pubs.acs.org/Langmuir © 2009 American Chemical Society
Organic-Inorganic Hybrid Supramolecular Gels of Surfactant-Encapsulated Polyoxometalates Yinglin Wang, Wen Li, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Received May 26, 2009. Revised Manuscript Received July 9, 2009 A new type of hybrid supramolecular gel derived from a synergetic self-assembling of organic and inorganic components has been prepared through encapsulating a series of polyoxometalate (POM) clusters with ammonium surfactants bearing two alkyl chains. Among the general organic solvents we employed, some of surfactant-encapsulated POM (SEP) complexes readily gel with nonpolar solvents. Proper alkyl chain density, alkyl chain length, and shape of POM are proven to be favorable for fabricating stable SEP gels based on the results of critical gelation concentration and transition temperature from gel to solution. In addition, the shape of POM influences the aggregation morphologies of SEPs, stripes and spheres in gel phases, as confirmed by polarizing optical microscopic and scanning electron microscopic images. X-ray diffractions reveal that all the SEPs in gel states possess similar lamellar aggregation structures with POM layer inside and alkyl chain bilayers shielding from the nonpolar solvent on both sides. The combination of solvent effect, electrostatic, and dipole interaction is thought to be responsible for the formation of SEP gels.
Introduction Supramolecular gels, as the bicontinuous colloidal systems which are viscoelastic and keep their solidlike appearance in rheology on the time scale of experiment, are of interest over recent years.1 The current attention is not only focusing on the understanding of the aggregation structure and formation mechanism of general organic gels2,3 but also concerning the exploration of new building materials and the functional aspects.4,5 For instance, owing to the inherent characters of noncovalent interactions serving as the driving force, supramolecular gels can sensitively respond to the stimulation from the physical and chemical microenvironments.6-11 Such kinds of gels exhibit potentials in the fields like sensor,12 nanodevice,13 drug delivery and release,14 catalyst carrier,15,16 and template synthesis.17 Moreover, the incorporation of functional units into gelators provides the diversity of novel gels, because these units can self-assemble *To whom correspondence should be addressed. E-mail:
[email protected].
(1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3160. (2) George, M.; Weiss, R. G. Acc. Chem. Res. 2006, 39, 489–497. (3) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (4) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002–8018. (5) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (6) Moon, K.-S.; Kim, H.-J.; Lee, E.; Lee, M. Angew. Chem., Int. Ed. 2007, 46, 6807–6810. (7) Suzuki, T.; Shinkai, S.; Sada, K. Adv. Mater. 2006, 18, 1043–1046. (8) Zhou, S.-L.; Matsumoto, S.; Tian, H.-D.; Yamane, H.; Ojida, A.; Kiyonaka, S.; Hamachi, I. Chem.;Eur. J. 2005, 11, 1130–1136. (9) Liu, Q. T.; Wang, Y.; Li, W.; Wu, L. X. Langmuir 2007, 23, 8217–8223. (10) Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. Rev. 2007, 36, 1857–1867. (11) Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 8932–8938. (12) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 4164– 4165. (13) Yamaguchi, S.; Matsumoto, S.; Ishizuka, K.; Iko, Y.; Tabata, K. V.; Arata, H. F.; Fujita, H.; Noji, H.; Hamachi, I. Chem.;Eur. J. 2008, 14, 1891–1896. (14) Panda, J. J.; Mishra, A.; Basu, A.; Chauhan, V. S. Biomacromolecules 2008, 9, 2244–2250. (15) Tu, T.; Assenmacher, W.; Peterlik, H.; Werisbarth, R.; Nieger, M.; D€otz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368–6371. (16) Tu, T.; Bao, X.; Assenmacher, W.; Peterlik, H.; Schnakeburg, G.; D€otz, K. H. Angew. Chem., Int. Ed. 2008, 47, 7127–7131. (17) Xue, P.; Lu, R.; Huang, Y.; Jin, M.; Tan, C.; Bao, C.; Wang, Z.; Zhao, Y. Langmuir 2004, 20, 6470–6475.
