Synthetic High-Charge Organomica: Effect of the Layer Charge and

Article pubs.acs.org/Langmuir

Synthetic High-Charge Organomica: Effect of the Layer Charge and Alkyl Chain Length on the Structure of the Adsorbed Surfactants M. Carolina Pazos,† Miguel A. Castro,† M. Mar Orta,† Esperanza Pavón,‡ Jesús S. Valencia Rios,§ and María D. Alba*,† †

Instituto de Ciencia de Materiales de Sevilla, Consejo Superior de Investigaciones Científicas-Universidad de Sevilla, Avenida Américo Vespucio s/n. 41092 Sevilla, Spain ‡ Unité de Catalyse et de Chimie du Solide, UCCS CNRS, UMR8181, Université Lille Nord de France, 59655 Villeneuve d’Ascq, France § Laboratorio de Catálisis Heterogénea, Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Bogotá, D.C., Colombia ABSTRACT: A family of organomicas was synthesized using synthetic swelling micas with high layer charge (NanSi8‑nAlnMg6F4O20·XH2O, where n = 2, 3, and 4) exchanged with dodecylammonium and octadecylammonium cations. The molecular arrangement of the surfactant was elucidated on the basis on XRD patterns and DTA. The ordering conformation of the surfactant molecules into the interlayer space of micas was investigated by 13C, 27Al, and 29 Si MAS NMR. The arrangement of alkylammonium ions in these high-charge synthetic micas depends on the combined effects of the layer charge of the mica and the chain length of the cation. In the organomicas with dodecylammonium, a transition from a parallel layer to a bilayer-paraffin arrangement is observed when the layer charge of the mica increases. However, when octadecylammonium is the interlayer cation, the molecular arrangement of the surfactant was found to follow the bilayer-paraffin model for all values of layer charge. The amount of ordered conformation all-trans is directly proportional of layer charge.

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INTRODUCTION Organophilic clays (usually known as organoclays) are clay− surfactant hybrids derived from an ion exchange of hydrophilic clay minerals with quaternary ammonium salts.1 These hybrids have shown many applications such as catalysts,2−4 electrical materials,5−7 and nanoporous starting materials.8,9 Moreover, due to their high affinity for nonpolar organics, organoclays are especially useful as effective sorbents of organic pollutants in soil and water remediation programs.10−14 Most organoclays have been prepared using natural smectites such montmorillonite or hectorite15 as host materials, due to their ideal properties such as high cation exchange capacity, swelling behavior, adsorption properties, and large surface area. Recently, the synthesis of micas with swelling capacity similar to that of smectites or vermiculite but with higher interlayer charge densities, as brittle micas, has been reported.16−19 Moreover, they have higher crystallinity, controllable composition and have fewer impurities than natural clay minerals. Therefore, those synthetic micas should be more advantageous than natural clay minerals for use as host materials. Recently, Alba et al.20 reported the preparation of a synthetic high-swelling organomica, Cn-Mica-4, using as starting materials a high charge mica, Na-mica-4, and alkylammonium cations with different chain length. Those organomicas exhibited high purity and high surfactant adsorption capacity. However, it is well established that the adsorption capacity and the surfactant © 2012 American Chemical Society

structure are strongly dependent on the layer charge and surfactant length. Therefore, the aim of this study is to compare the adsorption amount and structure of surfactant and the hydrophobicity of the interlayer space of a set of organomicas using three different synthetic high-charged micas, Na-Mica-n (n is the interlayer charge ranging between 2 and 4) and two different organic cations (dodecyl- and octadecyl-ammonium). These organomicas could represent interesting technological materials due to the possibility of tuning specific features with the layer charge of host materials and length of chain of organic cation. To understand their thermal stability and the degradation mechanism of the Cn-Mica-n, conventional thermogravimetric analysis (TG-DTA), X-ray diffraction at room and at variable temperature (XRD and VT-XRD), and solid-state nuclear magnetic resonance (MAS NMR) are used. Moreover, an approach of the packing models adopted by the highly charged expandable micas is calculated on the basis on TG and XRD data, taking into account some geometrical considerations.21,22 Received: January 11, 2012 Revised: April 16, 2012 Published: April 19, 2012 7325

dx.doi.org/10.1021/la300153e | Langmuir 2012, 28, 7325−7332

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Table 1. Interlayer Water and Surfactant Contents of the Organomicas Na-Mica-n

