pubs.acs.org/Langmuir © 2009 American Chemical Society
Extended Investigation of the Aqueous Self-Assembling Behavior of a Newly Designed Fluorinated Surfactant S. Buscemi,† G. Lazzara,*,‡ S. Milioto,‡ and A. Palumbo Piccionello*,† †
Dipartimento di Chimica Organica “E. Patern o” and ‡Dipartimento di Chimica Fisica “F. Accascina”, Universit a degli Studi di Palermo, Viale delle Scienze, Parco D’Orleans II, 90128 Palermo, Italy Received June 1, 2009. Revised Manuscript Received July 14, 2009
The physicochemical behavior of the newly synthesized fluorinated 5-hydroxyamino-3-perfluoroheptyl-1,2, 4-oxadiazin-6-one (PFHO) surfactant was investigated. Thermal analysis showed that the pure surfactant is thermally stable under an inert atmosphere to 135 °C, which is several degrees higher than the melting point (99 °C). PFHO is rather active at the water/air interface where it assumes a standing up configuration. It exhibits an enhanced selfassembling behavior; accordingly, the critical micellar concentrations at some temperatures are 2 orders of magnitude lower than those of a similar surfactant having the same phobicity, such as sodium perfluorooctanoate. Even in the dilute domains, PFHO forms large micelles, detected by dynamic light scattering studies, that are precursors of the gel occurring at rather low composition (only 2.0% w/w at 25 °C). Optical microscopy evidenced cylindrical aggregates in gel systems whereas differential scanning calorimetry and viscosity showed that the gels are stable over a wide temperature range to ca. 70 °C where they undergo a reversible gel f fluid transition. Finally, percolation theory combined with data provided by the experimental studies enabled us to predict the PFHO gelation process correctly, in very good agreement with the experimental findings.
Introduction The synthesis of amphiphilic molecules based on fluorinated parts is still a modern issue in designing surfactants, with different molecular structure, hydrophilic heads, spacers of variable lengths, and functionalities for specific purposes. Perfluoroalkylated surfactants present a certain number of peculiar properties such as chemical and biological inertness, thermal stability, fire extinguishing, high gas solubility, and so on.1,2 Fluorinated biocompatible surfactants can be used as oxygen carriers, oxygen-transporting gels for surgery, and drug-delivery systems.3 A novel series of fluorinated bisammonium surfactants were recently4 synthesized to optimize their antimicrobical activity against Pseudomonas aeruginosa. Because of these interesting features, in the last decades several physicochemical investigations were carried out on fluorinated amphiphiles, providing exhaustive insight into the behavior at the molecular level. Fluorinated colloidal systems collect organized structures (micelles, bilayers, vesicles, tubules, etc.), the formation of which is driven by the enhanced fluorous-phase aggregation, being that the fluorinated chain is both lipophobic and hydro*Authors to whom correspondence should be addressed. E-mail: g.lazzara@ unipa.it;
[email protected]. (1) (a) Filler, R.; Kobayashi, Y.; Yagupolskii, L. M. Organofluorine Compounds in Medicinal Chemistry and Biomedical Applications; Elsevier: Amsterdam, 1993. (b) Chemistry of Organic Fluorine Compounds II. A Critical Review; Hudlicky, M., Pavlath, A. E., Eds.; ACS Monograph 187; American Chemical Society: Washington, DC, 1995. (2) See, for example, (a) Riess, J. G. Tetrahedron 2002, 58, 4113. (b) Krafft, M. P. Curr. Opin. Colloid Interface Sci. 2003, 8, 213. (c) Krafft, M. P. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 4251. (d) Emmanouil, V.; El Ghoul, M.; Andre-Barres, C.; Guidetti, B.; Rico-Lattes, I.; Lattes, A. Langmuir 1998, 14, 5389. (e) Pasc-Banu, A.; Blanzat, M.; Belloni, M.; Perez, E.; Mingotaud, C.; Rico-Lattes, I.; Labrot, T.; Od, R. J. Fluorine Chem. 2005, 126, 33. (3) Cirkva, V.; Polak, R.; Paleta, O.; Kefurt, K.; Moravcova, J.; Kodicek, M.; Forman, S. Carbohydr. Res. 2004, 339, 2177. (4) Massi, L.; Guittard, F.; Levy, R.; Geribaldi, S. Eur. J. Med. Chem. 2009, 44, 1615.
