interactions of a fluoroaryl surfactant with hydrogenated, partially fluorinated, and perfluorinated...

Interactions of a Fluoroaryl Surfactant with Hydrogenated, PartiallyFluorinated, and Perfluorinated Surfactants at the Air/Water Interface

Marcin Broniatowski and Patrycja Dynarowicz-a tka*

Jagiellonian UniVersity, Faculty of Chemistry, Ingardena 3, 30-060 Krakow, Poland

ReceiVed February 14, 2006. In Final Form: May 10, 2006

A novel surfactant containing pentafluorophenyl moiety attached at the terminal position of undecanol (11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, abbr. PBD) was synthesized and employed for the Langmuir monolayercharacterization and miscibility studies with a semifluorinated alkane (perfluorodecyleicosane, abbr. F10H20) and fouralcohols differing in the degree of fluorination in their hydrophobic chains: octadecanol (C18OH), perfluorooctyldecanol(F8H10OH), perfluoroisononyldecanol (iF9H10OH) and 1H,1H-perfluorooctadecanol (F18OH). Pure monolayers ofall of the investigated surfactants as well as their mixtures were investigated with surface pressure-area isothermscomplemented by Brewster angle microscopy (BAM) images. PBD was found to form stable Langmuir monolayersof liquid-expanded character. Characteristic dendritic structures were formed at the very early stage of compressionand remained up to the vicinity of collapse, where 3D crystallites appeared. 2D miscibility studies revealed that PBDforms mixed monolayers with the investigated semifluorinated alkane (F10H20) as well as with perfluorinated alcohol(F18OH) within the whole composition range, do not mix with octadecanol to the fully hydrogenated alcohol, whereasit is partially miscible (up to a certain surface pressure value) with the studied semifluorinated alcohols. The analysisof the miscibility derived from the surface pressure-area isotherms (collapse pressure vs composition dependencies)agrees well with BAM images. Molecular interactions in the investigated systems have been quantified with interactionparameter,R.

Introduction

Langmuir monolayer forming fluorinated surfactants wereoriginally investigated in the 1950s by the group of Zisman.1

Generally, the fluorinated surfactants can be divided onto twogroups: perfluorinated and semifluorinated. Perfluorinatedamphiphiles possess all carbon atoms in the hydrophobic chainsubstituted by fluorines, whereas in the case of semifluorinatedcompounds, the hydrophobic chain has a diblock structure, inwhich hydrogenated and perfluorinated moieties are covalentlybound. Perfluorinated and semifluorinated carboxylic acids canbe treated as model fluorinated surfactants and were subjectedto numerous research studies.2-5 Compounds with different polargroups, such as fluorinated alcohols and thiols, were alsoinvestigated and characterized in Langmuir monolayers6-9 andself-assembled layers.10-13

A very interesting class of fluorinated surfactants is representedbysemifluorinatedalkanes (SFAs)of thegeneral formulaF(CF2)m-(CH2)nH (abbr. FmHn). Although SFAs do not possess any polar

group, they were recognized to be surface active at the organicliquid/air, oil/water, and water/air interfaces.14-17 Because ofthe lack of any polar headgroup, SFAs were called primitivesurfactants.18 In 1991, Gaines proved that some representativesof SFAs are capable of the Langmuir monolayer formation andcan be transferred onto solid substrates.19 These interestingchemicals were also investigated in the past few years in ourlaboratory and a number of papers describing their surfacebehavior have already been published.20-23

Since the fluorination of the hydrocarbon chain has usuallya beneficial effect on surfactants properties, a semifluorinatedalkyl chain is a very common and well-known structural motifin surfactants chemistry. At present, a growing interest indeveloping new materials is observed, and therefore there is aneed for new structural motifs, so that other fluorinated moieties(e.g., aromatic) could be incorporated into surfactant molecules.To the best of our knowledge, there were no attempts so far toconstruct surfactants incorporating the perfluorobenzyl fragmentin their structure. Hexafluorobenzene is a very interestingmolecule, being a subject of numerous experimental andtheoretical publications. Interestingly, this very molecule formswith benzene a complex of equimolar stoichometry, which meltscongruently at 20C, whereas the melting temperatures of the

* Corresponding author. Tel.+48-12-6632082. Fax:+48-12-6340515.E-mail: [email protected].

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3860.(3) Shibata, O.; Yamamoto, S. K.; Lee, S.; Sugihara, G.J. Colloid Interface

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16, 1734-1743.(13) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.;

Graupe, M.; Shmakova, O. E.; Lee, T. R.Langmuir2001, 17, 1913-1921.