C12-Mica-n

C18-Mica-n

n

molecules H2O/u.c.a

molecules Cn/u.c.b

molecules H2O/u.c.a

molecules Cn/u.c.b,c

molecules H2O/u.c.a

molecules Cn/u.c.b,c

2 3 4

2.93 3.33 3.27

----

0.07 0.13 0.52

1.94 2.90 3.63

0.05 -0.07

2.44 3.48 4.78

Calculated from the weight loss between 30 and 170 °C. bCalculated from the weight loss between 170 and 900 °C. cThe molecules Cn/u.c. necessary to satisfy the charge deficit of Na-Mica-n (n = 2, 3, or 4) are 2, 3, and 4, respectively. a

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min−1) in an atmosphere of N2. Approximately 150 mg of sample was used and the DTA reference was pure aluminum oxide. 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 into 4 mm zirconia rotors and spun at 10 kHz. 27Al MAS NMR spectra were acquired at a frequency of 104.26 MHz, using a pulse width of 0.92 μs (π/2 pulse length = 9.2 μs) and a delay time of 0.1 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 0.1 M AlCl3 solution for 27Al and tetramethylsilane for 29Si and 13C.

EXPERIMENTAL DETAILS

a. Materials. Na-Mica-n (n = 2, 3, and 4) were synthesized using the NaCl-melt method following a similar procedure to that described by Alba et al. 19 Their structural formulas are Na n [Si 8‑n Al n ] Mg6O20F4·mH2O, where n represents the charge per unit cell and m is the number of water molecules. The starting materials employed were SiO2 (Sigma; CAS no. 112945−52−5, 99.8% purity), Al(OH)3 (Riedel-de Haën; CAS no. 21645−51−2, 99% purity), MgF2 (Aldrich; CAS no. 20831−0, 98% purity), and NaCl (Panreac; CAS no. 131659, 99.5% purity). Stoichiometric proportions of reactants were weighed and mixed in an agate mortar. The molar ratio between the reactants were (8 − n)SiO2:(n/2)Al2O3:6MgF2:(2n)NaCl. Twice the amount of NaCl was added to ensure complete charge balance with Na+ cation in the interlayer space.9 The optimal amount of reaction mixture for grinding was up to 2 g per batch during 30 min for ensuring the homogeneity of the mixture. The heat treatments were carried out in a closed Pt crucibles at 900 °C during 15 h using a heating rate of 10 °C·min−1. The product was washed with distilled water, and the solid was separated by filtration, dried at room temperature, and then ground in the agate mortar. b. Preparation of Organomicas. The organomicas were prepared by a cation-exchange reaction between the micas and an excess of alkylammonium salt (two times the amount of alkylammonium salt necessary to satisfy the total layer charge 2, 3, and 4 per unit cell of Na-Mica-2, Na-Mica-3, and Na-Mica-4, respectively). 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 dispersion was then mixed with 0.6 g of Na-Mica-n and stirred for 3 h at 80 °C. After adding hot deionized water, the mixture was stirred for 30 min at 50 °C, and then the dispersion was 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 at 50 °C and then centrifuged.21 The precipitate was dried at room temperature. c. 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 Kα 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 step time of 3.0 s. The effect of temperature on the mica interlayer swelling was monitored by thermal analysis and variable-temperature X-ray powder (VTXRD) analysis. VTXRD patterns were recorded at the CITIUS Xray 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·min−1 (target, Cu; voltage, 40 kV; current, 40 mA; θ:θ geometry combining divergence Göbel 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. Simultaneous TG/DTA measurements were performed at the ́ Mineralogiá y Quimica ́ ́ Departamento de Crystalografia, Agricola (University of Seville, Spain) using a NETZSCH (STA 409 PC/PG) instrument which is 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

