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Intercalation of vermiculite in presence of surfactants
Applied Clay Science 146 (2017) 7–13
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Intercalation of vermiculite in presence of surfactants
MARK
Sevim İşçi Istanbul Technical University, Department of Physics, Maslak 34469, Istanbul, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Vermiculite Surfactants Intercalation Colloidal and structural properties
Vermiculite was modified with surfactants in order to enable intercalation of vermiculite layers. Since vermiculite has negative charges on its surfaces, it was expected that cationic surfactant would expand the clay mineral layers more than an anionic surfactant. Nevertheless, negative parts of the anionic surfactants interacted with the positively charged edges of vermiculite and caused to super lattice structure so, unexpectedly the expansion of the layers was determined to be fully collapsed phase of vermiculite. Colloidal and structural properties of vermiculite dispersions were examined in presence of anionic, cationic and nonionic surfactants. The results showed that cationic surfactant covered the surface of the vermiculite with a second layer but the expansion of the clay mineral layer was limited compared to the anionic surfactant. The anionic surfactant produced electrostatic interaction with the positively charged edges of vermiculite and fully expanded the layer structure of the vermiculite.
1. Introduction Layered structure clay minerals have been extensively used as naturally nano-sized particles, low-cost filler to enhance the mechanical and physical properties of polymer composites (Pinnavaia & Beall, 2000; Ray and Okamoto, 2003; Utracki, 2004; Bergaya et al., 2006; Liu et al., 2006; Gul et al., 2016). Montmorillonite is mostly used as layered clay in many articles, and also in industrial uses. However, compared with montmorillonite, the clay mineral layers in vermiculite have a higher charge density, a key parameter facilitating the incorporation of organic modifiers to generate larger interlayer spacing (Slade and Gates, 2004; Bergaya et al., 2006). Vermiculite forms macroscopic crystals, that are potentially suitable for producing high aspect ratio nanofillers, and its natural abundance makes it economically attractive for industrial applications (Qian et al., 2011). Vermiculites, 2:1 phyllosilicates, are generally composed of macroscopic particles, similar in appearance to micas. (Meisinger, 1985; Suquet, 1988; Suquet et al., 1991; Martins and Fernandes, 1992; Lv et al., 2017). The clay mineral surfaces have negative charges due to the isomorphic substitutions which create a deficit of positive charge, compensated by interlayer cations that can be easily exchanged and solvated by positively charged particles. Additionally, clay minerals are hydrophilic but the clay minerals are modified by alkylammonium ions to change the surfaces as hydrophobic surfaces so, the adsorption of polymer molecules is enhanced by the clay minerals. Composites (LeBaron et al., 1999; Pinnavaia and Beall, 2000; Ray and Okamoto,
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.clay.2017.05.030 Received 14 February 2017; Received in revised form 22 May 2017; Accepted 24 May 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
2003; Utracki, 2004; Bergaya et al., 2006; Liu et al., 2006; Gul et al., 2016). Hence, with the purpose of increase the interaction of clay and polymer, clay particles are mostly modified with cationic surfactants to change the hydrophilic surface to the hydrophobic ones, also to expand the clay mineral interlayers. The expansion of the interlayer of vermiculite particles was studied detail in various articles with partially collapsed structure and fully collapsed phase of vermiculite alkyl ammonium complexes by Johns et al. Johns and A. S. G. P. K (1967); Serratosa et al., 1970; Lee and Solin, 1991; Williams et al., 1996; Syrmanova et al., 2017). The effects of the anionic surfactants on the properties of montmorillonite have been extensively studied in literature but not in particular for vermiculite (Lagaly et al., 1984; Kopka et al., 1988; Lagaly and F., 2001; Penner and Lagaly, 2001; Yalcin et al., 2002a,b; Yalcin et al., 2002a,b; Lagaly and Ziesmer, 2003; Gunister et al., 2004). There are three types of possible of interactions between negative charge-carrying clay particles and anionic surfactant. First, it is possible that ion exchange can take place between OH– ions on clay mineral surfaces and the anionic part of surfactants. Second, H-bonds can form between clay particles and surfactant molecules. Third, it is possible that Ca2 + cation can establish electrostatic bridges between the anionic part of surfactants and the surface of clay particles(Parfitt and D.J., 1970; Parfitt, 1978; Lagaly, 1986, 1989; Kopka et al., 1988; Lagaly and F., 2001; Penner and Lagaly, 2001; Yalcin et al., 2002a, b). In this study, the effects of the cationic surfactants hexadecyltrimethylammonium bromide (HDTABr), and octadecyltrimethylammonium bromide (ODTABr), the anionic surfactants sodium dodecyl
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S. İşçi
sulfate (SDS), and ammonium lauryl sulfate (ALS), and the nonionic surfactant N,N-Dimethyldodecylamine N-oxide (DDAO) were investigated on the colloidal and structural properties of vermiculite. The results unexpectedly showed that anionic surfactant is much more suitable to expand the interlayer of vermiculite than cationic or nonionic surfactants. Anionic or cationic also the nonionic surfactants caused to super lattice structure by entering the interlayer of vermiculite with ion exchange and expanded the interlayer. Rheological measurements were used to examine particle–particle interactions, electrokinetical measurements were used to determine the effects of the surfactants on the surface properties of vermiculite particles. XRD, SEM results showed that the intercalation of vermiculite layers in presence of surfactants, and finally FTIR results showed the ion exchange between vermiculite and the surfactants. 2. Experimental study 2.1. Materials Vermiculite sample were obtained from Gold Butte District, Clark County, Nevada, USA. Ore samples were identified as vermiculite clay minerals using X-ray diffractometer (Bruker D8 Advance model X-ray diffractometer), indicating trioctahedral structure of vermiculite. The vermiculite was subjected to a heat treatment at temperatures over 700 °C the mineral expands, multiplying its volume between 15 and 20 times, and the expanded form of the vermiculite was used in this study. Hexadecyltrimethylammonium bromide, HDTABr, (C16H33N(CH3)3Br, MW = 364.46 g/mol from Fluka), and Octadecyltrimethylammonium bromide, ODTABr, (C18H35N+(CH3)3Br, MW = 392.52 g/mol from Fluka) were used as cationic surfactants. Sodium dodecyl sulfate, SDS, (C12H25NaO4S, MW = 288.38 g/mol from Aldrich Chemical Co) and Ammonium lauryl sulfate, ALS, (CH3(CH2)11OSO3NH4, MW = 283.40 g/ mol from Fluka) were used as anionic surfactants. N,N-Dimethyldodecylamine N-oxide (C14H31NO, MW = 229.43 g/mol from Sigma) was used as nonionic surfactant.