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1995, 11, 977-983.(17) Napoli, M.; Conte, L.; Gambaretto, G. P.J. Fluorine Chem. 1997, 85,

163-167.(18) Turberg, M. P.; Brady, J. E.J. Am. Chem. Soc. 1988, 110, 7797-7801.(19) Gaines, G. L., Jr.Langmuir1991, 7, 3054-3056..(20) Broniatowski, M.; Sandez Macho, I.; Minones, J., Jr.; Dynarowicz-a tka,

P. J. Phys. Chem. B2004, 108, 13403-13411.(21) Broniatowski, M.; Dynarowicz-atka, P.J. Fluor. Chem.2004,125, 1501-

1507.(22) Broniatowski, M.; Sandez Macho, I.; Dynarowicz-a tka, P.Thin Solid

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6622 Langmuir2006,22, 6622-6628

10.1021/la060421f CCC: $33.50 2006 American Chemical SocietyPublished on Web 06/15/2006

pure chemicals are 4.0 and 5.5C, respectively.24,25The crystalstructure of this complex comprises infinite stocks of alternating,nearly parallel benzene and hexafluorobenzene molecules, incontrast to the herringbone packing present in the crystals ofpure components.26 Hexafluorobenzene has the same value ofquadrupolar moment as benzene, but of an opposite sign, thusthe quadrupolar interactions together with the van der Waalsforces are thought to be responsible for the complex formation.27

Perfluoroaryl-aryl interactions can find application for examplein the solid-state UV-initiated topological synthesis of stereo-regular polymers28,29as well as for designing of new electroopticalmaterials.30Moreover, many biologically active compounds, i.e.,aromatic amino acids, possess the benzene ring in their structure,so their interactions with surfactants having a perfluorobenzylmoiety are supposed to be of great interest.

To fulfill the gap in the literature concerning aromaticfluorinated surfactants, we have synthesized a new surfactant,namely 11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, whichpossesses the perfluorinated benzene ring in its structure (seeFigure 1). The IUPAC name of this surfactant is rathercomplicated, and therefore we tried to simplify it. Since11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol contains twomoieties, the fluorinated (perfluorobenzyl) and the hydrogenated(a fragment of decanol), thus we propose to name this compoundas 10-perfluorobenzyldecan-1ol, abbr. PBD.

This contribution presents a brief description of PBD spreadin Langmuir monolayers as well as its behavior in mixedfilms. For our investigations, the following compoundshave been selected to mix with PBD: perfluorodecyleicosane(1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-heneicosafluorotria-contane, abbr.F10H20), a semifluorinated alkane, well character-ized in our previous paper,20 as well as four alcohols differing

in the degree of fluorination in their hydrophobic chains,namely: octadecanol (C18OH), perfluorooctyldecanol(11,11,12,12,13,13,14,14,15,15,16,16,17,17,18,18,18-heptade-cafluorooctadecan-1-ol, abbr. F8H10OH), perfluoroisononyl-decanol (11,11,12,12,13,13,14,14,15,15,16,16,17,18,18,18-hexa-decafluoro-17-(trifluoromethyl)octadecan-1-ol, abbr. iF9H10OH),and 1H,1H-perfluorooctadecanol (F18OH).

In our research, we have applied surface pressure-areaisotherms complemented with Brewster angle microscopic(BAM) observations. The latter method has become the mostpowerful tool in investigations of the mixed systems. From the-A isotherms datapoints, such thermodynamic parameters asthe excess free energy of mixing (Gexc) and interaction parameter(R) were calculated and related to BAM results.

Experimental SectionMaterials. PBD and the following semiflluorinated alcohols,

perfluorooctyldecanol (F8H10OH) and perfluoroisononyldecanol(iF9H10OH), were synthesized by one of us (M.B.). Perfluoroalkyliodides react with olefins according to the mechanism of radicaladdition,31 forming after reductive dehaloganation the requiredsemifluorinated compound. The perfluorinated alkyl iodides, per-fluorobenzyl iodide, perfluorooctyl iodide, and perfluoroisononyliodide, were purchased from Fluorochem (98% purity) and wereapplied without preliminary purification. The-unsaturated alcohol,9-decen-1-ol (98%), was supplied by Aldrich, and AIBN (azo-bisisobutyronitrile) was used as the initiator of the radical additionreaction. The synthetic procedure of Rabolt et al.32was used withoutany significant modifications. The purity of the synthesizedfluorinated alcohols (>99%) was verified by mass spectrometry,IR, 1H and 13C NMR, and elemental analysis. 1H,1H-perfluoro-octadecanol (98%) was purchased from Fluorochem, and octadecanol(99%) was supplied by Aldrich. The semifluorinated alkane (F10H20)has also been synthesized according to the above-mentionedprocedure.32

