Effects of cetyltrimethylammonium bromide (CTAB) on the structural characteristic of non-expandable muscovite

Materials Chemistry and Physics 196 (2017) 324e332

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Effects of cetyltrimethylammonium bromide (CTAB) on the structural characteristic of non-expandable muscovite Nor Hafizah Che Ismail a, b, Nur Suraya Anis Ahmad Bakhtiar a, Hazizan Md. Akil a, * a b

School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia Faculty of Applied Sciences, Universiti Teknologi MARA (Perlis), 02600, Arau, Perlis, Malaysia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The non-expandable muscovite successfully synthesized using hydrothermal method.
Effect of (CTAB) concentrations on structural and morphology studied.
Number of stacked individual silicate decrease signifies the separation has occurred within muscovite layers.
FESEM show microstructural differences between modified and unmodified muscovite.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2017 Received in revised form 5 May 2017 Accepted 6 May 2017 Available online 8 May 2017

The non-expandable muscovite was treated with an alkaline salt and modified with cetyltrimethylammonium bromide (CTAB) at various concentrations in a cation exchange reaction. Basal spacing, interlamellar structure, morphological structure, and specific surface area (SSA) of this organomuscovite were characterized using X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDX) and the BrunauereEmmetteTeller (BET) method. The increase in basal spacing and SSA following lithium nitrate (LiNO3) treatment and organic modification indicates that natural muscovite can be expandable as the number of stacked individual layers keeps decreasing at high CTAB concentrations as suggested by XRD and BET. The appearance of symmetric and asymmetric stretching vibrations in FTIR analysis indicated the presence of the surfactant in muscovite layers. The FESEM organomuscovite formation images confirm the presence of intercalated structures. The present study shows that not only the basal spacing, but also the specific surface area and the number of the stacked individual silicate layers of the organoclay strongly depend on the packing density of the surfactant within the muscovite interlayer space. © 2017 Elsevier B.V. All rights reserved.

Keywords: Muscovite Cetyltrimethylammonium bromide Ion exchange Intercalation

1. Introduction

* Corresponding author. E-mail address: [email protected] (H. Md. Akil). http://dx.doi.org/10.1016/j.matchemphys.2017.05.007 0254-0584/© 2017 Elsevier B.V. All rights reserved.

The integration of inorganic particles into polymers as precursors of nanocomposites is the most active field in the filling

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industry. Recently, scientists have taken interest in layered silicates generally 2:1 phyllosilicate clay minerals due to their special features of received variation in charge. This variability induces occupancy of the interlayer space by exchangeable cations [1] which makes it suitable for the preparation of most types of nanocomposites. One such phyllosilicate clay minerals is muscovite KAl2(Si3Al)O10(OH)2. Several studies have supported the potential use of muscovite as an important raw material in various applications such as packaging [2,3], automotive [4,5], absorbent [6,7], and electrical appliances [8e10]. The extraordinary properties such as high aspect ratio, chemical inertness, good dielectric properties, high thermal stability, and excellent mechanical properties [11e13] make muscovite a potentially suitable raw material in various applications. The layered structure of muscovite is composed of one octahedral sheet sandwiched between two tetrahedral sheets. Although its layered structure is quite similar to those of montmorillonite, muscovite is considered a non-expandable clay mineral with minimum swelling and ion exchange properties. This is due to the isomorphic substitution of Al3þ ions for Si4þ ion at tetrahedral sites in muscovite. The resulting excess negative charge is balanced by non-hydrated potassium ion, creating a strong electrostatic force between the layers. Muscovite is believed to be non-exchangeable, with inherent expansion, and incompatible with most of the polymer systems. These problems require appropriate treatments to weaken the attraction force of potassium ion and to improve the swelling properties of muscovite by forming hydrophilic clay. However, problem may arise due to incompatibility between the hydrophilic clay and hydrophobic polymers. Therefore, subsequent step of modifying the surface layer by using organic surfactant as part of the ion exchange process is essential to overcome the incompatibility. According to previous researchers [14e16], muscovite is modified using the delamination procedure, which extracts the interlayer potassium ion by treatment with various complex solutions and molten alkaline salts. The early work conducted by Scott et al. [17] found that the interlayer Kþ ion in the muscovite could be depleted using sodium tetraphenylborate and sodium chloride (NaTPB-NaCl) solutions. Based on the existing literature, the next progression on the exchange of potassium ions showed that most researchers used molten LiNO3 solution [17e19] as this solution do not alter muscovite’s structure and chemical composition. Once the interlayer cations of muscovite have been exchanged with more hydrated cations, modification step is needed to change the surface properties of clays from hydrophilic to hydrophobic and to render the layers to be more compatible with the polymer chains. Quaternary ammonium ions with the alkyl chain of 16e20 have been frequently used to prepare organoclays [20]. In case of muscovite, Yu et al. [21] reported the preparation of organomuscovite with the use of CTAB, C16 as intercalating agent. They reported an increase in basal spacing of 27.4 Å. In a recent paper, Jia et al. [19] presented a study of intercalation of muscovite with octadecyl trimethyl ammonium ion (OTAC), C18 and found a progressive increase in basal spacing up to 29.2 Å. Although the intercalation with a large number of amines with different chain lengths has been studied, lack of information about the effect of surfactant concentration on the internal structure specifically on the stacked layers of muscovite and the specific surface area (SSA) of organoclay produced are reported. Therefore, an attempt has been made to convert nonexpandable muscovite to organoclay using CTAB as surfactant and systematically analyzed with the help of XRF, XRD, FTIR, BET, and SEM. In this work, CTAB at different concentrations (0.25, 0.6, 1, and 2 g CTAB/g Li-muscovite) was incorporated into Li-muscovite, and the effects of CTAB concentration on the structure and morphology