Fig. 1. The changes of the plastic viscosity of vermiculite-water systems in presence of surfactants.
2.2. Preparation of clay - surfactant dispersions Vermiculite was dispersed in distilled water in an ultrasonic bath and then shaken overnight. The vermiculite dispersions were mixed with 5.10− 5 to 5.10− 2 mol/L concentrations of each surfactant. Then, the dispersions were shaken for 24 h, and ultrasonicated for 5 min. Fig. 2. The changes of the zeta potential values of vermiculite-water systems in presence of surfactants.
3. Methods Rheological properties such as viscosity, shear rate (γ), and shear stress (τ) of the dispersions were measured using a Brookfield DVIII + type low-shear viscometer. The flow behavior of the clay dispersions was obtained by shear rate measurements within 0–330 s− 1 shear rates. Rheological measurements were carried out in duplicate. The electrophoretic mobility measurements were carried out using a Zetasizer 2000, Malvern Instruments. The optic unit contains a 5 mW HeeNe (638 nm) laser. The dispersions were prepared as explained above. Before the measurements, all the dispersions were centrifuged at 4500 rpm for 30 min. Supernatants were then used for zeta potential measurements. To make an electrophoretic mobility measurement in this instrument, laser beams are crossed at a particular point in the cell. Particles in the cell were illuminated by these beams. Electrophoretic mobility was measured, injecting a small portion of the dispersion into the cell of the Zetasizer 2000 instrument at 25 °C temperature. The electrophoretic mobility was then converted to zeta potential using Henry eq. X-ray diffraction (XRD) measurements were performed Bruker D8 Advance model X-ray diffractometer at room temperature using Nifiltered and Cu tube. The diffractograms were scanned in 2θ ranges from 2 to 40° at a rate of 2°/min.
The morphology of the fractured surfaces of the clay films was investigated with a FEI Quanta Feg 250 scanning electron microscope. SEM measurements were operated at 15 kV. The specimens were frozen under liquid nitrogen, and then fractured, mounted, and coated with gold on Edwards S 150B sputter coater. FTIR analyses (400–4000 cm− 1) were performed on Perkin Elmer Spectrum 100 FTIR spectrophotometer using KBr pellets with a concentration of 1% or film. Spectral outputs were recorded either in absorbance or transmittance mode as a function of wave number.
4. Results & discussion 4.1. The effect of the surfactants on the flow and the electrokinetical properties of vermiculite dispersions Vermiculite dispersions in water showed Bingham plastic behavior according to their concertina-like swelling properties. At low shear rates, the system exhibits non-Newtonian flow, and after a certain value of the shear rate, the flow curve becomes linear. Vermiculite has a lower swelling potential than other swelling clay minerals and in some 8
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Fig. 3. XRD diagrams of vermiculite and surfactant added vermiculites, a) Vk and Rehydrated Vk b) Vk and addition of 10-3mol/l of surfactants.
