16034
J. Phys. Chem. B 2005, 109, 16034-16039
Layered Inorganic-Organic Clay-like Nanocomposites Rearrange To Form Silsesquioxanes on Acid Treatment Guruswamy Kumaraswamy,*,† Yogesh Deshmukh,† Vikrant V. Agrawal,† and P. Rajmohanan‡ Polymer Science and Engineering DiVision, National Chemical Laboratory, Pune 411008, India, and Central NMR Facility, National Chemical Laboratory, Pune, 411008, India ReceiVed: May 20, 2005; In Final Form: July 7, 2005
The formation of talc-like compounds by the condensation of organotrialkoxy silanes with magnesium hydroxide has been recently reported. These represent layered hybrid nanomaterials that have a layer thickness of around 1 nm, have organic moieties covalently linked to the layer surfaces, and are called “organoclays.” We show that such compounds are sensitive to acid treatment. When a phenylclay is treated with hydrochloric acid, magnesium leaches out, destroying the layered structure. The extent to which magnesium is leached out is a function of the time of the acid treatment and the concentration of the acid used. Magnesium leaches out rapidly when the concentration of acid used to treat the phenyl-clay is higher, and the extent of structural magnesium that is leached out is also higher for higher acid concentrations. Removal of the magnesium rearranges the structure of the phenyl-clay to form oligomeric phenylsilsesquioxanes. FTIR and NMR suggest that the silsesquioxanes formed by acid treatment of the phenyl-clay comprise a mixture of ladderlike and cagelike structures.
1. Introduction Organically modified clays find applications in polymerclay nanocomposites1 as well as in wastewater remediation.2 The effectiveness of clays in these applications is a result of their structuresthe layered geometry and the nanometer-size thickness of the individual clay layers provide a large surface area. This is important for reinforcement of the polymer matrix in polymer-clay nanocomposites and for adsorption of effluents in water remediation. Further, the individual nanometer-thick clay layers need to be hydrophobic so that they can exfoliate and disperse in polymers to form nanocomposites and so that they can adsorb organic effluents from water streams. Natural clays are not hydrophobic but can be rendered hydrophobic1 by replacing their exchangeable counterions with organic cationic surfactants. Typically, a water-swellable smectite such as montmorillonite is dispersed in water and treated with an aqueous solution of a quaternary ammonium surfactant such as dimethyl ditallow ammonium chloride. Thus, smectites can be organically modified up to their cation exchange capacity, typically around 1 mequiv/g for montmorillonite. Recently, there has been renewed interest in synthesis of nanostructured inorganic-organic hybrids,3 and a sol-gel condensation route has been reported for the synthesis of organoclay-like layered compounds having a structure analogous to talc. Natural talc, unlike smectite, does not swell in water. Therefore, natural talc cannot be made hydrophobic by surfactant modification and thus cannot be used for wastewater treatment or as nanofillers for polymers. However, organo-talcs that might be suitable for these applications can be prepared by sol-gel condensation. Synthesis of organoclays involves hydrolysis of organotrialkoxy silanes to form organosilanol surfactants that * To whom correspondence should be addressed. Phone: 91-20-25893382. Fax: 91-20-2589-3234. E-mail:
[email protected]. † Polymer Science and Engineering Division. ‡ Central NMR Facility.
