Changes in the morphology of organoclays with HDTMA + surfactant loading

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1 Applied Clay Science 31 (2006) Changes in the morphology of organoclays with HDTMA + surfactant loading Hongping He a,b, Ray L. Frost b,, Thor Bostrom b, Peng Yuan a, Loc Duong b, Dan Yang a, Yunfei Xi b, J. Theo Kloprogge b a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou , China b Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane, QLD 4001, Australia Received 29 March 2004; received in revised form 31 May 2005; accepted 17 October 2005 Available online 1 December 2005 Abstract The detailed understanding of the interlayer structure of organoclays is of importance in the design of organoclay based materials and their industrial applications. In this study, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been used to provide new insights into the interlayer structure and morphology of HDTMA + /montmorillonite organoclays. XRD patterns show that thermal treatment has an important effect on the stability of organoclays, reflected by significant changes in the basal spacing. TEM and SEM micrographs demonstrate that the organoclays with lower surfactant packing density are mainly composed of irregular layer stacking with a number of curved organoclay layers, while those with higher surfactant packing density are mainly composed of regularly intercalated and flat layers. Variations of the interlayer distances exist in all organoclays and are more pronounced in the organoclays with lower surfactant packing density. This study demonstrates that not only the arrangement model of surfactant but also the morphology of organoclay strongly depend on the surfactant packing density within the montmorillonite interlayer space Elsevier B.V. All rights reserved. Keywords: Intercalation; Surfactant; Montmorillonite; Organoclay; TEM; SEM morphology 1. Introduction The study of organoclays is a vital subject in current research since various organoclays are widely used as nanocomposite precursors (Gu et al., 2001; Maiti et al., 2002), adsorbents for organic pollutants (Zhu and Su, 2002), rheological control agents (Manias et al., 1996) and electric materials (Wu and Lerner, 1993). The combination of the hydrophobic nature of the surfactant and the layered structure of the silicate layers leads to Corresponding author. address: r.frost@qut.edu.au (R.L. Frost). unique physicochemical properties. In these applications, the behavior and properties of the organoclays strongly depend on the structure and the molecular environment of the organic molecules within the galleries. During the intercalation of the surfactants, the interlayer of the clay minerals is expanded as shown by the XRD patterns. A number of previous studies have demonstrated that the d-spacings of the organoclays depend on the length of the alkyl chains and the packing density of the surfactants within the galleries of clay minerals (e.g., Lagaly, 1981; Beneke and Lagaly, 1982; Favre and Lagaly, 1991; Tamura and Nakazawa, 1996; /$ - see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.clay

2 H. He et al. / Applied Clay Science 31 (2006) Klapyta et al., 2001; Yui et al., 2002). Accordingly, various models for the surfactant structural arrangements have been proposed chiefly based on XRD results. Recently, new approaches to the study of the surfactant structural arrangements including Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, differential scanning calorimetry (DSC) and magic-angle-spinning nuclear-magic-resonance (MAS NMR) have been introduced to study the microstructure of organoclays and provided detailed conformational information (e.g., Vaia et al., 1994; Khatib et al., 1996; Harris et al., 1999; Wang et al., 2000; Li and Ishida, 2002, 2003; Madejova, 2003; Zhu et al., 2003; He et al., 2004a,b). However, these studies could not provide information about morphology and homogeneity of the organoclays. TEM complemented with SEM is the best techniques to investigate the morphology of clay minerals. TEM and SEM have been widely used in the study of organoclay based nanocomposites (Hoffmann et al., 2000; Gu et al., 2001; Maiti et al., 2002; Xie et al., 2003). In this research, the principal focus was on the distribution of organoclays in the nanocomposites and their effect on the nanocomposite properties. However, few studies have been performed on the microstructure and morphology of the organoclays (Lee and Kim, 2002a,b). Hoffmann et al. (2000) revealed that low molecular