Langmuir 1993,9, 1794-1800
1794
Cointercalation of A113 Polycations and Nonionic Surfactants in Montmorillonite Clay Laurent J. Michot,? Odile Barrds,t Eric L. Hegg,* and Thomas J. Pinnavaia’J Laboratoire “Environnement et Minkralurgie”, URA 236 du CNRS, BP 40,64501 Vandoeuvre Cedex, France, and Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Laming, Michigan 48824 Received December 21, 1992. In Final Form: March 31, 1993 A l l 3 polycationexchanged forms of montmorillonitehave been prepared in the presenceof alkyl polyether abbreviated C12-lrEx. Photon correlationspectroscopy surfactantsof the typeCISI~H~~-ZDO(CH~CH~O)~H, indicated that All3 polycationa and C 1 2 - 1 8 ~both undergo concentration-dependent self-aggregation in aqueousmedia, with the polycations formingaggregatesof -16 nm and the surfactantforming aggregates of 20-500 nm. Mixing the cl2-l& surfactant and the polycation at a 1:l molar ratio resulted in complex formation and in the appearance of aggregateseven larger thanthose formed by the parent end members. FTIR frequencyshiftsfor the C 1 2 1 8 6 - A l 1 3 complexes indicated that the surfactant binds to the polycation through the oxygen atoms of the ethylene oxide segment of the chain. At low surfactant loadings in the range 0 to -0.25 molecule/Om(OH)4unit cell, the surfactant binds to the gallery A l l 3 ions and f i i the microporous space between pillare, causing a dramatic loas of Nz Brunauel-Emmett-Teller surface area. Increasing the surfactant loading to 0.67 molecule/unit cell resulted in additional surfactant binding to the gallery surfaces. In this second binding domain the Surfactant acta in part as a pillaring reagent, causing an increase in basal spacing from 19 to 23 A and the reappearance of micropores. At even higher loadingsa third surfactantbinding mode was observed in whichthesurfactantbinds exclusivelytoexternal surfacesand blocks all micro- and mesopores. Qualitatively similar results were observed for the related surfactant C1214E12. Both surfactants greatly inhibited the hydrolysis of All3 polycationa in the clay galleriesand dramaticallyimproved the crystallographicorderingof the intercalatealong the layer stacking direction.
Introduction Sincetheir introduction in the late 1970s,l4 metal oxide pillared clays have been prepared by preintercalation of 2:l layered silicates with robust polycations and subsequent thermal conversionof the polycations to nanoscopic metal oxide-like aggregates. The most commonly used polycation is the Keggin-like All3 oligomer,4f6[Al1304(OH)24+x(Hz0)12-xl(7-=)+. More recently, the possibility of cointercalating organic species together with the pillaring All3 ions has been proposed using cationic and nonionic ~urfactants.~J The cointercalation of A113 ions and long-chain onium ions affords materials with interesting properties for the adsorption of toxic organics from aqueous solution.8 Nonionic alkyl polyether surfactants of the type RO(CH2CH20)xH intercalated into All3 montmorillonite also improve the affmity of the clay for organic molecule^.^ The alkyl polyether surfactants contain hydrophobic and hydrophilic chain segments that can be presented in abbreviated form as CnEx, where n is the number of carbon atoms in the alkyl chain and x is the number of ethylene oxide unite. The coadsorption of CnEz into All3 montmorillonite can dramaticallyalter the fundamental surface chemistry of the intercalate. For instance, the cointer-
calation of C12-1& has been shown to greatly improve both the hydrolytic stability of All3 ions in the gallery and the layer stackingorder.l0 In addition,the alumina pillared derivatives formed by calcining the intercalates exhibit micropore size distributions that are much more regular than those observed for pillared derivatives prepared in the absence of surfactant. Rslated surfactant-modified pillared clam have been reported in the patent literature to enhance methane storage capacity and to improve methanel ethane and ethane/propane molecular sieving proper tie^.^ In the present work we investigate in greater detail the binding of C12-14E6 in A13 montmorillonite. Since the ethylene oxide units are potential complexanta for the A 1 3 oligomer,we have included in our studies a surfactant derivative with an extended ethylene oxide unit, namely, C12-14E12. Also, we have utilized photon correlation spectroscopyto obtain information on complex formation between A13 polycations and Cl%l4E6in aqueous solution.
