A.; Hayman, A. R. Langmuir 2000, 16,2430-2435. Davey, T. W.; Warr, G. G.; Almgren, M.; Asakawa, T.Langmuir 2001, 17, 5283-5287.

10726 Langmuir 2003, 19, 10726-10731

10.1021/la0351921 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 11/18/2003

when the formation of small-sized aggregates is prevented,the larger surface tension reduction could be obtained.Moreover, increasing the hydrophobic driving force isexpected to lower the cmc of the unsymmetrical bolaformsurfactants. These properties, which are the lower limitingsurface tension and cmc, are proposed to be reached withthe addition of a fluorocarbon chain within the structureof the unsymmetrical bolaform surfactant.

Fluorocarbons are characterized by their exceptionalchemical and biological inertness, extreme hydrophobicity,lipophobicity, high gas-dissolving capacities, low surfacetensions, high fluidity, and spreading coefficients.11,12

These unique properties of fluorocarbons are the basis ofapplications such as biomedical applications.13,14 Althoughthere are several advantages of fluorinated surfactantsover hydrocarbon surfactants, their limited solubility inaqueous solutions and limited mixing ability with organicmaterials (even when used with hydrocarbon surfactantsthey tend to phase separate macroscopically or micro-scopically) limit the use of fluorinated surfactants inseveral processes.8 There are several studies present inwhich hybrid surfactants (i.e., surfactants having fluori-nated and hydrogenated parts) were designed.15-18

The fluorocarbon chain is more hydrophobic than thehydrocarbon chain.13,19 Hence, the presence of the fluoro-carbon chain within the surfactant reduces the cmc/criticalaggregation concentration (cac). The cationic dimericsurfactant with a small spacer (m-2-n, where m and nare numbers of carbons within the hydrophobic groups)was known to form aggregates at lower concentrationsthan classical ionic surfactants such as DTAB or SDS.17

The substitution of hydrocarbon chains (C12-2-C12) withfluorocarbon chains (C8

FC4-2-C8FC4) resulted in a dra-

matic decrease at the cac of the solution from 0.95 mM to28 µM.17 The hybrid dimeric surfactant in which one tailwas a fluorocarbon and the other tail was a hydrocarbonchain (C8

FC4-2-C12) formed aggregates at a 0.2 mMsurfactant concentration.17 These observations underlinethe hydrophobic character of fluorocarbon-containingsurfactants and explain partly the high aggregationcapacity of fluorocarbon-containing surfactants.

Recently, a detailed investigation of a hybrid anionicsurfactant sodium 1-oxo-1-[4-(fluoroalkyl)phenyl]-2-alka-nesulfonates [FCm-HCn, in which (m, n) are (6, 4), (6, 2),and (4, 6)] has been reported.17,18 From this study, it isconcluded that an increase in the fluorocarbon chain lengthof a hybrid surfactant gave rise to an increase at the Kraffttemperature and a decrease at the cmc and limiting surfacetension.17 The aggregation number of a FCm-HCn micellewas determined to be smaller than those of typical ionichydrocarbon surfactants.18 The value of area per moleculefor FCm-HCn was obtained as 81-122 Å2/molecule.18 Ithas been further demonstrated that a concentratedaqueous solution of FC6-HC4 exhibits unusual viscoelasticbehavior, which has not been observed in systems having

a hydrocarbon counterpart of FC6-HC4.20 The necessarycondition to exhibit thermoresponsive viscoelastic behav-ior was suggested as having a hybrid structure.20,21

In this paper, we present the interfacial and bulkbehavior of the new fluorocarbon-hydrocarbon hybridunsymmetrical bolaform surfactant (FHUB) (OH(CH2)11N+-(C2H4)2(CH2)2(CF2)5CF3 I-; see Figure 1). The structure ofthis surfactant is designed on the basis of the previousobservations of fluorocarbon surfactants, hybrid ionicsurfactants, and unsymmetrical bolaform surfactants. Inthe scope of this study, it is planned to combine severalfeatures of fluorocarbon surfactants15-18 and unsym-metrical bolaform surfactants5 into the structure of thisnew molecule. The proposed configuration of the FHUBmolecules within the air/water interface has been shownin Figure 2. Like the HTAB molecules, the presence of thehydroxyl group at the other end of the hydrocarbon chainis expected to result in changes in its configuration (loopedconfiguration).