organize in solution and condense with magnesium or aluminum ions under ambient4a-d or hydrothermal4e conditions to form layered structures that are analogous to 2:1 phyllosilicates (such as talc or hectorite). The “clays” prepared by this route are hydrophobic due to the presence of the organic group that is covalently bonded to each silicon atom in the compound, rather than due to noncovalent surfactant modification of a parent clay. As each silicon atom in the clay framework is bonded to an organic group, these synthetic organoclays contain a very high organic content. Synthetic clays containing a wide variety of chemical functionalities covalently bonded to the framework silicon have been prepared using this route.4 Recently, Lagadic et al.5 have reported the preparation of a thio-functionalized synthetic clay that shows outstanding capacity for adsorption of heavy metal ions from wastewaters. The exceptional adsorption properties of this clay can be attributed to the layered structure of the clay that provides a large accessible surface area and due to the high number density of thiol groups available for adsorption. Lagadic et al.5 claim that the adsorptive capacity of the thioclay adsorbent that is saturated by heavy metal ions can be regenerated by treatment of the clay with acid. When organoclays are used to prepare nanocomposites by compounding with condensation polymers such as nylons or polyesters, trace amounts of base residual from the synthesis of the clay could catalyze hydrolytic degradation of the polymer, leading to rapid reduction in molecular weight. Therefore, it might be beneficial to completely remove traces of the base by washing the clay with acid. While it appears that acid treatment of organoclays is important for its applications, the effect of acid treatment on the structure of the organoclays has not been reported in detail. It has been suggested4d that synthetic organoclays are sensitive to acidic pH and that the magnesium in the clay layers might be extracted by acid treatment. In 1:1 layered compounds such as chrysotile, it is known that magnesium can be leached out
10.1021/jp0526400 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005
Nanocomposite Rearrangement to Silsesquioxanes by treatment with strong mineral acids.6 Further, it is known that acid treatment of model low molecular weight metallosilsesquioxanes can destroy their cagelike structure.7 Feher et al. have shown that polyhedral cubic silsesquioxanes can be cleaved by the action of acid8a,b or base8c to form incompletely condensed cages. Here we show that treatment of layered, claylike magnesium silicate structures with acid extracts magnesium ions from the clay and destroys the layered structure. Specifically, when a phenyl organoclay is acid-treated, it transforms to form a mixture of phenylsilsesquioxane structures. 2. Experimental Section Materials. Phenyltriethoxysilane [C6H5Si(OC2H5)3] was used as obtained from Sigma-Aldrich. Sodium hydroxide and magnesium chloride hexahydrate were used as obtained from Merck. Distilled, deionized water with a resistivity of 18.2 MΩ‚cm was obtained from a Millipore MilliQ system for the synthesis and washing of clays. Laponite was obtained from Laporte Industries Ltd., USA, and used as received (trade name Laponite-RD). Laponite-RD is sold as a gel-forming clay. Talc was obtained from B. S. Mica Pvt. Ltd., India, and used as received. Synthesis. Phenyl organoclay (Ph clay) was prepared by cocondensing phenyltriethoxysilane with magnesium hydroxide under basic conditions, using a process similar to that described by Ukrainczyk et al.4c Phenyltriethoxysilane (3.244 g, 0.27 mol dm-3) and magnesium chloride hexahydrate (2.03 g, 0.20 mol dm-3) solutions were prepared in ethanol (50 mL) and then mixed together and stirred for 10 min. To this mixture, aqueous sodium hydroxide (0.50 mol dm-3) was added dropwise until the pH reached 11.5. A white precipitate was obtained that was then aged for a day at room temperature and then washed repeatedly with distilled deionized water until the pH of the filtrate became neutral and the filtrate free from chloride ions (confirmed by the addition of silver nitrate solution to the fresh filtrate). Chloride ions are below the level of detection when addition of silver nitrate to the fresh filtrate does not result in precipitation. The white clay precipitate was then air-dried, followed by vacuum-drying at 110 °C for 12 h, and then finally crushed in a mortar to get a fine white powder. Acid treatment of clay samples was done in the wet state, immediately after washing and before the samples were air-/ vacuum-dried. The experimental protocol for acid treatment was as follows: clay (325 mg) was stirred in a solution of HCl (50 mL, 0.55 mol dm-3) for various times. The time of the acid treatment was varied from 2 min to 1 h. All experiments are performed at an acid concentration of 0.55 mol dm-3, except for the investigations of the effect of