Ultrasound-Induced In Situ Formation of Coordination Organogels

Published on Web 12/18/2009 .... wed by dropwise addition of 92 mL of 34.65 mmol (1.944 g) methanolic potassium hydroxide .... Quick gelation of IBAs/...
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Ultrasound-Induced In Situ Formation of Coordination Organogels from Isobutyric Acids and Zinc Oxide Nanoparticles Atanu Kotal, Tapas K. Paira, Sanjib Banerjee, and Tarun K. Mandal* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India Received October 16, 2009. Revised Manuscript Received November 20, 2009 The discovery of ultrasound-induced in-situ formation of coordination organogels using various isobutyric acids (such as isobutyric acid or 2-methylisobutyric acid or 2-bromoisobutyric acid) and zinc oxide nanoparticles was described. FTIR and XRD results suggest that ultrasound irradiation triggers the quick dissolution of zinc oxide nanoparticles by isobutyric acids, resulting in the in-situ formation of zinc isobutyrate complexes that undergoes fast sonocrystallization into gel fibers. FESEM results clearly demonstrate the formation of well-defined networks of fibers with several micrometers in length, but the average diameter of the fiber ranges from 30 to 65 nm, depending upon the nature of the isobutyric acids used. A combination of single-crystal structure analysis and powder XRD result was used to envisage the molecular packing present in the gel state. This is probably a very rare case of ultrasound-induced organogelation where metal oxide NPs are used as the precursor.

Introduction Recently, there has been extensive research on the stimuliresponsive assembly of low-molecular-weight compounds.1-7 The major aim is to create new technologies for the precise control of the physical properties and functions of assembled materials. Among the physical stimuli, ultrasound is usually used to disrupt the weak noncovalent interactions or to disintegrate aggregated particles but seldom favors the assembly formation.8 But recent reports showed that ultrasound could act as a stimulus to induce gelation of organic liquids with low-molecular-weight gelators (LMGs) such as pallado-macrocycles, peptide-based palladium complexes, and dipeptides.8-11 In general, LMGs gels, no matter how they are prepared, are of great interest due to the simplicity of *Corresponding author: Fax þ91-33-2473 2805; e-mail psutkm@ mahendra.iacs.res.in. (1) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282–12283. (2) Zimenkov, Y.; Dublin, S. N.; Ni, R.; Tu, R. S.; Breedveld, V.; Apkarian, R. P.; Conticello, V. P. J. Am. Chem. Soc. 2006, 128, 6770–6771. (3) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. J. Am. Chem. Soc. 2005, 127, 11134–11139. (4) Tanaka, S.; Shirakawa, M.; Kaneko, K.; Takeuchi, M.; Shinkai, S. Langmuir 2005, 21, 2163–2172. (5) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922–13923. (6) Frkanec, L.; Jokic, M.; Makarevic, J.; Wolsperger, K.; Zinic, M. J. Am. Chem. Soc. 2002, 124, 9716–9717. (7) Kawano, S. I.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592– 8593. (8) Hasobe, T.; Oki, H.; Sandanayaka, A. S. D.; Murata, H. Chem. Commun. 2008, 724–726. (9) Naota, T.; Koori, H. J. Am. Chem. Soc. 2005, 127, 9324–9325. (10) Bardelang, D.; Camerel, F.; Margeson, J. C.; Leek, D. M.; Schmutz, M.; Zaman, M. B.; Yu, K.; Soldatov, D. V.; Ziessel, R.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 2008, 130, 3313–3315. (11) Isozaki, K.; Takaya, H.; Naota, T. Angew. Chem., Int. Ed. 2007, 46, 2855– 2857. (12) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (13) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (14) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (15) Camerel, F.; Faul, C. F. J. Chem. Commun. 2003, 1958–1959. (16) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021. (17) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Ojima, M.; Fujii, A.; Ozaki, M.; Shinkai, S. Chem.;Eur. J. 2007, 13, 4155–4162. (18) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 15954–15955.