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of Li-muscovite were investigated. Experimental results confirmed that CTAB can be intercalated and adsorbed onto the clay depending on its concentration. The information of crystallite size obtained from WAXD using Scherrer equation enabled the estimation of the number of the stacked individual silicate layers before and after modification. This study provides new insights into the structure and properties of organoclay and demonstrates that the orientation of alkylammonium molecules strongly depends on the surfactant packing density within muscovite interlayer space. The understanding of the CTAB’s orientation on layered silicates, its morphology and structural changes is of great importance, as this could help expand the application of organomuscovite to many new fields. 2. Experimental methods 2.1. Materials A complimentary sample of muscovite was supplied by Lingshou County Xinfa Mineral Industry Co. Ltd. Muscovite was subjected to two stages of ion exchange treatment. Lithium nitrate (LiNO3) and cetyltrimethylammonium bromide (CTAB) for the ion exchange treatment of the muscovite particles were supplied by Merck. 2.2. Preparation method (Ion exchange of muscovite) 2.2.1. Inorganic ion exchange using molten lithium nitrate The Li-muscovite and the organomuscovite (OM) were prepared using the cation exchange process as proposed by Yu et al. [21]. Muscovite powder (5 g) and LiNO3 powder (85 g) were mechanically mixed using a ball milling process. The resulting mixture was then heated in a furnace at 300  C for 12 h. The resulting product was washed with deionized water and vacuum-filtered. The filtrate was then dried in a vacuum oven at 110  C for 12 h. The product obtained was a strong, shiny, fine-grained silver powder and was labelled as Li-muscovite. 2.2.2. Organic ion exchange by intercalating the Li-muscovite with the cetyltrimethylammonium bromide The CTAB-modified muscovite was prepared under hydrothermal condition. The ion-exchange treatment was performed at various CTAB concentrations, while the Li-muscovite mass was kept constant. Firstly, 1.2 g of Li-muscovite was dissolved in 150 mL of deionized water and then mixed with 0.3 g of CTAB at room temperature. At this stage, sonication was introduced to improve the dispersion of muscovite in the surfactant while at the same time promoting the delamination of muscovite particles. Then, the mixtures were placed in a hydrothermal reactor (300 mL) and heated at 200  C for 12 h. Subsequently, all surfactantemuscovite products were filtered and washed at least three times with ethanol to remove any excess bromide anions as confirmed by silver nitrate (AgNO3) test. All organoclays were dried in a vacuum oven at room temperature, ground in an agate mortar, and stored in a desiccator. The obtained CTAB-modified muscovite samples were labelled as OM0.25. All the other CTAB-modified muscovite samples (OM0.6, OM1, and OM2) were prepared using different concentrations of CTAB solutions. The muscovite chemical composition was determined using automatic sequential X-ray fluorescence (XRF). X-ray diffraction (XRD) analysis was performed using Bruker D8 Advance diffractometer with a Cu target of k ¼ 1.5405 Å at a generator voltage and current of 35 kV and 30 mA, respectively. The wide-angle diffraction patterns at 2q ranging from 5 to 70 were collected at a step-