acid concentration on the kinetics of clay degradation investigated (using HCl concentrations of 0.01, 0.05, 0.10, and 0.50 mol dm-3). After acid treatment, the clay was filtered and washed thrice with distilled deionized water. To determine the magnesium extracted by acid treatment, the acid solution after treating the organoclay was collected and buffered at pH ) 10 using buffer solution (17.5 g of NH4Cl was dissolved in 100 mL of deionized water; 142 mL of ammonia solution (25%) was added to the solution; the mixture was further diluted to 250 mL).9 This was then titrated against ethylenediaminetetraacetic acid (EDTA; 0.01 mol dm-3) in the presence of eriochrome black T indicator. At the end point, the wine red color of the titrate turns green. The amount of magnesium in the filtrate can be calculated from the titration since 1 mL of EDTA (0.01 mol dm-3) corresponds to 0.243 mg of magnesium.9
J. Phys. Chem. B, Vol. 109, No. 33, 2005 16035 3. Characterization X-ray diffraction (XRD) was performed in reflection mode on a Rigaku Dmax 2500 using a rotating anode source and Nifiltered Cu radiation. FTIR was performed at a resolution of 4 cm-1 on a Perkin-Elmer 16PC. 29Si MAS NMR at 59.6 MHz was performed on a Bruker MSL 300 NMR spectrometer equipped with a standard CP/MAS probe using a 7 mm sample rotor. The spinning speed was maintained at 3.5 kHz. 13C CP/ MAS spectral data were collected on a Bruker DRX 500 NMR spectrometer using samples filled in a 4 mm rotor at a spinning speed of 8 kHz. The solution-state 13C NMR measurements were also done on the Bruker DRX 500 NMR spectrometer. The solid-state 13C chemical shifts were referred to the CH2 peak of adamantine taken as 28.7 ppm, whereas the 29Si chemical shifts were referred to neat tetraethyl orthosilicate taken as -82.4 ppm. The central peak of CDCl3 carbon was taken as reference (76.9 ppm) for the solution-state measurement. CHN analysis was performed using Vario EL from Elemental Analyzer GmbH. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was performed at the Sophisticated Analytical Instrument Facility at IIT Mumbai using model 8440 Plasmalab from Labtam, Australia. 4. Results and Discussion The Ph clay shows a mass loss of about 42% when heated between 200 and 600 °C in a thermogravimetric analyzer, indicating a large organic content that compares well with the value expected on the basis of the ideal structural formula (47%). Ph clay has an idealized structural formula4 of Ph8Si8Mg6O16(OH)4, where Ph represents the phenyl group, C6H5. ICP-AES of Ph clay shows that the percentages of Si and Mg in the clay are 10.42 and 16.88%, respectively, significantly different from the theoretically expected values based on the structure given above (Si, 17.1%; Mg, 11.12%). Similar results have been reported earlier in the literature4c and can be attributed to incomplete condensation of phenyltriethoxysilane during synthesis of Ph clay. On the basis of this analysis, we estimate that the approximate level of condensation relative to the maximum possible condensation is 60%. Thus, the Ph clay synthesized has structural imperfections and the magnesium hydroxide layers are incompletely condensed with the phenyltriethoxysilane added during organoclay formation. CHN analysis gave percentages of carbon (41.74%) and hydrogen (3.77%) present in the Ph clay. Ph clay does not disperse to form a clear solution in organic solvents such as toluene or choloroform. Attempts at preparing suitable samples for transmission electron microscopy (TEM) by sonication of Ph clay in methanol/water mixtures were unsuccessful, and we could not prepare samples that were appropriately thin for TEM. Therefore, it was not possible for us to use TEM to estimate the lateral extent of the platelets. However, scanning electron microscopy (SEM) of the organoclay as prepared showed platelike aggregates (see Supporting Information). The phyllosilicate-like layered structure of the Ph clay is confirmed by the 001 peak and the intralayer 110, 020; 130, 200; and 060, 330 peaks in the X-ray diffraction pattern near 2θ values of 10, 20, 40, and 60°, respectively (XRD, Figure 1a; peaks indexed by comparison with those observed in the corresponding natural clay10). As observed previously,4 the peaks for the Ph clay are much broader than those observed for natural clays, indicating considerable structural disorder and/or a small lateral extent of the platelets in the organoclay. Specifically, the 110, 020 peak is either very broad or is significantly
16036 J. Phys. Chem. B, Vol. 109, No. 33, 2005
Kumaraswamy et al. SCHEME 1: Effect of Acid Treatment of Layered Ph Claysa
Figure 1. XRD from the (a) Ph organoclay, (b) acid-treated Ph organoclay, and (c) phenylsilsesquioxane. The arrows around 40 and 60°, 2θ represent the 130/200 and 060/330 intralayer peaks, respectively.
a The magnesium is extracted from the clays and a variety of completely and incompletely condensed phenylsilsesquioxanes are formed.