not affect the flow properties of vermiculite dispersions. Cationic surfactants also were not effective until the concentration of 5.10− 4 mol/L. But after that concentration the plastic viscosity values start to increase due to the flocculation of vermiculite particles. Here, the positively charged head groups of the monomers of the cationic surfactant adsorb on the negatively charged surface of the clay particles. The hydrophobic tails of the adsorbed monomers on the clay particle link the tails on the other particles by hydrophobic interactions and this cause an increase in the viscosity. Fig. 2 shows the change of zeta potential values with increasing
instances has been classified as an intermediate between micas, and smectites (Ndlovu et al., 2011). A Bingham model was applied, where τ = τΒ + ηplγ. From the equation, the yield value τB was obtained by extrapolation of the linear portion of the shear stress-shear rate curve to γ = 0, and the plastic viscosity ηPL were obtained from the slope of the linear portion of the curve. The viscosity values of clay suspensions (Fig. 1) were plotted as a function of increasing surfactant concentrations. It was obviously seen that anionic surfactants SDS and ALS and nonionic surfactant DDAO did 9
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Table 1 Interlayer spacings (Å) of vermiculite with additive surfactants. Cationic Surfactants Surfactant concentrations (mol/L)
0 (Vk-rehydrated) 1.10− 5 1.10− 4 1.10− 3 5.10− 3 1.10− 2
HDTABr
Anionic surfactants ODTABr
SDS
Nonionic surfactant ALS
DDAO
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
11,54 11,53 11,53 11,50 11,72 11,71
23,37 24,54 23,53 23,25 23,83 23,36
11,54 11,49 11,48 11,62 11,73 11,91
23,37 24,30 24,61 24,51 23,96 24,02
11,54 – 11,42 11,41
23,37 – 24,03 28,60
11,54 11,47 11,37 11,45
23,37 24,51 25,00 27,87
11,54 11,52 11,37 11,55 11,55 11,59
23,37 24,06 25,04 25,75 24,02 24,18
given at Fig. 3b. According to the concentration of the surfactant that shoulder slid to the lower angels, meaning that increased the new interlayer spaces which are given at Table 1, (Johns and A. S. G. P. K, 1967; Qian et al., 2011). Ionic surfactants interact electrostatically with vermiculite particles. Cationic surfactants attracted by the negatively charged surfaces while anionic surfactants attracted by the positively charged edges. The interlayer spaces have negatively charged surfaces, so normally it was expected that the cationic surfactants enter the interlayer spaces and expand it. However, anionic surfactant interacted with the positively charged edges and caused to superlattice structure of vermiculite. Hence, the anionic surfactant surprisingly increased the interlayer space much more than the cationic surfactants according to the XRD results. Besides, the interlayer spaces were higher than 28 Å for some anionic surfactant additions with a possible orientation of surfactants at the edges, the new structure is called “fully collapsed phase” according to Johns, and Sen Gupta 1967. The XRD reflections of the anionic surfactants overlap of the vermiculite reflections after the concentrations 10− 3 mol/L so the interlayer spaces were not determined. Nonionic surfactants interact sterically with clay mineral surfaces. The steric interaction also caused to the superlattice structure of vermiculite and expanded the interlayer space. The nonionic surfactant can interact both the surfaces and the edges due to its neutral structure. The same behaviors of intercalation of vermiculite were obtained according to the kinds of surfactant. However, each surfactant increased the interlayer of the vermiculite according to their chain length. The schematic representation of the interaction of surfactants and vermiculite particles were given at Fig. 4. Scanning electron microscopic (SEM) pictures of the samples were used for characterization and to show the intercalation of the vermiculite dispersions with surfactants. Direct evidence of intercalated vermiculite dispersion can be found in SEM examination. Since vermiculite composed of macroscopic particles, its layer structure can be seen even in low magnifications. Forming different aggregates (heaps) of clay particles with each other could be seen in Fig. 5a for vermiculite particles. HDTABr coated particles exhibited appearance of layers with one on top of the other (Fig. 5b). SDS coated particles (Fig. 5c) showed full intercalation which is the visual evidence of XRD results. Vermiculite dispersions with surfactants were also characterized by FTIR spectroscopy. The FTIR spectrum of vermiculite sample showed characteristic bands (Fig. 6). The broad peak of OeH stretching centered at 3412 cm− 1, HeOeH bending at 1647 cm− 1, SieO stretching at 995 cm− 1 were determined for vermiculite. The bands between 917, and 648 cm− 1 are responsible from the OH deformations and translational due to the cations of the clay sample. Finally, the SieO bending peaks of vermiculite appeared at 455 cm− 1. The rehydrated form of vermiculite showed different peaks than the thermally expanded pristine sample. The water molecules changed the OH-deformation, HeOeH bending peaks due to the hydrogen bonding. Surfactant added vermiculite dispersions had some characteristic peaks of surfactants which were exactly the same with pristine surfactants like CH
surfactant concentrations. The surfactants were not being effective on the zeta potential values of the vermiculite surfaces until the 10− 3 mol/ L additions. This concentration is related with the cation-exchange capacity of vermiculite. Cationic surfactants quickly covered the whole surface of the vermiculite so the electrically neutral surfaces obtained at 5.10− 3 mol/L. Here, the positively charged head groups of the monomers of the cationic surfactant adsorb on the negatively charged surface of the clay particles. Besides, the plastic viscosity value at the same concentration decreased so both plastic viscosity and zeta potential values indicated the flocculation. With increasing cationic surfactant concentration, the absolute value of the zeta potential decreased to the zero point and then increased to the positive values with further addition. After zero point, the increase is attributed to the formation of a second layer, which involves the interaction of surfactant tails with each other. Hence, at the full coverage of the particles, the surface is recharged and the repulsive forces between positively charged particles cause to increase in zeta potential. The anionic surfactant SDS increased the zeta potential values after 10− 3 mol/L addition. The negatively charged head of the surfactants adsorb by the positively charged of the clay mineral edges which caused the increase on the number of negative charges on the total surface of the clay particle. With increasing SDS concentration, hydrophobic tails of the surfactants covered the surface of the vermiculite layers so the zeta potential values decreased until the zero potential. The adsorption effect of ALS on the zeta potential values of vermiculite was determined at the last concentration. Nonionic surfactant DDAO covered the vermiculite surfaces and caused to decrease of the zeta potential values almost to the zero point. Since there is no electrical interaction between nonionic surfactant and the vermiculite particles the formation of the interaction should be steric effect. 4.2. The structural characterizations vermiculite-surfactant dispersions The effect of surfactants on the vermiculite particles was investigated by XRD, SEM, and FTIR methods. XRD analyses were done by measuring the interlayer spacing of vermiculite in order to understand whether added surfactant enters into the interlayer space of vermiculite or not. The sheets in the layers in clay mineral structures bound to each other with covalent bonds and therefore, their crystal structure is stable. In contrast, the layers bound with Van der Waals bonds in the clay particles and water or organic molecules expand the interlayer spacing when it introduces in the interlayer space of the clay particle. Vermiculite showed a major broad reflection at d001 = 12,29 Å for thermally expanded sample and, d001 = 11,54 Å for rehydrated sample, corresponding to the interlayer spaces of vermiculite given at Fig. 3a. Almost no changes were observed at the main interlayer space of vermiculite in presence of surfactant. Incorporation of water and surfactants generates new interlayers which were appeared as a shoulder (Fig. 3 a and b). XRD patterns of vermiculite and 10− 3 mol/L addition of each kind of surfactants were 10
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Fig. 4. Schematic representation of the interactions of vermiculite and surfactants.