weight modifiers only promoted intercalation and failed to exfoliate the silicate particles during the synthesis of nanocomposites, whereas organophilic modification of clay with bulky surfactant molecules affords completely exfoliated silicate particles in the nanocomposites. This means the intercalated species of organoclay have an important effect on the properties of the organoclay and, therefore, on nanocomposite. Meanwhile, Lee and Kim (2002a,b) proposed that the charge value or its distribution on clay layer surface might be an important factor in expansion. Such characteristics of the organoclay would affect the function of the organoclay as a precursor in nanocomposite formation. In this study, HDTMA + /montmorillonite organoclays were prepared at different surfactant concentrations and investigated using TEM, SEM and XRD, to elucidate the relation between the morphology of organoclay and the surfactant packing density within the montmorillonite galleries. Such information is vital for the preparation of organoclay-based nanocomposites. 2. Materials and methods The montmorillonite used in this study originated from Hebei province, China. The sample was purified by sedimentation and the b2-μm fraction was collected and dried at 90 C. The sample was ground and sieved through 200 mesh and sealed in a glass tube for use. The collected montmorillonite contains very low concentrations of illite and kaolinite (Zhu et al., 2003). The cation exchange capacity (CEC) was 57.9 meq/ 100g. Its structural formula is [Na 0.05 Ca 0.18 Mg 0.10 ][Al 1.58 Fe 0.03 Mg 0.39 ][Si 3.77 Al 0.23 ]O 10 (OH) 2 nh 2 O. The surfactant used in this study is hexadecyltrimethylammonium bromide (HDTMAB) with a purity of 99%, and was obtained from YuanJu Chem. Co., China. The surfactant was used without any further purification. The preparation of Na montmorillonite (Na mont) was carried out by the following procedure. 10 g of the mixture of montmorillonite (9.4 g) and Na 2 CO 3 (0.6 g) was added into 100 ml of deionized water and stirred at 80 C for 3 h. Na mont was collected by centrifugation and washed with deionized water until the ph of the solution of Na mont was 7. The Na mont was dried at 105 C, ground and sieved through 200 mesh and kept in a sealed bottle. The synthesis of organoclay was performed by the following procedure: 2.5 g of Na montmorillonite was first dispersed in about 300 ml of deionized water, to which a desired amount of hexadecyltrimethylammonium bromide was slowly added. The concentrations of HDTMA + were 0.5 CEC, 1.5 CEC and 5.0 CEC of montmorillonite, respectively. The reaction mixtures were stirred for 10 h at 80 C. All products were washed free of bromide anions (tested by AgNO 3 ), dried at 90 C and ground in an agate mortar. The HDTMA + / montmorillonite organoclays prepared at the concentrations of 0.5 CEC, 1.5 CEC and 5.0 CEC were labeled as OM1, OM2 and OM3, respectively. For TEM observation, small drops of dilute suspensions of 0.1 g of the clays in 5 cm 3 of doubly distilled water were placed on Cu mesh grids which had been coated with a thin carbon film. The grids were air-dried then briefly placed in a 60 C oven to ensure complete drying prior to insertion into the instrument. The specimens were examined in a Philips CM200 transmission electron microscope operated at an accelerating voltage of 200 kv. Although this preparation is not generally optimal for high resolution imaging as compared to thinned or sectioned samples, the montmorillonite crystallite layers were thin enough that imaging of the interlayer spacings could be performed where suitable orientations could be obtained. The surface morphology of the prepared organoclays was examined by SEM. Small amounts of the dried powders (approximately 0.01 g) were placed on sticky carbon tape on standard Al mounts, then sputter coated with a thin conductive layer of gold. The samples were viewed in an FEI Quanta 200 scanning electron microscope at 20 kv. For XRD analysis, the organoclays were first dispersed in the deionized water, then dropped on the glass slides and dried in the air. XRD patterns were recorded between 2 and 20 (2θ) at a scanning speed of 2 /min, using Rigaku D/max-1200 diffractometer with CuKα radiation. In addition, the XRD analysis of the organoclay powder (before being dispersed in water) was performed under the same conditions as mentioned

