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Adsorption of conventional and gemini cationic surfactants in nonswelling and swelling layer silicate
Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 568–572
Adsorption of conventional and gemini cationic surfactants in nonswelling and swelling layer silicate Liyun Qi a,b,∗ , Wensheng Liao b , Zhichu Bi b a
School of Chemical and Material Engineering, Southern Yangtze University, Wuxi 214122, People’s Republic of China b Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 11 December 2006; received in revised form 13 March 2007; accepted 19 March 2007 Available online 23 March 2007
Abstract The adsorption of conventional (cetyltrimethylammonium bromide) and its corresponding gemini cationic surfactant [ethanediyl-␣,bis(cetyldimethylammonium bromide)] in both swelling (montmorillonite) and nonswelling (kaolinite) layer silicates was investigated by adsorption isotherms and X-ray diffraction techniques. Adsorption studies showed that the adsorption of gemini surfactant was more efficient than that of the corresponding conventional surfactant on both kaolinite and montmorillonite. The adsorption amount of both surfactants on montmorillonite was much higher than on kaolinite, which was related to the nature of the clays. Although the molar adsorption amount of both surfactants on the same clay were almost identical, there existed some differences in the microstructure of adsorption layer revealed by X-ray diffraction experiments. © 2007 Elsevier B.V. All rights reserved. Keywords: Gemini surfactants; Montmorillonite; Kaolinite; Adsorption isotherm; X-ray diffraction
1. Introduction The adsorption of surfactants onto clay plays an important role in a number of interfacially controlled processes such as flotation and flocculation, and especially in enhanced oil recovery [1]. In recent years modified natural clays with quaternary ammonium or some other onium ions have attracted great interest of researchers due to their high capability to remove hydrophobic contaminants from aqueous solutions and thus are very promising agents in environmental control and in the reduction of leaching, photodegradation, and volatilization of herbicides [2–6]. These applications are based on the phenomena that exchangeable inorganic cations within silicate clays layer can be readily replaced not only by inorganic ions but also by cationic surfactants such as quaternaryammonium compounds. Layered clay minerals such as nonswelling kaolinite and swelling montmorillonite are adsorbents used very widely in these applications [7]. There have been extensive investigations on the adsorption of conventional alkyl ammonium onto non∗ Corresponding author at: School of Chemical and Material Engineering, Southern Yangtze University, Wuxi 214122, People’s Republic of China. Tel.: +86 510 89880656; fax: +86 510 85917763. E-mail address: [email protected] (L. Qi).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.03.035
swelling and swelling clays [8–10]. However, only a few papers have reported on the adsorption behavior of gemini surfactants (surfactants containing two hydrophilic and two hydrophobic groups in the molecule) on clay, especially with alkyl chain as long as 16 carbon number. Gemini surfactants constitute a new class of surfactants that have become a topic of scientific interest in virtue of some of their excellent properties, such as smaller critical micelle concentrations (cmcs), higher surface activity than conventional monomeric counterparts, as well as remarkable rheological properties [11], foamability, wetting, solubilization, and antibacterial activity, etc. [12,13]. To our knowledge, only several papers reported the adsorption of gemini surfactants at the clay/aqueous solution interface [14–16]. Those papers focused on either the influence of the spacer of gemini surfactant on the amount of surfactant adsorbed or the effect of gemini surfactants on the removal of organic contaminants. Hence, a better understanding of the adsorption mechanism of conventional and gemini surfactants in silicate layer is still highly desired. Moreover, the differences of microscopic structure between the adsorbed layer of conventional and that of gemini surfactants have not been reported as yet. The investigation of the microstructure of intercalated clay is crucial for successful development of new adsorption materials. In the present study, we choose a typical nonswelling
L. Qi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 568–572
clay kaolinite and a typical swelling clay Na-montmorillonite as adsorbents. On the two kinds of clays, adsorption of the conventional surfactant cetyltrimethylammonium bromide (CH3 )3 N+ C16 H33 Br− (CTAB) and corresponding gemini surfactant ethanediyl-␣,bis-(cetyldimethylammonium bromide) C16 H33 (CH3 )2 N+ Br− (CH2 )2 N+ Br− (CH3 )2 C16 H33 (referred to as 16-2-16 surfactant, where 16 and 2 are the carbon numbers of the side alkyl chains and the alkanediyl spacer, respectively) was investigated with the purposes of examining the adsorption mechanism of conventional and gemini cationic surfactants in silicate layer. The arrangement of CTAB and 16-2-16 adsorbed in montmorillonite interlayer was determined and compared. 