Arrangements of Interlayer Quaternary Ammonium Ions in a Layered

Publication Date (Web): April 1, 2010. Copyright © 2010 American Chemical Society .... Tomohiko Okada , Yusuke Ide , Makoto Ogawa. Chemistry - An Asi...
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DOI: 10.1021/cg900922v

Arrangements of Interlayer Quaternary Ammonium Ions in a Layered Silicate, Octosilicate

2010, Vol. 10 2068–2072

Makoto Ogawa* and Daisuke Iwata Department of Earth Sciences and Graduate School of Creative Science and Engineering, Waseda University, Nishiwaseda 1, Shinjuku-ku, Tokyo 169-8050, Japan Received August 5, 2009; Revised Manuscript Received March 23, 2010

ABSTRACT: The ion exchange of octosilicate with a series of quaternary organoammonium ions was examined. The ion exchange proceeds almost quantitatively to replace the interlayer sodium ions. When n-alkyltrimethylammonium ions were exchanged with sodium ions, the basal spacings increased with the alkyl chain length, showing the formation of paraffin type surfactant aggregates in the interlayer space. When the organic cations with complex molecular geometry (dimethyldioctadecylammonium, N-(R-trimethylammonioacetyl)-didodecyl-L-glutamate, and O,O0 ,O00 -tridodecanoyl-N-(ω-trimethyl ammoniodecanoyl)tris(hydroxymethyl)aminomethane) were used, the basal spacings changed as a function of the molecular weight of the intercalated guest species. It is speculated that the molecular geometries of the intercalated species are flexible to form densely packed nanostructures in the interlayer space.

Introduction Self-organization of small objects such as organic molecules and nanoparticles with periodic structure is a topic of current interest. Among them, self-assembly of amphiphilic molecules (ions) has been extensively investigated due to the technological importance of surfactants as well as from the viewpoints of the structural relationship of surfactant aggregates and biomembranes.1 Basic understandings have been derived from the numerous research on the structures of surfactant assemblies in solutions of different concentrations, temperatures, pH, etc. Besides the studies on the surfactant aggregation in solutions, concentration of surfactant at the interface and the structure have also attracted a wide range of scientific and practical interests. Organization of surfactant in the interlayer nanospaces of layered solids has been investigated so far.2-4 The hybrid assemblies of smectite clays and cationic surfactants are a well-known example of this type, and their applications as rheology controlling agents, polymer additives, adsorbents, and supports of catalytic and electro/optically active sites have been documented so far.2,5,6 In addition, the ion exchange with a series of n-alkylammonium ions (of the type RNH3þ) has been utilized to estimate the layer charge density and the internal surface area of smectites and other layered materials.3-7 In the case of smectites, jumps were observed in the relationships between the basal spacing and the size of intercalated n-alkylammonium ions, which indicate monolayer to bilayer and bilayer to pseudotrimolecular layer transitions of intercalated n-alkylammonium ion’s arrangements as shown in Figure 1. When layered solids with the layer charge density higher than smectites were used, the n-alkylammonium ions tend to form paraffin type aggregates in the interlayer space, so that the basal spacing increased with the alkyl chain length and the inclination of the increase in the basal spacing as a function of the alkyl chain length reflects the layer charge density.3 Thus, the organization of *To whom correspondence should be addressed. E-mail: makoto@ waseda.jp. pubs.acs.org/crystal

Published on Web 04/01/2010

Figure 1. Schematic model of the arrangements of the intercalated alkylammonium caions in layered solids.

n-alkylammonium ions is a way to form well-structured assemblies and the structures are imaged from the X-ray diffraction results. On the other hand, the nanostructures of the intercalation compounds are difficult to expect when organic cations with complex molecular geometry were used. In the present study, we examined the ion exchange of a layered alkali silicate, octosilicate, with several bulky organoammonium ions to see how the nanostructures vary depending on the interlayer cation. Layered alkali silicates such as magadiite (the ideal formula is Na2Si14O29 3 10H2O), kenyaite (Na2Si22O45 3 11H2O), and octosilicate (Na8Si32O64[OH]8 3 32H2O) are a class of layered solids capable of incorporating guest species in the interlayer space to form intercalation compounds.8,9 The intercalation of organoammonium ions10 and polar molecules11 as well as the preparation of the organosilane grafted derivatives12-24 of layered alkali silicates have been reported previously. Besides the fundamental study on the intercalation chemistry of layered alkali silicates,8-11,25,26 the possible applications of organoammonium-silicates to silica-pillared materials,27-30 polymer-silicate nanocomposites31-33 and adsorbent for environmental purification34 have been reported so far. Besides the simple organoammonium ions, the intercalation of cationic dyes with the quarternary ammonium group has r 2010 American Chemical Society

