Langmuir 2006, 22, 2691-2696
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Semifluorinated Chains at the Air/Water Interface: Studies of the Interaction of a Semifluorinated Alkane with Fluorinated Alcohols in Mixed Langmuir Monolayers Marcin Broniatowski and Patrycja Dynarowicz-Ła¸ tka* Department of General Chemistry, Faculty of Chemistry, Jagiellonian UniVersity, Ingardena 3, 30-060 Krako´ w, Poland ReceiVed December 6, 2005. In Final Form: January 17, 2006 Mixtures of a semifluorinated alkane 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorotriacontane (abbr. F10H20) and different alcohols were investigated at the air/water interface using surface pressure-area isotherms complemented with BAM images. In our studies, octadecanol and its fluorinated derivatives differing in the degree of fluorination were researched. To verify the influence of an iso branching of the fluorinated segment in an alcohol molecule, the properties of perfluorooctyldecanol and perfluoro-iso-nonyldecanol in mixtures with F10H20 were compared. From the isotherms datapoints, the excess of free energy of mixing (∆Gexc) together with the interaction parameter (R) were calculated. On the basis of the additivity rule and BAM images, phase diagrams for all of the investigated systems were constructed. It occurs that F10H20 mixes with the fully hydrogenated alcohol, octadecanol, within the whole range of alcohol mole fractions, whereas it is completely immiscible with its perfluorinated analogue. Regarding the mixtures of F10H20 with semifluorinated alcohols, it turned out that these systems exhibit limited miscibility, i.e., are miscible at a low semifluorinated alcohol proportion, whereas upon increasing alcohol content, the systems start to demix. It may be concluded that the molecular packing in mixed monolayers is the key factor determining the miscibility of F10H20 of the investigated alcohols.
Introduction Langmuir monolayers formed by two or more components are subjects of continuously growing interest, since investigation of their properties can provide valuable information on the mutual interactions between film-forming molecules.1 Similarly to threedimensional solutions, surfactants in a mixed Langmuir monolayer can in some instances mix ideally, whereas in other cases, positive or negative deviations from the ideal can be observed, which can lead to phase separation at the interface.2 It is well-known that mixtures often exhibit superior properties than their individual components, and therefore, mixed systems are more frequently used as compared to single surfactants in many diverse branches of science and technology, especially as washing agents, in the food industry, or in medicine.3-5 Mixed monolayers are also frequently studied because they provide suitable models for cellular membranes.6,7 Semifluorinated alkanes (SFA) are diblock molecules of the general formula CmF2m+1-CnH2n+1, often abbreviated to FmHn. They were synthesized for the first time by Brace in the early 1960s.8 Their physical properties both in solutions and in the solid state were thoroughly investigated by Rabolt et al.,9 and liquid crystalline properties were researched by Viney et al.10 * Corresponding author. Tel. +48-12-6632082. Fax: +48-12-6340515. E-mail:
[email protected].
(1) Dynarowicz-Ła¸ tka, P.; Kita, K. AdV. Colloid Interface Sci. 1999, 79, 1-17. (2) Capuzzi, G.; Kulkarni, K.; Fernandez, J. E.; Vincieri, F. F.; Lo Nostro, P. J. Colloid Interface Sci. 1997, 186, 271-279. (3) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; WileyInterscience: New York, 1989. (4) Rodriguez Patino, J. M.; Ruiz Dominguez, M.; De La Fuente Feria, J. J. Colloid Interface Sci. 1992, 148, 223-229. (5) Dynarowicz, P.; Jawien´, W. Chem. Pharm. Bull. (Jpn.) 1992, 40, 33163318. (6) de Fontanges, A.; Bonte, F.; Taupin, C.; Ober, R. J. Colloid Interface Sci. 1984, 101, 301-307. (7) Galves Ruiz, M. J.; Cabrerizo Vilchez, M. A. Thin Solid Films 1992, 210/211, 127-131. (8) Brace, N. O. J. Org. Chem. 1962, 27, 3033-3038.