3 264 H. He et al. / Applied Clay Science 31 (2006) above, to elucidate the effect resulted from the dispersion of organoclay in water. 3. Results and discussion Before TEM and SEM observations of the three organoclays, XRD analyses were performed with similar preparation methods as for TEM and SEM. Fig. 1 shows the XRD patterns of Na montmorillonite and the three organoclays prepared at different surfactant concentrations. Upon intercalation, the basal spacings are expanded as expected depending on the surfactant concentrations. The d(001) values of OM1 and OM3 are at 1.48 nm and 2.26 nm, respectively, Fig. 1. XRD patterns of Na montmorillonite (Na mont) and HDTMA + /montmorillonite organoclays. while OM2 shows the main peak at 1.98 nm with a shoulder at 2.22 nm. However, the powder diffractions of the three organoclays before being dispersed in the deionized water show the basal spacings at 1.48 nm, 2.45 nm and 4.03 nm for OM1, OM2 and OM3 (not shown), respectively, which are identical to previous reports (Zhu et al., 2003). This indicates that heating the sample at 90 C during drying will result in the instability of surfactants within montmorillonite galleries, especially for the organoclays with higher surfactant packing density. As shown in Fig. 2, the shape of a perfectly straightchain of HDTMA + looks like a nail, where the long alkyl chain is the nail-body and the chain end holding three methyls is the nail-head. Based on the data of van der Waals radius, covalent bond radius and bond angle, the parameters of HDTMA + were calculated and are shown in Fig. 2. When the HDTMA + lies stretched out, the length of the nail is 2.53 nm, consisting of the nail-head (0.43 nm) and nail-body (2.1 nm). Our calculation result is similar to that in the literature (Yui et al., 2002). However, the height of the HDTMA + cation will vary with its orientation. When the plane of the zigzag arrangement of the carbon atoms of HDTMA + is perpendicular to the plane of the silicate layer as shown in Fig. 2a, the height of the nail-body is 0.46 nm and that of the nail-head is 0.51 nm. However, the heights of nail-body and nail-head are 0.41 nm and 0.67 nm, respectively, when the plane of the zigzag arrangement of the carbon atoms of HDTMA + is parallel to the plane of the silicate layer as shown in Fig. 2b. From the literature (Harris et al., 1999), the thickness of a single montmorillonite's TOT layer is 0.97 nm. The basal spacing for OM1 is 1.48 nm as shown in Fig. 1. Therefore, the interlayer distance is 0.51 nm. This value agrees well with the height of the nail-head of HDTMA + when the plane of the zigzag arrangement of the carbon atoms of HDTMA + is perpendicular to the plane of the montmorillonite layer. This implies that a lateral-monolayer arrangement of HDTMA + within the interlayer space of montmorillonite and the plane of the zigzag arrangement of the carbon atoms of HDTMA + is perpendicular to the plane of the montmorillonite layer. For OM3, the basal spacing of 2.26 nm reflects a paraffin-type monomolecular arrangement. Taking the length of HDTMA + and the thickness of TOT layer into account, it can be calculated that the angle (α) between the alkyl chain and basal surface is 31. In the XRD pattern of OM2, the main peak shifts to 1.98 nm, while a weak diffraction peak at 2.22 nm, corresponding to a paraffin-type monomolecular arrangement, is recorded. The basal spacing of 1.98 nm

4 H. He et al. / Applied Clay Science 31 (2006) Fig. 2. Molecular conformation of HDTMA + : (a) the plane of the zigzag arrangement of the carbon atoms of HDTMA + is perpendicular to the plane of the montmorillonite layer; (b) the plane of the zigzag arrangement of the carbon atoms of HDTMA + is parallel to the plane of the montmorillonite layer. indicates that there is an interlayer spacing of 1.01 nm. The pseudotrilayer arrangement can well explain the basal reflection at 1.98 nm. Beneke and Lagaly (1982) and Brindley and Moll (1965) proposed a model for the arrangement of alkyl chains based upon mutual interlocking. In the parallel packing of the chains, it was assumed that a CH 2 group of one chain lies between similar groups of neighboring chains. In this case, the height of the bilayer with interlocking chains should be smaller than that without interlocking. The decrease of the height is 0.1 nm when adding one layer with interlocking, compared with that without interlocking. Therefore, the height of pseudotrilayer should be 1.0 nm. This value is similar to our experimental results and that reported previously by other researchers (Choy et al., 1997). Based on the variable basal spacings and the broad profiles of XRD patterns, it is proposed that the HDTMA + /montmorillonite organoclays represent multi-phase systems consisting of both regularly and randomly intercalated layers. This is clearly demonstrated by the TEM micrographs as shown in Figs The morphologies of OM3 are shown in Fig. 3. Generally, OM3 is mainly composed of regularly

