DOI: 10.1021/la900593c 8075Langmuir 2009, 25(14), 8075–8082 Published on Web 05/18/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Langmuir Monolayers of a Hydrogenated/Fluorinated CatanionicSurfactant: From the Macroscopic to the Nanoscopic Size Scale

Elena Blanco,† �Angel Pi~neiro,† Reinhard Miller,‡ Juan M. Ruso,*,† Gerardo Prieto,† andF�elix Sarmiento†

†Biophysics and Interfaces Group, Department of Applied Physics, Faculty of Physics, University of Santiago deCompostela, 15782 Santiago de Compostela, Spain, and ‡Max-Planck-Institut f

::ur Kolloid- und Grenzflachen-

forschung, Am M::uhlenberg 1, 14424 Potsdam, Germany

Received February 17, 2009. Revised Manuscript Received April 22, 2009

Langmuir monolayers of the hydrogenated/fluorinated catanionic surfactant cetyltrimethylammonium perfluor-ooctanoate at the air/water interface are studied at room temperature. Excess Gibbs energies of mixing, ΔGE, as well astransition areas and pressures, were obtained from the surface pressure-area isotherm. The ΔGE curve indicates thattail-tail interactions are more important than head-head interactions at low pressures and vice versa. Atomic forcemicroscopy and molecular dynamics simulations allowed a fine characterization of the monolayer structure as afunction of the area per molecule at mesoscopic and nanoscopic size scales, respectively. A combined analysis of thetechniques allow us to conclude that electrostatic interactions between the ionic head groups are dominant in themonolayer while hydrophobic parts are of secondary importance. Overall, results obtained from the different techniquescomplement to each other, giving a comprehensive characterization of the monolayer.

1. Introduction

Depending on several external factors such as temperature,pressure, or component concentrations, self-assembled surfactantmonolayers (SAMs) can exist in a wide variety of topologicallydistinct phases. Pioneerworks in this area arose from their interestto the detergent, food, cosmetic, and petroleum industries. Nowa-days, new applications based on these structures, such as (theiruse as) drug delivery agents, chemical microreactors for nano-fabrication purposes, templates for nanolithography, compoundsfor organic solar cells, microelectronic, nanoelectromechanicaland microfluidic devices, nanosensors of different types, ormolecular coverings with specific properties, among others, havebeen proposed.1 However, the use of SAMs has been hindered byinsufficient control of the order, domain size, density, andstability of these templates.2 Recently, new approaches basedon the control of interactions among hydrophobic or/and hydro-philic parts of the components in mixed monolayers have beendeveloped for this purpose. In these studies, the control of domainsize and shape from the nanometer to the micrometer size scalesmay be achieved by changing the value of accessible externalparameters like the pressure, composition, temperature, pH, oreven the application of electric fields.3

The knowledge of the interactions among different surfactantsin a mixture or between oils and surfactants is of primaryimportance in most industrial applications. In general, surfac-tant-based commercial products are composed of a number ofingredients combined in specific proportions to attain the desiredproperties. Mixtures of molecules with fluorocarbon and hydro-carbon chains represent an optimal opportunity to investigate

how compounds with very different behavior have an effect uponeachother. In general,when a surfactant solutionwith at least twodifferent chain types is spreadonto a subphase, themoleculesmayseparate into two-dimensional domains;each rich in one of thecompounds;or, alternatively, they might mix leading to a uni-form monolayer or forming clusters of specific sizes and geome-tries. The pattern of the resulting monolayer depends on thebalance of a competition between the head groups and tails of thesolutes through intermolecular interactions. This also applies tothe bulk of a solution, but the nature of this competition becomesparticularly clear in Langmuir monolayers due to the fact thatsurfactant molecules are constrained to lie in a plane. In order todescribe the structure of a monolayer, several parameters like thechain tilting of surfactants chains, the relative azimuth betweenmolecules or domains, and their depth into the subphase areimportant.All these parameters depend on the nature of the polarhead and hydrophobic chain of the surfactants and, altogether,determine the packing of the film and the dependence between thesurface pressure and the average area per molecule.4 During thecompression of a surface film, the molecules at the interfacespontaneously reorganize into a state of lower Gibbs energyby changing these structural parameters. Thus, the knowledgeof the monolayer arrangement at different size scale as wellthe connection between the structural parameters andmacroscopic properties is relevant in this field. In this sense animportant number of researches have focused their interest in theapplication of molecular dynamic simulation offering a newperspective.5

*Corresponding author. E-mail: juanm.ruso@usc.es.(1) Zhang, G.; Marie, P.; Maaloum, M.; Muller, P.; Benoit, N.; Krafft, M. P. J.