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RESULTS AND DISCUSSION a. Hydrophobicity of the Interlayer Space. To investigate the structure and properties of the alkylammonium molecules adsorbed onto Na-Mica-n, it is necessary first to ensure that the cation exchange is complete and then to evaluate the amount of surfactant in the form of ion pairs, as alkylamonium chloride. The latter process was monitored by TGA, which is suitable for detecting both bonded and nonbonded organic molecules.23−25 Quantitative water and surfactant contents derived from TGA data (not shown) are reported in Table 1. The amount of interlayer water was determined from the weight loss in the temperature range 30−170 °C. The amount of adsorbed alkylammonium molecules was determined from the weight loss between 170 and 900 °C,26 with the assumption that the amount of structural hydroxyl groups could be ignored in the fluoromicas. Klapyta et al.27 observed only part of the inorganic (sodium or lithium) ions in the interlayer spaces of synthetic fluorotetrasilicic mica and fluoroetaeniolite even if an excess of the surfactants was used; the maximum organic ions incorporated was 0.92 mequiv/g, which is considerably lower than the exchangeable cation amount reported by the producer (1.70 mequiv/g). However, in Na-Mica-n, in general, the amount of adsorbed alkylammonium was proportional to the layer-charge deficit, which indicated that the main adsorption occurred by cation exchange. However, the total amount also depended on the length of the alkyl chain. Slightly less dodecylammonium was adsorbed than their CEC (0.97 CEC for n = 2 and 3 and 0.91 CEC for n = 4), and it decreased in Mica-4 due to the higher hydrophilic character of the interlayer space.28 For the C18-Mica-n, adsorption was beyond the CEC with limited additional adsorption of ca. 20% per mol. The additional alkylammonium cations exceeding the mica exchange capacity are likely to be absorbed between the silicate layers due to van der Waals interactions between alkyl chains.29−32 On one hand, this interaction is favored in Na-Mica-4 where the concentration of alkylammonium necessary to satisfy the layer 7326


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charge is higher and, thus, the packing density is higher and the hydrophobicity could be affected. On the other hand, since the van der Waals attraction is proportional to the number of −CH2− groups (1−1.5 kJ per −CH2−),33 this effect became more pronounced as the chain length increased.34,35 In addition, the lower solubility and mechanical trapping of the long alkylammonium salt may also play a role. The replacement of the hydrated Na+ by an organic cation may change the surface properties from hydrophilic to organophilic,36 which may cause a drastic diminishing of the water content. Thus, whereas Na-Mica-n was found to have water content between 2.93 and 3.33 molecules per unit cell, the water content of C12-Mica-n decreased to below 0.6 molecules per unit cell and of C18-Mica-n to below 0.1 molecules per unit cell. The remaining water molecules in the organomicas are considered to be due to the release of the residual free water retained in particle pores after drying.37 The nature and amount of organic cations occupying the exchange sites together with the packing density allow tuning of the hydrophilic/hydrophobic character of the interlayer space of phyllosilicates. The exchange on the high layer charge NaMica-n with long-chain surfactants produces high expansion of the interlayer space with a distribution of organic cations (dominated by hydrophobic interactions between alkyl chains) in an arrangement of the paraffinic bilayer (high packing density). In this confined organic phase, the hydrophobic properties should be related with the volume of absorption and the stability of arrangement during the adsorption process.38,39 b. Package Structure of the Alkylammonium in the Interlayer Space. The long-range order of the organomicas was analyzed using the 001 reflections of the XRD patterns (Figure 1). The basal spacing of Na-Mica-4 is 1.20 nm, which

Table 2. Geometrical and Package Parameters of the Organomicas Cn-Mica-2 Interlayer cation amount/Si8O20 unita Interlayer space height (nm)b Interlayer cation densityc Length of the chain (nm) Tilting angle (°) Layer area/ molecule, Ae (nm 2 ·molec−1)d Cross-sectional area of chain, Ac (nm 2)d % disorder conformatione

Cn-Mica-3

Cn-Mica-4

C12

C18

C12

C18

C12

C18

1.94

2.44

2.90

3.48

3.63

4.78

1.41 2.66 --

3.54

3.66

2.69

3.66

1.40

2.74 1.73 --

1.92

2.73

2.64

1.52

2.27

1.52

2.27

1.52

2.27

-61.3 0.50

0.83 40

51.1 0.40

64.6 -0.34

1.17

0.83

37

34

53.6

62.5

53.6

0.28

0.28

0.20

1.17

0.83

1.17

27

26

17

a Cn molecules/u.c. values from Table 1 have been used for the calculation. bd001(nm) − 0.94. cCalculated from the unit cell values: a = 0.534 nm and b = 0.925 nm. dDescription in the text. eIg‑t × 100/(Ig‑t + It), calculated from the integration of the 13C MAS NMR signals at ca. 30 ppm (mixed gauched and trans, Ig‑t) and 33 ppm (all-trans, It).