5 266 H. He et al. / Applied Clay Science 31 (2006) Fig. 3. TEM micrographs of OM3 with generally regular stacking of intercalated layers with a slight difference of layer height from layer to layer (a) and coexistence of regularly and irregularly intercalated layers (b and c). intercalated layers as shown in Fig. 3a in spite of a slight difference in the layer-to-layer distance for different intercalated clay particles (see Fig. 3a, b and c). This kind of difference does exist even for the neighboring interlayer spaces in the same particles (shown in Fig. 3a) and for different parts in the same interlayer space (shown in Fig. 3b). However, most layer-to-layer distances are between 2.2 and 2.3 nm, corresponding to relatively narrow basal reflection centered at 2.26 nm. We can also observe a characteristic swelling of montmorillonite containing termination of layers as shown by the arrows in Fig. 3b. This suggests that the swelling of silicate layers may be augmented by defects of the clay structure (Lee and Kim, 2002a). In sample OM3, most layers are flat with few curved layers. In sample OM2, irregularly intercalated layers can always be observed as shown in Figs. 4a and b. However, some regularly intercalated layers only occur in a small area (Fig. 4c). In addition, the slight difference of layer height for the regularly intercalated layers still can be observed as in OM3. Compared with sample OM3, the morphologies of OM2 show that there are much more variations for layer height of the organoclay, and the layer height is obviously decreased with most in the range of nm. This is in accordance with the XRD result for OM2, in which a broad basal reflection at 1.98 nm and a weak shoulder at 2.22 nm are recorded. Meanwhile, some curved and irregular organoclay Fig. 4. TEM micrographs of OM2: (a) curved and irregularly intercalated layers; (b) irregular stacking of intercalated layers; (c) relatively regular stacking of intercalated layers in a small area; (d and e) curved and irregular stacking of intercalated layers.

6 H. He et al. / Applied Clay Science 31 (2006) Fig. 5. TEM micrographs of OM1 with curved and irregularly intercalated layers. layers can be found in OM2 (Fig. 4e). This is obviously different from sample OM3. For sample OM1, it proved difficult to obtain detailed TEM micrographs since the small amount of surfactants within the montmorillonite interlayer made the sample susceptible to beam damage or radiolysis when electron beam was focused on the sample for any period of time. Fig. 5 shows the morphologies of sample OM1. Generally, they are similar to those of sample OM2, being mainly composed of regularly intercalated layers with regular layer-to-layer distance. In some area, the layer height remained about 1.3 nm, similar to that of Na montmorillonite. This indicates that not all the interlayers are intercalated by surfactants, resulting from lower surfactant concentration and/or irregular intercalation. Compared with OM2 and OM3, more curved layers can be found in sample OM1, similar to natural montmorillonite (Bauluz et al., 2002). This should be the result of the lower packing density of surfactants within the galleries. SEM micrographs (Fig. 6) show surface morphology of montmorillonite and the organoclay samples. It can be seen that the original Ca and Na montmorillonites have massive and curved plates (Fig. 6a, b and f), especially for Na montmorillonite (Lee and Kim, 2002a). However, the clay treated with organic surfactant shows significant changes in the morphology. Compared with the morphology of the Na montmorillonite, there are many small and aggregated particles and the plates become relatively flat in OM1 and OM2 (Fig. 6c and d), while OM3 demonstrates mostly flat and aggregated morphology (Fig. 6e and g). This is similar to that observed in the TEM micrographs in the present study and indicates that, with the increase of surfactant packing density in the interlayer, the curved plates in Na montmorillonite will transform to flat layers. Hence, the present study shows that not only the basal spacing but also the morphology of the organoclay strongly depends on the packing density of surfactant within the montmorillonite interlayer space. A number of