Materials and Methods1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8-iodooctane, 11-bromo-

1-undecanol, diethylamine, trimethylamine, 11-bromoundecane,and DTAB were purchased from Across (Belgium). The acetone,hexane, diethyl ether, and lithium bromide were purchased fromSigma (Germany). The hybrid surfactant (11-hydroxundecyl)-tridecafluorooctane diethylammonium iodide (OH(CH2)11N+-(C2H4)2(CH2)2(CF2)5CF3I-, or FHUB) was synthesized in ourlaboratory. During the synthesis, diethylamine (20 mL) wasadded to an acetone solution (20 mL) of 11-bromo-1-undecanol(2 g). The reaction mixture was stirred at room temperature for24 h. The precipitate, which was a salt, was separated by usingvacuum filtration. After the volatile substances were removed

(11) Lange, R. K. Surfactants: A Practical Handbook; HanserPublisher: Munich, 1999.

(12) Krafft, M. P. Adv. Drug Delivery Rev. 2001, 47, 209-229.(13) Riess, J. G.; Krafft, M. P. Biomaterials 1998, 19, 1529-1539.(14) Riess, J. G.; Krafft, M. P. Chem. Phys. Lipids 1995, 75, 1-14.(15) Thunemann, A. F.; Lochhaas, K. H. Langmuir 1999, 15, 4867-

4874.(16) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir

1996, 12, 5768-5772.(17) Ito, A.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir

1996, 12, 5768-5772.(18) Ito, A.; Sakai, H.; Kondo, Y.; Kamogawa, K.; Kondo, Y.; Yoshino,

N.; Uchiyama, H.; Harwell, J. H.; Abe, M. Langmuir 2000, 16, 9991-9995.

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

(20) Tobita, K.; Sakai, H.; Kondo, Y.; Yoshino, N.; Kamogawa, K.;Momozawa, N.; Abe, M. Langmuir 1998, 14, 4753-4757.

(21) Abe, M.; Ito, K.; Yoshihara, K.; Ogino, K.; Yoshino, N. Mater.Technol. 1994, 12, 81.

Figure 1. Molecular structures of the new fluorocarbon-hydrocarbon unsymmetrical bolaform surfactant FHUB (a) andthe unsymmetrical bolaform surfactant HTAB (b).

Figure 2. Schematic illustration of the likely conformationsof (A) HTAB and (B) FHUB.

New Bolaform Surfactant Langmuir, Vol. 19, No. 26, 2003 10727

by evaporation in a vacuum, the residue was washed by diethylether. 11-Hydroxyundecane diethylamine (2 g) was dissolved in20 mL of ethanol. 1,1,1,2,2,3,3,4,4,5,5,6,6-Tridecafluoro-8-iodo-octane (3.8 g) was added to that solution. The reaction mixturewas refluxed for 120 h at 90 °C. After the solvent was evaporated,the surfactant FHUB was purified via repeated recrystallizationfrom the hexane/ethanol mixture (8:2).22

Aqueous surfactant solutions were prepared freshly for eachexperiment using water from a water purification system(Barnstead, U.S.A.). The equilibrium surface tensions of aqueoussolutions of FHUB were measured using a Wilhelmy plate methodin a tensiometer (Kruss, Germany). All the surface tensionmeasurements were repeated at least twice. All the glasswarewas cleaned in piranha solution (18 M H2SO4, 30% H2O2, 70:30v/v). Warning: piranha solution should be handled with extremecaution; in some circumstances (most probably when it has beenmixed with significant quantities of an oxidizable organicmaterial), it has detonated unexpectedly. The Krafft temperatureof FHUB in water was measured as 38 °C.23 Because FHUB hasthe Krafft temperature higher than room temperature, all surfacetension measurements were performed at 40 °C. Surface tensionmeasurements were performed in 1 and 100 mM LiBr electrolyteat pH 2.