Figure 2. FTIR of (i) Ph clay, (ii) acid-treated Ph clay, and (iii) phenylsilsesquioxane synthesized by condensation of phenyltriethoxysilane.
overlapped by a broad amorphous humpsin either case, this reflects the structural disorder in these materials. While Ph clay represents an incompletely condensed structure, the magnesium hydroxide is not present as brucite since XRD peaks characteristic of brucite are not observed in the XRD of the phenyl organoclay. For Ph clay, we observe an unusual increase in scattered intensity at low 2θ (Figure 1, top curve). This upturn at low 2θ nearly disappears when the organoclay is wetted with a small amount of toluene (data not shown; while the organoclay does not disperse in organic solvents such as toluene or chloroform, it is wetted by them). An upturn in the X-ray scattered intensity at low 2θ can be due to scattering from the colloidal11 clay particles or from voids present between the clay particles.12 Scattering from microcracks or voids is known to increase intensity at low 2θ in polymers,12a,b,e and it is known that the low 2θ upturn can be decreased by decreasing the contrast13 between the matrix particles and the voids. In our experiments, the difference in electron density contrast for the clay versus air is decreased when the solvent wets the clay particles and displaces air to fill the interparticle spaces (analogous to “contrast matching” experiments13). Therefore, the upturn that is visible for the dry particulate clay sample decreases and is nearly eliminated when the clay is wetted with solvent. The upturn in scattered X-ray intensity is not observed for clays in generalswe speculate that it is observed for organoclays due to the small size of the clay particles. We first discuss how treatment of the organoclay with concentrated acid destroys the layered structure, then discuss the products that are formed on acid treatment of Ph clay, and,
Figure 3. GPC elution trace (RI detector) of acid-treated Ph clay. Note that the data are presented without normalization of the RI signal by molecular mass to clearly show the signal at high molecular mass.
finally, examine the kinetics of the change in the structure of the organoclay at various acid concentrations. The intralayer clay peaks for the Ph clay indicated by the arrows in Figure 1a disappear when the organoclay is treated with 0.55 mol dm-3 HCl for 2 min (Figure 1b). Titration indicates that the magnesium in the filtrate was 16.8 wt % of the original organoclay, in accord with the magnesium content determined using ICP-AES (16.88 wt %). The percentage of magnesium that is removed at the end of “complete” extraction depends on the degree of condensation of the brucite-like layers with the added silane and varies within (2-3% from batch to batch of prepared organoclays. Further acid washing of the Ph clay does not yield any further magnesium, indicating complete removal of the magnesium from the organoclay after the first acid wash. Complete removal of magnesium from Ph clay is further confirmed by the disappearance of the MgO-H stretching band4b,14 in the IR spectrum (3702 cm-1, Figure 2b) in the acid-treated compound. ICP-AES analysis of the acid-treated compound also shows that the magnesium has been almost entirely extracted (0.12 wt % Mg residual after acid treatment; silicon-to-magnesium ratio of over 110). We have investigated the effect of acid treating a synthetic hectorite, Laponite using the same acid concentration, to compare with our results on the Ph clay. We selected Laponite
Nanocomposite Rearrangement to Silsesquioxanes
J. Phys. Chem. B, Vol. 109, No. 33, 2005 16037
Figure 4. 59.6 MHz 29Si MAS spectra of the Ph clay (a) after acid treatment and (b) before acid treatment.