Am. Chem. Soc. 2005, 127, 10412–10419.(2) (a) Maaloum,M.; Muller, P.; Krafft, M. P. Angew. Chem., Int. Ed. 2002, 41,

4331–4334. (b) Fontaine, P.; Goldmann,M.;Muller, P.; Faure,M. C.; Konovalov,O.; Krafft, M. P. J. Am. Chem. Soc. 2005, 127, 512–513.(3) Oishi, Y.; Kato, T.; Narita, T.; Ariga, K.; Kunitake, T. Langmuir 2008, 24,

1682–1685.

(4) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. J. Chem. Phys. 1994,101, 7161–7168.

(5) (a) Bertrand, E.; Blake, T. D.; De Coninck, J.Langmuir 2005, 21, 6628–6635.(b) McMullen, R. L.; Kelty, S. P. J. Phys. Chem. 2007, 111, 10849–10852. (c)Petrov, J. G.; Polymeropoulos, E. E.;Mohwald, H.Langmuir 2007, 23, 2623–2630.(d) Kaznessis, Y. N.; Kim, S.; Larson, R. G. Biophys. J. 2002, 82, 1731–1742. (e)Knecht, V.; M

::uller, M.; Bonn, M.; Marrink, S. J.; Mark, A. E. J. Chem. Phys.

2005, 122, 024704. (f) Zheng, J.; Li, L.; Chen, S.; Jiang, S.Langmuir 2004, 20, 8931–8934. (g) He, Y.; Hower, J.; Chen, S.; Bernards, M. T.; Chang, Y.; Jiang, S.Langmuir 2008, 24, 10358–10364.

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Catanionic surfactants are salts of an amphiphilic anion withan amphiphilic cation that can form monolayers at air/waterinterfaces aswell asmicelles and vesicles in the bulk of a solution.6

The ability of catanionic surfactants to self-assemble producingthese structures is related to the strong synergism due to electro-static attraction between oppositely charged head groups.7,8 Veryscarce investigations on catanionic surfactant resulting frommixtures of hydrogenated and fluorinated surfactants have beenpublished.9,10 The most outstanding characteristic of fluorinatedsurfactants is their simultaneous hydrophobic and lipophobicnature. For this reason these compounds typically exhibit alimited miscibility with hydrocarbon amphiphiles, and often theintroduction of fluorine into a surfactant results in interestingproperties that depend on both the interaction between hydro-carbon/fluorocarbon chains and the interaction between the headgroups.11,12 On the other hand, although catanionic surfactantshave been studied by a variety of experimental techniques, theobtained results depend on the method employed to perform themeasurements. Discrepancies have been mainly found in the highsurface pressure region;short area per molecule values. Theyhave been explained in terms of metastable domains in themonolayer such as rippled regions and also due to the loss ofmaterial from the interface, i.e. partial monolayer collapse ormonolayer leakage. These factors are difficult to control.13 Thus,a detailed mesoscopic description of their structures compatiblewith macroscopic results is still lacking.14

Hence, our aim in this article is to build upon the knowledge inthis area in several ways. First, the stability of a Langmuirmonolayer of the hydrogenated/fluorinated catanionic surfactantcetyltrimethylammonium perfluorooctanoate at the air/waterinterface is studied at room temperature bymeasuring the surfacepressure-molecular area isotherm. The analysis of the isothermalso provides information on intermolecular interactions andsurface phase transitions during the compression of the mono-layer. On the other hand, the molecular arrangement of trans-ferred films atmesoscopic and nanoscopic levels is investigated byatomic force microscopy and molecular dynamics simulations,respectively. Altogether, the obtained results provide a multilevelcharacterization of the system. Some of the conclusions areexpected to be useful to better understand the behavior of similarSAMs and consequently to design monolayers of catanionicsurfactants with specific structures.

2. Methodology

2.1. Experimental Section. Materials. The studied cata-nionic system was synthetized by direct mixing of the anionicsurfactant sodium perfluorooctanoate (S-PFO, 97% from Lan-caster) and the cationic surfactant cetyltrimethylammonium

bromide (CTA-B, 99% from Sigma) at equimolar concentrations(0.5 � 10-3 M) in water. Both chemicals were used as received,without further purification. The salt precipitate formed aftermixing was removed by washing with Milli-Q water. The processwas repeated until the counterions Br- and Na+ could not bedetected by electrical conductivity.

The method used was electrical conductivity. Samples werecentrifuged until a precipitate was formed. Then the precipitatewas redissolved in water and centrifuged again. The processcontinues until the electrical conductivity of the sample was inthe same order than water.

Surface Pressure-Area Measurements. The surface pres-sure (π) vs area per molecule (A) isotherm at 295 K was obtainedby a homemade Langmuir Teflon through of dimensions 6.2 �24.7 cm with a movable barrier, coupled to a Wilhelmy typebalance that uses chromatography paper as plate material. Achloroform solution of the catanionic surfactant was carefullyspread onto the subphase with a Hamilton microsyringe. After10min for solvent evaporation the isothermswere recordedwith aconstant barrier compression rate of 55 cm2/min.