nm, an interlayer space height of 1.41 nm, which could be compatible with a monolayer conformation, and a small 001 reflection due to a basal spacing of 3.60 nm, an interlayer space height of 2.66 nm, which could be compatible with a paraffin conformation. Similar behavior was observed in C12-Mica-3, but the most intense 001 reflection had a lower 2θ. Finally, the C12-Mica-4, with an interlayer cation density of 2.73 (Table 1), again shows a unique 001 reflection sequence with a basal spacing of 3.63 nm and an interlayer space height of 2.69 nm, smaller than that of C18-Mica-4 (4.60 nm). The splitting of the 001 reflections in C12-Mica-2 and C12-Mica-3 cannot be explained by inhomogeneity of the charge distribution, which is not expected, as the starting material is the same for the C12 and C18 exchange, and in that case, a unique 001 sequence is observed. However, the splitting of the 001 reflections in the case of C12-Micas-n (n = 2 and 3) represent the mixed layer composed of two arrangements of the organic cations (monolayer and paraffinic bilayer).42 Ganguly et al.43 have observed the same heterogeneity in the interlayer arrangement of organic cations in the interlayer space of montmorillonites and concluded that both the cation exchange capacity and the layer charges of the silicates played a dominant role in the interlayer arrangements of the organic cations. Alkylammonium ions in the interlayer space of 2:1 phyllosilicates can acquire distinct arrangements,40,41,44,45 and the transition between parallel and more compact configurations can be calculated on the basis of geometrical considerations.41 This transition occurs if the area per alkylammonium ion (Ac = 0.0572 × nc + 0.14 (nm2))42 is greater than twice the equivalent area (Ae = [a × b]/ξ), where nc = the length of alkyl chain; ξ = the interlayer cation amount (average layer charge in eq/(Si,Al)4O10), and a and b are lattice parameters (0.534 and 0.925 nm for the micas).46 The interlayer alkylammonium molecules cannot form parallel layer arrangements in the interlayer space of the mica when

Figure 1. XRD patterns of organomica: (a) Cn-Mica-2, (b) Cn-Mica-3, and (c) Cn-Mica-4.

corresponds to Na+ in the interlayer space surrounded by one water pseudo-monolayer.19 The exchange reaction between Na+ and the alkylammonium cations causes an increase in the basal spacing (d001). For the C18-micas, a unique and wellordered sequence of the 001 reflections is observed and corresponds to a basal spacing between 4.48 and 4.60 nm, an interlayer space height ranging between 3.54 and 3.66 nm (Table 2), compatible with paraffin-type structures (>2.20 nm).40,41 However, in the case of C12-Mica-n (n = 2 and 3), where the CEC was not completely satisfied (Table 1), two 001 reflection sequences were observed. In C12-Mica-2, the most intense 001 reflection corresponds to a basal spacing of 2.35 7327


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Figure 2. 13C MAS NMR spectra of organomica: (a) Cn-Mica-2, (b) Cn-Mica-3, and, (c) Cn-Mica-4.