since it is a synthetic clay that has a small platelet size of around 25 nm.15 In this case, too, we find that the intralayer peaks for the phyllosilicate disappear after acid treatment (using 0.55 mol dm-3 hydrochloric acid), and the magnesium is extracted from the layer structure by the acid. After prolonged acid treatment of Laponite, the 001 peak disappears (data in Supporting Information), and we observe a broad hump characteristic of amorphous silica (obtained, for example, by condensation of tetraethyl orthosilicate using aqueous base). The time taken for the intralayer clay peaks to disappear in Laponite is longer than for Ph clay. This could be due to the greater structural perfection of the Laponite compared to the Ph clay and/or also due to the larger lateral size of the Laponite platelets as compared to the Ph clay. When we treat natural talc with acid, there is no change in the structure of the clay observable by XRD, even after treatment with concentrated acid (3 mol dm-3 HCl) for a prolonged duration (7 days; see data in Supporting information). We feel that the stability of the talc to acid treatment is primarily due to the large lateral size of the talc platelets, and perhaps also due to nonswelling nature of the clay that prevents the acid from efficiently contacting the clay surface. The XRD pattern for the acid-treated phenyl organoclay (Figure 1b) is virtually identical to that for polyphenylsilsesquioxane (Figure 1c) synthesized by condensation of triethoxyphenylsilane in ethanol by addition of aqueous base. Interestingly, the strong upturn observed for Ph clay at low 2θ disappears on acid treatment, suggesting that the acid-treated compound is a homogeneous material showing no variation in electron density at a length scale of tens of nanometers. Neither polyphenylsilsesquioxane nor acid-treated Ph clay show the upturn at low 2θ. Similar to polyphenylsilsesquioxane, the acidtreated Ph clay has a peak at slightly higher 2θ value as compared to the 001 peak for the organoclay (d spacing: 1.30 nm for the Ph clay, 1.21 nm for the acid treated compound; Figure 1a,b). The two XRD peaks for crystalline polyphenylsilsesquioxane have been interpreted in the literature16,17 as characteristic of the mean intramolecular chain-chain distance (intense peak at 2θ ≈ 7.3°, d ≈ 1.20 nm) and of the mean repeat distance of chains (broad peak at 2θ ≈ 19.2°, d ≈ 0.46 nm). The acid-treated compound comprises a mixture of ladderlike and cagelike polyphenylsilsesquioxanes (Scheme 1). Unambigu-
ous interpretation of the structure of polyphenylsilsesquioxanes is difficult and has led to controversies in the literature.18 The first reports on polyphenylsilsesquioxanes by Brown16 claimed a ladder-like structure of cis-syndiotactic linked units based on a consideration of bond angle, XRD, and IR18,19 data. This interpretation was subsequently questioned20 and it was claimed that polyphenylsilsesquioxanes were comprised of randomly linked polycyclic siloxane rings. FTIR of Ph clay, acid-treated Ph clay and the polyphenylsilsesquioxane show phenyl ring modes19 at 998 and 1026-1028 cm-1, and two absorption bands in the 1000-1200 cm-1 region corresponding to Si-O-Si vibrations.19 Structurally perfect symmetric Tn cage structures show18 only a single absorption around 1130 cm-1. Structurally regular clays such as talc or Laponite also show only a single strong absorption band in the 1000-1200 cm-1 region. The two bands observed in Figure 2 a suggest17,18,21 that Ph clay is structurally imperfect and that on acid treatment it rearranges to form a mixture of incompletely condensed cages and ladderlike structures. Interestingly, acid-treated Ph clay shows more pronounced IR absorptions at 870 and 910 cm-1 as compared to polyphenylsilsesquioxane. This suggests21 that Tn cages formed by acid-treated Ph clay are more structurally regular as compared to those in polyphenysilsesquioxanes synthesized by direct condensation of phenyltriethoxysilane. The acid-treated Ph clay is soluble in organic solvents such as toluene and chloroform, confirming16,18 the change in structure from organoclay to polyphenylsilsesquioxane. The gel permeation chromatography (GPC) trace of a solution of the acid treated compound in chloroform is multimodal, as would be expected for a mixture of polyphenylsilsesquioxanes (Figure 3). The molecular weights obtained for the acid-treated Ph clay are low, typically below 10 000 based on polystyrene standards. The multiple peaks in the GPC could result from elution of various Tn and incomplete cagelike or ladderlike structures. Phenyl-T8 has a molecular weight of about 1000, and ladderlike molecules with molecular weights up to 100 000 have been reported.18 The ladderlike polyphenylsilsesquioxanes are thought to be rigidstherefore, it is not appropriate to infer molecular weights based on PS standards. However, in the absence of other suitable standards, we report our GPC data in terms of PS standards. Polyphenylsilsesquioxanes synthesized by condensa-
16038 J. Phys. Chem. B, Vol. 109, No. 33, 2005
Kumaraswamy et al.