previous XRD studies have shown that the arrangement models of surfactant within the clay mineral galleries and the basal reflections strongly depend on the surfactant packing density. However, XRD patterns cannot provide the detailed information about the intercalated layers, including whether they are regularly intercalated layers or not. Our TEM micrographs for HDTMA + /montmorillonite organoclays prepared at higher surfactant concentrations have clearly demonstrated that they are mainly composed of regularly intercalated layers as proposed in the literature (Klapyta et al., 2001). Meanwhile, a slight difference of layer height does exist from layer to layer. This can well explain the observations in the literature that the basal reflections for organoclays are always relatively broad. The organoclays, prepared at lower surfactant concentrations, are mainly composed of randomly intercalated layers. The Na montmorillonite layer may be retained since there is not enough surfactant provided to exchange the sodium cations in the montmorillonite interlayer. In the study of fluorohectorite intercalated with quaternary phosphonium salt [C 16 H 33 P(C 4 H 9 ) 3 + ]Br at 0.5 CEC concentration, Ijdo and Pinnavaia (1999) proposed that the organoclay is composed of regular stacking of two types of galleries, C 16 H 33 P(C 4 H 9 ) 3 + fluorohectorite and Na + fluorohectorite. However, such regular stacking of two types of galleries, surfactant montmorillonite and Na montmorillonite, does not occur in HDTMA + /montmorillonite organoclays. Our

7 268 H. He et al. / Applied Clay Science 31 (2006) Fig. 6. SEM micrographs of Ca montmorillonite (a, g), Na montmorillonite (b), OM1 (c), OM2 (d) and OM3 (e, f). present study demonstrates that, at lower surfactant concentration, the intercalation of surfactant is random and the increase of surfactant packing density will result in the regular stacking of surfactant montmorillonite galleries. This may be different from the previous study reported by Lee and Kim (2002a), in which they found many disconnected, thin, relatively short and strongly disrupted layer stacks in the organoclay prepared at 2.5 CEC while the other organoclay prepared at 1.0 CEC shows a well-preserved repetitive layer-to-layer distance. As indicated in their paper, ultrasonic dispersion was used in the preparation of oriented films for XRD and alcohol used for the sample preparation for TEM. However, ultrasonic method can affect the microstructure of intercalated clays (Katdare et al., 2000) and alcohol organic solvent, which might also influence the structure of the organoclays. In addition, Lee and Kim (2002a,b) also proposed that the charge value or its distribution on clay mineral might be an important factor for expansion characteristics of organoclay. Hence, we believe the difference between our result and that from a previous study might have resulted from the different experimental conditions and characteristics of montmorillonite sample. Our present study also indicates that the morphologies of the organoclays strongly depend on the surfactant packing density within the interlayer, i.e., many curved plates in the organoclays prepared at lower surfactant concentrations while fewer occur in the ones prepared at higher surfactant concentrations. Fig. 7 shows schematically how the curved plates transform to flat ones with the increase of surfactant packing density. After alkylammonium cations enter into the silicate interlayers, they experience the interactions between the head group of alkylammonium cation silicate surface, the hydrocarbon chain of

8 H. He et al. / Applied Clay Science 31 (2006) Fig. 7. The schematic diagram for the curved plates transforming to flat ones with the increase of surfactant packing density and alkyl chain aggregation in the interlayers: (a) Na montmorillonite; (b) lateral monolayer (OM1); (c) pseudotrilayer with a small amount of paraffintype monolayer (OM2); (d) paraffin-type monolayer (OM3). The hydrated sodium cations are represented by special circles ( ) while single open circles ( ) represent methyls and nitrogen atoms in cationic head groups are represented by filled circles ( ). alkylammonium cation silicate surface and the hydrocarbon chain hydrocarbon chain of alkylammonium cations. The latter comprises the nonbonded van der Waals interactions, which are functions of alkylammonium concentration (Gupta et al., 1994). The strong electrostatic interaction between negative charged clay surface and the positively charged head group