Adsorption of dimeric surfactants in lamellar silicates

Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx

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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb

Adsorption of dimeric surfactants in lamellar silicates Mateusz Balcerzak a,1, Zuzanna Pietralik a,1, Ludwik Domka b, Andrzej Skrzypczak c, Maciej Kozak a,⇑ a

´ , Poland Department of Macromolecular Physics, Faculty of Physics, A. Mickiewicz University, Umultowska 85, 61-614 Poznan ´ , Poland Department of Metalorganic Chemistry, Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan c ´ University of Technology, Berdychowo 4, 60-965 Poznan ´ , Poland Institute of Chemical Technology, Poznan b

a r t i c l e

i n f o

Article history: Received 1 June 2015 Received in revised form 30 July 2015 Accepted 31 July 2015 Available online xxxx Keywords: Gemini surfactant Dicationic surfactant Montmorillonite Adsorption Organophilisation

a b s t r a c t The adsorption of different types of cationic surfactants in lamellar silicates changes their surface character from hydrophilic to hydrophobic. This study was undertaken to obtain lamellar silicates modified by a series of novel dimeric (gemini) surfactants of different length alkyl chains and to characterise these organophilised materials. Synthetic sodium montmorillonite SOMASIFÒ ME 100 (M) and enriched bentonite of natural origin (Nanoclay – hydrophilic bentoniteÒ) were organophilised with dimeric (gemini) surfactants (1,10 -(1,4-butanediyl)bis(alkoxymethyl)imidazolium dichlorides). As a result of surfactant molecule adsorption in interlamellar space, the d-spacing (d001) increased from 0.97 nm (for the anhydrous structure) to 2.04 nm. A Fourier transform infrared spectroscopy (FTIR) analysis of the modified systems reveals bands assigned to the stretching vibrations of the CH2 and CH3 groups and the scissoring vibrations of the NH group from the structure of the dimeric surfactants. Thermogravimetric (TG) and derivative thermogravimetric (DTG) studies imply a four-stage process of surfactant decomposition. Scanning electron microscopy (SEM) images provide information on the influence of dimeric surfactant intercalation into the silicate structures. Particles of the modified systems show a tendency toward the formation of irregularly shaped agglomerates. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Minerals from the smectite group have generated considerable interest due to their high swelling ability, large specific surface area and facility for chemical or physical surface modifications [1]. Bentonites are minerals from the smectite group whose main component is montmorillonite, most often of the sodium, potassium or magnesium type, representing the 2:1-type lamellar silicates [2,3], and are built of two tetrahedral siloxane layers covering a single metal-hydroxyl layer of octahedral symmetry [4]. These three layers are linked at their corners via oxygen atoms incorporated in the two structures [2]. Due to the isomorphous substitution of the atoms from the octahedral layer with cations of lower valence, the montmorillonite structure has an additional negative charge on the surface of the aluminosilicate packets [5]. The additional negative charge is compensated by the charge brought by cations adsorbed in the interpacket space, such as potassium, sodium, magnesium or calcium ions [4,6]. The specific surface area of sodium montmorillonite is Asp = 750–800 m2/g, which means that approximately every ⇑ Corresponding author. Tel.: +48 61 8295266; fax: +48 61 8295155. 1

E-mail address: [email protected] (M. Kozak). These authors contributed equally to this work.

second atom is on the surface, endowing montmorillonite with a high ability to bind large numbers of polymer particles [7]. Both the external surface of montmorillonite crystallites and the interlamellar surface can undergo chemical modification. The latter possibility is particularly attractive in terms of applications, as it permits the formation of polymer nanocomposites. This type of modification has typically been performed with the use of monomeric surfactants [1,8,9]. Little information has been published on the organophilisation of lamellar silicates with the use of bis (alkoxymethyl)imidazolium gemini surfactants. Surfactants of this type are covalent dimers of classical surfactants whose characteristic structural feature is the presence of a link (spacer) occurring among or near hydrophilic groups [10]. Gemini surfactants exhibit interesting properties, e.g., a low CMC value, and have been considered as potential agents for the organophilisation of lamellar silicates. The presence of intercalated cationic surfactants changes the surface character of smectites from hydrophilic to hydrophobic, which makes silicates organophilic and thus more easily dispersed in organic solvents [11]. The interlamellar space of layered silicates has also been modified by different compounds, including inorganic ions [12], polyamines [13], alkylamines [14], silanes [15], amino acids [16], poly(epsilon-caprolactone) [17,18], polypropylene [19] and other polymers [2].