stretching peaks at the 3020–2840 cm− 1, and CH bending peaks at 1650–1210 cm− 1. Hydroxyl groups located at broken edges, steps and related defects of clay minerals. At the crystal edges, an accumulation of charges is attenuated by the adsorption of protons by O2− or OH− ions. Generally, the compensation is not complete so that cations or
anions, as gegen ions, balance the edge charges. Anions can be adsorbed at positive edge sites by electrostatic interactions or exchanged for structural OH– groups at the edges (Parfitt, 1978; Lagaly, 1986, 1989, 1993; Penner and Lagaly, 2001; Bergaya et al., 2006). Shifted OH− deformation and HeOeH bending peaks of vermiculite appeared at
Fig. 5. SEM pictures of a) vermiculite, b) HDTABr/vermiculite, c) SDS/vermiculite.
11
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Fig. 6. FTIR spectra of vermiculite and surfactant added vermiculites.
Table 2 OH− deformation, HeOeH bending and SieO stretching peaks of samples. Samples
OH-deformation Pristine sample
Vermiculite Rehydrated-Vk HDTABr ODTABr SDS ALS DDAO
3412 3426 – 3366 3469 3137 3399
HeOeH bending Vermiculite + surfactant
Pristine sample
3400 3401 3467 3401 3420
1647 1632 – – – – –
SieO streching Vermiculite + surfactant
Pristine sample
Vermiculite + surfactant
1639 1639 1642 1637 1648
995 998 – 999 – – –
999 999 997 999 999
References
surfactant added samples as given in detail in Table 2 and Fig. 6. Especially OH− deformation peaks of vermiculite shifted to the OH− deformation peaks of corresponding surfactant. For SDS, it was clearly seen that the OH stretching of vermiculite almost completely shifted to the OH stretching of the additive surfactant which can be explained by the replacement of OH− of vermiculite by the surfactants via ion exchange. Besides, these shifts may be attributed to the formation of hydrogen bonds.
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5. Conclusions The interlayers of vermiculite can be intercalated and fully collapsed phase can be obtained with anionic surfactant SDS and ALS without changing the flow properties. The surface of the vermiculite particles can be covered by cationic surfactants even with a second layer of the surfactant but only partially collapsed structure with limited expansion of layers. Anionic and nonionic surfactants caused to superlattice structure and intercalation of the vermiculite much more than cationic surfactants.
Acknowledgement The author wish to thank Prof. Dr. O. Işık Ece and Prof. Dr. Paul Schroeder for their valuable comments and contributions. This research project is supported by the Istanbul Technical University, Research Fund(38443). 12
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10.1016/j.progpolymsci.2003.08.002. Serratosa, J.M., Johns, W.D., Shimoyama, A., 1970. I.R. study of alkyl-ammonium vermiculite compolexes. Clay Clay Miner. 18, 107. Slade, P.G., Gates, W.P., 2004. The swelling of HDTMA smectites as influenced by their preparation and layer charges. Appl. Clay Sci. 25 (1–2), 93–101. http://dx.doi.org/ 10.1016/j.clay.2003.07007. Suquet, C.a., 1988. Hydrous phyllosilicates (exclusive of micas). Rev. Mineral. 19, 455–492. Suquet, H., Chevalier, S., Marcilly, C., Barthomeuf, D., 1991. Preparation of porous materials by chemical activation of the Llano vermiculite. Clay Miner. 26, 49. Syrmanova, K.K., Suleimenova, M.T., Kovaleva, A.Y., Botabayev, N.Y., Kaldybekova, Z.B., 2017. Vermiculite absorption capacity increasing by acid activation. Orient. J. Chem. 33 (1), 509–513. http://dx.doi.org/10.13005/ojc/330160. Utracki, L.A., 2004. Clay-containing Polymeric Nanocomposites. Rapra Technology Ltd., Shrewsbury. Williams, G.D., Skipper, N.T., Smalley, M.V., Soper, A.K., King, S.M., 1996. Structure of alkyl ammonium solutions in vermiculite clays. Faraday Discuss. 104, 295–306. http://dx.doi.org/10.1039/fd9960400295. Yalcin, T., Alemdar, A., Ece, O.I., Gungor, N., 2002a. The viscosity and zeta potential of bentonite dispersions in presence of anionic surfactants. Mater. Lett. 57 (2), 420–424. http://dx.doi.org/10.1016/S0167-577x(02)00803-0. (Pii S0167-577x(02)00803-0). Yalcin, T., Alemdar, A., Ece, O.I., Gungor, N., Coban, F., 2002b. By particle interactions and rheological properties of bentonites plus ALS suspensions. Mater. Lett. 53 (3), 211–215. http://dx.doi.org/10.1016/S0167-577x(01)00478-5. (Pii S0167-577x(01) 00478-5).