2. Experimental 2.1. Material The gemini surfactant ethanediyl-␣,-bis(cetyldimethylammonium bromide) (16-2-16) was synthesized as Ref. [17]. The product was recrystallized three times from ethanol/hexane mixed solvent. The structure and purity of the compound were confirmed with elemental analysis, 1 H NMR spectrum and the absence of a minimum value in the surface tension–concentration curve. CTAB was recrystallized three times from acetone and freeze-dried before use. Other agents of analytical reagent grade were obtained commercially and used without further purification. Doubly deionized water was used for preparing all the aqueous solutions in this study. The SWy-2-Na-montmorillonit was purchased from Source Clays Responsitory, University of Missouri, Columbia, MO, USA. The cation exchange capacity (CEC) of the montmorillonite was 0.85 mmole (+)/g determined by Ca exchange method. The surface area of the SWy-2 was 25.5 m2 /g examined by nitrogen adsorption measurement. The kaolinite used was purchased from J.T. Baker Corporation, and the cation exchange capacity (CEC) was 0.052 mmole (+)/g, the surface area 7.5 m2 /g. The clays were treated with 30% H2 O2 for 18 h to remove organic matter, and then saturated with Na by washing 10 g of clay with 250 mL of 1 mol/L NaCl solution three times. The equilibrium time was 8 h for each washing. Different size fractions of Na-clay were separated by wet sedimentation. The <0.2 m size fraction was collected and rinsed with deionized water until no Cl− can be detected in supernatant. The clay obtained through centrifuge was dried at 110 ◦ C and stored in a desiccator over P2 O5 before use. 2.2. Adsorption isotherms Montmorillonite suspensions were prepared by dispersing 5 g of dehydrated montmorillonite in 250 mL of bidistilled water. The suspension was then stirred for about 48 h in order to swell the montmorillonite. Five millilitres of homogeneous colloidal clay suspension (20 g/L) was piped into an Erlenmeyer flask. For CTAB and 16-2-16 adsorption, corresponding surfactant aqueous solution (in the range of 0.1–20 mmol/L) was piped into the clay suspension and the whole volume of the mixture
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solution was fixed. The ionic strength of the initial solution was adjusted to 0.02 using standardized NaCl. The mixture was allowed to equilibrate for 24 h with continuous gently stir. The required time to reach equilibrium and the solid/liquid ratio were previous determined experimentally. The pH of the solutions was not adjusted but was about 6. For the determination of the adsorption isotherm, the samples were centrifuged in tightly stoppered Teflon containers for 30 min at 16,000 rpm to separate the clay particles. Since the surface tension of a given surfactant in equilibrium is a fixed value corresponding to the surfactant concentration below cmc, the concentration of supernatant was determined by the surface tension using the pendant drop technique (the apparatus, JC2000A POWEREACH, made by Shanghai powereach digital equipment company). In addition, the concentration of the surfactant in the aqueous phase was measured by the mixed indicator two-phase titration method [18]. The same results were obtained by either method. The amount of surfactant adsorbed onto the clays is given by the following equation: ns =
(C0 − Ceq ) × V g
(1)
where ns is the number of moles of adsorbate (surfactant) per gram of adsorbent (clay) (mol/g), C0 the initial concentration of adsorbate in suspension solution (mol/L), Ceq the equilibrium concentration of adsorbate in the supernatant (mol/L), V the volume of solution (L), and g is the weight of adsorbent (g). 2.3. X-ray measurements The samples of the CTAB-Na-montmorillonite and 162-16-Na-montmorillonite were centrifuged from the initial suspensions, washed thoroughly, and freeze-dried. The d0 0 1 val˚ radiation from an ues were recorded by using Cu K␣ (λ = 1.54 A) automated X-ray diffractometer (XRD) (Rigaku D-MAX2500, Tokyo, Japan). The XRD was operated at 40 kV and 30 mA in a step scan mode. The scanning speed was 0.0167◦ (2θ s−1 ). The distance between the lamellae, i.e. the so-called basal spacing was calculated by Bragg-reflections from data determined by X-ray diffraction. The orientation of the intercalated surfactant molecules was obtained on the basis of the basal d0 0 1 value. 3. Results and discussion 3.1. Adsorption isotherms Fig. 1 shows the adsorption isotherms of the both surfactant onto kaolinite (Fig. 1a) and SWy-2-Na-montmorillonite (SWy2) (Fig. 1b) at 25 ◦ C in aqueous solution. The four isotherms obtained in our studies are all typical S-shaped. In the low concentration region, all the adsorption increases linearly, which indicates that electrostatic interaction between the substrate and the adsorbed surfactant plays a dominant role in the adsorption process [19]. At the higher concentrations, the adsorption amount increases sharply with the increase of surfactant equilibrium concentration until the saturation surface excess plateau is
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clay (0.052 mmole /g for kaolinite and 0.85 mmole /g for SWy2, respectively). This implies that although cationic exchange plays a predominant role in the adsorption process, hydrophobic interaction between carbon chains also occurs and enhances the surfactant adsorption. It can also be seen from Fig. 1 that the adsorption isotherm of gemini surfactant 16-2-16 is more steep than CTAB in the high concentration region, indicating that the gemini surfactant with two hydrophobic tails leads to stronger hydrophobic interaction. The experimental results show that the observed maximum adsorption excess of both surfactants in kaolinite is only ca. one-seventeenth of that in SWy-2. In consideration of the fact that the surface area of the used SWy-2 is only 3.4 times of the kaolinite, and the CEC of the former is 16.1 times of the latter, we conclude that the exchange of the ions plays the major role in the adsorption process. 3.2. X-ray diffraction
Fig. 1. Adsorption isotherms of the conventional surfactant and the gemini surfactant on kaolinite (a) and SWy-Na-montmorillonite (b) at 25 ± 1 ◦ C.