and is compatible with the two 001 reflections families observed by XRD. Regarding the methylene groups (C2−Cn‑2), the resonance peaks can be deconvoluted and quantified to fit peaks at ca. 30 and 33 ppm. The area ratios of the disordered conformation at δ ≈ 30 ppm (mixed gauche and trans) to the ordered conformation at δ ≈ 33 ppm (all-trans), Ig‑t/It, depend on the chain length and the total layer charge of the mica. As the interlayer alkylammonium density increases, the proportion of disordered configuration diminishes as the alkyl chain freedom decreases (Table 2). These data are consistent with results reported in the literature, where He et al.52 observed that, in organomontmorillonite, the amount of all-trans conformer increases with an increase in amine concentration when amine chains radiate from the silicate layers. The high proportion of C12-Mica-n in disordered configuration may be explained by its high available layer area per alkylammonium molecule (Ae) (a decrease of the Ae value of 18−29% is observed in C18-Mica-n compared to C12-Mica-n), and it is more evident in C12-Mica4. c. Structural Study of the Organomica Framework. The short-range structural order of the mica framework after the intercalation of the alkylammonium ion into the interlayer space is analyzed by 29Si and 27Al MAS NMR. The 29Si MAS NMR spectra (data not shown) contain a set of bands in the range between δ = −70 to −95 ppm. These data are consistent with the existence of four single Q3(mAl) (0 ≤ m ≤ 3) environments, which is expected for 2:1 layered aluminosilicates.53 Figure 3 shows the variation of the Q3(mAl) (0 ≤ m ≤ 3) environments as a function of the interlayer cation. While the spectra of Na-Mica-nare consistent with those previously reported for these micas,20 the signals of Cn-Mica-n are shifted up to 5 ppm toward lower frequencies. According to the observed high tilt angle by XRD, this shift of the 29Si NMR signals is caused by the incorporation of the NH3+ moiety into the pseudohexagonal hole. A similar shift in the 29Si frequencies of M-Mica-n exchanged with different interlayer inorganic cations has already been observed and was explained by proposing that the cations form an inner-sphere

the interlayer cation density is higher than 1.94. This is due to steric effects and the hydrophilic character between the positively charged head of the alkylammonium and the negative charge of the layers, and hydrophobic interactions between alkyl chains. The high basal spacing value shown by C12-Mica-n and C18-Mica-n (Figure 1) 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 (α) 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 α + 0.94 (nm).47 The calculated α-values (see Table 2) show that the tilting angle increases as the alkyl chain length decreases, but no direct correlation with the layer charge of the mica was observed. A tilt angle between 50° and 51° allows an optimal interaction between the NH3+ groups and the surface oxygen atoms via three hydrogen bonds.42 However, we observed that this angle increases up to a range between 53.6° (C18-Mica-4 and C18-Mica-3) and 64.6° (C12-Mica-3). This angle increase should be facilitated by the incorporation of the NH3+ moiety into the pseudohexagonal hole. On the basis of experimental evidence, a linear relationship between the tilt angle and the layer charge was suggested by Ghabru et al.48 and Mermut et al.49 observed that the tilt angle of most highly charged phyllosilicates is 65°. Substantial information of the alkylammonium structure was obtained from the 13C MAS NMR analysis (Figure 2). 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.50,51 CH2(C1) appears as a broad signal at around δ = 40 ppm. Additionally, the organomicas C12-Mica-n (n = 2 and 3) show a second set of narrower signals (marked with asterisk Figure 2a,b left) that is likely corresponds to dodecylammonium that is packaged differently 7328


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Figure 4. 27Al MAS NMR spectra of organomica: (a) Cn-Mica-2, (b) Cn-Mica-3, and (c) Cn-Mica-4.

Cn-Mica-n, with the degradation temperature increasing with alkyl chain length and mica layer charge. C18-Mica-4 undergoes a two-step degradation in the temperature range 300−425 °C,

Figure 3. 29Si chemical shift of organomica.

complex where they are coordinated to water molecules and oxygen atoms from the clay mineral.54 The above results reinforce the proposed bilayer structure with alkyl chains in an all-trans configuration, and reinforce that the polar part of the surfactant is close to the basal oxygen plane. All the 29Si spectra of Cn-Mica-4 show similar relative peak intensities in comparison with that of Na-Mica-n. No differences in the 27 Al MAS NMR spectra (Figure 4) were observed. The exchange process, therefore, does not alter the Si and Al distribution. d. Thermodynamic Behavior of the Alkylammonium in the Interlayer Space. The thermodynamic stability of the phases and the chain dynamics were explored by DTA (Figure 5, solid lines). The DTA curves in the temperature range 25− 550 °C (Figure 5, solid line) show two endothermic changes. The strongest endothermic process that occurs between 339 and 416 °C is associated with an abrupt weight loss, and it has previously been assigned to the thermal degradation of intercalated alkylammonium ions.55,56 Thermal degradation of the intercalated alkylammonium cation is characteristic for each

