Interactions of a Fluoroaryl Surfactant with Hydrogenated, Partially

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Langmuir 2006, 22, 6622-6628

Interactions of a Fluoroaryl Surfactant with Hydrogenated, Partially Fluorinated, and Perfluorinated Surfactants at the Air/Water Interface Marcin Broniatowski and Patrycja Dynarowicz-Ła¸ tka* Jagiellonian UniVersity, Faculty of Chemistry, Ingardena 3, 30-060 Krako´ w, Poland ReceiVed February 14, 2006. In Final Form: May 10, 2006 A novel surfactant containing pentafluorophenyl moiety attached at the terminal position of undecanol (11,11difluoro-11-(pentafluorophenyl)undecan-1-ol, abbr. PBD) was synthesized and employed for the Langmuir monolayer characterization and miscibility studies with a semifluorinated alkane (perfluorodecyleicosane, abbr. F10H20) and four alcohols differing in the degree of fluorination in their hydrophobic chains: octadecanol (C18OH), perfluorooctyldecanol (F8H10OH), perfluoroisononyldecanol (iF9H10OH) and 1H,1H-perfluorooctadecanol (F18OH). Pure monolayers of all of the investigated surfactants as well as their mixtures were investigated with surface pressure-area isotherms complemented by Brewster angle microscopy (BAM) images. PBD was found to form stable Langmuir monolayers of liquid-expanded character. Characteristic dendritic structures were formed at the very early stage of compression and remained up to the vicinity of collapse, where 3D crystallites appeared. 2D miscibility studies revealed that PBD forms 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, whereas it is partially miscible (up to a certain surface pressure value) with the studied semifluorinated alcohols. The analysis of 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 interaction parameter, R.

Introduction Langmuir monolayer forming fluorinated surfactants were originally investigated in the 1950s by the group of Zisman.1 Generally, the fluorinated surfactants can be divided onto two groups: perfluorinated and semifluorinated. Perfluorinated amphiphiles possess all carbon atoms in the hydrophobic chain substituted by fluorines, whereas in the case of semifluorinated compounds, the hydrophobic chain has a diblock structure, in which hydrogenated and perfluorinated moieties are covalently bound. Perfluorinated and semifluorinated carboxylic acids can be treated as model fluorinated surfactants and were subjected to numerous research studies.2-5 Compounds with different polar groups, such as fluorinated alcohols and thiols, were also investigated and characterized in Langmuir monolayers6-9 and self-assembled layers.10-13 A very interesting class of fluorinated surfactants is represented by semifluorinated alkanes (SFAs) of the general formula F(CF2)m(CH2)nH (abbr. FmHn). Although SFAs do not possess any polar * Corresponding author. Tel. +48-12-6632082. Fax: +48-12-6340515. E-mail: [email protected]. (1) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1963, 67, 1534-1540. (2) Naselli, C.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1989, 90, 38553860. (3) Shibata, O.; Yamamoto, S. K.; Lee, S.; Sugihara, G. J. Colloid Interface Sci. 1996, 184, 201-208. (4) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K. H. Langmuir 1998, 14, 1786-1798. (5) Lehmler, H. J.; Jay, M.; Bummer, P. M. Langmuir 2000, 16, 1016110166. (6) Lehmler, H. J.; Bummer, P. M. J. Fluor. Chem. 2002, 117, 17-22. (7) Takiue, T.; Vollhardt, D. Colloids Surf. A 2002, 198-200, 797-804. (8) Lehmler, H. J.; Bummer, P. M. Colloids Surf. B 2005, 44, 74-81. (9) Vysotsky, Y. B.; Bryantsev, V. S.; Boldyreva, F. L.; Fainerman V. B.; Vollhardt, D. J. Phys. Chem. B 2005, 109, 454-462. (10) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (11) Schonherr, H.; Vansco, G. J. Langmuir 1997, 13, 3769-3774. (12) Tsao, M. W.; Rabolt, J. F.; Schonherr, H.; Castner, D. G. Langmuir 2000, 16, 1734-1743. (13) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913-1921.