Measurements of quasi-elastic light scattering were done todetermine the hydrodynamic radii of the aggregates. Quasi-staticlight scattering experiments were performed using a MalvernZetasizer-1000 with a 50-mW 532-nm laser (Malvern, U.K.).Aqueous solutions were prepared using water filtered througha 0.22-µm filter. The samples were centrifuged for 1 h and allowedto equilibrate at 40 °C for 20 min before the measurements.Measurements were repeated 2 and 10 h after preparation. Noincrease in the size of the aggregates is determined. The angleof the detector was set at 90°. The conductivity of the FHUBsolutions was measured with an electroconductometer (Jenway4020 Conductivity Meter). All the measurements are performedat 40 °C, and the measurements are repeated at least three times.UV-visible absorption spectra were recorded on a UV-visiblespectrometer (Hitachi 150-20 spectrometer). All the measure-ments are done at 40 °C. For the solubilization experiments, anexcess amount of water-insoluble dye (indigo) was added to theaqueous surfactant solutions. The solutions are mixed over 24h at 40 °C. Prior to measurements, surfactant solutions are eithercentrifuged or filtered to remove dispersed dye particles withinthe aqueous solution.

Results and Discussion

Figure3showsmeasurementsof theequilibriumsurfacetensions of aqueous solutions of FHUB that contain either1 or 100 mM LiBr electrolyte. Inspection of Figure 3 revealsthat each plot of the surface tension displays a sharp breakcorresponding to the onset of aggregation in the bulksolution. The surface tension reductions obtained fromFHUB are 25 and 29 mN/m for solutions containing 100and 1 mM LiBr electrolyte, respectively. The cac’s of FHUBare determined as 0.50 and 0.45 mM for 100 and 1 mMLiBr, respectively.

The summary of the properties of FHUB as well as thecomparison to several surfactants are given in Table 1.The chemical structure of FHUB (see Figure 1) resemblesthe dimeric cationic surfactant didodecyldimethylammo-nium bromide (DDAB). Unlike DDAB, FHUB bears afluorocarbon chain. The hydrophobic free energy contri-bution (ghyd) of a fluorocarbon chain to the free energy ofthe monolayer is reported to be larger than that of a

hydrocarbon chain having the same number of carbons.25

This high hydrophobicity of the fluorocarbon chain givesrise to the formation of a denser monolayer and formationof aggregates at a lower surfactant concentration.25 Thehydrophobic driving forces for adsorption (ghyd) of FHUBand DDAB are calculated by using the formulations thathave been shown previously to be successful in predictingghyd of surfactants.25 The hydrophobic free energy con-tribution to the free energy of the monolayer of FHUB iscalculated as -34.4 kT, which is larger than the ghyd ofDDAB (Table 1). This difference suggests that the FHUBshould start aggregation at a lower concentration thanDDAB. However, the experimentally determined cmc ofFHUB is larger than the cmc of DDAB. The seconddifference in the structure of FHUB compared to that ofDDAB other than the presence of the fluorocarbon chainis the presence of the hydroxyl group at the other end ofthe hydrocarbon chain. The hydroxyl group at the end ofthe hydrocarbon chain acts as a hydrophilic group whenthe carbon number of the hydrocarbon chain is less than12.26 Because one end of the hydrocarbon chain of FHUBis tethered by the ionic headgroup, the hydrophilicity ofthe OH group gives rise to the looped configuration of thehydrocarbon chain.5 Previously, it has been revealed thatthe surface tension reduction mechanism of unsym-metrical bolaform surfactants was shown to be dominatedby the configurational constraints and not the electrostaticinteractions, as expected from a classical ionic surfactantlike DTAB.5 This configurational constraints of FHUBprevent the adsorption of surfactant to the interface andprevent the formation of aggregates at a low surfactantconcentration. In other words, the cmc of FHUB is larger(22) Characterization of FHUB has been performed through 1H NMR

and elemental analysis. 1H NMR (DMSO) δ: 3.5 (2H, CH2OH), 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).Elem anal. Calcd.: C, 0.385; H, 0.052; N, 0.022. Found: C, 0.039; H,0.052; N, 0.022.

(23) The Krafft temperature of fluorocarbon surfactants is known tobe high. This problem can be solved by the addition of more hydrophilicgroups to the structure of the surfactant.24

(24) Sagisaka, M.; Ito, A.; Kondo, Y.; Yoshino, N.; Kwon, K. O.; Sakai,H.; Abe, M. Colloids Surf., A 2001, 183, 749-755.

