Ultrasound-Induced In Situ Formation of Coordination Organogels

Ultrasound-Induced In Situ Formation of Coordination Organogels

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DOI: 10.1021/la903923q ALangmuir X, X(X), X–X pubs.acs.org/Langmuir ©XXXX American Chemical Society

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.


Recently, there has been extensive research on the stimuliresponsive assembly of low-molecular-weight compounds.1-7 Themajoraimistocreatenewtechnologiesfortheprecisecontrol 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.8-1 In general, LMGs gels, no matter howtheyareprepared,areofgreatinterestduetothesimplicityof 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-bromoisobutyricacid(BIBA),withZnONPs. Recentreports show that gel can act as templates for generation of metal oxides NPs.2,27 However, to date, this is probably a very rare

*Corresponding author: Fax þ91-3-2473 2805; e-mail psutkm@ mahendra.iacs.res.in. (1) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.;

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B DOI: 10.1021/la903923q Langmuir X, X(X), X–X

Article Kotal et al.

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

Experimental Section

Materials. 2-Methylisobutyricacid(MIBA) (9%),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)236H2O] 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 Cf ollowed by dropwise addition of 92 mL of 34.65 mmol (1.944 g) methanolic potassium hydroxide solution within 15 min. Then thereactionmixturewasstirredcontinuouslyforanother2.25hat 60 C, and then it was cooled to room temperature. The asprepared ZnO nanoparticles were centrifuged at 15000 rpm (21000g) for 20min,washed with methanol and waterrepeatedly to remove unreacted compounds, and then heated at 100 Cf or 24 h in a vacuum oven to obtain a white powder (1.3 g). The average diameter of the synthesized ZnO NPs was ∼9n ma s 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,toprepare a0.5 wt%2-bromoisobutyricacid(BIBA) gel, 10mg(0.0598mmol)ofBIBAwasweighedinascrew-cappedvial, and then 2 mL of freshly distilled toluene was added to the vial to get the homogeneous solution of the acid. 4.87 mg (0.0598mmol) of zinc oxide nanoparticles was then added to this solution and sonicatedthoroughlyatroomtemperatureinaCole-Parmer8891 sonicator (42 kHz) until gelation.

Ratio (Isobutyric acids/ZnO NPs) Optimizations for

Gelation Experiment. To optimize the molar ratio between different isobutyricacids and zincoxide 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 methacrylateasreportedelsewhere.30Briefly, 0.54mL ofisobutyric acid was dissolved in 2 mL of water, and then 0.27 g of zinc carbonate was added portionwise uponvigorousvortexing. 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)andzinc2-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 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 filteredimmediately,andthefiltratewasdividedintwoparts.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 samplesaswellasallthexerogelswithKBrin1:100(w/w)ratiosin 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-I 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 = 0.71073 A ), and were corrected for Lorentzpolarization effects. A total of 10011 reflections were collected, ofwhich1902wereunique(Rint=0.0459),satisfyingthe(I>2σ(I)) criterion,andwereusedin subsequentanalysis.

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 ∼9nm)28andisobutyricacids(IBA/MIBA/BIBA)weredispersed in various organic solvents (Scheme 1, left vial). When these sols were irradiated with ultrasound (42 kHz) for a period of 20 s to

Scheme 1

(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, 115,1 19–124. (31) Farrugia, L. J. WinGX version 1.70.01 ed., Department of Chemistry, University of Glasgow, 2005.

DOI: 10.1021/la903923q CLangmuir X, X(X), X–X

Kotal et al. Article

10min(dependingontypeofgelatorsandtheirconcentrations)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

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