Methods.The spreading solutions for Langmuir experiments wereprepared by dissolving each of the investigated compound inchloroform (Aldrich, HPLC grade) with a typical concentration ofca. 0.5 mg/mL, except for F18OH which was dissolved in hexane/ethanol (3:1 v/v). Mixed solutions were prepared from the respectivestock solutions of both compounds. The number of molecules spreadon water subphase (7.5 1016 molecules), with a Hamiltonmicrosyringe, precise to(0.2 L, was kept constant in all theexperiments. Ultrapure water (produced by a Nanopure waterpurification system coupled to a Milli-Q water purification system(resistivity) 18.2 M cm) was used as a subphase. The subphasetemperature was 20C and was controlled to within 0.1C by acirculating water system from Haake. Experiments were carried outwith a NIMA 601 trough (Coventry, U.K.) (total area)600 cm2,equipped with two symmetrical barriers placed on an antivibrationtable. Surface pressure was measured with the accuracy of(0.1mN/m using a Wilhelmy plate made from chromatography paper(Wharman Chr1) as a pressure sensor. After spreading, themonolayers were left for 10 min for the solvent to evaporate, afterwhich compression was initiated with a barrier speed of 15 cm2/min.A brewster Angle Microscope BAM 2 plus (NFT, Germany) wasused for microscopic observation of the monolayer structure. It isequipped with a 50 mW laser, emittingp-polarized light of 532 nmwavelength, that was reflected off the air-water interface atapproximately 53.15 (Brewster angle). The lateral resolution of themicroscope was 2m. The images were digitized and processed toobtain the best quality of the BAM pictures. Each image correspondsto a 228m 170 m of the monolayers fragment.

ResultsThe-A isotherm of PBD registered at 20C together with

BAM images is presented in Figure 1. The surface pressure

(24) Patrick, C. R.; Prosser, G. S.Nature1960, 187, 1021.(25) Schroer, J. W.; Monson, P. A.J. Chem. Phys. 2003, 118, 2815-2823.(26) Collings, J. C.; Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.Cryst.

Eng. 2002, 5, 37-46.(27) Smith, C. E.; Smith, P. S.; Thomas, R. L.; Robins, E. G.; Collings, J. C.;

Dai, C.; Scott, A. J.; Borwick, S.; Batsanov, A. S.; Watt, S. W.; Clark, S. J.; Viney,C.; Howard, J. A. K.; Clegg, W.; Marder, T. B. J. Mater. Chem. 2004, 14, 413-420.

(28) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Dougherty, D. A.; Grubbs,R. H. Angew. Chem., Int. Ed. Engl.1997, 36, 248-251.

(29) Coates, G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky,E. B.; Grubbs, R. H.J. Am. Chem. Soc.1998, 120, 3641-3649

(30) Renah, M. L.; Bartholomew, G. P.; Wang, S.; Ricatto, P. J.; Lachicotte,R. J.; Bazan, R. J.J. Am. Chem. Soc.1999, 122, 7787-7799.

(31) Brace, N. O.J. Fluor. Chem. 1999, 93, 1-25.(32) Rabolt, J. F.; Russell, T. P.; Twieg, R. J.Macromolecules1984, 17, 7,

2786-2794.

Figure 1. -A isotherm of PBD (chemical formula- top leftcorner) registered at 20C, together with representative BAM images.Inset: -A isotherms registered upon the compression of puremonolayers of: F10H20 and the four alcohols used in this study.

Interactions of a Fluoroaryl Surfactant Langmuir, Vol. 22, No. 15, 20066623

starts to rise at ca. 0.6 nm2/molecule and the isotherm has theshape characteristic for a monolayers in the liquid-expandedstate. At the beginning of the surface pressure rise, foamliketextures, typical for the gaseous/liquid expanded states equi-librium, were observed (image a). Upon compression, at ca. 2mN/m, characteristic dendritic domains of the liquid-expandedmonolayer of PBD start to appear, which are present during thewhole compression (image b) until the monolayer collapses atca. 32 mN/m and white spots, characteristic of the collapsed 3Dphase, become cleraly visible (image c). To obtain morequantitative information about the monolayer physical state, thecompression modulusCS-1 (CS-1 ) -A d/dA, whereA is themolecular area, is the surface pressure)33 was calculated. Inthe case of the PBD monolayer, compressed at 20C on purewater,CS-1 reaches its maximum of 70 mN/m in the vicinity ofthe film collapse, which is still a value characteristic of the liquid-expanded state of the monolayer.

