Determination of the surface area of smectite in water by ethylene oxide chain adsorption

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Journal of Colloid and Interface Science 285 (2005) 443 447 www.elsevier.

1 Journal of Colloid and Interface Science 285 (2005) Determination of the surface area of smectite in water by ethylene oxide chain adsorption Paul-Cheng Yuang, Yun-Hwei Shen Department of Resources Engineering, National Cheng Kung University, Tainan, Taiwan 70101, Republik of China Received 4 December 2003; accepted 7 December 2004 Available online 29 January 2005 Abstract This study investigates the feasibility of using ethylene oxide (EO) chain adsorption to determine the surface area of smectite in water. Experimental results indicate that high-molecular-weight poly(ethylene oxide) (PEO) should be used to provide reasonable estimations for monolayer capacity of PEO on smectite. The surface areas of smectites in water are calculated from the monolayer capacity of PEO adsorbed on smectite by taking the area per EO unit as 8.05 Å 2. The method measures the actual surface area of smectite exposed when dispersed in water, which is important to applications of smectite under aqueous conditions Elsevier Inc. All rights reserved. Keywords: Surface area; Smectite; Adsorption; Ethylene oxide chain 1. Introduction The determination of the exposed surface area of smectite, a swelling clay, deliminated in water is important in many industrial applications such as organoclays, drilling muds, binding agents, plasticizers, and soil technology. Surface area measurements are typically based on the amount of probe molecule that a solid adsorbs at monolayer coverage and the molecular area of the probe molecule. Nitrogen, a nonpolar molecule, is often used in the Brunauer, Emmett, and Teller (BET) method to obtain the surface area of fine particles [1]. However, it is widely recognized that nitrogen does not interact with or have access to the interlayer surfaces of expandable clay minerals, such as smectite, which collapse when the clay is dried during pretreatment. In the presence of water, exchangeable cations that are associated with expandable clays are hydrated; this forces a separation of interlayer surface or delimination. The interlayer surfaces of the clays then become accessible and are therefore considered an important component of the surface area of clays. * Corresponding author. Fax: address: yhshen@mail.ncku.edu.tw (Y.-H. Shen). The amount of some polar solvent (e.g., water or ethylene glycol) retained by a unit mass of the clay under certain evacuating conditions has been used to determine the total surface area of clay [2,3]. However, appropriate adsorbates for surface area determination must be chemically inert so that they neither alter the structure of the solid nor penetrate the molecular network of the solid [4]. Because some polar solvents can potentially alter the solid structure, such as by a solvation process with some clay minerals, the resulting surface area is often a measure of phenomena other than physical adsorption. For example, it has been found that the retention of polar molecules may be influenced by the species of exchangeable cation of the clay [5] and polar molecules tend to form multilayers at cation-exchange sites prior to complete monolayer coverage [6]. The adsorption of methylene blue dye by clay minerals in water has also been used to determine either their cation exchange capacity (CEC) or their surface area [7]. However, due to the cation exchange nature of this adsorption process, what properties are actually being measured is unclear. Poly(ethylene glycol) (PEG) is a water-soluble nonionic polymer, HO (CH 2 CH 2 O ) n H, which is available commercially in a wide range of molecular weights. Higher /$ see front matter 2004 Elsevier Inc. All rights reserved. doi: /j.jcis