group, they were recognized to be surface active at the organic liquid/air, oil/water, and water/air interfaces.14-17 Because of the lack of any polar headgroup, SFAs were called “primitive surfactants”.18 In 1991, Gaines proved that some representatives of SFAs are capable of the Langmuir monolayer formation and can be transferred onto solid substrates.19 These interesting chemicals were also investigated in the past few years in our laboratory and a number of papers describing their surface behavior have already been published.20-23 Since the fluorination of the hydrocarbon chain has usually a beneficial effect on surfactants properties, a semifluorinated alkyl chain is a very common and well-known structural motif in surfactants chemistry. At present, a growing interest in developing new materials is observed, and therefore there is a need 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 to construct surfactants incorporating the perfluorobenzyl fragment in their structure. Hexafluorobenzene is a very interesting molecule, being a subject of numerous experimental and theoretical publications. Interestingly, this very molecule forms with benzene a complex of equimolar stoichometry, which melts congruently at 20 °C, whereas the melting temperatures of the (14) Hopken, J.; Pugh, C.; Richtering, W.; Moller, M. Makromol. Chem. 1988, 189, 9111-932. (15) Lo Nostro, P.; Chen, S. H. J. Phys. Chem. 1993, 97, 6535-6540. (16) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 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. Langmuir 1991, 7, 3054-3056.. (20) Broniatowski, M.; Sandez Macho, I.; Min˜ones, J., Jr.; Dynarowicz-Ła¸ tka, P. J. Phys. Chem. B 2004, 108, 13403-13411. (21) Broniatowski, M.; Dynarowicz-Ła¸ tka, P. J. Fluor. Chem. 2004, 125, 15011507. (22) Broniatowski, M.; Sandez Macho, I.; Dynarowicz-Ła¸ tka, P. Thin Solid Films 2005, 493, 249-257. (23) Broniatowski M.; Sandez Macho, I.; Min˜ones, J.; Dynarowicz-Ła¸ tka, P. Appl. Surface. Sci. 2005, 246, 342-347.

10.1021/la060421f CCC: $33.50 © 2006 American Chemical Society Published on Web 06/15/2006

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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-heptadecafluorooctadecan-1-ol, abbr. F8H10OH), perfluoroisononyldecanol (11,11,12,12,13,13,14,14,15,15,16,16,17,18,18,18-hexadecafluoro-17-(trifluoromethyl)octadecan-1-ol, abbr. iF9H10OH), and 1H,1H-perfluorooctadecanol (F18OH). In our research, we have applied surface pressure-area isotherms complemented with Brewster angle microscopic (BAM) observations. The latter method has become the most powerful tool in investigations of the mixed systems. From the π-A isotherms datapoints, such thermodynamic parameters as the excess free energy of mixing (∆Gexc) and interaction parameter (R) were calculated and related to BAM results. Experimental Section Figure 1. π-A isotherm of PBD (chemical formula - top left corner) registered at 20 °C, together with representative BAM images. Inset: π-A isotherms registered upon the compression of pure monolayers of: F10H20 and the four alcohols used in this study.

pure chemicals are 4.0 and 5.5 °C, respectively.24,25 The crystal structure of this complex comprises infinite stocks of alternating, nearly parallel benzene and hexafluorobenzene molecules, in contrast to the herringbone packing present in the crystals of pure components.26 Hexafluorobenzene has the same value of quadrupolar moment as benzene, but of an opposite sign, thus the quadrupolar interactions together with the van der Waals forces are thought to be responsible for the complex formation.27 Perfluoroaryl-aryl interactions can find application for example in the solid-state UV-initiated topological synthesis of stereoregular polymers28,29 as well as for designing of new electrooptical materials.30 Moreover, many biologically active compounds, i.e., aromatic amino acids, possess the benzene ring in their structure, so their interactions with surfactants having a perfluorobenzyl moiety are supposed to be of great interest. To fulfill the gap in the literature concerning aromatic fluorinated surfactants, we have synthesized a new surfactant, namely 11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol, which possesses the perfluorinated benzene ring in its structure (see Figure 1). The IUPAC name of this surfactant is rather complicated, and therefore we tried to simplify it. Since 11,11-difluoro-11-(pentafluorophenyl)undecan-1-ol contains two moieties, the fluorinated (perfluorobenzyl) and the hydrogenated (a fragment of decanol), thus we propose to name this compound as 10-perfluorobenzyldecan-1ol, abbr. PBD. This contribution presents a brief description of PBD spread in Langmuir monolayers as well as its behavior in mixed films. For our investigations, the following compounds have 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-heneicosafluorotriacontane, abbr.F10H20), a semifluorinated alkane, well characterized in our previous paper,20 as well as four alcohols differing (24) Patrick, C. R.; Prosser, G. S. Nature 1960, 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, 413420. (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.

