Indian Journal of Chemical Technology Vol. 12, May 2005, pp. 259-262
Modification of layered double hydroxides by short chain organic surfactants via ion-exchange method R Anbarasan* & S S Ima Department of Polymer Technology, Kamaraj College of Engineering and Technology, Virudhunagar 626 001, India a Department of Fiber and Polymer Engineering, Hanyang University, Seoul, Korea Received 27 October 2004; revised received 21 February 2005; accepted 16 March 2005 The layered double hydroxides (LDH), hydrotalcite (HT) was modified through ion-exchange method with two different organic short chain anionic surfactants in the presence and absence of ultrasound (US). Hydroxy ethane sulphonic acid (HESA) modified HT prepared in the presence of US showed increase of peak intensity in XRD analysis. US assisted sulphoacetic acid (SAA) ion-exchanged HT exhibited completely delaminated/degraded amorphous HT structure. This was authenticated by XRD profiles. The presence of anionic surfactants in the LDH structure is evidenced by FTIR spectroscopy. Keywords: Layered double hydroxides, organic surfactants, ion-exchange, XRD, FTIR IPC Code: B01J39/04
Recently, the academists and industrialists turned their attention towards the synthesis, characterization and applications of various LDH due to their versatile applications as anionic adsorbents, catalysts and cocatalysts1,2. Nowadays the use of LDH has increased in the area of chemistry, particularly in the field of polymer chemistry to impart certain physical and chemical properties to the polymer backbone. Unfortunately, due to the existence of incompatibility between the polymer backbone and nanosized LDH, it cannot interact with polymer further. The compatibility of LDH with polymer backbone can be well developed by the introduction of a functionalised organic compound through an anionic exchange process. Crepaldi and co-workers3 studied the intercalation reactions of various sulphonated and sulphated surfactants with Zn-Cr LDH. Chlorine substituted methoxy benzoic acid was intercalated4 into LDH-CO3. Reports of succinate and tartrate intercalated Zn-Al, Zn-Cr LDH are available in the literature5. Kooli et al.6 reported the benzoate and terephthalate intercalated Mg-Al LDH. Recently, Li et al.7 reported the glycine intercalated LDH, which was treated with poly(vinyl alcohol) to prepare polymernanocomposites. The intercalation capability of biomolecules such as pentose, deoxyribonucleic acid and polypeptide into LDH has also been checked8-10. _______________ *For correspondence (E-mail:
[email protected])
Stereoselective intercalation of hexose into LDH by calcination/rehydration reaction was reported by a Japanese workers11. In the present communication, the ion-exchange reactions of LDH with short chain organic anionic surfactants such as HESA and SAA are reported. The role of US in the ion-exchange process of LDH was also tested. Experimental Procedure Materials
LDH with carbonate ion as an inter layer anion (Mg/Al=3) was a gift sample from KICET, Korea and had a layer thickness of 0.48 nm. Sodium salts of 2hydroxy ethane sulphonic acid (HESA) (Merck, Korea) and sulphonic-1-acetic acid (SAA) (Aldrich Chemicals, Korea) were used for modification purpose. Branson 2210 Ultrasonication bath was used. Modification method
HT (5g) was placed in 500 mL two way round neck flask with 250 mL de-ionized water. 10 g of sodium salt of anionic surfactant (HESA or SAA) was added with vigorous stirring under inert atmosphere at 70 °C for 48 h in an alkaline medium. The ratio of carbonate to anionic surfactant was kept to be 1:2, because the divalent carbonate anion was ion-exchanged by the monovalent anionic surfactant. After 48 h of ionexchange process the contents were filtered and washed several times by de-ionized water to remove
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sodium carbonate formed during the ion-exchange reaction. Finally, the precipitate was dried at room temperature for 48 h under vacuum. The white solid thus obtained was grounded and stored in a vial bottle. In the case of ultrasonication treatment, the ion-exchange process was carried out under ultrasonic irradiation. Characterization
HT before and after ion-exchange process was characterized by XRD (powder sample, Rigaku Rint 2000 Japan) diffractometer at room temperature with CuKα radiation. The angle 2θ was scanned from 2 to 70°. FTIR spectroscopy for the powder samples was carried out using Nicolet 760 magna IR spectrometer by KBr pelletisation method. Length of the organic anionic surfactants can be calculated based on the Isupov’s12 semi-empirical formula. The basal spacing can be calculated from Bragg’s equation as λ d = ⎯⎯ 2 Sinθ where d⎯basal λ-wavelength. Gallery height HT layer
Fig. 1⎯XRD of (a) HT, (b) HT-HESA, (c) HT-HESA-US
…(1) spacing, =
Basal
θ⎯diffraction
angle,
spacing-thickness of … (2)
Results and Discussion Generally, after modification with anionic surfactants, the d003 plane peak of HT shifts towards lower 2 theta value (due to increase of basal spacing) and the increase of inter layer space depends on the intercalation nature and alkyl chain length of anionic surfactants. After ion-exchange process with HESA, the d003 plane of HT was not shifted towards lower 2 theta value (Fig. 1b). This indicates that the basal spacing is not increased due to HESA. The gallery height of the HT (0.279 nm) is lower than that of the length of HESA (0.718 nm) even then the basal spacing of HT is not increased. This suggests that HESA is adsorbed on the surface of HT through hydrogen bonding. This is in accordance with the observation drawn by Crepaldi et al.3. Normally, the sulphonated surfactants act as a good adsorbent rather than a good intercalant. Presence of HESA in HT structure was confirmed by observing the FTIR peaks. Fig. 2a exhibited the