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phobic.2 As a general feature, the phobicity5 of one CF2 group is 1.5 times larger than that of one CH2 group, conferring to a given surfactant with a fixed number of carbon atoms a hydrophobicity larger than that of the hydrogenated ones. The self-assembly tendency of various fluorinated macromolecules6 has also been used for hydrogel preparation. Gel formation represents an attractive research area because of the unique properties of these soft materials that present many applications ranging from biomedicals, tissue engineering, drug delivery, and cell immobilization to food, cosmetic technology, and so forth.7 Molecules alternative to the most common polymeric gelators, which predominantly form chemical gels, are low-molecular-weight (LMW) gelators that form essentially physical gels where the molecules are self-assembled into 3D structures, held together by noncovalent interactions.8 Because of the supramolecular assembly, these gels are present as well-ordered arrays, are thermoreversible, and exhibit a high tolerance toward additives; moreover, low minimal gelation concentrations are often found.8
(5) Shinoda, K.; Hato, M.; Hayashi, T. J. Phys. Chem. 1972, 76, 909. (6) See, for example, (a) Roovers, J. Macromolecules 1986, 18, 1361. (b) Matsuda, A.; Kaneko, T.; Gong, J.; Osada, Y. Macromolecules 2000, 33, 2535. (c) Matsumoto, K.; Nishimura, R.; Mazaki, H.; Matsuoka, H.; Yamaoka, H. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3751. (d) Tae, G.; Kornfield, J. A.; Hubbell, J. A.; Johannsmann, D.; Hogen-Esch, T. E. Macromolecules 2001, 34, 6409. (e) Kuroda, K.; Fujimoto, K.; Sunamoto, J.; Akiyoshi, K. Langmuir 2002, 18, 3780. (f) Lee, W. F.; Liu, T. M. J. Appl. Polym. Sci. 2006, 100, 4661. (g) Kilbinger, A. F. M.; Grubbs, R. H. Angew. Chem., Int. Ed. 2002, 41, 1563. (h) Babu, S. S.; Praveen, V. K.; Prasanthkumar, S.; Ajayaghosh, A. Chem.;Eur. J. 2008, 14, 9577. (7) (a) Hydrogels in Medicine and Pharmacy; Peppas, N. A., Ed.; CRC Press: Boca Raton, FL, 1987; Vol. 3. (b) Polymer Gels: Fundamentals and Biomedical Applications; De Rossi, Kajiwara, K., Osada, Y., Yamauchi, A., Eds.; Plenum Press: New York, 1991. (c) Jen, A. C.; Wake, M. C.; Mikos, A. G. Biotechnol. Bioeng. 1996, 50, 357. (d) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99, 3181. (e) Lee, K. Y.; Mooney, J. D. Chem. Rev. 2001, 101, 1869. (8) (a) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201. (b) de Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 3615. (c) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (d) Molecular Gels. Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006.
Published on Web 08/18/2009
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Scheme 1. Structure of Fluorinated Surfactant PFHO
To the best of our knowledge, just two examples of fluorinated LMW hydrogelators have been previously studied.9,10 In this article, we report an extended physicochemical investigation aimed at elucidating the aqueous self-assembling behavior of a newly designed fluorinated surfactant;5-hydroxyamino-3perfluoroheptyl-1,2,4-oxadiazin-6-one (PFHO);the chemical structure of which is reported in Scheme 1. This surfactant was recently synthesized by some of us,11 but no insights into its behavior are available. In the first step, we characterized the thermal response of pristine PFHO by determining the melting and thermal degradation characteristics by means of differential scanning calorimetry (DSC) and thermogravimetric experiments. These properties were compared to those of conventional fluorinated surfactants such as sodium perfluorooctanoate (NaPFO) having the same tail size. The surface tension at 25 and 40 °C showed that PFHO is rather active at the interface. PFHO exhibits an enhanced tendency to self-assemble in water. In fact, compared to NaPFO, the critical micellar concentration values are 2 orders of magnitude smaller, evidencing the relevant role of the unusual uncharged polar headgroup (i.e., the hydroxyamino group). The micellization process was evidenced by means of density, dynamic light scattering, surface tension, and fluorescence experiments. What is also striking is the high PFHO gelling performance being exhibited at low concentration (2 wt % at 25 °C). Fluorescence spectroscopy, viscosity, and DSC experiments showed that the gel is stable over a wide temperature range and it undergoes a gel f fluid transition at 72 °C. Optical microscopy evidenced cylindrical aggregates in the gel system. The PFHO gelation process was very well predicted by the percolation theory using data provided by the DSC and density studies.