Experimental Section Materials. The clay wed for thisstudy waa naturalWyoming sodium montmorillonite purchased from the Source Clay Minerala Repository at the University of Missouri, Columbia, MO. The idealized unit cell formula was Neoaa(Alade~Mg~)(Si7.ss&.l~)Op(OH)4~~andthe cationexchangecapacitywaa 8Omequiv/ * To whom correspondence should be addressed. 100 g. Prior to use the >2-~mfraction was removed by t URA 236 du CNRS. sedimentation, exchanged three times in 1.0 M sodium chloride, t Michigan State University. and washed until free of chloride ion. (1)Brindley, G. W.;Sempela, R. E. Clay Miner. 1977,12,229. (2) Lahav, N.; Shani, U.; Shabtai, J. Clays Clay Miner. 1978,26,107. The two nonionic surfactants used in this study, namely, (3) Vaughan,D. E.W.;Lwier,R. J.A.eprintsofthebthZnternatio~1 Tergitoll6-S-6andTergitol 16-8-12,were derivativegof eecondary Conference on Zeolites, Naples, Italy, June 2-6, 1980. alcohols. Both compoundswere provided by Union CarbideCorp. (4) Johaneeon, G. Acta Chem. S c a d . 1960,14, 771. Tergitol15-5-5has a general chemical formula of C l l - 1 a ~ 2 6 ~ (5) Bottero, J. Y.;Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980,84,2933. O(CH2CHzO)aand an averagemolecular weight of 420. Tergitol (6) Sriniaavan, K. R.; Fogler, S. H. Clays Clay Miner. 1990,38, 277. 15-5-12, C I ~ - I ~ H ~ ~ - Z ~ O ( C H Z has C Han Z Oaverage)~~H, (7)Fahey,D.R.;Williame,K.A.;Harris,R.J.;Stapp,P.R.U.S.Patent 4 845 068, 1989.
(8) Srinieavan, K. R.; Fogler, S. H. Clays Clay Miner. 1990,38,287. (9) Michot, L. J.; Pinnavaia, T. J. Clays Clay Miner. 1991,39,634.
(10) Michot, L. J.; Pinnavaia, T. J. Chem. Mater. 1992,6, 1433. Clays Clay Miner. 1998,38,343.
(11) Whitney, G.
0743-746319312409-1794$04.00/0 0 1993 American Chemical Society
All3
Langmuir, Vol. 9, No.7, 1993 1795
Polycatiom a n d Nonionic Surfactants
molecular weight of 730. These two compounds will be abbreviated as ClzlrEa and Clz1rE12, respectively.
