Physico-Chemical Characterizations of Tunisian ... - ACS Publications

Organoclays were prepared from two tunisian purified Na-bentonites using ... from two different regions (Berka and Chbika) located in the south of Tun...
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J. Phys. Chem. C 2007, 111, 10869-10877

10869

Physico-Chemical Characterizations of Tunisian Organophilic Bentonites H. Othmani-Assmann,†,‡ M. Benna-Zayani,‡ S. Geiger,† B. Fraisse,§ N. Kbir-Ariguib,‡ M. Trabelsi-Ayadi,‡ N. E. Ghermani,†,§ and J. L. Grossiord*,† Laboratoire d’Applications de la Chimie aux Ressources et Substances Naturelles et a` l’EnVironnement, De´ partement de Chimie, Faculte´ des Sciences de Bizerte, Tunisie, Laboratoire de Physique Pharmaceutique (UMR 8612), Faculte´ de Pharmacie de Chaˆ tenay-Malabry, France, Laboratoire Structure, Proprie´ te´ s et Mode´ lisation des Solides, Ecole Centrale Paris, France ReceiVed: December 21, 2006; In Final Form: March 21, 2007

Organoclays were prepared from two tunisian purified Na-bentonites using benzyltetradecyldimethylammonium chloride (C14) and benzyldodecyldimethylammonium chloride (C12). The added quantities of organic salts, expressed as a function of the clay’s cationic exchange capacity (CEC), have been varied from 0.3 to 4 CEC. Two different methods were investigated in order to determine the influence of the preparation method on the adsorption properties: on one hand, dry powder clay (method I) and, on the other hand, aqueous clay suspensions (method II) were mixed with the organic salt solutions. The adsorption of organic cations on clays was studied by adsorption isotherms, FTIR spectroscopy and X-ray diffraction. It appears that more salts are adsorbed in the clay interlayer space when using method II. The arrangement of salts within clay is rather complicated. It depends on clay composition, nature of tensioactive molecules, CEC of the clay, and preparation method. According to these parameters, the inserted surfactants can be arranged in monolayer, paraffin, or admicelles structures.

I. Introduction The mixed systems “clay-tensioactive” are of great interest for industrial applications. Nowadays, they are used in various fields as thickening agents in inks,1 in the preparation of silver nanoparticles,2 as thickening/suspending/thixotropic additives in cosmetic formulations,3 as sorbents of nonionic organic contaminants (NOCs) from wastewater,4 as rheology controlling agents,5 etc. Their major interest comes from the properties conferred by the intercalation of organic cations in the interlayer space of clay. Replacement of hydrated interlayer cations strongly enhances the hydrophobicity of the clay interlayers and consequently enables clay to swell in nonaqueous systems. The adsorption of alkyl ammonium molecules or cations on clay exhibits different structures depending on the clay cationic exchange capacity (CEC), the size of the alkyl chain, or the method of preparation. These molecules or cations can adopt various arrangements in the interlamellar spacing of clay. Recent studies on the adsorption of 1 CEC equivalent quantity of decyltrimethylammonium bromide on montmorillonite show that the surfactant is arranged in a lateral-monolayer mode within clay interlayers.6 At 1.38 CEC, the benzyldodecyldimethylammonium chloride intercalated in Na-montmorillonite displays a bilayer arrangement parallel to the basal spacing of montmorillonite particles.7 With the increase of the concentration of hexadecyltrimethylammonium (2.2 to 5 CEC) in montmorillonite, the latter adopts a paraffin-type bilayer arrangement.8 The adsorption of cationic surfactant was extensively studied for several types of clay such as montmorillonite,7 saponite,9 kaolinite,10 and mica.11 In contrast, few papers are dedicated to * Corresponding author. Tel: +33(0)146835614. Fax: +33(0)146835882. E-mail: [email protected]. † Faculte ´ des Sciences de Bizerte. ‡ Faculte ´ de Pharmacie de Chaˆtenay-Malabry. § Ecole Centrale Paris.

the adsorption of organic ammonium cations on natural interstratified clays composed of smectite, kaolinite, and illite.7 The mixture of various kinds of clay explains the fact that the adsorption process and the arrangement of organic derivatives are more complex than in the case of only one kind. The aim of the present study is to try to elucidate the mechanism by which tensioactive molecules or cations are intercalated in the interlayer space of such clays, through analysis based on adsorption isotherms, FTIR spectroscopy, and X-ray diffraction. For this purpose, two methods conducted under the same reaction conditions (precursor concentration, reaction time, temperature) are used to prepare the organoclays: the first one consists in mixing dry clay powder with organic salt solutions and the second one uses clay aqueous suspensions. II. Materials II.1. Clays. The studied clays originated from two different regions (Berka and Chbika) located in the south of Tunisia. After sampling, these clays were crushed and sieved at 100 µm. In a first treatment, they were purified by sodium exchange :12 the NaCl solution-clay mixture was stirred for 12 h at 250 rpm by a mechanical stirrer (Edmund Bu¨hler, Labortechnik, Materialtechnik, Jokanna Otto GMBH. KS-15). In order to separate the clay particles from the exchanging solution, the suspension was centrifuged at 3000 rpm for 5 min. The whole process was repeated seven times. Then the washing stage with water was achieved by a centrifugation at 4500 rpm for 30 min. The centrifugal machine was a Sigma 3-15 bioblock scientific. After dialysis in bidistilled water, the obtained clay was dried at 60 °C, crushed, and sieved at 80 µm. The two purified clays are hereafter noted B and CH for Berka and Chbika respectively. Physicochemical techniques showed that they have different structural characteristics: B is an interstratified smectite-illite (25% illite and 72.5% smectite) with a smaller amount of free