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Scheme 1. Molecular Structure of N-(R-Trimethylammonioacetyl)-didodecyl-L-glutamate (abbreviated as DCC)

been reported.35-37 Photoresponsive change in the basal spacing of an cationic azeobenzene intercalated magadiite has been reported so far.35,36 Compared with smectite,4-6 layered alkali silicates possesses some unique properties for organizing guest species. (1) The cation exchange capacity (calculated values from the chemical formula are 200 and 290 meq/100 g of silicate for magadiite and octosilicate, respectively) is expected to be higher than that (ca. 50100 meq/100 g of clay) of smectite. (2) It can be conveniently prepared in the laboratory by hydrothermal synthesis. The intercalation of guest species is not easy if it is compared with that into smectite probably due to the higher layer charge density, so that the studies on the preparation of intercalation compounds from layered alkali silicates have been limited. In order to introduce bulky organic species, organoammoniumexchanged forms, which have been prepared by conventional ion exchange reactions in aqueous media, were used as the intermediates. By utilizing the intermediate route, cationic organic dye38 and organometallic cation39 have successfully been intercalated into the interlayer space of magadiite. It has been reported that the presence of a crown ether promotes the cation exchange of magadiite.40,41 Despite successful reports on the intercalation of organic cations, the systematic understanding of the nanostructures of the intercalation compounds is still lacking. Among available layered alkali silicates, octosilicate is characterized by the thin silicate layer (the thickness is less than 1 nm) which directly correlates with the high CEC (the CEC value derived from the chemical formula is 290 meq/ 100 g of silicate) and well-shaped (square) platy particle’s morphology.42,43 These characteristics of octosilicate are attractive for the applications as fillers, adsorbents, and hostguest chemistry. In addition, the ion exchange properties of octosilicate have scarcely been investigated if compared with other layered alkali silicates (magadiite, kenyaite, and kanemite). 22,23,43,44 Therefore, the studies on their ion exchange are worth conducting. Experimental Section Materials. Fumed silica (Particle size of 7 nm) was purchased from SIGMA. Sodium hydroxide was obtained from Kanto Kagaku Ind. Co. Dodecyltrimethylammonium chloride (abbreviated as C12TMACl), hexadecyltrimethylammonium chloride (abbreviated as C16TMACl), octadecyltrimethylammonium chloride (abbreviated as C18TMACl), and dimethyldioctadecylammonium chloride (2C18NCl) were purchased from Tokyo Kasei Co. Behenyltrimetylammonium chloride (abbreviated as C22TMACl) was donated from Daiichi Kougyo Seiyaku Co. N-(R-Trimethylammonioacetyl)-didodecyl-L-glutamate (abbreviated as DCC, Scheme 1), O,O0 ,O00 -tridodecanoyl-N-(ω-trimethyl ammoniodecanoyl)-tris(hydroxymethyl)aminomethane (abbreviated as TCC, Scheme 2) were products of Sogo Yakko Co. All the chemicals were reagent grade used as received. Sample Preparation. Na-octosilicate was prepared by a method reported previously by Endo et al.22 Fumed silica (25 g), sodium hydroxide (8.32 g), and distilled water (48.3 g) were mixed (the molar ratio of SiO2:NaOH:H2O = 4:1:25.8), and the mixture was sealed in a Teflon-lined autoclave (Taiatsu Glass Co., TAF-SR-100,

Scheme 2. Molecular Structure of O,O0 ,O00 -TridodecanoylN-(ω-trimethyl ammoniodecanoyl)-tris(hydroxymethyl)aminomethane (abbreviated as TCC)

