Journal of Colloid and Interface Science222,125–136 (2000)doi:10.1006/jcis.1999.6550, available online at http://www.idealibrary.com on

Adsorption Mechanism and Structure of the MontmorilloniteComplexes with (CH3)2XO (X = C, and S),

(CH3O)3PO, and CH3–CN Molecules

G. Dios-Cancela,1 L. Alfonso-Mendez, F. J. Huertas, E. Romero-Taboada, C. I. Sainz-D´ıaz, and A. Hern´andez-Laguna

Estacion Experimental del Zaidın (CSIC), C/ Prof. Albareda no. 1, 18008 Granada, Spain

Received June 8, 1999; accepted September 15, 1999

The formation of complexes with different ligands in the in-terlayer space of montmorillonite saturated in Na+, Mg2+, Ca2+,Co2+, Cu2+, Ni2+, Fe3+, and Cr3+ was studied. Acetone, acetoni-trile, dimethyl sulfoxide, and trimethylphosphate were used asligands. The nature of the complexes was studied by means ofX-ray diffraction, infrared spectroscopy, thermogravimetric analy-sis, microcalorimetry, and ab initio quantum mechanical methods.In all cases, the organic ligands penetrate into the interlayer spaceat room temperature, forming complexes stable in vacuum with theinterlayer cations. The ligand–cation ratio depends on the valence ofthe saturating cation. The cation–ligand interaction in these com-plexes has an ion–dipole electrostatic nature. The complexes areformed by the direct interaction of the oxygen or nitrogen atom ofligand and the interlayer cation. Using the quantum mechanical ap-proach, allow us to determine the disposition of the ligand in thesecomplexes. In all cases, only one layer of ligands is present in thestable complexes. C© 2000 Academic Press

Key Words: interaction; dimethylsulfoxide; trimethylphosphate;acetone; acetonitrile; montmorillonite; heats of adsorption; struc-ture; ab initio calculations.

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INTRODUCTION

Montmorillonites are present in most soils and sedimeThey are very important minerals in chemistry, the environmagriculture, and industry. They play a relevant role in soil potion because of their important adsorption capacity and lowmeability. Adsorption of polar organic molecules is a top subin clay research and is being studied by many authors; an esive review cannot be given in this Introduction. The study ofphysical–chemical properties related to the organic–inorginterface can be very useful for understanding the macroscbehavior of organic molecules in soils (mobility, diffusion, etand in industrial applications, as catalysts and paints.

Many organic molecules with polar functional groups canadsorbed in the interlayer space of montmorillonite, producthe swelling. McEwan (1) reported that these polar groupsdiate with the exchange cations and layer effective charges

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erating energy. This energy is able to overcome the cation-lattraction forces and produce the interlayer swelling. Nonpadsorbates, wherein only van der Waals forces mediate, dproduce enough energy to surmount the cation-layer attracforces in montmorillonites. Other scientists studied the swelof montmorillonite by means of X-Ray diffraction. These worconfirm the McEwan’s hypothesis and furthermore they shthat the complex formation and the basal spaces are a funof the drying level of the sample (2) and the phase (gas or liqof the organic molecules (3–5).

Barshad (3) reported that the complexation capacity of mmorillonite with polar organic molecules depends on theelectric constant of the molecule. This capacity decreases wthe dielectric constant decreases for molecules with compadipole moment. Recently, van Ascheet al. (4) discussed this facfinding that the greater the dielectric constant value of theganic molecule, the greater is the work to expand the layers osilicate. Bissada and Johns (6) determined the adsorption heacetone and methanol in montmorillonite by means of gas pchromatography, they proposed that the ion–dipole interacis the specific mechanism for the adsorption in montmorillonVan Ascheet al. (4) discussed once again those conclusiobecause the adsorption of amines by Na+-montmorillonites ismore effective than that of alcohols,in spite of that amines hlower dipolar moment than alcohols.

In the 1960s, infrared (IR) spectroscopy studies of orgamolecules adsorption in montmorillonite revealed that distmechanisms are possible. The most probable adsorption manisms of polar molecules are due to ion–dipole forces or foing coordination complexes (7). Probability of complex formtion and the interaction type depend basically on the drylevel of the sample and the crystal-chemistry properties ofexchange cations.

To describe quantitatively the adsorption mechanism, we mknow the interaction energy between exchange cations anders, the energy developed during the adsorption process,suitable hypothesis of the complex nature. For these therdynamic requirements, heats of adsorption must be determin a reliable way. In order to study the adsorption mechaniswe determine the adsorption heats of polar neutral molecule

5 0021-9797/00 $35.00Copyright C© 2000 by Academic Press

All rights of reproduction in any form reserved.

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126 DIOS-CANC

montmorillonite, with different polar moment and dielectric costants, by means of microcalorimetric methods. The followmolecules are studied in this work: acetone (ACO), acetoni(ACN), dimethyl sulfoxide (DMSO), and trimethylphospha(TMP). In this series of molecules, different polar functiongroups are present. All of them have a common characterof a polar bond and lone pairs: carbonyl (ACO), nitrile (ACNsulphonyl (DMSO), and phosphoryl (TMP) functional groupLone pairs of these molecules can form coordination compleand besides, due to their high dipole moment, they can besorbed by means of an ion–dipole mechanism. This workhelp to clear up the adsorption mechanism type. Also, thfunctional groups are presented in many pesticides and intrial pollutants, which contaminate soils. So, this study canuseful for the treatment and evaluation of polluted soils.