Synthesis of Surfactant-Modified Pillared Clays. A
solution containing CA1130r(OH)~+t(Hz0)lzr1~~)+ ions was prepared by slowly adding a 0.4 M solution of sodium hydroxide to a 0.4 M solution of aluminum chloride in order to obtain a fiial hydrolysis ratio OH-:A13+= 2.4. At this latter ratio, aluminum exists principally as All3 units, whereas larger oligomers form at higher hydrolysis ratios.12,13 To a 200-mL aliquot of the A l l 3 solution was added 500 mL of a surfactant solution containingvarious amounts of surfactant. The total volume of the pillaring solution was kept constant to avoid any effect arising from the dilution of the pillaring solution?' In the case of C ~ Z & ,the , amounts used were 100,200,300,400, 600, 800, 1000, 1200, and 1600 mg, whereas for C~%&Z,the amounts were 340,690,1000,1380, and 2080 mg. The modified pillared clays were then prepared by adding dropwise to the pillaring solution 330 mL of a 6.0 g/L suspension of sodium montmorillonite under vigorous stirring. The ratio of aluminum to clay was then 15 mmol/mequiv of clay. The suspensionswere allowed to age overnight. The products were collected by centrifugation,washed with deionized water until free of chloride, as judged by the silver nitrate test, and, finally, air dried on a glass plate. Characterization Methods. Chemical analyseswere carried out on samples prepared using the lithium metaborate fusion technique. A 50-mg sample was mixed with 300 mg of lithium metaborate and heated for 10min at 1000OC. The fused product was then diluted in a 3% HNO3 solution. This solution was analyzed by ICP emission spectroscopy at the Michigan State University Toxicology Laboratory. Carbon analyses were performed by the AnalyticalChemistry Laboratory of the University of Illinois, Urbana. X-ray diffraction patterns were obtained for oriented f i i samples using a Rigaku diffractometer with a rotating anode and CuKa radiation. Nitrogen adsorption-desorption experiments were carried out at 77 K on a Coulter Omnisorb 36OCX sorptometer using the quasi-equilibrium volumetric method.'"'' The samples were o u t g d at 150OC overnightunder a vacuum of lodtorr. Surface areas were obtained by using the Brunauer-Emmett-Teller (BET) equation. The t-plot methodla was applied to determine the total micropore volume of the sample, as well as the nonmicroporous surfacearea. The desorption branch was treated accordingto a parallel pore model developedfor phyllosilicatee.lB Photon correlation spectraecopyexperiments were carried out on a Malvern Instruments PCS 4700 spectrometer which allows the measurement of sizes between 5 and 3000 nm. All of the correlation spectra were obtained at a fixed angle of No. Infrared spectra (KBr pellets, 3-mg sample, 150 mg of KBr) were obtained in the transmission mode on a Bruker IFS 88 Fourier transform infrared spectrometer equipped with a deuterium triglycine sulfide (DTGS) detector. A total of 200 scans were coadded at a resolution of 4 cm-'.
0
500
loo0
1500
2000
2500
3000
Size (nm)
Figure 1. Aggregate size distributions for 1:lmolar mixtures of ClzlJZaand A l l 8 in aqueous solution after agingfor varioust h e e at ambient temperature.
complexescould associate into larger aggregates. In order to investigate the possibility of aggregate formation, the interaction of A113 ions with surfactant was studied by photon correlation spectroscopy (PCS). The PCS size distribution obtained for an aqueous solution of All3 polycations in the absence of surfactant was centered around 15 nm, indicating that some of the 4 1 3 ions are clustered. This result is consistentwith previous 27AlNMR studiesmat an OH-:A13+hydrolysisratio of 2.4, where 60% of the totalaluminum was not detectad due to condensation into aggregateslarger than N13. Since the aggregated A l l 3 units most likely are linked through chlorine ions in a nearly linear arrangement with very little branching,I2and since the distance between All3 subunits is approximately 15 A, the aggregates we observe by PCS most likely are composed of approximately 10 All3 units. PSC studies indicate that Cle& solutions also contain self-aggregated units. At very low concentrations (2 X 1V M), the size of the surfactant units is centered near 20 nm, indicating the presence of micelles. At higher concentrations (8 X lV M and above), the average aggregate size is much larger, around 500 nm, suggesting the presence of highly aggregated phases. The aggregates did not exhibit substantial changes in size upon aging over 30 min. Figure 1illustrates