volume of 100 mL) and was treated hydrothermally at 100 °C for 4 weeks. The product was collected by centrifugation and dried at 40 °C for 2 days. Na-octosilicate powder thus obtained was characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), thermogravimetric-differential thermal analysis (TG-DTA), and scanning electron microscopy (SEM). As shown in Figure 2a, the basal spacing was 1.12 nm and the pattern coincided with that of the previous report on the structural characterization of octosilicate.42 Diffractions due to impurities were not detected. Dodecyltrimethylammonium chloride (C12TMACl), hexadecyltrimethylammonium chloride (C16TMACl), octadecyltrimethylammonium chloride (C18TMACl), behenyltrimetylammonium chloride (C22TMACl), dimethyldioctadecylammonium chloride (2C18NCl), N-(R-trimethylammonioacetyl)-didodecyl-L-glutamate (abbreviated as DCC, Scheme 1), and O,O0 ,O00 -tridodecanoylN-(ω-trimethyl ammoniodecanoyl)-tris(hydroxymethyl)aminomethane (abbreviated as TCC, Scheme 2) were used for the preparation of intercalation compounds. Na-octosilicate (100 mg) powder was dispersed in an aqueous solution (50 mL) of organoammonium salts (the amount is equal to the amount of sodium in the 100 mg of Na-octosilicate) and the mixture was mixed with a magnetic stirrer at room temperature for 1-2 days. Then, the solid product was separated by centrifugation and was washed with deionized water. Finally, the powder was dried under a reduced pressure (rotary pump was used) at room temperature. For the ion exchange of TCC, repeated ion exchange procedure and heating the solution at 60 °C were examined in order to achieve the quantitative ion exchange. Characterization. X-ray powder diffraction patterns were obtained on a Rigaku RAD IIB diffractometer using monochromatic Cu KR radiation operated at 40 kV and 20 mA. TG-DTA was recorded on a Rigaku Thermo Plus 2 TG8120 instrument at a heating rate of 10 °C min-1 using R-alumina (R-Al2O3) as the standard material. IR spectra were recorded on a Shimadzu FT-8200 instrument by KBr disk method. Scanning electron micrographs were obtained on a HITACHI S-2380N scanning electron microscope for the samples coated with Au (20 nm). CHN analysis was performed on a Perkin-Elmer 2400 II instrument.

Results and Discussion The IR spectra of the products showed the peaks due to C-H stretching at 3650-3660 cm-1, and the TG curves showed weight losses accompanied by the exothermic peaks in the corresponding DTA curves, confirming the introduction of the CnTMA between the layers. On the basis of the TG results of the products, the exchanged amounts of CnTMA were estimated to be 2.03, 1.45, and 1.76 mol of CnTMA per Si8O17 unit of octosilicate for C16TMA, C18TMA, and C22TMA, respectively. As a typical example of the TG-DTA curves of the CnTMA-octosilicates, the TG-DTA curves of the

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Figure 2. TG-DTA curves of the C16TMA-octosilicate. Table 1. The Exchanged Amounts of CnTMA and the Basal Spacings of the CnTMA-Octosilicates alkyl ammonium C12TMA C14TMA C16TMA C18TMA C22TMA

exchanged amounts/mol per Si8O17

basal spacing/nm

2.03 1.45 1.76

2.37 2.54 2.74 3.01 3.31

Figure 3. XRD patterns of (a) Na-octosilicate, (b) C12TMA-octosilicate, (c) C16TMA-octosilicate, (d) C18TMA-octosilicate, and (e) C22TMA-octosilicate. Inset: the variation of the basal spacing as a function of the number of carbon atoms in the alkyl chain.

C18TMA-octosilicate are shown in Figure 2. Table 1 lists the exchanged amounts of CnTMA and the basal spacings of the CnTMA-octosilicates. The ideal value of the exchanged amount is 2.0, while the values are slightly variable in the present study as well as in the previous publication.22,23,44 Although it is difficult to explain the variation (from 1.45 to 2.03 observed in the present study) clearly, the presence of the unexchanged phases (sodium form as well as protonated form) may be concerned. In order to discuss the points, careful 29Si NMR studies of the exchange products are worth conducting. Figure 3 shows the X-ray diffraction patterns of the products obtained by the reactions of octosilicate with CnTMACl. The