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scanning speed of 10 /min. The wide-angle diffraction patterns (WAXD), ranging from 1 to 10 , were collected using a step size of 0.05 and a scan speed of 3 /s. The interlayer distance of organoclay in the composite was calculated using Bragg’s equation. The specific surface area was calculated using the BrunauereEmmetteTelle (BET) method. Fourier transform infrared (FTIR) was conducted using Perkin Elmer System 2000 FTIR spectrometer (Waltham, MA, USA), using KBr pellet method. The ratio about 1:300 mg of sample and KBr was used in the preparation of the pellets. The spectrum resolution was 4 cm1 in a spectral range of 4000 to 500 cm1. The muscovite composition and the OM morphologies were analyzed using Ziess Supra-35VP field emission scanning electron microscope (FESEM) coupled with energy dispersive X-ray spectroscopy (EDX). 3. Results & discussion 3.1. X-ray fluorescence The XRF analysis was carried out to determine the chemical composition of the minerals and the chemical changes resulting from the treatment. The composition of the muscovite before and after the treatment is presented in Table 1. Chemical analysis shows high levels of SiO2 and Al2O3, whereas K2O and Fe2O3 are only present in small amounts. All other oxides such as NaO2, MgO, NiO, Rb2O, and ZrO2 are present in trace amounts. After LiNO3 treatment with, the muscovite composition changed considerably, where the contents of MgO, CaO, and K2O decreased while Al2O3 and SiO2 contents increased. The increased in alumina and silica content can be easily explained by the effects of milling and grinding steps, which caused an increase in the lateral surface area of the muscovite plates where hydroxyl groups (Si-OH, Al-OH) were located. However, these results contradict those obtained by Osman et al. [22]. This discrepancy is due to the difference in the treatment method used. An important part of carrying out the XRF analysis is to identify the ion exchange mechanism between Kþ and Liþ ions by examining the changes in the Kþ ions concentration as the result of the exchange process. The decreasing amount of Kþ ions (8.81 and 6.08 wt% of K2O in the untreated and treated muscovite, respectively) confirms that the ion exchange process fundamentally changes the muscovite composition. This 45% of Kþ ions were exchanged with Liþ ions. In this study, the presence of an appreciable amount of Kþ ions after LiNO3 treatment indicates an incomplete ion exchange because there are still counterions present in the interlayers. The result shows Liþ ions caused changes in the muscovite composition, which is not the case in the study conducted by Friedrich et al. [23]. No significant decrease in the potassium concentration was detected during the treatment with the use of polyvalent cation Cu2þ. However, in this analysis the exchangeable amount of Liþ ions cannot be detected due to the limitation of the instrument to

measure elements with the atomic number, Z, of less than 11 (the atomic number of Liþ ion is 3). 3.2. X-ray diffraction analysis The best way to study the dispersion level of organoclay in a matrix is investigated using XRD analysis. It is a very important characterization tool capable of tracking the changes in basal spacing and providing information about the degree of intercalation or the exfoliation of the organoclay. The XRD patterns of muscovite before and after the treatment are shown in Fig. 1. Based on the basal reflections and hkl values, muscovite can be classified as a monoclinic related structure (ICDD Data Card No. 98-0034853). The d(002) (9.9 Å) of the main peak indicates that the original sample is muscovite. The XRD peaks are sharp and narrow confirming the high quality and good crystallinity of the sample. 3.2.1. Effect of alkaline treatment The main purpose of conducting this treatment is to change the non-hydrated cations (Kþ) with more hydrated cations (Liþ) through ion exchange process. After LiNO3 treatment, the peak intensities in Fig. 1 increases and widens as 2q decreases. The existence of a new diffraction peak at 2q ¼ 7.30 does not indicate a new compound formation as these results are caused by the presence of the alkaline salt LiNO3 and in good agreement with the work conducted by Yu et al. [18]. Although some of the peaks have shifted after the LiNO3 treatment they are still narrow and sharp. This is due to the heat treatment at 200  C, causing the muscovite lattice to absorb more energy thus enhancing the lattice vibration. Therefore, the exchange process was initiated, where Kþ ions were replaced by Liþ ions at the interlayer. This can be seen at the d(002) plane which shifted from 2q ¼ 8.87 e7.30 corresponding to 9.9 Å and 12.1 Å, respectively. The interlayer distance is determined from the diffraction peak using Bragg’s Law equation as shown in Equation (1):