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into mesostructures regularly, and the synergy of the units with gel matrixes may significantly enhance their intrinsic properties in functional materials. Oligo(p-phenylenevinylene),18 thiophene,19 fullerene,20 anthracene,21 8-quinolinol,22 and even metal ions23 have been incorporated into functional gels. Especially, because of the unique features, ordered assemblies of inorganic nanosized clusters in three-dimensional architectures induced by gelation are of significance for exceptional soft materials.24,25 Polyoxometalates (POMs) are a class of important inorganic transition-metal oxide nanoclusters with various chemical structures and display considerable functionalities in medicine, magnetism, optics, and catalysis.26 To perform the unique properties of POMs, much effort has been paid to incorporate them into organic matrixes through chemical and physical approaches, while the fabrication of POM-based supramolecular gels still remains a challenge. Covalently modified POMs were reported to form supramolecular gels through hydrogen bonding.27,28 In general, those modified POMs are uncommon, and the synthetic procedures are relatively complicated. To find convenient routes, a controllable photochromic gel has been prepared through mixing alkylammonium-polyoxomolybdate composites with efficient gelators.29 (18) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J. Angew. Chem., Int. Ed. 2007, 46, 6260–6265. (19) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393–1397. (20) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2003, 125, 9902– 9903. (21) Wang, C.; Zhang, D.; Xiang, J.; Zhu, D. Langmuir 2007, 23, 9195–9200. (22) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Shinkai, S. Chem. Commun. 2005, 4149–4151. (23) Miravet, J. F.; Escuder, B. Chem. Commun. 2005, 5796–5798. (24) Kimura, M.; Kobayashi, S.; Kuroda, T.; Hanabusa, K.; Shirai, H. Adv. Mater. 2004, 16, 335–338. (25) Bhattacharya, S.; Srivastava, A.; Pal, A. Angew. Chem., Int. Ed. 2006, 45, 2934–2937. (26) Hill, C. L. Chem. Rev. 1998, 98 (the entire issue). (27) Favette, S.; Hasenknopf, B.; Vaissermann, J.; Gouzerh, P.; Roux, C. Chem. Commun. 2003, 2664–2665. (28) Carraro, M.; Sartorel, A.; Scorrano, G.; Maccato, C.; Dickman, M. H.; Kortz, U.; Bonchio, M. Angew. Chem., Int. Ed. 2008, 47, 7275–7279. (29) Yi, T.; Sada, K.; Sugiyasu, K.; Hatano, T.; Shinkai, S. Chem. Commun. 2003, 344–345.
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Figure 1. (a) Photos of the phase transition between hybrid gel and sol and (b) schematic representation of the formation mechanism of SEP gels.
Nevertheless, how to control and quantify the dispersion of inorganic clusters in gels is still an unresolved issue from the mesostructure point of view. To develop a new approach, we tried to design POM-based building blocks and fabricate the even hybrid supramolecular gels through a simple dispersion of the precisely designed materials in selected solvents. POMs can be conveniently modified by exchanging their counterions with cationic surfactants, generating surfactant-encapsulated POM (SEP) complexes.30 The improvement of the surface properties of the inorganic clusters makes the complexes applicable for hybrid self-assembling31-33 and grafting into organic34 and inorganic matrixes.35 We believe that these features of SEPs are also suitable for the fabrication of organogels. In this article, we report a simple but unique method to construct POMbased hybrid gels through appropriate choice of SEPs and solvents (Figure 1a) for the first time. A systematical investigation has been done on the influence of alkyl chain density, alkyl chain length of SEP, and the shape of POM on the gelation. These results not only help to control the formation of gels but also provide the trace for constructing the functional organic-inorganic hybrid gels based on surfactant covered POMs. Furthermore, since the POMs can perform photochromism and redox reaction without changing their framework structures, the hybrid gel structures may facilitate the application of POMs in sensor and catalysis.