10.1021/jp068814a CCC: $37.00 © 2007 American Chemical Society Published on Web 06/30/2007

10870 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Othmani-Assmann et al. zyltetradecyldimethylammonium chloride (C14) were prepared in demineralized water, at a temperature higher than their corresponding Krafft temperatures (TK ) 12 °C for C12 and TK ) 23 °C for C14). The concentrations of the solutions used for determination of adsorption isotherms as well as for organoclays physicochemical characterizations were calculated according to the clay’s CEC. They vary from 0.3 to 4 CEC. The average pH value of the solutions is 6.6 ( 0.2. In order to evaluate the influence of the preparation method on adsorption properties, two methods were used: the first one consists of using clay in the powder form (method I), whereas the second one uses a 10% w/w clay aqueous suspension (method II). Clay (powder or aqueous suspension) was then put in contact with surfactant solution. The obtained mixtures were stirred for 24 h at 250 rpm. Thereafter, the suspensions were centrifuged in polypropylene PPCO tubes at 40000 rpm for 50 min under an average acceleration gj ) 1.17 × 105 m‚s-2. A standard Beckman Coulter Optima LE-80 was then used to separate organoclay from solution. The obtained organoclay was dried at 70 °C for 48 h and crushed using an agate mortar. III. Methods of Characterization

Figure 1. Representation of C12, tensioctive molecule: (a) Longitudinal view of C12; (b) Tansversal view of C12.

TABLE 1: Dimensions of C12 and C14 Tensioactive Molecules

C12 C14

length of surfactant (Lt) (Å)

maximum height (Hmax) (Å)

height of alkyl chain (Å)

minimum height (Hmin) (Å)

20 23

10 10

4.0 4.0

6.0 6.0

kaolinite (2.5%). Its dioctahedral smectite fraction has a pronounced montmorillonitic character;13 CH clay also consists of smectite, illite, and kaolinite but the smectite part has a beidellitic character14 and the proportion of kaolin is higher than in B. Chemical analysis yield the average compositions of these clays: (Si7.649 Al0.351)IV (Al2.600 Fe0.784 Mg0.551 Ti0.044 Mn0.001)VI O20 (OH)4 Na0.618 Ca0.022 K0.254 for B.13 (Si7.149 Al0.850)IV (Al2.86 Fe0.883 Mg0.387 Ti0.034 Mn0.007)VI O20 (OH)4 Na0.285 Ca0.068 K0.18 for CH.14 The cationic exchange capacities (CEC), measured by adsorption of copper ethylene diamine complex,15 are 101.86 and 72.09 meq/100 g of burnt clay for B and CH, respectively. II.2. Surfactants. The benzyltetradecyldimethylammonium chloride and the benzyldodecyldimethylammonium chloride (Figure 1a,b) are the surfactants used here to prepare the organoclays. They are provided from Sigma Aldrich (purity ) 99%). According to the number of carbon atoms in their alkyl chains, these two compounds will be noted C14 and C12 for benzyltetradecyldimethylammonium and benzyldodecyldimethylammonium respectively. The critical micellar concentration (cmc) values obtained from conductimetry measurements are 2.07 × 10-3 mol‚L-1 for C14 and 9.10 × 10-3 mol‚L-1 for C12. Dimensions of C12 and C14 have been determined from crystallographic data in relation to the length of chemical C-C and C-N bonds as well as the C-C-C and C-N-C angles.16 They are listed in Table 1. II.3. Preparation of Organoclays. Aqueous solutions of benzyldodecyldimethylammonium chloride (C12) and ben-