2 d sinq ¼ l;

(1)

where l is the wavelength, q is the angle between the incident rays and the surface of the crystal, and d is the spacing between the layers of atoms. The shift in the location of the peaks shows an increase in the d spacing which in turn, demonstrates the

Table 1 Chemical composition of muscovite and Li-muscovite. Elemental oxide

Muscovite

Li-muscovite

NaO2 MgO Al2O3 SiO2 K2O Fe2O3 NiO Rb2O ZrO2 Nb2O5

0.15 0.71 30.93 52.74 8.81 5.68 0.01 0.05 0.07 0.01

e 0.44 32.40 54.52 6.08 5.72 e 0.02 0.067 e

Fig. 1. Powder XRD diffraction patterns of (a) muscovite and (b) Li-muscovite.

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separation between the adjacent muscovite layers and results in an intercalated muscovite structure. The reaction of the molten LiNO3 with the muscovite is presented in Equation (2). The net effect of this reaction would lower the muscovite layer charge due to the partial rejection of octahedral cations or the partial hydroxyl structure loss as mentioned by Newman and Brown [24]. Once the layer charges decrease, the replacement of interlayer Kþ ion with Liþ ions will occur

KAl2 ðAlSi3 O10 ÞðOHÞ2 þ LiNO3 /LiAl2 ðAlSi3 O10 ÞðOHÞ2 þ KNO3 (2) þ

The exchange process occurs due to the ability of Li ions to penetrate the muscovite crystal lattice and occupy the vacant octahedral sites to partially neutralize the charge as the position on the periodic table indicates a lithium cation is smaller than a potassium cation. Liþ ion is a strong reducing agent due to its ability to easily lose one of its valence electrons. Therefore, the interlayer attraction forces result in the reversible uptake of water. This causes interlayer spacing to expand. Liþ ions are able to take up water due to high hydration enthalpy and this improves the muscovite swelling characteristic. Hence, the results prove that LiNO3 had not only been adsorbed onto the muscovite surface but caused the layers to move apart by increasing the basal spacing. However, not all ions are fully exchanged during this treatment, indicating incomplete removal of Kþ ions. In fact the diffraction patterns of both samples are similar with some peaks that remain in their original positions and the main peaks at d(002) still exist even though the muscovite crystallinity decreases slightly. It can be concluded that the alkaline solution activated the muscovite crystal lattice, increasing the crystallinity without destroying its structure through delamination of muscovite layers.

Fig. 2. Powder XRD diffraction patterns of (a) OM0.25, (b) OM0.6, (c) OM1, and (d) OM2.

concentration and basal spacing. Two plateaus are observed once the CTAB concentration increases above its equilibrium concentration. From the WAXD patterns, the crystallite size present in untreated and treated muscovite were focused on the main diffraction peak 8.87, corresponding to the interplanar spacing of 9.98 Å, as the peak depicts the major changes after surface layer being modified. The size is calculated using Debye-Scherrer formula [28].

D¼ 3.2.2. Effect of the cetyltrimethylammonium bromide concentration on the Li-muscovite Modification of muscovite’s surface is required to enlarge the basal spacing and improve the compatibility with polymer matrices. The changes in basal spacing indicate the organic surfactant has been intercalated within the interlayer of organoclay. Surface treatment of Li-muscovite with various concentrations of CTAB was investigated (0.25, 0.6, 1, and 2 g/g Li-muscovite). The changes in the diffraction patterns of all OM samples are depicted in Fig. 2. The peaks in all the OM diffraction patterns are very intense confirming a high degree of order for the organoclay produced. As the CTAB concentration increases, the basal spacing of the resultant OM also increases in the following order: 25 Å (OM0.25), 27.9 Å (OM0.6), 28.20 Å (OM1), and 28.20 Å (OM2). The increase in the d spacing was calculated using the Bragg’s law equation by considering the thickness of the muscovite aluminosilicate layer of approximately 6.7 Å [25]. The distance between the adjacent aluminosilicate layers in the muscovite increased 5.4 Å for Limuscovite. For samples with lower concentration at OM0.25, the interlayer spacing increased 18.3 Å and 21.2 Å at OM0.6, which is the optimum value of basal spacing. The observed optimum values at OM0.6 can be attributed to effective interactions between the charged sites in the muscovite layer and the long chain of CTAþ ions. In addition, the ions are more mobile due to a larger number of unoccupied active sites at the interlayer region [26]. Meanwhile, at OM1 and OM2, two progressive plateaus are observed. This observation is due to additional increase in the equilibrium CTAB concentration, resulting in the surface saturation through ion exchange and the ion pairing mechanism [27]. However, the results still show an increase in the interlayer spacing to 21.5 Å in height. This means that there is a direct correlation between CTAB

