Langmuir 2003, 19, 2313-2317
2313
Inclusion Compound from a Semifluorinated Alkane and β-Cyclodextrin Pierandrea Lo Nostro, Ilaria Santoni, Massimo Bonini, and Piero Baglioni* Department of Chemistry and CSGI, University of Florence, via della Lastruccia 3, 50019 Sesto Fiorentino (Florence), Italy Received June 28, 2002. In Final Form: November 14, 2002 Inclusion compounds (ICs) were obtained for the first time from a semifluorinated n-alkane (F(CF2)8(CH2)16H, abbreviated F8H16) with β-cyclodextrin (β-CD) in water. Because of the hydrophobic nature of the two constituent blocks, the semifluorinated alkane readily penetrates the hydrophobic CD’s cavity, to avoid contact with water, and produces a fine crystalline powder. The crystalline ICs were studied through differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and atomic force microscopy, and they were identified as novel systems, with different structure and behavior from the two parent molecules. According to X-ray diffractometry, the host-guest system presents the channel type structure that is typical of polymer-CD inclusion compounds. Atomic force microscopy confirms the presence of tubular structures obtained from dispersion of the F8H16/β-CD inclusion compound. The formation of such supramolecular assemblies may be useful for the encapsulation of fluorinated hydrophobic materials and for the insertion of specific polymeric chains onto the surface of cyclodextringrafted fibers and textiles.
Introduction Cyclodextrins (CDs) as hosts form a large variety of supramolecular, host-guest inclusion compounds with several different organic and inorganic guest materials, and the fact that they are commercially available, nontoxic, and water soluble explains their success in pharmaceutical formulations for drug delivery, cosmetics, food manufacturing, chromatographic separations, and textile processing.1-3 Besides their relevant industrial applications, these shallow, somewhat flexible, truncated cone hosts represent a valid probe for studying some of the most interesting topics in the nanosciences and molecular biology, that is, molecular recognition, selectivity, molecular encapsulation, chemical stabilization, and for the investigation of noncovalent binding forces. The secret of such successful performances ultimately resides in their peculiar structure: a ring of cyclic glucose units linked through 1,4-glycosidic bonds. Besides the well-known representatives of this category, namely R-, β-, and γ-CD (with six, seven, and eight glucose moieties, respectively), bigger and more flexible CDs have been synthesized, along with a large number of different derivatives (more or less hydrophobic), that possess different water solubility, ionization capacity, inclusion capability, and so forth.4-6 Figure 1 depicts the schematic structure of CDs. The internal cavity of CD is hydrophobic and can accommodate suitable hydrophobic chemicals, depending on their size, while the upper and lower rims are formed by the secondary and primary -OH groups and therefore are hydrophilic. In the presence of hydrophobic species, * Corresponding author. Fax: +39 055 457-3032. E-mail:
[email protected]. Internet: http://www.csgi.unifi.it. (1) Huang, L.; Gerber, M.; Lu, J.; Tonelli, A. E. Polym. Degrad. Stab. 2001, 71, 279. (2) Loftsson, T.; Olafsson, J. H. Int. J. Dermatol. 1998, 37, 241. (3) Loftsson, T.; Masson, M. Int. J. Pharm. 2001, 225, 15. (4) Skiba, M.; Wouessidjewe, D.; Puisieux, F.; Ducheˆne, D.; Gulik, A. Int. J. Pharm. 1996, 142, 121. (5) Ravoo, B. J.; Darcy, R.; Mazzaglia, A.; Nolan, D.; Gaffney, K. Chem. Commun. 2001, 827. (6) Cucinotta, V.; Giuffrida, A.; Grasso, G.; Maccarrone, G.; Vecchio, G. J. Chromatogr., A 2001, 916, 61.