III.1. Adsorption Isotherms. The adsorption isotherms are determined by mixing in centrifuge tubes 0.4000 ( 0.0005 g of clay with 8.00 ( 0.05 mL of surfactant solution at a required concentration (ranging from 0.3 to 4 CEC). The tubes are first stirred and then centrifuged at a temperature of 25 °C which is above the Krafft temperatures of the two surfactants. The supernatants carefully recovered after centrifugation are quantified by UV-vis spectrophotometry (Perkin Elmer UV/vis spectrometer Lamda 11). The absorbance primarily due to the aromatic ring contained in the salt molecule reaches its maximum at a wavelength of λ ) 265 nm. III.2. Infrared Spectroscopy. Infrared spectra are collected on BRUCKER YEWS 66 Fourier Transform infrared spectrometer. OPUS software allows the band intensity to be normalized by the most intense one. Spectra are collected over the spectral range 600-4000 cm-1. Samples consist of pellets of anhydrous KBr mixed with 1% powder clay. III.3. X-ray Measurements. In the present study, in order to explore the full Bragg angular zone, two types of diffractometers are used: 1. A prototype high-resolution diffractometer is required for angles 2θ g 2.5°. It uses a rotating anode X-ray tube (λCuKβ ) 1.392 Å and λCuKβ ) 1.5405 Å) at a voltage of 50 kV and an intensity of 300 mA. The high incident intensity is useful in order to follow the evolution of the smectite, the kaolinite, and the illite fraction in the 2θ g 2.5° zone; the explored angular zone is defined from 2.5° to 13°. The recording of the diffractograms is carried out with steps of 0.06°, during 30 s each. The measured distances d(00l) are determined with TOPAZ software. For this analysis, organoclay is dried at 70 °C, crushed using an agate mortar, and directly put into the sample holder. 2. The second diffractometer scans the small angles. It is equipped with an Enrae Vernier generator at a voltage of 40 kV and an intensity of 20 mA. It uses an X-ray source with linear collimation (λCuKR ) 1.5405 Å) and covers angles 2θ < 8°. Samples have been crushed before being put in glass tubes 1.5 mm in diameter. The time of acquisition is 600 s. IV. Results IV.1. Adsorption Isotherms. In order to compare the two intercalation methods used for organoclays preparation, adsorp-

Characterization of Tunisian Organophilic Bentonites

Figure 2. Adsorption of C12 and C14 surfactants on B versus their concentration (related to the CEC): (a) B/C14 prepared by method II; (b) B/C12 prepared by method II; (c) B/C14 prepared by method I; (d) B/C12 prepared by method I.

Figure 3. Adsorption of C12 and C14 surfactants on CH versus their concentration (related to the CEC): (a) CH/C14 prepared by method II; (b) CH/C12 prepared by method II; (c) CH/C14 prepared by method I; (d) CH/C12 prepared by method I.

tion isotherms for each of them have been determined (Figures 2 and 3). These isotherms show a first region of adsorption which is independent of the preparation method. It corresponds to a linear variation of the molar quantity of adsorbed surfactant (nads) as a function of its initial quantity in solution (ni) expressed by the CEC. The adsorption is total where ni ) nads, and this region spreads until a quantity of clay of 1 and 1.5 CEC respectively for B and CH treated by C12 and of 1.5 and 2 CEC respectively for B and CH treated by C14. Two adsorption mechanisms characterize this concentration domain: up to 1 CEC, the adsorption occurs by ion exchange between surfactant cations and sodium exchangeable cations in stoechiometric proportions.17-20 This exchange is almost total. The Na+ ions expelled from the interlayer spaces are thus found in solution; their positive charge is balanced by the chlorides ions (counterion of ammonium) which are not adsorbed on the surface.7-21 Beyond one CEC, the anionic sites of the clay surfaces are electrically neutralized, and a new adsorption of surfactants is then realized by hydrophobic interactions between the alkyl chains of the resident cations and of the incoming ones;21-23 the binding energy between the alkyl chains is due to van der Waals interactions. At this stage, the adsorption is inevitably molecular, and the counterion (Cl-) helps to reduce the headgroup repulsions.7,11 The particular affinity that the two clays have toward C14 is due to the length of its alkyl chain which is higher than that of C12.23 Indeed, hydrophobic

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10871 attractions between the alkyl chains will be more effective in the case of C14 than in the case of C12. In this concentration domain, surfactant adsorption depends on the number of accessible sites and on the length of the alkyl chain. Above the first concentration domain, the variation of adsorption isotherm behaviors strongly depends on the method of organoclay preparation. Indeed, for the clays prepared by the method I, adsorption reaches 2 and 2.3 CEC respectively for B and CH modified by C12 or C14. This second interval corresponds to the curved part of adsorption isotherm where nads is lower than ni. In addition, these concentrations (2 and 2.3 CEC) indicate the beginning of the adsorption steady state where nads reaches its maximum value. These values depend on the nature of salt as well as of clay. Indeed, the maximum quantities of C12 and C14 adsorbed on B are 5.0 × 10-4 and 5.5 × 10-4 mol respectively for 0.4 g of clay, and are 4.2 × 10-4 and 4.6 × 10-4 mol respectively for 0.4 g of CH. The results show that the quantity of adsorbed salts is higher on B than on CH. This is predictable considering the CEC of B which is more significant than that of CH. The limit of the organic intercalation is probably due to the saturation of the clay’s interlayer space. The adsorption isotherms resulting from method II show a different behavior characterized by the absence of a steady state. The adsorption continues up to the highest investigated values (4 CEC for B and 3.8 CEC for CH), with nads lower than ni. The maximum quantities of adsorbed C12 on B and CH are 7.0 × 10-4 and 5.6 × 10-4 mol, respectively; the maximum quantities of adsorbed C14 on B and CH are 7.7 × 10-4 and 5.7 × 10-4 mol, respectively. In this region, a little change of variation is observed at 2.3 CEC for the modified clay either for C12 or for C14; this point may indicate a change of the adsorption nature. It is worth noting that this was not observed with method I. Method II is thus characterized by the absence of an adsorption steady state and by quantities of adsorbed salt more significant than those in the case of method I. This is probably due to the hydration state of the clay in the suspension. Indeed, in aqueous suspension (method II), clay is well swollen (it is already above the osmotic swelling at the concentration used) and shows a stable hydration state in which the layers are expanded and already open when alkyl ammonium cations penetrate; allowing an easier adsorption, and at the CEC, all the Na+ cations can be replaced by alkyl ammonium cations. Moreover, in a stable aqueous clay suspension there are many accessible external surfaces on which surfactants can be adsorbed. However, when clay powder is added to the alkyl ammonium solution (method I), the swelling of clay occurs simultaneously with the intercalation of alkyl ammonium cations. This leads, probably, to more stacked clay particles and so to fewer accessible external surfaces and consequently to a lack in the adsorption comparing to method II. This hypothesis can explain the adsorption steady state observed with method I and not with method II. IV.2. Infrared Spectroscopy. Infrared spectroscopy gives a qualitative idea on the adsorption process; the bands were normalized with respect to the most intense one corresponding to the stretching mode of Si-O-Si at 1030 cm-1. In addition, it provides some accurate information about the conformational changes of alkyl chains within interlayer space. Figures 4 and 5 show examples of FTIR spectra of the organoclays recorded in the spectral range of 2700-3100 cm-1. They correspond to CH/C14 organoclays prepared by methods I and II, respectively. The intensities of the two intense