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the gelator molecules, their physical properties, and various potential applications.7,12-19 Some examples of such gelators are derivatives of long-chain hydrocarbons,12 amino acids,10 steroids,14 and carbohydrates,20 the molecules of which are assembled mainly through noncovalent interactions during gelation. However, the synthesis of such gelator molecules requires complex chemistries and time-consuming synthesis. The use of two-component gelator system including metal complexes is an alternative, but still relatively less studied, approach to avoid such difficulties.16,17,21-24 However, the use of zinc coordination complex with very-low-molecular-weight ligands for gelling organic fluid is very rare.25,26 To date, the majority of LMGs were discovered by chance. To our surprise, during the course of functionalization of zinc oxide (ZnO) nanoparticles (NPs) with 2-bromoisobutyric acid (BIBA) by sonication, we discovered that the system showed unexpected gelation of organic solvents. This observation prompted us to explore the gelation abilities of other related acids with ZnO NPs in different organic fluids. In this article, we report our observations on the ultrasound-induced gelation of several organic fluids using isobutyric acid (IBA) or its alpha-derivatives, namely 2-methylisobutyric acid (MIBA) or 2-bromoisobutyric acid (BIBA), with ZnO NPs. Recent reports show that gel can act as templates for generation of metal oxides NPs.22,27 However, to date, this is probably a very rare (19) Ziessel, R.; Pickaert, G.; Camerel, F.; Donnio, B.; Guillon, D.; Cesario, M.; Prange0 , T. J. Am. Chem. Soc. 2004, 126, 12403–12413. (20) Buhler, G.; Feiters, M. C.; Nolte, R. J. M.; Dotz, K. H. Angew. Chem., Int. Ed. 2003, 42, 2494–2497. (21) Tu, T.; Assenmacher, W.; Peterlik, H.; Weisbarth, R.; Nieger, M.; Dotz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368–6371. (22) Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S. Chem.;Eur. J. 2008, 14, 6534– 6545. (23) Anderson, K. M.; Day, G. M.; Paterson, M. J.; Byrne, P.; Clarke, N.; Steed, J. W. Angew. Chem., Int. Ed. 2008, 47, 1058–1062. (24) Leong, W. L.; Tam, A. Y.-Y.; Batabyal, S. K.; Koh, L. W.; Kasapis, S.; Yam, V. W.-W.; Vittal, J. J. Chem. Commun. 2008, 31, 3628–3630. (25) Hui, J. K.-H.; Yu, Z.; MacLachlan, M. J. Angew. Chem., Int. Ed. 2007, 46, 7980–7983. (26) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663–11672. (27) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999.

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case of ultrasound-induced organogelation where metal oxide NPs are used as the precursor.

Scheme 1

Experimental Section Materials. 2-Methylisobutyric acid (MIBA) (99%), 2-bromoisobutyric acid (BIBA) (98%), and zinc acetylacetonate [Zn(acac)2] were obtained from Aldrich and used as received. Isobutyric acid (IBA) was received from Riedel de Haen AG Seelze-Hannover, Germany, and was distilled prior to the reaction. Zinc nitrate hexahydrate, potassium hydroxide, zinc carbonate, and zinc oxide powder were obtained from Merck, India, and used as received. All the solvents used were distilled prior to gelation. Synthesis of Zinc Oxide Nanoparticles. Zinc oxide nanoparticles (ZnO NPs) were prepared by the hydrolysis of zinc nitrate hexahydrate [Zn(NO3)2 3 6H2O] instead of zinc acetate in the presence of potassium hydroxide (KOH) in methanol as reported elsewhere.28 In a typical procedure, 17.8 mmol (5.3 g) of zinc nitrate hexahydrate was dissolved in 168 mL of methanol containing 1 mL of water under vigorous stirring at 60 °C followed by dropwise addition of 92 mL of 34.65 mmol (1.944 g) methanolic potassium hydroxide solution within 15 min. Then the reaction mixture was stirred continuously for another 2.25 h at 60 °C, and then it was cooled to room temperature. The asprepared ZnO nanoparticles were centrifuged at 150 00 rpm (21 000g) for 20 min, washed with methanol and water repeatedly to remove unreacted compounds, and then heated at 100 °C for 24 h in a vacuum oven to obtain a white powder (1.3 g). The average diameter of the synthesized ZnO NPs was ∼9 nm as determined by Scherer’s equation.29 Gelation Procedure. For gelation, ZnO nanoparticles were added to the homogeneous solution of different isobutyric acids (molar ratio of acid and ZnO is 1:1) in various organic liquids. Briefly, to prepare a 0.5 wt % 2-bromoisobutyric acid (BIBA) gel, 10 mg (0.0598 mmol) of BIBA was weighed in a screw-capped vial, and then 2 mL of freshly distilled toluene was added to the vial to get the homogeneous solution of the acid. 4.87 mg (0.0598 mmol) of zinc oxide nanoparticles was then added to this solution and sonicated thoroughly at room temperature in a Cole-Parmer 8891 sonicator (42 kHz) until gelation.