Experimental Section Materials. Inorganic clusters, H3PW12O40 (POM-1a) and H4SiW12O40 (POM-1b), were purchased from Sinopharm Chemical Reagent Co., Ltd. Dimethyldioctadecylammonium bromide (30) Bu, W.; Wu, L.; Zhang, X.; Tang, A.-C. J. Phys. Chem. B 2003, 107, 13425– 13431. (31) Li, H.; Sun, H.; Qi, W.; Xu, M.; Wu, L. Angew. Chem., Int. Ed. 2007, 46, 1300–1303. (32) Sun, H.; Li, H.; Bu, W.; Xu, M.; Wu, L. J. Phys. Chem. B 2006, 110, 24847– 24854. (33) Li, W.; Yin, S.; Wang, J.; Wu, L. Chem. Mater. 2007, 20, 514–522. (34) Li, H.; Qi, W.; Sun, H.; Li, P.; Yang, Y.; Wu, L. Dyes Pigm. 2007, 79, 105– 110. (35) Qi, W.; Li, H.; Wu, L. Adv. Mater. 2007, 19, 1983–1987.
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(DODA 3 Br, 99%) and dimethyldidodecylammonium bromide (DDDA 3 Br, 99%) were products of Acros Organics. All of the chemicals, unless specially mentioned, were used as received. Dimethylditetradecylammonium bromide (DTDA 3 Br) and dimethyldihexadecylammonium bromide (DHDA 3 Br) were synthesized through a modified procedure as reported in the literature.36,37 K7PW11O39 (POM-2a), K8SiW11O39 (POM-2b), K9BW11O39 (POM-2c), K11[Eu(PW11O39)2] 3 13H2O (POM-3a), K13[Eu(SiW11O39)2] 3 20H2O (POM-3b), and K15[Eu(BW11O39)2] 3 16H2O (POM-3c) were prepared according to literature procedures.38,39 Preparation of SEPs. SEPs were synthesized according to the previously reported method.30 Exemplified by (DODA)3PW12O40 (SEP-1a), POM-1a was dissolved in water, and then the chloroform solution of DODA 3 Br was added slowly under stirring. The initial molar ratio of DODA 3 Br to POM-1a was controlled at 3:1, according to the charge ratio. After 2 h of stirring, POMs were transferred into chloroform spontaneously, and the electrostatic complexes with defined ratio of components formed. The organic phase was then separated and dried with anhydrous sodium sulfate, giving the solid SEP-1a by evaporating the solution to dryness. The product was placed in vacuum for the measurements. Fourier transformation infrared (FT-IR) spectra for SEP-1a (KBr, cm-1): ν = 2956, 2918, 2850, 1484, 1468, 1080, 977, 897, 808, 721, 596, 522, 512. Anal. Calcd for SEP-1a (C114H240N3O40W12P, 4530.18): C 30.22, H 5.34, N 0.93. Found: C 30.52, H 5.32, N 0.74. The SEP-1a should correspond to a tentative formula (DODA)3PW12O40. Other SEPs, (DODA)4SiW12O40 (SEP-1b), (DODA)7PW11O39 (SEP-2a), (DODA)8SiW11O39 (SEP-2b), (DODA)9BW11O39 (SEP-2c), (DODA)11[Eu(PW11O39)2] (SEP-3a), (DODA)13[Eu(SiW11O39)2] (SEP-3b), (DODA)15[Eu(BW11O39)2] (SEP-3c), (DDDA)9BW11O39 (SEP4), (DTDA)9BW11O39 (SEP-5), and (DHDA)9BW11O39 (SEP-6), were prepared by the same method. The fundamental characterizations of these SEPs, including elemental analysis and FT-IR spectra, are summarized in the Supporting Information. Preparation of Gels. A certain weight of SEP samples and a certain volume of organic solvents were placed in a glass vial and heated in water bath until the solids were completely dissolved. The solutions were then cooled to room temperature slowly and kept for ca. 2 h under the ambient condition or at 4 °C. Finally, the vials were turned upside down to check whether the solution became viscoelastic, in which the cooled sample solutions showing no flowing down could confirm gelation.