Langmuir-Blodgett Deposition. The floating films at theair-subphase interface were transferred to freshly cleaned silicasubstrates by the vertical dipping method.15,16 The substrateswere clamped parallel to the throughbarriers and immersed in thesubphase before spreading the monolayer material. After com-plete evaporation of the solvent, the floating layer was com-pressed up to the target surface pressure. After a relaxationperiod, the deposition was performed at constant surface pres-sure, with a dipping speed of 5 mm/min.

Atomic Force Microscopy (AFM). The monolayer deposi-tions were surveyed with an atomic force microscope (D3100,Nanoscope IIIa controller, Digital Instruments, Santa Barbara,CA) in tapping mode. The resonance frequency of the employedcantilever is 250-300 kHz. The nominal curvature radius ofthe tip is in the 5-10 nm range, and the typical force constantis∼40 Nm-1. Images were analyzed using the software providedwith the AFM instrument. Manipulation of images was mini-mized to avoid artifacts; only brightness and contrast adjustmentsto whole images were performed.2.2. Molecular Dynamics Simulations. Setup of the

Simulation Boxes. The method employed to build the simula-tion boxes was similar to that recently reported for perfluoroalk-ane monolayers.17 Briefly, a random mixture consisting of 2511water molecules, 20 CTA, and another 20 PFO molecules wasintroduced in boxes of 4 � 4 � 24 nm3. The larger size of thez dimension was chosen to allow the formation of two air/waterinterfaces perpendicular to this axis. A 10 ns longMD simulationat 298 K and constant volume produced two monolayers due tothe fast adsorption of the surfactant molecules to the surface ofthe solution. Then, the system was enlarged by a factor of 4,pasting two inverted copies of itself adjacent to the original box inthe xy plane and an exact replica in the diagonal. The resultingbox consists of 10044 water molecules in total with two mono-layers of exactly 80 CTA and 80 PFOmolecules each. The size ofsuch a simulation box is 8 � 8 � 24 nm3, and the average areaavailable per surfactant molecule is 80 A2. An energy minimiza-tion of the final system using the steepest descent method wascarried out. Then a MD simulation at constant box height withsemi-isotropic control pressure in the x and y dimensions coupled

(6) (a) Jonsson, B.; Jokela, P.; Khan, A.; Lindman, B.; Sadaghiani, A. Langmuir1991, 7, 889–895. (b) Marques, E. F.; Regev, O.; Khan, A.; Lindman, B. Adv.Colloid Interface Sci. 2003, 100-102, 83–104. (c) Song, Y.; Dorin, R. M.; Garcia,R. M.; Jiang, Y.-B.; Wang, H.; Li, P.; Qiu, Y.; van Swol, F.; Miller, J. E.; Shelnutt,J. A. J. Am. Chem. Soc. 2008, 130, 12602–12603.(7) Caillet, C.; Hebrant, M.; Tondre, C. Langmuir 2000, 16, 9099–9102.(8) Khan, A.; Marques, E. F. In Specialist Surfactants; Robb, I. D., Ed.;

Blackie: Academic and Professional: London, 1998; p 37.(9) Gonz�alez-P�erez, A.; Schmutz, M.; Waton, J. G.; Romero, M. J.; Krafft, M.

P. J. Am. Chem. Soc. 2007, 129, 756–757.(10) L�opez-Font�an, J. L.; Blanco, E.; Ruso, J. M.; Prieto, G.; Schulz, P. C.;

Sarmiento, F. J. Colloid Interface Sci. 2007, 312, 425–431.(11) Nishida, J.; Brizard, A.; Desbat, B.; Oda, R. J. Colloid Interface Sci. 2005,

284, 298–305.(12) Blanco, E.; Olsson, U.; Ruso, J. M.; Schulz, P. C.; Prieto, G.; Sarmiento, F.

J. Colloid Interface Sci. 2009, 331, 522–531.(13) Baoukina, S.; Monticelli, L.; Marrink, S. J.; Tieleman, D. P. Langmuir

2007, 23, 12617–12623.(14) Rodriguez, J.; Clavero, E.; Laria, D. J. Phys. Chem. B 2005, 109, 24427–

24433.

(15) Peterson, I. R. J. Phys. D 1990, 23, 379–395.(16) Chen, X.; Lenhert, S.; Hirtz, M.; Lu, N.; Fuchs, H.; Chi, L.Acc. Chem. Res.

2007, 40, 393–401.(17) Pi~neiro, �A.; Prieto, G.; Ruso, J. M.; Verdes, P. V.; Sarmiento, F. J. Colloid

Interface Sci. 2009, 329, 351–356.