reached. The sudden rise in adsorption in this region is attributed to the formation of surfactant aggregates on the solid surface [19]. In the case of CTAB, the amount adsorbed at the saturation is about 0.071 mmol/g on kaolinite and 1.22 mmol/g on SWy-2. On the other hand, the maximum of adsorption amount of 16-2-16 on kaolinite and on SWy-2 is 0.070 and 1.20 mmol/g, respectively. Although the 16-2-16 has two quaternary ammonium headgroups that can adsorb onto negative sites of the clay, the above data show that the molar maximum adsorption of the gemini and conventional surfactants is almost identical either on the kaolinite or on SWy-2. The results suggest that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic group is presumably oriented towards the aqueous phase. This may be resulted from the low density of the clays which cannot offer enough ion exchange location within the limited scope of which the rigid spacer of 16-2-16 extends. Noteworthy is the much lower equilibrium concentration of 16-2-16 than CTAB (about 1/40 that of CTAB) when the maximum adsorption is reached (Fig. 1). This indicates that the gemini surfactant 16-2-16 is much more efficient than the corresponding conventional surfactant CTAB at achieving the maximum adsorption on both kaolinite and SWy-2. The maximum adsorption of both surfactants on kaolinite and SWy-2 is slightly higher than the measured cation exchange capacity (CEC) of the
The most widely used technique for the study of intercalated surfactants structure in the silicate galleries is X-ray diffraction (XRD), which provides the information on the layer structure [20]. In our experiments, the SWy-2 clay samples, the CTABSWy-2 and 16-2-16-SWy-2 complex were examined by X-ray diffraction. The X-ray diffractiongram of SWy-2 with different CTAB loadings is presented in Fig. 2. With the increase of the CTAB loadings in SWy-2, the d0 0 1 diffraction peak of SWy-2 shifts toward the lower 2θ region showing the expansion of the interlayer space. The changes of d0 0 1 basal spacing with the increase of CTAB adsorbed amount are presented in Fig. 3. According to the model of Refs. [21,22], long-chain alkylammonium cations can conceivably adopt monolayer, bilayer, pseudotrimolecular, or paraffin-type arrangements in the swelling layer silicate depending on the density of the clay charge and the proportion of the CEC occupied. The thickness of a montmorillonite platelet is 0.96 nm and the gallery height of a flat-lying alkyl chain in the clay interlayer is 0.4 or 0.45 nm, depending on
Fig. 2. Selected X-ray diffraction patterns of CTAB-Na-SWy-2, SWy-2MMt denoted the montmorillonite and the numbers immediately over the X-ray diffraction patterns indicate the CTAB loadings (normalized to CEC).
L. Qi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 568–572
Fig. 3. d0 0 1 -spacings of CTAB-Na-SWy-2 at different loadings (the CTAB loadings were normalized to CEC).
