(2007) 194–200www.elsevier.com/locate/clay

Applied Clay Science 35

Preparation and characterization of phosphonium montmorillonitewith enhanced thermal stability

Hasmukh A. Patel, Rajesh S. Somani, Hari C. Bajaj ⁎, Raksh V. Jasra ⁎

Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, G.B. Marg, Bhavnagar, 364 002, Gujarat, India

Received 5 January 2006; received in revised form 15 September 2006; accepted 23 September 2006Available online 20 November 2006

Abstract

Quaternary phosphonium cations (hexadecyl tributyphosphonium; tetradecyl tributylphosphonium; tetraphenylphosphonium;methyl triphenylphosphonium; ethyl triphenylphosphonium and propyl triphenylphosphonium) were intercalated intomontmorillonite (MMT) rich bentonite of Indian origin, by ion exchange reaction. The phosphonium MMT were characterizedby Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction analysis (PXRD), particle size distribution (PSD) andthermogravimetric analysis (TGA). The phosphonium cations significantly influenced the particle size distribution. With longeralkyl chain finer particles were formed. The tetrabutylphosphonium and tetraphenylphosphonium MMT showed enhanced thermalstability (300–400 °C) and may be potentially useful materials for melt processing of polymer/layered silicates nanocomposites.© 2006 Elsevier B.V. All rights reserved.

Keywords: Montmorillonite; Organoclays; Nanocomposites; Thermal stability; Phosphonium; Intercalation

1. Introduction

Organoclays have attracted substantial attention bothin fundamental research and industrial applicationsbecause of their superior reinforcement properties(Carrado, 2001; Deniss et al., 2001; Lee and Lee,2004). Compared to conventional filled polymers, clay/polymer nanocomposites can have enhanced mechan-ical properties, increased heat distortion temperature,improved thermal stability, decreased gas/vapour per-meability and reduced flammability (Hasegawa et al.,1998; Takekoshi et al., 1998; Garces et al., 2000; Meieret al., 2001; Maguy et al., 2005). Several methods have

⁎ Corresponding authors. Tel.: +91 278 2471793, +91 278 2567760;fax: +91 278 2566970, +91 278 2567562.

E-mail addresses: hcbajaj@csmcri.org,hcbajaj13@rediffmail.com (H.C. Bajaj),rvjasra@csmcri.org (R.V. Jasra).

0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.clay.2006.09.012

been reported to synthesize clay/polymer nanocompo-sites; however, three methods (in situ polymerization,intercalation in solutions and melt processing) devel-oped during the early stages of this field are widelyapplied (Garces et al., 2000). The melt processingtechnique is mostly used because this process played animportant role in speeding up the progress of thecommercial production of clay/polymer nanocompo-sites. Most of the commercially available organoclaysare produced by exchange of alkali or alkali earthcations in the interlayer space of clay minerals with alkylammonium salts (Hasegawa et al., 1998; Garces et al.,2000; Klapyta et al., 2001; Maguy et al., 2005). Othercations, such as phosphonium, pyridinium and immi-nium have also been used due to their higher thermalstability (Takekoshi et al., 1998). Alkyl ammoniummodified clays are thermally not very stable above250 °C and start to degrade at nanocompositesprocessing temperature (200–300 °C). Therefore,

Fig. 1. FTIR spectra of upgraded bentonite and phosphonium MMT (P1, P2, P3, P4, P5, P6 and P7).

Fig. 2. PXRD of upgraded bentonite and phosphonium MMT (P1, P2,P3, P4, P5, P6 and P7).

195H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

organoclays prepared using quaternary alkyl ammoniumsalts are less suitable for most engineering plastics withhigh processing temperature (Wu and Lerner, 1993; Weiet al., 2001a,b; Gao, 2002; Li and Ishida, 2002; Wangand Wilkie, 2003). The thermal stability of organoclaysis improved by intercalating quaternary phosphoniumsalts (Wei et al., 2002). Degradation pathways andthermal stability depend on molecular structure. Never-theless, the stability of phosphonium MMT is substan-tially decreased (70–80 °C) with regard to the parentphosphonium salt, necessitating future studies of thespecific influence of the interlayer environment andaluminosilicate surface on reaction pathways.