The stability of PBD monolayer has also been investigated,by monitoring the drop of surface pressure after the barriers werehalted. Two values of surface pressure were examined: 6 and20 mN/m. It appears that the PBD monolayer is quite stable,since after 30 min the surface pressure value falls to 71% of itsinitial value in the case of the higher pressure or to 66% in thecase of the lower initial pressure.

The inset in Figure 1 presents the isotherms of pure surfactants,investigated later in mixtures with PBD. The studied alcoholsas well as the semifluorinated alkane F10H20 form homogeneousmonolayers at the air/water interface as proved by structurelessBAM images within the full compression, except for a very lowsurface pressure region where foamlike structures, similar tophoto a in Figure 1, were observed. Because of the lack of anyinteresting textures, the BAM images of the pure fluorinatedalcohols and F10H20 are not shown.

After characterizing pure PBD monolayer, mixed systems wereinvestigated. Figure 2 shows-A isotherms registered for allof the studied five systems containing PBD mixed with F10H20and the investigated alcohols. The measurements were carriedout with the increment of the mole fraction of a given surfactantof 0.1; however, for the clarity of presentation,only the isothermsfor the following mole fractions are shown: 0, 0.2, 0.4, 0.6, 0.8,and 1.

Figure 2a presents the results obtained for the system PBD/F10H20. At the low mole fraction of F10H20, the isotherm forXSFA) 0.2 (open circles) is more expanded than the isothermsof both pure components; however, at higher mole fraction ofF10H20, the-A isotherms of mixtures are shifted to the lowermolecular areas as compared to the isotherms of pure components,indicating a condensing effect of SFA on the PBD monolayer.Figure 2b illustrates the behavior of the PBD/C18OH system.The isotherms registered for mixtures are situated on the rightside of the PBD isotherm, which proves that the mixed monolayersare more expanded that the monolayers of the pure components.However, the situation is different for the system PBD/F18OH,Figure 2c. The isotherms registered for the mixtures nearly overlapwith PBD isotherm, and at higher mole fractions of F18OH (i.e.,X ) 0.8), the isotherm is shifted toward lower molecular areasthan the isotherm of PBD. Figure 2d shows-A isothermsregistered for the system PBD/F8H10OH. At lower surfacepressure values, the isotherms are visibly more expanded thanthose registered for pure components, however, upon compressionthe situation changes, and the-A isotherms recorded formixtures are located between of the isotherms of the pure

components, at higher surface pressure values. In the case of thesystem PBD/iF9H10OH (Figure 2e), the isotherms of mixtureslie generally between the isotherms of the pure components,except for X(iF9H10OH)) 0.8, where the isotherm is shiftedtoward greater molecular areas than the isotherm of iF9H10OH.

A valuable indicator of the mutual miscibility in Langmuirmonolayers is the dependence of the collapse pressure (C) vscomposition of the 2D mixture. According to the two-dimensionalphase rule,34 if two surfactants are miscible in Langmuirmonolayers, only one collapse pressure is observed, the valueof which lies between the collapse pressures of pure components.However, if the components mixes ideally, according to theadditivity rule, thecoll-Xdependence is linear. The dependenciesof the collapse pressure versus mole fraction of the surfactantmixed with PBD are plotted in Figure 3.

Figure 3a shows theC-X dependence of the system PBD/F10H20. At eachX(F10H20), there is only one collapse observedin the course of the isotherms. At lower mole fraction of F10H20(up to 0.4), an increase of theC value is observed, whereas atgreater proportion of F10H20,C starts to decrease. The collapsepressure of the pure PBD monolayer is ca. 32 mN/m, whereasfor F10H20,C reaches 16 mN/m. Values higher than 32 mN/m,observed forX(F10H20) ranging from 0.1 to 0.4, indicate thatthe mixed monolayers are more stable than the films formed bythe pure components. Figure 3b illustrates the situation of thesystem PBD/C18OH. At X (C18OH) greater than 0.3 twocollapses are visible in the course of the isotherms, which impliesthat the components are immiscible at least at such mole fractions.This can be corroborated by the fact that the first collapse pressure

(33) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed, AcademicPress: New York, 1963.