its orientation [23]. The step increases of d0 0 1 -spacings in Fig. 3 suggest three kinds of arrangements with different loadings. At CTAB loading levels ≤ 0.4 CEC, the d-spacing is 1.43–1.48 nm. This implies a lateral-monolayer arrangement of CTA+ in the interlayer space of the SWy-2. For 0.5 CEC and 0.7 CEC, the d0 0 1 values are 1.79 and 1.83 nm, respectively, which reflects a lateral-bilayer arrangement. As the CTAB loadings increase from 0.8 CEC to 1.1 CEC, the d-spacing increased from 1.87 to 1.98 nm, suggesting the transitional structure between the bilayer and pseudotrimolecular layer. From 1.1 CEC to 2.5 CEC, the d0 0 1 spacings reach over 2.0 nm which is corresponding to a pseudotrimolecular layer. The vertical configuration (i.e. paraffin complex) is not observed even at higher CTAB loadings. This may be attributed to the low charge density of SWy-2. Fig. 4 shows the XRD patterns of 16-2-16-Na-SWy-2 montmorillonite hybrids with different 16-2-16 loadings. At low loadings, the diffraction peak of the complexes also shifts toward lower 2θ region with the increase of adsorption excess (Fig. 4),
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Fig. 5. d0 0 1 -spacings of 16-2-16-Na-SWy-2 at different loadings (the 16-2-16 loadings were normalized to CEC).
just similar to the instance of CTAB-Na-SWy-2. Unexpectedly, XRD pattern of 16-2-16-Na-Swy-2 beyond 0.6 CEC loadings shows a minor new peak except d0 0 1 -spacing peak in the higher region (Fig. 4), which did not appear in the case of CTAB. At 0.2 CEC there exists a peak corresponding to d0 0 1 of 1.5 nm, and at 0.7 CEC of surfactant coverage, two peaks corresponding to interlamellar distances of both 1.87 and 1.36 nm (Fig. 5) were detected. This should be explained by the delamination of silicate sheets upon the adsorption of 16-2-16 molecules in excess of 0.6 CEC. With the increase of the adsorption excess of 16-2-16 in SWy-2 both peaks of 16-2-16-Na-SWy-2 shift to lower 2θ region. The results suggest that the minor peak is also formed by the interlayer arrangement of flexible 16-2-16 carbon chains. The change of diffraction peak d0 0 1 value with 16-2-16Na-SWy-2 loadings is showed in Fig. 5. The stepwise d-spacing curve of 16-2-16-Na-SWy-2 is also observed. In comparison with the CTAB-Na-SWy-2, the layer expansion of 16-2-16-NaSWy-2 is a little bigger under the same adsorption loadings. The behavior is attributed to the stronger hydrophobicity of 16-216 with two carbon chains. Gemini surfactant organocalys were reported to have better adsolubility than conventional ones [15]. The relationship between microstructure and the adsolubilization capability should be studied substantially. More details are under the investigation. 4. Summaries
Fig. 4. Selected X-ray diffraction patterns of 16-2-16-Na-SWy-2. The numbers immediately over the X-ray diffraction patterns indicate the 16-2-16 loadings (normalized to CEC).
The equilibrium adsorption behavior of CTAB and its corresponding gemini surfactant 16-2-16 on kaolinite and montmorillonite (SWy-2) in aqueous solution was studied using adsorption isotherm method. The results suggested that the cation exchange function dominantly determined the adsorption amount of CTAB and 16-2-16 on kaolinite and SWy-2. However, hydrophobic interaction also played an assistant role in the adsorption process. Gemini surfactant 16-2-16 was more efficient than CTAB in adsorption, whereas the maximum adsorption excess of the two surfactants was almost identical. The basal spacing obtained from X-ray diffraction gave
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the microstructure details of surfactant-intercalated organoclay. The delamination of SWy-2 with high loadings of 16-2-16 was observed. Acknowledgement This research project was supported by the National Fundamental Research Project (G1999022506). References [1] M.J. Rosen, Surfactants and Interfacial Phenomena, 2nd ed., John Wiley & Sons, New York, 1989. [2] S.A. Boyd, J.F. Lee, M.M. Mortland, Nature 333 (1988) 345–347. [3] J.M. Brixie, S.A. Boyd, J. Environ. Qual. 23 (1994) 1283–1290. [4] Y. El-Nahhal, S. Nir, T. Polebusova, L. Margulies, B. Rubin, J. Appl. Clay Sci. 14 (1999) 105–119. [5] S. Nir, T. Undabeytia, D. Yaron-Marcovich, Y. El-Nahhal, T. Polebusova, C. Serban, G. Rytwo, G. Lagaly, C. Rubin, Environ. Sci. Technol. 34 (2000) 1269–1274. [6] Y. El-Nahhal, S. Nir, C. Serban, O. Rabinowitz, C. Rubin, J. Agric. Food Chem. 49 (2001) 5364–5371.