Mater. Manuf. Process. 21 (2), 143–151. http://dx.doi.org/10.1081/Amp200068646. Lv, P.Z., Liu, C.Z., Rao, Z.H., 2017. Review on clay mineral-based form-stable phase change materials: preparation, characterization and applications. Renew. Sustain. Energy Rev. 68, 707–726. http://dx.doi.org/10.1016/j.rser.2016.10.014. Martins, J., Fernandes, R., 1992. Hydrophobic expanded vermiculite as a cleaning agent for contaminated waters. Water Sci. Technol. 26, 2297. Meisinger, A.C., 1985. Vermiculite. In: Mineral Facts and Problems. US Bureau of Mines, In, pp. 917. Ndlovu, B., Becker, M., Forbes, E., Deglon, D., Franzidis, J.P., 2011. The influence of phyllosilicate mineralogy on the rheology of mineral slurries. Miner. Eng. 24, 1314–1322. Parfitt, R.L., 1978. Anion adsorption by soils and soil materials. Adv. Agron. 30, 1–50. Parfitt, R.L.G., D.J., 1970. The adsorption of poly(ethylene glycols) on clay minerals. Clay Minerals 8, 305. Penner, D., Lagaly, G., 2001. Influence of anions on the theological properties of clay mineral dispersions. Appl. Clay Sci. 19 (1–6), 131–142. http://dx.doi.org/10.1016/ S0169-1317(01)00052-7. Pinnavaia, T.J., Beall, G.W., 2000. Polymer-clay Nanocomposites. Wiley, Chichester, England; New York. Qian, Y., Lindsay, C.I., Macosko, C., Stein, A., 2011. Synthesis and properties of vermiculite-reinforced polyurethane nanocomposites. ACS Appl. Mater. Interfaces 3 (9), 3709–3717. http://dx.doi.org/10.1021/Am2008954. Ray, S.S., Okamoto, M., 2003. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog. Polym. Sci. 28 (11), 1539–1641. http://dx.doi.org/
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Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Intercalation of vermiculite in presence of surfactants
MARK
Sevim İşçi Istanbul Technical University, Department of Physics, Maslak 34469, Istanbul, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Vermiculite Surfactants Intercalation Colloidal and structural properties
Vermiculite was modified with surfactants in order to enable intercalation of vermiculite layers. Since vermiculite has negative charges on its surfaces, it was expected that cationic surfactant would expand the clay mineral layers more than an anionic surfactant. Nevertheless, negative parts of the anionic surfactants interacted with the positively charged edges of vermiculite and caused to super lattice structure so, unexpectedly the expansion of the layers was determined to be fully collapsed phase of vermiculite. Colloidal and structural properties of vermiculite dispersions were examined in presence of anionic, cationic and nonionic surfactants. The results showed that cationic surfactant covered the surface of the vermiculite with a second layer but the expansion of the clay mineral layer was limited compared to the anionic surfactant. The anionic surfactant produced electrostatic interaction with the positively charged edges of vermiculite and fully expanded the layer structure of the vermiculite.
1. Introduction Layered structure clay minerals have been extensively used as naturally nano-sized particles, low-cost filler to enhance the mechanical and physical properties of polymer composites (Pinnavaia & Beall, 2000; Ray and Okamoto, 2003; Utracki, 2004; Bergaya et al., 2006; Liu et al., 2006; Gul et al., 2016). Montmorillonite is mostly used as layered clay in many articles, and also in industrial uses. However, compared with montmorillonite, the clay mineral layers in vermiculite have a higher charge density, a key parameter facilitating the incorporation of organic modifiers to generate larger interlayer spacing (Slade and Gates, 2004; Bergaya et al., 2006). Vermiculite forms macroscopic crystals, that are potentially suitable for producing high aspect ratio nanofillers, and its natural abundance makes it economically attractive for industrial applications (Qian et al., 2011). Vermiculites, 2:1 phyllosilicates, are generally composed of macroscopic particles, similar in appearance to micas. (Meisinger, 1985; Suquet, 1988; Suquet et al., 1991; Martins and Fernandes, 1992; Lv et al., 2017). The clay mineral surfaces have negative charges due to the isomorphic substitutions which create a deficit of positive charge, compensated by interlayer cations that can be easily exchanged and solvated by positively charged particles. Additionally, clay minerals are hydrophilic but the clay minerals are modified by alkylammonium ions to change the surfaces as hydrophobic surfaces so, the adsorption of polymer molecules is enhanced by the clay minerals. Composites (LeBaron et al., 1999; Pinnavaia and Beall, 2000; Ray and Okamoto,
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.clay.2017.05.030 Received 14 February 2017; Received in revised form 22 May 2017; Accepted 24 May 2017 0169-1317/ © 2017 Elsevier B.V. All rights reserved.
