ARTICLE pubs.acs.org/Langmuir
Formation of Organo-Highly Charged Mica María D. Alba,*,† Miguel A. Castro,† M. Mar Orta,† Esperanza Pavon,† M. Carolina Pazos,‡ and Jesus S. Valencia Rios‡ †
Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas-Universidad de Sevilla, Avenida Americo Vespucio s/n. 41092 Sevilla, Spain ‡ Laboratorio de Catalisis Heterogenea, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota, D.C., Colombia ABSTRACT: The interlayer space of the highly charged synthetic Na-Mica-4 can be modified by ion-exchange reactions involving the exchange of inorganic Na+ cations by surfactant molecules, which results in the formation of an organophilic interlayer space. The swelling and structural properties of this highly charged mica upon intercalation with n-alkylammonium (RNH3)+ cations with varying alkyl chain lengths (R = C12, C14, C16, and C18) have been reported. The stability, fine structure, and evolution of gaseous species from alkylammonium Mica-4 are investigated in detail by conventional thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), in situ X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (MAS NMR) techniques. The results clearly show the total adsorption of n-alkylammonium cations in the interlayer space which expands as needed to accommodate intercalated surfactants. The surfactant packing is quite ordered at room temperature, mainly involving a paraffin-type bilayer with an all-trans conformation, in agreement with the high density of the organic compounds in the interlayer space. At temperatures above 160 °C, the surfactant molecules undergo a transformation that leads to a liquid-like conformation, which results in a more disordered phase and expansion of the interlayer space.
’ INTRODUCTION The synthetic highly charged mica Na-Mica-4,1 which has a high swelling property and ion-exchange capacity, is potentially useful for the elimination of harmful pollutants such as radioactive cations, heavy metals, organic compounds, and biomolecules.2,3 The mica structure consists of stacked twodimensional layers. Each layer formed by an octahedral sheet between two tetrahedral silica sheets has a thickness of ca. 0.96 nm.4 The interlayer space is occupied by Na+ cations, which balance the charge deficiency that is generated by isomorphous substitution of Si4+ by Al3+ in the tetrahedral sheet.5 The chemical composition of the interlayer space can be modified by ion-exchange reactions5 involving the exchange of inorganic cations by surfactant molecules6,7 to form a highly charged organomica. Organic inorganic hybrid materials are an increasingly important and rapidly growing research field.8 The arrangement, orientation, and mobility of surfactant cations depends on the layer charge, the total surface area, and the alkyl chain length.9,10 A complete understanding of the adsorption of cationic surfactants on layered silicates is of great importance due to the widespread use of these organoclays in both household and industrial activities11 and the occurrence of layered silicates in soils, subsoils, and sediments. Furthermore, several studies have hinted at the potential use of cationic surfactants in the remediation of contaminated subsoils and aquifers.12 15 Indeed, the exchange of inorganic cations in the interlayer space by surfactant r 2011 American Chemical Society
molecules confers the hydrophobic character needed for optimal adsorption and elimination of organic wastewater contaminants.12 However, the success of this application depends on the complete replacement of inorganic cations and the stability of the resulting silicate surfactant complex. It is well-known that the adsorption of surfactants from water onto mineral substrates is governed mainly by electrostatic and hydrophobic interactions16 and that the main factors involved are (i) electrostatic interactions between the surfactant headgroup and the surface, (ii) interaction between the tails, and (iii) electrostatic repulsion between the headgroups. The structural characteristics of the precursor clay mineral, such as a high charge on the aluminosilicate and/or the chain length of the alkylammonium cations, can therefore determine the swelling and the hydrophobic properties of the organoclay. Yang et al.17 reported the swelling capacity and the interstratified structure of synthetic fluorine mica, fluorphlogopite type, after the ion-exchanged of nalkylammonium with alkyl chain length ranging between 6 and 18. Tamura and Nakazawa18 reported the intercalation of alkylammonium, length ranging between 4 and 22, into a synthetic Li-fluorotaenolite, low-charged swelling mica, and they found that the fundamental structure of the complexes were the “paraffin-type bilayers” of TMA+ with an inclination of the chains Received: December 21, 2010 Revised: June 1, 2011 Published: June 02, 2011 9711
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Langmuir of ca. 30°. The new synthetic expansible brittle mica Na-Mica-4, with a higher calculated CEC (468 mequiv/100 g on an anhydrous basis19) than natural swelling phyllosilicates (80 130 mequiv/100 g) still possesses the other beneficial properties of clay minerals,20 and hence is a potential precursor for the generation of nanocomposites with enhanced properties for being used as adsorbents in the removal of toxic metals and organic pollutants. The aim of this study was therefore to optimize the synthesis of the mica surfactant complex. As the structure and properties of the organo-modified layered silicates depend on the chain length of the surfactant, swollen highly charged mica was intercalated with n-alkylammonium cations (RNH3)+ with varying alkyl chain lengths (R = C12, C14, C16, and C18). To understand the thermal stability and degradation mechanism of the alkylammonium Mica-4, they were investigated by conventional thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), in situ X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (MAS NMR) techniques.