the water molecules that are usually associated with CDs are readily depleted, and the guest penetrates the empty ring, leading to the formation of an inclusion compound, thermodynamically stabilized by relevant hydrophobic interactions, while water molecules reenter the aqueous phase resulting in an overall positive ∆S contribution. When aqueous CD solutions are mixed with polymers (such as PEG, PEO, PPG, Pluronics, and so on), the threading action continues with a significant increment of turbidity; during this complexation process several cyclodextrin rings are penetrated by a single polymeric chain, thereby forming the so-called polypseudorotaxanes that can then be converted into molecular necklaces or molecular tubes.7-11 More recently, the formation of cyclodextrin tubular aggregates in solution has been proposed by Antonietti and co-workers, and represents a new interesting feature that may explain some still unsolved questions that arise from the formation of cyclodextrin-based supramolecular structures.12 Semifluorinated n-alkanes (SFAs) are short-chain block copolymers where a fluorinated chain is covalently bound to a hydrogenated tail, F(CF2)m(CH2)nH (abbreviated FmHn).13 The peculiar chemical architecture of these compounds results in interesting physicochemical properties, such as the formation of supramolecular assemblies (micelles or gels) in selective solvents.14-18 Being composed of two highly hydrophobic residues, SFAs are totally (7) Ceccato, M.; Lo Nostro, P.; Baglioni, P. Langmuir 1997, 13, 2436. (8) Ceccato, M.; Lo Nostro, P.; Rossi, C.; Bonechi, C.; Donati, A.; Baglioni, P. J. Phys. Chem. B 1997, 101, 5094. (9) Lo Nostro, P.; Lopes, J. R.; Cardelli, C. Langmuir 2001, 17, 4610. (10) Lo Nostro, P.; Lopes, J. R.; Ninham, B. W.; Baglioni, P. J. Phys. Chem. B 2002, 106 (9), 2166. (11) Nepogodiev, S. A.; Stoddart, J. F. Chem. Rev. 1998, 98, 1959. (12) Polarz, S.; Smarsly, B.; Bronstein, L.; Antonietti, M. Angew. Chem., Int. Ed. 2001, 40, 4417. (13) Lo Nostro, P. Adv. Colloid Interface Sci. 1995, 56, 245. (14) Wang, S.; Lunn, R.; Krafft, M. P.; Leblanc, R. M. Langmuir 2000, 16, 2882. (15) Lo Nostro, P.; Chen, S. H. J. Phys. Chem. 1993, 97, 6535. (16) Ku, C. Y.; Lo Nostro, P.; Chen, S. H. J. Phys. Chem. B 1997, 101, 908. (17) Lo Nostro, P.; Ku, C. Y.; Chen, S. H.; Lin, J. S. J. Phys. Chem. 1995, 99, 10858.
10.1021/la026147g CCC: $25.00 © 2003 American Chemical Society Published on Web 01/30/2003
2314
Langmuir, Vol. 19, No. 6, 2003
Lo Nostro et al.
Figure 1. Schematic structure of cyclodextrins. CPK minimized structure of β-CD and of F8H16.
insoluble in water. Due to the typical inertness of fluorocarbons, they are very resistant to physical, chemical, and biochemical attack,13 and they possess a peculiar behavior in the solid state.19-22 For their unusual properties, SFAs are now being carefully investigated for their interesting biomedical applications as oxygen carriers23-25 and in ophthalmology.26 In this paper we report for the first time the occurrence of the typical threading phenomenon that leads to the formation of an inclusion adduct obtained upon mixing cyclodextrins with a semifluorinated n-alkane, namely F(CF2)8(CH2)16H (F8H16). Materials and Methods F8H16 was synthesized in our laboratory, according to the literature.15 β-CD was purchased from Sigma-Aldrich-Fluka (Milan, Italy), and used as received. Differential scanning calorimetry (DSC) experiments were carried out with a Perkin-Elmer DSC 7 power compensation instrument, equipped with PE PC Pyris 3.52 software. Measurements were performed up to 100 °C, in both