http://dx.doi.org/10.1016/j.nimb.2015.07.135 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: M. Balcerzak et al., Adsorption of dimeric surfactants in lamellar silicates, Nucl. Instr. Meth. B (2015), http://dx.doi.org/ 10.1016/j.nimb.2015.07.135

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The organophilised aluminosilicates obtained via chemical modification are capable of removing non-ionic organic compounds from water solutions and have been used for the removal of pesticides and other environmental pollutants. Modified lamellar silicates have become an important group of fillers used for the production of polymer nanocomposites. Even a small admixture of such materials to the polymer matrix can significantly change its mechanical, barrier or thermal properties [1,5]. This study was undertaken to obtain lamellar silicates modified by a series of dimeric (gemini) surfactants with different alkyl chain lengths and to characterise these organophilised materials. We have also studied the influence of the ion exchange process on the structure and morphology of the products. The new organophilised aluminosilicates can be of potential use as nanofillers of polymer nanocomposites or as effective adsorbents for the removal of organic pollutants from the environment.

montmorillonite (MMT) or bentonite Nanoclay (BNT) was supplemented with 2 mL of deionised water and 40 mg of a given surfactant at room temperature. The suspensions were homogenised in a sonicator for 5 h at 80 °C. Then, the modified aluminosilicates were centrifuged and washed with distilled water, and this procedure was repeated four times. The filtrate was dried on Petri dishes for 90 min at 110 °C.

2. Materials and methods

2.3. Fourier transform infrared spectroscopy

The materials studied were synthetic sodium montmorillonite SOMASIFÒ ME 100 (M) (CO-OP Chemical, Japan), labelled MMT, and enriched bentonite of natural origin (98% of sodium montmorillonite) – Nanoclay – hydrophilic bentoniteÒ (Aldrich Chemistry, USA), labelled BNT. The cation-exchange capacities (CEC) of MMT and BNT were 0.8 and 0.145 meq/g, respectively. The materials were modified with a series of dimeric surfactants (1,10 -(1,4-butanediyl) bis(alkoxymethyl)imidazolium dichlorides) with the general formula n-4-n (Fig. 1, see list below), where n varies from 8 to 16 and is the number of methylene groups in the hydrophobic chain.

FTIR spectra were recorded on a Tensor 27 spectrometer made by Bruker Optics. One milligram of each sample of unmodified and modified aluminosilicate was ground in a mortar with 120 mg of KBr. The material was then pressed by 12 T of pressure into a thin tablet that was 12 mm in diameter and 0.5 mm in thickness. Each spectrum was composed of 512 scans with a resolution of 2 cm1 in the range of 4000–600 cm1.

 1,10 -(1,4-Butanediyl)bis(octyloxymethyl)imidazolium dichloride (IMI_Cl_C4_C8).  1,10 -(1,4-Butanediyl)bis(decyloxymethyl)imidazolium dichloride (IMI_Cl_C4_C10).  1,10 -(1,4-Butanediyl)bis(dodecyloxymethyl)imidazolium dichloride (IMI_Cl_C4_C12).  1,10 -(1,4-Butanediyl)bis(tetradecyloxymethyl)imidazolium dichloride (IMI_Cl_C4_C14).  1,10 -(1,4-Butanediyl)bis(hexadecyloxymethyl)imidazolium dichloride (IMI_Cl_C4_C16). The structures of the above-listed surfactants are given in Fig. 1, and the detailed procedure of their synthesis has been described by Pietralik et al. [20]. 2.1. Synthesis of organophilised lamellar silicates Organophilised lamellar silicates were obtained via the ionexchange reaction. A portion of the 200 mg of synthetic

2.2. Scanning electron microscopy The morphology of unmodified and modified samples of MMT and BNT was studied under a Philips SEM-515 scanning electron microscope. The powdered samples were dispersed in t-butanol, deposited on a holder and subjected to gold evaporation. Images were collected with a working distance of 14 mm and at an excitation voltage of 3–10 kV.