of alkylammonium cation will hold the head group of alkylammonium cation close to the clay surface as indicated by molecular simulation (Zeng et al., 2003). On the other hand, since the silicate surface of clay is hydrophilic whereas the hydrocarbon chains of alkylammonium cation are hydrophobic, the silicate surface hydrocarbon chain interaction is of a repulsive nature. The transformation of the curved plates to flat ones might be mainly controlled by the interactions between the hydrocarbon chain silicate surface and the hydrocarbon chain hydrocarbon chain of alkylammonium cations. Previous reports (Klapyta et al., 2001; Li and Ishida, 2003; Zeng et al., 2003) have demonstrated that, in the organoclays with lower surfactant packing density, the alkyl chains within the interlayer space are parallel within the interlayer space and are individually separated. In this case, the repulsive interaction between the hydrocarbon chain silicate surface is dominant whereas the interaction among the hydrocarbon chains is very weak. This repulsive interaction is an important factor to result in the transformation from the curved plates to flat ones. In addition, Li and Ishida (2003) proposed that alkyl chains have the tendency to take more ordered trans-conformers to minimize the total energy of organoclay. However, the bent clay plates will result in the increase of gauche conformers, which will increase the total energy of organoclays. Accordingly, the transformation from the curved plates to flat ones will decrease the total energy of organoclays. Here, we can find that both of the two above-mentioned factors would lead to flattening the organoclay plates. With the increase of surfactant packing density, the interchain interaction among the alkylammonium cations becomes the dominant force and the orientation of the hydrocarbon tail changes from parallel to the silicate surface within the interlayer space to parallel but at an angle to the silicate surface, as shown by our XRD patterns and previous studies (Li and Ishida, 2003). The force from the silicate surface to the confined alkyl chains is limited by the distance (Gupta et al., 1994). Accordingly, when alkyl chains radiate at an angle away from the silicate surface, the interaction among hydrocarbon chains is relatively strong while that between the hydrocarbon chain silicate surface is very weak or does not exist any more. The interaction among alkyl chains will increase with the increase of the surfactant packing density (Lee and Kim, 2002a) and this will result in the ordered packing of the alkyl chains as indicated by FTIR and Raman (Vaia et al., 1994; Li and Ishida, 2003; Venkataraman and Vasudevan, 2000, 2001; He et al., 2004a,b). Reasonably, the stacking of the organoclay layers will be regular and flat rather than

9 270 H. He et al. / Applied Clay Science 31 (2006) random and curved as shown in the TEM and SEM micrographs of OM3. 4. Conclusions In this paper, TEM and SEM in conjunction with XRD have been used to investigate and characterize the interlayer structure and morphology of HDTMA + / montmorillonite organoclays with different surfactant packing density. XRD patterns indicate that the surfactants in OM1 and OM3 adopt lateral-monolayer and paraffin-type monomolecular arrangement models, respectively, while pseudotrilayer and paraffin-type monomolecular arrangements coexist in OM2. The surfactant arrangement model strongly depends on the packing density of surfactant within the montmorillonite galleries. In addition, thermal treatment has an important effect on the stability of surfactants in the organoclays. TEM and SEM micrographs demonstrate that the organoclays with lower surfactant packing density are mainly composed of irregular layer stacking with a number of curved organoclay layers, while those with higher surfactant packing density are mainly composed of regular layer stacking with fewer curved organoclay layers. The variation of the layer height from layer to layer exists in all three organoclays while more obvious variation is found in the organoclays with lower surfactant packing density than the one with higher packing density. This can well explain the broad basal reflections reported in the present study and in the literature. Our present study demonstrates that not only the arrangement model of surfactant but also the morphology of the organoclay strongly depend on the surfactant packing density within the montmorillonite interlayer space. 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