Materials. PBD and the following semiflluorinated alcohols, perfluorooctyldecanol (F8H10OH) and perfluoroisononyldecanol (iF9H10OH), were synthesized by one of us (M.B.). Perfluoroalkyl iodides react with olefins according to the mechanism of radical addition,31 forming after reductive dehaloganation the required semifluorinated compound. The perfluorinated alkyl iodides, perfluorobenzyl iodide, perfluorooctyl iodide, and perfluoroisononyl iodide, were purchased from Fluorochem (98% purity) and were applied without preliminary purification. The ω-unsaturated alcohol, 9-decen-1-ol (98%), was supplied by Aldrich, and AIBN (azobisisobutyronitrile) was used as the initiator of the radical addition reaction. The synthetic procedure of Rabolt et al.32 was used without any significant modifications. The purity of the synthesized fluorinated alcohols (>99%) was verified by mass spectrometry, IR, 1H and 13C NMR, and elemental analysis. 1H,1H-perfluorooctadecanol (98%) was purchased from Fluorochem, and octadecanol (99%) was supplied by Aldrich. The semifluorinated alkane (F10H20) has also been synthesized according to the above-mentioned procedure.32 Methods. The spreading solutions for Langmuir experiments were prepared by dissolving each of the investigated compound in chloroform (Aldrich, HPLC grade) with a typical concentration of ca. 0.5 mg/mL, except for F18OH which was dissolved in hexane/ ethanol (3:1 v/v). Mixed solutions were prepared from the respective stock solutions of both compounds. The number of molecules spread on water subphase (7.5 × 1016 molecules), with a Hamilton microsyringe, precise to (0.2 µL, was kept constant in all the experiments. Ultrapure water (produced by a Nanopure water purification system coupled to a Milli-Q water purification system (resistivity ) 18.2 MΩ cm) was used as a subphase. The subphase temperature was 20 °C and was controlled to within 0.1 °C by a circulating water system from Haake. Experiments were carried out with a NIMA 601 trough (Coventry, U.K.) (total area )600 cm2, equipped with two symmetrical barriers placed on an antivibration table. Surface pressure was measured with the accuracy of (0.1 mN/m using a Wilhelmy plate made from chromatography paper (Wharman Chr1) as a pressure sensor. After spreading, the monolayers were left for 10 min for the solvent to evaporate, after which compression was initiated with a barrier speed of 15 cm2/min. A brewster Angle Microscope BAM 2 plus (NFT, Germany) was used for microscopic observation of the monolayer structure. It is equipped with a 50 mW laser, emitting p-polarized light of 532 nm wavelength, that was reflected off the air-water interface at approximately 53.15° (Brewster angle). The lateral resolution of the microscope was 2µm. The images were digitized and processed to obtain the best quality of the BAM pictures. Each image corresponds to a 228 µm × 170 µm of the monolayer’s fragment.

Results The π-A isotherm of PBD registered at 20 °C together with BAM images is presented in Figure 1. The surface pressure (31) Brace, N. O. J. Fluor. Chem. 1999, 93, 1-25. (32) Rabolt, J. F.; Russell, T. P.; Twieg, R. J. Macromolecules 1984, 17, 7, 2786-2794.

