borane nanocomposite

Applied Clay Science 118 (2015) 295–300

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A new luminescent montmorillonite/borane nanocomposite Zdeňka Kolská a,⁎, Jindřich Matoušek a, Pavla Čapková a, Jakub Braborec a,b, Monika Benkocká a, Hana Černá a, Michael G.S. Londesborough b a b

Faculty of Science, J. E. Purkyně University in Usti nad Labem, České Mládeže 8, 40096 Usti nad Labem, Czech Republic Institute of Inorganic Chemistry of the AS CR, v.v.i., 250 68, Husinec-Řež, Czech Republic

a r t i c l e

i n f o

Article history: Received 27 March 2015 Received in revised form 7 October 2015 Accepted 8 October 2015 Available online 21 October 2015 Keywords: Luminophore Montmorillonite/borane nanocomposite X-ray photoelectron spectroscopy X-ray diffraction UV–Vis spectra Electrokinetic analysis

a b s t r a c t Herein we present the first luminescent montmorillonite/borane nanocomposite materials, formed by the modification of montmorillonite phyllosilicate matrices with cysteamine and the highly fluorescent boron hydride (borane) compound anti-B18H22. Immobilization of anti-B18H22 in small quantities into the montmorillonite matrix leads to materials with a luminescence that is dependent on the borane concentration. At the lower concentration exceeds even the fluorescence intensity of the original highly fluorescent borane compound. The use of layer silicate – a cheap and naturally-available mineral – as a carrier of the borane, which, on the contrary, does not occur naturally, reduces the amount of expensive borane required for useful luminescence in the solid-state and thus provides a less expensive luminophore. Is the first usage of a blue borane laser compound for nanocomposite with montmorillonite. For purposes of comparison we have also studied a bentonite/borane composite. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Due to their unique structure and crystallochemical properties, phyllosilicates are very useful matrices for the preparation of various functional nanostructures via their intercalation and/or surface modification (for a detailed review see, e.g.: Čapková et al., 2003; Lagaly, 1986; Ogawa and Kuroda (1995); Schoonheydt, 2003). Such structural modifications of phyllosilicates by organic and/or inorganic species are usually affected either by thermal and/or chemical treatment (Matoušek et al., 2015). Intercalation and surface-anchoring of organic dyes into and/or onto phyllosilicates are a good means to tune the optical properties of these dyes, especially their photoluminescence, by the selection of a suitable phyllosilicate with appropriate silicate layer charge (Čapková et al., 2004; Čeklovský et al., 2009; Martynková et al., 2007; Klika et al., 2011). Such studies have revealed that the magnitude of the layer charge, and its distribution, are important parameters for determination of the structure and properties of phyllosilicate/organic dye nanocomposite materials. For example, (Klika et al., 2011), achieved a dramatic increase in the fluorescence intensity of methylene blue (MB) saturated montmorillonite (Mt) nanocomposites by reducing the Mt layer charge through the intercalation of lithium ions that subsequently limited the MB load in the matrix (Klika et al., 2009). Of these layered silicates, montmorillonites are especially convenient matrices for the preparation of nanocomposites that have a wide range of ⁎ Corresponding author at: Materials Centre of Ústí nad Labem, Faculty of Science, J. E. Purkyně University, České mládeže 8, 40096 Ústí nad Labem, Czech Republic. E-mail address: [email protected] (Z. Kolská).

http://dx.doi.org/10.1016/j.clay.2015.10.009 0169-1317/© 2015 Elsevier B.V. All rights reserved.

applications such as, for example, adsorbents, catalysts, photocatalysts, drug carriers, luminophores, antibacterial nanocomposites, etc. (Ray and Okamoto, 2003; Liu and Zhang, 2007). The particular suitability of Mt as a guest matrix for the intercalation of organic dyes is due to the facile isomorphous replacement of cations (Al → Mg) that occurs predominantly in the octahedral sheet. Consequently the intercalation of Mt is easily compared to other phyllosilicates, for example, vermiculite, that have mainly tetrahedral substitutions (Si → Al) (Tokarský et al., 2013). One of the advantages of clay mineral based nanocomposites is the low cost of the clay mineral matrix that can accommodate various guest molecules, which keep their functions even at lower concentration then in their original state. In case of clay minerals the guest concentrations and their arrangement is ruled by the magnitude and distribution of the silicate layer charge (Klika et al., 2007; Klika et al., 2009). For intercalation and guest arrangement it is then very important to know whether the isomorphous substitution is in the octahedral or in the tetrahedral sheet. We have recently become interested in the luminescence and photophysical properties of the boron hydride (borane), anti-B18H22 (Londesborough et al., 2012) and its derivatives (Sauri et al., 2013). Anti-B18H22 is a highly fluorescent material; its solutions emitting blue light on excitation with UV irradiation with a quantum yield close to unity (Londesborough et al., 2012). This fluorescence has been shown to withstand the stringent excitation pumping conditions required to achieve laser light emission, making anti-B18H22 the first boron hydride laser and, indeed, the first molecular inorganic laser material (Cerdán et al., 2015). The quasi-aromatic polyhedral structures that the boron