10872 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Figure 4. FTIR spectra of CH/C14 prepared by method I (27003100 cm-1).

Figure 5. FTIR spectra of CH/C14 prepared by method II (27003100 cm-1).

absorption bands at 2925 and 2854 cm-1, corresponding to the antisymmetric and the symmetric -CH2 stretching mode, respectively, gradually raise with the increase of the concentration of surfactant within the interlayer space. Their intensities strongly depend on the density of the alkyl chains within the interlayer space.24 Indeed, for the clays prepared by method I, these intensities remain constant for the high concentrations of surfactant, indicating the existence of an adsorption steady-state level. The limit of adsorption is 2 CEC for B/C12 and B/C14 and 2.3 CEC for CH/C12 and CH/C14, respectively. On the other hand, in the case of clays prepared by method II, the proportional increase of these intensities as a function of the quantity of added surfactant indicates the absence of adsorption steady-state level. These results are in perfect agreement with those of the adsorption isotherms obtained by UV-visible spectrophotometry. The wave numbers corresponding to -CH2 stretching absorption bands of alkyl chain are extremely sensitive to the conformationnal changes of the alkyl chains in the interlayer space.25-27 For pure salts C12 and C14, these bands appear at 2852 and 2923 cm-1 indicating that the alkyl chains adopt an essentially all-trans conformation. However, the appearance of left-handed conformations shifts these wave numbers to higher values, according to the quantity of disordered chains. In our study, at relatively low concentrations (0.5 CEC), these wave numbers appear at 2863 and 2935 cm-1 which proves the

Othmani-Assmann et al.

Figure 6. FTIR spectra of CH/C14 prepared by method II (6503800 cm-1).

existence of left-handed conformation in the interlayer space; these wave numbers shift to 2857 and 2929 cm-1 for the highest concentrations but never reach the values found in pure C12 or C14. Different results were obtained in previous studies when, for high concentrations of hexadecylamine on montmorillonite, these wave numbers were equal to those found in the pure amine.25 The infrared absorption bands relative to the scissoring modes of -CH2 show triplet bands at 1454, 1471, and 1479 cm-1 (in pure C12 and C14). This feature is due to the intermolecular interactions between two adjacent hydrocarbon chains28,29 and to the increase of the number of alkyl chains in all-trans conformations.30 In the case of our organoclays (Figure 6 gives an example of CH modified by C14 according to the second preparation method), the triplet band is not well-defined for low concentrations of surfactant. This indicates a disordered structure where hydrocarbon chains are poorly linked together within the silicate interlayers25 and freely rotate around their longitudinal axis.30 They do not have the same three-dimensional arrangement as the pure surfactant. At this level, alkyl chains mainly show left-handed conformations. The appearance of well-solved bands is, however, observed above 1 CEC. IV.3. X-ray Diffraction. The diffractograms, resulting from the two apparatuses previously described, allow a complete and accurate study of the clay interlayer evolution after its modification by different amounts of organic salts. The detailed X-ray diffraction study will focus on organoclays prepared by method II which is considered, according to the results derived from the adsorption isotherms, to be the most favorable for intercalation of tensioactive molecules. Figures 7-10 show the variation of the basal interlayer distance d(00l) according to the initial amount of surfactant (expressed in CEC of the clay). Considering the various arrangements that surfactant can adopt in the interlayer space, the different possible clay structures are referred by the following numbers: curves 3-5 indicate the interlayer distance evolution of the smectite fraction, curve 1 relates to the kaolinite fraction, and we will discuss the nature of the clay’s structure on curve 2. IV.3.1. Intercalation in the Smectite Part. Curve 4 shows the swelling of the smectite part following the intercalation of surfactants: • At low concentrations (0.3 CEC), the basal spacing varies from 12.5 Å for sodium purified clay to 14.3 Å for B treated by C12 or C14 and to 13.3 Å and 16.6 Å for CH treated by C12 and C14, respectively. The obtaining of a broad peak in

Characterization of Tunisian Organophilic Bentonites

Figure 7. Change of d(001) for B/C12, prepared by method II as function of C12 concentration (related to the CEC).