Ratio (Isobutyric acids/ZnO NPs) Optimizations for Gelation Experiment. To optimize the molar ratio between different isobutyric acids and zinc oxide nanoparticles (ZnO NPs) for gelation, we carried out gelation experiments (2 wt %) with different mole ratio in four solvents (toluene, nitrobenzene, dichloroethane, and decalin) under similar conditions. From this study we may conclude that equimolar ratio is excellent for organogelation in terms of lowering the MGC as well as the sonication time for all three systems. Synthesis of Neat Zinc Isobutyrate (Zn-IBA). Zinc isobutyrate was synthesized from isobutyric acid and zinc carbonate in water analogous to the procedure of the synthesis of zinc methacrylate as reported elsewhere.30 Briefly, 0.54 mL of isobutyric acid was dissolved in 2 mL of water, and then 0.27 g of zinc carbonate was added portionwise upon vigorous vortexing. After the addition of zinc carbonate, the cloudy solution was filtered and divided into two parts. One part was freeze-dried immediately, and the other part was allowed to stand for several days to obtain single crystals of zinc isobutyrate.

Synthesis of Neat Zinc 2-Bromoisobutyrate (Zn-BIBA) and Zinc 2-Methylisobutyrate (Zn-MIBA). Zinc 2-bromoisobutyrate (Zn-BIBA) and zinc 2-methylisobutyrate (Zn-MIBA) were prepared by following the similar procedure as used for Zn-IBA, but a water-ethanol (1:1) mixture was used as solvent (28) Sun, B.; Sirringhaus, H. Nano Lett. 2005, 5, 2408–2413. (29) Cullity, B. D. In Elements of X-ray Diffractions; Addison-Wesley: Reading, MA, 1978. (30) Parrish, C. F.; Kochanny, J. G. L. Makromol. Chem. 1968, 115, 119–124.

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instead of water, since both the BIBA and MIBA were insoluble in water. A typical synthesis procedure was as follows. At first, 0.489 g of 2-bromoisobutyric acid (BIBA) was dissolved in 2 mL of a water-ethanol mixture (1:1). 0.135 g of zinc carbonate was then added in a portion to the above mixture with vigorous vortexing as described above for the synthesis of Zn-IBA. After completion of the addition of zinc carbonate, the solution was filtered immediately, and the filtrate was divided in two parts. One part was freeze-dried immediately to obtain the neat salt. The other part was allowed to stand for several days to obtain single crystals of zinc 2-bromoisobutyrate. However, a suitable quality single crystal was not obtained. A similar procedure was followed for the synthesis of zinc 2-methylisobutyrate. Here 0.48 mL of 2-methylisobutyric acid (MIBA) and 0.206 g of zinc carbonate were used as the starting materials. A needlelike fine crystal was obtained but is not suitable for single crystal diffraction analysis. Characterization. Fourier-transformed infrared (FTIR) spectra was recorded from KBr pellets, prepared by mixing neat samples as well as all the xerogels with KBr in 1:100 (w/w) ratios in a Nicolet Magna IR 750 spectrometer. For field-emission scanning electron microscopic (FESEM) analysis, the xerogel samples were directly put on the carbon tape, and then the samples were coated with gold to avoid charging. The micrographs were recorded by placing gold-coated ample under a JEOL field emission electron microscope (model JSM 7600F) operated at an accelerating voltage of 5 kV. Atomic force microscopic (AFM) study of the gel was carried out on the thin film of a cleaned glass slide. The solvent was evaporated at room temperature. AFM images of the films were taken in a VEECO Digital Instrument CP-II microscope operating in tapping mode at room temperature. Wide-angle X-ray diffraction studies (WAXS) of neat samples and all the xerogel (obtained by slow evaporation of the solvents from gels) samples were performed on a glass slide by Bruker D8 XRD instrument (SWAX) at an acceleration voltage of 35 kV with 30 mA current intensity using Cu tube (λ = 0.154 nm) as radiation source. Single crystal data were collected on a Bruker SMART APEX diffractometer, equipped with graphite monochromatic Mo KR radiation (λ = 0.710 73 A˚), and were corrected for Lorentzpolarization effects. A total of 10 011 reflections were collected, of which 1902 were unique (Rint = 0.0459), satisfying the (I > 2σ(I)) criterion, and were used in subsequent analysis. The crystal structure was solved by direct methods using SHELXS-97 and refined by full-matrix least-squares methods based on F2 using SHELXL-97, incorporated in the WINGX 1.70.01 crystallographic collective package.31 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were added at the calculated positions and refined isotropically.