2.4. X-ray diffractometry X-ray diffractometry (XRD) measurements were made on a modified HZG-4 powder diffractometer with an X-ray tube with Cu anode as the source of radiation (k = 0.154178 nm), monochromatised by a nickel filter. X-ray radiation was supplied by a TUR M62 generator producing a voltage of 30 kV and a current of 30 mA. XRD measurements were performed for 2h from 3.2° to 60° at a scanning rate of 0.02°/s. Diffraction maxima were fitted with a Gaussian shape function. 2.5. Thermogravimetric analysis Thermogravimetric (TG) and derivative thermogravimetric (DTG) measurements were made on a Setsys TG-DSC 15 (Setaram) instrument for samples of approximately 13 mg in helium atmosphere and at temperatures from 10 to 1000 °C at a heating rate of 10 °C/min. The data were analysed by the programme Origin (http://www.originlab.com). 2.6. Small angle X-ray scattering (SAXS) SAXS studies of selected surfactants solutions (15.2 mM) were performed on the BM29 beamline of ESRF (Grenoble, France) [21] using synchrotron radiation (k = 0.9919 nm). The scattering data (for each sample ten 10-s frames) were collected at 15 °C with sample-to-detector distance of 2.867 m, using flow cell (sample volume = 30 lL) and Pilatus3 1 M detector (169  179 mm2). 3. Results and discussion

Fig. 1. Chemical structure of dicationic (gemini) surfactants – 1,10 -(1,4-butanediyl)bis(alkoxymethyl)imidazolium dichlorides used for the modification of layered silicates.

In the initial stage of the study, the association ability of selected surfactants in solution was characterised using SAXS technique. For the study, three surfactants (IMI_Cl_C4_C8, IMI_Cl_C4_C12 and IMI_Cl_C4_C16) at a concentration of 15.2 mM were selected, which corresponds to approximately half of the initial concentration of surfactants in the modification of the studied silicates. SAXS curves obtained for the tested solutions

Please cite this article in press as: M. Balcerzak et al., Adsorption of dimeric surfactants in lamellar silicates, Nucl. Instr. Meth. B (2015), http://dx.doi.org/ 10.1016/j.nimb.2015.07.135

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Fig. 2. SAXS data recorded for solutions of selected dicationic surfactants.

are presented in Fig. 2. Surfactants of long chain (hexadecyl and dodecyl) in solution form the characteristic multilayer micellar systems.

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For the solution of the IMI_Cl_C4_C16 surfactant were observed strong diffraction maxima: d001 = 42.7 nm, d002 = 21.3 nm, d003 = 14.1 nm, d004 = 10.6 nm, d005 = 8.7 nm, d006 = 7.08 nm, d007 = 6.21 nm, d008 = 5.53 nm, d0010 = 4.23 nm. While for solution of the IMI_Cl_C4_C12 surfactant the diffraction peaks are already much weaker (except d001 and d005) and are characterised by dspacing values as follows: d001 = 29.9 nm, d002 = 14.9 nm, d005 = 6.13 nm, d006 = 5.03 nm, d007 = 4.57 nm, d008 = 4.32 nm. On the other hand the IMI_Cl_C4_C8 surfactant, in the studied concentration, does not form any micelles. The changes in the morphology of lamellar silicates appearing as a result of modification with dimeric surfactants are shown in Figs. 3 and 4. Crystallites of unmodified MMT or BNT typically have irregular shapes and sizes not exceeding 15 lm. The particles in unmodified MMT samples are well separated, and the lamellar structure of the material is clearly observed (Fig. 3A). The samples organophilised with the three gemini surfactants with the shortest alkyl chains are made of the smallest particles relative to those in the unmodified materials. The particle size in MMT_IMI_Cl_C4_C8 and MMT_IMI_Cl_C4_C10 is 10 lm (Fig. 3B and C), and that in MMT_ IMI_Cl_C4_C12 is 8 lm (Fig. 3D). Despite a significantly smaller size, some crystallites form highly disorganised agglomerates as large as 25 lm. The lamellar structure typical of montmorillonite is collapsed, which leads to the formation of the large agglomerates observed in the SEM images [22]. The materials modified with IMI_Cl_C4_C14 and IMI_Cl_C4_C16 (Fig. 3E and F) have morphologies that differ considerably from those of the other systems with the presence of agglomerates as