[7] M. Ogawa, K. Kuroda, Bull. Chem. Soc. Jpn. 70 (1997) 2593–2618. [8] S.Y. Lee, W.J. Cho, K.J. Kim, J.H. Ahn, M. Lee, J. Colloid Interf. Sci. 284 (2005) 667–673. [9] Y. Yamaguchi, H. Hoffmann, Colloid Surf. A: Physicochem. Eng. Aspects 121 (1997) 67–74. [10] S.Y. Lee, S.J. Kim, Clay Miner. 38 (2003) 225–232. [11] F. Kern, F. Lequeux, R. Zana, S.J. Candau, Langmuir 10 (1994) 1714–1723. [12] M.J. Rosen, Chem. Technol. 30 (1993) 23–33. [13] R. Zana, C. Opin, Colloid Interf. Sci. 1 (1996) 566–571. [14] K. Esumi, Y. Tajeda, M. Goino, K. Ishiduki, Y. Koide, Langmuir 13 (1997) 2585–2587. [15] F. Li, M.J. Rosen, J. Colloid Interf. Sci. 224 (2000) 265–271. [16] K. Esumi, M. Goino, Y. Koide, J. Colloid Interf. Sci. 183 (1996) 539– 545. [17] R. Zana, M. Benrraou, R. Rueff, Langmuir 7 (1991) 1072–1075. [18] M. Tsubouchi, H. Mitsushio, N. Yamasaki, Anal. Chem. 53 (1981) 1959–1961. [19] R. Atkin, V.S.J. Craig, E.J. Wanless, S. Biggs, Adv. Colloid Interf. Sci. 103 (2003) 219–304. [20] Y. Li, H. Ishida, Langmuir 19 (2003) 2479–2484. [21] G. Lagaly, A. Weiss, Kolloid. Z. Z. Polym. 243 (1971) 48–55. [22] J.L. Bonczek, W.G. Harris, P. Nkedi-Kizza, Clays Clay Miner. 50 (2002) 11–17. [23] G.W. Brindley, R.W. Giffnabb, Clays Clay Miner. 9 (1962) 546–556.
Adsorption of conventional and gemini cationic surfactants in nonswelling and swelling layer silicate Liyun Qi a,b,∗ , Wensheng Liao b , Zhichu Bi b a
School of Chemical and Material Engineering, Southern Yangtze University, Wuxi 214122, People’s Republic of China b Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received 11 December 2006; received in revised form 13 March 2007; accepted 19 March 2007 Available online 23 March 2007
Abstract The adsorption of conventional (cetyltrimethylammonium bromide) and its corresponding gemini cationic surfactant [ethanediyl-␣,bis(cetyldimethylammonium bromide)] in both swelling (montmorillonite) and nonswelling (kaolinite) layer silicates was investigated by adsorption isotherms and X-ray diffraction techniques. Adsorption studies showed that the adsorption of gemini surfactant was more efficient than that of the corresponding conventional surfactant on both kaolinite and montmorillonite. The adsorption amount of both surfactants on montmorillonite was much higher than on kaolinite, which was related to the nature of the clays. Although the molar adsorption amount of both surfactants on the same clay were almost identical, there existed some differences in the microstructure of adsorption layer revealed by X-ray diffraction experiments. © 2007 Elsevier B.V. All rights reserved. Keywords: Gemini surfactants; Montmorillonite; Kaolinite; Adsorption isotherm; X-ray diffraction
1. Introduction The adsorption of surfactants onto clay plays an important role in a number of interfacially controlled processes such as flotation and flocculation, and especially in enhanced oil recovery [1]. In recent years modified natural clays with quaternary ammonium or some other onium ions have attracted great interest of researchers due to their high capability to remove hydrophobic contaminants from aqueous solutions and thus are very promising agents in environmental control and in the reduction of leaching, photodegradation, and volatilization of herbicides [2–6]. These applications are based on the phenomena that exchangeable inorganic cations within silicate clays layer can be readily replaced not only by inorganic ions but also by cationic surfactants such as quaternaryammonium compounds. Layered clay minerals such as nonswelling kaolinite and swelling montmorillonite are adsorbents used very widely in these applications [7]. There have been extensive investigations on the adsorption of conventional alkyl ammonium onto non∗ Corresponding author at: School of Chemical and Material Engineering, Southern Yangtze University, Wuxi 214122, People’s Republic of China. Tel.: +86 510 89880656; fax: +86 510 85917763. E-mail address: [email protected] (L. Qi).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.03.035
swelling and swelling clays [8–10]. However, only a few papers have reported on the adsorption behavior of gemini surfactants (surfactants containing two hydrophilic and two hydrophobic groups in the molecule) on clay, especially with alkyl chain as long as 16 carbon number. Gemini surfactants constitute a new class of surfactants that have become a topic of scientific interest in virtue of some of their excellent properties, such as smaller critical micelle concentrations (cmcs), higher surface activity than conventional monomeric counterparts, as well as remarkable rheological properties [11], foamability, wetting, solubilization, and antibacterial activity, etc. [12,13]. To our knowledge, only several papers reported the adsorption of gemini surfactants at the clay/aqueous solution interface [14–16]. Those papers focused on either the influence of the spacer of gemini surfactant on the amount of surfactant adsorbed or the effect of gemini surfactants on the removal of organic contaminants. Hence, a better understanding of the adsorption mechanism of conventional and gemini surfactants in silicate layer is still highly desired. Moreover, the differences of microscopic structure between the adsorbed layer of conventional and that of gemini surfactants have not been reported as yet. The investigation of the microstructure of intercalated clay is crucial for successful development of new adsorption materials. In the present study, we choose a typical nonswelling