(25) Tanford, C. The Hydrophobic Effect: Formation of Micelles andBiological Membranes, 2nd ed.; John Wiley and Sons: New York, 1980.

(26) Israelachvilli, J. Intermolecular and Surface Forces, 2nd ed.;Academic Press Limited: London, 1991.

Figure 3. Surface tension of the aqueous FHUB solutions in(O) 100 and (b) 1 mM LiBr.

Table 1. Comparison of Interfacial and Bulk Propertiesof FHUB with Those of Other Types of Surfactants

surfactantcmc

(mM)γlim

(mN/m)Alim(Å2)

ghyd(kT)

TKrafft(°C) reference

FHUBa 0.450 25 88 ( 5 -34.4 38DDABb 0.014 19 103 -32.0 <25 28HTABa 21.000 48 68 ( 5 -16.0 28 5DTABa 2.500 37 41 ( 5 -18.5 <0 19FCmCn

c

(m ) 6)0.045 22 100 ( 5 -37.8 26 20

a In the presence of 100 mM LiBr at 40 °C. b In water at 25 °C.c Anionic hybrid surfactant in water at 25 °C (see ref 20).

10728 Langmuir, Vol. 19, No. 26, 2003 Aydogan et al.

than that of DDAB because of the differences in theirconfiguration at the air-water interface even though thehydrophobic free energy (which favors micellization) ofFHUB is larger than that of DDAB.

The limiting area per FHUB molecule at the interfaceis determined by using the Gibbs adsorption equation.18,19

At the presence of 100 mM LiBr, the minimum area permolecule of FHUB is determined to be 88 ( 5 Å2/molecule,which is larger than the minimum area of a classical ionicsurfactant (DTAB) and smaller than the minimum areaof a cationic dimeric surfactant (DDAB). Because FHUB,DTAB, and DDAB bear one ionic headgroup, this differ-ence at the area per molecules is caused by differences inthe configuration of surfactants at the interface. Moreover,the minimum area per FHUB molecule within theinterface is larger than the minimum area per moleculeof HTAB but smaller than the minimum area per moleculeof DDAB, which can be explained simply by the presenceof the fluorocarbon chain.

One of the properties that distinguishes the unsym-metrical bolaform surfactants from the classical ionicsurfactants is their salt-insensitive interfacial behavior.5This behavior is related to the dominant contribution tothe lowering of the surface tension.5 The effect of theelectrolyte concentration on the behavior of FHUB issummarized in Table 2. It has been revealed from Figure3 that at a low surfactant concentration region (C < 0.1mM) the surface activity of FHUB is not affected by thechange in the electrolyte concentration. However, at ahigher surfactant concentration (0.1 < C < 0.45), the effectof the electrolyte concentration on the surface activity isobserved. It is known that the addition of electrolyte tothe solution of a classical ionic surfactant results inscreening of the electrostatic repulsions between thesurfactant molecules. As a result, a decrease at the cmcis observed. The cmc of HTAB, on the other hand, isdemonstrated to be less sensitive to changes in theelectrolyte concentration than that of the classical ionicsurfactants (Table 2). The FHUB molecule is proposed tobehave like an unsymmetrical bolaform surfactant. Theminimum area per FHUB molecule, which is calculatedas88Å2/molecule in thepresenceof100 mM LiBr, indicatesthe unsymmetrical bolaform structure of that molecule.Because the area per molecule of FHUB is larger thanthat of HTAB in the presence of 100 mM LiBr, theelectrostatic interactions between FHUB molecules areexpected to be smaller than those of HTAB, which furtherreduces the effect of the electrolyte concentrations on theinterfacial behavior of FHUB. In parallel to our expecta-tions, the cmc of FHUB is determined to be less sensitiveto change in electrolyte concentrations, as is seen fromTable 2. The addition of salt in this case causes the slightincrease in the cmc of FHUB. This unexpected behaviorof FHUB is proposed to be related to the different aggregateproperties of FHUB at 1 and 100 mM LiBr. The limitingsurface tension of FHUB, however, shows a decrease asa result of the addition of electrolyte. This change in the

limiting surface tension of FHUB with the addition ofelectrolyte is the largest compared to those of DTAB andHTAB. As mentioned before, the limiting surface tensionof a surfactant solution is actually determined by theconcentration at which aggregates are starting to form.The cmc of FHUB is determined to increase by the additionof the electrolyte. The increase of the cmc’s and thedecrease of the limiting surface tensions of FHUB solutionswith the increase of the electrolyte concentration arehypothesized result from the differences in the aggregationstates of FHUB compared to those of DTAB and HTAB.