Experimental Section Materials. Hydroxylamine hydrochloride (reagent grade),
1,2,4-oxadiazole (synthetized as described elsewhere12) was added, and the mixture was stirred for 3 h at room temperature. The reaction mixture was then diluted with water (50 cm3), neutralized with 1 M HCl, and extracted with EtOAc (6 100 cm3). The combined organic layers were dried over Na2SO4, filtered, and concentrated in vacuo. The chromatography performed on the residue gave 5-hydroxylamino-3-pentadecafluoroheptyl-6H-1,2,4-oxadiazin-6-one (PFHO) with a yield of 75 wt %. Equipment. Thermogravimetry. The experiments were carried out on pure PFHO, PFOAc, and NaPFO by using a Q5000 IR apparatus (TA Instruments) under nitrogen flows of 25 cm3 min-1 for the sample and 10 cm3 min-1 for the balance. The sample weight was ca. 15 mg. The experiments were performed from 25 to 400 °C at a heating rate of 10 °C min-1. The degradation temperature was calculated as the onset of the weight loss versus temperature signal.
Differential Scanning Calorimetry (DSC) on Pure Surfactant. The enthalpy (ΔHm) and the melting temperature (tm) for pure PFHO, PFOAc, and NaPFO were determined by using a TA Instruments DSC (2920 CE) under a nitrogen atmosphere (flow rate=60 cm3 min-1) at a heating rate of 10 °C min-1. The pans that we used were aluminum and contained ca. 5 mg of the compound. The tm value was defined as the onset of the melting endotherm peak. The calibration was carried out by using an indium standard, the temperature and the enthalpy of melting of which are 156.51 °C and 28.71 J g-1, respectively. Surface Tension. A programmable tensiometer (KSV Sigma 7) via the Wilhemy plate was employed to determine the surface tension of the solutions. The apparatus is an electrobalance with a computer-controlled lifting system. The measurements were carried out by systematically adding a concentrated surfactant solution to water. For each solution, the surface tension was measured five times. The precision was 0.05 mN m-1. The temperatures were set at 25.0 ( 0.1 and 40.0 ( 0.1 °C. Density. The densities of the aqueous surfactant mixtures were measured at 25 °C by using a vibrating tube flow densimeter (model 03D, Sodev Inc.) with a sensitivity of 3 10-6 g cm-3. The temperature stability ((0.001 °C) was controlled with a closed-loop temperature controller (model CT-L, Sodev Inc.). The densimeter was calibrated according to the procedure described elsewhere.13 The apparent molar volume (VΦ) of PFHO in water was calculated by means of the following equation VΦ ¼
Mw 103 ðd -do Þ mddo d
ð1Þ
potassium tert-butylate (t-BuOK) (reagent grade), dried dimethylformamide (DMF), and ethyl acetate (EtOAc) were purchased from Sigma-Aldrich and used as received. Perfluorooctanoic acid (PFOAc) from Fluka was crystallized from carbon tetrachloride and dried at room temperature. Its sodium salt (NaPFO) was prepared by neutralizing it with an aqueous sodium hydroxide solution. The product was crystallized twice from an ice-cold solution and dried in a vacuum oven at 60 °C for at least 4 days before use. All of the mixtures were prepared by mass ((0.01 mg). The aqueous solutions were prepared using water from reverse osmosis (Elga model Option 3) with a specific resistivity higher than 1 MΩ cm. All of the mixtures were placed in a sonicating bath for 2 h in order to ensure the complete solubilization of the fluorinated surfactant. Synthesis of PFHO. PFHO was prepared according to the procedure described elsewhere.11 Briefly, t-BuOK (5 mmol) was added to a solution of hydroxylamine hydrochloride (5 mmol) in dried DMF (3 cm3). The mixture was stirred at room temperature for 30 min. Then 1 mmol of 5-perfluoroheptyl-3-ethoxycarbonyl-
where m and Mw are the molality and the molecular weight of PFHO, respectively; d and do are the densities (g cm-3) of the solution and water, respectively. Fluorescence Spectroscopy. The experiments were carried out by using pyrene as a fluorescent probe that is sensitive to the nature of its microenvironment.14 The emission spectrum consists mainly of five bands. The ratio between the intensity of the first (at 373 nm) and third (at 384 nm) vibrational bands (I1/I3) strongly depends on the medium polarity: it is ∼1.8 in aqueous media and 1.2/0.8 in hydrophobic media. The steady-state pyrene fluorescence spectra were registered with a Fluoromax 4 (Jobin-Yvon) spectrofluorometer (right angle geometry, 1 cm 1 cm quartz cell) at 25.0 ( 0.1 °C. The excitation wavelength was 333 nm, and the emission spectra were recorded from 350 to 500 nm. The widths of the slits were set at 1.5 and 1.0 nm for excitation and emission, respectively. The mixtures for the measurements were prepared as described in the following text. Known aliquots of a solution of pyrene in acetone
(9) Pang, S. F.; Zhu, D. B. Chem. Phys. Lett. 2002, 358, 479. (10) Imae, T.; Funayama, K.; Krafft, M. P.; Giulieri, F.; Tada, T.; Matsumoto, T. J. Colloid Interface Sci. 1999, 212, 330. (11) Palumbo Piccionello, A.; Pace, A.; Buscemi, S.; Vivona, N.; Giorgi, G. Tetrahedron Lett. 2009, 50, 1472.