2003; Utracki, 2004; Bergaya et al., 2006; Liu et al., 2006; Gul et al., 2016). Hence, with the purpose of increase the interaction of clay and polymer, clay particles are mostly modified with cationic surfactants to change the hydrophilic surface to the hydrophobic ones, also to expand the clay mineral interlayers. The expansion of the interlayer of vermiculite particles was studied detail in various articles with partially collapsed structure and fully collapsed phase of vermiculite alkyl ammonium complexes by Johns et al. Johns and A. S. G. P. K (1967); Serratosa et al., 1970; Lee and Solin, 1991; Williams et al., 1996; Syrmanova et al., 2017). The effects of the anionic surfactants on the properties of montmorillonite have been extensively studied in literature but not in particular for vermiculite (Lagaly et al., 1984; Kopka et al., 1988; Lagaly and F., 2001; Penner and Lagaly, 2001; Yalcin et al., 2002a,b; Yalcin et al., 2002a,b; Lagaly and Ziesmer, 2003; Gunister et al., 2004). There are three types of possible of interactions between negative charge-carrying clay particles and anionic surfactant. First, it is possible that ion exchange can take place between OH– ions on clay mineral surfaces and the anionic part of surfactants. Second, H-bonds can form between clay particles and surfactant molecules. Third, it is possible that Ca2 + cation can establish electrostatic bridges between the anionic part of surfactants and the surface of clay particles(Parfitt and D.J., 1970; Parfitt, 1978; Lagaly, 1986, 1989; Kopka et al., 1988; Lagaly and F., 2001; Penner and Lagaly, 2001; Yalcin et al., 2002a, b). In this study, the effects of the cationic surfactants hexadecyltrimethylammonium bromide (HDTABr), and octadecyltrimethylammonium bromide (ODTABr), the anionic surfactants sodium dodecyl
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sulfate (SDS), and ammonium lauryl sulfate (ALS), and the nonionic surfactant N,N-Dimethyldodecylamine N-oxide (DDAO) were investigated on the colloidal and structural properties of vermiculite. The results unexpectedly showed that anionic surfactant is much more suitable to expand the interlayer of vermiculite than cationic or nonionic surfactants. Anionic or cationic also the nonionic surfactants caused to super lattice structure by entering the interlayer of vermiculite with ion exchange and expanded the interlayer. Rheological measurements were used to examine particle–particle interactions, electrokinetical measurements were used to determine the effects of the surfactants on the surface properties of vermiculite particles. XRD, SEM results showed that the intercalation of vermiculite layers in presence of surfactants, and finally FTIR results showed the ion exchange between vermiculite and the surfactants. 2. Experimental study 2.1. Materials Vermiculite sample were obtained from Gold Butte District, Clark County, Nevada, USA. Ore samples were identified as vermiculite clay minerals using X-ray diffractometer (Bruker D8 Advance model X-ray diffractometer), indicating trioctahedral structure of vermiculite. The vermiculite was subjected to a heat treatment at temperatures over 700 °C the mineral expands, multiplying its volume between 15 and 20 times, and the expanded form of the vermiculite was used in this study. Hexadecyltrimethylammonium bromide, HDTABr, (C16H33N(CH3)3Br, MW = 364.46 g/mol from Fluka), and Octadecyltrimethylammonium bromide, ODTABr, (C18H35N+(CH3)3Br, MW = 392.52 g/mol from Fluka) were used as cationic surfactants. Sodium dodecyl sulfate, SDS, (C12H25NaO4S, MW = 288.38 g/mol from Aldrich Chemical Co) and Ammonium lauryl sulfate, ALS, (CH3(CH2)11OSO3NH4, MW = 283.40 g/ mol from Fluka) were used as anionic surfactants. N,N-Dimethyldodecylamine N-oxide (C14H31NO, MW = 229.43 g/mol from Sigma) was used as nonionic surfactant.
Fig. 1. The changes of the plastic viscosity of vermiculite-water systems in presence of surfactants.
2.2. Preparation of clay - surfactant dispersions Vermiculite was dispersed in distilled water in an ultrasonic bath and then shaken overnight. The vermiculite dispersions were mixed with 5.10− 5 to 5.10− 2 mol/L concentrations of each surfactant. Then, the dispersions were shaken for 24 h, and ultrasonicated for 5 min. Fig. 2. The changes of the zeta potential values of vermiculite-water systems in presence of surfactants.
3. Methods Rheological properties such as viscosity, shear rate (γ), and shear stress (τ) of the dispersions were measured using a Brookfield DVIII + type low-shear viscometer. The flow behavior of the clay dispersions was obtained by shear rate measurements within 0–330 s− 1 shear rates. Rheological measurements were carried out in duplicate. The electrophoretic mobility measurements were carried out using a Zetasizer 2000, Malvern Instruments. The optic unit contains a 5 mW HeeNe (638 nm) laser. The dispersions were prepared as explained above. Before the measurements, all the dispersions were centrifuged at 4500 rpm for 30 min. Supernatants were then used for zeta potential measurements. To make an electrophoretic mobility measurement in this instrument, laser beams are crossed at a particular point in the cell. Particles in the cell were illuminated by these beams. Electrophoretic mobility was measured, injecting a small portion of the dispersion into the cell of the Zetasizer 2000 instrument at 25 °C temperature. The electrophoretic mobility was then converted to zeta potential using Henry eq. X-ray diffraction (XRD) measurements were performed Bruker D8 Advance model X-ray diffractometer at room temperature using Nifiltered and Cu tube. The diffractograms were scanned in 2θ ranges from 2 to 40° at a rate of 2°/min.