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hydrides form give compounds such as anti-B18H22 large potential for novel properties. In the case of the laser emission from anti-B18H22, this is manifested in its exceptionally high photostability compared to commercially available blue-emitting laser materials (Cerdán et al., 2015). In this study we combined montmorillonite and anti-B18H22 to develop a new fluorescent composite. According to the previous experience and to the generally known fact the layer charge of silicates located in tetrahedral sheet of silicate layer very complicates the intercalation of large guest molecules. That is the reason why we have chosen the montmorillonite (Mt) with octahedral replacements as host matrix (Klika et al., 2007; Klika et al., 2009; Klika et al., 2011) which plays the most important role for the intercalation behavior and guest arrangement. In addition the composition of silicates affects a charge and the charge subsequently influences the concentration of guest molecules. The higher concentration of guest molecules leads to and aggregation and subsequent quenching of fluorescence (Klika et al., 2007; Klika et al., 2009). Therefore we have also studied the dependence of borane concentration and we confirmed the above mentioned relation between concentration and luminescence intensity. Consequently, it became our intention to combine the advantageous properties of Mt and anti-B18H22 (see Fig. 1 for graphical depictions) and prepare a novel thermally and mechanically stable Mt/borane luminescent nanocomposite material. Here the primary objective is to investigate whether the luminescent properties of anti-B18H22 are retained after introduction to the Mt matrix (and, for comparison, into bentonite as well), a goal that would provide a low-cost solid-state medium for the attractive photophysical properties of the borane. Hence, this work describes the preparation of such materials and their characterization by UV–Vis spectra, X-ray photoelectron microscopy (XPS), X-ray diffraction (XRD) and electrokinetic analysis. 2. Materials and methods 2.1. Material Natural Na-montmorillonite (Mt) with the basal spacing d = 12.5 Å and crystallochemical formula: Na0.25 K0.07 Ca0.10 (Si4.0) (Al1.45 Fe3+0.21 Mg0.24 Ti0.01) O10 (OH)2 was used for the present experiments. Fig. 1 (right) presents the fragment of Mt structure, where Si – tetrahedra share oxygens with Al(Mg) – octahedra and Al → Mg isomorphous replacements cause the negative layer charge, compensated with interlayer cations. Also bentonite, the basal spacing also d = 12.5 Å

(composition: SiO2, 55–57%, Al2O3, 15.7–17.3%, Fe2O3, 0.1–1%, TiO2, 3.8–6.3%, MnO, 0.1–0.3%, Na2O, 0.1–0.4%, K2O, 0.3–1.2%, Li2O, 0.1%, P2O5, 0.1% and H2O, 5.3–6.3%) was used for comparison. The chemical composition of Mt was determined by XRF (X-ray fluorescence analysis) and crystallochemical formula was calculated using method of Köster (1977). 2.2. Borane compounds preparation Octadecaborane(22), anti-B18H22 (see Fig. 1, left) was synthesized by the hydrolysis of the hydronium ion salt of [B20H18]2 according to the method described in the literature (Pitochelli and Hawthorne, 1962). Pure samples were obtained by repeated crystallizations from hot saturated n-hexane solutions and had 1H and 11B NMR and mass spectroscopic characteristics in agreement with the published data (Pitochelli and Hawthorne, 1962; Li and Sneddon, 2005; Plešek et al., 1967), experimental solution was characterized previously (Londesborough et al., 2010). 2.3. Montmorillonite and bentonite modification 2-Aminoethanethiol, (Cysteamine, NH2(CH2)2SH, Sigma Aldrich, purity N 98%, CYS) was used for the first step of Mt intercalation and surface modification to obtain the amino-groups into/onto Mt, which facilitated the effective anchorage of borane compounds to the Mt (the second step of Mt modification). 500 mg of CYS was dissolved in 10 ml of distilled water and added to the vigorously stirred Mt (1.0 g, pH = 6.5). This mixture was subsequently stirred for 5 h, after which the mixture was filtered, washed three times with 10 ml of distilled water and the solid portion dried in air at 50 °C to a constant weight. To this powder 15 ml of freshly distilled dichloroethane was added and stirred. The dependence of borane compound as the guest molecule was tested. Solid borane, anti-B18H22, of three different amount, (i) 0.2, (ii) 0.6 and (iii) 1.1 mmol, was added into this mixture and the reaction mixture was stirred for 2 h, after which the mixture was filtered and washed three times with 10 ml of dichloromethane. Samples were dried to the constant weight under vacuum evaporation at 35 °C. The bentonite was modified in the same way using only 0.3 mmol of anti-B18H22. 2.4. Characterization methods The relative atoms concentration of all samples was determined by X-ray photoelectron spectroscopy (XPS). Omicron Nanotechnology