isothermal and dynamic mode with a dry nitrogen flow of 16 cm3/min. The scanning rate was 5 °C/min for each sample, after isothermal conditioning at -30 °C for 1 min. Instrument calibration was performed with an indium standard. About 10 mg of the sample was weighed in standard aluminum sample pans and sealed. X-ray diffractometry was carried out with a Philips PW 105/25 instrument, equipped with a copper KR radiation source (λ ) 1.5405 Å), with 2° < 2θ < 30°. Voltage and current were set at 30 kV and 20 mA, respectively. (18) Viney, C.; Russell, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63. (19) Ho¨pken, J.; Mo¨ller, M. Macromolecules 1992, 25, 2482. (20) Sloutskin, E.; Kraack, H.; Ocko, B.; Ellmann, J.; Mo¨ller, M.; Lo Nostro, P.; Deutsch, M. Langmuir 2002, 18 (6), 1963. (21) Geppi, M.; Pizzanelli, S.; Veracini, C. A.; Cardelli, C.; Tombari, E.; Lo Nostro, P. J. Phys. Chem. B 2002, 106 (7), 1598. (22) Marczuk, P.; Lang, P.; Mo¨ller, M. Colloids Surf., A 2000, 163, 103. (23) Riess, J. G. Tetrahedron 2002, 58 (20), 4113. (24) Ferro, Y.; Krafft, M. P. Biochim. Biophys. Acta 2002, 1581 (12), 11. (25) Krafft, M. P.; Giulieri, F.; Fontaine, P.; Goldmann, M. Langmuir 2001, 17 (21), 6577. (26) Meinert, H.; Roy, T. Eur. J. Ophthalmol. 2000, 10, 189.
Figure 2. Formation of the F8H16/β-CD inclusion compound in water (right vial) compared to a clear β-CD solution at the same temperature (left). Fourier transform infrared spectra (FTIR) were obtained with an FTS 40-PC Bio-Rad instrument between 4000 and 500 cm-1, gain ) 1, and scans ) 128. Samples were mixed with potassium bromide and pressed into pellets. The F8H16/β-CD inclusion compound was obtained by spreading some crystals of the semifluorinated alkane onto the surface of a concentrated β-CD aqueous solution (16 mM), at 70 °C and sonicated. Figure 2 shows the formation of the F8H16/β-CD IC (right vial). The white crystalline inclusion compound was then filtered and washed with cold water and acetone. At room temperature, precipitation of the same product was attained after about 48 h. Molecular mixtures were prepared by dissolving suitable amounts of the two components in methanol (in which inclusion compounds are not formed), and then by evaporating the solvent under vacuum; the final powder was checked with DSC, FTIR, and X-ray diffractometry and compared to the data obtained for the corresponding IC. Atomic force microscopy (AFM) imaging was performed with an Explorer TMX 2000 microscope (Topometrix) using a 130 µm air scanner and a silicon “I”-shaped cantilever with an integrated pyramidal tip for noncontact imaging. Images were taken at room temperature in noncontact mode with on-line filtering, and they were processed by flattening in order to remove the background slope. Both topographic and internal sensor images were collected. Mica was used as a flat substrate: a 5 µL drop of cyclodextrin inclusion compound (0.1 mM in aqueous 0.2 M KOH) was
Inclusion Compound from F(CF2)8(CH2)16H and β-CD
Langmuir, Vol. 19, No. 6, 2003 2315
Table 1. Structural Parameters for Cyclodextrins (see Figure 1) R-CD β-CD γ-CD
a (Å)
b (Å)
h (Å)
14.6 15.4 17.5
4.9 6.2 7.9
7.9 7.9 7.9
deposited on freshly cleaved mica. Samples were dried in air at room temperature and imaged in the next 24 h. Images of cyclodextrin samples were obtained in the same way. To avoid highly concentrated regions, we scanned the samples near the border of the drop. A preliminary 5 × 5 µm2 scanning was always performed to detect the presence of filamentous structures. These structures were then imaged at higher magnifications.