Results and Discussion For gelation experiments, an equimolar amount (see Experimental Section for details) of spherical ZnO NPs (diameter ∼9 nm)28 and isobutyric acids (IBA/MIBA/BIBA) were dispersed in various organic solvents (Scheme 1, left vial). When these sols were irradiated with ultrasound (42 kHz) for a period of 20 s to (31) Farrugia, L. J. WinGX version 1.70.01 ed., Department of Chemistry, University of Glasgow, 2005.

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Table 1. Gelation Study of Isobutyric Acid (IBA) with Zinc Oxide Nanoparticles (ZnO NPs) at a Molar Ratio of 1:1 in Various Organic Solvents at 25 °Ca solvent

MGC (wt %)

sonication time at MGC (s)

appearance at MGC

sonication time for making 2 wt % gel (s)

benzene 1.50 300 CG 120 toluene 1.75 480 CG 360 xylene 1.25 120 CG 30 ethylbenzene 1.50 120 CG 60 nitrobenzene 0.50 240 CG 60 chlorobenzene 1.25 300 CG 180 n-hexane P cyclohexane 0.20 240 TG 60 decalin 0.15 240 TG 20 carbon tetrachloride P chloroform P 1,2-dichloroethane 1.50 240 TG 120 dichloromethane P ethyl acetate P a MGC = minimum gelator concentration with respect to IBA, CG = clear gel, TG = turbid gel, and P = precipitation.

appearance of 2 wt % gel CG CG CG CG CG CG P TG TG P P TG P P

Table 2. Gelation Study of 2-Bromoisobutyric Acid (BIBA) with Zinc Oxide Nanoparticles (ZnO NPs) at a Molar Ratio of 1:1 in Various Organic Solvents at 25 °Ca solvent

MGC (wt %)

sonication time at MGC (s)

appearance at MGC

sonication time for making 2 wt % gel (s)

benzene 0.15 480 CG 25 toluene 0.15 120 CG 20 xylene 0.25 180 CG 30 ethylbenzene 0.75 900 CG 75 nitrobenzene 0.25 120 CG 120 chlorobenzene 0.15 420 CG 25 n-hexane P cyclohexane 0.25 1200 TG 240 decalin 0.50 360 TG 150 carbon tetrachloride 0.50 300 TG 180 chloroform 1.00 240 CG 80 1,2-dichloroethane 0.75 180 TG 30 dichloromethane 0.75 720 TG 100 ethyl acetate P a MGC = minimum gelator concentration with respect to BIBA, CG = clear gel, TG = turbid gel, and P = precipitation.

appearance of 2 wt % gel CG CG CG CG CG CG P TG TG TG CG TG TG P

Table 3. Gelation Study of 2-Methylisobutyric Acid (MIBA) with Zinc Oxide Nanoparticles (ZnO NPs) at a Molar Ratio of 1:1 in Various Organic Solvents at 25 °Ca solvent

MGC (wt %)

sonication time at MGC (s)

appearance at MGC

sonication time for making 2 wt % gel (s)

benzene P toluene 2 900 TG 900 xylene P ethylbenzene P nitrobenzene 2 480 CG 480 chlorobenzene 2 540 CG 540 n-hexane P cyclohexane P decalin 2 600 TG 600 carbon tetrachloride P chloroform P 1,2-dichloroethane P dichloromethane P ethyl acetate P a MGC = minimum gelator concentration with respect to MIBA, CG = clear gel, TG = turbid gel, and P = precipitation.