basal spacings of CnTMA-octosilicates were 2.74, 3.01, and 3.31 nm for n = 16, 18, and 22. According to the previous report,22,23 the basal spacings of the C12TMA and C14TMAoctosilicates were 2.37 and 2.54 nm, respectively. These values are plotted as a function of the alkyl chain length (Figure 2 inset) to show a linear relationship between the basal spacing and the number of the C in the alkyl chain (n in CnTMA). From the inclination of the relationship between the basal spacing and n, the arrangement or orientation of the intercalated alkylamines has been discussed.3 On the basis of the discussion by Lagaly on the orientation of alkylamines in the interlayer space of various layered materials,3 the orientation of the intercalated CnTMA in octosilicate was discussed. Taking the molecular structures and the observed basal spacing, the intercalated CnTMA is thought to take a paraffin type arrangement of the possible two types; one is a monomolecular (interdigitating) layer with a tilt angle of ca. 65 deg and the other is bimolecular layer with a tilt angle of ca. 35 deg. The gallery heights of the CnTMA-octosilicates, which were determined by subtracting the thickness of the silicate layer (0.74 nm) from the observed basal spacings, were in good agreement with those reported for analogous CnTMAmagadiites and C12TMA-kenyaite.14-20 The gallery height of C12TMA exchanged products are compared as an example. The gallery height of the C12TMA-magadiite was determined to be ca. 1.7 nm by subtracting the thickness (1.12 nm) of the silicate sheet from the observed basal spacing (2.8 nm).14 The gallery height of the C12TMA-kenyaite was also ca. 1.7 nm, which was determined by subtracting the thickness (1.64 nm) of the silicate layer from the observed basal spacing (3.4 nm).14 The gallery height of C12TMA-octosilicate was ca. 1.7 nm, which was determined by subtracting the thickness of silicate layer (0.74 nm)42 from the basal spacing (2.37 nm).23 These facts indicate that the layer charge density of octosilicate is almost same as that of magadiite and kenyaite (0.27 nm2/e-). Consequently, CnTMA intercalation compounds possess a similar interlayer nanostructure, which is a kind of paraffintype surfactant aggregates. The variation of the basal spacing mentioned above follows the general tendency of the nanostructures of the intercalated simple ionic surfactants such as alkylammonium and alkyltrimethylammonium in cation exchangeable layered solids and alkylsulfonate in anion exchangeable layered solid as summarized by Lagaly.3 In order to see how the nanostructures were imaged when organoammonium ions with complex geometries were intercalated in the interlayer space of octosilicate, the ion exchange of DCC, TCC, and 2C18N into ocotosilicate was investigated. The size of DCC, TCC, and 2C18N are much larger than C22TMA, so that the greater interlayer expansion can be expected after the ion exchange with them. In addition, due to the complex molecular structures of DCC and TCC (Schemes 1 and 2) if compared with simple CnTMA ions, the expansion is difficult to be expected from such a relationship as shown in Figure 2 inset. We expect very large interlayer expansion, which may be interesting for the filler and adsorbents applications. When DCC and 2C18NCl were used as the guest species, the ion exchange proceeded almost quantitatively to give intercalation compounds. The XRD patterns of the products after the ion exchange with DCC and 2C18N are shown in Figure 4. The basal spacing of the DCC-octosilicate was 4.3 nm. From the mass loss in the TG curve, the exchanged DCC amount was determined to be 1.6 mol/Si8O17, to confirm the quantitative ion exchange. The CHN analysis showed the

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Figure 4. XRD patterns of (a) Na-octosilicate, (b) DCC-octosilicate, and (c) 2C18N-octosilicate.

Figure 5. The variation of the XRD patterns the products after the ion exchange with TCC. (a) Na-octosilicate, (b-g) TCC-octosilicates where the exchanged amounts of TCC are 0.5 (b), 0.7 (c), 1.2 (d), 1.4 (e), 1.6 (f ), and 2.8 (g) per Si8O17 unit of octosilicate.

following chemical composition: carbon: 47 mass%, hydrogen: 8.3 mass%, nitrogen mass: 3.0%. From the carbon and nitrogen contents, the C/N ratio was determined to be 15, which was close to the value (15) calculated from the chemical formula of DCC. The exchanged DCC amount was also derived from the CHN anlaysis result (carbon content) to be 1.7 mol/Si8O17, which is also close to the value determined from the TG result (1.6 mol/Si8O17). The basal spacing of 2C18N-octosilicate was 4.26 nm. From the TG results, the exchanged 2C18N amount was determined to be 1.3 mol/Si8O17. This value is slightly smaller than those observed for the CnTMA-octosilicates while the reason is not clear at present. On the other hand, when TCC was used, the ion exchange under similar experimental conditions (at room temperature) resulted in the splitting of the XRD pattern as shown in Figure 5. Repeated ion exchange led the intercalation compounds with various basal spacings (splitted; suggesting segregation) and the exchanged amounts (which are given in Figure 5). The variation of the XRD patterns of the products after the reaction of octosilicate with TCC and after the washing with ethanol is shown in Figure 6. Thus, the ion exchange of octosilicate with TCC does not proceed homogeneously by the reactions at room temperature.