ðK lÞ bcosq

(3)

where D is the mean crystallite size, K is the grain shape dependent constant of 0.89, l is the wavelength of the incident beam, q is the Bragg reflection angle, and b is the line broadening at half the maximum intensity in radians. Table 2 clearly establishes that crystallite size which is the thickness (d clay ¼ D) of the intercalated layers in muscovite decreases with increasing CTAB concentration. The decrease in crystallite size explains that the lattice becomes less rigid. Hence, lower energy is required to distort the lattice, allowing the ease of polymer insertion. In addition, by dividing the value of D by d spacing value, the number of stacked individual silicate layers for muscovite, Li-muscovite, and organoclays of OM0.6 and OM1 were determined to be 80, 48, 11, and 9, respectively. The decreasing number of stacked individual layers signifies the separation has occurred within the muscovite layers. Fig. 3 displays Li-muscovite behavior in the aqueous solution of CTAB. In contrast to the pristine muscovite when Li-muscovite came into contact with CTAB aqueous solution, the muscovite layers changed significantly. At this stage the hydrophobicity of muscovite increased. This result is supported by the work conducted by Liao et al. [29] proving that the hydrophobicity will

Table 2 Characteristic parameter of untreated muscovite and various OMs. Characteristic parameter

Muscovite

Li-muscovite

OM0.6

OM1

d spacing (nm) D, crystallite size (nm) Number of stacked individual layers (D/d spacing)

0.99 79 80

1.21 59 48

2.78 32 11

2.82 26 9

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Fig. 3. Schematic of LiNO3 treatment and modification procedure of muscovite in the aqueous solution of CTAB.

increase once CTAB intercalates into clay layers. Upon intercalation with CTAB, the interlayer spacing increases more indicating the formation of an intercalated structure. CTAB has a high tendency to facilitate the exfoliation of Li-muscovite by lowering the surface energy, thus enlarging the interlayer spacing. The increase in the interlayer spacing is important as the OM formation from this reaction is essential during polymer insertion to interact with this type of filler. The head of CTAB will interact with the surface of muscovite while the tail will attach to polymer matrix. CTAB will improve the hydrophobicity of the OM and the adhesion between the filler and the polymer matrix [30]. By varying CTAB concentration, the orientation of alkylammonium molecules between the clay unit layers can be observed. According to Xi et al. [31], the lateral arrangement of alkylammonium molecules may adopt a monolayer (13.6 Å), a bilayer (17.7 Å), a pseudotrimolecular layer (21.7 Å), or a paraffin layer (>22 Å). However, in this study, the bilayer and the pseudotrimolecular layer arrangement were absent, because the basal spacing was greater than 21.7 Å in most locations. These results suggest that the important step of LiNO3 treatment greatly influences the muscovite gallery expansion. In most modification steps, the paraffin layer is the most desirable orientation because this arrangement may ease the process of intercalation and exfoliation within the polymer

matrix. In our study, the CTAþ ions in the muscovite interlayer space were arranged in a paraffin complex when OM concentrations ranged from OM0.25 to OM2. This means the CTAþ ions assume a tilted position at the right angle to the aluminosilicate surface [31]. The latter peak is sharp, suggesting the paraffin type monomolecular arrangements are the major constituents in the muscovite layers. This result is in good agreement with the experimental results reported by Yu et al. [21]. It suggests that the expansion of muscovite layers occurs, resulting in a given alkylammonium ions arrangement. Therefore, by understanding the initial basal changes and the surfactant arrangement in the prepared organoclay, the potential for polymer intercalation can be tailored to a specific purpose. 3.3. BrunauereEmmetteTeller analysis BET analysis is a very useful method for determining the surface area and is strongly dependent on the degassing condition. As clays are hydrophilic materials, high-vacuum conditions are required to remove all weakly absorbed water molecules. N2 absorption measurements using BET method were conducted on all samples. In this study, the relationship between basal spacing and specific surface area (SSA) was investigated as shown in Table 3.