Detection of Gel-Solution Transition Temperature (Tg). Gel-solution transition temperatures were determined by the “falling drop” method.40 Typically, a gel sample in a sealed glass vial was inverted and immersed in thermostated water bath. Then the sample was slowly heated in a rate of 1 °C/min until reaching the transition temperature, Tg, at which the gel began to flow down due to the gravity. Measurements. FT-IR spectra were carried out on a Bruker IFS 66v FT-IR spectrometer equipped with a DTGS detector (32 scans) with a resolution 4 cm-1 from pressed KBr pellets. Element analysis (C, H, N) was carried out on a Flash EA1112 analyzer from ThermoQuest Italia SPA. Scanning electron microscopic (SEM) images were performed on a JEOL JSM-6700F field emission scanning electron microscope. X-ray diffraction (XRD) was collected on a Rigaku X-ray diffractometer (D/max rA, using Cu KR radiation at a wavelength of 1.542 A˚), and the data were recorded from 0.7° to 10°. Polarizing optical micrographs were taken with an Axioskop 40POL polarized optical microscope from Zeiss. The samples were prepared by adding hot solution of (36) Matsumoto, Y.; Ueoka, R. J. Org. Chem. 1984, 49, 3774–3778. (37) Okahata, Y.; Ando, R.; Kunitake, T. Bull. Chem. Soc. Jpn. 1979, 52, 3647– 3653. (38) Haraguchi, N.; Okaue, Y.; Isobe, T.; Matsuda, Y. Inorg. Chem. 1994, 33, 1015–1020. (39) Ballardini, R.; Chiorboli, E.; Balzani, V. Inorg. Chim. Acta 1984, 95, 323. (40) Takahashi, A.; Sakai, M.; Kato, T. Polym. J. 1980, 12, 335–345.
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SEPs into a quartz cell and then sealing the cell. All species were cooled slowly and aged for 12 h before taking micrographs.
Scheme 1. (a) Coordination Polyhedral Representations of POMs Applied for Encapsulation, Classified by Their Structures, and (b) a Typical Schematic Structure of SEPs, SEP-2a ((DODA)7PW11O39)
Results and Discussion Gelation Behavior of SEPs. The as-prepared organicinorganic hybrid complexes as the gelator candidates are composed of two parts, organic surfactants and inorganic cores, which are combined together through electrostatic interaction. As demonstrated already, the molecules with simple alkyl chains,41 such as n-alkanes42 and dioctadecylamine,43 normally have a tendency to gel with various organic solvents through van der Waals interaction. The POMs with different shapes and numbers of negative charges have been proved to be promising units adjusting the interaction between alkyl chains around them, even the interaction between SEPs, and providing various aggregations under selected conditions.32,33 Thus, the complexes could be effective gelators, controlling the stabilities of gel structures through tuning the chemical composition of SEPs. First, to understand the effect of alkyl chain density and the POM shape on the gelation properties, we employed the same cationic surfactant DODA 3 Br to encapsulate a series of POMs. To make the comparison among the SEP gels convenient, we classified the SEPs with DODA into three classes by the types of POMs (as shown in Scheme 1). SEP-1a, -1b and SEP-2a, -2b, -2c correspond to the complexes with Keggin-type ([XW12O40]n-, X = P, Si, n = 3, 4) and lacunary Keggin-type POMs ([XW11O39]n-, X = P, Si, B, n = 7, 8, 9), respectively. SEP-3a, -3b, and -3c contain twinsphere shaped POMs possessing two lacunary Keggin-type POMs linked by a europium ion ([Eu(XW11O39)2]n-, X = P, Si, B, n = 11, 13, 15). The gelation behaviors of SEPs with DODA were evaluated in various organic solvents at the same concentration, 3 wt %. The data summarized in Table 1 reveal that nonpolar solvents are in favor of the construction of gels. The first type of SEPs consisting of Keggin-type POMs with few charges cannot dissolve in nonpolar solvents even after heating. In contrast to the case of the first type of SEPs, the second type of SEPs composed of lacunary Keggin-type POMs with appropriate numbers of negative charges can dissolve in nonpolar solvents such as n-hexane, cyclohexane, isooctane, and carbon tetrachloride by heating due to the increased ratio of organic component in the complexes, and the gels can be observed when their heated solutions are cooled to room temperature gradually. However, the gelation of SEP-2 series was not found in benzene. For the third type of SEPs bearing POMs with relatively larger numbers of negative charges, the gelation becomes complicated. All the third type of SEPs is insoluble in n-hexane. The important difference among them is SEP-3b and -3c generate typical gels in cyclohexane and isooctane, but SEP-3a shows no gel behavior (insoluble). In addition, SEP-3b and -3c form solution and jelly in carbon tetrachloride and benzene, respectively, while SEP-3a presents jelly and solution in these solvents. All SEPs can dissolve in toluene, dichlormethane, and chloroform as well as the strong polar solvents such as ethanol and methanol quite well, whereas the solubility is too good to form gels. We also tried the gelation behaviors for the SEPs in some other solvents which were not listed in Table 1. Dodecane and silicon oil as the nonpolar solvents are too viscous to disperse SEPs efficiently, which always bring jelly with phase-separated aggregations rather than gels. Like the case of general supramolecular gels, (41) Tu, T.; Bao, X.; Assenmacher, W.; Peterlik, H.; Daniels, J.; D€otz, K. H. Chem.;Eur. J. 2009, 15, 1853–1861. (42) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352–335. (43) Abdallah, D. J.; Lu, L.; Weiss, R. G. Chem. Mater. 1999, 11, 2907–2911.