10 min (depending on type of gelators and their concentrations) at ambient conditions, organogelation resulted (Scheme 1, right vials). Besides these three IBAs, several other related acids (see Table S1 in Supporting Information) were also checked for organogelation with ZnO NPs and found that, except for 2-bromopropionic acid (BPA), all are failed to form a gel in any conditions. However, BPA produces loose gels in the presence of ZnO NPs in selective solvents (see Table S1 in Supporting 6578 DOI: 10.1021/la903923q

appearance of 2 wt % gel P TG P P CG CG P P TG P P P P P

Information). Gelation was confirmed by simply inverting the vial, and the formed gels were thermoreversible, robust, transparent/opaque (depending on the solvents), and stable for months in a sealed vial (see Tables 1-3). Note that the individual component (either IBAs or ZnO NPs) did not produce any organogel under similar conditions. Quick gelation of IBAs/ ZnO systems occurred only under sonication but not with other stimuli such as fast heating/cooling or under microwave Langmuir 2010, 26(9), 6576–6582

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Figure 1. Plot of sonication time vs concentration (wt %) of various isobutyric acids (IBAs) used for preparing gel with ZnO NPs (at 1:1 molar ratio): (A) in toluene and (B) in decalin.

irradiation. However, vigorous magnetic stirring resulted gelation, but only after prolonged time, e.g., the BIBA/ZnO NPs (1 wt %) system takes ∼90 min to gelate toluene. The efficiency of gelation of the IBAs/ZnO systems was examined, and results are depicted in Tables 1-3. IBA/ZnO and BIBA/ZnO systems can gelate a wide range of organic solvents, while the MIBA/ZnO system can gelate only some selective solvents as evidenced from Tables 1-3. It is also very clear that the minimum gelator concentration (MGC) with respect to IBAs and the sonication time for gelation of these systems depend vastly on the nature of IBAs and solvents. Notably, the BIBA/ZnO system shows excellent gelation ability (MGC = 0.15-0.2 wt %) in both aromatic and cyclic hydrocarbon solvents (see Table 2). But, the MGC values of the IBA/ ZnO system in these aromatic solvents are quite high (ca. 1.252 wt %), while those in cyclic hydrocarbons are again very low (ca. 0.15-0.2 wt %) (see Table 1). The MGC values of the MIBA/ ZnO system in the above-mentioned solvents are also very high (ca. 2 wt %) (see Table 3). However, none of these systems form gel in n-alkane solvents. Sonication time required for gelling a particular solvent by any of these systems decreases with the increase of concentrations of gelators as shown for two solvents (Figure 1A for toluene and Figure 1B for decalin). These organogels are stable at room temperature but can be converted into sol state upon heating at above gel melting temperature (tgel). The value of tgel depends on the nature and concentrations of isobutyric acids as can be clearly seen in Table S2 of the Supporting Information. For example, as examined for BIBA/ZnO gel in toluene, the tgel value increases with increasing gelator’s concentration (see Figure S1 in Supporting Information). To check the effect of diameter of the used ZnO NPs on their gelation ability, we performed the gelation experiments with various ZnO NPs of different diameters as well as with commercial ZnO powder (Merck, India) in the presence of BIBA. These results were shown through a bar diagram in Figure S2 of the Supporting Information, which clearly revealed that with decrease in diameter of the ZnO particles sonication time decreases for gelation in a particular solvent. Note that the time required for gelation of toluene using ZnO powder is much higher than that required using ZnO nanoparticles in the presence of BIBA. The different surface area of different sized ZnO particles might be responsible for such variation of sonication time. To check the importance of use of ZnO NPs as precursor for the present gelation systems, we also performed some control gelation experiments in toluene using BIBA and any one of other zinc precursors such as zinc nitrate hexahydrate, zinc acetate dihydrate, zinc chloride, and with zinc sulfide nanoparticles under Langmuir 2010, 26(9), 6576–6582

Figure 2. XRD patterns of (a) neat zinc acetylacetonate [Zn(acac)2] and (b) xerogel of BIBA/Zn(acac)2 in toluene at 1:1 molar ratio (1 wt % with respect to acid). Upper inset shows the FESEM image of BIBA/Zn(acac)2 xerogel.