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starts to rise at ca. 0.6 nm2/molecule and the isotherm has the shape characteristic for a monolayers in the liquid-expanded state. At the beginning of the surface pressure rise, foamlike textures, typical for the gaseous/liquid expanded states equilibrium, were observed (image a). Upon compression, at ca. 2 mN/m, characteristic dendritic domains of the liquid-expanded monolayer of PBD start to appear, which are present during the whole compression (image b) until the monolayer collapses at ca. 32 mN/m and white spots, characteristic of the collapsed 3D phase, become cleraly visible (image c). To obtain more quantitative information about the monolayer physical state, the compression modulus CS-1 (CS-1 ) -A dπ/dA, where A is the molecular area, π is the surface pressure)33 was calculated. In the case of the PBD monolayer, compressed at 20 °C on pure water, CS-1 reaches its maximum of 70 mN/m in the vicinity of the film collapse, which is still a value characteristic of the liquidexpanded state of the monolayer. The stability of PBD monolayer has also been investigated, by monitoring the drop of surface pressure after the barriers were halted. Two values of surface pressure were examined: 6 and 20 mN/m. It appears that the PBD monolayer is quite stable, since after 30 min the surface pressure value falls to 71% of its initial value in the case of the higher pressure or to 66% in the case of the lower initial pressure. The inset in Figure 1 presents the isotherms of pure surfactants, investigated later in mixtures with PBD. The studied alcohols as well as the semifluorinated alkane F10H20 form homogeneous monolayers at the air/water interface as proved by structureless BAM images within the full compression, except for a very low surface pressure region where foamlike structures, similar to photo a in Figure 1, were observed. Because of the lack of any interesting textures, the BAM images of the pure fluorinated alcohols and F10H20 are not shown. After characterizing pure PBD monolayer, mixed systems were investigated. Figure 2 shows π-A isotherms registered for all of the studied five systems containing PBD mixed with F10H20 and the investigated alcohols. The measurements were carried out with the increment of the mole fraction of a given surfactant of 0.1; however, for the clarity of presentation, only the isotherms for 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 for XSFA) 0.2 (open circles) is more expanded than the isotherms of both pure components; however, at higher mole fraction of F10H20, the π-A isotherms of mixtures are shifted to the lower molecular 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 right side of the PBD isotherm, which proves that the mixed monolayers are 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 overlap with PBD isotherm, and at higher mole fractions of F18OH (i.e., X ) 0.8), the isotherm is shifted toward lower molecular areas than the isotherm of PBD. Figure 2d shows π-A isotherms registered for the system PBD/F8H10OH. At lower surface pressure values, the isotherms are visibly more expanded than those registered for pure components, however, upon compression the situation changes, and the π-A isotherms recorded for mixtures are located between of the isotherms of the pure

components, at higher surface pressure values. In the case of the system PBD/iF9H10OH (Figure 2e), the isotherms of mixtures lie generally between the isotherms of the pure components, except for X(iF9H10OH) ) 0.8, where the isotherm is shifted toward greater molecular areas than the isotherm of iF9H10OH. A valuable indicator of the mutual miscibility in Langmuir monolayers is the dependence of the collapse pressure (πC) vs composition of the 2D mixture. According to the two-dimensional phase rule,34 if two surfactants are miscible in Langmuir monolayers, only one collapse pressure is observed, the value of which lies between the collapse pressures of pure components. However, if the components mixes ideally, according to the additivity rule, the πcoll-X dependence is linear. The dependencies of the collapse pressure versus mole fraction of the surfactant mixed with PBD are plotted in Figure 3. Figure 3a shows the πC-X dependence of the system PBD/ F10H20. At each X(F10H20), there is only one collapse observed in the course of the isotherms. At lower mole fraction of F10H20 (up to 0.4), an increase of the πC value is observed, whereas at greater proportion of F10H20, πC starts to decrease. The collapse pressure of the pure PBD monolayer is ca. 32 mN/m, whereas for F10H20, πC reaches 16 mN/m. Values higher than 32 mN/m, observed for X(F10H20) ranging from 0.1 to 0.4, indicate that the mixed monolayers are more stable than the films formed by the pure components. Figure 3b illustrates the situation of the system PBD/C18OH. At X (C18OH) greater than 0.3 two collapses are visible in the course of the isotherms, which implies that 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, Academic Press: New York, 1963.