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Figure 6. The relationship between the Δd of the intercalation compounds vs molecular weights of the organoammonium ions.

When the ion exchange with TCC was conducted at 60 °C, the product showed a single phase XRD pattern with a basal spacing of 6.0 nm. In this case, the exchanged TCC amount was derived from the TG curve to be 1.6 mol/Si8O17. From the CHN analysis result (carbon: 55 mass%, hydrogen: 9.6 mass%, nitrogen: 2.2 mass%), the C/N molar ratio was determined to be 25.6, which is close to the value (23.1) calculated from the chemical formula of TCC. The exchanged amount of TCC was determined from the CHN analysis result (carbon content) to be 1.6 mol/Si8O17, which is close to the value (1.6 mol/Si8O17) determined from the TG result, confirming the quantitative ion exchange between octosilicate and TCC. As far as we know, the gallery height (5.3 nm) of the TCC-octosilicate, which was derived by subtracting the thickness of the silicate layer from the observed basal spacing (6.0 nm), was the largest expansion achieved by the intercalation of molecular cation. Table 1 also summarizes the basal spacings and the exchanged amounts of the organoammonium ions of the 2C18N-, DCC-, and TCC-octosilicates after the quantitative ion exchange. Figure 6 shows the relationship between the gallery heights of the intercalation compounds (CnTMA-, 2C18N-, DCC-, and TCC-octosilicates) and the molecular weights of the organoammonium ions. There is a linear relationship between the gallery heights and the molecular weights as shown in Figure 6, indicating that, in the interlayer space, the packing density of the guest species used in the present study is similar and is probably as dense as possible. The present idea on the packing density can be extended to several examples of intercalation compounds of magadiites.9,16 It was also reported that the exchanged amounts of the interlayer octylsilyl groups determined the gallery heights of organosilyl derivatives of magadiite and layered titanates.17,20,21,45-48 On the contrary, as expected from the results of the ion exchange with TCC (shown in Figure 5), it is difficult to control the gallery heights by changing the amounts of the organoammonium cations. The ion exchange with semiquantitative amounts of organoammoium cation normally results in segregration as reported for the cation exchange of smectites.49,50 In addition, in the case of cation exchange of octosilicate with TCC, the quantitative exchange was difficult even when ion exchange using the excess amount of TCC was conducted. The discussion of the interlayer expansion based on the molecular weight of the guest organoammonium cations is a

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versatile way to estimate the basal spacings, when the guest cations have a similar density. The intercalation of organoammonium with a more rigid and bent molecular structure may give a result deviating from the relationship shown in Figure 6. The intercalation compounds of layered silicate with organic dye cations seem to be possible targets for this purpose because of the variation of the complex molecular structures of available dyes. The introduction of dyes into various layered solids has been a topic of interest for the preparation of photofunctional hybrids;50 however, the quantitative ion exchange is still difficult when layered alkali silicates were used as the host materials.37-39 In order to discuss the nanostructures of the dye intercalated species, the comparison of the basal spacing with the presently reported values seems to be interesting. Conclusions The ion exchange of octosilicate, a layered alkali silicate with the chemical formula of Na8Si32O64[OH]8 3 32H2O, with a series of quaternary organoammonium ions (dodecyltrimethylammonium, hexadecyltrimethylammonium, octadecyltrimethylammonium, behenyltrimetylammonium, dimethyldioctadecylammonium, N-(R-trimethylammonioacetyl)didodecyl-L-glutamate, and O,O0 ,O00 -tridodecanoyl-N-(ω-trimethyl ammoniodecanoyl)-tris(hydroxymethyl)aminomethane) in aqueous solutions at room temperature was examined. The cation exchange resulted in the intercalation compounds of the following chemical composition: (organoammonium)x 3 Si8O17, where x ranges from 1.3 to 2.0. Among tested ammonium ions, the ion exchange with O,O0 ,O00 -tridodecanoyl-N-(ω-trimethyl ammoniodecanoyl)-tris(hydroxymethyl)aminomethane was difficult and needed a reaction temperature of 60 °C to obtain a single phase intercalation compound. We proposed a versatile way to estimate the interlayer expansion that the basal spacings of the intercalation compounds were put in order as a function of the molecular weights of the intercalated species. Acknowledgment. The work was partially supported by a Grant-in-Aid for G-COE research, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and by a Grant-in-Aid for Scientific Research (B) (19350103) from Japan Society for the Promotion of Science.

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