(12) Buscemi, S.; Pace, A.; Palumbo Piccionello, A.; Pibiri, I.; Vivona, N.; Giorgi, G.; Mazzanti, A.; Spinelli, D. J. Org. Chem. 2006, 71, 8106. (13) De Lisi, R.; Lazzara, G.; Milioto, S.; Muratore, N. J. Phys. Chem. B 2003, 107, 13150. (14) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
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were carefully added to dark flasks by means of a Hamilton microsyringe. After solvent evaporation, the sample solutions were added and equilibrated at room temperature for ca. 2 days under vigorous stirring. For all of the mixtures, the final concentration of pyrene was 5 10-7 mol dm-3 (i.e., the solubility in water). Dynamic Light Scattering. The measurements were performed at 25.0 ( 0.1 °C in a sealed cylindrical scattering cell at a scattering angle of 90° by means of a Brookhaven Instruments apparatus composed of a BI-9000AT correlator and a He-Ne laser (75 mW) with a wavelength of 632.8 nm. Each experiment was repeated four times. The solvent was filtered through a Millipore filter with 0.45 μm pore size. The intensity-time correlation functions were treated by means of the cumulants15 method, which provides the polydispersity index and the decay rates (Γ) correlated to the apparent diffusion coefficient (D) as D ¼ Γ=q2
tm
ΔHm
RW
tD b
c
0.32 PFHO 98.7 55.3 135, 143.0 0.00 PFOAc 44.5 47.4 106,b 104.0c 9.75 NaPFO 278.6 92.6 285,b 283.3c a Units are tm and tD, °C; ΔHm, J g-1; and RW, wt %. b From DSC. c From TGA.
ð2Þ
where q is the scattering vector (4πnλ-1 sin(θ/2) with n being the water refractive index, λ is the wavelength, and θ is the scattering angle. The apparent hydrodynamic radius (Rh) of the aggregates was calculated by using the diffusion coefficient experimentally determined from the correlation function and applying the Stokes-Einstein equation where the water viscosity value was introduced. The polydispersity index is nearly independent of concentration, being ca. 0.35 over the investigated range. Viscosity. Tube inversion measurements were performed to evidence gel formation. Toward this aim, tubes with an internal diameter of 10 mm, containing ca. 1 g of the solution, were kept at 25.0 ( 0.2 °C. For the mixtures at 0.0422, 0.0543, and 0.0664 mol kg-1, gels were obtained whereas solutions at 0.0282 and 0.0342 mol kg-1 were flowing. On the basis of these findings, the aqueous surfactant solution at 0.0408 mol kg-1 was selected to measure the viscosity. In particular, a Bohlin Visco 88 rotation viscosimeter was used, and variable shear rates (14/1000 s-1) and temperatures (20/85 °C) were set. For temperatures larger than 70 °C, the lowest shear rate was set at 200 s-1 because of the low viscosity of the mixture. μDSC on Aqueous Surfactant Solutions. The self-assembling process in water induced by temperature was determined by means of a μDSC III SETARAM in the 5-90 °C range at a heating rate of 0.6 °C min-1. The sample cell was filled with ca. 500 mg of aqueous surfactant solution, and the reference cell was filled with the corresponding amount of water. The surfactant concentration changed between 0.001 and 0.041 mol kg-1. The baseline was subtracted according to a literature16 procedure. Optical Microscopy. The self-assembled structures were observed in transmitted light by means of a Leitz Laborlux 12 equipped with a Leica DC180 camera. The software employed for image acquisition was Leica IM50.