Semifluorinated alkanes are surface active11,12 and, as proved by Gaines, some of them are capable of Langmuir monolayer formation.13 The investigation of monolayer properties of SFAs were continued by other researchers.14-18 It is well-known that perfluorinated liquids do not mix with normal hydrocarbons and other typical organic solvents as well as water. This issue was originally investigated by Hildebrandt and Scott19,20 and was important in the regular solutions theory formulation. Semifluorinated alkanes dissolve in normal hydrocarbons, chloroform, and even slightly in methanol (because of their large dipole moment of about 2.8 D).21 Although their miscibility in bulk was thoroughly investigated, only a few reports have been published so far regarding their miscibility in 2D systems. For example, the behavior of F8H16 mixed with other surfactants (like phospholipids,22,23 diunsaturated polimerisable fatty acid24 or poly(ethylene oxide) diblock copolymer25) at the air/water interface was investigated. In all of these studies, the hydrogenated component of the binary 2D-mixtures had a large and complex headgroup22,23,25 or possessed unsaturation (two alkyne triple bonds) in the hydrogenated chain.24 (9) Rabolt, J. F.; Russel, T. P.; Twieg, R. J. Macromolecules 1984, 17, 27862794. (10) Viney, C.; Russel, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63-82. (11) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797-7801. (12) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 1995, 11, 977-983. (13) Gaines, G. L., Jr. Langmuir 1991, 7, 3054-3056. (14) Huang, Z.; Acero, A. A.; Lei, N.; Rice, S. A.; Zhang, Z.; Schlossman, M. L. J. Chem. Soc., Faraday Trans. 1996, 92, 545-552. (15) El Abed, A.; Faure´, M. C.; Pouzet, E.; Abillon, O. Phys. ReV. E 2002, 65, art. no. 051603. (16) Lo Nostro, P. Curr. Opin. Colloid Interface Sci. 2003, 8, 223-226. (17) Broniatowski, M.; Sandez Macho, I.; Min˜ones, J., Jr.; Dynarowicz-Ła¸ tka, P. J. Phys. Chem. B 2004, 108, 13403-13411. (18) Broniatowski, M.; Dynarowicz-Ła¸ tka, P. J. Fluor. Chem. 2004, 125, 15011507. (19) Scott, R. L. J. Am. Chem. Soc 1948, 70, 4090-4093. (20) Hildebrand, J. H. J. Am. Chem. Soc. 1950, 72, 4348-4351. (21) Ho¨pken, J.; Pugh, C.; Richtering, W.; Mo¨ller, M. Makromol. Chem. 1988, 189, 911-925.
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So far, no systematic studies have been undertaken on the interactions of semifluorinated alkanes with hydrogenated, semifluorinated, and perfluorinated chains at the air/water interaface. Such studies would be of utmost importance because of broad applications of fluorinated surfactants (e.g., as drug carriers26 alone as well as in mixtures, where they can be more effective than in one-component systems. Aiming at investigating chain-chain interactions, n-alcohols differing in the degree of fluorination in their chains were selected. Alcohols were chosen for our studies because the hydroxyl group is the smallest and the simplest polar headgroup, and therefore, the head-head interactions should be reduced to the possible minimum. Four different alcohols have been investigated, namely octadecan1-ol (abbr. C18OH), 18,18,18,17,17,16,16,15,15,14,14,13,13,12,12,11,11-heptadecafluoroocta- decan-1-ol (abbr. F8H10OH), 18,18,18,17,16,16,15,15,14,14,13,13,12,12,11,11-hexadecafluoro17-(trifluoromethyl)-octadecan-1-ol (abbr. iF9H10OH), and 1H,1H-perfluoro -octadecan-1-ol (abbr. F18OH). The investigated alcohols can be treated as derivatives of octadecanol, differing in the degree of fluorination. The number of fluorine atoms in their chains can profoundly influence their interactions with SFA. In the case of perfluorooctyl-decanol and perfluoroisononyldecanol, we wished to verify how the iso branching of the perfluorinated moiety affects the interactions with semifluorinated alkane in mixed Langmuir monolayers. As far as the selection of a semifluorinated alkane is