’ EXPERIMENTAL DETAILS Materials. Na-Mica-4 was synthesized by the NaCl melt method following a similar procedure to that described by Alba et al.3 Its cationexchange capacity (CEC) is 468 mequiv/100 g and its structural formula is Na4[Si4Al4]Mg6O20F4 3 nH2O. The starting products employed were SiO2 (Sigma; CAS no. 112945 52 5, 99.8% purity), Al(OH)3 (Riedel-de Ha€en; CAS no. 21645 51 2, 99% purity), MgF2 (Aldrich; CAS no. 20831 0, 98% purity), and NaCl (Panreac; CAS no. 131659, 99.5% purity). The primary alkylamines octadecylamine (CAS no. 124 30 1, g99% purity), hexadecylamine (CAS no. 143 27 1, g99% purity), tetradecylamine (CAS no. 2016 42 4, g98.5% purity), and dodecylamine (CAS no. 124 22 1, g99.5% purity) were purchased from Sigma-Aldrich. Preparation of Organomicas. The organomicas were prepared by a cation-exchange reaction between the mica and an excess of alkylammonium salt (2 CEC of Na-Mica-4). Thus, the primary amines were dissolved in an equivalent amount of HCl (0.1 M) and the resulting mixture stirred for 3 h at 80 °C. The alkylammonium solution was then mixed with 0.6 g of Na-Mica-4 and stirred for 3 h at 80 °C. After this time, hot deionized water was added, the mixture stirred for 30 min at 50 °C, and the dispersion centrifuged at 10 000 rpm for 20 min. The product was dissolved in a hot ethanol water mixture (1:1) and stirred for 1 h, then centrifuged, and the precipitate dried at room temperature. Techniques. X-ray diffraction (XRD) patterns were obtained at the CITIUS X-ray laboratory (University of Seville, Spain) on a Bruker D8 Advance instrument equipped with a Cu KR radiation source operating at 40 kV and 40 mA. Diffractograms were obtained in the 2θ range 1 70° with a step size of 0.05° and a time step of 3.0 s. The effect of temperature on the mica swelling was monitored by thermal analysis and variable-temperature X-ray powder (VTXRD) analysis. VTXRD patterns were recorded at the CITIUS X-ray laboratory (University of Seville, Spain) on a Bruker D8 Advance diffractometer (Bruker, Germany) fitted with a high-temperature camera (Anton Paar XRK 900, Austria) and a position-sensitive detector (Bruker Vantec PSD, Germany) in the temperature range 30 480 °C at a heating rate of 5 °C 3 min 1 (target, Cu; voltage, 40 kV; current, 40 mA; θ:θ geometry combining divergence G€obel mirrors configuration; detector, radial Soller slits; scan speed, 1.583° 2θ/min; step size, 0.025°; time step, 0.1 s). Approximately 100 mg of sample was weighed into the sample holder.