We continue our discussion by commenting on thecounterion and its effect on the bulk behavior of FHUB.The counterion of a surfactant will affect the aggregationnumber and cmc.18,27,28 The degree of counterion bindingand the aggregation number for single-chain and double-tailed surfactants increase in the order of OH- < CH3COO-

< Cl- < Br- < I-. The degree of counterion binding is alsodependent on the charge density of the micelles. Becausethe area per FHUB molecule at the interface is largecompared to that of the classical ionic surfactant, al-though iodine has a strong binding capacity, in theseconditions we believe comparing the behavior of FHUBwith that of surfactants having a bromide counterion doesnot create significant error. The conductivities of thesurfactant solutions in 100 mM LiBr, NaBr, and NaCl aregiven in Figure 4. The break at the curve is attributed tothe formation of aggregates.18,19 Moreover, the degree ofcounterion binding (â) will be obtained from the ratios ofthe slopes of micellar and pre-micellar phases.29 In thepresence of 100 mM LiBr, the cmc of FHUB is determinedas 0.52 mM, which is close to the value obtained fromsurface tension measurements. The counterion bindingin the presence of LiBr electrolyte is calculated as 0.3.Previously, the counterion binding of a classical ionicsurfactant, DTAB, was reported as 0.67.29 This differencesin the counterion binding of FHUB and DTAB as well asthe differences in the minimum area occupied by asurfactant molecule at the interface also support ourproposition of FHUB molecules having a looped config-uration at the interface and within the aggregate. Differenttypes of electrolytes have also been used to furtherinvestigate the behavior of this new surfactant. In thepresence of 100 mM NaCl, the cmc and the counterionbinding of FHUB is determined to be 0.5 mM and 0.36,respectively. However, the cmc and â are determined fromconductivity experiments in the presence of 100 mM NaBras 0.42 mM and 0.5, respectively. From these results, itis concluded that, with the increase in the counterionbinding, the cmc of the surfactant decreases.

The bulk properties of unsymmetrical bolaform surf-actants are reported to be different than those of theclassical ionic surfactants.10 For example, HTAB moleculesare demonstrated to form small aggregates with a smallaggregation number.10 The fluorocarbon chain of FHUBis hypothesized to make the formation of small aggregatesharder as a result of the more rigid structure of afluorocarbon chain, which makes the formation of ag-gregates with high curvature (small aggregates) lesspossible. To evaluate this hypothesis, the sizes of theaggregates formed by FHUB are determined using quasi-elastic light scattering. The hydrodynamic diameters ofFHUB, HTAB, and DTAB are reported in Table 2. The

(27) Proverbio, Z. E.; Bardavid, S. M.; Arancibia, E. L.; Schulz, P. C.Colloids Surf., A 2003, 1-3, 167-171.

(28) Gaillon, L.; Lelievre, J.; Gaboriaud, R. J. Colloid Interface Sci.1999, 213, 287-297.

(29) Shah, S. S.; Jamroz, N. U.; Sharif, Q. M. Colloids Surf., A 2001,178, 199-206.

Table 2. Effect of the Electrolyte Concentration on theSurface Activity and Aggregate Size of FHUB and