concerned, we have chosen a molecule possessing a large perfluorinated moiety as well as a long hydrogenated chain, namely 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10-henicosafluorotriacontane (abbr. F10H20). The considerable length of both parts of SFA should maximize the interactions with the chains of the studied alcohols (regardless of their attractive or repulsive character). In our research, we have applied classical methods such as surface pressure measurements upon monolayer compression (π-A isotherms) complemented with the modern optical methods, namely Brewster angle microscopy. The data obtained from π-A isotherms were analyzed qualitatively, according to the additivity rule, and quantitatively with the excess free energy of mixing (∆Gexc) values as well as the interaction parameter (R). Plots of the compression modulus and collapse pressure as a function of mole fraction of a given alcohol are presented and discussed. Finally, phase diagrams for all of the four mixed systems investigated here are proposed. Experimental Section The semifluorinated alkane as well as the two semifluorinated alcohols used in this research were synthesized according to the procedure of Rabolt et al.,9 which was originally invented by Brace.8 More synthetic details and appropriate references can be found in the works by Lehmler et al.27,28 It is a radical addition reaction in which perfluoroalkyl iodide reacts with an alkene. AIBN (asobisiso-butyronitrile) is used an initiator of the radical reaction. The first stage of the procedure (addition) is followed by a reductive detachment of the iodine atom. Hydrogen in statu nascendi generated in the reaction of gaseous hydrogen chloride with zinc powder is used as a reductor. The synthesis of F10H20 is described elsewhere,17 whereas semifluorinated alcohols were synthesized according to the (22) Krafft, M. P.; Giulieri, F.; Fontaine, P.; Goldmann, M. Langmuir 2001, 17, 6577-6584. (23) Maaloum, M.; Muller, P.; Krafft, M. P. Langmuir 2004, 20, 2261-2264. (24) Wang, S.; Lunn, R.; Krafft, M. P.; Leblanc, R. M. Langmuir 2000, 16, 2882-2886. (25) Simoes Gamboa, A. L.; Filipe, E. J. M.; Brogueira, P. Nano Lett. 2002, 2, 1083-1086. (26) Riess, J. G. Tetrahedron 2002, 58, 4113-4131. (27) Lehmler, H. J.; Bummer, P. M. J. Fluor. Chem. 2002, 117, 17-22. (28) Lehmler, H. J.; Bummer, P. M. Colloids Surf. B 2005, 44, 74-81.
Broniatowski and Dynarowicz-Ła¸ tka above-described procedure in a reaction of 9-decen-1-ol (98% from Aldrich) with perfluorooctyliodide (99%) (synthesis of F8H10OH) or perfluoro-iso-nonyliodide (98%) (synthesis of iF9H10OH). Both perfluorinated alkyliodides were purchased from Fluorochem and used as obtained. The crude semifluorinated alcohols were purified in subsequent crystallizations from hexane (Spectroscopy grade, Aldrich). Their final purity was grater than 99%, which was proved by 1H and 13C NMR spectra, mass spectrometry, DSC analysis, IR-spectra, as well as elemental analysis (F8H10OH calculated C 37.51; H 3.67, found C 37.57; H 3.55; iF9H10OH calculated C 36.44; H 3.38; found C 36.38; H 3.43). n-Octanol (99%) was supplied by Aldrich, and 1H,1H-perfluorooctadecanol (99%) was supplied by Fluorochem. These compounds were used without preliminary purification. The spreading solutions for Langmuir experiments were prepared by dissolving each compound in chloroform (Aldrich, HPLC grade) with a typical concentration of ca. 0.5 mg/mL. 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 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 with a NIMA 601 trough (Coventry, U.K.) (total area )600 cm2, equipped with two symmetrical barriers placed on an anti-vibration 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 monolayers 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 53.1° (Brewster angle). The lateral resolution of the microscope was 2 µm. 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 fragment.