Fig. 3. SEM micrographs of unmodified MMT (A) and MMT modified by IMI_Cl_C4_C8 (B), IMI_Cl_C4_C10 (C), IMI_Cl_C4_C12 (D), IMI_Cl_C4_C14 (E), IMI_Cl_C4_C16 (F).

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Fig. 4. SEM micrographs of unmodified BNT (A) and BNT modified by IMI_Cl_C4_C8 (B), IMI_Cl_C4_C10 (C), IMI_Cl_C4_C12 (D), IMI_Cl_C4_C14 (E), IMI_Cl_C4_C16 (F).

Fig. 5. XRD diffraction data for unmodified MMT and MMT modified by gemini surfactants. Inset – magnification of the observed diffraction peak of the interlayer space.

large as 250 lm but with a mean size of 30 lm. In addition, the weaker fragmentation of the material is observed by a lack of crystallites in the submicron size range. Fig. 4 presents a series of SEM images taken for samples based on hydrophilic bentonite (BNT).

The crystallites of unmodified BNT (Fig. 4A) are well-separated spherical particles with regular shapes and porous surfaces whose mean size is 13–17 lm. After the modification of BNT-based samples, the spherical morphology of particles was conserved only in

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Fig. 6. XRD diffraction data for unmodified BNT and BNT modified by gemini surfactants. Inset – magnification of the observed diffraction peak of the interlayer space.

Table 1 Basal spacing observed for unmodified layered silicates (MMT, BNT) and samples modified with gemini surfactants. Sample

Basal spacing (nm)

MMT MMT_IMI_Cl_C4_C8 MMT_IMI_Cl_C4_C10 MMT_IMI_Cl_C4_C12 MMT_IMI_Cl_C4_C14 MMT_IMI_Cl_C4_C16

0.97 1.39 1.42 1.36 1.30 1.28

BNT BNT_IMI_Cl_C4_C8 BNT_IMI_Cl_C4_C10 BNT_IMI_Cl_C4_C12 BNT_IMI_Cl_C4_C14 BNT_IMI_Cl_C4_C16

1.28 1.39 1.42 1.41 1.43 1.41

1.21

1.27

1.97 1.58 1.65 1.66

2.04

the sample modified with IMI_Cl_C4_C10 (sample BNT_IMI_Cl_C4_C10, Fig. 4C). However, the crystallites are no longer well separated and no longer have porous surfaces. An SEM image of this sample reveals the appearance of smaller crystallites. The structures of the other four samples after modification with surfactants are not similar to the structure of the initial bentonite (Fig. 4B, D–F). As a result of modification, the characteristic spherical shape of particles disappears, and irregular layered agglomerates are formed. The maximum size of the particles is also changed, reaching sizes as large as 100 lm. Depending on the type of surfactant used, the size of the crystallites varies from 30 to 40 lm for BNT_IMI_Cl_C4_C8, from 30 to 90 lm for BNT_IMI_Cl_C4_C12, from 20 to 90 lm for BNT_IMI_Cl_C4_C14 and from 20 to 100 lm for BNT_ IMI_Cl_C4_C16. The changes in morphology and particle size are caused by surfactant adsorption in the interlamellar space (intercalation) [23]. This effect has been verified by a powder XRD study. Figs. 5 and 6 present diffractograms obtained for unmodified MMT and BNT as well as for a series of modified samples. The lattice constants characterising these systems are summarised in Table 1. The lattice constant d001 obtained for anhydrous unmodified MMT is 0.97 nm, which is in agreement with the previously determined value [5]. The other lattice constants obtained for MMT are 1.21 and 1.27 nm, and the value d001 = 1.28 nm determined for BNT corresponds to