the PCS size distribution obtained Rssults and Discussion for a 1:lmolar mixture of C12-14Es and 4 1 3 ions in aqueous Surfactant-A113 Interactions. Previous studies7J'J0 solution as a function of aging time at ambient temperof the All3 montmorillonite-Clz-l& system suggestedthat ature. The initial suspension exhibited the same size there could be a specificinteraction between the surfactant distribution a~ a pristine surfactant suspension, centered and the A13 ion, leading to the formation of a surfactantaround 500 nm. The particle size increased with aging modified intercalation complex in the gallery region of time and reached a maximum of about 3 Nm after 1h. It the host clay. It was also possible that the resulting was possible to visually observe phase separation in the suspension after 1h of aging, as the bulk of the solution (12)Bottero,J.Y.;Axeloe,M.A.V.;Tchoubar,D.;Caees,J.M.;Fripiat, became nearly clear and the surface of the suspension J. J.; Fiessinger, F.J. Colloid Interface Sci. 1987, 117,47. turned very turbid. (13)Fui,G.; Nazar, L. F.;Daeh, A. I). Chem. Mater. 1991,3,602. These resulta suggest that the surfactant-All3 ion (14)Harris, J. R.In Perspectives in Molecular Sieves; Flank,W. H., Whyte, E., Eds.;American Chemical Society: Waehington, DC, 1988; p interaction occurs through the hydrophilic part of the 253. surfactant and then hydrophobic alkyl chain segments of (15)Grillet, Y.;Rouquerol, F.;Rouquerol, J. J. Chim. Phys. 1977,74, the surfactant-All3 complex aggregate to form a low 179. (16)Northrop, P.S.;Flagan, R. C.; Gavalas, G. R. Langmuir 1987,3, density segregated phase. It is worth noting that the 300. aggregates formed are very fragile. Indeed, stirring the (17)Michot, L. J.; Franqoie, M.; Cases, J. M. Langmuir 1990,6,677. (18)De Boer, J. H.; Lippena, B. C.;Linsen, B. G.; Broekhoff, J. C. P.; Van den Heuvel,A.; O s i a ,Th. J. J.Colloid Interface Sci. 1966,21,405. (19)Delon, J. F.;Dellyes, R. C. R. Acad. Sci., Ser. D 1967,265,1161.
(20)Bottero,J.Y.;Marchal,J.P.;Poirier,J.E.;Cases,J.M.;Fieseiager, F. Bull. SOC.Chim. Fr. 1982,11-12,439.
Michot et al.
1796 Langmuir, Vol. 9,No. 7, 1993
I
C12.14E5 0 BETsurfacc area
0 Non microporous
250
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0 0 50
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00 O
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.
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0,O I
I
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2500
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1500
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Figure 4. Dependence of surface area on surfactant loading in
WAVENUMBERS, cm -1
ClZl4E!6-k13
Figure 2. FTIR spectra for (A) neat ClZ;&, (B)a 1:1, mo+ mixture of CM& and 4 1 3 , and (C) A l l 3 ions (hydrolysis ratio r = 2.4).
montmorillonites. (molecules/unitcell) 0.193
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-
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Figure 3. Final loadings of Cls& and C1slJ312 in intercalated A l l 3 montmorillonites as a function of the total surfactantcontent of the pillaring solution. The A l l 3 loading is 0.091 0.005
molecule/unit cell.
60
80
100 120 140 160 180 200
morillonites containing different C ~ Z & loadings.
0
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Figure 5. Mesopore size distributionfor NU-exchangedmonb
0
ob
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Pore Diameter, A
0
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solution obtained after 1 h resulted in a total breakdown of the structure, as the PCS spectrum obtained after stirring was the same as the initial spectrum. In order to seek evidencefor binding of the polar segment of the surfactant to Ah3, we investigated the FTIR spectra for C12-1&, in the presenceand absenceOf Al13. The results are provided in Figure 2. Included in the figure is the spectrum for the All3 oligomer, obtained by evaporation of an A13+ aolution at OH-:A13+ = 2.4. The spectrum for the pristine surfactant, as shown in Figure 2A, was consistent with literature data.21.22 The surfactant exhibited (i) bands characteristic of the ethylene oxide chain at 1350 cm-l (CH2 wagging), 1326 cm-l (CH2 wagging/ twisting), 1250 cm-' (CH2 twisting), 1140 and 1106 cm-' (CH2 rocking), 950 and 845 cm-1 (rocking), 885 cm-l (21) Matsuura, H.;Miyazawa, T. Spectrochim. Acta 1967,23A, 2433. (22) Nadeau, H.G.;Siggia, S. In Nonionic Surfactants;Schick, M. J., Ed.; Marcel Dekker Inc.: New York, 1967; Chapter 26, p 860.