Results and Discussion Thermal Response of the Pure Surfactant. The melting behavior of the newly synthesized PFHO is compared to that of PFOAc and NaPFO, which differ in their polar heads. Each compound exhibited an endothermic peak from which ΔHm and tm values were obtained (Table 1). At a temperature larger than tm, each thermogram presented a strong deviation from the baseline ascribable to the degradation of the molecule. This hypothesis was confirmed by TGA experiments that evidenced a single-step degradation process. The values of the degradation temperature (tD), obtained from both TGA and DSC techniques, and the residual weight (RW) at 400 °C are reported in Table 1. (15) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814. (16) Malakhov, D. V.; Khatwa, M. K. A. J. Therm. Anal. Calorim. 2007, 87, 595.
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Table 1. Melting and Degradation Data of Some Fluorinated Surfactantsa
Figure 1. Surface tension as a function of the PFHO concentration at 25 °C (b) and 40 °C (2).
The RW is nearly null for PFHO and PFOAc, showing that all of the decomposition products are volatile at temperatures higher than tD; in the case of NaPFO, the value of RW may reflect the presence of a nonvolatile compound based on sodium. By assuming that the residue is formed either by Na2O or NaF, from simple calculations one obtains the RW values of 14.2 and 9.6 wt %, respectively. From the data in Table 1, one may state that our residue corresponds to NaF. It is interesting that tD of NaPFO is only a few degrees larger than tm whereas for the nonionic molecules tD is much larger than tm; furthermore, the hydroxyamino group strongly enhances the thermal stability of the surfactant compared to that of the OH group. The ΔHm values for PFOAc and PFHO are comparable but lower than the corresponding value for NaPFO; this finding reflects the ionic nature of NaPFO. The tm values follow the order PFOAc < PFHO < NaPFO; the difference between the nonionic surfactants is ascribable to the stronger interactions between the polar head groups of PFHO. Dilute Surfactant Domain. Behavior at the Air/Water Interface. Figure 1 shows the surface tension (γ) as a function of the PFHO concentration (m) at 25 and 40 °C. The observed trends are typical of surfactants in water; namely, γ decreases with m reaching a constant value at high concentration. The γ values at 40 °C are larger than the corresponding ones at 25 °C so that one can argue that temperature plays a role in the adsorption at the interface enhanced at lower temperature. This finding is in agreement with the general idea that water assumes a less structured configuration if temperature is raised and accordingly the amphiphilic molecules are expelled from the interface. The absence of minima in the γ versus m trends proves the high purity of the surfactant and straightforwardly allows the determination of the critical micellar concentration (cmc) from the break in the γ versus m trend. The cmc values are 1.02 10-4 and 1.26 10-4 mol kg-1 at 25 and 40 °C, respectively. These values indicate that the aggregation process is only slightly affected by temperature, being enhanced at the lower temperature. It is interesting to Langmuir 2009, 25(23), 13368–13375
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Figure 2. Apparent molar volume of PFHO in water as a function of concentration at 25 °C.