In this work, montmorillonites were saturated by cations wan electronic structure of noble gas (Na+, Mg2+, Ca2+) and tran-sition metal cations (Fe3+, Cr3+, Co2+, Ni2+, and Cu2+). Thecomplex nature was studied by means of the following expmental techniques: X-ray diffraction (XRD), microcalorimetinfrared spectroscopy (IR), differential thermal analysis (DTthermogravimetric analysis (TG), and differential thermogrametric analysis (DTG).Ab initio quantum-mechanical methodwere used in order to study the structure of these ligandstheoretical–experimental way.

METHODS AND MATERIALS

Samples

The<2µ fraction of the Wyoming montmorillonite (Swy-1was used. The surface area (S) determined by the Dyal anHendricks method (8) is 753 m2/g, its cation exchange capaci(dried sample at 120◦C in vacuum) was 112 meq/100 g, andcharge density was 4.07× 104 esu/cm2.

Organic Compounds

All solvents were used with a purity of 99.9%. The ACO aACN (analytical grade) were supplied by Sigma, and Fluka splied the TMP and DMSO. The dipole moments of ACO, ACTMP, and DMSO are 2.85, 3.94, 3.02, and 3.96 D, respectivand their dielectric constants (9) are 20.7, 37.5, 2.57, and 4respectively.

Homoionic Sample Preparation

Homoionic samples (Na+, Mg2+, Ca2+, Fe3+, Cr3+, Co2+,Ni2+, and Cu2+) were prepared by treating 10 g of montmrillonite with 50 ml of 1Eq/L chloride solutions for the corresponding cations. The treatment was repeated five times ansamples were washed each time with water/methanol (60mixture, until no Cl− was detected (with AgNO3 solution) inthe washing solutions. Subsequently, samples were oven-

◦

at 60 C and ground until the powder could be passed throug480-mesh sieve.

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Treatment of Samples

Some researchers (10) have dehydrated montmorillonit10−5 mmHg and room temperature, finding this method tomore effective than desorption on P2O5. However, other authors(11, 12) treated vermiculite at 10−6 mmHg for a week at 25 and50◦C and found by XRD that small quantities of water molecuremain within the interlayer space of the clay. We dehydrathe samples at 120◦C in a 10−3 mmHg vacuum for 4 h and underthese conditions found that the interlayer space of the montrillonite measured 0.98 nm, thus confirming that dehydratiovirtually total. All experimental data are referred to dry sampunder these conditions.

Preparation of Complexes

Approximately 0.5 g of each sample was treated at 120◦C ina 10−3 mmHg vacuum, and later it was wetted with an excessolvent (ACO, ACN, DMSO, and TMP). The crucibles with thwetted samples were placed in a desiccator of high vacuumP2O5 during 48 h. Afterward, the complexes were desorbedvacuum of 10−2 mmHg until constant weight. In each case tquantity of liquid retained was determined. The complexesformed, were studied by means of XRD, DTA, TG, and IR.

X-Ray Diffraction

The complexes stable in vacuum were analyzed using a PhPW-1750 X-ray diffractometer, with CuKα radiation, a Ni filter,and a graphite monochromator, by means of the powder meon a plane sample holder.

Thermogravimetric and Differential Thermal Analysis

The TG, DTG, and DTA of the stable complexes were pformed in a Netzsch STA-409EP, using Al2O3 as referencewith a heating rate of 10◦C/min in air atmosphere. The sample amount was 40 mg approximately in crucibles of 50capacity. A baseline correction was applied for all curves.

Infrared Spectra

IR spectra of the stable complexes in vacuum were performin a Zeiss IMR-16 spectrometer from 4000 to 625 cm−1. Thefollowing procedure was used: the Na+, Mg2+, Cr3+, and Co2+

(as head member of samples saturated with cations with able gas electronic structure and transition metal cations, restively) saturated samples were prepared as a film on alumifoil. Air-dried films were removed from the aluminum foil anagain dried at 120◦C in 10−3 mmHg vacuum. The dried filmswere introduced in the solvating liquids during 48 h, andterward they were desorbed at 10−2 mmHg of vacuum during8 h. The complexes formed in these conditions were stud

h aby IR in the above range. The ACO and ACN complexes werehygroscopic.

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STRUCTURE OF THE MONT

Microcalorimetric Methods

The heat of immersion on the sample dried at 120◦C under10−3 mmHg and the stable complex in vacuum were determiin an adsorption calorimeter LKB-2101-1221, previously cabrated by the Joule effect. All determinations were repeathree times. The sample amount used in each determinationapproximately 60 mg.

Theoretical Calculations

Ab initiomolecular orbital calculations on the molecules chsen as ligands in these complexes were performed usingGAUSSIAN 94 package (13). Restricted Hartree–Fock appimation was used by means of the triple-ζ basis set with polarization diffuse functions on the heavy atoms (6-311+G∗) (14, 15).The electron correlation effects were included by performsecond-order Moeller–Plesset (MP2) perturbation calculat(16) (MP2/6-311+G∗). Critical points of the potential-energsurface were determined optimizing the whole molecule wout restrictions by using the Berny method (17). Dipole momewere also calculated.

Maximal distances were calculated for different spatial dpositions of the ligands in the critical point geometries. Tdistances were matched with variation of the spacing d(0of the swelled montmorillonites. Distances of a nucleus tospecial plane were calculated by using the Lagrange multipmethod. The corresponding Van der Waals radii (9) were adto these distances.