Fig. 1. The molecular structure of anti-B18H22 (left) as determined by single-crystal X-ray analysis (Simpson and Lipscomb, 1963), and a schematic of the layered structure of montmorillonite (right).

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ESCAProbe spectrometer (Omicron Nanotechnology GmbH, Germany) was used to measure photoelectron spectra. Samples were measured after preparation (as prepared). Additionally, all samples were measured after a period of 3 months without significant changes. Fluorescence optical emission spectra of the unmodified Mt (or bentonite) powder, intercalated by CYS and anchoraged with borane compound, anti-B18H22 (of all tested concentrations) were measured using a fluorospectrometer Fluoromax-4 (Horiba). The powder was placed into a sample holder and was covered by a fused silica glass slide (FSM5021 from UQG Optics) which has a very high transmission in the range from 180 nm to 2000 nm and it is fluorescence free. The sample was placed tilted 30° to the incident beam, which is the position recommended by the manufacturer of the fluorospectrometer. The emission spectra were measured for various excitation wavelengths ranging from 350 nm to 440 nm. Each sample needs different excitation to achieve optimal (most intense) emission but in all cases the optimum was close to the 370 nm, therefore this excitation wavelength was chosen for the comparison of individual samples. Especially, at this wavelength the pure borane anti-B18H22 exhibits the maximum fluorescence peak and we would like to compare our new composite with this. More detail characterization of this anti-B18H22 was presented earlier (Cerdán et al., 2015; Londesborough et al., 2012). X-ray powder diffraction (XRD) analysis was carried out using a Philips X-pert powder diffractometer in a Bragg-Brentano arrangement with radiation CuKb αN (λ = 1.5418 Å). The XRD diffractograms were measured with X-ray tube set to 30 kV and 40 mA, the slit setting was 1/8 and 1/4 for antiscatter slit. Small amount of the analyzed powder was placed on the amorphous Si sample holder (has no background) and the excessive powder was removed prior to the measurement. Electrokinetic analysis was carried out using the dynamic light scattering (DLS) method. DLS is an analytical method commonly used for the determination of the zeta potential of the particles dispersed in a liquid medium — a key indicator of the stability of colloidal dispersions. The basis of this method is to measure the fluctuation of the intensity of scattered light from a light source. The analysis was performed with a Zetasizer Ver. 6.32, Malvern software was used. As a source of light a laser of wavelength 366 nm was applied. 5 mg of all samples were dispersed in 4 ml of KCl water solution, concentration of 0.01 mol/dm3. The zeta potential was measured three times for all samples at temperature 25 °C and at constant pH = 6.2. Samples were measured 2 days after preparation and again after a period of 3 months without any changes in the chemistry and surface charge, confirming the high stability of the system.