L. Qi et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 302 (2007) 568–572
clay kaolinite and a typical swelling clay Na-montmorillonite as adsorbents. On the two kinds of clays, adsorption of the conventional surfactant cetyltrimethylammonium bromide (CH3 )3 N+ C16 H33 Br− (CTAB) and corresponding gemini surfactant ethanediyl-␣,bis-(cetyldimethylammonium bromide) C16 H33 (CH3 )2 N+ Br− (CH2 )2 N+ Br− (CH3 )2 C16 H33 (referred to as 16-2-16 surfactant, where 16 and 2 are the carbon numbers of the side alkyl chains and the alkanediyl spacer, respectively) was investigated with the purposes of examining the adsorption mechanism of conventional and gemini cationic surfactants in silicate layer. The arrangement of CTAB and 16-2-16 adsorbed in montmorillonite interlayer was determined and compared. 2. Experimental 2.1. Material The gemini surfactant ethanediyl-␣,-bis(cetyldimethylammonium bromide) (16-2-16) was synthesized as Ref. [17]. The product was recrystallized three times from ethanol/hexane mixed solvent. The structure and purity of the compound were confirmed with elemental analysis, 1 H NMR spectrum and the absence of a minimum value in the surface tension–concentration curve. CTAB was recrystallized three times from acetone and freeze-dried before use. Other agents of analytical reagent grade were obtained commercially and used without further purification. Doubly deionized water was used for preparing all the aqueous solutions in this study. The SWy-2-Na-montmorillonit was purchased from Source Clays Responsitory, University of Missouri, Columbia, MO, USA. The cation exchange capacity (CEC) of the montmorillonite was 0.85 mmole (+)/g determined by Ca exchange method. The surface area of the SWy-2 was 25.5 m2 /g examined by nitrogen adsorption measurement. The kaolinite used was purchased from J.T. Baker Corporation, and the cation exchange capacity (CEC) was 0.052 mmole (+)/g, the surface area 7.5 m2 /g. The clays were treated with 30% H2 O2 for 18 h to remove organic matter, and then saturated with Na by washing 10 g of clay with 250 mL of 1 mol/L NaCl solution three times. The equilibrium time was 8 h for each washing. Different size fractions of Na-clay were separated by wet sedimentation. The <0.2 m size fraction was collected and rinsed with deionized water until no Cl− can be detected in supernatant. The clay obtained through centrifuge was dried at 110 ◦ C and stored in a desiccator over P2 O5 before use. 2.2. Adsorption isotherms Montmorillonite suspensions were prepared by dispersing 5 g of dehydrated montmorillonite in 250 mL of bidistilled water. The suspension was then stirred for about 48 h in order to swell the montmorillonite. Five millilitres of homogeneous colloidal clay suspension (20 g/L) was piped into an Erlenmeyer flask. For CTAB and 16-2-16 adsorption, corresponding surfactant aqueous solution (in the range of 0.1–20 mmol/L) was piped into the clay suspension and the whole volume of the mixture
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solution was fixed. The ionic strength of the initial solution was adjusted to 0.02 using standardized NaCl. The mixture was allowed to equilibrate for 24 h with continuous gently stir. The required time to reach equilibrium and the solid/liquid ratio were previous determined experimentally. The pH of the solutions was not adjusted but was about 6. For the determination of the adsorption isotherm, the samples were centrifuged in tightly stoppered Teflon containers for 30 min at 16,000 rpm to separate the clay particles. Since the surface tension of a given surfactant in equilibrium is a fixed value corresponding to the surfactant concentration below cmc, the concentration of supernatant was determined by the surface tension using the pendant drop technique (the apparatus, JC2000A POWEREACH, made by Shanghai powereach digital equipment company). In addition, the concentration of the surfactant in the aqueous phase was measured by the mixed indicator two-phase titration method [18]. The same results were obtained by either method. The amount of surfactant adsorbed onto the clays is given by the following equation: ns =
(C0 − Ceq ) × V g
(1)