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Table 1. Molecules of Water and Surfactants Per Unit Cell molecules H2O/u.ca
molecules Cn/u.cb
Na
3.28
0.6
C12
0.52
3.63
C14
0.03
3.97
C16
0.09
3.90
C18
0.07
4.78
Calculated from the weight loss between 30 and 170 °C. b Calculated from the weight loss between 170 and 900 °C. a
Simultaneous TG/DTA measurements were performed at the Departamento de Crystalografía, Mineralogía y Química Agrícola (University of Seville, Spain), using a NETZSCH (STA 409 PC/PG) instrument equipped with a Pt/Pt Rh thermocouple for direct measurement of the temperature at the sample/reference crucible from room temperature up to 900 °C (heating rate: 10 °C 3 min 1) in N2 atmosphere. Approximately 150 mg of sample was used and the DTA reference was pure aluminum oxide. FTIR spectra were recorded in the range 4000 300 cm 1 by the Spectroscopy Service of the ICMS (CSIC-US, Seville, Spain), as KBr pellets, using a Nicolet spectrometer (model 510P) with a nominal resolution of 4 cm 1. Single-pulse (SP) MAS NMR experiments were recorded by the Spectroscopy Service of the ICMS (CSIC-US, Seville, Spain) using a Bruker DRX400 spectrometer equipped with a multinuclear probe. Powdered samples were packed in 4 mm zirconia rotors and spun at 10 kHz. 1H MAS spectra were obtained using a typical π/2 pulse width of 4.1 μs and a pulse space of 5 s. 29Si MAS NMR spectra were acquired at a frequency of 79.49 MHz, using a pulse width of 2.7 μs (π/2 pulse length = 7.1 μs) and a delay time of 3 s. 13C MAS NMR spectra were recorded at 104.26 MHz with proton decoupling, a pulse width of 2.5 μs (π/2 pulse length = 7.5 μs), and a delay time of 2 s. The chemical shift values are reported in ppm with respect to tetramethylsilane.
’ RESULTS AND DISCUSSION Thermogravimetric Analysis. To investigate the structure and properties of the alkylammonium molecules adsorbed onto Na-Mica-4, first of all it is necessary to ensure that the cation exchange is complete and then evaluate the amount of surfactant in the form of ion pairs. The latter were monitored by TGA, which is a suitable method for detecting both bonded and nonbonded organic molecules.21 23 The thermal decomposition of alkylammonium-Mica-4 was analyzed in two well-separated regions: the first of which, between 30 and 170 °C, corresponds to the loss of hydration water, and the second, up to around 900 °C, corresponds to surfactant decomposition. A quantitative analysis of the TGA data can be found in Table 1 and the TG/DTA plots are shown in Figure 1. The amount of interlayer water was determined from the weight loss in the temperature range 30 170 °C. Thus, whereas Na-Mica-4 was found to have a water content of up to 3.28 molecules per unit cell, the water content in C12-Mica-4 decreased to 0.52 molecules and to below 0.1 molecules per unit cell for Cn-Mica-4 (14 e Cn e 18). The weight loss between 170 and 900 °C corresponds to dehydroxylation of the aluminosilicate framework and decomposition of the surfactant.24 Assuming that the mica is fluorinated, the amount of structural hydroxyl groups can be ignored and the number of alkylammonium molecules adsorbed by the mica calculated from the weight loss in this region, taking into account the molecular weight of the 9712
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Figure 1. DTA (solid line) and TG (dashed line) plots of Cn-Mica-4.