The morphology of the fractured surfaces of the clay films was investigated with a FEI Quanta Feg 250 scanning electron microscope. SEM measurements were operated at 15 kV. The specimens were frozen under liquid nitrogen, and then fractured, mounted, and coated with gold on Edwards S 150B sputter coater. FTIR analyses (400–4000 cm− 1) were performed on Perkin Elmer Spectrum 100 FTIR spectrophotometer using KBr pellets with a concentration of 1% or film. Spectral outputs were recorded either in absorbance or transmittance mode as a function of wave number.
4. Results & discussion 4.1. The effect of the surfactants on the flow and the electrokinetical properties of vermiculite dispersions Vermiculite dispersions in water showed Bingham plastic behavior according to their concertina-like swelling properties. At low shear rates, the system exhibits non-Newtonian flow, and after a certain value of the shear rate, the flow curve becomes linear. Vermiculite has a lower swelling potential than other swelling clay minerals and in some 8
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Fig. 3. XRD diagrams of vermiculite and surfactant added vermiculites, a) Vk and Rehydrated Vk b) Vk and addition of 10-3mol/l of surfactants.
not affect the flow properties of vermiculite dispersions. Cationic surfactants also were not effective until the concentration of 5.10− 4 mol/L. But after that concentration the plastic viscosity values start to increase due to the flocculation of vermiculite particles. Here, the positively charged head groups of the monomers of the cationic surfactant adsorb on the negatively charged surface of the clay particles. The hydrophobic tails of the adsorbed monomers on the clay particle link the tails on the other particles by hydrophobic interactions and this cause an increase in the viscosity. Fig. 2 shows the change of zeta potential values with increasing
instances has been classified as an intermediate between micas, and smectites (Ndlovu et al., 2011). A Bingham model was applied, where τ = τΒ + ηplγ. From the equation, the yield value τB was obtained by extrapolation of the linear portion of the shear stress-shear rate curve to γ = 0, and the plastic viscosity ηPL were obtained from the slope of the linear portion of the curve. The viscosity values of clay suspensions (Fig. 1) were plotted as a function of increasing surfactant concentrations. It was obviously seen that anionic surfactants SDS and ALS and nonionic surfactant DDAO did 9
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Table 1 Interlayer spacings (Å) of vermiculite with additive surfactants. Cationic Surfactants Surfactant concentrations (mol/L)
0 (Vk-rehydrated) 1.10− 5 1.10− 4 1.10− 3 5.10− 3 1.10− 2
HDTABr
Anionic surfactants ODTABr
SDS
Nonionic surfactant ALS
DDAO
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
Main peak
Appeared shoulder
11,54 11,53 11,53 11,50 11,72 11,71
23,37 24,54 23,53 23,25 23,83 23,36
11,54 11,49 11,48 11,62 11,73 11,91
23,37 24,30 24,61 24,51 23,96 24,02
11,54 – 11,42 11,41
23,37 – 24,03 28,60
11,54 11,47 11,37 11,45
23,37 24,51 25,00 27,87
11,54 11,52 11,37 11,55 11,55 11,59
23,37 24,06 25,04 25,75 24,02 24,18
given at Fig. 3b. According to the concentration of the surfactant that shoulder slid to the lower angels, meaning that increased the new interlayer spaces which are given at Table 1, (Johns and A. S. G. P. K, 1967; Qian et al., 2011). Ionic surfactants interact electrostatically with vermiculite particles. Cationic surfactants attracted by the negatively charged surfaces while anionic surfactants attracted by the positively charged edges. The interlayer spaces have negatively charged surfaces, so normally it was expected that the cationic surfactants enter the interlayer spaces and expand it. However, anionic surfactant interacted with the positively charged edges and caused to superlattice structure of vermiculite. Hence, the anionic surfactant surprisingly increased the interlayer space much more than the cationic surfactants according to the XRD results. Besides, the interlayer spaces were higher than 28 Å for some anionic surfactant additions with a possible orientation of surfactants at the edges, the new structure is called “fully collapsed phase” according to Johns, and Sen Gupta 1967. The XRD reflections of the anionic surfactants overlap of the vermiculite reflections after the concentrations 10− 3 mol/L so the interlayer spaces were not determined. Nonionic surfactants interact sterically with clay mineral surfaces. The steric interaction also caused to the superlattice structure of vermiculite and expanded the interlayer space. The nonionic surfactant can interact both the surfaces and the edges due to its neutral structure. The same behaviors of intercalation of vermiculite were obtained according to the kinds of surfactant. However, each surfactant increased the interlayer of the vermiculite according to their chain length. The schematic representation of the interaction of surfactants and vermiculite particles were given at Fig. 4. Scanning electron microscopic (SEM) pictures of the samples were used for characterization and to show the intercalation of the vermiculite dispersions with surfactants. Direct evidence of intercalated vermiculite dispersion can be found in SEM examination. Since vermiculite composed of macroscopic particles, its layer structure can be seen even in low magnifications. Forming different aggregates (heaps) of clay particles with each other could be seen in Fig. 5a for vermiculite particles. HDTABr coated particles exhibited appearance of layers with one on top of the other (Fig. 5b). SDS coated particles (Fig. 5c) showed full intercalation which is the visual evidence of XRD results. Vermiculite dispersions with surfactants were also characterized by FTIR spectroscopy. The FTIR spectrum of vermiculite sample showed characteristic bands (Fig. 6). The broad peak of OeH stretching centered at 3412 cm− 1, HeOeH bending at 1647 cm− 1, SieO stretching at 995 cm− 1 were determined for vermiculite. The bands between 917, and 648 cm− 1 are responsible from the OH deformations and translational due to the cations of the clay sample. Finally, the SieO bending peaks of vermiculite appeared at 455 cm− 1. The rehydrated form of vermiculite showed different peaks than the thermally expanded pristine sample. The water molecules changed the OH-deformation, HeOeH bending peaks due to the hydrogen bonding. Surfactant added vermiculite dispersions had some characteristic peaks of surfactants which were exactly the same with pristine surfactants like CH