Figure 5. DTA signals (solid line) and evolution of d001 (line and circle) with temperature of organomica: (a) C12-Mica-2, (b) C12-Mica3, (c) C12-Mica-4, (d) C18-Mica-2, (e) C18-Mica-3, and (f) C18-Mica-4. 7329


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the interlayer space depend on both the inorganic adsorbent (interlayer charge) and the organic adsorbate (alkyl chain length). The structural arrangement of the dodecylammonium cation in the interlayer space of high-charge micas is more sensitive to the effect of the mica layer charge than to octadecylammonium cations, because the sorbed amount of dodecylammonium is slightly less than the cation exchange capacity of the mica. The arrangement of the surfactant in Mica-n is a mixture between ordered (all-trans) and disordered (mixed gauche−trans) configurations, but the amount of the disordered arrangement is higher in C12-Mica-n and in mica with a layer charge below 4. Finally, the nature of the alkylammonium and of the highcharge mica influences the decomposition process of the surfactant in the interlayer space. The decomposition temperature of alkylammonium surfactant is higher for C18-Mica-n than for C12-Mica-n.

which may be related to the simultaneous presence of both exchanged and ion-pair alkylammonium ions in this mica (Table 1). A small endothermic change is observed below 170 °C in CnMica-n, although a small weight loss is only evident in this region for C12 (Table 1), due to the hydration water of the remaining sodium in the interlayer space. This is because the CEC is not satisfied in C12-Mica-n. Finally, C18-Mica-4 shows several endothermic changes at temperatures different from that of dehydration, which could be due to the elimination of nonadsorbed or weakly adsorbed organic molecules.56 Discrete endothermic peaks between 61 and 165 °C in the DTA curves of C18-Mica-4 indicate that the small water loss in this temperature range is not caused by interlayer water but by hydration superficial water.57 e. Thermal Stability of the Alkylammonium in the Interlayer Space. The evolution of d001 with the temperature of the organomica (Figure 5, line and circle) are obtained from the XRD patterns measured from room temperature up to 480 °C. 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. At temperatures below 200 °C, the 001 distances (d001), which increase as a consequence of the organic film increasing in thickness, indicate an increase in the volume of the confined molecules and a decrease in their density. The basal spacing increases with an increasing number of methylene groups (up to 2% in C12 and up to 6.5% in C18). Inomata et al.58 observed an increase in the basal space of LDH−stearate intercalation compounds attributed to a phase transition from gel to liquid crystalline state, supported by an endothermic peak after heating at 78 °C. However, no endothermic peak is observed in the organomica DTA curves at this temperature, indicating that the structural swelling observed is due to an increase in the mobility freedom of alkyl chain as a consequence of the temperature increase. This assumption is also consistent with the literature; a gradual increase of the d001 spacing of HDTMA-smectite as the temperature is increased to 160 °C has been reported previously and was attributed to conformational changes in the alkyl chains.57 The temperature associated with this conformational change increases as the length of the alkyl chain and the layer charge of the micas increases. A soft decrease of interlayer space between 160 and 250 °C is caused by the formation and rearrangement of alkyl chain conformations (gauche and kinks conformations) produced for the possibility of rotation around the C−C bond.57,59 The structures remain fully expanded up to ca. 220 °C, but at higher temperatures, between 220 and 235 °C, an abrupt decrease in the basal space is observed depending on the nature of the surfactant and the mica, in accordance with the abrupt weight loss. This reduction in the interlayer space height is due to the decomposition of the alkylammonium cation, which leads to smaller molecules in the interlayer space. The final d001 value of the organomicas ranges between 1.6 nm for C12 and 2.4 nm for C18, but it is not dependent on the mica layer charge as was previously observed in organoclays prepared from montmorillonite with different cation exchange capacity.60

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the DGICYT (project no. CTQ 201014874) for financial support and FEDER funds. We would also like to thank the X-ray laboratory at CITIUS (Universidad de Sevilla) for its help recording the XRD patterns.

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REFERENCES

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CONCLUSIONS For the first time, dodecylammonium and octadecylammonium have been intercalated in the interlayer space of a whole family of synthetic high-charge micas. The amount of adsorbed organic matter and the arrangement of the organic cations in 7330


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