Figure 3. FESEM images of xerogels (2 wt %) of (a) IBA/ZnO, (b) MIBA/ZnO, and (c) BIBA/ZnO systems obtained from toluene.

similar conditions but resulted no gelation. Only the BIBA/zinc acetylacetonate system successfully gelated toluene after prolonged ultrasonication. Figure 2 represents the XRD pattern and FESEM image of the obtained xerogel showing the formation of fibrillar morphology and details of which will be published later. Thus, we have chosen ZnO NPs as precursor for the present study. FESEM images of the xerogels of these three systems obtained from toluene clearly showed well-defined networks of fibers with several micrometers in length (Figure 3). But, the average diameter (D) of fibers varies for system to system, e.g., D ∼ 30, 45, and 65 nm as measured from the FESEM images of BIBA/ZnO (Figure 3C), MIBA/ZnO (Figure 3B), and IBA/ZnO (Figure 3A) system, respectively. As the gelator’s (BIBA/ZnO) concentration in toluene was increased, the obtained fibers were seen to be more assembled, but the diameter of the fibers remained almost unchanged (Figure 4). FESEM images also reveal that the xerogels of the BIBA/ZnO system obtained from toluene and dichloromethane show fibrillar morphology, but fibers from dichloromethane are more assembled than that obtained from toluene (see Figure S3 in Supporting Information). An AFM image of the xerogel of the BIBA/ZnO system in toluene (0.5 wt %) also showed similar fibriller morphology with D ∼ 25-30 nm (Figure 5), which correlates well with that obtained from FESEM image mentioned above (Figure 3C) for the BIBA/ZnO system in toluene. To obtain insight into this gelation mechanism, FTIR characterization was performed on the xerogel of the BIBA/ZnO DOI: 10.1021/la903923q

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Figure 4. FESEM images of the xerogel synthesized from 2-bromoisobutyric acid (BIBA) and zinc oxide nanoparticles (ZnO NPs) of varying acid concentrations: (a) 0.25, (b) 1, and (c) 2 wt % in toluene.

Figure 6. FTIR spectra of (a) neat 2-bromoisobutyric acid (BIBA), (b) xerogel of BIBA/ZnO system obtained from toluene (1 wt % with respect to acid), and (c) neat zinc 2-bromoisobutyrate (Zn-BIBA).

Figure 5. AFM image and the corresponding height profile of BIBA/ZnO NPs gel (0.5 wt %) in toluene.

system obtained from toluene, and the corresponding characteristic peaks are summarized in Table S3 of the Supporting Information. The spectrum (Figure 6b) of this xerogel did not exhibit a peak due to the carbonyl group at 1708 cm-1 as observed for neat BIBA (Figure 6a), indicating the absence of free acid in the gel. However, the xerogel exhibits peaks at 1608, 1578, and 1406 cm-1 that are characteristic of carboxylate asymmetric and symmetric stretching vibrations, respectively (Figure 6b and Table S3),32 which correlates well with that of neat zinc 2-bromoisobutyrate (Zn-BIBA) as shown in Figure 6c. These results indicate that Zn-BIBA salt is formed in situ in the gel. Splitting of the carboxyl band (1608 and 1578 cm-1) suggests bridging of carboxylate bidentate ligands which provides further evidence for the formation of Zn-BIBA.33 Similar conclusions can also be drawn by comparing the FTIR results (Table S3) of the xerogels of IBA/ZnO and MIBA/ZnO systems obtained from toluene with that of neat zinc isobutyrate (Zn-IBA) and zinc (32) Si, S.; Dinda, E.; Mandal, T. K. Chem.;Eur. J. 2007, 13, 9850–9861. (33) Shafi, K. V. P. M.; Ulman, A.; Lai, J.; Yang, N.-L.; Cui, M.-H. J. Am. Chem. Soc. 2003, 125, 4010–4011.