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3. Results and discussion 3.1. Intercalation and anchorage of cysteamine and borane onto Mt Initial attempts to prepare Mt/borane luminophores without any pretreatment, were not successful. This is reasonably due to the slight negative surface charge of Mt that results in the deprotonation of acidic anti-B18H22 (pKa = 2.68) and the generation of anionic [anti-B18H21]−, which is known not to be fluorescent (Sauri et al., 2013). Consequently, prior to the anchorage of the borane, we endeavored to modify the Mt with cysteamine (CYS), the amino-groups of which engender a positively charged chemical environment that mitigates the deprotonation of anti-B18H22. This approach indeed resulted in the successful preparation of composite Mt/borane luminophores, from which clear fluorescence is visible as shown in Fig. 2. Cysteamine was intercalated in a cationic form in order to functionalize the silicate layers and consequently to anchorage the boranes. The cysteamine in fact provides links between negatively charged silicate layers and anionic species — boranes. All the subsequent analyses confirmed the successful intercalation of cysteamine (CYS) and the anchorage of the borane compound, antiB18H22, onto the montmorillonite (Mt). Fig. 2 shows samples under a UV lamp (λ = 356 nm) that clearly shows the luminescence not only of pure anti-B18H22, which has been presented earlier (Londesborough et al., 2012, Cerdán et al., 2015), but also of Mt modified with CYS and subsequently with anti-B18H22 with similar intensity of light emission, which depends on borane concentration as discussed below. 3.2. XPS measurements nanocomposites

of

Mt/borane

and

bentonite/borane

The relative concentrations of selected atoms, C(1s), O(1s), B(1s), S (2p), Si(2p), Al(2p), Mg(2s) and N(1s) determined by XPS are presented in Table 1. Measurements were made on all the samples, montmorillonite (Mt), Mt modified with cysteamine (Mt + CYS), Mt modified with CYS and subsequently with anti-B18H22 (Mt + CYS + B18H22), at freshly prepared samples. While unmodified Mt and bentonite consists of only O, Si, Al, Mg, Na and Ca elements, Mt and bentonite modified with CYS includes also the presence of N and S, and anchorage of anti-B18H22 results also in the emergence of B, which is clear from Table 1. As it is apparent from Table 1, there is an emergence of N and S atoms in the data for the Mt modified with CYS that were missing in

Fig. 2. Samples under UV lamp (λ = 356 nm). From the left: montmorillonite (Mt); Mt modified with cysteamine (CYS); Mt anchorage with CYS and subsequently with B18H22; only B18H22.

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Table 1 Concentration of all atoms, C(1s), O(1s), B(1s), S(2p), Si(2p), Al(2p), Mg(2s), N(1s) and Cl (2p) determined by XPS for montmorillonite (MMT, above) and bentonite (BNT, below) grafted by cysteamine (CYS) and by anti-B18H22. Sample

MMT + CYS

MMT + CYS

BNT + CYS

+ B18H22 Elements, at.% C(1s) 24.0 O(1s) 49.0 B(1s) − S(2p) 2.3 N(1s) 3.3 Si(2p) 14.8 Al(2p) 4.9 Mg(2s) 1.7 Fe(2p) − Ca(2p) − Cl(2p) −

37.7 39.8 4.7 2.6 1.9 11.6 4.3 0.5 − − 1.5

BNT + CYS + B18H22

21.6 50.3 − 2.9 3.0 13.9 4.8 1.7 1.0 1.0 −

25.5 40.6 16.7 0.4 3.7 8.7 3.2 0.6 0.4 0.3 1.8

unmodified Mt. These data confirm the successful bonding of CYS onto Mt due to a presence of amino- (N) and thiol- (S) groups from CYS. This rationale is also confirmed by XRD measurement. Probably the CYS can be bonded onto Mt (i) via thiol-groups with “free” aminogroup on the surface or (ii) via amino-group with “free” thiol-group on the surface. The first possibility is a prerequisite to gain a positive charge on surface that mitigates the deprotonation of anti-B18H22 and thus facilitates its anchorage to the Mt. This preferential bonding of CYS via thiol- or amino-groups above the surface depends on surface charge and polarity (Kolská et al., 2014). After the anchorage of anti-B18H22 the presence of B element arises, which indicates that the borane was linked onto the Mt surface. (The nature of this linkage is unsure, but unlikely to be covalent as the fluorescence properties are unchanged. One reasonable explanation is the presence of hydrogen bonds between amino or thiol groups of cysteamine and –BH groups of borane (Kar and Scheiner, 2003). However, whatever the nature of the linkage, the intermolecular interaction between the modified clay mineral matrix and the borane is seemingly sufficient for the desired optical functions. Table 1 also shows data after anchorage of CYS and borane onto bentonite. From these data it is evident that the intercalation and modification by bentonite with CYS and subsequently with anti-B18H22 was successful. After CYS addition the presence of N and S is evident and after anchorage of anti-B18H22 the presence of B dramatically increases. The different amount of B at Mt and bentonite are caused by different tested borane concentrations on both of clay minerals. While for Mt we used concentrations of 0.2, 0.6 and 1.1 mmol, for bentonite only the concentration of 0.3 mmol was applied.