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to a Berendsen barostat and thermostat18 at 1 bar and at 298 Kwas performed. As expected, a compression of the system in thexyplanewas observed. The compression ratewas constant duringthe first nanosecond, and then it slowly decreased until thecollapse area (27.7 A2) was reached (see Figure 1). From thistrajectory, frames with an average of 75, 70, 65, 60, 55, 50, 45, 40,35, 30, and 27.7 A2 per CTA or PFO molecule were extracted.Eachof these latter systems togetherwith the original one (with 80A2 per molecule) was simulated at constant volume and 298K for10 ns in order to observe the rearranging of the solutes on thewater surface.

MD Simulation Parameters. All MD simulations were per-formed using the GROMACS package19-21 version 3.3.3. Thetwo surfactant molecules were modeled using the GROMOS96(53a6) force field22 with the bonded parameters that involvefluorine in PFO taken from Borodin et al.23 as detailed in ourprevious work.17 Following the same authors, no partial chargewas associated with fluorine atoms. The partial charges of theatoms consisting the CTA cationic head were estimated by usingthe PETRA package (http://www2.chemie.uni-erlangen.de/soft-ware/petra/) and then slightlymodified tohave a net chargeof+1for the whole head with the final 15 CHn groups of the hydro-phobic tail uncharged. Thus, the charge considered for the Natomand the attached three CH3 and oneCH2 groups in theCTAmolecule were +0.1760, +0.2152, and +0.1784, respectively.Bonded parameters and charges for the CFO anionic head(COO-) were extrapolated from glutamic acid in the GROMOS(53a6) force field. The extended simple point charge (SPC/E)model24 was utilized for water molecules. Three dimensionalperiodic boundary conditions with rectangular boxes were usedfor all the trajectories. Except for the case of the semi-isotropiccompression described above, all simulations were performed at298 K and at constant volume. Water and surfactant molecules

were separately coupled to a Berendsen thermostat with acoupling constant of 0.1 ps.25 Long-range electrostatic interac-tions were calculated using the particle mesh Ewald method26,27

with a real-space cutoff of 0.9 nm, a 0.12 nm spaced grid, andfourth-order B-spline interpolation. The Ewald sum in threedimensions with a correction term (EW3DC)28,29 was used toavoid artifacts due to interactions between replicas in thez direction. Random initial velocities were assigned to the systemsfromaMaxwell-Boltzmanndistributionat 298K.The equationsofmotionwere integratedusing the leapfrogmethod30with a timestep of 2 fs. Bond lengths and angles in water were constrainedusing the SETTLEalgorithm,31while the LINCS algorithm32wasused to constrain bond lengths within the surfactant molecules.During the MD simulations, coordinates, velocities, and energieswere stored every 2 ps for further analysis.

Analysis of MD Trajectories. The viewers RASMOL 2.7,33

VMD 1.8.2,34 and PyMOL 0.9935 were employed to roughlyinspect the arrangement of surfactant molecules on the watersurface and to capture images throughout the trajectories. Thedistributions of the CTA hydrocarbon and PFO fluorocarbonatoms, as well as that of water molecules, as a function of the boxheight, obtained at the end of each trajectory, i.e., after 10 ns ofmolecular dynamics simulation, were plotted and employed tocalculate the amount of watermolecules in direct contact with thecatanionic system for all the simulations. The latter analysis wasperformed for each simulation by integrating the intersection areabetween the distribution of water molecules and the distributionof CTA or PFO atoms for each of the two monolayers. Thedistributions were determined by counting the number of atomsof each type in 1.2 A width slides perpendicular to the z axis of thesimulation boxes. Average deuterium order parameters for thecarbon atoms of the surfactant molecules were also calculated atthe end of the trajectories. The radial distribution functions ofhydrogen and oxygen atoms ofwater around the ionic head of thesurfactants were also determined. The analysis was performedusing programs from the GROMACS package and scriptsspecifically designed for this work.

3. Results and Discussion

3.1. π-A Isotherms for Catanionic Semifluorinated Sur-factant. The π-A isotherm at 295 K for the catanionic semi-fluorinated surfactant CTA/PFO with pure water as subphase isshown in Figure 2.Generally two scenarios are possible when twodifferent amphiphile molecules A and B are mixed to form asurface film: (i) one of the amphiphile is incorporated in theassembly reached by the second one, and (ii) A and B formseparated phases or assemblies.36 Assuming that sharp slopechanges in the isotherm are associated with structural or con-formational molecular rearrangements of the catanionic systemthat forms the monolayer, the derivative dπ/dA should be useful

Figure 1. Available area per molecule as a function of time,obtained from MD simulations during a semiisotropic compres-sion of the monolayer. The inset shows a close-up of the regionwhere the slope becomes negligible; i.e., the surface collapses.

(18) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.;Haak, J. R. J. Chem. Phys. 1984, 81, 3684.(19) Berendsen, H. J. C.; van der Spoel, D.; van Drunen, R. Comput. Phys.