(34) Dynarowicz-a tka, P.; Kita, K.AdV. Colloid Interface Sci. 1999, 79,1-17.

Figure 2. -A isotherms registered for the investigated systems:(a) PBD/F10H20; (b) PBD/C18OH; (c) PBD/F18OH; (d) PBD/F8H10OH, and (e) PBD/iF9H10OH.

6624 Langmuir, Vol. 22, No. 15, 2006 Broniatowski and Dynarowicz--a tka

virtually does not change upon the increase of the C18 proportion,having the value of ca. 32 mN/m, characteristic of the pure PBDmonolayer. The system of PBD/F18OH is illustrated in Figure3c. Only one collapse pressure can be observed in the-Aisotherms at each proportion of F18OH, the value of which risesslowly with the increase of F18OH proportion. The collapsepressures of PBD and F18OH are comparable (ca. 32 and 38mN/m, respectively), which can explain the small differencesobserved between the values of the collapse pressures of themixtures and pure components. Figure 3d,e presents theC-Xdependencies for the systems PBD/F8H10OH and PBD/iF9H10OH, respectively. In both cases, two collapses are visible,in the former case forX(F8H10OH) ranging from 0.4 to 0.7 andin the latter case forX(iF9H10OH) ranging from 0.3 to 0.6. Inthe discussed cases, the first collapse pressure is not constant,contrary to the PBD/C18OH system (Figure 3b), but rises slowlyupon increasing the proportion of a semifluorinated alcohol. Sucha behavior can lead to a conclusion that at some proportions ofthe components in the monolayer, the mixtures of PBD with asemifluorinated alcohol are miscible, whereas at the others, phaseseparation takes place.

As shown in Figure 1, PBD monolayers visualized by BAMhave a very specific texture of long dendritic domains. It seemsthat the addition of a second component to the Langmuirmonolayer can profoundly change its texture, especially if PBDdoes not mix with the second component. All mixed monolayersinvestigated here were visualized with BAM microscopy, andthe results are presented in the following figures.

Figure 4 shows the results obtained for the system PBD/F10H20. The observed BAM structures were very similar

regardless the proportion of F10H20, and therefore we onlypresent the images registered of low SFA content (XF10H20) 0.2;photo a and b) and high SFA proportion (XF10H20) 0.8; photoc and d). Images a and c were taken below the collapse pressure:at 32 and 18 mN/m, respectively, whereas photos b and d wererecorded at surface pressures exceeding the value of the collapsepressure (at 36 and 21 mN/m, respectively). The images a andc are very similar to the textures observed for the pure PBDmonolayer, i.e., long dendritic domains are clearly visible. Imagesb and d illustrate the collapsed monolayer, where white stripesof a mixed multilayer are present. The photos corroborate theconclusion drawn on the bases of the analysis of-A isothermsas well as theC-X(F10H20) dependence, namely that the twosurfactants are miscible within the whole range of mole fractions.However, BAM images shed some light at this mixed system;that is, it seems that F10H20 molecules incorporate themselvesinto the dendritic domains characteristic of the pure PBDmonolayer. Moreover, the structure of the collapsed monolayeris different for PBD/F10H20 monolayers (where long white stripsof mixed multilayer were observed) versus pure PBD monolayer(where white spots were visible in the collapse region).

BAM images registered for the system PBD/C18OH arepresented in Figure 5. We have selected images for the followingmole fractions of C18OH: 0.2, photos a and b; 0.5, photos c andd; and 0.8, photos e and f. Images a, c, and e were taken at 15mN/m i.e., in the middle between the isotherms lift-off and themonolayer collapse, whereas images b, d, and f were registeredat 40 mN/m, i.e., above the first collapse. All of the presentedphotos differ to those described above for the system PBD/F10H20. A large number of small circular domains are visible.At low concentration of C18OH below the collapse (Figure 5a),large white islets were present, surrounded by small circulardomains. The islets are likely to be the clusters of long dendriticdomains characteristic of PBD monolayers, whereas the whitecircular domains can be attributed to C18OH. AtX(C18OH))0.5 Gy dendritic domains of PBD can be visible in the backgroungof the photo, embedded in the large number of whire circulardomains of C18OH monolayer. AtX(C18OH)) 0.8, only whitecircular domains are visible, which can indicate that at suchlarge proportion of C18OH, either the two components begin tomix in the monolayer, or the system is immiscible but graydendritic domains of PBD are not visible, because of theoverwhelming number of the bright circular domains of C18OHmonolayer. The monolayer of pure C18OH is homogeneous,and if at large mole fraction of C18OH and small of PBD, the

Figure 3. Plots of collapse pressures vs mole fraction of theinvestigated surfactant: (a) PBD/F10H20; (b) PBD/C18OH; (c) PBD/F18OH; (d) PBD/F8H10OH; and (e) PBD/iF9H10OH.