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Figure 7. [A] XRD patterns of (a) xerogel of BIBA/ZnO in toluene (1 wt %) and (b) Zn-BIBA salt, (c) neat BIBA, and (d) ZnO NPs. [B] XRD patters of (a) xerogel of IBA/ZnO in toluene (5 wt %), (b) Zn-IBA salt, (c) xerogel of Zn-IBA salt in toluene (5 wt %), and (d) simulated XRD pattern obtain from single crystal data of Zn-IBA salt.

2-methylisobutyrate (Zn-MIBA) salts (see Figures S4 and S5 in Supporting Information). All these results suggested that ZnO Langmuir 2010, 26(9), 6576–6582

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Figure 8. ORTEP and molecular packing diagram of the zinc isobutyrate single crystal. All the hydrogen atoms are omitted for clarity.

NPs reacts with IBAs to produce respective Zn-IBAs, following the equation ZnO þ IBAs f Zn-IBAs during gelation process, which are actually acting as gelators in these systems. To gain structural information, the powder XRD data of the xerogels were compared with those of ZnO NPs and neat Zn-IBAs. The xerogel of BIBA/ZnO obtained from toluene (1 wt %) exhibited a well-resolved diffraction peak at 2θ = 6.58° (Figure 7Aa) having d-spacing of 1.342 nm, which indicates the formation of ordered lamellar in the gel as also obtained for other systems, reported by earlier researchers.21,22 The broad peak at 2θ = 20.48° may be due to the stacking of the molecules or the long-range ordering of the molecules in the xerogel (Figure 7Aa). All other peaks of the xerogel correlate well with that of neat Zn-BIBA as revealed from Figure 7Ab. The xerogel does not exhibit any characteristic diffraction peaks of neat BIBA (2θ = 17.69°, 19.65°, and 30.47°), as shown in Figure 7Ac. These XRD results further confirmed the absence of free BIBA and the in-situ formation of Zn-BIBA salt in the gel as also evidenced from the FTIR results mentioned above. The XRD pattern (Figure 7Ba) of xerogel of the IBA/ZnO system obtained from toluene also showed lamellar peak at 2θ = 7.62°, and the rest of the peaks are similar to that of neat Zn-IBA (Figure 7Bb). These results again revealed the formation of zinc carboxylate, i.e., Zn-IBA in the gel obtained from IBA/ZnO in toluene. A similar conclusion can also be made from the XRD results of the MIBA/ ZnO system (see Figure S6 in Supporting Information). To prove further that the Zn-IBAs salts (Zn-IBA, Zn-BIBA, and Zn-MIBA) are the actual gelators in our systems, as a representative case, we prepared separately zinc isobutyrate from zinc carbonate and isobutyric acid in water as described in the Experimental Section. Then we irradiated the toluene solution of neat Zn-IBA (5 wt %) with an ultrasound for ∼30 min. Interestingly, we observed gelation of toluene. One can use Zn-IBAs as gelators instead of IBAs/ZnO NPs systems to prepare such gels; however, higher concentration of salts (>3 wt %) and longer sonication time (>30 min) are required to make such gels. The XRD profile of this xerogel (Figure 7Bc) correlates well with that of xerogel from the IBA/ZnO system, showing a lamellar peak at almost the same position 2θ = 7.62° (see Figure 7Ba). These results confirmed that Zn-IBA salts are the actual gelators in these systems. In these systems, gels are formed by the irradiation with ultrasound, and the XRD results of the formed xerogels exhibit a lamellar peak suggesting that the gelation may proceed through sonocrystallization process. Ultrasound triggers the dissolution of ZnO NPs by isobutyric acids and giving rise to a supersaturated Langmuir 2010, 26(9), 6576–6582