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all of the gels fabricated by SEP complexes can decompose and reform reversibly under heating and cooling runs. The gelation behaviors of SEPs with DODA in various organic solvents can be well regulated, rooting in the difference of the chemical composition of SEPs. On the basis of the corresponding crystal structures,44 Keggin-type POMs can be approximately abstracted to a spherical structure with a diameter of 1.04 nm. So, the Keggin-type POMs possess a surface area around 3.4 nm2, calculated according to the formula of spherical area. Because POM-3a, -3b, and -3c are composed of two Keggin units, the surface area is ca. 6.8 nm2, approximately 2-fold of Keggin-type POMs’ surface area. Following this, the laccunary Keggin-type POMs can be approximately abstracted to a spherical crown with area of ca. 3.1 nm2, deriving from removing a laccunary part from a spherical structure of Keggin-type POMs. Because of the fit charge neutralization, the proportions of organic component in SEP-1a and -1b are smaller than the other two types of SEPs, making the complexes less hydrophobic and insoluble in nonpolar solvents. In contrast to this, along with increasing the charge numbers of POMs, more positive charged surfactant molecules cover on the surface of inorganic cores, resulting in the good solubility of the SEP-2a, -2b, and -2c in the heated nonpolar solvents. Although the POMs in the third type of SEPs possess even more charges, their surface area also becomes larger, and the elongated shape may prevent alkyl chains from covering the hydrophilic surface of inorganic cores effectively. As a result, SEP-3a, -3b, and -3c with more alkyl chains could not dissolve in n-hexane, and SEP-3b and -3c gel fewer solvents than SEP-2 series complexes. But in polar solvents, the amphiphilic SEPs can readily dissolve, and the influence of the proportion of organic component to the solubility becomes negligible, so that all of the SEPs with DODA employed in the experiment form transparent solutions at room temperature. Hence, compared with polar solvents, the SEPs are more willing to gel with nonpolar solvents. Besides, to study the influence of the alkyl chain length on the construction of hybrid gels, we selected three other surfactants with shorter alkyl chain length and structures similar to that of DODA 3 Br, DDDA 3 Br, DTDA 3 Br, and DHDA 3 Br to encapsulate the same inorganic core, POM-2c. The gel behaviors of resulting complexes (SEP-4, SEP-5, SEP-6), as well as SEP-2c, are compared and summarized in Table 2. Among the three (44) Rocchoccioli-Deltcheff, C.; Fourier, M.; Franck, R.; Thouvenot, R. Inorg. Chem. 1983, 22, 207–216.
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Article Table 1. Gelation Behaviors of SEP-1, -2, and -3 Series in Various Organic Solventsa SEP-1a
SEP-1b
SEP-2a
SEP-2b
n-hexane I I G G cyclohexane I I G G isooctane I I G G I I G G CCl4 benzene I I S S toluene S S S S S S S S CH2Cl2 S S S S CHCl3 a I = insoluble when heated; G = gel; S = solution; J = jelly (thick solution).
Table 2. Gelation Behaviors of SEP-4, -5, -6, and -2c in Various Organic Solventsa SEP-4
SEP-5
SEP-6
SEP-2c
n-hexane I I G