(rockinginthe terminal ethylene oxide unit), and 526cm-l (CCObending);(ii) a band correspondingto the OH group of the terminal alcoholfunction at 3444cm-I; (iii) shoulders characteristic of symmetricand antisymmetricstretching of the CH3 groups on the alkyl chains at 2953 and 2876 cm-l and CH3 bending bands at 1377 and 1457 cm-l; and (iv) bands due to the CH2 groups of both the alkyl chains and the ethylene oxide chain at 2926 and 2858 cm-l (antisymmetric and symmetric stretching), 1466 cm-' (bending), and 723 cm-' (rocking). The FTIR spectrum of the A l l 3 hydrolysis product (cf. Figure 2C) exhibited bands at 3438 and 3180 cm-l (OH stretching), 2450 cm-l (combination),1640cm-', 1085and 979 cm-' (OH bending), and 772,695,582,and 530 cm-l (OH rocking, twisting, or wagging). It is difficult to aeeign these bands more precisely. Indeed, the band positions are almost certainly influenced by the presence of aurrounding chlorine ions. The infrared spectrum of the product obtained by drying a 1:l C12-14Es-Al13 mixture, as shown in Figure 2B, exhibited only the most intense bands characteristic of the ethylene oxide chain (1352,1250, and 1102cm-9, along with the bands corresponding to the CH3 groups of the alkyl chains (2954,2871,1458, and 1377cm-9. The bands correspondingto the CH2groups of both the ethyleneoxide and alkyl chains were also present in the spectrum. It is particularly noteworthythat an intensity ratio of 1.62 was observed for the antisymmetric and symmetric CH2 stretching bands in the 2900-cm-' region of the complex,
A113 Polycations
1
Langmuir, Vol. 9, No. 7,1993 1797
and Nonionic Surfactants 11000
19.1)
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A
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Figure 6. X-ray diffraction patterns for C l & 3 ~ A l l ~montmorillonites containing different levels of surfactant loading (moleculeel unit cell): (A) 0.0-0.255, (B) 0.424-0.672, (C) 0.884-1.159.
whereas the ratio was 1.28 for the pure surfactant (cf. Figure 2A). A t the same time, the band corresponding to the symmetric CH2 stretching frequencywas shifted from 2858 cm-l in the neat surfactant to 2853 cm-l in the 1:l complex. The frequency and intensity shifts for the 1:l complex were consistent with the surfactant binding to
the A113 ion through the ethylene oxide chain. The bands corresponding to the All3 ion were also modified upon complex formation, as they were shifted toward higher wavenumbers. The above PCS and FTJR results for surfactant-Al13 mixtures are indicative of complex formation with the
1798 Langmuir, Vol. 9,No. 7, 1993 binding interaction most likely occurring through H bonding between the ethylene oxide segment of the surfactant and the OH groups of the A113 ion. We next consider how surfactant binding affects the properties of the A113 montmorillonite intercalate. Surfactant-Modified All3 Montmorillonite. Na+ montmorillonite in aqueous suspension reacts at room temperature with A13 polycations in the presence of C12-14E6 to form surfactant-modified intercalation compounds. As was noted in our earlier work,1° the products derived from the surfactant-modified procedure are wellflocculated and are readily washed free of excesselectrolyte with approximatelytwo volumes of water, i.e., three times less than needed when the synthesis is carried out in the absence of surfactant. Thus, the surfactants greatly facilitate dewatering of the clay intercalate. Lengthening the hydrophilic segment of the chain to 12 ethylene oxide units, as in C12-14E12, increases the solubility of the surfactant, but the same general cointercalation and floculation phenomena are observed upon reaction with Na+ montmorillonite in the presence of A113 ions. Figure 3 presents a plot of the surfactant loadings for intercalated surfactant-All3 clay complexes versus