Classical Ionic and Unsymmetrical Bolaform Surfactants(T ) 40 °C, pH 2)

cmc (mM) γlim (mN/m) Dh (nm)

surfactant1 mMLiBr

100 mMLiBr

1 mMLiBr

100 mMLiBr

1 mMLiBr

100 mMLiBr

FHUB 0.4 0.45 29.0 25.0 65 ( 5;258 ( 20

388 ( 20

HTAB 29.0 21.0 47.0 47.0 <1 <1DTAB 11.0 2.5 36.0 35.5 3 ( 1 3 ( 1

New Bolaform Surfactant Langmuir, Vol. 19, No. 26, 2003 10729

aggregate size of a classical ionic surfactant such as DTABis determined to form globular micelles. The instrumentthat we used in this study was not able to detect theaggregates of HTAB (Dh < 1 nm). The aggregate size ofFHUB is expected to be different than those of theaggregates of DTAB and HTAB. First, FHUB formsaggregates that are larger in size with less curvature,which is parallel to our expectations from the fluorocarbon-containingsurfactant.Second, thechange in theelectrolyteconcentration from 100 to 1 mM LiBr gives rise to atransition in the aggregate size, as seen from Table 2. Atlow electrolyte concentrations (1 mM LiBr), aggregateswith 65 and 250 nm diameters coexist. When theelectrolyte concentration is raised to 100 mM, aggre-gates with a diameter of 388 nm are formed. In a previousstudy, aggregates formed by a fluorocarbon-hydrocarbonhybrid anionic surfactant are reported to have interestingthermoresponsive behavior, which is explained by thechange in the aggregate properties.21 When the aggregateshave been investigated in detail, it has been observedthat close to the cmc (∼0.5 wt %) small spherical aggregateswith an aggregation number of ∼22 are present.30 Anincrease in the surfactant concentration from 2 to 10%results in the formation of larger aggregates. In the 10%solution, rodlike micelles and multilayer vesicles (∼500

nm in diameter) have been determined.30 Moreover, theanionic hybrid surfactant has been reported to show atransition in the aggregate size as a result of the increasein the temperature from large aggregates with a 500 nmdiameter to a state in which small and large sphericalaggregates coexist.20 A further increase in the temperatureresults in the formation of small aggregates (80 nm).20

Within these large aggregates, the hydrocarbon- andfluorocarbon-rich region is self-organized. However, whenthe electrolyte concentration is decreased at the sametemperature or the temperature is increased at the sameelectrolyte solution, aggregates with a smaller size arefavored and a mix of large and small aggregates isdetermined. To characterize the bulk properties of FHUBto identify the similarities and differences with anionichybrid surfactant, the effects of the surfactant concentra-tion and temperature on the aggregate size are furtherinvestigated.

The aggregate size of FHUB, which is determined closeto the cmc (1 mM), is measured as 323 ( 20 nm by lightscattering (Figure 5). Although FHUB is a hybrid surf-actant, the configuration of FHUB within the aggregateis expected to be different than the anionic hybridsurfactant. The presence of ω-OH group constrains theconfiguration of the hydrocarbon group (looped configu-ration). The combination of the looped configuration ofthe hydrocarbon chain with the presence of the fluoro-carbon chain is expected to prevent the formation of smallaggregates. The formation of large aggregates even at alow surfactant concentration is evaluated as the indicationof differences in the aggregation state or differences inthe configuration of surfactants that are adopted withinthe aggregates of FHUB and anionic hybrid surfactant.An increase in the surfactant concentration from 1 to 3mM leads to the formation of larger aggregates with a 388( 20 nm diameter (Figure 5). The further increase in thesurfactant concentration to 5 mM results in the formationof smaller aggregates with a 234 ( 20 nm diameter. Thismaximum could be the indication of a change in theaggregate type of the surfactant. The aggregate geometryof this new surfactant is still under investigation; to givea better understanding and better comparison, the tem-perature-dependent change at aggregate sizes below andafter the maximum point needs to be examined.

We would like to finish our investigation by indicatingthe solubilization capacity of this new surfactant. TheUV-visible spectrum of the FHUB solution indicates theappearance of a second peak at concentrations larger than0.5 mM. This concentration (0.5 mM) has been determinedas the cmc from the surface tension and the conductivitymeasurements. This second peak in the UV-visiblespectrum of FHUB is attributed to the formation of

(30) Danino, D.; Weihs, D.; Zana, R.; Oradd, G.; Lindblom, G.; Abe,M.; Talmon, Y. J. Colloid Interface Sci. 2003, 259, 382-390.

Figure 4. Conductivities of surfactant solutions in the presenceof 100 mM (A) LiBr, (B) NaCl, and (C) NaBr. The dashed linesare drawn to guide to eyes. The break point in the conductivityvalue is taken as the cmc of the solution.