2 444 P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) molecular weights are referred to as poly(ethylene oxide) (PEO). The ether oxygen in the polymer chain interacts with water, causing the polymer to be water-soluble, while the CH 2 CH 2 groups are hydrophobic in nature. Adsorption behavior of PEO and PEG for several different powders in aqueous systems has been reported. Rubio and Kitchener [8] proposed that the mechanism of PEO adsorption onto silica was hydrogen bonding between the surface silanol (SiOH) groups and the ether oxygen in the polymer. Koksal et al. [9] discovered that no PEO adsorption occurred on alumina and hematite surfaces and postulated that the polymer could not displace strongly adsorbed water molecules. Shen [10] also confirmed that the ethylene oxide chain has strong adsorption on SiO 2, but not on oxides such as Al 2 O 3 and Fe 2 O 3. Given that the ethylene oxide chain adsorbs strongly onto SiO 2 via hydrogen bonding in water, it is reasonable to speculate that the ethylene oxide chain will fully coat the interlamellar space of smectite, which is composed predominately of siloxane ( Si O Si ) surface. This caused us to investigate the feasibility of using nonionic ethylene oxide chain adsorption to determine the surface area of an expandable clay, smectite in water. 2. Experimental 2.1. Materials Smectites (SWy-1, SAz-1, and STx-1, obtained from the Clay Source Repository, University of Missouri) were used as received. Montmorillonit K-10 was obtained from Aldrich Chemical Co. and used as received. Table 1 lists the identification and CEC for the studied smectites. PEGs and PEOs were obtained from Aldrich Chemical Co. and used as received. Table 2 lists the molecular formulae, molecular weights, and abbreviations of these compounds. Table 1 Identification and CEC for studied smectites Sample Smectite CEC (meq/100 g) Smectite A SWy Smectite B K Smectite C SAz Smectite D STx Table 2 Molecular formulas, molecular weights, and abbreviations of polymers used Formula MW (g/mol) Abbreviation HO(CH 2 CH 2 O) n H 300 PEO300 HO(CH 2 CH 2 O) n H 600 PEO600 HO(CH 2 CH 2 O) n H 4000 PEO4000 HO(CH 2 CH 2 O) n H 20,000 PEO20000 HO(CH 2 CH 2 O) n H 100,000 PEO HO(CH 2 CH 2 O) n H 400,000 PEO HO(CH 2 CH 2 O) n H 1,000,000 PEO PEO adsorption isotherms All adsorption tests were conducted at ph 7. Adsorption was conducted in 50-ml Pyrex beakers by contacting the smectite suspension with polymer solution using magnetic stirrer at 600 rpm in a constant temperature chamber of 22 C. A control with no smectite addition was included. After a predetermined contact time (typically 8 h to reach equilibrium), the sample was centrifuged at 10,000 rpm for 25 min and the supernatant was withdrawn. A total organic carbon analyzer (TOC 5000, Shimazdu) determined the residual polymer concentrations. The standard errors of the mean for the TOC concentrations were in the range of , which corresponds to a standard deviation of mg TOC/L. No detectable change in solution concentration of polymer was found in the control samples with no smectite addition. The difference in the amount before and after sorption gives the amount sorbed. All samples were run in duplicate, and the average is reported EO unit area determinations The surface area of a single EO unit was determined based on the amount of PEO that a nonswelling clay, kaolinite, adsorbs at monolayer coverage and the BET surface area of kaolinite measured with a Micromeritics ASAP 2000 instrument. Kaolinite is a nonswelling clay with 1:1 structure, comprising an octahedral aluminum layer overlying a tetrahedral silicon sheet. This 1:1 structure implies an equal area of exposed Si-oxide and Al-oxide surface. In other words, one-half of the BET surface area of kaolinite arises from the exposed Si-oxide surface. Assuming that the ethylene oxide chain has strong adsorption only on the Si-oxide surface, PEO will form monolayer coverage only on that half of the BET surface area of kaolinite. Thus, the surface area of a single EO unit may be obtained by dividing one-half of the BET surface area of kaolinite by the total number of EO units adsorbed at monolayer coverage X-ray diffraction analysis The d 001 spacing of smectite was examined using an X-ray diffractometer (Model Rad II A, Rigaku, Tokyo) with CuK α radiation and Ni filter, operated at 30 kv, 200 ma, and a scanning rate of 2 /min. The smectites were freeze-dried and ground to pass a 100-mesh screen before XRD analysis. 3. Results and discussion 3.1. Monolayer coverage of PEO on smectite surface Fig. 1 depicts the adsorption isotherms of the various PEG and PEO polymers on smectite A (SWy-1). Notably, as the molecular weight of the polymer increases, the isotherm