Fig.2⎯FTIR spectrum of (a) HT, (b) HT-HESA, (c) HT-HESA-US
FTIR spectrum of pristine HT. The important peaks are assigned as follows: OH stretching, 3500; bending vibration of water, 1641; carbonate ion stretching, 1377; and metal-hydroxide stretching, 666 cm-1. FTIR spectrum of HT-HESA is shown in Fig. 2b. The spectrum showed some new peaks other than HT peaks. Peak due to carbonate anion stretching (1377 cm-1) is also observed in the spectrum. This indicates that complete exchange of carbonate ion by HESA is not possible due to high charge density of HT and the CO2 can be readily adsorbed from the atmosphere during the filtration process. The high charge density of HT is due to the partial exchange of Al3+ by Mg2+. Peaks appeared at 1182 and 1044 cm-1 are assigned to the sulphonate anion of HESA. C-H deformation vibration is noticed at 736 cm-1. Figure 3 includes the XRD of SAA modified HT as Fig 3b. It showed no more crystalline peaks in the 2
ANBARASAN & IM: MODIFICATION OF LAYERED DOUBLE HYDROXIDE BY ORGANIC SURFACTANTS
theta range of 2–70°. This confirms that SAA can degrade the HT layer structure. The degraded products were found to be amorphous in nature {formation of ill crystalline products such as Al(OH)3 and Mg(OH)2}. This can be proved by the absence of crystalline peaks in XRD. For the sake of comparison XRD of pristine HT is included as Fig. 3a. The presence of SAA in the degraded HT structure can be supported by FTIR spectrum. The FTIR spectrum of HT-SAA is shown in Fig. 4b. Peaks appearing in the range 1188 – 950 cm-1 evidenced the presence of sulphonate group in the degraded HT structure. XRD data confirms the degradation of HT by SAA and the FTIR spectrum supports the presence of sulphonate group of SAA in the degraded HT structure. These two combined facts point to the fact, that, the adsorption of SAA onto HT leads to layer degradation process. Recently, Lee et al.13 recommended the non-aqueous medium for the modification of LDH with the perpendicular approach of surfactants to the hydoxyl layer of LDH without layer structure degradation. For the past few decades, the use of US has increased. In polymer chemistry, US is used as a free radical initiator. Recently, Anbarasan and coworkers14 reported the effect of US initiation on the acrylic acid and acrylamide polymerization in the presence of a redox system. The application of US is slightly extended to nanoparticle system. Poly(aniline)-zirconium phosphate nanocomposite was prepared by ultrasonic irradiation method15. The purpose of using US in nanoparticle/solvent system is to receive a well dispersed system16. In the present work the, role of US in the ionexchange process was checked. XRD report of US assisted HT-HESA is shown in Fig. 1c. There is no change in the 2θ value of d003 plane peak. This confirms that during the US assisted ion ex-change process, the HESA is not intercalated into HT structure. At the same time, the intensity of the d003 plane peak has increased sharply. This explains the fine or well order arrangement of HESA onto the surface of the HT through hydrogen bonding (i.e) existence of hydrogen bonding between the sulphonate group of HESA and hydroxyl layer of LDH. When the intensity of d003 plane peak of HT before and after modification is compared with HESA in the presence of US, one can observe the high intensity d003 plane peak for HT-HESA-US system. From these results one can say that US can increase
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Fig.3⎯XRD of (a)HT, (b) HT-SAA, (c) HT-SAA-US
c
Fig.4⎯FTIR spectrum of (a) HT, (b) HT-SAA, (c) HT-SAA-US
the dispersion of nanoparticles in the reaction medium uniformly and simultaneously the crystallinity of HT has increased by making the well ordered arrangement of HESA on the surface of HT. The FTIR spectrum of US assisted HESA-HT system is shown in Fig. 2c. Noticeable differences in the FTIR spectrum are not observed when compared with HESA-HT system. XRD of US assisted ion exchange process of SAA with HT is shown in Fig.3c. In this case, peaks due to LDH could not be observed. This gives an additional support to the degradation of HT by SAA. This may be due to the strong acidic nature of SAA. This fact indirectly proves that after the modification of HT with SAA, the final product becomes completely amorphous with HT layer degradation and with the
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adsorption of SAA onto de-layered HT through hydrogen bonding. Presence of SAA in the HT structure is further supported by FTIR spectra (Fig. 4c). The sulphonate peaks appeared between 1188 and 950 cm-1. The –OH peak appeared at 3500 cm-1 and has separated into twin peaks corresponding to –OH stretch (3440 cm-1) and C-H anti-symmetric stretch (3144 cm-1). On comparing HESA and SAA systems, both are leading to adsorption phenomenon onto HT backbone during the ion-exchange process. For the sake of comparison the FTIR spectrum of pure HT is also included as Fig. 4a. Conclusion This work clearly explains that short chain organic anionic surfactants with sulphonate group led to adsorption process during the ion-exchange process with HT through hydrogen bonding. Ultrasonication assists the ion-exchange process by arranging the surfactant in a well ordered manner on the surface of HT and hence increase of crystalline peaks. Strong acidic nature of SAA degraded the layered structure of HT followed by the adsorption on their surface.
Acknowledgement Thanks are due to BK 21 Office and Hanyang University Office for their financial support during the course of work done at Korea. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
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