N.H. Che Ismail et al. / Materials Chemistry and Physics 196 (2017) 324e332 Table 3 XRD and BET results of the pristine muscovite and the OM samples. Sample

Surface area (m2 g1)

Basal spacing (nm)

Muscovite Li-muscovite OM0.25 OM0.6 OM1 OM2

3.8389 5.5018 2.2269 2.2985 3.8495 8.2290

0.99 1.21 2.50 2.79 2.82 2.82

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Fig. 4 demonstrates the relationship between the basal spacing of all OMs and the SSA. There is a direct relationship between CTAB concentration and basal spacing, where at certain concentration above the threshold value, the basal spacing became plateau. Meanwhile, a decreasing correlation was observed mostly at low concentration of CTAB. As CTAB is added, the surfactant and Limuscovite particles interact and the interaction is usually polar. CTAB is an amphiphilic compound which consists of two parts of hydrophilic (head) and hydrophobic (tail). In this case, CTAB hydrophilic head will attach itself to the polar muscovite surfaces, while the hydrophobic tail will be oriented toward the polymer matrix. The surface area decreases at low CTAB concentrations but increases at high CTAB concentrations. There are two reasons for this phenomenon: 1. The interaction between the OH ions on the muscovite surface with the Nþ ions in the surfactant. 2. The strong repulsive forces between Liþ and Nþ ions as Liþ ions have a high strong positive charge per volume ratio.

Fig. 4. A plot of basal spacing (filled circles) to specific surface area (filled squares) of pristine muscovite and OM samples.

After the LiNO3 treatment, the SSA increased by 43%, from 3.8389 m2 g1 to 5.5018 m2 g1. This can be attributed to the particle size reduction following the treatment. In contrast, the SSA decreased to 2.22 m2 g1 at the early stage of CTAB introduction (OM0.25). The SSA increased further to 2.2985 m2 g1 and 3.84 m2 g1 for the OM0.6 and OM1.0, respectively. These results demonstrate that the increased in SSA of OM was still below the pristine muscovite mainly at low concentration of CTAB. For the OM2.0 samples, the surface area increased significantly to 8.22 m2 g1.

At low CTAB concentrations, the repulsive forces between Liþ and Nþ ions are dominant, making the surfactant inaccessible. At this stage, the surfactant is only starting to adsorb onto the surface, mainly by the ion exchange mechanism while at the same time forming aggregates. The concentration of CTAþ molecules in the bulk solution is very low. After CTAB concentration increases slightly to 0.6, the slope increases but still below the SSA of pristine muscovite. At this point, the adsorption must overcome the electrostatic repulsion between the incoming ions and the similarly charged surface [27]. Therefore, the specific surface area decreases at low CTAB concentrations due to strong repulsive forces. At high CTAB concentrations, however, the stack density of organic cations in the interlayer space increases and the arrangement of the organic cations changes. The attraction force between the OH ions, which is the main effective sites for adsorption from muscovite surface onto Nþ of CTAB, is very strong. Due to the high attracting forces, the surface area increases and the basal spacing expands. The changes in SSA also may induce degradation in the crystallinity of muscovite layers as observed from the XRD patterns. This correlation is reported by Murray & Lyons [32] which first suggested that there was a general relationship between decreasing crystallinity with an increase of SSA.

Fig. 5. FTIR spectra of (a) Muscovite and (b) OM.

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Fig. 6. FESEM images of (a) muscovite and (b) Li-muscovite, with a magnification of 1000.

Table 4 EDX data for muscovite and Li-muscovite elemental compositions. Elements

Muscovite (wt%)

Li-muscovite (wt%)

C O Mg Al Si K Fe

06.71 35.63 01.07 17.54 24.22 10.79 04.04

22.29 33.64 00.68 13.57 18.92 05.06 05.84

3.4. Fourier transform infrared analysis FTIR spectroscopy is an effective analytical instrument for detecting functional groups and characterizing the covalent bonding information. As shown in Fig. 5, the FTIR spectra reveal the structural differences between muscovite and organomuscovite. New absorption bands at 2922, 2855, 1653, and 1474 cm1 appeared after the muscovite was modified by CTAB. The bands at 2922 and 2855 cm 1 correspond to the CH2 asymmetric and symmetric stretching vibration modes of alkyl chain. These bands are absent in the original muscovite spectrum which indicates the incorporation of CTAB into the muscovite structure. Meanwhile, the absorption bands at 1653 and 1474 cm1 are from methylene scissoring mode [33]. The absorption band at 3624 cm 1 is due to the OH stretching vibration. Based on the transmittance bands, the

structure of original muscovite is similar with organomuscovite, suggesting that the structure of aluminosilicate layers is intact even after the exchange process with surfactant.