Figure 6. Kinetics of acid extraction of magnesium from the layered Ph clays at various concentration of hydrochloric acid. The time axis represents the time for which the Ph clay is acid-treated, while the legend indicates the concentration of acid used (0.01 mol dm-3, filled squares; 0.05 mol dm-3, filled diamonds; 0.10 mol dm-3, open triangles; 0.50 mol dm-3, open circles). Magnesium removed is reported as a percentage of the original weight of the organoclay. All experiments to study effects of acid concentration on kinetics of magnesium extraction were done on the same batch of Ph claystherefore, the maximum magnesium removal is comparable for all experiments (around 13-14%). Note that there is a 2-3% variation in the maximum Mg removed from batch-to-batch of the Ph clay.
Figure 5. 125 MHz 13C CP/MAS spectra of Ph-clay (a) after acid treatment and (b) before acid treatment. Comparison of the region of isotropic peaks of acid treated (lower spectrum) and untreated Ph clay (upper spectrum) is shown in (c).
tion of phenyltriethoxysilanes are also soluble in toluene and chloroform and have molecular weights that are comparable to the acid-treated Ph clay (data not shown). Interestingly, similar to polyphenylsilsesquioxanes, the acid-treated Ph clay shows a softening transition as it is heated to around 200 °C (dynamical mechanical experiments show a dramatic increase in tan δ ) G′′/G′ as the sample is heated through the softening point; data not shown), and the acid-treated compound flows to form a brittle, transparent film. Solid-state 29Si NMR for the Ph clay, the acid-treated Ph clay (Figure 4a,b), and the polyphenylsilsesquioxane (data not presented) show a strong peak around -79 ppm and a broad peak in the region of 60-70 ppm. The former peak can be assigned to the T3 type of silicon in the Ph clay while the latter corresponds to the T2 type. No peaks are observed in the region of 95-110 ppm (the “Q”-type peaks), indicating that the Si-C bond is intact in both Ph clay and the acid-treated compound. It is interesting to note that the relative proportion of the T2 environments is higher in the acid-treated Ph clay (Figure 4), clearly indicating a decrease in the degree of condensation. This suggests the presence of incompletely condensed polyphenyl-
silsesquioxane structures indicated in Scheme 1. The solid-state 13C CP/MAS spectra also show changes on the removal of magnesium from the layered Ph clay structure (Figure 5). The acid-treated sample shows only three signals at ∼133, 130, and 127 ppm that can be assigned respectively to the carbons at ortho, para, and meta positions to the substituent (Figure 5a). The carbon to which the silicon is attached is not seen separately. The intensity of this signal is expected to be weak and hence may be merged with the low field signal. The 13C spectrum of the Ph clay prior to the acid treatment is shown in Figure 5b for comparison. The differences between parts a and b of Figure 5 can be seen when the region of isotropic signals is magnified for comparison (Figure 5c). The signal at ∼130 ppm is found to be broader for the Ph clay relative to the acid-treated compound, and a broad shoulder can also be seen at ∼138 ppm, which may be due to the carbon attached to the Si atom. These differences after acid treatment are most likely due to the removal of magnesium from the compound. Solution spectra of the Ph clay in deuterated chloroform provide better resolved data that accords with the solid-state spectra (see Supporting information). Thus, solid-state and solution NMR taken together support the structure proposed in Scheme 1. The extraction of magnesium from Ph clays is a function of the acid concentration. Using 0.01 mol dm-3 hydrochloric acid, even prolonged acid treatment is incapable of completely removing all the framework magnesium from the Ph clay, but higher concentrations of acid (greater than about 0.10 mol dm-3) completely extract all the magnesium (Figure 6). The extraction of magnesium is more rapid at higher acid concentrations, and within 2-5 min of acid treatment, all the magnesium is extracted (Figures 1, 6). In fact, aggressive acid treatment using mineral acids such as hydrochloric acid is not required to extract magnesium from hectorites or organotalc analogues. Even acidification by dissolution of carbon dioxide from the atmosphere to form carbonic acid has been reported22 to be sufficient to extract magnesium from stored aqueous solutions of hectorites such as Laponite. The ionic strength of aqueous solutions of Laponite and organotalcs that are exposed to the atmosphere increases with time over a period of months due to the dissolution of the Laponites and the extraction of magnesium.