Also anchorage of the borane in small concentration (0.3 mmol) on the bentonite exhibits slightly higher fluorescence intensity in comparison with only anti-B18H22. These observations are in agreement the previous studies concerning the influence of concentration of guest molecules on the properties of modified silicates (Klika et al., 2007; Klika et al., 2009). The composition of Mt and bentonite affect the charge and this charge subsequently influences the concentration of guest molecules (borane in this case). The higher concentration of guest molecules leads to aggregation and subsequent quenching of fluorescence (Klika et al., 2007; Klika et al., 2009). The rate of quenching depends on the arrangement of guests, determined by layer charge as the result of competition between host (Mt)-guest and guest-guest interactions. Concordantly, decreasing the guest concentration prevents their aggregation and consequently the fluorescence intensity can increased 103– 104 times (Klika et al., 2009). The emissions are slightly shifted somewhat towards higher energy for the higher borane concentrations when compared to the emission from the free compound in solidstate (λmax = 425 nm vs. λmax 455 nm for only anti-B18H22). It is reasonable that a dihydrogen bond linkage between the Mt and borane inhibits, to a degree, rotational and vibrational molecular movement, leading to the observed blue-shift in the emission band. And also due to the aggregation of guest molecules of borane (at higher borane concentration discussed above) the emission maximum is slightly different in comparison with the non-aggregated “smaller” molecules of borane of lower concentration anchored into Mt matrix. These observations are also in the good agreement with the previous study of intercalation of methylene blue (MB) into Mt. It was found not only fluorescent intensity increased dramatically after decrease of guest molecules (MB) but also that is the crucial factor that affects the quantum yield (Klika et al., 2009). It is expected the same influence at composite Mt/antiB18H22. These results confirmed also testing on bentonite, as it is clear, the UV–Vis spectra for Mt and bentonite modified with only CYS are the same (see Fig. 3, red and violet dash lines).

3.3. Fluorescence UV–Vis measurements of new Mt/borane and bentonite/ borane nanocomposites The UV–Vis emission spectra of individual samples irradiated at 370 nm, for (i) Mt modified with CYS, (ii-iv) Mt anchoraged with CYS and anti-B18H22, (ii) 0.2 mmol, (iii) 0.6 mmol, (iv) 1.1 mmol, (v) pure anti-B18H22, (vi) bentonite modified with CYS and (vii) bentonite modified with CYS and anti-B18H22 for purposes of comparison, are presented in Fig. 3. From these data, it is evident that the fluorescence intensity of Mt modified with CYS and subsequently with anti-B18H22 is significant, especially considering the concentration of anti-B18H22 in the Mt matrix. As it is clear the fluorescent intensity of nanocomposite strongly depends on borane concentration. The lowest concentration (0.2 mmol) gives the highest fluorescent intensity. Even this intensity exceeds the fluorescent intensity of original highly fluorescent antiB18H22 compound. On the other hand the highest borane concentration (1.1 mmol) reduced the fluorescent intensity significantly (see Fig. 3).

Fig. 3. Fluorescence emission spectra of samples irradiated at 370 nm wavelength of montmorillonite modified with cysteamine (red dash line) and subsequently with borane compound, anti-B18H22 ((i) 0.2 mmol — green solid line, (ii) 0.6 mmol — blue dash dot line, (iii) 1.1 mmol — brown dot line) and their comparison with only borane compound, antiB18H22 (black solid line), bentonite modified with cysteamine (violet dash line) and subsequently with borane compound, anti-B18H22 of concentration 0.3 mmol (pink dash dot line).