solution of Zn-IBAs, which upon fast sonocrystallization resulted gelation of organic liquids. This mechanism is well resembled with that proposed by Bardelang et al.10 but is quite different from that of proposed conformational change of the molecules due to ultrasonication by Naota et al.9,11 To ascertain the mechanism responsible for gelation, we have performed single crystal XRD study of gelator salts, as it is virtually impossible to determine the crystal structure of gel networks.34,35 Thus, we made several attempts to obtain single crystals of three gelator salts (Zn-BIBA/Zn-IBA/Zn-MIBA). However, at this point, suitable quality single crystal is only obtained for Zn-IBA salt for diffraction analysis. XRD data of single crystal of Zn-IBA salt are provided in Table S4 of the Supporting Information.36 Structural analysis of Zn-IBA single crystal reveals that in the complex each zinc atom is tetrahedrally coordinated by four oxygen atom of isobutyrate group (Figure 8).36 While three of the four isobutyrate unit bridges between the two zinc atoms of the same molecule, the fourth one bind the zinc atom of another molecule and thus resulting in the formation of a one-dimensional polymeric network along the crystallographic axis “b” (green line) (Figure 8). The stimulated XRD pattern (Figure 7Bd), as obtained from the single crystal analysis, correlates well with that of xerogel of the IBA/ZnO system obtained from toluene (see Figure 7Ba). Therefore, we may conclude that the xerogel of the IBA/ZnO system have quite similar molecular packing to that of zinc isobutyrate salt but not identical. From the above-mentioned FTIR, powder XRD, and single crystal XRD results, it is confirmed that the Zn-IBA salts are the actual gelators in these organogel systems. Therefore, one could expect that these organogels could also be prepared by using a molar ratio of 2:1 (IBAs:ZnO NPs) instead of using equimolar ratio. To resolve this issue, we have carried the gelation experiment by varying the molar ratio of IBAs and ZnO and found that the organogels are in fact formed at molar ratio of 2:1 (IBAs:ZnO NPs). However, as we have mentioned in the Experimental Section that the equimolar ratio of IBAs and ZnO NPs is excellent for organogelation in terms of lowering the MGC value as well as the sonication time for all three systems. At this moment, we are (34) Menger, F. M.; Yamasaki, Y.; Catlin, K. K.; Nishimi, T. Angew. Chem., Int. Ed. 1995, 34, 585–586. (35) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. 1996, 35, 1324–1326. (36) Crystal data for zinc isobutyrate: C16H28O8Zn2, M = 479.16, monoclinic, space group P21, a = 9.174(2), b = 12.858(3), c = 9.593(3) A˚, V = 1131.5(5) A˚3, T = 150(2) K, Z = 2, unique data: 1902, parameters: 235. The final R1 (all data) and wR2 were 0.0716 and 0.1129, respectively.

DOI: 10.1021/la903923q

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unable to provide proper explanation for such experimental observations. More details study regarding the optimization of molar ratio of these two components is currently underway in our laboratory. In conclusion, in these systems, triggerable organogelations are based on the principle of in-situ coordination complex (Zn-IBAs) formation by the dissolution of ZnO NPs in nonaqueous solution of a simple organic acid such as isobutyric acid or its derivatives through the irradiation of ultrasounds. Ultrasound irradiation triggers fast in-situ formation of Zn-isobutyrate complexes that undergoes fast sonocrystallization into gel fibers. Xerogel fibers were micrometers in length and nanometers (30-65) in diameters. Combinations of single crystal structure analysis and powder XRD result were used to anticipate the molecular packing present in the gel state. Presently, we are exploring the possibility of formation of other such coordination organogels by the proper choice of organic acids and other metal oxide NPs.

6582 DOI: 10.1021/la903923q

Kotal et al.

Acknowledgment. T.K.P. and S.B. thank CSIR, India, for providing fellowships. This research was supported by grants from CSIR, India. We also thank Dr. Sudipta Chatterjee, Dept. of Inorganic Chemistry, IACS, for solving the crystal structure. Crystallography was performed at the National Single Crystal Diffractometer Facility, IACS. Supporting Information Available: List of other carboxylic acids tested for gelation, crystallographic data of zinc isobutyrate salt, variation of sonication time with diameter of ZnO particles, tgel of 2 wt % gel prepared in toluene with different IBAs, variation of tgel with gelator concentration, FESEM images of xerogels of prepared in different solvents, FTIR spectra and peak assignment table of all the gel samples, powder XRD pattern, crystallographic data in CIF format. This material is available free of charge via Internet at http://pubs.acs.org.

Langmuir 2010, 26(9), 6576–6582