Figure 5. Effect of the FHUB concentration on the aggregatesize (100 mM LiBr, 40 °C at pH 2).

10730 Langmuir, Vol. 19, No. 26, 2003 Aydogan et al.

aggregates. The appearance of a second peak has not beenobserved for the DTAB and HTAB molecules. Thisdifference in the UV-visible spectrum of FHUB, DTAB,and HTAB is attributed to differences in their aggregatestates. The solubilization capacities of FHUB, DTAB, andHTAB are also determined by utilizing water-insolubledye (indigo). Figure 6 indicates the concentrations of indigosolubilized in DTAB micelles. As observed from the figure,the increase in the DTAB concentration results in theincrease in the dye concentration solubilized in the micelle.This increase in the solubilization capacity of DTAB isexplained by the increase in the number of globularmicelles. The HTAB molecules are reported to form smalleraggregates compared to the DTAB molecules. As a result,their capacity to solubilize dye molecules within theaggregates is expected to be lower than that of the DTABmolecules. The amount of indigo solubilized within theHTAB aggregates (at 25 mM) is determined to be aboutfive times smaller than that within the DTAB aggregates(3 mM). For the case of FHUB, the amount of dyesolubilized within the aggregate first declines with theincrease in the surfactant concentration and then startsto increase after a 3 mM surfactant concentration (Figure6). As noted previously, at this concentration (3 mM) amaximum in the aggregate size is observed. This UV-visible study also support the proposition of change in theaggregate state of the FHUB molecules with the surfactantconcentration. On the basis of the results of light scatteringand UV-visible spectroscopy, the proposed configurationof the surfactant within the aggregate is shown in Figure7. Surfactant molecules are expected to have a configu-ration similar to its configuration at the air-waterinterface. Because aggregate sizes obtained from lightscattering are larger than those of globular aggregates,bilayers and vesicles are the expected aggregate types ofFHUB. A detailed investigation of the aggregate state ofFHUB is going to be published in our following article.

Conclusions

In this study, we demonstrated the surface and bulkbehavior of a new fluorocarbon-hydrocarbon hybrid

unsymmetrical bolaform surfactant. This surfactant com-bines the properties of unsymmetrical bolaform surfac-tants and fluorocarbon-hydrocarbon hybrid surfactantsinto one molecule. Although this molecule has a cationicheadgroup, its surface properties were demonstrated tobe insensitive to change in the electrolyte concentration.This salt-insensitive behavior of FHUB is demonstratedto be related to its unsymmetrical bolaform structure.Unlike the unsymmetrical bolaform surfactant, FHUBwas able to form aggregates at lower concentrations thanthe classical ionic surfactants. This behavior was simplyrelated to the presence of the fluorocarbon chain, whichismorehydrophobic thanthehydrocarbonchain.Theotherfunction of the fluorocarbon chain within the structure ofFHUB was to prevent to formation of small aggregates sothat the surface tension of the surfactant solution willcontinue to decrease. This new surfactant was able toreduce the surface tension to a value that is lower thanthe limiting surface tension of the classical ionic surfactantDTAB (25 vs 35 mN/m). The aggregation behavior ofFHUB was determined to be different than that of thehybrid anionic surfactants. This new surfactant was ableto form large aggregates with less curvature. Although adetailed investigation on the aggregate behavior of FHUBis in progress, from preliminary study it is concluded thataggregate state of FHUB is affected by its concentration.Under these circumstances, it is expected that FHUB alsohas thermoresponsive viscoelastic and thermoresponsivebulk properties.

Acknowledgment. This work is supported by TheScientific and Technical Research Council of Turkey(TUBITAK) through the grant of the project MISAG-208and Hacettepe University Research Fund (0102602006).N. Aldis also acknowledges the support from TUBITAK.

LA0351921

Figure 6. Effect of DTAB (A) and FHUB (B) concentrationson the solubilized dye concentration within the aggregate (100mM LiBr, 40 °C at pH 2).

Figure 7. Proposed surfactant configurations and aggregategeometries of FHUB.

New Bolaform Surfactant Langmuir, Vol. 19, No. 26, 2003 10731