In the present paper, we report the preparation andcharacterization of phosphonium MMT by the interac-tion of quaternary phosphonium salts with MMT richbentonite clay of Indian origin with an objective toprepare organoclays with higher thermal stability. Thethermal stability of the phosphonium MMT werecorrelated with different parameters of the phosphoniumsalts. We have selected three quaternary phosphoniumsalts similar to those reported by Wei et al. (2002) andcompared them with four other quaternary phospho-nium salts, which are not reported so far.

2. Experimental

2.1. Materials and methods

The montmorillonite rich bentonite clay was collected atAkli mines, Barmer, Rajasthan, India. The bentonite waspurified by sedimentation technique. The clay fraction wasobtained by dispersing 150 g of bentonite in 10 l de-ionizedwater and collecting the supernatant dispersion of particles

<2 μm after the calculated time (10 h) at a height of 15 cm at30 °C according to the Stokes law of sedimentation. The slurrywas then dried at 90 °C. The cation exchange capacity (CEC)of purified bentonite, measured by the standard ammoniumacetate method, was 91 meq/100 g on dry basis (dried at110 °C). The chemical composition of the purified bentonitewas: 55.9% SiO2, 20.9% Al2O3, 9.15% Fe2O3, 2.1% MgO,2.8% CaO, 0.71% K2O and 0.73%Na2O with loss on ignitionof 7.7%, and the ionic formula: (Si3.7 Al0.3) (Al1.33 Mg0.21Fe0.46) O10 (OH)2nH2O·Na0.094, K0.06, Ca0.20. The purifiedbentonite was rich in montmorillonite with Na+ and Ca+2 asexchangeable cations. The purified bentonite contained minor

Scheme 1. Optimized energy structures of phosphonium salts (molecular modeling software; Accelreys, MS Modeling 3.2).

Fig. 3. Correlation of basal spacing with chain length of organicmoiety in phosphonium MMT.

196 H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

amount of kaolinite (1–2%w/w). The phosphonium salts(mainly bromides), were of 97% purity (Aldrich, USA) andused as received.

2.2. Preparation of phosphonium MMT

10 g of upgraded bentonite was dispersed in 1 l of de-ionized water. To this solution 9.1 meq per phosphonium salts(as 0.01 M solution) was slowly added per gram of bentonite100 under continuous stirring at 80 °C within 1 h. Theproducts were washed free from halide ions as tested usingAgNO3 solution, dried at 35 °C followed by overnight dryingat 110 °C and then pulverized to pass through 300 mesh sieve.The phosphonium MMTwas designated as P1 (tetrabutylpho-sphoniumMMT), P2 (hexadecyl tributylphosphonium MMT),P3 (tetradecyl tributylphosphonium MMT), P4 (tetraphenyl-phosphonium MMT), P5 (methyl triphenylphosphoniumMMT), P6 (ethyl triphenylphosphonium MMT) and P7(propyl triphenylphosphonium MMT).

2.3. Characterization

The phosphoniums MMT were characterized by thermo-gravimetric analysis (Mettler-Toledo, TGA/SDTA 851e).

Samples were heated to 500 °C for aliphatic and 800 °C foraryl phosphonium MMT at 10 °C/min heating rate in the airflow of 40 ml/min. Powder X-ray diffraction (XRD) analysiswas carried out with a Phillips powder diffractometer X' PertMPD using PW3123/00 curved Cu-filtered Cu-Kα radiationwith slow scan of 0.3°/s. Fourier transform infrared spectra

Fig. 4. Particle size distributions of phosphonium MMT (P1, P2, P3, P4, P5, P6 and P7).

197H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

(FTIR) were measured with the Perkin-Elmer-Spectrum GX-Spectrophotometer using KBr pellet. The particle size analysis(as dry powder) was done on Malvern Instrument-Master sizer2000 at feed rate of 50% and air pressure of 1 bar.

3. Results and discussions

In the FTIR spectra of the purified bentonite (Fig. 1),the bands between 3500 and 3700 cm−1 and near3400 cm−1 are indicative of montmorillonite. The broadband centered near 3400 cm−1 is due to –OH stretchingmode of interlayer water. The bands at 3620 and3698 cm−1 are due to –OH stretching mode of Al–OHand Si–OH of montmorillonite structure. The bands at3698 cm−1 due to –OH stretching vibration may also bedue to the presence of small quantity of kaolinite. Theshoulders and the broadness of the –OH bands aremainly due to contributions of several structural –OHgroups occurring in this smectite. The overlaid absorp-tion peaks in the region of 1640 cm−1 is attributed to –OH bending mode of adsorbed water. The characteristicpeak at 1115 cm−1 is due to Si–O stretching, out-of-plane Si–O stretching mode for montmorillonite. Theband at 1035 cm−1 is attributed to Si–O stretching (in-plane) vibration for layered silicates. The IR peaks at