where ns is the number of moles of adsorbate (surfactant) per gram of adsorbent (clay) (mol/g), C0 the initial concentration of adsorbate in suspension solution (mol/L), Ceq the equilibrium concentration of adsorbate in the supernatant (mol/L), V the volume of solution (L), and g is the weight of adsorbent (g). 2.3. X-ray measurements The samples of the CTAB-Na-montmorillonite and 162-16-Na-montmorillonite were centrifuged from the initial suspensions, washed thoroughly, and freeze-dried. The d0 0 1 val˚ radiation from an ues were recorded by using Cu K␣ (λ = 1.54 A) automated X-ray diffractometer (XRD) (Rigaku D-MAX2500, Tokyo, Japan). The XRD was operated at 40 kV and 30 mA in a step scan mode. The scanning speed was 0.0167◦ (2θ s−1 ). The distance between the lamellae, i.e. the so-called basal spacing was calculated by Bragg-reflections from data determined by X-ray diffraction. The orientation of the intercalated surfactant molecules was obtained on the basis of the basal d0 0 1 value. 3. Results and discussion 3.1. Adsorption isotherms Fig. 1 shows the adsorption isotherms of the both surfactant onto kaolinite (Fig. 1a) and SWy-2-Na-montmorillonite (SWy2) (Fig. 1b) at 25 ◦ C in aqueous solution. The four isotherms obtained in our studies are all typical S-shaped. In the low concentration region, all the adsorption increases linearly, which indicates that electrostatic interaction between the substrate and the adsorbed surfactant plays a dominant role in the adsorption process [19]. At the higher concentrations, the adsorption amount increases sharply with the increase of surfactant equilibrium concentration until the saturation surface excess plateau is
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clay (0.052 mmole /g for kaolinite and 0.85 mmole /g for SWy2, respectively). This implies that although cationic exchange plays a predominant role in the adsorption process, hydrophobic interaction between carbon chains also occurs and enhances the surfactant adsorption. It can also be seen from Fig. 1 that the adsorption isotherm of gemini surfactant 16-2-16 is more steep than CTAB in the high concentration region, indicating that the gemini surfactant with two hydrophobic tails leads to stronger hydrophobic interaction. The experimental results show that the observed maximum adsorption excess of both surfactants in kaolinite is only ca. one-seventeenth of that in SWy-2. In consideration of the fact that the surface area of the used SWy-2 is only 3.4 times of the kaolinite, and the CEC of the former is 16.1 times of the latter, we conclude that the exchange of the ions plays the major role in the adsorption process. 3.2. X-ray diffraction
Fig. 1. Adsorption isotherms of the conventional surfactant and the gemini surfactant on kaolinite (a) and SWy-Na-montmorillonite (b) at 25 ± 1 ◦ C.
reached. The sudden rise in adsorption in this region is attributed to the formation of surfactant aggregates on the solid surface [19]. In the case of CTAB, the amount adsorbed at the saturation is about 0.071 mmol/g on kaolinite and 1.22 mmol/g on SWy-2. On the other hand, the maximum of adsorption amount of 16-2-16 on kaolinite and on SWy-2 is 0.070 and 1.20 mmol/g, respectively. Although the 16-2-16 has two quaternary ammonium headgroups that can adsorb onto negative sites of the clay, the above data show that the molar maximum adsorption of the gemini and conventional surfactants is almost identical either on the kaolinite or on SWy-2. The results suggest that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic group is presumably oriented towards the aqueous phase. This may be resulted from the low density of the clays which cannot offer enough ion exchange location within the limited scope of which the rigid spacer of 16-2-16 extends. Noteworthy is the much lower equilibrium concentration of 16-2-16 than CTAB (about 1/40 that of CTAB) when the maximum adsorption is reached (Fig. 1). This indicates that the gemini surfactant 16-2-16 is much more efficient than the corresponding conventional surfactant CTAB at achieving the maximum adsorption on both kaolinite and SWy-2. The maximum adsorption of both surfactants on kaolinite and SWy-2 is slightly higher than the measured cation exchange capacity (CEC) of the
The most widely used technique for the study of intercalated surfactants structure in the silicate galleries is X-ray diffraction (XRD), which provides the information on the layer structure [20]. In our experiments, the SWy-2 clay samples, the CTABSWy-2 and 16-2-16-SWy-2 complex were examined by X-ray diffraction. The X-ray diffractiongram of SWy-2 with different CTAB loadings is presented in Fig. 2. With the increase of the CTAB loadings in SWy-2, the d0 0 1 diffraction peak of SWy-2 shifts toward the lower 2θ region showing the expansion of the interlayer space. The changes of d0 0 1 basal spacing with the increase of CTAB adsorbed amount are presented in Fig. 3. According to the model of Refs. [21,22], long-chain alkylammonium cations can conceivably adopt monolayer, bilayer, pseudotrimolecular, or paraffin-type arrangements in the swelling layer silicate depending on the density of the clay charge and the proportion of the CEC occupied. The thickness of a montmorillonite platelet is 0.96 nm and the gallery height of a flat-lying alkyl chain in the clay interlayer is 0.4 or 0.45 nm, depending on
Fig. 2. Selected X-ray diffraction patterns of CTAB-Na-SWy-2, SWy-2MMt denoted the montmorillonite and the numbers immediately over the X-ray diffraction patterns indicate the CTAB loadings (normalized to CEC).