Results One component monolayers of pure semifluorinated alkanes have already been investigated previously.17,18 Generally, SFAs with long perfluorinated moiety and long hydrogenated chain, like F10H20, form stable films at the air/water interface. The structure of their monolayers was found to be homogeneous until collapse pressure, at which white crystallites of a new 3D phase appear and can be visible in BAM images. As far as the normal saturated hydrogenated alcohols are concerned, their Langmuir monolayers properties are described in a large number of papers and some general information can be found, for example, in the monography by Gaines.29 On the contrary, not many reports on monolayer properties from perfluorinated alcohols (as 1H,1H perfluorooctadecanol) can be found in the literature;27,28,30 however, some papers reporting surface behavior of related compounds like semifluorinated thiols31,32 or semifluorinated carboxylic acids33-38 have been published. Generally, monolayers (29) Gaines, G. L., Jr. Insoluble Monolayers at the Liquid-Gas Interfaces; Willey Interscience: New York, 1966. (30) Vysotsky, Y. B.; Bryantsev, V. S.; Boldyreva, F. L.; Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 2005, 109, 454-462. (31) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913-1921. (32) Naudi, C.; Calas, P.; Commeyras, A. Langmuir 2001, 17, 4851-4857. (33) Barton, S. W.; Goudot, A.; Bouloussa, O.; Rondelez, F.; Lin, B.; Novak, F.; Acero, A.; Rice, S. A. J. Chem. Phys. 1992, 96, 1343-1351. (34) Shin, S.; Rice, S. A. Langmuir 1994, 10, 262-266. (35) Kato, T.; Kameyama, M.; Kawano, M. Thin Solid Films 1996, 273, 232235.
Semifluorinated Chains at the Air/Water Interface
Figure 1. π-A isotherms for the investigated systems: (a) F10H20/ C18OH; (b) F10H20/F18OH; (c) F10H20/F8H10OH; (d) F10H20/ iF9H10OH; X (alcohol) denotes the increment of the mole fraction of the respective alcohol.
formed by 1H,1H-perfluorooctadecanol (F18OH) are homogeneous at lower surface pressures; however, upon compression, some 3D crystallites appear, the number of which grows with rising surface pressure. Such instability has also been observed for perfluorocarboxylic acids.39 Namely, upon increasing the perfluorinated chain length from perfluorodecanoic to perfluorooctadecanoic acids, the collapse pressure was found to diminish, indicating gradual decrease of monolayers stability. Regarding semifluorinated alcohols used in this investigation, their monolayers are homogeneous upon compression until collapse is reached. Figure 1 shows π-A isotherms registered for the studied four systems containing F10H20 mixed with the investigated alcohols. The measurements were carried out with the increment of the mole fraction of a given alcohol 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. In the case of mixtures of F10H20 with C18OH and F18OH (Figure 1, panels a and b), the addition of alcohol has a condensing effect; that is, with the increasing ratio of alcohol, the π-A isotherms shift toward lower molecular area; and as a result, the isotherms for mixed monolayers lie between the isotherms of pure components. The situation is different for mixtures containing semifluorinated alcohols (Figure 1, panels c and d), because at lower surface pressures the π-A isotherms obtained for mixed systems are of more expanded character than the isotherms for pure components (are shifted toward greater molecular areas). Interesting behavior occurs for mixtures containing semifluorinated alcohol in excess (X ) 0.7, 0.8, and 0.9) for the monolayers are apparently expanded and the shape of their isotherms seems to be different as compared to the remaining ones. On the basis of the π-A dependencies, compression moduli were calculated for all of the investigated four systems. The compression modulus, CS-1, defined as CS-1 ) -A dπ/dA, where A is molecular area and π surface pressure, is a valuable tool, which can be used to classify the monolayer physical state.40,41 (36) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K. Langmuir 1998, 14, 1786-1798. (37) Lehmler, H. J.; Jay, M.; Bummer, P. M. Langmuir 2000, 16, 1016110166. (38) Arora, M.; Bummer, P. M.; Lehmler, H. J. Langmuir 2003, 19, 88438851. (39) Shibata, O.; Yamamoto, S. K.; Lee, S.; Sugihara, G. J. Colloid Interface Sci. 1996, 184, 201-208.