RESULTS AND DISCUSSION

X-Ray Studies

The d(001) spacing of the complexes stable in vacuumtermined by XRD are shown in Table 1. The adsorbed quanof solvent (g of solvent / g of dried sample) and the moleculigand/cation ratio are also presented in Table 1. The samtreated with DMSO and desorbed at vacuum produce stable

plexes, where d(001) basal spacing oscillate between 1.35

Ni 0.1262 2.9 1.46 0.2163 2.8Cu2+ 0.1155 2.8 1.46 0.2171 2.8

MORILLONITE COMPLEXES 127

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spacing of the TMP complexes oscillate between 1.53 nmthe Na+ sample and 1.63 nm with Cu2+, meanwhile the basaspacing of the ACO and ACN complexes vary among 1.221.36 nm, and 1.25 and 1.35 nm, respectively, depending onexchange cation. Except for the Na+ samples, the d(001) spaing does not change significantly for the same ligand withdifferent cations, even when the cation/ligand ratio changes

Therefore, the DMSO, TMP, ACO, and ACN molecules petrate in the interlayer space of the montmorillonite at room teperature, and they produce stable complexes at vacuum.ligand–cation ratio depends on the valence of the saturacation, being one for monovalent cations, two to three forvalent cations, and three to five for trivalent cations, dependon the solvating liquid (Table 1).

Thermal Study

In order to investigate the thermal stability of the complexwe performed DTA and TG studies of the stable compleat vacuum and analyzed the d(001) spacing coming fromthermal treatment in samples dried at different temperatuIn Figs. 1a–1d, we present the DTA and TG diagrams ofcomplexes of Mg2+, Ca2+, Cr3+, and Fe3+ with the differentligands.

The thermal decomposition of the complexes of DMSOMg2+, Cr3+, and Fe3+ are similar each other (Fig. 1a). At 80120◦C an initial weight loss is observed along with an endothmic effect. This fact can be attributed to a loss of hydration wamolecules, which does not affect to the d(001) spacing ofcomplexes (1.46–1.50 nm). Another deeper weight loss witendothermic effect at 220–260◦C, followed by an exothermiceffect at 260–300◦C in the DTA curve, can be attributed tDMSO molecules loss. This loss corresponds to three molecper cation in trivalent cations and one molecule per cation invalent cations. After this weight loss, a d(001) spacing of 1.35was observed, similar to that found in the original Na+ complex.The increasing of spacing of this latter complex with respecthe dried sample spacing is 0.4 nm, similar to those foun

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the kaolinite–DMSO complexes (18, 19). Finally, the TG curveby a

for the Na+ sample and 1.50 nm with Mg2+ and Ca2+. The basal

TABLE 1d(001) Spacings (nm) of the Montmorillonite Complexes Stable in Vacuum, Amount (x) Ligand/Sample,

and Ratio Ligand/Cation (Molecules/Cation)

DMSO TMP ACO ACN

Cation x (g/g) ligand/cation d (001) x (g/g) ligand/cation d(001) x (g/g) ligand/cation d(001) x (g/g) ligand/cation d(001)

Na+ 0.0873 1.0 1.35 0.1551 1.0 1.53 0.0649 1.0 1.22 0.0462 1.0 1Mg2+ 0.1262 2.9 1.50 0.2328 3.0 1.62 0.0691 2.1 1.32 0.0431 1.9 1Ca2+ 0.1436 3.3 1.50 0.3108 4.0 1.62 0.0740 2.2 1.36 0.0462 2.0 1Cr3+ 0.1327 4.6 1.45 0.1680 3.2 1.61 0.0650 3.0 1.33 0.0401 2.6 1Fe3+ 0.1436 4.9 1.45 0.1859 3.5 1.61 0.0654 3.0 1.33 0.0386 2.5 1Co2+ 0.1185 2.8 1.47 0.2299 3.0 1.61 0.0690 2.1 1.33 0.0436 1.9 1

2+

presents another deep weight loss, which is accompanied

1.62 0.0660 2.0 1.33 0.0468 2.0 1.331.63 0.0681 2.1 1.33 0.0436 1.9 1.33

f

128 DIOS-CANCELA ET AL.

FIG. 1. (a) DTA and TG diagrams of complexes of DMSO with Mg2+, Ca2+, Cr3+ and Fe3+-montmorillonite. (b) DTA and TG diagrams of complexes o

TMP with Mg2+, Ca2+, Cr3+ and Fe3+-montmorillonite. (c) DTA and TG diagrams of complexes of ACO with Mg2+, Ca2+, Cr3+ and Fe3+-montmorillonite.

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(d) DTA and TG diagrams of complexes of ACN with Mg2+, Ca2+, Cr3+ and F

strong exothermic effect in the DTA curve (340◦C for Cr3+ and402◦C for Mg2+). This weight loss can be due to the combustof the two last adsorbed DMSO molecules. In Ca2+ sample theloss of water is found at 90◦C, and afterward there is a gra

ual loss of DMSO molecules until reaching high temperatuwhere the DMSO molecules are oxidized.