Figure 4. BAM images registered for the system PBD/F10H20OH: (a)X(F10H20)) 0.2, ) 32 mN/m; (b)X(F10H20)) 0.2, ) 36 mN/m; (c)X(F10H20)) 0.8, ) 18 mN/m; (d)X(F10H20)) 0.8, ) 21 mN/m.


two substances were miscible, the monolayers should be alsohomogeneous, which is not the case. On the basis of the imagesa, c, and e, it can be supposed that the mixed monolayers of PBDand C18OH are phase separated below the first collapse, so thephotos registered after the first collapse (b, d, and f) illustratealso a separated 2D system.

BAM textures registered for the system PBD/F18OH areillustrated in Figure 6. Simillary to the PBD/F10H20 system, thetextures of the mixtures of PBD with F18OH are very similar,regardless the mole fraction of F18OH. Because of this fact, wepresent here only the photos registered atX(F18OH)) 0.2 and0.8. Similarly to the previously discussed figures, images a andc in Figure 6 were registered below the collapse pressure (at 20mN/m), whereas b and d were taken at 40 mN/m (in the vicinity

of the molecular collapse). Long, dendritic domains can beobserved in photos a and c. They are longer and wider as comparedto those observed for pure PBD monolayer, but they are still verysimilar. This can suggest that PBD and F18OH are miscible inLangmuir monolayers regardless of the composition of the mixedfilm. The appearance of the collapsed 3D phase in the discussedcase (photos b and d) is slightly different than for the systemPBD/F10H20 and pronouncedly different than for pure PBDmonolayer, which can be caused by the presence of F18OHmolecules in the collapsed multilayer.

BAM textures for the mixtures of PBD with F8H10OH andiF9H10OH are shown in Figures 7 and 8, respectively. Bothsystems are very similar and therefore will be discussed together.The photos presented in these figures were taken at the followingmole fractions of the semifluorinated alcohols: 0.2 (photos aand b), 0.5 (photos c and d), and 0.8 (photos e and f). Imagesa, c, and e were taken at 20 mN/m, i.e., below the first collapsepressure, whereas b, d and f were registered at 40 mN/m, thatis at a surface pressure value between the two collapse pressuresobserved for the mixed systems. There is a profound differencebetween the discussed systems and the immiscible PBD/C18OHsystem. There are no white circular domains visible similarilyto the former case, only some long, dendritic domains charac-teristic of PBD can be seen in the images. The domains are lessexpressive as compared to pure PBD monolayer. On the contrary,the photos registered after the first collapse represent phaseseparated monolayers and are very similar to the collapsed filmsshown for the system described above. It seems that at lowersurface pressure values the components (PBD and semifluorinatedalcohols) are miscible, however, with the increasing surfacepressure the tendency for a phase separation starts to prevail andat a particular surface pressure value the PBD molecules areexpelled from the monolayer. The image of the collapsed phasesin both systems containing a semifluorinated alcohol are differentto that for pure PBD monolayer. Therefore, it can be inferred

Figure 5. BAM images observed for the system PBD/C18OH: (a)X(C18OH)) 0.2, ) 15 mN/m; (b)X(C18OH)) 0.2, ) 40mN/m; (c)X(C18OH)) 0.5, ) 15 mN/m; (d)X(C18OH)) 0.5, ) 40 mN/m; (e)X(C18OH)) 0.8, ) 15 mN/m; (e)X(C18OH)) 0.8, ) 40 mN/m.

Figure 6. BAM images registered for the system PBD/F18OH: (a)X(F18OH) ) 0.2, ) 20 mN/m; (b)X(F18OH) ) 0.2, ) 40mN/m; (c)X(F18OH)) 0 8, ) 20 mN/m; (d)X(F18OH)) 0.8, ) 40 mN/m.