Table 1Particle size distribution of phosphonium MMT (P1, P2, P3, P4, P5, P6 and

Particle size/organoclays P1

d(0.1), μm 1.1d(0.5), μm 5.4d(0.9), μm 25.8d(0.99), μm 46.7Molecular surface area of organic phosphonium salts, Å2/molecule 300.1

915, 875 and 836 cm−1 are attributed to AlAlOH,AlFeOH and AlMgOH bending vibration (Madejova,2003; Tyagi et al., 2006).

In the FTIR spectra (Fig. 1) of P1, P2 and P3, thepeaks at 2940 and 2850 cm−1 are ascribed to theasymmetric and symmetric vibration of methylenegroups (CH2)n of the aliphatic chain (Vaia et al., 1994;Xi et al., 2005). Tetrabutylphosphonium montmorillo-nite (P1) shows a weak intensity of the –OH bendingvibration at 1640 cm−1 due to adsorbed water. Thephenyl ring attached to the phosphonium atom displayedan unusually sharp and relatively strong vibration bandat 1430 cm−1 as shown in the infrared spectrum of P4,P5, P6 and P7. In addition to this, there is also the HCHstretching vibration band at 1465 cm−1 in IR spectrum ofall phosphonium montmorillonite except tetraphenyl-phosphoniumMMT (P4). The IR absorption bands in thelow frequency region of the upgraded bentonite and thephosphonium MMT analogues were largely comparableindicating that the clay mineral has not changed uponexchange of the interlayer sodium/calcium ions by thequaternary phosphonium ions.

The XRD data show that basal spacing increased withthe alkyl chain length (Fig. 2). For the MMT, the spacingwas 1.21 nmwhile the basal spacing for the P1 is 1.4 nm,

P7) and molecular surface area of phosphonium salts

P2 P3 P4 P5 P6 P7

9 1.15 1.31 1.15 1.74 1.49 1.410 3.88 8.32 6.75 14.72 10.30 10.156 13.34 26.89 28.47 39.71 33.44 36.861 39.35 43.52 46.91 51.00 49.94 50.664 510.67 490.65 384.03 329.76 336.54 345.66

Scheme 2. Smaller phosphonium MMT particles by breaking the house of cards structure by phosphonium ions.

198 H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

and for the P2 and P3, 2.32 and 2.19 nm. The basalspacing of the samples P4, P5, P6 and P7 was nearly thesame (1.76±0.065 nm) as the cations are of similar size.The molecular shape of the phosphonium cations weredetermined using molecular modeling software (Accel-reys, MS Modeling 3.2) (Scheme 1). It is expected thatphosphonium cations will occupy configuration inside

Fig. 5. (a) Thermogravimetric analysis of P1, P2 and P3 up to 500 °C;(b) differential thermogravimetric analysis of P1, P2 and P3 up to500 °C.

the interlayer space so that the positively chargedphosphorous is closer to the negatively charged silicatelayer to maximize electrostatic interaction. The basalspacing values were plotted (Fig. 3) against themaximum molecular length for all phosphonium cationswith this configuration. The linear variation observedbetween the basal spacing and the molecular length do

Fig. 6. (a) Thermogravimetric analysis of P4, P5, P6 and P7 up to800 °C; (b) differential thermogravimetric analysis of P4, P5, P6 andP7 up to 800 °C.

Table 2Weight loss (%) at different temperatures related to the total weightloss for the phosphonium MMT

Temperature(°C)

Weight loss (%)

P1 P2 P3 P4 P5 P6 P7

50 0.00 0.00 0.00 0.00 0.00 0.00 0.00100 1.09 0.00 0.66 0.00 1.52 0.42 0.90150 2.03 0.08 1.34 0.03 2.38 1.14 1.19200 2.54 0.54 1.77 0.39 2.98 1.35 1.42250 2.96 1.13 3.09 1.16 3.47 1.63 1.85300 3.51 2.55 9.40 1.97 4.08 2.38 2.52350 4.63 7.46 16.27 3.18 4.82 4.10 3.48400 6.52 15.54 18.78 5.60 7.55 8.07 5.94450 9.65 21.79 23.09 8.72 11.01 12.92 9.47500 12.67 28.52 27.00 12.74 17.07 18.37 16.65600 – – – 19.15 18.16 19.22 17.47700 – – – 22.67 19.02 20.23 19.16Loss on

ignition a15.29 31.13 28.6 23.67 19.09 20.96 20.54

a Loss on ignition was measured at 850 °C for 2 h.