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3.4. XRD measurements of new Mt/borane and bentonite/borane nanocomposites It is evident from the Fig. 4, that unmodified Mt with a mixture of interlayer cations Na0.25, K0.07, Ca0.10 per structure formula (Si4.0)·(Al1.45Fe3+0.21Mg0.24Ti0.01)·O10·(OH)2 exhibits a large profile broadening (FWHM = 1.16°) due to the disorder in the interlayer structure with basal spacing 12.36 Å (black solid line). After modification of Mt with CYS (dash red line) the 001 peak significantly shifted to lower angles, i.e. to the higher basal spacing 13.31 Å with a narrower profile (FWHW = 0.450°). This observation suggests that the CYS was not only bonded on the Mt surface, but also intercalated into the interlayer space of the Mt structure, creating a homogeneous interlayer distance and expanding the interlayer space. This expansion of the interlayer space increased the total surface of Mt particles and creates more space for anchorage of boranes also on the edge of plate-like Mt particles. This means the modification with CYS has a double benefit for the effective anchorage of boranes: (1) it brings amino-groups into/ onto Mt and (2) it creates better steric conditions increasing Mt surface due to solvent molecules of water solution of CYS. It corresponds also with the data from the XPS analysis. The subsequent modification of Mt + CYS with the borane compound, anti-B18H22, led to very small shifts of the 001 peak position (blue dash dot line) and peak profiles (FWHM) remained nearly the same as for Mt + CYS, which indicates that the borane molecules are indeed anchored on the Mt surface and yet do not affect the interlayer structure significantly. The slight increase of basal spacing up to 13.77 Å of Mt + CYS after anchorage with boranes may be due a water uptake into the interlayer space. As it is clear form Fig. 4 the similar results of successful intercalation and modification of bentonite with CYS (green dot line) and anti-B18H22 (violet short dash line) is also evident in comparison with XRD profile of unmodified bentonite (e.g. Zhirong et al., 2011).

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3.5. Electrokinetic analysis Zeta potential values for Mt unmodified and modified indicate the changes in surface chemistry and charge after all modification steps. Samples were measured both 2 days after preparation and subsequently 3 months after preparation without any changes in surface chemistry and charge. This confirms the high stability of the system. For unmodified Mt the zeta potential is to −37.6 mV. After modification with CYS the zeta potential slightly changed to −40.3 mV. This change is not so significant, suggesting that the CYS was bonded not only on the Mt surface but also (and preferentially) intercalated into the interlayer space of Mt structure. It was confirmed also by XRD and XPS determinations. Significant changes were obtained after anchorage of the borane compound a dramatic change of zeta potential to the positive value of 2.4 mV. This indicates that the anti-B18H22 molecules were successfully bonded to the Mt and were immobilized preferentially onto the Mt surface. This finding is consistent with the data obtained from the XRD measurements. In addition, the anchorage of anti-B18H22 onto the Mt results in a positively-charged surface. Presence of water electrolyte leads to the deprotonation of borane anchoraged on Mt and therefore to creation of the negative charge of surface. Then the positive cations from electrolyte are attracted to the negatively charge the surface. This results in the slightly positive zeta potential. 4. Conclusions Montmorillonite was successfully intercalated and surface-modified by bonding of cysteamine to create an ‘activated’ Mt matrix suitable for the subsequent anchorage of the borane compound, anti-B18H22. The luminescence properties of the resulting Mt/borane nanocomposite materials were determined and explained. The new material was characterized by X-ray photoelectron spectroscopy, X-ray diffraction, electrokinetic analysis and UV–Vis spectroscopy. These results confirmed the successful binding of anti-B18H22 to the montmorillonite and that anti-B18H22 anchorage on Mt modified by CYS renders blue light fluorescent emission. It was also confirmed the fluorescent properties of new composite depends dramatically on the concentration of the borane and the lowest tested borane concentration creates the composite material with the intensive fluorescence intensity than that of the pure borane compound in solid state. The similar observation and results were also confirmed for other silicate, bentonite. This work opens the doors to the development of new stable low-cost nanocomposite materials potentially capable of laser emission. Acknowledgment This work was partially supported by the GACR project nos. 1306989S and 13-06609S and by the internal grant of the J. E. Purkinje University in Ústí nad Labem in the Czech Republic (SGA project 5322215000101). References

Fig. 4. XRD profile of basal reflection 001 for the unmodified and modified montmorillonite (black solid line), with CuKα rad. Montmorillonite (Mt) and bentonite (bentonite) modified with cysteamine (red dash line, green dot line, resp.), Mt and bentonite anchorage with cysteamine and subsequently with anti-B18H22 (blue dash dot line, violet short dash dot line, resp.).

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