Figure 7. BAM images registered for the system PBD/F8H10OH:(a) X(F8H10OH)) 0.2, ) 20 mN/m; (b)X(F8H10OH)) 0.2, ) 40 mN/m; (c) X(F8H10OH) ) 0.5, ) 20 mN/m; (d)X(F8H10OH)) 0.5, ) 40 mN/m; (e)X(F8H10OH)) 0.8, )20 mN/m; (f) X(F8H10OH)) 0.8, ) 40 mN/m.


that the 3D-phase formed at the first collapse contains not onlyPBD molecules but also some amount of the semifluorinatedalcohol molecules. It seems that in the surface pressure regionranging from the first collapse to the second one, a mixed 3D-multilayer phase coexists with a monolayer of a given semi-fluorinated alcohol.

From the-A isotherms, a thermodynamic analysis can beperformed, which can also be important in analyzing themiscibility/immiscibility of the investigated systems. In our study,we have calculated the excess free energy of mixingGexc aswell as interaction parameterR. These quantities are defined asfollows:34

whereA12 is the average surface area per molecule in the mixedmonolayer,A1(A2) stands for the molecular area of singlecomponent monolayer at the same surface pressure as it is appliedto determine A12 in the mixture, andX1 and X2 are the molefractions of component 1 and 2 in the mixed film.R is the gasconstant, andT is the absolute temperature.34 All experimentswere performed at 20C, so 293.16 K was taken for the calculationof R. The interaction parameterR is directly derived from theGexcvalues and brings similar thermodynamic information aboutthe investigated systems asGexc; therefore, we decided to presentand discuss here only theR-X(surfactant) dependencies, whichare gathered in Figure 9. The negative sign of the interactionparameter means that the interaction between the unlike moleculesin a mixed monolayer are more attractive than the interactionsbetween the like molecules in a pure one-component monolayer,whereas the positive sign ofR denotes that the interactions

between the unlike molecules are less attractive or even repulsive,as compared to the interactions between like molecules in a pureone-component monolayer.

Generally negative values ofR are observed for the systemswhich were defined to be miscible at all proportions of thecomponents, i.e., for PBD/F10H20 and PBD/F18OH (Figure 9photos a and c). For the other three systems, the values ofR arepositive, indicating than the interactions between PBD and thegiven alcohol are less attractive than the interactions betweenPBD molecules in its pure monolayer. The situation for thesystems PBD/F8H10OH and PBD/i9H10OH is different as faras theR-X(alcohol) plots are concerned. The less attractiveinteractions in the system PBD/F8H10OH are observed atX(F8H10OH)) 0.3, whereas in the system PBD/iF9H10OH,the less attractive interactions are present at both ends of theX(iF9H10OH) range, that is at 0.1 and 0.9, whereas atX(iF9H10OH) theR-X(iF9H10OH) has a minimum close to 0,which means that at this very concentration the interactionsbetween PBD and iF9H10OH are similar to the interactionsbetween like molecules in pure PBD and iF9H10OH monolayers.

Discussion

It is important to stress that all three methods applied in ourinvestigations, that is surface pressure-area isotherms, BAMimaging, and the thermodynamic approach, are complementaryand together give a deeper insight into the behavior of mixedmonolayers containing PBD. Because of very especial BAMtextures of PBD monolayer, BAM seems to be a very valuabletool in investigations of such systems.

Figure 8. BAM images registered for the system PBD/iF9H10OH:(a) X(iF9H10OH)) 0.2, ) 20 mN/m; (b)X(iF9H10OH)) 0.2, ) 40 mN/m; (c) X(iF9H10OH) ) 0.5, ) 20 mN/m; d)X(iF9H10OH)) 0.5, ) 40 mN/m; (e)X(iF9H10OH)) 0.8, ) 20 mN/m; (f) X(iF9H10OH)) 0.8, ) 40 mN/m.

Gexc) 0 (A12 - (A1X1 + A2X2))d (1)R ) G

exc

RT(X1X22 + X2X1

2)(2)

Figure 9. Interaction parameter (R) vs mole fraction of the surfactantused dependencies for the investigated systems: (a) PBD/F10H20;(b) PBD/C18OH; (c) PBD/F18OH; (d) PBD/F8H10OH; (e) PBD/iF9H10OH.


We have investigated the interactions of PBD, a novel surfactantcontaining a perfluorobenzyl moiety, with five different sur-factants, analyzing whether and at which conditions the two-dimensional binary systems are miscible, and at which conditionsimmiscibility and phase separation takes place. It has occurredthat PBD forms mixed monolayers with F10H20 and F18OH ateach proportion of the monolayer components, do not mix withoctadecanol to the fully hydrogenated alcohol, whereas it formsmixed monolayers with semifluorinated alcohols up to a particularsurface pressure value, at which the mixed monolayer collapsesand the mixed 3-D multilayer exists later in equilibrium with amonolayer of pure semifluorinated alcohol.