199H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

support the proposed configuration of phosphoniumcations inside the interlayer space.

The particle size distributions significantly dependedon the alkyl chain length of phosphonium cations (Fig. 4and Table 1). P2 showed finer particle size as comparedto P1 and P3. P4 showed a finer particle size distributioncompared to P5, P6 and P7. These observations can beexplained in terms of the aggregation of the montmor-illonite particles in “house of cards” structure of clayobserved in aqueous dispersion. When MMT is dis-persed in de-ionized water at slightly acidic conditions,positively charged edges are attracted to negativelycharged surfaces of the platelets to form a threedimensional “house of cards” structure (Scheme 2)which contains hundreds or thousands of silicateplatelets (Lagaly, 1981; Dijkstra et al., 1995). Theparticle size of clay in dispersion depends on the numberof the platelets forming this structure. When alkyl/arylphosphonium salts are added to aqueous dispersions, thepositively charged phosphonium cations will alsointeract with negatively charged surface of the claymineral and reduce its interactions with positivelycharged edges of other platelets and thereby decreasethe number of platelets in the “house of cards” structure.Subsequent drying process will, therefore, yield finerparticles. The alkyl phosphonium cations with longercarbon chains (hexadecyl tributyl) are likely to cover theclay mineral surface more efficiently than phosphoniumcations with smaller carbon chains (tetra butyl ortetradecyl tributyl) and lead, therefore, to finer particle.Aryl phosphoniumMMT P4, P5, P6 and P7 have similarnumbers of carbon atoms as P2 and P3, however, these

show broader particle size distribution because ofsmaller organic moieties. The phosphonium MMT withshorter alkyl chains or cyclic groups (tetra phenyl,methyl/ethyl/propyl triphenyl) will cover the surface of aplatelet to a smaller degree compared to the longer chainderivatives. As a result, organoclays prepared using arylphosphonium salts will have a “house of cards” structurewith a higher number of platelets that on drying results ina broader particle size distribution. The molecularsurface areas (Conolly surface) determined using themolecular modeling software (Accelreys, MS Modeling3.2), also reveal the dependence of the particle sizedistribution on the surface area of molecular cations(Table 1) showing finer particle size distribution forhigher molecular surface areas except tetrabutylpho-sphonium MMT.

Comparison of the thermal stability of the three alkylsubstituted phosphoniumMMT (P1, P2 and P3, Fig. 5a, b)and four aryl and/or alkyl substituted phosphoniumMMT (P4, P5, P6 and P7, Fig. 6a, b) is summarized inTable 2. The TGA data show improvement in thermalstability for all aryl and/or alkyl substituted phospho-nium MMT as compared to only alkyl phosphoniumMMT. The tetraphenylphosphoniumMMT (P4) showedthe highest thermal stability at 350–400 °C (5%decomposition), while substitution of phenyl group bya methyl, ethyl or propyl group (P5, P6 and P7) ledlower thermal stability of 300–350 °C (5% decomposi-tion, Fig. 6a, b). The alkyl chain length also affects thethermal stability (Fig. 5a, b). The thermal stabilityincreases with the alkyl chain length. However, P1having tetrabutylphosphonium as interlayer ion showsthermal stability up to 350 °C, almost equivalenttetraphenylphosphonium MMT. This is due to the highthermal stability of tetrabutylphosphonium cation.

4. Conclusions

The finer particle size distribution was observed forthe intercalated MMT with longer chain phosphoniumions because the alkyl chains prevented silicate plateletsfrom aggregation. The phenyl group substituted phos-phonium MMT showed the highest thermal stability of350–400 °C (5% decomposition). Tetrabutylphospho-nium MMT was stable as tetraphenylphosphoniumMMT due to the high thermal stability of thetetrabutylphosphonium cation.

Acknowledgements

We thank Dr. P.K. Ghosh, Director, Central Salt andMarine Chemicals Research Institute, Bhavnagar for

200 H.A. Patel et al. / Applied Clay Science 35 (2007) 194–200

taking keen interest in this work and also providing thefacilities; to CSIR for funding under Network Project:Organic–Inorganic Hybrids and Nanocomposites COR0004.