3 P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) Fig. 1. Sorption isotherms of the various PEO polymers on smectite A. Fig. 2. The amount of PEO adsorbed on smectite A at the adsorption isotherms platforms as a function of the square root of the PEO s molecular mass. also increases more steeply at low concentrations and attains a higher plateau region of adsorption. In general, it is more difficult to desorb larger polymer molecules because more segments must detach simultaneously for the larger polymer molecule to desorb. Thus, for higher-molecularweight PEO, the isotherm is steeper at low solution concentrations. For a given fraction of surface area that adsorbed polymer segments cover, larger polymers will have a greater mass in loops and tails, and therefore the amount adsorbed will have a greater mass. Scheutjens and Fleer [11,12] theoretically predicted that a linear correlation existed between the saturated amount of polymer adsorbed and the square root of polymer molecular mass. However, when we plot the amount of polymers adsorbed on smectite A at the adsorption isotherms platforms against the square root of the polymer s molecular mass (Fig. 2), a linear relationship is not obtained. Fig. 2 indicates that the saturated amount of polymer adsorbed initially increases with the increasing molecular weight of low-molecular-weight polymers but is almost independent of the molecular weight Fig. 3. X-ray diffraction patterns of PEO-loaded smectite A. for high-molecular-weight polymers. The convergent saturated amount of polymer that was adsorbed for smectite A is 305 mg/g (Fig. 2). This observation obviously indicated that for high-molecular-weight PEOs the polymer conformation adsorbed on smectite takes train segments predominantly and a monolayer coverage is formed on smectite surface. Apparently, this is in direct conflict with accepted polymer adsorption models, which indicate that the formation of loops and tails increases with molecular weight. Research results [13 16] indicated that a large part of the siloxane ( Si O Si ) surface in smectites has a hydrophobic nature. Thus, it is not surprising to observe a different PEO adsorption model on smectites. We speculate that the hydrophobic interaction between the siloxane ( Si O Si ) surface and the ethylene group (CH 3 CH 2 )ofthepeois the major adsorption mechanism of PEO on smectite surface. In this context, high-molecular-weight PEOs are less soluble in water and tend to bind more strongly on hydrophobic siloxane surfaces, leading to a rapid approach to monolayer coverage on smectite. The results from X-ray diffraction analyses of PEO-loaded smectites shown in Fig. 3 provide additional evidence for monolayer coverage of PEO on smectite surface. At 125% saturated capacity loading of PEO MW 100,000 and PEO MW 1,000,000 the d spacing of both PEO-loaded smectites increase from 13.2 to 18.2 Å. If it were not monolayer coverage of PEO, i.e. the adsorbed polymer takes train segments predominantly on smectite, the higher-molecular-weight PEO would have a greater mass in loops and tails, and therefore a larger d spacing. At 50 and 125% saturated capacity loading of PEO MW 1,000,000 the d spacing of PEO-loaded smectites increase from 13.2 to 17.2 and 18.2 Å, respectively. The slightly smaller expansion of d spacing for smectite with 50% saturated capacity loading of PEO probably results from inadequate coverage of PEO on the upper and lower surface of the interlamellar region in smectite. A reviewer of this manuscript raised a concern with the speculation that high-molecular-weight polymers adsorb essentially flat on a solid surface, especially at high surface coverage. However, without further

4 446 P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) Fig. 4. Sorption isotherms of various PEO polymers on kaolinite. Fig. 6. The amount of PEO adsorbed on various smectites at the adsorption isotherms platforms as a function of the square root of the PEO s molecular mass. Table 3 Specific surface areas of smectites in water Sample Smectite Surface area (m 2 /g) Smectite A SWy Smectite B K Smectite C SAz Smectite D STx posed Si-oxide surface, the area of a single EO unit was determined to be 8.05 Å 2. This value is consistent with the area of a single EO unit reported in other research [17,18]. Fig. 5. The amount of PEO adsorbed on kaolinite at the adsorption isotherm platforms as a function of the square root of the PEO s molecular mass. evidence from in situ surface analysis, we believe that this admittedly crude model logically explain the experimental observations Area of a single EO unit The surface area of a single EO unit was obtained by dividing one-half of the BET surface area of kaolinite by the total number of EO units adsorbed at monolayer coverage. The total number of EO units adsorbed at monolayer coverage on kaolinite was obtained from Figs. 4 and 5. Fig. 4 depicts the adsorption isotherms of the various PEG and PEO polymers on kaolinite and Fig. 5 plots the number of polymers adsorbed on kaolinite at the adsorption isotherm platforms against the square root of the polymer s molecular mass. The convergent saturated amount of PEO that was adsorbed for kaolinite is 6.32 mg/g and is a reasonable estimate of the monolayer capacity of EO on kaolinite. Quantity of 6.32 mg of PEO is equivalent to mole of the EO unit. Taking one-half of the BET surface area of kaolinite ( m 2 /g) as the area of ex Surface area of smectite in water At this stage, two requirements for surface area measurements, the amount of probe molecule that a solid adsorbs at monolayer coverage and the molecular area of the probe molecule, have been established for the adsorption of PEO on smectite. In the following, the surface areas of smectites in water are calculated from the monolayer capacity of PEO adsorbed on smectite by taking the area per EO unit as 8.05 Å 2. The monolayer capacity of PEO was obtained by the convergent saturated amount of PEO adsorbed on smectites form Fig. 6, which plots the saturated amount of PEO adsorbed against the square root of PEO s molecular mass. The resulting areas obtained for four smectites are listed in Table 3. The calculated surface area for a typical smectite is m 2 /g [19] and there is a significant difference between the areas determined by PEO adsorption in this study and by calculation. The calculated surface area of smectite ( m 2 /g) is the total area exposed when individual smectite sheets are fully deliminated. Sheets of smectite dispersed in water are unlikely to be fully deliminated due to the natural heterogeneity of smectite. Instead, several smectite sheets combine to form tactoids, groups of aligned sheets, in water. This produces a small number of