surfactants and dehydrated micas. The results indicate that adsorption of alkylammonium occurs but that satisfaction of the CEC (4 molecules of cationic surfactant per unit cell) depends on the alkyl chain length: (i) The CEC is not satisfied in C12-Mica-4, which explains the remnant weight loss in the temperature range 30 170 °C. (ii) The CEC is completely satisfied in C14-Mica-4 and C16-Mica-4. (iii) The CEC is completely satisfied and a limited additional adsorption, an excess of 20% per mol, occurs beyond the CEC in C18-Mica-4. The additional alkylammonium cations exceeding the mica exchange capacity are likely to be adsorbed between the silicate layers due to van der Waals interactions between alkyl chains.25 Since the van der Waals attraction is proportional to the number of CH2 groups (1 1.5 kJ per CH2 ),26 this effect becomes more pronounced as the chain length increases. Additionally, the lower solubility and mechanical trapping of the long alkylammonium salt may also play a role. XRD Analysis. Figure 2 shows the evolution of the XRD patterns for the layered hybrids as a function of the number of carbons in their alkyl chain. The basal spacing of Na-Mica-4 is 1.20 nm, which corresponds to Na+ in the interlayer space surrounded by one water pseudomonolayer.3 The exchange reaction between Na+ and alkylammonium cations causes an increase in the basal spacing (d001) from 3.626 nm (C12) up to 4.602 nm (C18). Moreover, it is possible to observe up to five 001
reflections in C18-Mica-4. The unique and well-ordered sequence of the 001 reflections suggests a homogeneous distribution of the alkylammonium cations in the interlayer space and, therefore, a homogeneous charge distribution in the mica layer.27 Finally, the XRD pattern of C18-Mica-4 shows two small reflections, corresponding to a space of 3.398 and 1.179 nm, due to the presence of extra alkylammonium molecules in the form of ion pairs. This observation is in good agreement with the excess of 20% per mole beyond the CEC observed by TGA. It is remarkable that all the organomicas show a much greater interlayer spacing than those observed in smectite or vermiculite, in good agreement with the much higher layer charge of the mica. Alkylammonium ions in the interlayer space of vermiculite and smectite can acquire distinct arrangements, including a lateral monolayer conformation (1.38 nm), a lateral bilayer conformation (1.77 nm),17,28 a pseudotrimolecular layer (2.17 nm),29 and paraffin-type structures (>2.20 nm).30,31 The transition between configurations can be calculated on the basis of geometrical considerations.32 Thus, one-layer/two-layer and two-layer/ three-layer transitions occur if the area per alkylammonium ion (Ac = 0.0572 nc + 0.14 (nm2))33 is equal or twice the equivalent area (Ae = [a b]/ξ), where nc = length of alkyl chain; ξ = interlayer cation density (average layer charge in eq/(Si,Al)4O10) and a and b are lattice parameters (0.534 and 0.925 nm for the micas).34 The intercalation of alkylammonium 9713
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Table 2. Geometrical and Package Parameters of the Organomica C12
C14
interlayer cation density/Si8O20 unit
0.363
interlayer space height (nm) length of the chain (nm)
2.69 1.52
a
tilting angle (°)
62.5
C16
C18
0.397
0.390
0.478
3.11 1.78
3.41 2.03
3.66 2.29
60.9
57.1
53.0
layer area/molecule (nm2)
0.140
0.130
0.130
0.100
cross-sectional area of chain (nm2)
0.830
0.940
1.060
1.170
a
The Cn molecules/u.c. values from Table 1 have been used for the calculation.
Figure 2. XRD pattern of Na-Mica-4 and its organomicas.