References

Carrado, K.A., 2001. Synthetic organo- and polymer-clays: prepara-tion, characterization, and materials applications. Applied ClayScience 17, 1–23.

Deniss, H.R., Hunter, D.L., Chang, D., Kim, S., White, J.L., Chow,J.W., Paul, D.R., 2001. Effect of melt processing conditions onthe extent of exfoliation in organoclay based nanocomposites.Polymer 42, 9513–9522.

Dijkstra, M., Hansen, J.P., Madden, P.A., 1995. Gelation of a claycolloid suspension. Physical Review Letters 75, 2236–2239.

Gao, F., 2002. The current problems with the use of reactive meltprocessing to produce clay/polymer nanocomposites. Presented atOrganic-Inorganic Hybrid II. PRA, Guildford.

Garces, J.M., Moll, D.J., Bicerano, J., Fibiger, R., McLeod, D.G.,2000. Polymeric nanocomposites for automotive applications.Advanced Materials 12, 1835–1839.

Hasegawa, N., Kawasumi, M., Kato, M., Usuki, A., Okada, A., 1998.Preparation and mechanical properties of polypropylene–clayhybrids using a maleic anhydride-modified polypropylene oligo-mer. Journal of Applied Polymer Science 67, 87–92.

Klapyta, Z., Fujita, T., Iyi, N., 2001. Adsorption of dodecyl- andoctadecyltrimethylammonium ions on smectite and syntheticmicas. Applied Clay Science 19, 5–10.

Lagaly, G., 1981. Characterization of clays by organic compounds.Clay Minerals 16, 1–21.

Lee, J.Y., Lee, H.K., 2004. Characterization of organo-bentonite usedfor polymer nanocomposites. Materials Chemistry and Physics 85,410–415.

Li, Y.Q., Ishida, H., 2002. A differential scanning calorimetry study ofthe assembly of hexadecylamine molecules in the nanoscaleconfined space of silicate galleries. Chemistry of Materials 14,1398–1404.

Madejova, J., 2003. FTIR techniques in clay mineral studies. VibrationalSpectroscopy 31, 1–10.

Maguy, J., Jocelyne, M., Luc, D., Ronan, L.D., 2005. Formation oforganoclays by a one step synthesis. Solid State Sciences 7, 610–615.

Meier, L.P., Nueesch, R., Madsen, F.T., 2001. Organic pillared clays.Journal of Colloid and Interface Science 238, 24–32.

Takekoshi, T., Khouri, F., Campbell, J.R., Jordan, T.C., Dai, K.H.,(1998) Layered minerals and compositions comprising the same.US Patent (5 707 439).

Tyagi, B., Chudasama, C.D., Jasra, R.V., 2006. Determination ofstructural modification in acid activated montmorillonite clay byFT-IR spectroscopy. Spectrochimica Acta Part A, Molecular andBiomolecular Spectroscopy 64 (2), 273–278.

Vaia, R.A., Teukolsky, R.K., Giannelis, E.P., 1994. Interlayer structureand molecular environment of alkyl ammonium layered silicates.Chemistry of Materials 6, 1017–1022.

Wang, Dongyan, Wilkie, Charles A., 2003. A stibonium-modified clayand its polystyrene nanocomposite. Polymer Degradation andStability 82, 309–315.

Wei, X., Zongming, G., Kunlei, L., Wei-Ping, P., Vaia, R., Doug, H.,Singh, A., 2001a. Thermal characterization of organically modifiedmontmorillonite. Thermochimica Acta 367–368, 339–350.

Wei, X., Zongming, G., Wei-Ping, P., Doug, H., Singh, A., Vaia, R.,2001b. Thermal degradation chemistry of alkyl quaternary ammo-nium montmorillonite. Chemistry of Materials 13, 2979–2990.

Wei, Rongcai, X., Wei-Ping, P., Doug, H., Bryan, K., Loon-Seng, T.,Vaia, R., 2002. Thermal stability of quaternary phosphoniummodified montmorillonites. Chemistry of Materials 14, 4837–4845.

Wu, J.H., Lerner, M.M., 1993. Structural, thermal and electricalcharacterization of layered nanocomposites derived from Na-montmorillonite and polyethers. Chemistry of Materials 5,835–838.

Xi, Y., Ding, Z., Hongping, H., Frosr, R.L., 2005. Infrared spectro-scopy of organoclays synthesized with the surfactant octadecyl-trimethylammonium bromide. Spectrochimica Acta Part A 61,515–525.