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Figure 2. Compression modulus vs alcohol mole fraction for the investigated binary Langmuir monolayers: (a) F10H20/C18OH; (b) F10H20/F18OH; (c) F10H20/F8H10OH; (d) F10H20/iF9H10OH.
Values of CS-1 ranging from 0 to 12 mN/m indicate the gaseous state of a monolayer, varying from 12 to 100 mN/m prove the liquid-expanded state, from 100 to 250 mN/m are characteristic of the liquid-condensed state, whereas greater than 250 mN/m indicate the solid state of a film. In Figure 2, plots of CS-1 vs mole fraction of alcohol at three different surface pressure vales are presented. As it is visible in Figure 2, the values of CS-1 oscillate between 20 and 80 mN/m, which is characteristic for the liquid-expanded state of a monolayer. At X (alcohol) ranging from 0.3 to 0.6, monolayers are most expanded, whereas for a larger alcohol proportion, a slight condensing effect can be observed. Simultaneously with π-A measurements, BAM observations were performed upon full compression, and representative images for each mixture were registered. The images shown in the following figures were taken at π) 7 mN/m, because this value lies just in the middle of the compression range between lift-off area of surface pressure and collapse of pure F10H20 monolayer (ca. 15.5 mN/m). The photos are presented for four different mole fraction of a given alcohol: 0.2, 0.4, 0.6, and 0.8. For mixtures of F10H20 and C18OH, the monolayers were completely homogeneous for all of the mixture compositions and in the whole range of compression until collapse, and therefore, the images are not shown herein. The behavior of the F10H20/F18OH system was completely different to that described above (Figure 3). A large number of white spots is visible, especially for X (F18OH) greater than 0.2, which can be a indication of a phase separation. Figure 4 presents BAM images for mixtures of F10H20 with semifluorinated alcohol: F8H10OH. For an alcohol mole fraction below 0.5, the images are homogeneous (not shown), whereas for X ) 0.6 (Figure 4a), a large number of white spots is visible, indicating proceeding nucleation of the monolayer. Figure 4b presents a constellation of small white nuclei proving proceeding phase separation in the system. Figure 5 concerns the last system investigated here, namely mixtures of F10H20 with a semifluorinated alcohol possessing an iso branched perfuorinated moiety. At the mole ratio of iF9H10OH smaller than 0.3, the registered images are characteristic for homogeneous monolayers (Figure 5a), but upon (40) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold Publishing Co., New York, 1954; p 107. (41) Davies, J. T.; Rideal, E. K. Interfacial Phenomena, 2nd ed.; Academic Press: New York, 1963; pp 265 and 65.
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Figure 3. BAM images for the system F10H20/F18OH registered at 7 mN/m: (a) X (F18OH) ) 0.2; (b) X (F18OH) ) 0.4; (c) X (F18OH) ) 0.6; (d) X (F18OH) ) 0.8.
Figure 4. BAM images for the system F10H20/F8H10OH registered at 7 mN/m: (a) X (F8H10OH) ) 0.6; (b) X (F8H10OH) ) 0.8.
Figure 5. BAM images for the system F10H20/iF9H10OH registered at 7 mN/m: (a) X (iF9H10OH) ) 0.2; (b) X (iF9H10OH) ) 0.4; (c) X (iF9H10OH) ) 0.6; (d) X (iF9H10OH) ) 0.8.
increasing the proportion of alcohol, white crystallites appear (Figure 5b and c). At X (iF9H10OH) greater than 0.6 (Figure 5d), a large number of white flocks of the 3D phase was seen, corroborating phase separation. A valuable indicator of mutual miscibility in Langmuir monolayers is the dependence of collapse pressure vs composition of the 2D mixture. According to the two-dimensional phase rule1 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, and if the components
Figure 6. Collapse pressure vs alcohol mol fraction for the investigated systems: (a) F10H20/C18OH; (b) F10H20/F18OH; (c) F10H20/F8H10OH; (d) F10H20/iF9H10OH. Filled squares indicate the value of the first collapse pressure, whereas stars correspond to the values of the second collapse (if present).