the initial surfactant content in the pillaring solution. At relatively low total surfactant to clay ratios C12-14E~and C12-14E12 behave similarly; i.e., -75% of the surfactant initially present in the pillaring solution is incorporated into the intercalate. When the total surfactant to clay ratios reach values above 355 mmol/mol of clay, the surfactants behave very differently. The binding of C12.14E12 to the intercalated complex reaches a plateau around 320mmol, whereasthe loadingof C12-14Escontinues to increase linearly with increasingtotal surfactant. This differencemost likely is due to the differencein solubilities for the two surfactants. The more soluble C12-14E12 surfactant can be desorbed by washing the intercalation complex with water. On the other hand, the relatively insoluble C12-14E6 remains bound at the surface of the clay platelets even after extensive washing. It is particularly noteworthy that the A113 content of the complexes remains essentially constant near a value of 0.091 f 0.005mol/020(OH)4unit cell of clay, regardless of the nature of the surfactant or the surfactant loading. That is, the presence of the surfactant does not affect the quantity of A113 intercalated in the complex. Surface area and pore size determinations provided helpful insights into the nature of surfactant binding. The relationship between surface area and surfactant loading is shown in Figure 4. A113 montmorillonite prepared in the absence of surfactant exhibits a BET surface area of 305 m2 g-' and a liquid microporous volume of 0.10 cm3 gl,when outgassed at 150 "C. Most of this surface area (-90%) arises from the presence of micropores, the nonmicroporous surface area being only 32 m2 g'. The addition of C12-1& surfactants causes dramatic changes in surfacearea and pore structure. For surfactant loadings up to approximately 0.3 molecule/Ozo(OH)4unit cell, the totalsurfacearea decreasesalmost linearly with increasing loading of surfactant to a nonnicroporous value of 55 m2/g1. Analogous behavior was observed for C12-14E12 over the same loadingrange. Thereduction in microporous surface area at low surfactant loadings verifies that the surfactant preferentially binds to the gallery surfaces between the pillaring A113 ions. Increasing the loading of C1214E6 surfactant from 0.30 to 0.55-0.67 molecule/unit cell increased the surface area from 55 to -160 m2/g1(cf. Figure 4). This increase in surfacearea was due to an increase in both the microporous
Michot et al. Table I. C~rtallographicP ~ o m e t e nfor Alia Montmorillonites Containing A d s o r b ClZ-l4& and clZ-14El2 Surfactants surfactant loading layers per (molecules/unit cell) dm1 (A) L (A) scatteringdomain A.Cl&l& Surfactant 0 0.079 0.132 0.913 0.255 0.424 0.566 0.672 0.84 1.159
18.8 18.7 18.8 19.1 19.1 19.6' 22.5,19.6 22.7,19.6 23" 23
B.C1%&2 0 0.067 0.130 0.223 0.239 0.283 0.320 0.306
81 98 186 233 245
4.3 5.2 9.9 12.2 12.8
316
13.7
Surfactant
18.8 18.8 19.1 19.2 19.2 19.5' 22.5,lg.l 22.3,19.4
81 221 259 259 275
4.3 11.7 13.5 13.5 14.3
'Asymmetric diffraction peak. volume (from 0.0057 to 0.040 cm3 gl) and the nonmicroporous surface area from 55 to 80 m2 gl). The nonmicroporous surface area over this loading range was related to the appearance of mesopores. This was evident from the results given in Figure 5, which displaysthe pore size distributionsdetermined from the desorptionbranchea of the isotherms using a parallel plates model.19 Samples prepared in the absence of surfactant or at low loadings (0.193molecule/unitcell) exhibited little or no mesopores larger than 25 A. In contrast, a sample containing 0.672 molecule of C12-14E6 per unit cell showed the presence of mesopores in the range 25-70 A. The appearance of mesopores was attributable to interparticle aggregation arising in part from the binding of