molecules into other synthetic micas leads to an interstratified phase with the 001 reflections at 3.9617 and 4.38 nm,35 which are assigned to a parallel bilayer and a pseudotriple layer structure, respectively. These synthetic micas therefore consisted of regularly alternating layers with high and low charge. In order to elucidate the packing model adopted by the highly charged expandable micas, some calculations based on TG and XRD data have been carried out and summarized in Table 2. Table 2 lists some parameters related to steric limitations, a comparison of the layer area of the mica per alkylammonium molecule, and the cross-sectional area of the cation (Ac). It is evident that the interlayer alkylammonium molecules cannot form parallel layer arrangements in the interlayer space of the mica due to steric effect and hydrophilic character, between the positive charged head of the alkylammonium and the negative charge of the layers, and hydrophobic, between alkyl chains, interactions. The high layer space value shown by Mica-4 (Figure 2) can be explained if a tilted arrangement involving a bilayer paraffin-type structure is considered. The basal spacing in such a case will depend directly on the length of the alkylammonium axis and the tilt angle (R) with the solid surface, which can be calculated from the experimental basal spacing using the equation d001 = 2 [(nc 1) 0.126 + 0.131] sin R + 0.94 (nm).36 The calculated R values for Cn-Mica-4, which are also included in Table 2, show that the tilting angle increases as the alkyl chain length decreases. This allows the surfactant to be accommodated in a closed and ordered package. Experimental evidence which supports this model will be provided below. FTIR Analysis. FTIR as well as 1H and 13C MAS NMR (show later) spectroscopy are used to directly probe the conformational changes of interlayer alkyl chains and provide a better insight into the interlayer structure. Figure 3 shows two regions of the FTIR spectra of Cn-Mica-4. All Cn-Mica-4 samples have similar FTIR spectra, with two bands at 2919 and 2848 cm 1 due to the asymmetric [νas(CH2)] and symmetric [νs(CH2)] methylene stretching and a band at 2954 cm 1 due to νas(CH3). The band at around 1469 cm 1 is assigned to the scissoring vibration of the methylene groups [δ(CH2)].37
It is well-known that C H stretching frequencies, νas(CH2), are significantly affected by conformational changes in the alkyl chains and lateral interchain interactions. Therefore, the value of νas(CH2) is sensitive to the gauche/trans conformer ratio and the methylene chain packing density,38 41 shifting to higher frequency as the number of gauche conformations along the hydrocarbon chain (chain disorder) increases.42 No shift of this band as a function of the alkyl chain length is observed here. A shift in the absorption maximum is only observed after a certain gauche population is reached, which leads to a late recognition of the transformation.43 These results infer that, if gauche configuration exits, the population is not high enough to be observed by this technique and 13C MAS NMR analysis is demanded. Additional structural and phase-state information may be obtained from the location and shape of the δ(CH2) mode, as this band is sensitive to interchain interactions and the chain packing arrangement. The observed absorption at around 1469 cm 1 is characteristic of a partially ordered phase where the chains are mobile while maintaining some orientational order.42 The decrease of the interlayer space height (from 3.66 nm in C18 to 2.69 nm in C12) is accompanied by a decreased packing density as the chains are no longer fully stretched in an all-trans conformation but progressively adopt a more disordered structure. A tilted arrangement containing all-trans chains, although consistent with the XRD measurements, seems unequivocal in view of the IR data. Furthermore, the progressive adoption of a more disordered structure in C12-Mica-4 could explain the anomalous tilting angle of C12 calculated from the basal spacing. MAS NMR Analysis. Figure 4 shows the 1H MAS NMR spectra of Na-Mica-4 and Cn-Mica-4. The spectrum of Na-Mica4 shows a broad signal at δ = 4.5 ppm due to the water involved in interlayer Na+ hydration.3 The presence of alkylammonium ions in the interlayer space leads to the following changes: (i) The signal at around δ = 4.5 ppm is no longer observed. An absence of interlayer water is indicative of the hydrophobic character of the interlayer space after alkylammonium ion incorporation. Despite the small weight loss observed in the first region of the TGA plot for C12-Mica-4, no signal at around δ = 4.5 ppm is detected due to the low portion of water protons in comparison with alkyl chain protons (1:100). (ii) Two sets of new signals appear, one at around δ = 8.6 ppm due to the protons of the surfactant head (NH4+)44, and a broad signal at around δ = 1 ppm due to the protons of the alkyl chain. However, whereas the lowest frequency signal appears as a broad band in C14 C18, it is a well-resolved, narrow signal for C12. The special behavior of the alkylammonium ion with the shortest chain length is a combination of several factors: An increase in alkyl chain length implies a 9714
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Figure 3. IR/FT spectra of the organomicas.
Figure 5.