Commun. 1995, 91, 43.(20) Lindahl, E.; Hess, B.; van der Spoel, D. J. Mol. Model. 2001, 7, 306.(21) van der Spoel, D.; Lindahl, E.; Hess, B.; Groenhof, G.; Mark, A. E.;

Berendsen, H. J. C. J. Comput. Chem. 2005, 26, 1701.(22) Oostenbrink, C.; Villa, A.; Mark, A. E.; van Gunsteren, W. F. J. Comput.

Chem. 2004, 25, 1656.(23) Borodin, O.; Smith, G. D.; Bedrov, D. J. Phys. Chem. B 2002, 106, 9912.(24) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. J. Phys. Chem. 1987, 91,

6269.

(25) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; DiNola, A.;Haak, J. R. J. Chem. Phys. 1984, 81, 3684.

(26) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen,L. G. J. Chem. Phys. 1995, 103, 8577.

(27) Darden, T.; York, D.; Petersen, L. J. Chem. Phys. 1993, 98, 10089.(28) Bostick, D.; Berkowitz, M. L. Biophys. J. 2003, 85, 97.(29) Yeh, I. C.; Berkowitz, M. L. J. Chem. Phys. 1999, 111, 3155.(30) Hockney, R.W.; Eastwood, J.W. Computer Simulation Using Particles, 1st

ed.; Adam Hilger: Bristol, 1988.(31) Miyamoto, S.; Kollman, P. A. J. Comput. Chem. 1992, 9.(32) Hess, B.; Bekker, H.; Berendsen, H. J. C.; Fraaije, J. G. E. M. J. Comput.

Chem. 1997, 18, 1463.(33) Sayle, R. A.; Milnerwhite, E. J. Trends Biochem. Sci. 1995, 20, 374.(34) Humphrey, W.; Dalke, A.; Schulten, K. J. Mol. Graphics 1996, 14, 33.(35) De Lano, W. L. The PyMOLMolecular Graphics System, San Carlos, CA,

2002, http://www.pymol.org.(36) Imae, T.; Takeshita, T.; Kato, M. Langmuir 2000, 16, 612–621.

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to identify such transitions. dπ/dA vs A profiles, also plotted inFigure 2, exhibit two clear minima at 36 and 61 A2 per surfactantmolecule, which could presumably be related to the onset oftransitions between two phases. Thus, three different zonesseparated by those minima are present in this system althoughno equilibrium regions;with no pressure dependence on thearea;were detected in the plot. Comparison of this plot withothers previously studied37points to two different phases, a liquidexpanded (LE) phase and a liquid condensed (LC) phase, con-nectedwith a regionwhere the two phases coexist. The CTA/PFOsystem shows typical characteristics of stable monolayers: largeareas at the pressure onset and high surface pressure near the filmcollapse. Area permolecule value at the LE-LC transition aswell

as the limiting area, the area at the collapse, and the collapsepressure observed for this system are reported in Table 1.

Different two-dimensional crystal phases exhibiting a varietyof packing patterns and molecular tilts have been reported forsimilar systems.38 In the present work the structure of themonolayers as a function of the area per molecule at differentsize scales has been characterized by AFM and moleculardynamics simulations (see below). A comparison between thecompression and expansion isotherm is also shown as an inset ofthe plot displayed in Figure 2. The hysteresis or differencebetween the compression and expansion isotherms could berelated to some molecular desorption from the catanionic film.In order to get further insight into this matter, consecutivecompression-expansion cycles at pressures ranging from 0 to35 mN/m, well below the collapse, were performed (see Figure S1in the Supporting Information). These results reflect that thecatanionic semifluorinated surfactant CTA/PFO monolayer isunstable at the air-water interface. A number of factors mayprovoke the gradual loss of molecules from the monolayerincluding the desorption of molecules into the subphase, surfacechemical reactions like polar group hydration, or conformationalchanges of the monolayer molecules induced by the simultaneousmotion of the monolayer and the liquid substrate due to thesurface pressure gradient.

The stability of monolayers consisting of a mixture of twosurfactants, lets say a and b, can be analyzed in terms of excessGibbs energy of mixing:39

ΔGE ¼Z π

0

ðAab -XaAa -XbAbÞ dπ ð1Þ

where Ai is the molecular area (0.49 and 0.42 nm2 for CTA andPFO, respectively),40 Xi the molar fraction of component i, andAab is themeanmolecular area in themonolayer. Idealmixtures;where the interactions between the a and b species are given by alinear concentration-based interpolation of the pure com-pounds;lead to ΔGE = 0. In contrast, a-b interactions lessattractive than the value obtained from the interpolation of a and

Figure 2. Top: π-A (solid line) and dπ/dA-A (dashed line) iso-therms at the air-water interface for the system CTA/PFO.(a)-(d) labels correspond to the samples chosen for the AFMimages shown below. An example of compression-expansionisotherms is shown in the inset. Bottom: AFM images of theCTA/PFO monolayer obtained by tapping mode at 15 (a),27 (b), 30 (c), and 45 mN m-1 (d). The scale bars represents100 nm for (a) and 200 nm for (b)-(d). The grayscale palettesrepresent a height range of 5 nm for (a)-(c) and 8 nm for (d).