Figure 8. Change of d(001) for CH/C12, prepared by method II as function of C12 concentration (related to the CEC).

Figure 9. Change of d(001) for B/C14, prepared by method II as function of C14 concentration (related to the CEC).

the corresponding diffractograms (Figure 11) indicates a nonhomogeneous structure, where surfactants occupy only few clay interlayers. • Between 0.5 and 0.7 CEC, the corresponding basal spacing vary between 17.5 and 19.4 Å indicating a more significant intercalation of surfactant; • At 1 CEC, the clays modified by C12 and C14 do not exhibit the same extension characteristics. Indeed, in the case of B/C12 and CH/C12, the basal distance is equal to 18.2 and 19.5 Å, respectively, whereas the intercalation of C14 at 1 CEC leads to an interlayer spacing rise corresponding to 44.3 Å for B and 24.3 Å for CH. The increase in inserted surfactant

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10873

Figure 10. Change of d(001) for CH/C14, prepared by method II as function of C14 concentration (related to the CEC).

Figure 11. X-ray diffraction patterns of de B/C12 at 0.3CEC.

TABLE 2: Values of d(001) in Å at the Steady-state Level organoclay

B/C12

B/C14

CH/C12

CH/C14

d(001)

41-45

43-45

46

46

quantity induces the expansion of interlayer spacing. This may be due to a change of arrangement of C14 in the interlayer space which is more marked for B than for CH because of its relatively high CEC value. • Beyond 1 CEC, the curves (4), representing d(001) as a function of the CEC, reveal a limit of the interlayer spacing of the clay after intercalation of surfactant. This limit begins at 1.5 CEC for B/C12, at 1 CEC for B/C14, and at 2 CEC for CH/C12 or C14. The values of d(001) at this steady-state level are reported in Table 2. It is important to note the absence of any smectite peak on the diffractograms of CH modified at 1.5 CEC. This indicates an order-disorder transition for which the structure of the smectite is probably very disordered. Curve 3 appears when curve 4 reaches the steady-state level. They indicate another possible arrangement of surfactant molecules in the interlayer space of smectite. Indeed, the appearance of a peak around 17 Å on the diffractograms, beginning at a surfactant amount of 1.5 CEC for B and 2 CEC for CH, cannot be attributed to a second order (d(002)) diffraction peak of the first structure. It does characterize a new structure independent of the nature of both clay and salt. This peak was observed on the diffractograms collected on the two apparatuses, which confirms its reliability. Curve 5 indicates a particular reorganization of surfactants within interlayer space. On the diffractogram, it corresponds to a small narrow peak which appears at 2.3, 2.5, and 2.7 CEC

10874 J. Phys. Chem. C, Vol. 111, No. 29, 2007

Figure 12. Small-angle X-ray diffraction patterns of B/C14 at 2.7CEC.

Figure 13. X-ray diffraction patterns of B/C12 (7-13°).

for B and at 2.3, 3.1, and 3.8 CEC for CH clay; it is characterized by a same value of d(001) equal to 20.5 Å. Figure 12 gives an example of diffractogram corresponding to B treated by C14 at 2.7 CEC. IV.3.2. Other Clay Components. The diffractograms measured between 7° and 13°, represented by curves 1 and 2, permit us to follow the kaolinite part, and probably the illite part too, of the studied organoclays. The results show that, in addition to the characteristic peak of the pure kaolin at 7.2 Å (curve 1) which appears on all of the diffractograms, a not well-defined peak appears around 9.9 Å at 0.7 CEC for B and at 1 CEC for CH. This peak moves toward 10.5 Å for the high concentrations where it becomes more defined (curve 2). For example, Figure 13 gives the diffractograms corresponding to B treated by C12. This peak cannot be attributed to the kaolinite fraction since (i) its characteristic peak persists, (ii) kaolinite does not present a basal negative charge allowing the adsorption of cations, and (iii) it is known that intercalation of organic matters on kaolinite surfaces needs a preliminary surface treatment of the clay. V. Discussion As mentioned above, studied tunisian clays consist of interstratified smectite-illite with a small quantity of kaolinite. These different parts do not show the same behavior during the surfactant intercalation. Consequently, we will divide the discussion into two parts: the first one relates to the swelling part of the clay (smectite), and the second one to its nonswelling part (kaolinite and illite). V.1. Swelling Part (Smectite). Adsorption mechanisms of tensioactives on swelling part as well as the possible arrange-