Figure 4. 1H MAS NMR spectra of the Na-Mica-4 and Cn-Mica-4.
higher proton concentration and consequently stronger dipolar interactions. The alkyl chain adopts a progressively more ordered structure, as previously observed by IR spectroscopy, which does not allow the dipolar interactions to be averaged. 13 C NMR spectroscopy was used to probe the structure, conformation, and dynamics of the alkyl chains. The results obtained complement the FTIR results and give an insight into
13
C MAS NMR spectra of the Na-Mica-4 and Cn-Mica-4.
the conformational heterogeneity and differences in chain packing at the interfaces. Figure 5 shows the 13C MAS NMR spectra of Cn-Mica-4. The terminal methyl group (Cn) of the alkyl chains appears at around δ = 15 ppm, whereas the Cn 1 carbon (last methylene group) appears at around δ = 25 ppm. The other CH2 groups (C2 Cn 2) are not well-resolved and give rise to a signal at around δ = 33 ppm, thus indicating the presence of a highly ordered all-trans domain.45,46 CH2(C1) appears as a broad signal at around δ = 40 ppm. A detailed analysis of the 13C resonances provided information regarding the molecular conformation and packing. The width of the C1 resonance, for example, increased markedly for alkyl chains with more than 12 carbon atoms, thus indicating a decreased mobility for this atom. These changes are more 9715
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Figure 6.
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29
Si MAS NMR spectra of the Na-Mica-4 and Cn-Mica-4.
noticeable for the methylene groups (C2 Cn-2), for which a shoulder appears at lower frequency (δ ≈ 30 ppm), thus showing that the alkyl chain is predominately in an all-trans conformation (chemical shift of around δ = 33 ppm) but exists as a dynamic average between the gauche and trans conformations.47 This is more evident for C12-Mica-4. Finally, two signals are observed at δ = 16 and 28 ppm in the spectra of C12-Mica-4 and C18-Mica-4. These signals are likely due to the alkylammonium ion being present as an ion pair, as anticipated from the TGA analysis of C18-Mica-4. 29 Si MAS NMR spectra were recorded to gain a greater understanding of the mica framework (Figure 6). The 29Si MAS NMR spectra contain a set of bands in the range δ = 70 to 95 ppm, which is compatible with the existence of four single Q3(mAl) (0 e m e 3) environments, as expected for 2:1 layered aluminosilicates.44 However, whereas the spectrum of Na-Mica-4 is consistent with those previously reported for this mica,3 the signals of Cn-Mica-4 are shifted by 2.5 ppm toward lower frequencies. We have previously observed a similar shift of the 29Si frequencies in mica-4 exchanged with different interlayer inorganic cations and explained it by proposing that the cations form an inner-sphere complex where they are coordinated to water molecules and oxygen atoms from the clay mineral. The similarity between all Cn-Mica-4 spectra reinforces the proposed bilayer structure with alkyl chains in an all-trans configuration and the polar part of the surfactant close to the basal oxygen plane. All the 29Si spectra show similar relative peak intensities, thus demonstrating that the exchange process does not alter the Si and Al distribution.
Figure 7. Molecular package of Cn-Mica-4.
All the above results indicate that the alkylammonium molecules pack tail-to-tail in a bimolecular arrangement, with the chain ends of two opposing molecules lying next to each other (Figure 7). The organic film thickness, calculated from the X-ray diffraction angle, is larger than the length of an extended alkylammonium molecule but smaller than the thickness of a bimolecular layer oriented orthogonally to the substrate surface. A tilt angle of 50 51° allows an optimal interaction of the NH3+ groups via three hydrogen bonds to the surface oxygen atoms.30 However, it has previously been reported that this angle increases to about 65° in highly charged clay minerals.34 This angle increase is facilitated by incorporation of the NH3+ moiety into the pseudohexagonal hole, as demonstrated by the shift of the 29 Si NMR signals. The more disordered configuration and anomalous tilt angle observed for C12-Mica-4 can be explained by the fact that the molecular packing of the chain is low and a considerable proportion of gauche/trans configuration coexists with the all-trans configuration. Thermodynamic Stability. The thermodynamic stability of the phases and the chain dynamics were explored by DTA and XRD at different temperatures. The DTA curves in the temperature range 170 900 °C (Figure 1) show one exothermic and two endothermic changes. The first endothermic change at 339 355 °C, which is associated with an abrupt weight loss, has previously been assigned to the thermal degradation of intercalated alkylammonium ions. The second endothermic change, which is accompanied by a smooth weight loss, is due to decomposition of the remaining organic components.48,49 9716
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Figure 8. VTXRD patterns: (a) C12-Mica-4, (b) C14-Mica-4, (c) C16Mica-4, and (d) C18-Mica-4.The upper graphs represent the comparison between the xy-projection of the 3D VTXRD graphs and the TG plots.