Table 1. Characteristic Parameters of the Catanionic SurfactantFilm: Ai (Initial Area), A (Limiting Area), Ac (Area at Collapse), and

πc (Collapse Pressure)

phases Ai (A2/molecule) A (A2/molecule) Ac (A

2/molecule) πc (mN/m)

LE, LC 99 57 28 50

Figure 3. ExcessGibbs energyofmixing for the systemCTA/PFOas a function of surface pressure, calculated using eq 1.

(37) Wang, Y.; Pereira, C. M.; Marques, E. F.; Brito, R. O.; Ferreira, E. S.;Silva, F. Thin Solid Films 2006, 515, 2031–2037.

(38) Hamley I. W. Introduction to Soft Matter; John Wiley & Sons: New York,2000.

(39) Nakamura, S.; Nakahara, H.; Krafft,M. P.; Shibata, O.Langmuir 2007, 23,12634–12644.

(40) (a) Bownes, N.; Ottewill, G. A.; Ottewill, G. H. Colloids Surf., A 1995, 102,203–211. (b) Sehgal, P.; Wimmer, R.; Mogensen, J. E.; Doe, H. J. Dispersion Sci.Technol. 2007, 28, 1262–1271.

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b at a given concentration provide ΔGE > 0, and an increase ofthe attractive interactions in the mixture with respect to the purecompounds gives ΔGE < 0.41 ΔGE values obtained from thecompression isotherm are plotted in Figure 3. Positive values areobserved for low surface pressure values (high area per molecule)while ΔGE < 0 for surface pressures higher than 45 mN m-1.Because of the nature of the molecular groups entailed in thissystem, relative unfavorable interactions are expected between thehydrocarbon/fluorocarbon tails while very attractive electrostaticinteractions are present between the charged head groups. Theexcess Gibbs energy curve indicates that tail-tail interactions aremore important than the head-head interactions at low pressuresand vice versa. A similar behavior has been found for hydro-carbon/fluorocarbonmixtures of lipids and surfactants, i.e., morenegative excessGibbs energies at high surface pressure valueswitha clear surface pressure-dependent balance between the interac-tions of polar heads and hydrophobic tails.42 It is convenient toremind that these results were obtained for an equimolar mixtureof the cationic and anionic surfactant. Different proportions ofthese salts are expected to affect seriously the balance of thedifferent contributions to ΔGE.3.2. Langmuir-Blodgett Monolayers. AFM Images.

Catanionic CTA/PFO monolayers at different surface pressuresselected from the π-A isotherm were transferred onto freshlycleaned silicon substrate and explored by atomic force micro-

scopy in tappingmode. Sample images obtained at representativemolecular areas (see labels a-d in the plot on top of Figure 2) areshown in Figure 2 (bottom). These AFM images show aggrega-tion patterns of the system at themesoscopic size scale. Increasinglevels of order are observed at higher compressions, reachingbranched structures with fractal geometry at∼60 A2 permolecule(π∼ 30mNm-1). At even lower areas, close to the collapse, starryisland integrated in the branched pattern are observed. Suchislands probably result from the coalescence of the less compactstructures found at lower pressures. The force levels observed inthe AFM images were correlated with the height of the observedstructures as can be observed in Figure 4. Typical domainsobtained at pressures lower than 30 mN m-1 exhibit heights ofnearly 3 nm, which is slightly higher than the length of CTA, thelargest molecule integrated in the monolayer. The surface rough-ness increases proportionally to the molecular two-dimensionalconcentration. The islands mentioned above, which appear closeto the collapse, correspond to greater heights, between 3 and 6nm, suggesting the presence of bilayers. In section 3.1 we havepostulated the possibility of molecular desorption (based oncompression-expansion cycles plots) or conformational changesof the monolayers. On the basis of AFM results, conformationalchanges in monolayer structure could be the main response ofpressure changes.3.3. Molecular Dynamics Simulations. The molecular dy-

namics simulations performed in the present work were designedto reproduce the conditions of the laboratory experiments, so theinformation obtained from this technique is expected to becompatible and to complement π-A isotherms and atomic force

Figure 4. AFM images of the monolayer and surface plot analysis of the selected area at surface pressures of (A) 27 mN m-1 and (B)45 mN m-1.