Othmani-Assmann et al. ments, which the alkyl chains can adopt in the interlayer space, will be discussed as function of surfactant’s quantity, expressed by the CEC : V.1.1. ni < 1 CEC. This interval of concentrations corresponds to a total adsorption of the added surfactants, whose mechanism is conducted essentially by electrostatic interactions between the organic cations and the negative interlayer sites. The anhydrous smectite sheet thickness is 9.5 Å, and the surfactant minimum height is 6 Å (the maximum is 10 Å). An increase of 6 Å (10 Å) in the basal spacing implies a lateral monolayer of surfactant with its alkyl chains parallel to the silicate sheets (Figure 14a). For very low concentrations (0.3 CEC), only 30% of the cationic exchange of sodium cations by organic cations is reached. Thus, because the smectite sheets are flexible, the basal distances must vary between 12.5 Å (with monolayer hydrated Na+ cations) and 16 Å or little higher (for monolayer of surfactant). This explains the obtained values of 14.3, 13.3, and 16.6 Å (curve 4). However, for the highest concentrations (0.5 and 0.7 CEC), the rise in the d spacing of approximately 9 Å (curve 4) supposes two possible monolayer arrangements: in the first one, surfactants are laying flat with alkyl chain parallel to the silicate sheet24,31,32 in their maximum height (Figure 14a), whereas in the second one, they are rather laying nearly flat with alkyl chains slightly tilted with respect to the silicate sheet25 (Figure 14b). The FTIR spectra recorded on this interval of concentrations (Figure 6) show that alkyl chains adopt a lefthanded conformation leading to the more probable second arrangement. Indeed, for covering rates lower than one CEC, the surfactants lay separately, parallel to the clay surface, so hydrophobic attractions between alkyl chains are weak. Moreover, the hydrocarbon tails have two different kinds of interaction with the clay surface: van der Waals attractions between carbon atoms of alkyl chain and siloxane group of the clay surface and hydrophobic-hydrophilic repulsions. The presence of a small amount of water adsorbed on the clay surfaces makes the repulsive interactions stronger, thus dominating the hydrocarbon-clay surface interactions.25 These two phenomena induce the mobility of the alkyl chains into the interlayer space, resulting in a high concentration of left-handed conformer and leading to their tilt with respect to the clay layers. In this interval of concentrations, only part of Na+ is exchanged by organic cations, which gives two possibilities of interlayer space: separate organic and inorganic (Na+) interlayers,33,34 on one hand, or layers where organic and inorganic cations coexist32,35 and whose basal spacing is governed by monolayer coverage of organic cations, on the other hand. In our study, the disappearance of the peak at 12.5 Å (Figure 11) corresponding to sodic clay and the existence of a unique broad peak which characterizes the structure of prepared organoclay show that both organic and inorganic cations coexist in the same interlayer space. V.1.2. ni ) 1 CEC. The clays treated by C12 show a basal spacing of about 18.2-19.5 Å (curve 4); the adsorption isotherms always correspond to ni ) nads. In spite of the increase in the quantity of adsorbed salt, there was no change in the arrangement of the organic cations within the interlayer space but simply a high intercalation level into the accessible sites. Surfactants always lay in the form of monolayers tilted with respect to silicate sheets; which is not the case for C14. The increase in basal spacing to 35 Å, observed in the case of B treated by C14 (curve 4), shows the beginning of paraffin-type arrangement: the cations start to arrange in two layers whose alkyl chains are linked by hydrophobic attractions and tilted

Characterization of Tunisian Organophilic Bentonites

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10875

Figure 14. Schematic structures of clay/tensioactive systems. Silicate sheets are indicated in rectangles. Dark spheres and broken lines correspond to the headgroup and alkyl chain of tensioactive molecules: (a) Monolayer arrangements with alkyl chain parallel to the silicate sheets; (b) monolayer arrangements with alkyl chain tilted with respect to the silicate sheets; (c) beginning of paraffin arrangement; (d) paraffin arrangement; (e) selfassociation of alkyl chains leading to admicelle structures.

with respect to clay interlayers (Figure 14c). Such a structure was observed in the study of Yui et al. on saponite treated by polyfluorinated surfactants whose perfluoroalkyl group contains 3 carbons.22 In the case of CH treated by C14, the increase to 15 Å in basal spacing (curve 4) indicates a more significant tilt of the alkyl chains within the interlayer space. The rearrangement observed during the treatment by C14, is inevitably due to the length of the alkyl chain which is relatively important. Indeed, to allow a higher adsorption by preventing hydrophobic attractions, the alkyl chains increase their slope compared to basal surfaces clay sheets in order to leave free space for other incoming surfactant molecules. The different arrangements adopted by the surfactant in the two clays (paraffin for B, and sheet-tilted monolayer for CH) primarily come from the nature of clay: the CEC of B (101.86 meq/100 g) is more significant than the CEC of CH (72.09 meq/ 100 g), so at 1 CEC the number of surfactant molecules placed in the interlayer space of B is higher than in CH. Consequently, chains are compelled to adopt a more complex arrangement in B than a simple tilt in CH. V.1.3. ni > 1 CEC. The displacement of the wave numbers, in FTIR spectra, corresponding to the antisymmetric and symmetric -CH2 stretching mode from 2863 and 2935 cm-1 (at 0.5 CEC) to 2857 and 2929 cm-1 (up to 1CEC) indicates an increase of the number of all-trans conformations. Indeed, the hydrophobic interactions between alkyl chains of resident cations and incoming ones limit their movements and lead to the installation of an order, which increases the all-trans conformations. Even for high concentrations, these vibrations never reach the values corresponding to the three-dimensional