3+-montmorillonite.

on

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The thermal behavior of the complexes of TMP with Mg2+,Cr3+, and Fe3+ was similar to those of DMSO (Fig. 1b). A smaweight loss appears near 85◦C, which can be attributed to smaquantities of water. This water does not influence the basal s

resing of original complexes. Another more important weight lossfollows the previous weight loss, which corresponds to 1.5 and

129

last TMP molecuwas confirmed b

STRUCTURE OF THE MONTMORILLONITE COMPLEXES

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FIG. 1—C

1 molecule/cation for trivalent and divalent cations, respectivThe complex basal spacing in samples treated at this tempervaries from 1.60 to 1.30 nm, when the temperature increaThe third weight loss is very deep and is accompanied byexothermic effect at temperatures varying between 345◦C forCr3+ to 390◦C for Mg2+, corresponding to the loss of the tw

les by oxidation of the methyl groups. This fay chemical analysis of samples treated to th

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temperatures, which detected important P quantities, whichnot exist in the original samples. In the Ca2+ complex, the lossof TMP molecules is gradual and continuous, a basal spacin1.0 nm appearing at 450◦C, which should correspond to the sample with small phosphorous quantities in the interlayer spaci

The thermal behavior of the complexes of ACO and ACN wi

cteseMg2+, Cr3+, and Fe3+ is similar (Figs. 1c and 1d). In all cases,there is an important weight loss, followed by an endothermic

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130 DIOS-CANC

effect at 120◦C, which must correspond to hydration waterthe complexes (the complexes are very hygroscopic), becthe samples treated at this temperature present the samespacing to the original complexes. Afterward, there is a dweight loss, which corresponds to 2 molecules/cation for Aand ACN complexes, respectively. This loss is accompaniean exothermic effect, which oscillates between 400◦C in Cr3+,420◦C in Mg2+, and 248–340◦C in the Fe3+ complex. At the endthe basal spacing of the samples is 1.0 nm. The Ca2+ complexloses the water and the ACO and ACN molecules at the stime with an endothermic effect at 140◦C for the ACO complexand 140–220◦C for the ACN complex.

Therefore, from these results we can conclude thatmolecules which form the complexes stable at vacuum (Tabwith a determined d(001) spacing are necessary to maintainspacing. This fact is important for the calculation of the swellenergy cited below.

Spectroscopic Studies

In order to determine the interaction between ligandcation, the complexes of ACO, ACN, and TMP with Na+, Mg2+,Cr3+, and Co2+ stable at vacuum were studied by means ofspectroscopy. The most characteristic bands of the free ligand the complexes spectra are included in Table 2.

In TMP complexes, the stretching vibration band of P=O(ν(P=O)) shifts from 1269 cm−1, in free ligand, to 1250, 12401238, and 1210 cm−1 in the Na+, Mg2+, Co2+, and Cr3+ com-plexes, respectively. This shift is due to the interaction betwthe oxygen atom of the phosphoryl group and the cations.interaction weakens the covalent character of the P=O, whichdecreases the frequency of the stretching band. Also, the mC–H stretching band shifts to higher wavenumbers, which icates some interaction of the hydrogens with the layers of cThis result is consistent with previous studies of TMP adsorpin montmorillonite (20).

In the ACO complexes, IR spectra are characterized by aand splitting of theν(C=O) stretching band from 1714 cm−1,for the free ligand, to 1696 cm−1 for the Na+ complex, 1700–1685 cm−1 for Mg2+, 1700–1682 cm−1 for Co2+, and 1695–

−1 3+ −1

1680 cm for Cr . The 18-cm shift in C=O stretching for+

Co 1235 3010, 2960, 2920, 2860 1700–168Cr3+ 1210 3010, 2960, 2920, 2860 1695–168

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the cation (21). In the rest of the complexes, two bands appin the absorption C=O zone. One of the bands oscillates fro1700 to 1695 cm−1; we consider that to be due to the interactiof the ACO molecules with the exchange cations through wabridges (21). The other band oscillates from 1685 to 1680 cm−1,which should correspond to direct bonds of the ACO molecwith the exchange cations (21). This interpretation agrees wthe following facts: when the complexes are continuouslyhydrated in vacuum, the 1700 cm−1 band lowers its intensity,and the 1600–1685 cm−1 band increases its intensity. Frequenshifts with respect to the free ligand are also observed in obands in all spectra of the montmorillonite–ACO complexSo, theδ(CH3) band, which in the free ligand appears at 14and 1424 cm−1, yields a simple band at 1422 cm−1 in the com-plexes. The band, which appears at 1366 cm−1 in the free ligand,splits at 1370–1360 cm−1 in the complexes. The C–C stretching band, which appears at 1225 cm−1 in the free ligands, shiftsto 1235 cm−1 in the complexes. These modifications were afound by Parfitt and Mortland (21).

The liquid ACN spectrum shows two characteristic bantheν(CN) band at 2257 cm−1 and the 2296 cm−1 band, whichis assigned to a combination ofν(C–C)+ ν(C–H) (22). In theNa+, Mg2+, and Co2+ complexes, theν(CN) band shifts to ahigher wavenumber (2270 cm−1 for Na+, 2280 cm−1 for Mg2+

and Co2+), while the 2296 cm−1 band shifts to 2300 cm−1

for the Na+ complex and 2310 cm−1 for the Mg2+ and Co2+

complexes. Thisν(CN) band shift is due to the increasing othe force constant of the bond C≡≡N (23). The formation of thecomplex with the electron acceptor cations perturbs the lpair of the nitrogen atom, increasing its s character (23). Tstronger the cation–ligand interaction, the higher the frequeshift. In the complex with Cr3+, four bands are observed at 2272306, 2333, and 2360 cm−1. The first two bands corresponto theν(CN) vibration. This shows the existence of two kindof coordinated ACN molecules. The 2306 cm−1 band willcorrespond to the molecules coordinated directly to interlacations. In contrast, the 2273 cm−1 band can correspond to ACNmolecules coordinated to the cations by water bridges (24).