(41) Chimote, G.; Banerjee, R. Colloids Surf., B 2008, 65, 120–125.(42) Hoffmann, H.; Klaus, J.; Thurn, H. Colloid Polym. Sci. 1983, 261, 1043–

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microscopy images. As seen in Figure 1, the minimum areareached during the semiisotropic compression performed onthe catanionic monolayer (27.7 A2 per molecule) agrees with thecollapse area of the experimental π-A isotherm (28 A2 permolecule). Unfortunately, surface tension profiles obtained fromall the simulations exhibit high fluctuations and do not allow toquantitatively reproduce the isotherm. Although this energeticanalysis was not successful, several structural parameters can berelated with the transitions observed during the compression.Four views of the final conformation, after 10 ns of MDsimulation, of each of the 12 trajectories (with areas of 80, 75,70, 65, 60, 55, 50, 45, 40, 35, 30, and 27.7 A2 per molecule) areshown as Supporting Information. Some representative imagesare shown in Figure 5. Lateral views illustrate the arrangementsand tilts of the surfactants while the views perpendicular to thetwo monolayers allow observing the relative distribution offluorocarbon and hydrocarbon chains as well as the ratio ofsurface coverage attained at each area. The more the area isdecreased, the more the distribution of both types of moleculesseems to be homogeneous. In spite of the lipophobic character offluorinated carbon chains, neither hydrocarbon nor fluorocarbondomains are clearly observed regardless of the available area permolecule. As in previousworks,42 the absence of domains;whichimplies the miscibility of hydrocarbon and hydrocarbon tails;is

probably due to the attractive electrostatic interactions betweenthe ionic heads of the molecules involved in the catanionicsurfactant. This conclusion is also supported by a reportbased on molecular dynamics simulations of fluorocarbon-hydrocarbon diblocks18 performed using the same force field ofthe present paper. In that work, clear hydrocarbon-richand fluorocarbon-rich domains were spontaneously formed atair/water interfaces. The main difference between the moleculesemployed in both studies is the absence or presence of the polarhead. Another solid argument against the formation of domainsfor the studied system is that, since the atomic counterions of eachsurfactantwere removed from the system (both in the experimentsand also in the simulations), a domain consisting of only one typeof the ionic surfactants would involve a charge concentration ofthe same sign leading to a strong electrostatic repulsion.

Water holes were formed for areas above 50-55 A2 permolecule, the surface being fully covered by the catanionicsurfactant at higher compressions. Fluorinated chains appear tobe more straight and vertical on the surface than hydrocarbonchains. Order parameter profiles for both chains confirm thisobservation (see Figure 6). Order parameters for PFO chainremain almost constant, probably due to its inherent stiffnesswhen compared to hydrocarbon chains.1 (The minor dependenceof the fluorinated PFO chain on the order parameters is probablydue to its inherent stiffness when compared to hydrocarbonchains.1) Considering the difference in the chain length of bothsurfactants, the presence of PFO is expected to affect essentiallythe first half of the hydrocarbon chain of CTA. This is clearlyobserved in the order parameters, mainly for low area permolecule values. Between 27.7 and 30 A2 per molecule, the firsthalf of the CTAhydrophobic tail is straight and almost parallel tothe PFO molecule (perpendicular to the water surface). At 35 A2

permolecule,CTA is still ordered although a bitmore tilted,whilefor larger areas only the first 2-3 carbon atoms of the CTA tailare perpendicular to the water surface, increasing the extent ofdisorder and tilting proportionally to the distance between theC atom and the ionic head of the surfactant. The changes in theorder parameter profiles suggest a transition phase of the hydro-carbon chain of the CTA between 35 and 40 A2 per molecule, inagreement with one of the slope changes observed in the π-Aisotherm (Figure 2).

Figure 5. Representative images of the final conformation, after10 ns of MD simulation, of the CTA/PFOmonolayer at 27.7 (A),55 (B), and 65 A2 (C). Lateral views (left) illustrate the arra-ngements and tilts of the surfactants while the views fromthe perpendicular of the monolayer (right) allow to observethe relative distribution of fluorocarbon and hydrocar-bon chains as well as the ratio of surface coverage attained ateach area.

Figure 6. Left: averagedeuteriumorder parameters for the carbonatoms of the CTA (large lines) and PFO (short lines) molecules at27.7 (black dashed), 35 (red solid), 45 (orange solid), 55 (bluedashed), 65 (black solid), and 75 A2 (red dashed). Right: idem at 30(black dashed), 40 (red solid), 50 (orange solid), 60 (blue dashed),70 (black solid), and 80 A2 (red dashed).

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Thus, the results obtained from the structural analysis of theMD trajectories could explain the three slope changes observedin the π-A compression isotherm: (i) the transition observed at∼57 A2 per molecule could be related with the disappearance ofwater holes mentioned above; (ii) between 35 and 40 A2 permolecule the order parameters clearly showa transition of the firsthalf of the CTA hydrocarbon chain; and (iii) at 27.7 A2 asidentified by MD simulations, or 28 A2 as observed in the π-Aisotherm, the monolayer collapses. Direct comparisons betweenAFM images and MD simulations are not possible for tworeasons: first, AFM images are in the micrometric scale whileMD simulations are performed in the nanometric size scale; andsecond, although AFM images demonstrated to give usefulinformation for many systems in the past, the observationsperformed by this technique are tackled on a deposition of themonolayer onto a solid substratewhileMDsimulationsmimic theexperimental conditions;involving a liquid subphase;at whichπ-A isotherms are obtained.