Results and Discussion Once spread on the water surface, F8H16 does not dissolve and forms stable monolayers at the air/liquid interface,13,14 with a molecular area of about 33 Å2/molecule and a quite low collapse pressure (below 15 mM/m).14 Because of the high hydrophobicity of the two blocks, and of the rigiditiy of the fluorinated segment, SFAs usually form condensed films at the air/water interface.14 Several studies on their phase behavior in the solid state have shown that these molecules undergo some solid-solid phase transitions, and that the hydrocarbon chain is in a more fluid state than the more rigid fluorinated tail.13 Although fluorocarbons are well-known for the weakness of the intermolecular interactions they establish, the relevant incompatibility (i.e., immiscibility) with their hydrogenated counterparts is at the origin of their typical behavior, both in the solid state (interdigitated tilted structures) and in the liquid state (aggregation in selective solvents, and formation of microdomains in the gel state).13 In this study we found that at room temperature F8H16 does not interact with a concentrated water solution of β-CD, as it remains undisturbed at the air/water interface. On the other hand, by increasing the solution temperature above the melting point of F8H16, this copolymer rapidly interacts with the subphase and precipitates as a fine crystalline powder. Because of the strong repulsions between water molecules and the two hydrophobic blocks of the SFA, contact between F8H16 molecules and the solvent can be avoided through the formation of an F8H16/ β-CD inclusion compound, and we can reasonably expect that each single F8H16 molecule will be totally covered by cyclodextrins. The fact that the formation of the IC can be observed only above the melting point of F8H16 is simply due to the increase in the free motion of the SFA molecules, to the lowering of the intermolecular interactions, and therefore to the threading of the more fluid semifluorinated alkane molecule in the host’s cavity. Calculation of the chain lengths for the fluorinated and hydrogenated segments in F8H16 provides values of about 11.5 and 21.7 Å, respectively,15 and a total length of about 33.2 Å in the fully stretched conformation. By considering that the depth of a single CD is about 8 Å, the CPK (CoreyPauling-Koltun) models of β-CD and F8H16 (see Figure 1) suggest that about four cyclodextrin rings can be threaded by a single chain of the semifluorinated alkane. Hydrophobic and van der Waals interactions will be established between the host and the guest molecules, and hydrogen bonds will ensure a strong binding between the piled-up CD rings. Moreover, from the structural parameters reported in Table 1 and Figure 1, the area at the larger upper rim of β-CD is about 30.2 Å2, which is large enough for accommodating the SFA molecule, which has a cross section of about 28.3 Å2.22
Figure 3. DSC thermograms of β-CD, F8H16, and F8H16/β-CD.
Figure 4. X-ray diffractograms of β-CD, F8H16, F8H16 + β-CD (molecular mixture), and F8H16/β-CD in the powder state. The profile for F8H16 has been reported on a logarithmic y-axis scale.
Differential Scanning Calorimetry. Figure 3 shows the DSC curves for the F8H16/β-CD system. The melting endotherm appears only for the pure F8H16 at 53.3 °C (onset), while no peak is shown in the case of the inclusion compound. This means that no free F8H16 is present in the precipitated crystalline powder, thereby demonstrating the formation of the inclusion compound. X-ray Diffraction. XRD is a widely used technique in the study of inclusion compounds for assessing the kind of structure they form, and to check whether a new compound has been produced from the parent molecules.27-30 Figure 4 reports the wide-angle X-ray diffraction profiles for the F8H16/β-CD system. Due to the rigid fluorinated chain, F8H16 possesses a strong peak at 2.1° (d ) 42.4 Å, about twice the length of the fluorocarbon tails plus that of the interdigitated hydrogenated chains)16 and two peaks at 6° and 7.5°, respectively. The F8H16/βCD IC produces a different pattern, with signals at about 7°, 12°, 20°, and 23°. X-ray diffractometry confirms that the F8H16/β-CD inclusion compound crystallizes with a structure that is different from its components and their mixture. (27) Huang, L.; Allen, E.; Tonelli, A. E. Polymer 1998, 39 (20), 4857. (28) Rusa, C. C.; Luca, C.; Tonelli, A. E. Macromolecules 2001, 34, 1318. (29) Lu, J.; Mirau, P. A.; Tonelli, A. E. Macromolecules 2001, 34 (10), 3276. (30) Porbeni, F. E.; Edeki, E. M.; Shin, I. D.; Tonelli, A. E. Polymer 2001, 42, 6907.