Although the DMSO–montmorillonite complexes have nbeen studied by IR spectrometry in this paper, previous wo

7

showed that the DMSO molecules interact with the interlayer26).

the Na complex indicates interaction of the ACO oxygen with

TABLE 2Infrared Studies of the Intercalated Complexes in Montmorillonite (cm−1)

TMP ACO ACN

Sample ν(P=O) ν(C–H) ν(C=O) δ(CH3) δ(C–C) ν(C–H)+ ν(C–C) ν(C≡N)

Pure liq. 1269 3000, 2958, 2920, 2850 1714 1440, 1424 1225 2296 225Na+ 1250 3010, 2960, 2920, 2860 1700–1696 1425 1235 2305 2270Mg2+ 1235 3010, 2960, 2920, 2860 1700–1685 1422 1235 2310 2280

2+

cation through the oxygen atom of the sulfoxide group (25,

5 1422 1235 2310 22800 1422 1235 2333–2360 2273–2306

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STRUCTURE OF THE MONTMORILLONITE COMPLEXES 131

TABLE 3Immersion Heats in Dimethylsulfoxide (DMSO), Trimethylphosphate (TMP), Acetone (ACO), and Acetonitrile (ACN) of the Degasified

Montmorillonites (120◦C, 10−3 mmHg) (QIo), and the Stable Complexes in Vacuum (QI) and Heat of Adsorption (qa) in kJ/mol

DMSO TMP ACO ACN

Cation QIo J/g QI J/g x (g/g) qa QIo J/g QI J/g x (g/g) qa QIo J/g QI J/g x (g/g) qa QIo J/g QI J/g x (g/g) qa

Na+ 61.5 25.7 0.0873 32.0 49.8 16.9 0.1551 29.9 45.2 8.6 0.0649 32.7 53.2 16.0 0.0462Mg2+ 174.0 42.7 0.1262 81.1 126.0 20.2 0.2328 64.1 108.4 19.4 0.0691 74.8 127.0 27.8 0.0460Ca2+ 157.0 55.2 0.1436 55.3 140.0 26.2 0.3108 51.6 105.0 48.3 0.0740 44.4 100.3 39.6 0.0552Cr3+ 105.9 17.3 0.1327 52.0 98.5 16.0 0.1680 69.2 62.0 9.1 0.0650 47.2 95.0 25.0 0.0401Fe3+ 109.9 17.3 0.1436 50.3 100.4 13.3 0.1859 66.1 63.0 9.1 0.0654 48.0 99.2 20.0 0.0386Co2+ 139.9 33.5 0.1186 70.2 110.6 26.7 0.2300 51.4 76.1 15.4 0.0690 51.0 98.0 19.8 0.0436Ni2+ 124.9 19.0 0.1262 65.4 115.7 26.0 0.2163 58.5 87.2 17.5 0.0660 61.2 109.0 20.5 0.0468

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Calorimetric Studies

In order to determine the mechanism and strength ofligand–cation interaction, the adsorption heats were also demined in this work. The following expression was used.

qa = (QIo − QI )/x, [1]

whereQIo andQI are the heat of immersion (J/g) of the monmorillonite dehydrated and the stable complexes in vacuumspectively, andx is the quantity of retained ligand (g of ligand/of sample) in the vacuum stable complexes (Table 3).

This heat of adsorption,qa, was corrected with the energy ointerlayer swelling. This energy can be calculated by the expsion proposed by Norrish (27) and later applied by other auth(28, 29):

Eexp= 2.39× 10−8S[(σze)/(εD)− (σze)/(ε′D′)], [2]

in cal/g, whereS is the clay specific surface in cm2/g, z is thecation valence,e is the electron charge in uee,ε and ε′ (inDebyes) are the dielectric constants of the dehydrated claythe complex, respectively, andD andD′ are the distance (in cm

between the center of the cation charge and the center of the

Cu2+ 66.2 41.2 107.4 59.4 48.9 108.3

59.4 87.3 17.0 0.0681 60.0 100.1 20.5 0.0436

theter-

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and

tively. The center of the interlayer negative charge was plaat 0.42 nm from the surface of the network oxygen atoms (2The dielectric constantε had the value of 3.5 (30, 31). In order tapply Eq. [2], the constantε′must be known. Obviously, the linkage of the distinct ligands to the exchange cations can proddielectric saturation phenomena, and the complexes shoulda lower dielectric constant than that in the pure liquids. Sincevalue ofε′ cannot be determined experimentally, these dieltric saturation phenomena can show that the dielectric consof these complexes is close to the nonhydrated clayε value.Then, the same value ofε was taken forε′ (ε= ε′ = 3.5 D). Thisexpansion energy calculated would be the minimal energyseparating the silicate layers in the complexes formation.