order that they had in pure surfactant. This point suggests that left-handed conformations remain. These results are confirmed by the X-ray diffraction analysis since no peak corresponding to the pure surfactant appears on the diffractograms of organoclays. Curve 4, related to the intercalated smectite fraction, shows a limit of basal space. The beginning of this limit depends on the nature of the clay and on the number of carbons in the alkyl chain. However, when method II is used, the adsorption isotherms (Figures 2 and 3) show a continuous adsorption up to 4 CEC for B and 3.8 CEC for CH. This can be explained in terms of the hydration state of initial clay particles in aqueous suspension when intercalation occurs. Indeed, more adsorption can occur on external surfaces like on the edges of particles. These adsorptions do not change the values of observed basal distances. Thus, because external surfaces and exposed edges of the well dispersed particles prepared by method II are much higher than those accessible by method I, the adsorption isotherms determined by method I reach a plateau, whereas those by method II show a continuous adsorption at least until the investigated quantities of salts. The values of d(001) at the steady-state level, mentioned in Table 2, show an increase in the basal spacing of 31-35, 3436, and 36.5 Å for B/C12, B/C14, CH/C12, and CH/C14, respectively (curve 4). Surfactant molecules inserted in the interlayer space adopt a paraffin arrangement directed by hydrophobic attractions between alkyl chains; this conformation tends to minimize their contact with water. The adsorption of counterion decreases the repulsions between surfactant charged

10876 J. Phys. Chem. C, Vol. 111, No. 29, 2007 heads, and leads to a higher adsorption until the saturation of interlayer space (Figure 14-d) is reached. A particular rearrangement of surfactant is observed only in the case of clays treated by C14 and prepared by the second method; it corresponds to an increase basal spacing to 11 Å (curves 5). This rearrangement is detected for quantities of C14 equivalent to 2.3, 2.5, and 2.7 CEC for B and 2.3, 3.1, and 3.8 CEC for CH. Only the high-resolution diffractometer with the intense source allows the detection of a narrow peak corresponding to d(001) equal to 20.5 Å. This peak is attributed to a probable admicelle arrangement of the surfactant in the interlayer space (Figure 14e). Indeed, organoclays were prepared at a temperature higher than their TK preventing from any crystallization of the adsorbed surfactant. This was also confirmed by FTIR spectra which show that even with significant quantities of salt, the symmetrical and asymmetrical valence vibrations of -CH2 do not coincide with those of crystalline salt. Consequently, the 20.5 Å distance corresponds probably to a new reorganization of adsorbed surfactants in the form of admicelles. The formation of admicelles occurs preferentially on external surfaces which are exposed to water36-38 without excluding the possibility that they can be formed in interlayer space. Indeed, the admicelles adsorbed on external surfaces cannot generate any distance d(hkl) and they are thus the interlayer admicelles which are probably at the origin of the observed basal spacing of 20.5 Å. Moreover, Mishael et al. have observed a basal spacing of 20.7 Å only when they carry out the intercalation in a dialysis membrane which ensures the elimination of the monomers and only allows the adsorption of micelles.39,40 Beside the existence of monolayer, paraffin, and admicelle structures in the interlayer space of the smectite part, another surfactant arrangement appears at a concentration equivalent to 1.5 CEC for B and 2 CEC for CH; it is characterized by an increment in the basal spacing of 7 Å. This structure (curve 3) does not depend on the nature of salt or clay, but it corresponds to a monolayer arrangement with laying chains in the interlayer space (Figure 14a); those are probably in all-trans position as indicated by FTIR analysis. V.2. Nonswelling Part (Kaolinite and Illite). Curve 2 shows a basal spacing of about 10 Å which can be attributed to either an intercalation of the alkyl chain in the interlayer space of kaolinite layers or diffraction peaks of the illite part as a consequence of delamination of smectitic fraction. Kaolinite is a 1:1 phyllosilicate, formed after superposition of tetrahedral and octahedral layers. These layers are linked together by hydrogen bonds between oxygen atoms of the tetrahedral layer and hydrogen atoms of the nearest-neighbor octahedral layer. The multiple hydrogen bonds between two successive layers lead to a strong binding energy which leaves an almost closed interlayer space.41 Consequently, a limited number of polar species such as N-methylformamide (NMF) or dimethylsulfoxide (DMSO) can be directly inserted. Intercalation reactions in kaolinite are followed by displacement of the already inserted elements. Several intermediaries such as kaolinite-methanol,10 kaolinite-NMF,42 kaolinite-acetate ammonium,43 or kaolinite-DMSO2 were used to prepare a modified kaolinite. This difficultly enables the direct intercalation of molecules of significant size such as C12 or C14 in the kaolinite part of the clays. Moreover, the persistence of the characteristic peak of pure kaolinite at 7.2 Å (curve 1) on all diffractograms of the prepared organoclays reinforces the assumption of the attribution of the 10 Å peak to the illite part contained in the interstratified initial clays. This result is