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

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

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

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

virtually does not change upon the increase of the C18 proportion, having the value of ca. 32 mN/m, characteristic of the pure PBD monolayer. The system of PBD/F18OH is illustrated in Figure 3c. Only one collapse pressure can be observed in the π-A isotherms at each proportion of F18OH, the value of which rises slowly with the increase of F18OH proportion. The collapse pressures of PBD and F18OH are comparable (ca. 32 and 38 mN/m, respectively), which can explain the small differences observed between the values of the collapse pressures of the mixtures and pure components. Figure 3d,e presents the πC-X dependencies for the systems PBD/F8H10OH and PBD/ iF9H10OH, respectively. In both cases, two collapses are visible, in the former case for X(F8H10OH) ranging from 0.4 to 0.7 and in the latter case for X(iF9H10OH) ranging from 0.3 to 0.6. In the discussed cases, the first collapse pressure is not constant, contrary to the PBD/C18OH system (Figure 3b), but rises slowly upon increasing the proportion of a semifluorinated alcohol. Such a behavior can lead to a conclusion that at some proportions of the components in the monolayer, the mixtures of PBD with a semifluorinated alcohol are miscible, whereas at the others, phase separation takes place. As shown in Figure 1, PBD monolayers visualized by BAM have a very specific texture of long dendritic domains. It seems that the addition of a second component to the Langmuir monolayer can profoundly change its texture, especially if PBD does not mix with the second component. All mixed monolayers investigated here were visualized with BAM microscopy, and the 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 only present the images registered of low SFA content (XF10H20 ) 0.2; photo a and b) and high SFA proportion (XF10H20 ) 0.8; photo c and d). Images a and c were taken below the collapse pressure: at 32 and 18 mN/m, respectively, whereas photos b and d were recorded at surface pressures exceeding the value of the collapse pressure (at 36 and 21 mN/m, respectively). The images a and c are very similar to the textures observed for the pure PBD monolayer, i.e., long dendritic domains are clearly visible. Images b and d illustrate the collapsed monolayer, where white stripes of a mixed multilayer are present. The photos corroborate the conclusion drawn on the bases of the analysis of π-A isotherms as well as the πC-X(F10H20) dependence, namely that the two surfactants 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 themselves into the dendritic domains characteristic of the pure PBD monolayer. Moreover, the structure of the collapsed monolayer is different for PBD/F10H20 monolayers (where long white strips of 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 are presented in Figure 5. We have selected images for the following mole fractions of C18OH: 0.2, photos a and b; 0.5, photos c and d; and 0.8, photos e and f. Images a, c, and e were taken at 15 mN/m i.e., in the middle between the isotherms lift-off and the monolayer collapse, whereas images b, d, and f were registered at 40 mN/m, i.e., above the first collapse. All of the presented photos 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 circular domains. The islets are likely to be the clusters of long dendritic domains characteristic of PBD monolayers, whereas the white circular domains can be attributed to C18OH. At X(C18OH) ) 0.5 Gy dendritic domains of PBD can be visible in the backgroung of the photo, embedded in the large number of whire circular domains of C18OH monolayer. At X(C18OH) ) 0.8, only white circular domains are visible, which can indicate that at such large proportion of C18OH, either the two components begin to mix in the monolayer, or the system is immiscible but gray dendritic domains of PBD are not visible, because of the overwhelming number of the bright circular domains of C18OH monolayer. The monolayer of pure C18OH is homogeneous, and if at large mole fraction of C18OH and small of PBD, the

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

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

two substances were miscible, the monolayers should be also homogeneous, which is not the case. On the basis of the images a, c, and e, it can be supposed that the mixed monolayers of PBD and C18OH are phase separated below the first collapse, so the photos registered after the first collapse (b, d, and f) illustrate also a separated 2D system. BAM textures registered for the system PBD/F18OH are illustrated in Figure 6. Simillary to the PBD/F10H20 system, the textures of the mixtures of PBD with F18OH are very similar, regardless the mole fraction of F18OH. Because of this fact, we present here only the photos registered at X(F18OH) ) 0.2 and 0.8. Similarly to the previously discussed figures, images a and c in Figure 6 were registered below the collapse pressure (at 20 mN/m), whereas b and d were taken at 40 mN/m (in the vicinity