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Fig. 3. d0 0 1 -spacings of CTAB-Na-SWy-2 at different loadings (the CTAB loadings were normalized to CEC).
its orientation [23]. The step increases of d0 0 1 -spacings in Fig. 3 suggest three kinds of arrangements with different loadings. At CTAB loading levels ≤ 0.4 CEC, the d-spacing is 1.43–1.48 nm. This implies a lateral-monolayer arrangement of CTA+ in the interlayer space of the SWy-2. For 0.5 CEC and 0.7 CEC, the d0 0 1 values are 1.79 and 1.83 nm, respectively, which reflects a lateral-bilayer arrangement. As the CTAB loadings increase from 0.8 CEC to 1.1 CEC, the d-spacing increased from 1.87 to 1.98 nm, suggesting the transitional structure between the bilayer and pseudotrimolecular layer. From 1.1 CEC to 2.5 CEC, the d0 0 1 spacings reach over 2.0 nm which is corresponding to a pseudotrimolecular layer. The vertical configuration (i.e. paraffin complex) is not observed even at higher CTAB loadings. This may be attributed to the low charge density of SWy-2. Fig. 4 shows the XRD patterns of 16-2-16-Na-SWy-2 montmorillonite hybrids with different 16-2-16 loadings. At low loadings, the diffraction peak of the complexes also shifts toward lower 2θ region with the increase of adsorption excess (Fig. 4),
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Fig. 5. d0 0 1 -spacings of 16-2-16-Na-SWy-2 at different loadings (the 16-2-16 loadings were normalized to CEC).
just similar to the instance of CTAB-Na-SWy-2. Unexpectedly, XRD pattern of 16-2-16-Na-Swy-2 beyond 0.6 CEC loadings shows a minor new peak except d0 0 1 -spacing peak in the higher region (Fig. 4), which did not appear in the case of CTAB. At 0.2 CEC there exists a peak corresponding to d0 0 1 of 1.5 nm, and at 0.7 CEC of surfactant coverage, two peaks corresponding to interlamellar distances of both 1.87 and 1.36 nm (Fig. 5) were detected. This should be explained by the delamination of silicate sheets upon the adsorption of 16-2-16 molecules in excess of 0.6 CEC. With the increase of the adsorption excess of 16-2-16 in SWy-2 both peaks of 16-2-16-Na-SWy-2 shift to lower 2θ region. The results suggest that the minor peak is also formed by the interlayer arrangement of flexible 16-2-16 carbon chains. The change of diffraction peak d0 0 1 value with 16-2-16Na-SWy-2 loadings is showed in Fig. 5. The stepwise d-spacing curve of 16-2-16-Na-SWy-2 is also observed. In comparison with the CTAB-Na-SWy-2, the layer expansion of 16-2-16-NaSWy-2 is a little bigger under the same adsorption loadings. The behavior is attributed to the stronger hydrophobicity of 16-216 with two carbon chains. Gemini surfactant organocalys were reported to have better adsolubility than conventional ones [15]. The relationship between microstructure and the adsolubilization capability should be studied substantially. More details are under the investigation. 4. Summaries
Fig. 4. Selected X-ray diffraction patterns of 16-2-16-Na-SWy-2. The numbers immediately over the X-ray diffraction patterns indicate the 16-2-16 loadings (normalized to CEC).
The equilibrium adsorption behavior of CTAB and its corresponding gemini surfactant 16-2-16 on kaolinite and montmorillonite (SWy-2) in aqueous solution was studied using adsorption isotherm method. The results suggested that the cation exchange function dominantly determined the adsorption amount of CTAB and 16-2-16 on kaolinite and SWy-2. However, hydrophobic interaction also played an assistant role in the adsorption process. Gemini surfactant 16-2-16 was more efficient than CTAB in adsorption, whereas the maximum adsorption excess of the two surfactants was almost identical. The basal spacing obtained from X-ray diffraction gave
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