mixes ideally, according to the additivity rule, the πcoll - X dependence is linear. Figure 6 contains πcoll - X (alcohol) dependencies for the systems investigated here. In the case of F10H20/C18OH mixtures, the πcoll - X (C18OH) dependence is a sigmoid curve, and only one collapse pressure can be observed throughout the course of the compression isotherm. This corroborates the results obtained with BAM, i.e., that the system is completely miscible for all of the mixed films compositions. Similarly, πcoll - X (F18OH) dependence (Figure 6b) is in accord with BAM observations described above (Figure 3). The πcoll value is constant and independent of X (F18OH) concentration and, moreover, at larger proportion of F18OH a second collapse is visible. This proves that these two components are completely immiscible in 2D films at the air/water interface. Figure 6c is similar to Figure 6a, since πcoll rises with increasing X (F8H10OH), and only one collapse is visible in the whole range of F8H10OH mole fractions. Such a behavior can lead to the same conclusion as in the case of F10H20/C18OH, that F10H20 is miscible with F8H10OH at all proportions. The situation is slightly more complicated for the system of F10H20/iF9H10OH. At low mole fractions of alcohol (until 0.2), the πcoll rises with X (iF9H10OH); however, at X (iF9H10OH) ) 0.3, a fall of πcoll below the value of pure F10H20 monolayer is observed, and at X ) 0.4, a second collapse occurs. Two collapses are present till X (iF9H10OH) ) 0.9, where once again only one collapse pressure can be noticed. Such a behavior indicates that both compounds mix in Langmuir monolayer at low as well as at high proportion of iF9H10OH, whereas at other proportions of that alcohol, the compounds are not miscible. From the π-A dependencies obtained for mixed binary systems, thermodynamic parameters, which quantify the molecular interactions, can be obtained. 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:1
∆Gexc )
∫0π (A12 - (A1X1 + A2X2)) dπ
R)
∆Gexc RT(X1X22 + X2X12)
(1) (2)
wherein A12 is the average surface area per molecule in the mixed
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monolayer, defined as follows:
A12 ) A1X1 + A2X2
(3)
In the above formula, A1 (A2) stands for the molecular area of a single component monolayer at the same surface pressure as it is applied to determine A12 in the mixture, whereas X1 and X2 are the mole fractions of components 1 and 2 in the mixed film. R is the gas constant, and T is the absolute temperature. All experiments were lead at 20 °C, so 293.16 K was taken for calculation of R. The excess of free energy of mixing, ∆Gexc, can be treated as an indicator of interactions and the stability of the mixed monolayer. The negative sign of ∆Gexc is considered as a criterion of monolayer stability, whereas a positive value can suggest phase separation in the monolayer. The interaction parameter R is closely related to ∆Gexc and has a similar sense. The minimum value of R indicates the composition of a mixed monolayer at which the strongest interaction between both components occurs. The negative sign of R means that the interactions between unlike molecules in a binary 2D-mixture are more attractive than between like molecules, whereas a positive sign of this parameter indicates that the interactions between unlike molecules are repulsive or at least less attractive than between like molecules in a onecomponent monolayer.1,42 Plots of R vs X (alcohol) dependencies can provide some additional information to that from ∆Gexc X (alcohol) and to affirm at which proportion of both components the interactions are most attractive/repulsive. Of course, there is no sense in calculating ∆Gexc and R parameter for immiscible systems. It was proved with BAM images and πcoll - X (alcohol) dependency that the system of F10H20/F18OH is completely immiscible, and therefore, this very system will be excluded from ∆Gexc - X (alcohol) and R - X (alcohol) dependencies discussion. In Figure 7, ∆Gexc - X (alcohol) dependencies are presented. It is visible in Figure 7a that for the system F10H20/C18OH ∆Gexc is negative in the whole range of compositions, proving previously suggested miscibility of F10H20 and C18OH in Langmuir monolayers. In the ∆Gexc - X (F8H10OH) plot (Figure 7b), two maxima are present, and the sign of ∆Gexc is positive for all compositions. The positive sign of ∆Gexc does not necessary mean that phase separation takes place in a mixed monolayer but indicates that the interactions between F10H20 and F8H10OH are less attractive than between like molecules. The plot for the mixtures of F10H20 with iF9H10OH is different, because for the mole fraction of iF9H10OH from 0 to 0.5 the value of ∆Gexc is negative or close to 0, whereas for a larger proportion of iF9H10OH the value of ∆Gexc is positive. Such a tendency can mean that, up to a certain proportion of a semifluorinated alcohol, this system is miscible, whereas for a larger mole fraction of iF9H10OH, phase separation begins. The above results can be complemented by the R - X (alcohol) plots, which are compiled in Figure 8. It is clearly visible in Figure 8a that the interactions between F10H20 and C18OH are most attractive at X (C18OH) ) 0.1, and for a larger X (C18OH), the values are close to 0 and independent of film composition. In the case of the F10H20/F8H10OH system (Figure 8b), a distinct minimum can be noticed at the equimolar proportion of the components and the value of R at minimum is close to 0. Therefore, at such a composition, the interactions between unlike molecules are of similar strength as between like molecules. Figure 8c for the mixtures of F10H20 and iF9H10OH gives (42) Chen, K. B.; Chang, C. H.; Yang, Y. M.; Maa, J. R. Colloids Surf. A 2000, 170, 199-208.
Figure 7. Excess of free energy of mixing vs alcohol mole fraction for (a) F10H20/C18OH; (b) F10H20/F8H10OH; (c) F10H20/ iF9H10OH.
Figure 8. Interaction parameter (R) vs alcohol mole fraction for (a) F10H20/C18OH; (b) F10H20/F8H10OH; (c) F10H20/iF9H10OH.
similar information as ∆Gexc - X (iF9H10OH) plot, which was interpreted previously.
Discussion On the basis of the π-A isotherms, BAM images, πcoll - X (alcohol) dependencies, as well as thermodynamic quantities
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2696 Langmuir, Vol. 22, No. 6, 2006
Figure 9. Phase diagrams for the investigated systems: (a) F10H20/ C18OH; (b) F10H20/F18OH; (c) F10H20/F8H10OH; (d) F10H20/ iF9H10OH.
calculated from the π-A istotherms, it was possible to construct phase diagrams of the investigated four systems containing F10H20 and alcohol molecule in mixed Langmuir monolayers, at 20 °C. These diagrams are presented in Figure 9. The system F10H20/C18OH shows the simplest phase behavior of all of the mixtures discussed here: both components mix well in the whole range of compositions and up to the collapse pressure the system contains only one phasesthe homogeneous mixed monolayer. At surface pressures above the collapse, two phases in equilibrium are present: a mixed monolayer and a mixed multilayer (3D mixed crystals). The system F10H20/F18OH is an example of completely separated phases. Below the collapse pressure of F10H20, the system can be treated as having the following two phases: aggregates of F18OH (visualized with BAM as white spots, Figure 3) and monolayer of F10H20. Above πcoll of F10H20, three phases can be distinguished: aggregates of F18OH and a collapsed multilayer of F10H20 being in equilibrium with a monolayer of F10H20. As far as the mixtures of F10H20 with F8H10OH are concerned, the system seems to be miscible up to the mole fraction of the semifluorinated alcohol of 0.5. Below the collapse pressure, the system is single phase, whereas at larger surface pressure, two phases are present: mixed 3D aggregates, which are in equilibrium with the mixed monolayer. The range of X (F8H10OH) from 0.6 to 1 can be treated as a region of immiscibility. Below the collapse pressure, three phases are present: separate domains of the F8H10OH monolayer, domains of the F10H20 monolayer that are in equilibrium with the 3D aggregates. In the discussed region, the semifluorinated alcohol is in excess, and the observed πcoll is characteristic of this compound. If the situation was different and the aggregates were formed by F8H10OH, the collapse pressure would be characteristic of F10H20, which is not the case, because πcoll values at these mole fractions of F8H10OH are greater than 50 mN/m and correspond to the semifluorinated alcohol monolayer. The mixtures of F10H20 and iF9H10OH have a similar phase diagram to F10H20/F8H10OH; however, the region of alcohol mole fractions in which the components mix in monolayer is narrower and ranges only from 0 to 0.2. At a higher proportion of iF9H10OH, the system starts to demix, and it seems that three surface phases are present: separate domains of the F10H20 monolayer, domains of iF9H10OH, which are in equilibrium with 3-D aggregates of the alcohol. Such a phase situation is corroborated by a low value of the first collapse pressure, which is, unlike in the previous system, characteristic