3.5. Morphology analysis The morphological images of the materials show the surface morphologies of muscovite and OM samples. Some differences in the shape of the muscovite and the changes occurring in Limuscovite and OM at different CTAB concentrations are observed. Fig. 6a shows that the shape of muscovite particles are plate-like and the aggregated plates are compact from the edges with the face dimension of most particles being less than 100 mm. The muscovite layers are closely stacked together, due to strong intermolecular forces compared to those of Li-muscovite (Fig. 6b). Following LiNO3 treatment, the Li-muscovite surface particles become loosely packed and expanded due to the increased basal spacing as calculated using the XRD results. These results suggests an increase in basal spacing is due to the inorganic cations exchange process, where Kþ ions are replaced by Liþ ions. The introduction of hydrated cations Liþ not only promotes swelling but it also helps to reduce the number of agglomerations within muscovite stacked layers. This observation is supported by energy dispersion X-ray spectroscopy (EDX) results. Table 4 shows the elements in the

Fig. 7. FESEM images of the resultant organoclay (a) OM0.25, (b) OM0.6,(c) OM1, and (d) OM2.

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muscovite before and after the LiNO3 treatment. It is clear that the weight percentage of K decreased by about 50% following the treatment, due to the migration of Liþ ions formed as a result of the isomorphous substitution in the muscovite tetrahedral site. In addition, the EDX spectra endorse muscovite purity and demonstrate that the particle is composed of four main elements (Si, Al, O, and K). Thus, EDX spectra indicate that the chemical composition not changed as the result of the LiNO3 treatment. However, upon intercalation with CTAB, the organoclay morphology changes. The intercalation with CTAþ cations reduces the grain dimensions. In other words, the exchange process with CTAB breaks the Li-muscovite particles into smaller parts. Fig. 7 shows the micrographs of the organoclay basal spacing. The separation within the muscovite layers is more intense, given their mainly flat morphology. The morphology of the opening layer of OM2 is clearly seen as being mostly flat and aggregated. Overall, the images at both low and high CTAB concentrations show that the layers are well separated and peeled. These micrographs demonstrate that CTAB addition expands the basal spacing in the muscovite layers and at high CTAB concentrations, the flat-layer morphology becomes more prevalent. Hence, the agglomerated platelets separates and more refined exfoliation can be observed. Therefore, the present study shows that the organoclay morphology strongly depends on the packing density of surfactant and this observation was also reported by Tiwari et al. [34]. The main reason for this observation is that at high packing density, the clay layers are observed clearly due to the significant muscovite expansion, resulting from the penetration of CTAþ molecules into the muscovite layers. 4. Conclusions Li-muscovite is successfully synthesized using the hydrothermal preparation method wherein silicate layers of the clay were intercalated and the number of stacked silicate layers decreased with increasing CTAB concentrations. The orientation of alkylammonium adopted paraffin complex arrangement as a major constituent in the intercalated muscovite. The absence of bilayer and pseudotrimolecular layer represents that the important step of the LiNO3 treatment has a significant effect on the enlargement in the muscovite gallery without destructing the muscovite structure as confirmed by FTIR. The XRF and EDX proved that about half of Kþ ions were exchanged for Liþ ions. The FESEM analyses demonstrated microstructural differences between the modified and the unmodified muscovite and confirmed that a coupled of LiNO3 treatment and addition of organic surfactant (CTAB) through ion exchange process promoted delamination in muscovite layers. The decreasing in the number of stacked individual silicate signifies the separation has occurred within the muscovite layers. The effect of CTAB concentrations on SSA was such that the surface areas were reduced due to strong repulsive forces but increases tremendously at OM2 as well as basal spacing expansion. Thus, detailed understanding of the interlayer structure of the organoclay produced is important in the design and the implementation of organoclaybased materials in industry. The chemical modification of the Limuscovite surface layers warrants further study in the view of producing a new class of layered silicate nanocomposites. Acknowledgments This work was supported by the Research University (Grant No. 1001/PKT/8640013) of Universiti Sains Malaysia (USM) and the

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