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

of the molecular collapse). Long, dendritic domains can be observed in photos a and c. They are longer and wider as compared to those observed for pure PBD monolayer, but they are still very similar. This can suggest that PBD and F18OH are miscible in Langmuir monolayers regardless of the composition of the mixed film. The appearance of the collapsed 3D phase in the discussed case (photos b and d) is slightly different than for the system PBD/F10H20 and pronouncedly different than for pure PBD monolayer, which can be caused by the presence of F18OH molecules in the collapsed multilayer. BAM textures for the mixtures of PBD with F8H10OH and iF9H10OH are shown in Figures 7 and 8, respectively. Both systems are very similar and therefore will be discussed together. The photos presented in these figures were taken at the following mole fractions of the semifluorinated alcohols: 0.2 (photos a and b), 0.5 (photos c and d), and 0.8 (photos e and f). Images a, c, and e were taken at 20 mN/m, i.e., below the first collapse pressure, whereas b, d and f were registered at 40 mN/m, that is at a surface pressure value between the two collapse pressures observed for the mixed systems. There is a profound difference between the discussed systems and the immiscible PBD/C18OH system. There are no white circular domains visible similarily to the former case, only some long, dendritic domains characteristic of PBD can be seen in the images. The domains are less expressive as compared to pure PBD monolayer. On the contrary, the photos registered after the first collapse represent phase separated monolayers and are very similar to the collapsed films shown for the system described above. It seems that at lower surface pressure values the components (PBD and semifluorinated alcohols) are miscible, however, with the increasing surface pressure the tendency for a phase separation starts to prevail and at a particular surface pressure value the PBD molecules are expelled from the monolayer. The image of the collapsed phases in both systems containing a semifluorinated alcohol are different to that for pure PBD monolayer. Therefore, it can be inferred

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

that the 3D-phase formed at the first collapse contains not only PBD molecules but also some amount of the semifluorinated alcohol molecules. It seems that in the surface pressure region ranging from the first collapse to the second one, a mixed 3Dmultilayer phase coexists with a monolayer of a given semifluorinated alcohol. From the π-A isotherms, a thermodynamic analysis can be performed, which can also be important in analyzing the miscibility/immiscibility of the investigated systems. In our study, we have calculated the excess free energy of mixing ∆Gexc as well as interaction parameter R. These quantities are defined as follows:34

∆Gexc )

∫0π (A12 - (A1X1 + A2X2))dπ

R)

∆Gexc RT(X1X22 + X2X12)

(1) (2)

where A12 is the average surface area per molecule in the mixed monolayer, A1(A2) stands for the molecular area of single component monolayer at the same surface pressure as it is applied to determine A12 in the mixture, and X1 and X2 are the mole fractions of component 1 and 2 in the mixed film. R is the gas constant, and T is the absolute temperature.34 All experiments were performed at 20 °C, so 293.16 K was taken for the calculation of R. The interaction parameter R is directly derived from the ∆Gexc values and brings similar thermodynamic information about the investigated systems as ∆Gexc; therefore, we decided to present and discuss here only the R-X(surfactant) dependencies, which are gathered in Figure 9. The negative sign of the interaction parameter means that the interaction between the unlike molecules in a mixed monolayer are more attractive than the interactions between the like molecules in a pure one-component monolayer, whereas the positive sign of R denotes that the interactions

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

between the unlike molecules are less attractive or even repulsive, as compared to the interactions between like molecules in a pure one-component monolayer. Generally negative values of R are observed for the systems which were defined to be miscible at all proportions of the components, i.e., for PBD/F10H20 and PBD/F18OH (Figure 9 photos a and c). For the other three systems, the values of R are positive, indicating than the interactions between PBD and the given alcohol are less attractive than the interactions between PBD molecules in its pure monolayer. The situation for the systems PBD/F8H10OH and PBD/i9H10OH is different as far as the R-X(alcohol) plots are concerned. The less attractive interactions in the system PBD/F8H10OH are observed at X(F8H10OH) ) 0.3, whereas in the system PBD/iF9H10OH, the less attractive interactions are present at both ends of the X(iF9H10OH) range, that is at 0.1 and 0.9, whereas at X(iF9H10OH) the R-X(iF9H10OH) has a minimum close to 0, which means that at this very concentration the interactions between PBD and iF9H10OH are similar to the interactions between like molecules in pure PBD and iF9H10OH monolayers.

Discussion It is important to stress that all three methods applied in our investigations, that is surface pressure-area isotherms, BAM imaging, and the thermodynamic approach, are complementary and together give a deeper insight into the behavior of mixed monolayers containing PBD. Because of very especial BAM textures of PBD monolayer, BAM seems to be a very valuable tool in investigations of such systems.