compare these values with the cmc’s of NaPFO, which are 0.03017 and 0.027 mol kg-1 at 25 and 40 °C, respectively. Note that the cmc at 40 °C was calculated by using the enthalpy18 and the heat capacity19 of micellization. Compared to NaPFO, the nonionic head renders the aggregation strongly favored, but its role in the cmc, the values of which are about 2 orders of magnitude smaller, is striking. By applying the Gibbs isothermal adsorption equation to γ data at a given temperature, the surface concentration of the PFHO-saturated monolayer (Γ) was calculated. From Γ, areas per molecule (a) of 36.4 ( 1.1 and 30.7 ( 1.3 A˚2 molecule-1 were computed at 25 and 40 °C, respectively. The results are in agreement with the computed fluorocarbon tail cross-sectional area20 (31.5 A˚2 molecule-1), indicating that the surfactant molecule assumes a stand up configuration at the interface. Self-Assembling in Water: Volume and Fluorescence Studies. Figure 2 illustrates the plot of VΦ as a function of m. Because of the limit of the technique, it was not possible to determine VΦ values at concentrations lower than 5 10-4 mol kg-1. Therefore, the data are available in the micellar region, and they slightly depend on m. For m > 0.008 mol kg-1, the fraction of the micellized surfactant calculated as (m - cmc)/m is nearly unitary; consequently, VΦ corresponds21,22 to the partial molar volume of the micellized surfactant (VM), the value of which is 254.1 ( 0.8 cm3 mol-1. The lack of data in the premicellar region did not allow us to determine the partial molar volume of the monomeric surfactant (Vm). Notwithstanding, if the cmc was very low, then Vm was reasonably set equal to the standard partial molar volume (Vo). By assuming the additive rule and the group contributions,23 a Vo value of 235.9 cm3 mol-1 was computed. The volume of micellization (ΔVm), calculated as (VM - Vo), gave a value of 18.2 cm3 mol-1, which is larger than that for NaPFO24 (13.4 cm3 mol-1). This difference likely reflects the desolvation of the polar head occurring during the micellization, which is very important for the nonionic surfactant because of its unusual large hydroxyamino group. Fluorescence spectroscopy is in general a very useful technique for detecting the micellization process because the fluorescent (17) De Lisi, R.; Milioto, S.; De Giacomo, A.; Inglese, A. Langmuir 1999, 15, 5014. (18) Milioto, S.; De Lisi, R. Langmuir 1994, 10, 1377. (19) De Lisi, R.; Inglese, A.; Milioto, S.; Pellerito, A. Langmuir 1997, 13, 192. (20) Wang, K.; Karlsson, G.; Almgren, M.; Asakawa, T. J. Phys. Chem. B 1999, 103, 9237. (21) Desnoyers, J. E.; Perron, G.; Roux, A. H. Surfactant Solutions. New Methods of Investigation, Zana, R., Ed.; Marcel Dekker: New York, 1987; pp 2-51. (22) Desnoyers, J. E.; Caron, G.; De Lisi, R.; Roberts, D.; Roux, A.; Perron, G. J. Phys. Chem. 1983, 87, 1397. (23) Lepori, L.; Gianni, P. J. Solution Chem. 2000, 29, 405. (24) Milioto, S.; Crisantino, R.; De Lisi, R.; Inglese, A. Langmuir 1995, 11, 718.
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Figure 3. (Top) Dependence of the I1/I3 ratio for pyrene fluorescence spectra in aqueous solutions at 25 °C on the PFHO concentration. (Bottom) Best fit according to eq 6.
pyrene probe is sensitive to the nature of its microenvironment.14 Typical variations of the I1/I3 ratio with surfactant concentration are reported for hydrogenated,25,26 fluorinated,27-29 and polymeric25,30-34 surfactants. Usually, the addition of the surfactant generates a small change in the premicellar region whereas a sharp decrease takes place at the cmc as a result of the transfer of pyrene from the aqueous phase to the micelles. Indeed, when fluorinated surfactants are involved, anomalous results are also obtained. A recent study27 on new gemini surfactants having partially fluorinated chains showed a slight increase in I1/I3 values at the cmc, which was enhanced by further fluorination of the chains. Our data (Figure 3) show a nearly linear variation of the intensity/ vibronic peaks ratio as a function of m that, on one side, shows that the pyrene environment becomes more hydrophobic upon surfactant addition but, on the other side, does not evidence micellization. From the cmc value, one predicts that in the concentrated domain covered by fluorescence experiments, micellar aggregates are present and at the highest concentration the aggregated fraction is nearly unitary. This result is striking in light of pyrene fluorescence data28 for perfluoroalkyl-carboxylates that are typical of surfactant behavior. Thus, it appears that pyrene is solubilized into the PFHO micelle palisade layer that is hydrated to some extent. Consequently, the hydrophobic association of PFHO cannot be monitored by pyrene. Notwithstanding, one can attempt to evaluate the pyrene affinity for the micelles quantitatively by determining its distribution constant between the aqueous and the micellar phases (KPy). Toward this aim, eq 3 was used25 I 1 =I 3 ¼ ðI 1 =I 3 Þw ð1 -xpy, M Þ þ ðI 1 =I 3 ÞM xpy, M
ð3Þ
(25) Lazzara, G.; Milioto, S.; Muratore, N. J. Phys. Chem. B 2008, 112, 5616. (26) Zhou, W.; Zhu, L. J. Hazard. Mater. B 2004, 109, 213. (27) Asakawa, T.; Okada, T.; Hayasaka, T.; Kuwamoto, K.; Ohta, A.; Miyagishi, S. Langmuir 2006, 22, 6053. (28) Kalyanasundaram, K. Langmuir 1998, 4, 942. (29) Chen, J.; Jiang, M.; Zhang, Y.; Zhou, H. Macromolecules 1999, 32, 4861. (30) Lazzara, G.; Milioto, S.; Gradzielski, M. Phys. Chem. Chem. Phys. 2006, 8, 2299. (31) Kozlov, M. Y.; Melik-Nubarov, N. S.; Batrakova, E. V.; Kabanov, A. V. Macromolecules 2000, 33, 3305. (32) Wen, X. G.; Verrall, R. E.; Liu, G. J. J. Phys. Chem. B 1999, 103, 2620. (33) Grant, C. D.; DeRitter, M. R.; Steege, K. E.; Fadeeva, T. A.; Castner, E. W., Jr. Langmuir 2005, 21, 1745. (34) Su, Y.; Wei, X.; Liu, H. Langmuir 2003, 19, 2995.