of the F10H20 monolayer. Above the collapse pressure of F10H20, a fourth phase occurs: F10H20 3-D aggregates. Upon further compression, the aggregates of iF9H10OH grow, which finally leads to the collapse of the iF9H10OH monolayer. Different kinds of interactions between F10H20 and four investigated alcohols can be explained based on differences in alcohols structure. It was proven in our previous papers17,18,43 that SFAs are oriented at the air/water interface with their perfluorinated moiety toward the air. Thus, F10H20 has 20 carbon atoms in the hydrogenated fragment, the terminal groups of which are in contact with water. In mixed monolayers with C18OH, only the hydrogenated chain of the alcohol, which is shorter by two CH2 groups versus the hydrogenated part of F10H20, is in contact with the hydrogenated fragment of the SFA molecule. The CH2-CH2 interactions are attractive, and the presence of C18OH molecules facilitates more packed arrangement between the hydrogenated chains of unlike molecules as compared to pure monolayers. The immiscibility in the system F10H20/F18OH can be explained basing on similar argumentation: in mixed F10H20/F18OH monolayers, the hydrogenated moiety of the semifluorinated alkane is in contact with a long perfluorinated chain. The interactions between CF2 and CH2 groups are much less attractive than in the pairs CH2-CH2 and CF2-CF2, leading to phase separation in 3D-systems, which was described more than 50 years ago in bulk, using the hexane/perfluorohexane system as an example.20 Contact of F10H20 with F18OH induces aggregation of the perfluorinated alcohol, because in this way the unfavorable interactions between hydrogenated and perfluorinated chains are minimized. The situation for mixtures of F10H20 with partially fluorinated alcohols is between the two limiting cases described above. In perfluorooctyldecanol (F8H10OH), eight carbon atoms from the air side are fluorinated. The long hydrogenated chain of F10H20 is in contact both with 10 CH2 groups as well as with 8 CF2 groups. The interaction with CH2 groups is favorable, whereas with CF2 groups, it is unfavorable. At low and mid mole fractions of F8H10OH, the system is miscible, but when the concentration of F8H10OH is greater than 0.5 phase separation occurs and F10H20 is expelled for the monolayer. Such a behavior can result from molecular packing; that is, the presence of F10H20 introduces disorder to the F8H10OH monolayer, and when the number of alcohol molecules is large enough, SFA molecules aggregate. It seems that the key factor leading to the phase diagram of the system F10H20/iF9H10OH is the structure of the semifluorinated alcohol. In this case, the alcohol has similar number of CF2 groups as F8H10OH, but its perfluorinated moiety has an iso branching at the ω position. Such a branching can be considered as a steric hindrance and can limit the distance to which the F10H20 molecule can approach to the iF9H10OH molecule. Such a tendency leads to the situation in which at some mole fraction of iF9H10OH the unfavorable interactions start to prevail and the system is no longer single phase. Our research sheds some new light on the interactions of hydrogenated and perfluorinated chains at the air-water interface. These investigations enrich the results concerning interactions between fluorinated and hydrogenated surfactants in micelles and generally in bulk solutions. The obtained results can be of interest in such fields as drug-delivery systems and blood substitutes development as well as in the designing of new materials. LA0533009 (43) Broniatowski, M.; Sandez Macho, I.; Dynarowicz-Ła¸ tka, P. Thin Solid Films 2005, 493, 249-257.