As specified in the Methods section, atomic distributions ofhydrocarbon, fluorocarbon, and water molecules were also de-termined for the last conformation of each trajectory. Tworepresentative plots, corresponding to the collapse area and to

55 A2 per molecule, are shown in Figure 7 while the distributionsobtained from all the trajectories are presented in the SupportingInformation (Figure S3). Distributions of both fluorinated andhydrogenated chains seem to be similar regardless of the mole-cular area available. As expected from the visual analysis of thefinal conformations (see pictures in Figure 5 and in the Support-ing Information), the distribution of the PFO atoms is slightlyshifted to the water surface; the lower the molecular area avail-able, the more shifted the distribution, due to the larger length ofthe CTA hydrocarbon chain. The intersection area between thewater molecules distribution and the hydrocarbon or fluorocar-bon chain distributions was determined and taken as a measure-ment of the depth of the monolayer in the liquid (see Figure 8). Itis observed that the higher the compression of the monolayer, theless water appeared at the same box height. The number of watermolecules found at the same height as hydrocarbon atoms washigher than those obtained from the intersectionbetween the PFOatoms and water molecules distributions, as expected due to thehigher rigidity and lower tilting of the fluorinated surfactant onthe water surface.

Radial distribution functions of oxygen and hydrogen wateratoms around different atomic groups of the PFOandCTApolarheads for the final conformation of the 12 trajectories are shownin Figure S4 (see Supporting Information). Similar profiles werefound regardless of themolecular area.Amaximumof g(r) for thedistributionofwater hydrogenatoms around the oxygen atoms ofPFO was observed at 1.5 A2. The distributions of water oxygenatoms around the N of CTA or the carbon atom of the PFOheadgroup exhibit maxima at about 4 A2.

4. Conclusions

Langmuir monolayers of the hydrogenated/fluorinated cata-nionic surfactant cetyltrimethylammonium perfluorooctanoateat the air/water interface were studied at room temperature bysurface pressure versus molecular area measurements, atomicforce microscopy, and molecular dynamics simulations. Analysisof the π-A isotherm revealed two transitions during the com-pression of the monolayer that were explained by the MDsimulations in terms ofmolecular rearrangements at the interface.

Figure 7. Numberofwatermolecules (dotted),CTAhydrocarbonatoms (dashed), andPFO fluorocarbonatoms (black) as a functionof the boxheightobtainedat the endof the trajectories (after 10ns),performed at the molecular areas indicated in each plot. Thedistributions were determined by counting the number of mole-cules or atoms of each type in 1.2 A width slides perpendicular tothe z axis of the simulation boxes. The origin of the z axis (z= 0)was taken in the middle of the slab.

Figure 8. Number of water molecules in direct contact with theCTA (triangles and circles) and PFO (squares and diamonds)molecules for all the simulations performed at constant area permolecule. Results for the monolayers orientated toward positive(solid) and negative (empty) z values are separately shown to checkfor reproducibility. This analysis is based on the integral of theintersection between distributions, as indicated in the text.

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The agreement between the collapse area obtained from bothtechniques was excellent, indicating that the results provided bythe simulations are reliable. Additionally, structural informationon the monolayer at nanoscopic and mesosopic levels as afunction of the available area per molecule was obtained fromMD simulations and atomic force microscopy, respectively. Acombined analysis of the results attained from the differenttechniques allows us to conclude that electrostatic interactionsbetween the ionic head groups are dominant in the monolayerwhile hydrophobic parts are of secondary importance. Overall,the present paper provides new insight into mixed monolayersconsisting of surfactants of very different nature.

Acknowledgment.The authors acknowledge financial supportfrom the Spanish “Ministerio de Ciencia e Innovaci�on”, PlanNacional de Investigaci�on (I+D+i), MAT2008-04724. �A.

Pi~neiro thanks Xunta de Galicia for his Isidro Parga Pondalposition. Juan M. Ruso thanks Direcci�on Xeral de Promoci�onCientıfica e Tecnol�ogica do Sistema Universitario de Galicia forfinancial support. We are grateful to the Centro de Super-computaci�on de Galicia (CESGA) for computing time and fortheir excellent services.

Supporting Information Available: Representative imagesof the final conformation of CTA/PFO momolayer; π-Aisotherms forCTA/PFO;AFMimages ofCTA/PFO; atomicdistributions of hydrocarbon, fluorocarbon, and water mo-lecules; and radial distribution functions of oxygen andhydrogen water atoms around different atomic groups ofthe PFO and CTA polar heads for the final conformation ofthe 12 trajectories. This material is available free of chargevia the Internet at http://pubs.acs.org.

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