5 P.-C. Yuang, Y.-H. Shen / Journal of Colloid and Interface Science 285 (2005) particles in water, so that the exposed surface area is reduced. We speculate that the surface area obtained by PEO adsorption reveals the actual exposed area of smectite in water. Comparing Tables 1 and 3, it is evident that the surface area of smectite in water decrease with increasing CEC of smectite. The suspensions of SAz-1 form larger particles than SWy-1 in water due to their larger charge (higher CEC) [20]. As the layer charge increases, the cohesion energy that holds the lamellae closer also increases, so that the dispersion of smectite in water becomes more difficult, resulting in larger tactoids and decreasing the available surface area. However, smectite K-10, with the lowest CEC, shows relatively low surface area in water, presumably due to the interlayer K + ions effectively preventing delimination of smectite K-10 in water. This is confirmed by the observation of the fast settling of smectite K-10 particles in suspension. 4. Conclusion This study demonstrates the feasibility of using the adsorption of on ethylene oxide chain to determine the surface area of smectite in water. The method measures the actual surface area of smectite exposed when dispersing in water and is important to the application of smectite under aqueous conditions. High-molecular-weight PEO (MW 100,000 or higher) should be used to provide reasonable estimates for monolayer capacity. Acknowledgment The authors thank the National Science Council of the Republic of China for financially supporting this study. References [1] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [2] R.W. Mooney, A.C. Keenan, L.A. Wood, J. Am. Chem. Soc. 74 (1952) [3] R.S. Dyal, S.B. Hendricks, Soil Sci. 69 (1950) 421. [4] C.T. Chiou, Partition and Adsorption of Organic Contaminants in Environmental Systems, Wiley Interscience, New York, [5] A. Kellomaki, P. Nieminen, L. Ritamaki, Clay Miner. 22 (1987) 297. [6] K.D. Pennell, R.D. Rhue, A.G. Hornsby, J. Environ. Qual. 21 (1992) 419. [7] P.T. Hang, G.W. Brindley, Clays Clay Miner. 15 (1970) 259. [8] J. Rubio, J.A. Kitchener, J. Colloid Interface Sci. 57 (1976) 132. [9] E. Koksal, R. Ramachandran, P. Somasundaran, C. Maltesh, Powder Technol. 62 (1990) 253. [10] Y. Shen, Chemosphere 41 (2000) 711. [11] J.M.H.M. Scheutjens, G.J. Fleer, J. Phys. Chem. 83 (1979) [12] J.M.H.M. Scheutjens, G.J. Fleer, J. Phys. Chem. 84 (1980) 178. [13] W.F. Jaynes, S.A. Boyd, Clays Clay Miner. 39 (1991) 428. [14] R.K. Kukkadapu, S.A. Boyd, Clays Clay Miner. 43 (1995) 318. [15] G. Sheng, S.A. Boyd, Clays Clay Miner. 46 (1998) 10. [16] C. Breen, R. Watson, J. Colloid Interface Sci. 208 (1998) 422. [17] S. Partyka, S. Zaini, M. Lindheimer, B. Burn, Colloids Surf. 12 (1984) 255. [18] P. Somasundaran, E.D. Snell, E. Fu, Q. Xu, Colloids Surf. 63 (1992) 49. [19] L.D. Baver, Soil Physics, Wiley, New York, [20] M.G. Neumann, F. Gessner, C.C. Schmitt, R. Sartori, J. Colloid Interface Sci. 255 (2002) 254.

Clays and Clay Minerals

Clays and Clay Minerals Fields of interest for clays Various definitions Acients: Earths in the earth-air-fire-water system Definition of clay depends on discipline: Geologist grain size