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Figure 4. Dependence on temperature of Δ(I1/I3) for pyrene fluorescence spectra in aqueous solutions of 9.19 10-3 (b) and 5.29 10-3 (2) mol kg-1 PFHO. Lines are guides for the eye.
where xpy,M is the fraction of pyrene in the micelles and (I1/I3)w and (I1/I3)M are the ratios in the aqueous and micellar phases, respectively. KPy is related to xpy,M as xpy, M ¼
K Py mM ð1þK Py mM Þ
Figure 5. Dependence on PFHO concentration of the scattering intensity (a) and the apparent hydrodynamic radius (b) for the aqueous PFHO solutions at 25 °C.
ð4Þ
where mM is the concentration of micellized surfactant given by (m - cmc). By combining eqs 3 and 4, one obtains 1 1þK Py mM ¼ ðI 1 =I 3 Þ ðI 1 =I 3 Þw þðI 1 =I 3 ÞM K Py mM
ð5Þ
based on the assumption that (I1/I3)w . (I1/I3)MKPymM, which is the case of low KPy, because (I1/I3)w > (I1/I3)M, eq 5 becomes 1 1 K Py ¼ þ mM ðI 1 =I 3 Þ ðI 1 =I 3 Þw ðI 1 =I 3 Þw
ð6Þ
Equation 6 predicts a linear trend in (I1/I3)-1 versus mM, which is observed for our data (Figure 3). From the linear fit of the experimental data, the values of 1.80 ( 0.01 and 48 ( 2 kg mol-1 for (I1/I3)w and KPy were determined, respectively. The KPy value confirms the low affinity of pyrene toward fluorinated micelles. On the contrary, pyrene is well solubilized into hydrogenated micelles as the KPy values of (8 ( 3) 104 kg mol-1 for sodium dodecanoate,25 1.7 104 dm3 mol-1 for sodium dodecylsulfate,26 and 1.7 105 dm3 mol-1 for Brij 3526 indicate. To study the effect of temperature on the PFHO aggregation process, the fluorescence experiments were performed on two surfactant solutions (5.29 10-3 and 9.19 10-3 mol kg-1) over a wide temperature range. Given that the I1/I3 ratio for pyrene in water slightly decreases with temperature,30 to evidence the effect of temperature on pyrene solubilization into PFHO aggregates, the difference between the I1/I3 ratio for pyrene in PFHO solution and in water, Δ(I1/I3), was calculated. Whatever the concentration, Δ(I1/I3) augments with temperature, indicating a more waterlike environment around the pyrene molecule at higher temperatures. Such behavior can be explained by assuming that PFHO aggregation is hindered by the temperature increase as predictable from the cmc values and/or the KPy value may decrease with temperature. Going further, one may observe that the Δ(I1/I3) versus temperature trends are almost linear and both of them tend toward the Δ(I1/I3) null value at ca. 72 °C. At 13372 DOI: 10.1021/la9019487
Figure 6. Viscosity of the PFHO aqueous solution (0.0408 mol kg-1) as a function of the shear rate at 80.4 °C (b) and 35.8 °C (2).
this temperature, it is likely that the micellar aggregates are disappearing. Concentrated Surfactant Domain. Dynamic Light Scattering Studies. The DLS experiments at 25 °C elucidated the diffusion behavior of PFHO aggregates. Unfortunately, the low scattering intensity (I) made it impossible to study the dilute surfactant domain (m