surfactant concentrations. The surfactants were not being effective on the zeta potential values of the vermiculite surfaces until the 10− 3 mol/ L additions. This concentration is related with the cation-exchange capacity of vermiculite. Cationic surfactants quickly covered the whole surface of the vermiculite so the electrically neutral surfaces obtained at 5.10− 3 mol/L. Here, the positively charged head groups of the monomers of the cationic surfactant adsorb on the negatively charged surface of the clay particles. Besides, the plastic viscosity value at the same concentration decreased so both plastic viscosity and zeta potential values indicated the flocculation. With increasing cationic surfactant concentration, the absolute value of the zeta potential decreased to the zero point and then increased to the positive values with further addition. After zero point, the increase is attributed to the formation of a second layer, which involves the interaction of surfactant tails with each other. Hence, at the full coverage of the particles, the surface is recharged and the repulsive forces between positively charged particles cause to increase in zeta potential. The anionic surfactant SDS increased the zeta potential values after 10− 3 mol/L addition. The negatively charged head of the surfactants adsorb by the positively charged of the clay mineral edges which caused the increase on the number of negative charges on the total surface of the clay particle. With increasing SDS concentration, hydrophobic tails of the surfactants covered the surface of the vermiculite layers so the zeta potential values decreased until the zero potential. The adsorption effect of ALS on the zeta potential values of vermiculite was determined at the last concentration. Nonionic surfactant DDAO covered the vermiculite surfaces and caused to decrease of the zeta potential values almost to the zero point. Since there is no electrical interaction between nonionic surfactant and the vermiculite particles the formation of the interaction should be steric effect. 4.2. The structural characterizations vermiculite-surfactant dispersions The effect of surfactants on the vermiculite particles was investigated by XRD, SEM, and FTIR methods. XRD analyses were done by measuring the interlayer spacing of vermiculite in order to understand whether added surfactant enters into the interlayer space of vermiculite or not. The sheets in the layers in clay mineral structures bound to each other with covalent bonds and therefore, their crystal structure is stable. In contrast, the layers bound with Van der Waals bonds in the clay particles and water or organic molecules expand the interlayer spacing when it introduces in the interlayer space of the clay particle. Vermiculite showed a major broad reflection at d001 = 12,29 Å for thermally expanded sample and, d001 = 11,54 Å for rehydrated sample, corresponding to the interlayer spaces of vermiculite given at Fig. 3a. Almost no changes were observed at the main interlayer space of vermiculite in presence of surfactant. Incorporation of water and surfactants generates new interlayers which were appeared as a shoulder (Fig. 3 a and b). XRD patterns of vermiculite and 10− 3 mol/L addition of each kind of surfactants were 10
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Fig. 4. Schematic representation of the interactions of vermiculite and surfactants.
stretching peaks at the 3020–2840 cm− 1, and CH bending peaks at 1650–1210 cm− 1. Hydroxyl groups located at broken edges, steps and related defects of clay minerals. At the crystal edges, an accumulation of charges is attenuated by the adsorption of protons by O2− or OH− ions. Generally, the compensation is not complete so that cations or
anions, as gegen ions, balance the edge charges. Anions can be adsorbed at positive edge sites by electrostatic interactions or exchanged for structural OH– groups at the edges (Parfitt, 1978; Lagaly, 1986, 1989, 1993; Penner and Lagaly, 2001; Bergaya et al., 2006). Shifted OH− deformation and HeOeH bending peaks of vermiculite appeared at
Fig. 5. SEM pictures of a) vermiculite, b) HDTABr/vermiculite, c) SDS/vermiculite.
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Fig. 6. FTIR spectra of vermiculite and surfactant added vermiculites.
Table 2 OH− deformation, HeOeH bending and SieO stretching peaks of samples. Samples
OH-deformation Pristine sample
Vermiculite Rehydrated-Vk HDTABr ODTABr SDS ALS DDAO
3412 3426 – 3366 3469 3137 3399
HeOeH bending Vermiculite + surfactant
Pristine sample
3400 3401 3467 3401 3420
1647 1632 – – – – –
SieO streching Vermiculite + surfactant
Pristine sample
Vermiculite + surfactant
1639 1639 1642 1637 1648
995 998 – 999 – – –
999 999 997 999 999
References
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5. Conclusions The interlayers of vermiculite can be intercalated and fully collapsed phase can be obtained with anionic surfactant SDS and ALS without changing the flow properties. The surface of the vermiculite particles can be covered by cationic surfactants even with a second layer of the surfactant but only partially collapsed structure with limited expansion of layers. Anionic and nonionic surfactants caused to superlattice structure and intercalation of the vermiculite much more than cationic surfactants.
Acknowledgement The author wish to thank Prof. Dr. O. Işık Ece and Prof. Dr. Paul Schroeder for their valuable comments and contributions. This research project is supported by the Istanbul Technical University, Research Fund(38443). 12
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