Thermal degradation of the intercalated alkylammonium cation is characteristic of each Cn-Mica-4, with the degradation temperature increasing with alkyl chain length. C18-Mica-4 undergoes a two-step degradation in the temperature range 300 425 °C, which can be related to the simultaneous presence of both exchanged and ion-pair alkylammonium ions in this mica. A small endothermic change is observed below 170 °C in CnMica-4 (12 e n e 16), although a small weight loss is only evident in this region for C12. This is due to the fact that the CEC is not satisfied in C12-Mica-4. Finally, C18-Mica-4 shows several endothermic changes at temperatures different from the dehydration one, which could be due to the elimination of non- or weakly adsorbed organic molecules.49 Discrete endothermic peaks between 61 and 165 °C in the DTA curves of C18-Mica4 indicate that the small water loss in this temperature range is not caused by adsorbed water but by hydration superficial water.50 The XRD patterns of the 001 reflections, measured from room temperature up to 480 °C, are shown in Figure 8. To better observe the changes produced in the 001 reflections, the 3D graphs are accompanied of the xy-projection of the 3D VTXRD graphs (upper plot in each graph). It can be seen how the 001 reflections of all the organoclays studied shift to lower angles at around 160 °C, after which they progressively shift to higher angles and their widths increase due to a loss of long-range order and decomposition of the surfactant, as observed previously by DTA. The fully expanded structure and the well-ordered sequence of the 001 reflections remain up to ca. 280 °C; at higher temperatures, only one 001 reflection remains for all the organoclays. The temperature dependence of d001 for the organomica is plotted in Figure 9. At room temperature, the alkylammonium molecules pack tail-to-tail in a bimolecular arrangement, with the chain ends of two opposing molecules lying next to each other. The increased thickness of the organic film indicates an increase
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Figure 9. Cn-Mica-4 basal space evolution with temperature.
in the volume of the confined molecules and a decrease in their density. The basal spacing increases with increasing number of methylene groups (4.3% in C12 and 7.6% in C18). A gradual increase of the d001 spacing of HDTMA-smectite up to 200 °C has been reported previously and was attributed to conformational changes in the alkyl chains.50 The temperature associated with this conformational change increases as the length of the alkyl chain increases. An abrupt decrease in the basal spacing is observed at temperatures between 220 and 235 °C, depending on the nature of the surfactant, a similar temperature to that of the abrupt weight loss (see Figure 1). This reduction in the interlayer space height is due to decomposition of the alkylammonium cation, which leads to smaller molecules in the interlayer space. The final basal spacing of the organomicas ranges between 1.5 nm for C12 and 2.0 nm for C18.