Broniatowski and Dynarowicz-Ł-a¸ tka

6628 Langmuir, Vol. 22, No. 15, 2006

We have investigated the interactions of PBD, a novel surfactant containing a perfluorobenzyl moiety, with five different surfactants, analyzing whether and at which conditions the twodimensional binary systems are miscible, and at which conditions immiscibility and phase separation takes place. It has occurred that PBD forms mixed monolayers with F10H20 and F18OH at each proportion of the monolayer components, do not mix with octadecanol to the fully hydrogenated alcohol, whereas it forms mixed monolayers with semifluorinated alcohols up to a particular surface pressure value, at which the mixed monolayer collapses and the mixed 3-D multilayer exists later in equilibrium with a monolayer of pure semifluorinated alcohol. We will begin with the system PBD/F18OH, because it seems to be easiest to interpret. F18OH (1H,1H-perfluorooctadecanol) possess only CF2 groups in its hydrophobic chain. The fluorine atoms of the chain of F18OH are in contact with the fluorines from the perfluorobenzyl moiety, and it seems that the rule like dissolves 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 cross section of the hydrocarbon chain, which is 0.185 nm2.36 Therefore, in the vicinity of monolayer collapse, when the molecules approach each other to the closest possible in this system distance, the CH2 groups from the hydrocarbon chain of PBD have still some freedom and are not very closely situated to the CF2 groups of the hydrophobic chain of the F18OH molecule. On the contrary, the system PBD/C18OH is completely immiscible. The interactions between the CH2 groups of the hydrocarbon chains of C18OH molecules are much more attractive than the interactions between the CH2 groups and the fluorines of the perfluorobenzyl moiety. Moreover, the CH2 groups of the hydrogenated chain of PBD cannot approach closely the CH2 groups of the hydrophobic chain of C18OH, because of the bulky cross-sectional area of the hexafluorobenzene ring. The system PBD/F10H20 seems to be the most interesting of all of the five systems discussed here. These two compounds are not only miscible in mixed Langmuir monolayers, but also the mixed monolayers are more stable than the monoalyers of the pure components (at least at some mole fractions of F10H20). In one of our former papers,20 it was proved that semifluorinated alkanes are oriented in Langmuir monolayers with their perfluorinated moiety directed toward the air. F10H20 has a long (35) Batsanov, A. S.; Howard, J. A. K.; Marder, T. B.; Robins, E. G. Acta Crystallogr. C 2001, 57, 1303-1305. (36) Gaines, G. L., Jr. Insoluble Monolayers at the Liquid-Gas Interfaces; Willey Interscience: New York, 1966.

hydrogenatd chain containing 20 carbon atoms, which is longer than the whole molecule of PBD. It is of interest that the CF2 groups of F10H20 seems not to have any contact with the fluorine atoms of the perfluorobenzyl moiety. It should be underlined here that both pure monolayers of PBD and the mixed monolayers of F10H20 are of liquid-expanded character. It was shown in our previous papers20,22 that SFA molecules can be tilted in their monolayers. If this is the case here, the perfluorinated chains of F10H20 from two molecules can approach each other over the perfluorinated ring of PBD. In such an arrangement, the CH2 groups of F10H20 do not approach the fluorine atoms of the hexafluorobenzene ring, as it can place for the system PBD/ C18OH. Both semifluorinated alcohols used in our research have 10 carbon atoms in their hydrogenated moiety, similarly to PBD. Thus, in the mixed systems, the fluorinated parts of these alcohols are in contact with the perfluorobenzyl moiety of PBD, whereas the methylene groups from the hydrogenated fragments have some freedom, as the cross-sectional areas of the perfluorinated parts are greater than those of the hydrogenated parts. On the basis of the R-X(alcohol) dependence, it can be inferred that the interactions between like molecules in pure monolayers are more attractive than in the mixtures with PBD; however, only at high surface pressure value does phase separation takes place. Some differences between the behavior of the mixtures of PBD with F8H10OH and iF9H10OH, clearly visible in the R-X(alcohol) plots, can originate from the structural differences between the perfluorinated moieties of these two alcohols. F8H10OH possesses its perfluorinated moiety in normal constitution, whereas iF9H10OH has the perfluorinated moiety iso-branched.

Conclusion The present contribution can be summarized in the statement that PBD interacts stronger with highly fluorinated surfactants than with their hydrogenated analogues. However, these results can only be treated as a preliminary step in characterizing the surfactants containing a perfluoaryl moiety in their structure. As it has already been described in the Introduction, the interactions between perfluorinated and hydrogenated aromatic moieties are of utmost interest. We are currently synthesizing new surfactants containing a hydrogenated benzene ring at the termination of the hydrophobic chain. The interactions between these surfactants and the PBD molecule will be the subject of a future contribution. LA060421F