In order to compareqa and Eexp, the last one should be expressed as kJ/mol of adsorbed ligand and the amount of solvamolecule necessary to separate the layers should be knownabove DTA–TG studies show that the loss of one moleculea ligand can produce an alteration of the d(001) spacing ofcomplexes. This means that all molecules adsorbed in the scomplexes in vacuum are necessary for maintaining the d(0spacing (Table 1). This amount was used in the calculationEexp in kJ/mol. The experimental and theoretical energetic v

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ues and the total heats of adsorption (qt = qa+Eexp) are included

terlayer charge in the anhydrous clay and in the complex, respec-

TABLE 4Heat of Adsorption (qa), Swelling Heat (qh), and Total Heat (qt) for Adsorption of DMSO, TMP, ACO,

and ACN in Montmorillonite (kJ/mol)

DMSO TMP ACO ACN

Cation qa qh qt qa qh qt qa qh qt qa qh qt Z/r

Na+ 32.0 14.8 46.8 29.9 18.3 48.2 32.7 11.9 44.6 33.0 11.8 44.8 1Mg2+ 81.1 49.2 130.3 64.1 48.9 113.0 74.8 44.7 119.5 88.6 47.3 135.9 3Ca2+ 55.3 36.2 91.5 51.6 36.6 88.2 44.4 41.7 86.1 45.0 47.2 92.2 2Cr3+ 52.0 133.9 185.9 69.2 100.0 170.0 47.2 107.1 154.3 71.6 107.1 178.7 4Fe3+ 50.3 133.9 184.2 66.1 91.8 157.9 48.0 107.1 155.1 84.1 107.1 191.2 4Co2+ 70.2 41.2 111.4 51.4 48.9 100.3 51.0 44.7 95.7 73.5 46.6 120.1 2Ni2+ 65.4 41.2 106.6 58.5 48.9 107.4 61.2 44.7 105.9 77.5 46.6 124.1 2

in Table 4.

60.0 44.7 104.7 74.8 46.6 121.4 2.85

132 DIOS-CANCELA ET AL.

ti

)

a

g

d

i

e-on–on.nic

iallynd-en

the-for

si-yerin-ge-

FIG. 2. Total adsorption heat of DMSO, TMP, ACO, and ACN

It can be considered that the total heat of adsorption reflthe energies that are produced during the solvating of the caand the montmorillonite surface. Then, if this last contributis constant for different samples in the same polar liquid, a grelationship betweenqt andZ/r (ionic potential of the cationscan be expected. This assumption is observed plottingqt versusionic potential for the different cations and solvating liqui(Fig. 2). A linear relationship is found between both magnitudThis means that the interaction ligand–cation has the same nin all cases, and this interaction is electrostatic and is proban ion–dipole interaction. The correlation lines cross they axisat different values ofqt in the range of 4 kJ/mol (DMSO) to17 kJ/mol (ACO). This fact means that these ligands also intewith the interlayer surfaces by means of hydrogen bondinvan der Waals interactions.

If the ligand–cation interaction is of ion–dipole nature, arect relationship should exist between the solvating energcations and the dipole moment of the adsorbed molecules. Sthe slopes of the correlation lines of Fig. 2 represent an ind

measurement of the solvating energy of cations, it would bepected that these slopes vary lineally with the dipole mom

in montmorillonite vs ionic potential of the different interlayer cations.

ectsionsonood

dses.aturebly

ractor

i-y ofince

rect

of the solvating liquids. In Fig. 3, we found a good lineal rlationship between both magnitudes. This fact reveals the idipole electrostatic character of the ligand–cation interactiThis is consistent with previous studies of adsorption of orgamolecules by montmorillonite (29). Van Ascheet al.(4) suggestthat the solvation energy of exchange cations comes partfrom the electron-donor character of the lone pairs of the boing atoms (4). However, we did not find any relationship betwethe HOMO (highest occupied molecular orbital) energies ofligands from ourab initio calculations and their heats of solvation. We suggest that other factors, like negative net chargethe electrostatic interactions, could act in this energy.

Disposition of Ligands in the Interlayer Space

In order to obtain some information about the spatial dispotion of the organic molecules that act as ligands in the interlaspace of montmorillonite, it is necessary to know the d(001)terlayer space of the complexes and the conformations and

ex-entometry of these molecules. In order to obtain a better insight ofthe molecular structure of these ligands, the electronic structure

d

pa,

earta

3

p

r

nl

n

reer

ryl

ilarty.

om-

theely)ec-

ri-up

an

s,

STRUCTURE OF THE MONT

FIG. 3. Relationship between the solvation energy and dipole momenthe ligands. Slope for water taken from a previous work (29).

of these molecules are studied by means ofab initio quantummechanics calculations. In all cases studied with MP2/6-31+G∗//MP2/6-311+G∗ basis set, similar geometrical featuresthose from 6-31+G∗//6-31+G∗ calculations were obtaineshowing only energy differences. Geometries optimized atMP2/6-311+G∗ level of the main conformers for all moleculeare shown in Fig. 4.