2316
Langmuir, Vol. 19, No. 6, 2003
Lo Nostro et al. Table 2. Wavenumbers (in cm-1) of the Main Absorption IR Peaks O-H C-H C-H, C-C, C-O bending, stretching stretching other vibr modes β-CD F8H16 F8H16/β-CD IC
Figure 5. FTIR spectra for β-CD, F8H16, and F8H16/β-CD between 4000 and 500 cm-1.
Until now formation of an inclusion compound between a semifluorinated alkane and cyclodextrins has never been reported in the literature. However, Druliner and Wasserman stated that β-CD is able to weakly bind perfluorooctane and form 1:1 inclusion complexes.31 On the other hand, the formation of supramolecular adducts between cyclodextrins and linear or branched alkanes has already been studied by Harada and reported in previous works.32 These findings support the hypothesis that CD and a semifluorinated alkane may form an inclusion compound. FTIR. Figure 5 reports FTIR absorbance spectra between 4000 and 500 cm-1 for the F8H16/β-CD systems. In general, the IR spectra show differences between the inclusion compound and the parent molecules in the 40003000 cm-1 region, where the asymmetric and symmetric
3375 3368
2925 1155, 1028 2922, 2853 1471, 1150, 1116, 704, 658 2922, 2851 1155, 1030, 710, 570
O-H stretching bands appear, and in the region where C-H, CH2, and O-H bending vibrational modes (15001000 cm-1) absorb. Table 2 reports the main FTIR peaks: small differences between the O-H frequencies for β-CD and its ICs are usually ascribed to hydrogen bonding or to intermolecular interactions with the guest molecule.27 The FTIR curves in the fingerprint region (below 1300 cm-1) confirm that the F8H16/β-CD IC is different from the originating parent molecules, as they possess different spectroscopic signals. Atomic Force Measurements. Parts a and b of Figure 6 show two images obtained from the F8H16/β-CD IC. Both nanoaggregates and tubular structures are present. The nanotube, indicated by the arrow, has been magnified in the upper right corner: the width is about 20 nm, while the length is approximately 1.3 µm. Several AFM experiments have been performed and produced results similar to that reported in the figure. Clusters were always irregular in shape and dimensions, while all the tubular structures were characterized by the same width and different lengths. For pure β-CD we found structures similar to those already reported in the literature.33 Conclusions We report for the first time the formation of inclusion compounds (IC) between a short-chain diblock semifluorinated alkane (F8H16) and β-cyclodextrin in water at high temperature (70 °C). Because of their hydrophobicity, FmHn compounds repel water and penetrate the lipophilic cavity of cyclodextrins. Differential scanning calorimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and atomic force microscopy experiments confirm the presence of a novel product, with properties different from those of originating host and guest molecules. AFM experiments show the presence of large nanotubes in the dried samples, which result from the aggregation of several F8H16/β-CD ICs. These supramolecular structures are
Figure 6. AFM noncontact images for F8H16/β-CD IC. (a) 2500 × 2500 nm2, bar 500 nm, with magnified detail in the upper right corner (500 × 500 nm2, bar 100 nm); (b) 1000 × 1000 nm2, bar 200 nm. Images were taken at the 50% free-oscillation amplitude set point of the cantilever.
Inclusion Compound from F(CF2)8(CH2)16H and β-CD
produced from the precipitation of large IC aggregates from water solutions, once the solvent has evaporated. Acknowledgment. The authors are thankful to Prof. Piergiorgio Malesani (Department of Earth Sciences, (31) Druliner, J. D.; Wasserman, E. J. Fluorine Chem. 1995, 72, 75. (32) Harada, A. Carbohydr. Polym. 1997, 34, 183.
Langmuir, Vol. 19, No. 6, 2003 2317
University of Florence) for X-ray diffractometry. Partial financial support from CSGI and MIUR is greatly acknowledged. LA026147G (33) Munoz-Botella, S.; Martin, M. A.; Del Castillo, N.; Vazquez, L. Biophys. J. 1996, 71, 86.