surfactant to external surfaces. The peak at 25 A was indicative of connectivity problems and thus has very little physical significance. When the C12-14E6 loading was increased still further, beyond 0.7molecule/unit cell, the intercalateagain became totally nonmicroporous (cf. Figure 4). The total surface area then decreased to -45 m2g', and no mesopores were present. The C12-14E12surfactant exhibited similar effects on the surface area of 4 1 3 montmorillonite, at least for loadings up to 0.35 molecule/unit cell. The above results for the dependence of the surface area on surfactant content revealed the following trends. For low loadings up to approximately 0.25 molecule/unit cell, the surfactant is mainly bound to the A13 ions in the gallery space of the clay. This results in a progressive blocking of the internal surfaces,as evidencedby the strong decrease in surface area and microporosity. For higher loadings, more surfactant molecules apparently enter the gallery spaceacting as pillars, and the microporositybegins to reappear. At the sametime, surfactant moleculesadsorb on the external surfaces of the clay plateleta where they create a few mesopores, perhaps through interparticle segregation. At even higher surfactant loadings, the surfactant adsorbs on the external faces and micro- and mesoporous systems are then completelyblocked or fad. The X-ray diffraction patterns obtained for oriented films of clays intercalated with A113 ions in the presence of C12-14E6 were consistent with the above binding mechanism. As shown by the data in Figure 6A and Table IA, at C1214E6 loadings less than 0.30molecule/unit cell, Le.,
All3 Polycatiohs and Nonionic Surfactants
0:
Langmuir, Vol. 9, No. 7, 1993 1799 R
Aged 6 weeks - Aged 8 months
3500
N
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WAVENUMBERS, cm"
-1000J
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Figure 8. Infraredspectra for Allrexchanged montmorillonites containing different C l d 3 6 loadings (moleculedunitcell) (A) 0, (B)0.079,(C) 0.132,(D) 0.193,(E)0.424,(F)0.566, (G)0.672, (H)0.884,(I) 1.159.
26
Figure 7. X-ray diffraction patterns of Clz-lJ3~Al1~ montmorillonitm after aging at ambient temperature for the times indicated and the following surfactant loadings (molecules/unit cell): (A) 0, (B)0.132, (C) 0.424.
in the range where the surfactant is present mainly in the gallery microporesof the clay, the 001reflection was located near 19 A, as expected for intercalatesconsisting of 9.6-A layers and lo-A A 1 3 polycations. With increasing surfactantloadingsabove0.30 molecule/ unit cell, the 001 reflection became sharper and higher order harmonics appeared. Thus, the presence of surfactantat these loadings improved the layer stackingalong the c axis. An index of crystallinity can be obtained by using Schemer's formula L = X / ( bcos q) where L is the size of the scattering domain, X the radiation wavelength, b the line width at half-height in radians, and q the diffractionangle. Table I presents the basal spacings and values of L as a function of the surfactant content. From these values of L it was possible to derive the average number of sheets along the c axis scattering domain. For surfactant contents between 0 and 0.255 molecule/unit cell, the size of the scattering domain increased from 81 to 245 A, indicating a substantial increase in the stacking order along the c axis. At a surfactant content of 0.424 molecule/unit cell, the 001 reflection became asymmetrical on the low angle side (cf. Figure 6B). This indicated the presence of a second intercalatedphase, which became more and more apparent a t higher surfactant loadings. At a loading of 0.672, two phases were clearly formed, with d spacings near 19 and 23 8. These two phases exhibited 002 reflections at 9.5 and 11.5 A, respectively. The P A increase in basal spacing suggests that additional surfactant molecules enter the