’ CONCLUSIONS Swollen highly charged micas have been intercalated with nalkylammonium ions with different alkyl chain lengths. FTIR and NMR spectroscopy, in conjunction with X-ray diffraction and thermal analysis studies, have provided a new experimental insight into the interlayer structure and phase state of intercalated alkylammonium-Mica-4. Thus, our experiments clearly show that the mass ratio of intercalated surfactant and the d-spacing increase with alkyl chain length, with the mica interlayer spaces expanding as needed to accommodate intercalated surfactants. The surfactant packing is quite ordered at room temperature, in agreement with the high density of the organic material in the interlayer space. A double all-trans conformation with a certain degree of gauche conformation has been observed for C12-Mica4, where the density is smaller. These organomicas undergo a transformation at temperatures above 160 °C that leads to a liquid-like conformation and therefore a more disordered phase and expansion of the interlayer space. The high charge density of these expandable brittle micas in comparison with other expandable layer silicates, such as smectite 9717
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Langmuir or vermiculite, is the responsible factor of the surfactant packing structure adopted in organic highly charged mica which must overcome the steric effect and the hydrophilic between the positive charged head of the alkylammonium and the negative charge of the layers, and hydrophobic, between alkyl chains, interactions.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We would like to thank the Ministerio del Medio Ambiente y Medio Rural y Marino (project no. 300/PC08/3-01.1) and the DGICYT (project no. CTQ 2010-14874) for financial support. ’ REFERENCES (1) Gregorkiewitz, M.; Rausell-Colom, J. A. Am. Mineral. 1987, 72, 515. (2) Ravella, R.; Komarneni, S.; Martinez, C.E.. Environ. Sci. Technol. 2008, 42, 113. (3) Alba, M. D.; Castro, M. A.; Naranjo, M.; Pavon, E. Chem. Mater. 2006, 18, 2867. (4) Schmeltzer, J. M.; Porter, L. A.; Stewart, M. P.; Buriak, J. M. Langmuir 2002, 18, 2971. (5) Vaia, R. A.; Ishii, H; Giannelis, E. P. Chem. Mater. 1993, 5, 1694. (6) Ogawa, M.; Wada, T.; Kuroda, K. Langmuir 1995, 11, 4598. (7) Osman, M. A.; Ploetze, M.; Skrabal, P. J. Phys. Chem. B. 2004, 108, 2580. (8) Hussain, S. A.; Islam, Md N.; Bhattacharjee, D. J. Phys. Chem. Solids 2010, 71, 323. (9) Bergaya, F.; Theng, B. K. G.; Lagaly, G. In Handbook of Clay Science. Developments in Clay Science; Elsevier: Oxford, 2006; Vol 1. (10) Lagaly, G. Solid State Ionics 1986, 22, 43. (11) Linfield, W. M. In Cationic surfactants; Jungermann, Ed.; Marcel Dekker, Inc.: New York, 1970. (12) Boyd, S. A.; Lee, J. L.; Mortland, M. M. Nature 1988, 33, 345. (13) Burris, D. R.; Antworth, C. P. J. Contam. Hydrol. 1992, 10, 325. (14) Boyd, S. A.; Mortland, M. M.; Chiou, C. T. Soil Sci. Soc. Am. J. 1988, 52, 652. (15) Jaynes, W. F.; Boyd, S. A. Soil Sci. Soc. Am. J. 1991, 55, 43. (16) Cases, J. M.; Villieras, F. Langmuir 1992, 8, 1251. (17) Yang, J. H.; Han, Y. S.; Choy, J. H.; Tateyama, H. J. Mater. Chem. 2001, 11, 1305. (18) Tamura, K.; Nakazawa, H. Clays Clay Miner. 1996, 44, 501. (19) Senturk, H. B.; Ozdes, D.; Gundogdu, A.; Duran, C.; Soylak, M. J. Hazard. Mater. 2009, 172, 353. (20) Komarneni, S.; Pidugu, R.; Hoffbauer, W.; Schneider, H. Clays Clay Miner. 1999, 47, 410. (21) Osman, M. A.; Ploetze, M.; Suter., U. W. J. Mater. Chem. 2003, 13, 2359. (22) Osman, M. A.; Seyfang, G.; Suter, U. W. J. Phys. Chem. B 2000, 104, 4433. (23) Osman, M. A.; Suter., U. W. Chem. Mater. 2002, 14, 4408. (24) Xie, W; Gao, Z.; Pan, W. P.; Hunter, D.; Singh, A.; Vaia, R. Chem. Mater. 2001, 13, 2979. (25) Narine, D. A.; Guy, R. D. Clays Clay Miner. 1981, 29, 205. (26) Streitwiesser, A.; Heathcock, C. H.; Kosower, E. M. Introduction to Organic Chemistry, 4th ed.; Macmillan: New York, 1992; p 75. (27) Lee, S. Y.; Kim, S. J. Colloids Surf., A. 2002, 211, 19. (28) Brindley, G. W.; Hofmann, R. W. (1962). Orientation and packing of aliphatic chain molecules on montmorillonite, in Clays and Clay Minerals, Proc. 9th Natl. Conf. West Lafayette, Indiana, 1960, Swineford, Ada , Ed.; Pergamon Press: New York; pp 546 556.
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