Acetonitrile presents only one conformer due to its C3v sym-metry (Fig. 4a). This theoretical geometry reproduces the eximental structure (0.108, 0.145, and 0.113 nm for C–H, C–C,CN bond lengths, respectively) (32). In the case of acetoneminimal energy conformer corresponds to the syn conformwhere the H–C–C=O dihedral angles are 0◦ (Fig. 4b). A secondconformer is detected as being 0.6 kcal/mol less stable, whermethyl coplanar hydrogens with the carbonyl group are in andisposition (θ (HCCO)= 180◦). Both conformers have similageometrical parameters, dipole moments, and electronic stures. In this molecule we have considered only the most stconformer. The main calculated geometry features are content with the experimental data (0.1215, 0.1515, and 0.1086for C=O, C–C, and C–H bond lengths, respectively, and 121.9◦

and 110.2◦ for OCC and CCH bond angles, respectively) (3Concerning the DMSO molecule (Fig. 4c), only one conformwas found, which shows a pyramidal configuration with the sfur atom as a top vertex, corresponding to asp3 hybridizationfor the sulfur atom. Methyl hydrogen atoms are in gauchesition with respect to S=O bond (θ (H–C–S=O)= 68◦). Thistheoretical geometry is consistent with the experimental ctal structure of DMSO (34), where a similar pyramidal structuwas found. The theoretical values of S–C bond length (0.181and OSC bond angle (106.8◦) reproduce the experimental vaues (0.180 nm, and 106.7◦, respectively). The calculated S=Obond length (0.151 nm) is slightly shorter than the experimevalue (0.153 nm). This fact can be explained by the existe

of intermolecular interactions in the crystal structure, which anot included in theoretical calculations.

MORILLONITE COMPLEXES 133

t of

1to

thes

er-ndtheer,

thenti

ruc-blesis-nm

).erul-

o-

ys-rem)

-

talnce

With respect to the TMP molecule, three conformers wefound (Figs. 4d, 4e, and 4f)). The minimal energy conform(TMP1) has a high symmetry (C3) and all three methyl groupsare in a syn-gauche disposition with respect to the phosphogroup (dihedral angleθ (O=P–O–C)= 44◦). This is consistentwith the results reported by Ewiget al. (35) for the phosphoricacid molecule, where the OH groups were oriented in a simsyn-gauche disposition with respect to the phosphoryl moieSimilar results were found in analogues organophosphorus cpounds (36, 37). The theoretical values of P=O and P–O bondlengths (0.145–0.146 and 0.157 nm, respectively) reproduceexperimental values (0.145–0.147 and 0.157 nm, respectivof similar organooxyphosphorus compounds (36, 37). The sond conformer (TMP2) is 1.08 kcal/mol (at 6-31+G∗//6-31+G∗

level) less stable than TMP1, where one methyl group is oented in an anti disposition with respect to the phosphoryl gro(θ (O=P–O–C)= 176◦). A third conformer was found (TMP3)being 2.37 kcal/mol (at 6-31+G∗//6-31+G∗ level) less stablethan TMP1. This conformer has also one methyl group inanti disposition (θ (O=P–O–C)= 180◦) and two methyl groups

FIG. 4. Geometry of the main conformers of the solvating molecule

re(a) ACN, (b) ACO, (c) DMSO, and TMP ((d) TMP1, (e) TMP2, (f) TMP3)optimized at the MP2/6-311+G∗ level.

if

n

h

p

gfdft

hPef

l

o

i

P

nttheore

er,

telyallns,dryforN6–the

g

theldmedhe-e abe-is).e ofn-

ersithyerthe

eetheorylrentcon-

134 DIOS-CANC

in a syn-gauche disposition with respect to the phosphoryl gr(dihedral angleθ (O=P–O–C)= ±44◦). However, this thirdconformer is more symmetric than TMP2 with the existencea symmetry plane. The higher energy of TMP3 can be explaby a possibly higher repulsive effect between the lone pairs oP–O–C oxygens, which are oriented between each other,in TMP2. Other conformations with more methyl groups in adisposition were searched, but they were not stationary poprobably due to possible steric effects. Including electron crelation, the interconformational energy differences are lowbeing TMP2 and TMP3 0.44 and 2.13 kcal/mol less stable tTMP1, respectively.

The calculated (gas phase under vacuum) values of dimoment of these molecules are larger than the experimevalues (liquid phase). Dipole moment is sensitive to theometry, basis set, and conformation. Therefore, small difences in the geometry and conformational angles can prochanges in dipole moment, especially in TMP, where conmational changes move a methyl group to an anti disposiwith respect to the phosphoryl group, ranging the dipole mment from 1.22 to 4.15 D. However, a good linear relations(R= 0.9965) is found in all molecules (Fig. 5) except for TMwhere the dipole moment of each conformer is very differeach other (Table 5). Taking into account the energy difences among these conformers, the conformational populaat room temperature (300 K) can be calculated approximawith a Bolztman population considering only the three conforers (TMP1/TMP2/TMP3= 65/33/2). So, the average dipomoment value can be calculated at vacuum, without intermolelar interactions (µ= 2.25 D). Our previous studies on phosphnates showed us that the rotational barriers of these confoers are not high and the conformational population can chawith the polarity of the media (38). In our case, taking inaccount the linear relationship between experimental andculated dipole moment values of most molecules (Fig. 5),

FIG. 5. Relationship between the calculated (at 6-31+G∗//6-31+G∗ level)and experimental values of the dipole moment of the solvating molecules.

ELA ET AL.

oup

ofnedthethanti

intsor-er,an

olentale-

er-uceor-iono-ip,nt

er-tiontelym-ecu--rm-ngetocal-t is

TABLE 5Comparison of Experimental and Calculated Values of Dipole

Moment of the Ligands (in Debyes)

Molecule µ exp. (liq.)a µ calc.b

ACN 3.55 4.20ACO 2.87 3.38DMSO 3.96 4.87TMP 3.02 1.22 (TMP1), 4.15 (TMP2, TMP3)c

2.25d

3.56e

a From Ref. (9).b Calculated at 6-31+G∗//6-31+G∗ level.c Values for each conformer.d Average value taking into account the conformational population of TM

at room temperature.e Value obtained by extrapolation in the correlation straight line of Fig. 5.

reasonable to think that the TMP will have a dipole momeconsistent with this relationship. Therefore, extrapolatingTMP value to this correlation line, we can consider that a mrealistic value of the TMPµ will be 3.56 D. This means that thepopulation of the conformers TMP2 and TMP3 will be high(TMP1/TMP2/TMP3= 20/75/5) than that considered initiallymaking TMP2 the most stable.