Othmani-Assmann et al. interesting since it can be used to identify the presence of illite in interstratified smectite-illite clay. VI. Conclusion The intercalation of surfactants in the smectite part of clays leads to several possible arrangements. In our study, these arrangements depend on different parameters such as the number of carbons in the alkyl chain, the amount of intercalated salt, the CEC of clay, and the method of preparation. As long as the quantity of salt put in contact with clay is lower than the CEC of the clay, the adsorption of organocations is the total and increases with the initial concentration of salt. This point indicates that the adsorption mechanism is mainly governed by electrostatic attractions (cation exchange). Surfactants adopt monolayer, parallel, or tilted type configurations in the clay interlayer space. Beyond the clay CEC, adsorption becomes partial as of 1, 1.5, 1.5, and 2 CEC for respectively B/C12, B/C14, CH/C12, and CH/C14 and the mechanism is linked to hydrophobic interactions; inserted surfactants within interlayer space adopt a paraffin arrangement directed by hydrophobic attractions between alkyl chains. Moreover, organoclays obtained by method II (well-swelled suspension) and modified with C14 (the longest alkyl chain) exhibit a probable admicelle configuration into the interlayer space. Beyond 2.3 CEC, it appears that method II allows a higher adsorption of salt. Surfactant arrangement depends on several parameters whos existence explains the complexity of the study of clay structure. Finally, kaolinite and illite fractions are not concerned with the intercalation of alkyl ammoniums even if the original purified clay is an interstratified smectite illite. References and Notes (1) Wasilewski, O.; Durand, J. R.; Krishnan, R.; Do, T. N. U.S. Patent 5,372,635, 1994. (2) Patakfalvi, R.; De´ka´ny, I. J. Appl. Clay Sci. 2004, 25, 149-159. (3) Clarke, M. T. New York. NY. U.S.A. 1993, 55-152. (4) Koh, S. M.; Dixon, J. B. J. Appl. Clay Sci. 2001, 18, 111-122. (5) Jones, T. R. Clay Miner. 1983, 18, 399-410. (6) Baldassari, S.; Komarneni, S.; Mariani, E.; Villa, C. J. App. Clay Sci. 2006, 31, 134-141. (7) Tahani, A.; Karroua, M.; Van Damme, H.; Levitz, P.; Bergaya, F. J. Colloid Interface Sci. 1999, 216, 242-249. (8) Zhu, J. X.; He, H. P.; Guo, J. G.; Yang, D.; Xie, X. D. Chin. Sci. Bull. 2003, 48, 368-372. (9) Ogawa, M.; Wada, T.; Kuroda, K. Langmuir 1995, 11, 4598-4600. (10) Komori, Y.; Sugahara, Y.; Kuroda, K. J. Appl. Clay Sci. 1999, 15, 241-252. (11) Chen, Y. L.; Chen, S.; Frank, C.; Israelachvili, J. J. Colloid Interface Sci. 1992, 153, 244-265. (12) Bergaya, F. Ph.D. Thesis, Universite´ d’Orle´ans: Orle´ans, France, 1978. (13) Benna-Zayani, M. Ph.D. thesis, Faculte´ des sciences de Bizerte: Bizerte, Tunisia, 2001. (14) Othmani-Assmann, H. thesis work. Faculte´ de pharmacie: Chaˆtenay-Malabry, France, 2004. (15) Bergaya, F.; Wayer, M. J. Appl. Clay Sci. 1997, 12, 275-280. (16) Rodier, N.; Dugue´, J.; Ce´olin, R.; Baziard-Mouysset, G.; Stigliani, J. L.; Payard, M. Acta Crystallogr. 1995, C51, 954-956. (17) Trompette, J. L.; Zajac, J.; Keh, E.; Partyka, S. Langmuir 1994, 10, 812-818. (18) Xu, S.; Boyd, S. A. Langmuir 1995, 11, 2508-2514. (19) Gherardi, B. Ph.D. thesis, Universite´ d’Orle´ans: Orle´ans, France, 1998. (20) Klapyta, Z.; Fugita, T.; Iyi, N. J. Appl. Clay Sci. 2001, 19, 5-10. (21) Patzko, A.; Dekany, I. Colloids Surf. 1993, 71, 299-307. (22) Yui, T.; Yoshida, H.; Tachibana, H.; Tryk, D. A.; Inoue, H. Langmuir 2002, 18, 891-896. (23) Kwolek, T.; Hodorowicz, M.; Stadnicka, K.; Czapkiewicz, J. J. Colloid Interface Sci. 2003, 264, 14-19. (24) Peker, S.; Yapar, S.; Bes¸ u¨n, N. Colloids Surf. 1995, 104, 249257. (25) Li, Y. Q.; Ishida, H. Langmuir 2003, 19, 2479-2484.

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