We will begin with the system PBD/F18OH, because it seemsto be easiest to interpret. F18OH (1H,1H-perfluorooctadecanol)possess only CF2 groups in its hydrophobic chain. The fluorineatoms of the chain of F18OH are in contact with the fluorinesfrom the perfluorobenzyl moiety, and it seems that the rule likedissolves like can be transferred into this two-dimensional system.Moreover, the cross sectional area of the hexafluorobenzene ring(ca. 0.3 nm2, crystallographic data35) is greater that the crosssection of the hydrocarbon chain, which is 0.185 nm2.36Therefore,in the vicinity of monolayer collapse, when the moleculesapproach each other to the closest possible in this system distance,the CH2 groups from the hydrocarbon chain of PBD have stillsome freedom and are not very closely situated to the CF2 groupsof the hydrophobic chain of the F18OH molecule.

On the contrary, the system PBD/C18OH is completelyimmiscible. The interactions between the CH2 groups of thehydrocarbon chains of C18OH molecules are much moreattractive than the interactions between the CH2 groups and thefluorines of the perfluorobenzyl moiety. Moreover, the CH2groups of the hydrogenated chain of PBD cannot approach closelythe CH2 groups of the hydrophobic chain of C18OH, becauseof the bulky cross-sectional area of the hexafluorobenzene ring.

The system PBD/F10H20 seems to be the most interesting ofall of the five systems discussed here. These two compounds arenot only miscible in mixed Langmuir monolayers, but also themixed monolayers are more stable than the monoalyers of thepure components (at least at some mole fractions of F10H20).In one of our former papers,20 it was proved that semifluorinatedalkanes are oriented in Langmuir monolayers with their per-fluorinated moiety directed toward the air. F10H20 has a long

hydrogenatd chain containing 20 carbon atoms, which is longerthan the whole molecule of PBD. It is of interest that the CF2groups of F10H20 seems not to have any contact with the fluorineatoms of the perfluorobenzyl moiety. It should be underlinedhere that both pure monolayers of PBD and the mixed monolayersof F10H20 are of liquid-expanded character. It was shown in ourprevious papers20,22 that SFA molecules can be tilted in theirmonolayers. If this is the case here, the perfluorinated chains ofF10H20 from two molecules can approach each other over theperfluorinated ring of PBD. In such an arrangement, the CH2groups of F10H20 do not approach the fluorine atoms of thehexafluorobenzene ring, as it can place for the system PBD/C18OH.

Both semifluorinated alcohols used in our research have 10carbon atoms in their hydrogenated moiety, similarly to PBD.Thus, in the mixed systems, the fluorinated parts of these alcoholsare in contact with the perfluorobenzyl moiety of PBD, whereasthe methylene groups from the hydrogenated fragments havesome freedom, as the cross-sectional areas of the perfluorinatedparts are greater than those of the hydrogenated parts. On thebasis of theR-X(alcohol) dependence, it can be inferred that theinteractions between like molecules in pure monolayers are moreattractive than in the mixtures with PBD; however, only at highsurface pressure value does phase separation takes place. Somedifferences between the behavior of the mixtures of PBD withF8H10OH and iF9H10OH, clearly visible in theR-X(alcohol)plots, can originate from the structural differences between theperfluorinated moieties of these two alcohols. F8H10OH pos-sesses its perfluorinated moiety in normal constitution, whereasiF9H10OH has the perfluorinated moiety iso-branched.

Conclusion

The present contribution can be summarized in the statementthat PBD interacts stronger with highly fluorinated surfactantsthan with their hydrogenated analogues. However, these resultscan only be treated as a preliminary step in characterizing thesurfactants containing a perfluoaryl moiety in their structure. Asit has already been described in the Introduction, the interactionsbetween perfluorinated and hydrogenated aromatic moieties areof utmost interest. We are currently synthesizing new surfactantscontaining a hydrogenated benzene ring at the termination of thehydrophobic chain. The interactions between these surfactantsand the PBD molecule will be the subject of a future contribution.

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(35) Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.; Robins, E. G.ActaCrystallogr. C2001, 57, 1303-1305.

(36) Gaines, G. L., Jr.Insoluble Monolayers at the Liquid-Gas Interfaces;Willey Interscience: New York, 1966.


interactions of a fluoroaryl surfactant with hydrogenated, partially fluorinated, and perfluorinated...

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