The experimental d(001) interlayer distance of the compledry montmorillonites studied is approximately 0.96 nm incases with the different interlayer cations. In our calculatiowe consider the difference of d(001) spacing between theclay and the clay with the complexes, as the space availablethe complexes (Table 1). For the TMP, DMSO, ACO, and ACcomplexes, this variation of spacing will be 0.65–0.67, 0.40.54, 0.36–0.40, and 0.37–0.39 nm, respectively, except forsamples saturated with Na+ cations, for which smaller spacinvariation values are found.

Taking into account the van der Waals size of the ligands,only disposition for conformer TMP1, which could exist woube that represented in Fig. 6a, where the complex can be foronly when the interlayer cation is localized near of the tetradral sheet. Also, the oxygens of the P–O bonds would havstrong repulsion with the negative charge of the clay surface,ing unlikely this disposition. Another disposition avoiding threpulsion will yield a too high interlayer thickness (0.85 nmHence the conformer TMP1 cannot exist in an interlayer spacmontmorillonite consistent with the increment of the experimetal d(001) spacing (0.65–0.67 nm). Then only the conformTMP2 and TMP3 can be swelled in this interlayer space. Wthese conformers, different orientations can fit into the interlaspace, consistent with the experimental value, depending onlocalization of the interlayer cation, close to tetrahedral sh(Fig. 6b) or in the center of the interlayer space (Fig. 6c). Tlast is the most probable disposition since it has the phosphoxygen oriented to the center of interlayer space and diffeligand–cation ratios are allowed. From these results, we can

clude that only one layer of these molecules can be present inthe interlayer space.

n

i

gt

p

l

gw

ectoreallm-

yer

s-the

ith

na-ionlig-

es,thein-e ofac-erter-ainen-ip

undntria-steds,xes

aree

una,

m-

STRUCTURE OF THE MONT

FIG. 6. Disposition of the solvating molecules in the montmorillonite iterlayer space, (a–c) TMP, (d) ACO, (e) ACN, and (f, g) DMSO.

For the acetone and acetonitrile cases, the only disposconsistent with the increment of the interlayer space (0.36–0and 0.37–0.39 nm, respectively) will be that where the oxyof carbonyl group and the nitrogen of nitrile group are oriento the center of the interlayer space toward the cation for formthe complex (Figs. 6d and 6e). In both cases, only one layerbe present in the interlayer space.

In the case of DMSO, different orientations consistent wthe increment of the interlayer space (0.46–0.54 nm) aresible (Figs. 6f–6g). However, the disposition of Fig. 6g will bthe most probable, where the oxygen of the sulphonyl grouoriented to the center toward the cation and the repulsionween this oxygen and the tetrahedral silicate oxygens wilminimal. Also, the highly positive sulfur atom will have an atractive electrostatic interaction with these negatively charsilicate oxygens. Again only one layer of these moleculesbe present in the complexes formed in the interlayer space oclay.

Going back to the experimental data, we find that the d(0interlayer spacing does not change with the cation nature, e

changing the cation/ligand ratio. This fact shows us that thcomplexes are more or less planar maintaining the same mo

MORILLONITE COMPLEXES 135

-

tion.40enedingwill

ithos-e

p isbet-be

t-edill

f the

01)ven

layer disposition. The higher coordination of the Ca with respto Mg can be explained by the higher ionic radius of Ca and mspace available for a higher number of ligands. Therefore, incases with all cations used and for all ligands studied, the coplexes formed in the interlayer space comprise only one laof ligands.

CONCLUSIONS

Acetone, acetonitrile, dimethylsulfoxide, and trimethylphophate penetrate, in all cases, into the interlayer space ofmontmorillonite at room temperature, forming complexes wthe interlayer cations stable in vacuum. Except for Na+, thed(001) interlayer spacing does not change with the cationture, even changing the cation/ligand ratio. The ligand–catratio depends on the valence of the saturating cation and theand type. The thermal stability results show that all moleculwhich form the complexes, are necessary for maintainingd(001) spacing of the original complexes. The cation–ligandteraction in all complexes studied has an electrostatic naturion-dipole type. The complexes are formed by the direct intertion of the oxygen or nitrogen atom of ligand and the interlaycation. The quantum mechanical approach allowed us to demine the disposition of the ligand in these complexes. The mcalculated geometry features are consistent with the experimtal geometry of the ligands studied. A good linear relationshbetween experimental and calculated dipole moments is foin all ligand molecules, except for TMP. Taking into accouthe van der Waals radii of ligands and the clay spacing vations, possible ligand–cation complex structures are suggein order to fit them in the interlayer space of clay. In all caseonly one layer of ligands can be present in the stable compleinside the interlayer space.

ACKNOWLEDGMENTS

This work was supported by DGICYT Grant PB97-1205. The authorsgrateful to the Centro de C´alculo de la Universidad de Granada for allowing thuse of their computational facilities.

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