Nanocomposite Rearrangement to Silsesquioxanes The effect of carbon dioxide dissolution on relatively less stable organoinorganic clay analogues such as Ph clay is even more severe. When distilled, deionized water (at an initial pH of 7) containing Ph clay is left exposed to the atmosphere for about 10 days, we observe that about 4% of the magnesium is leached out. Therefore, hectorites and, especially, organoinorganic clay analogues are stable only when their solutions are buffered at basic pH. Talc and its organomodified analogues such as Ph clay can be considered as condensation products formed by acidic sheets of silicic acid and basic sheets of magnesium hydroxide (brucite). Therefore, we can conceive that the alkali metal hydroxides or silica can be leached out by controlled treatment of phyllosilicates with strong acid or base, respectively. Recently, Choy et al.23 have reported a method of reacting micalike phyllosilicates with strong base to leach the silicate sheets and effect selective topochemical transformations to chlorite or brucite. Hydrolysis of oligo(phenylsilsesquioxane) to form cyclic tetramers has been reported in the literature.24 Shchegolikhina et al.24 have reported that the Si-O-Si bonds in oligo(phenylsilsesquioxanes) can be cleaved by strong base in alcohol to form a tetrameric salt of phenylsiloxanolate. The acid treatment reported in our work too results in alteration of the layered structure of the organoclay; however, in our case, removal of the framework magnesium destroys the layered structure and, the presence of the organic substituents on the silicon atom results in transformation of the layered structure to form a mixture of cagelike and ladderlike oligomeric phenylsilsesquioxanes. The effect of this structural transformation is dramaticsin experiments using thio-functionalized clays similar to those synthesized by Lagadic et al.,5 we observe, after acid treatment, the efficiency of the clay for heavy metal ion removal decreases dramatically. Thus, contrary to literature reports,5 acid treatment cannot be used to regenerate the adsorption capacity of an organoclay. 5. Conclusions Acid treatment of organoclay talc analogues result in the extraction of magnesium from the framework of the layered hybrid. The extraction of the magnesium is a function of the acid concentration. While the framework magnesium is extracted even at low acid concentrations (lower than about 0.10 mol dm-3 hydrochloric acid), even prolonged treatment at these concentrations is incapable of complete extraction of magnesium from Ph clay. However, at higher acid concentrations, the kinetics of magnesium extraction is rapid, and almost all the framework magnesium is extracted. After complete extraction of the magnesium, the presence of the organic group bonded to the silicon results in rearrangement of the layered compound to form oligomeric silsesquioxanes with a variety of structuress including ladderlike and cagelike structures. Acknowledgment. We acknowledge Dr. C. Ramesh for use of the X-ray facility, Prof. Anil Kumar, and Sarada P. Mishra (IITB) for help with GPC. We also acknowledge Dr. Sujata S. Biswas (NCL) for help with CHN analysis. We are grateful to Prof. R. Murugavel (IITB) for helpful discussions on the pH sensitivity of metallosilsesquioxanes. This work was initiated