Fig. 7. Schematic of ordered structures formed by gemini surfactants in the interpacket space: monolayer (A), bilayer (B), pseudo-trilayer (C) and paraffin-type monolayer (D), the representation prepared based on the Lagaly model [33].

the structure of hydrated silicate at a different degree of hydration [1,5,11]. When the modifier is not able to penetrate into the interlamellar space, the lattice constant does not change [5]. The alkyl chains of the molecules of low-molecular surfactants adsorbed in the interpacket space can assume different arrangements, organising into monolayers (1.37 nm), bilayers (1.77 nm), highly disordered trilayers (2.17 nm), paraffin-type monolayers (>2.2 nm) or paraffin-type bilayers [2,5]. A similar model of gemini surfactant packing in the interpacket space is presented in Fig. 7. The basal spacing of modified MMT changes by Dd = 0.3–0.45 nm, and that of modified BNT changes by 0.32–0.46 nm in reference to that of their anhydrous structures (approximately 0.95–0.98 nm) (for detailed values, see Table 1, column 1). Such an increase in the lattice constant is likely a result of the formation of a surfactant

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Fig. 9. FTIR spectra of unmodified BNT and BNT modified with gemini surfactants – 2500–600 cm1 (A) and 4000–2500 cm1 (B) regions. Fig. 8. FTIR spectra of unmodified MMT and MMT modified with gemini surfactants – 2500–600 cm1 (A) and 4000–2500 cm1 (B) regions.

monolayer in the interpacket space, strongly bonded to the packet surface [2,24,25]. The high-intensity, sharp peak appearing in the XRD pattern of MMT_IMI_Cl_C4_C8 suggests the presence of a well-ordered monolayer of dimeric surfactant and a large number of intercalated particles [26]. For samples MMT_IMI_Cl_C4_C12, MMT_IMI_Cl_ C4_C14 and MMT_IMI_Cl_C4_C16, an additional increase in the lattice constant by 0.61–0.68 nm relative to that of the anhydrous structure is observed. These values likely correspond to the appearance of a highly disordered surfactant bilayer [2,27]. The XRD pattern of sample MMT_ IMI_Cl_C4_C10 reveals a slightly asymmetric peak corresponding to a lattice constant of 2.0 nm. After fitting with two Gaussian profiles, this peak can be split into two peaks (dominant at 1.97 and 2.04 nm) that can be associated with the formation of a pseudo-trilayer (2.05 nm) and a paraffin-type monolayer (1.94 nm) in the interpacket space [1]. Assuming the presence of a small amount of water filling the space as well as flexibility and steric effects of dimeric surfactant molecules, this peak likely corresponds to a paraffin-type layer (as a dominant form) inclined at a certain angle to the packet surface [1]. The cationic hydrophilic groups of surfactants are attached to the aluminosilicate surface, whereas the hydrophobic tails spread away