galleryspace a t high concentration. This increasedgallery loading of surfactant is correlated with the increase in microporous volume at the same loadings, as discussed above. At C121.&5 surfactant contents above 0.8 molecule/unit cell, where surfactant molecules adsorb on the external faces and block both micro- and mesopores, the X-ray basal spacings changed again (cf. Table IA and Figure 6C). The 001 reflection was observed near 23 A, and the stacking order along the c axis was again very high. It is particularly interestingto note from the data in Table IA that for samples loaded with 0.255 and 1.159 molecules of surfactant/unit cell the scattering domain was the same, -13 clay layers. This result suggested that the changes in pore structure with surfactant loading did not involve a change in the size of the clay tactoids. Very similar changes in basal spacings were observed for C12-&12 as the surfactant (cf. Table IB). As we have noted previously, the binding of CxEy surfactantsto All3 montmorillonitedramaticallyimproved the hydrolyticstability of the pillared clay. The effect of C121& on the stability of All3 montmorillonite is illustrated by the X-ray diffractionpatterns in Figure 7. After only 2 weeks of aging at room temperature under ambient conditions, the All3 montmorillonite prepared in the absence of surfactant (Figure7A) experiencedsubstantial hydrolysis, as evidenced by the broadened diffraction peaks. A new chlorite-likephase with a basal spacing of 15.9 A became clearly evident after 6 weeks of aging. This phase totally replaced the original 19-A phase after 8 months of aging. In comparison, a c121&-&3 montmorillonite containing0.13 moleule of surfactant/unit cell (Figure7B) exhibited little or no 001X-rayline broadening after 2 weeks of aging. A t least 6 weeks of aging was
1800 Langmuir, Vol. 9, No. 7, 1993
Michot et al.
Table 11. Methyl and Methylene Group Stretching Frequenaier
ClZlrElZ 0.079 0.132 0.193 0.424 0.666 0.672 0.884 1.159 pure surfactant 0.069 0.130 0.223 0.283 0.306 0.320 pure surfactant
va(CH3) position (cm-l) intensity 2954 2954 2954 2954 2954 2954 2954 2964 2954 2954 2954 2954 2954 2954 2954 2954
0.054 0.078 0.069 0.168 0.228 0.258 0.297
0.480 0.352 0.054 0.078 0.108 0.174 0.174 0.198 0.378
vs(CH.4 position (cm-) intensity 2875 2873 2872 2872 2871 2870 2870 2870 2870 2875 2875 2874 2873 2873 2872
v.(CHd position (cm-l) inteneity
A.C1&14Surfactant 0.057 2928 0.072 2930 0.060 2930 0.150 2930 0.240 2929 0.282 2929 0.297 2927 0.480 2926 0.455 2926 B.C12&2 Surfactant 0.054 2932 0.090 2930 0.132 2929 0.186 2928 0.192 2929 0.246 2927 2925
v~CHZ) position (cm-1) intensity
relative v(CH2) intemities
0.646
2858 2868 2858 2858 2858 2858 2856 2856 2858
0.048 0.081 0.069 0.162 0.270 0.318 0.360 0.570 0.502
1.69 1.52 1.62 1.35 1.22 1.16 1.26 1.28 1.28
0.066 0.111 0.162 0.234 0.222 0.288 0.683
2858 2868 2858 2858 2858 2858 2863
0.042 0.072 0.120 0.168 0.174 0.228 0.685
1.67 1.54 1.35 1.39 1.28 1.26 0.997
0.081 0.123 0.106 0.219 0.330 0.372 0.450 0.732
Surfactant loading in molecules per Oa(OH)4 unit cell.
required before broadening became significant, which indicated an important reduction of the kinetics of hydrolysis. When the C12-18~loading was increased to 0.424 molecule/unit cell (Figure 7C), the initial 19-A structure was retained even after 8 months of aging. This result confirmed that the presence of surfactant molecules surrounding the A13 units in the galleries inhibits hydrolysis,presumably by reducing the hydrophiliccharacter of the galleries. Finally, we have investigated the infrared spectral properties of our surfactant-modified A13 montmorillonites. Typical spectra for the c12.186-&3 montmorillonite system are given in Figure 8. At c12-14&loadings