J. Phys. Chem. B, Vol. 109, No. 33, 2005 16039 after a vigorous debate with Dr. S. Charati, Dr. S. Bandyophadhyay, and Dr. S. Rajagopalan on the structure of the acid-treated organoclays. We are grateful to them for stimulating this work. Chandrashekar V. Kulkarni is acknowledged for preliminary experiments on Ph clays. Supporting Information Available: SEM of the Ph clay showing platelike aggregates, XRD of Laponite and natural talc before and after treatment with hydrochloric acid (the layered structure of the Laponite is destroyed by acid treatment, and prolonged acid treatment results in the formation of an amorphous material; the structure of talc is not affected even by prolonged acid treatment with 3 mol dm-3 hydrochloric acid), and the proton-decoupled 13C and DEPT spectrum of the acidtreated Ph clay. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: Amsterdam, 1979. (b) Gianellis, E. P. AdV. Mater. 1996, 8, 2935. (2) Mercier, L.; Detellier, C. EnViron. Sci. Technol. 1995, 29, 1318. (3) (a) Kuroda, F.; Shimojima, A.; Kuroda, K. Chem. Mater. 2003, 15, 4768. (b) Shea, K. J.; Loy, D. A. Chem. Mater. 2001, 13, 3306. (4) (a) Fukushima, Y.; Tani, M. J. Chem. Soc., Chem. Commun. 1995, 241. (b) Burkett, S. L.; Press: A.; Mann, S. Chem. Mater. 1997, 9, 1071. (c) Ukrainczyk, L.; Bellman, R. A.; Anderson, A. B. J. Phys. Chem. B 1997, 101, 531. (d) Silva, C. R.; da Fonseca, M. G.; Barone J. S.; Airoldi, C. Chem. Mater. 2002, 14, 175. (e) Carrado, K. A.; Xu, L.; Csencsits, R.; Muntean, J. V. Chem. Mater. 2001, 13, 3766. (5) Lagadic, I. L.; Mitchell, M. K.; Payne, B. D. EnViron. Sci. Technol. 2001, 35, 984. (6) (a) Fonseca, M. G.; Oliviera, A. S.; Airoldi, C. J. Colloid. Interface Sci. 2001, 240, 533. (b) Wypych, F.; Schreiner, W. H.; Mattoso, N.; Mosca, D. H.; Marangoni, R.; da S. Bento, C. A. J. Mater. Chem. 2003, 13, 304. (7) (a) Murugavel, R.; Voigt, A.; Walawalkar M. G.; Roesky, H. W. Chem. ReV. 1996, 96, 2205. (b) Murugavel, R. Personal communication. (8) (a) Feher, F. J.; Soulivong D.; Eklund, A. G. Chem. Commun. 1998, 399. (b) Feher, F. J.; Soulivong D.; Nguyen F. Chem. Commun. 1998, 1279. (c) Feher, F. J.; Terroba, R.; Ziller, J. W. Chem. Commun. 1999, 2309. (9) Vogel A. I. Textbook of QuantitatiVe Inorganic Analysis, 4th ed.; Longman Group: London, 1978; p 321. (10) Brindley, G. W., Brown, G., Eds. Crystal Structures of Clay Minerals and Their X-ray Identification; Mineralogical Society Monograph No. 5; Mineralogical Society: London, 1984; pp 330-356 (11) Winans, R. E.; Thiyagarajan, P. Energy Fuels 1988, 2, 356. (12) (a) Wu, W.-L. Polymer 1982, 23, 1907. (b) Steger. T. S.; Nielsen, L. E. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 713. (13) Hedden, R. C.; Lee, H.-J.; Bauer, B. J. Langmuir 2004, 20, 416. (14) Kermarec, M.; Carriat, J. Y.; Burattin, P.; Che, M.; Decarreau, A. J. Phys. Chem. 1994, 98, 12008. (15) Balnois, E.; Durand-Vidal, S.; Levitz, P. Langmuir 2003, 19, 6633. (16) (a) Brown, J. F., Jr.; Vogt, L. H., Jr.; Katchman, A.; Eustance, J. W.; Kiser, K. M.; Krantz, K. W. J. Am. Chem. Soc. 1960, 82, 6194. (b) Liu, C.; Liu, Y.; Shen, Z.; Xie, P.; Zhang, R.; Yang, J.; Bai, F. Macromol. Chem. Phys. 2001, 202, 1581. (17) Prado, L. A. S.; Radovanovic, E.; Pastore, H. O.; Yoshida, I. V. P.; Torriani, I. L. J. Polym. Sci., Part B: Polym. Chem. 2000, 38, 1580. (18) Baney, R. H.; Itoh, M.; Sakakibara, A.; Suzuki, T. Chem. ReV. 1995, 95, 1409. (19) Brown, J. F., Jr.; Vogt, L. H., Jr.; Prescott, P. I. J. Am. Chem. Soc. 1964, 86, 1120. (20) Frye, C. L.; Klosowski, J. M. J. Am. Chem. Soc. 1971, 93, 4599. (21) Frye, C. L.; Collins, W. T. J. Am. Chem. Soc. 1970, 92, 5586. (22) Mourchid, A.; Levitz, P. Phys. ReV. E 1998, 57, R4887. (23) Choy, J.-H.; Lee, S.-R.; Park, M.; Park, G.-S. Chem. Mater. 2004, 16, 3206. (24) Shchegolinkhina, O.; Pozdniakova, Y.; Antipin, M. Organometallics 2000, 19, 1077.