from the surface. Similar behaviour has been observed in the systems modified with monomeric CTAB (hexadecyltrimethylammonium bromide) and SDS (dodecyl sulphate) surfactants [9]. The simultaneous presence of a monolayer and paraffin-type layer has been earlier reported in calcium montmorillonite modified with a monomeric cationic surfactant (octadecyltrimethylammonium bromide) [28] and sodium montmorillonite modified with octadecyldihydroxy ethyl methylammonium bromide (ODEM) [29]. To confirm the chemical modification of aluminosilicates, the samples were studied with FTIR. Figs. 8 and 9 present the FTIR spectra of all samples studied. The absorption spectra of unmodified layered silicates (both synthetic and natural) exhibit peaks corresponding to the twisting vibrations of adsorbed –HOH (1636 cm1), stretching vibrations of –SiO from amorphous silica (1070 cm1) and stretching vibrations of –SiO characteristic of the crystalline structure (1003 cm1) [6]. No significant changes of shape and location were observed for the relevant absorption bands attributed to the structure of aluminosilicates due to modification with dimeric surfactants. The absorption spectra of the samples modified with surfactants exhibit a number of peaks assigned to the vibrations of groups from the surfactant structures, including the stretching asymmetric vibrations of –CH3 (2957–2949 cm1) [30], stretching asymmetric vibrations of –CH2 (2927–2926 cm1) [31], stretching

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Fig. 10. Results of thermogravimetric analysis of unmodified MMT and MMT modified with gemini surfactants – TG (A) and DTG (B) thermograms.

symmetric vibrations of –CH3 (2875–2872 cm1) [30], stretching symmetric vibrations of –CH2 (2857–2855 cm1) [30] and scissoring vibrations of the –NH group in imidazole (1556–1554 cm1) [32]. All of these bands are found in the spectra of all organophilised aluminosilicates. The effectiveness of the modification was also evaluated by TG and DTG. Figs. 10 and 11 present the TG and DTG curves recorded for MMT and BNT before and after modification. The first minimum in the range of 90–110 °C, occurring in each thermogram, corresponds to desorption of water from the interlamellar space in the aluminosilicates studied [1,5]. Unmodified BNT contained the largest amount of water adsorbed in the interlayer space. The water content was significantly lower in the other samples due to the appearance of dimeric surfactant molecules in the interlayer space to compensate the negative charge and contribute to the reduction in the amount of water adsorbed. A small minimum at 125–185 °C is observed in some TG/DTG curves, which is associated with the dehydration of water molecules associated with the crystalline structure of silicates. Several minima appear in the temperature range of 190–580 °C on the thermograms of both organophilised materials. This effect corresponds to the four stages of the decomposition processes of dimeric surfactants: (1) decomposition of surfactant molecules weakly (or not)

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Fig. 11. Results of the thermogravimetric analysis of unmodified BNT and BNT modified with gemini surfactants – TG (A) and DTG (B) thermograms.

associated with the clay surface (225–275 °C), (2) decomposition of molecules adsorbed on the external surfaces (275–360 °C), (3) decomposition of surfactants adsorbed in the interlayer space (360–460 °C), and (4) decomposition of surfactants intercalated in the interlayer space (460–540 °C) [1,23]. Changes in the mass of BNT-based samples during heating at temperatures of 400– 650 °C can be attributed to the montmorillonite dehydroxylation process, which breaks down the structure of aluminosilicate [1]. 4. Conclusions  The intercalation of dimeric surfactant molecules caused significant changes in the morphologies of the investigated systems, especially in the BNT. The modified clays tend to form agglomerates.  Changes in d-spacing indicate the presence of various layered structures formed by surfactant molecules in the interlayer space of silicate, including monolayers, bilayers, pseudotrilayers and paraffin-type monolayers.  The organophilisation of layered silicates by gemini surfactants was confirmed by FTIR studies. The IR spectra of modified systems revealed symmetric and asymmetric stretching vibrations of the CH2 and CH3 groups and scissoring vibrations of the NH groups from the imidazolium rings.

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 The four-step decomposition process of the intercalated dimeric surfactant, visible in thermogravimetric spectra, confirms the presence of surfactant molecules in the interlayer space.

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The research was supported by a research grant from the Polish Ministry of Science and Higher Education (Grant No. N N204 135738).

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Please cite this article in press as: M. Balcerzak et al., Adsorption of dimeric surfactants in lamellar silicates, Nucl. Instr. Meth. B (2015), http://dx.doi.org/ 10.1016/j.nimb.2015.07.135