Dendrimers-containing organoclays: Characterisation and interaction with methylene blue

Dendrimers-containing organoclays: Characterisation and interaction with methylene blue

Applied Clay Science 136 (2017) 142–151 Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/cla...

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Applied Clay Science 136 (2017) 142–151

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Dendrimers-containing organoclays: Characterisation and interaction with methylene blue Abdellah Beraa a,b,c, Mohamed Hajjaji a,⁎, Régis Laurent b,c, Anne-Marie Caminade b,c a Laboratoire de Physico-chimie des Matériaux et Environnement, Unité Associée au CNRST (URAC 20), Faculté des Sciences Semlalia, Université Cadi Ayyad, B.P. 2390, Av. Pce My Abdellah, 40001 Marrakech, Morocco b CNRS, LCC (Laboratoire de Chimie de Coordination du CNRS), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France c Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France

a r t i c l e

i n f o

Article history: Received 12 May 2016 Received in revised form 14 November 2016 Accepted 15 November 2016 Available online xxxx Keywords: Montmorillonite Stevensite Dendrimers Adsorption Methylene Blue Characterisation

a b s t r a c t The structure of organoclays composed of synthesized phosphorus-based dendrimers, which was endowed with peripheral ammonium groups, and stevensite- or montmorillonite-rich clays was examined using X-ray diffraction, high resolution transmission electron microscopy, thermal analysis and solid-state nuclear magnetic resonance. The interactions between methylene blue (MB) and the organoclays or Na+-saturated clay minerals were investigated. The results showed that both dendrimers adsorbed on Na+-saturated clay minerals by cation exchange, and that the organoclays, particularly those of the first generation dendrimers, were mainly composed of intercalated nanocomposites. As a result of the adsorption of the dendrimers, the clay hydrophilicity reduced and the dehydroxylation temperature of the clay minerals increased. Regarding the adsorption of MB on the organoclays, the results pointed out that MB mainly adsorbed as MB+ and MBH2+, and the electronic environments of AlIV of montmorillonite and that of AlVI of stevensite were disturbed. For some organoclays, ordered structures were formed, and the Si4+ substituent (Al3+) left the tetrahedral sheet. The thermodynamic data indicated that MB adsorbed spontaneously (− 11.3 b Δ G° b − 0.9 kJ/mol) and the uptake of MB by montmorillonite-based organoclays was believed to occur by chemisorption. In contrast, for stevensite-based organoclays MB adsorption seemed to be accomplished by physisorption. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Dendrimers are synthesized monodisperse macromolecules with globular shape and defined size. They have internal cavities and may acquire selected terminal groups, depending on their eventual uses. Because of their specific characteristics and good physical and chemical properties, as compared with some linear or branched polymers, dendrimers are suitable materials for various applications (Caminade et al., 2011). Owing to their expandable interlayer space and cation exchange ability, clay minerals of the smectite group interact with various organic species including polymers. In the presence of polymers, intercalated and exfoliated organoclays can be obtained (Ray and Okamoto, 2003; Okada and Usuki, 2006; Decker et al., 2011). Organoclays are useful industrial materials because of their good thermal and mechanical properties, and their ability to adsorb chemical pollutants (He et al., 2014). If the interactions between conventional branched polymers and clay minerals, particularly montmorillonite, have been investigated (Wang et al., 2010; Juang et al., 2010; Jincheng et al., 2012; Liu et al., 2016; Ma et al., 2016), few studies were carried out on those occurring ⁎ Corresponding author. E-mail address: [email protected] (M. Hajjaji).

http://dx.doi.org/10.1016/j.clay.2016.11.017 0169-1317/© 2016 Elsevier B.V. All rights reserved.

between dendrimers and clay minerals (Juang et al., 2010). Moreover, a very little attention was paid to the use of such hybrid materials in environmental remediation, particularly the removal of chemical pollutants from waste water. Dendrimers have flexible structures and can deform to fit the interlayer of swelling clay minerals. However, phosphorus-based dendrimers, which are involved in the present study, have a semi-rigid structure because of the presence of the phosphorhydrazone scaffold. The only flexible part was around the P\\O bond.

Table 1 Chemical compositions (wt.%) and some physical characteristics of the clays used. Chemical compositions RH AT

SiO2 52.3 66.4

Al2O3 3.9 22.0

MgO 26.8 4.2

Physical properties BET surface area (m2/g) RH 133 AT 77

Fe2O3 1.3 4.2

Na2O 1.0 0.4

CaO 10.6 1.7

K2O 0.8 0.5

TiO2 0.1 0.7

Cation exchange capacity (meq/g) 0.2 0.82

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Fig. 1. Schematic representations of the first (GC1) and second (GC2) generation dendrimers.

The aim of this work was to investigate the structure of organoclays composed of cationic phosphorus-based dendrimers and montmorillonite (dioctahedral clay mineral) or stevensite (trioctahedral clay mineral), and study the interactions between the organoclays and methylene blue. This study could be helpful in the preparation of nanoadsorbents for removing dyes from aqueous solutions. 2. Materials 2.1. Compositions and characteristics of the clays used The basic clays, labelled here as RH and AT, were from Jbel Rhassoul (Fès-Boulmane province, Morocco) and Tassaout area (Demnate, Morocco) respectively. Both clays were used in popular cosmetic recipes. RH was composed of stevensite (~ 65 wt.%), while that AT comprised montmorillonite (~ 85 wt.%). The chemical compositions and some of the physical characteristics of these clay materials are given in Table 1. The clays were the subject of purification by etching with dilute solutions of H2O2 and HCl (Alami et al., 1988). They were saturated with sodium, using a sodium chloride solution, and dried. Then, they were sieved (b80 μm) and stored at 110 °C until handling. 2.2. Dendrimers synthesis The first and second generations of phosphorus-based dendrimers were synthesized by using H2N-N(Me)-P(S)Cl2 and 4-hydroxybenzaldehyde for the branches and hexachlorocyclotriphosphazene for the core. For water solubility and interaction facility with clay mineral particles, the dendrimers were endowed with peripheral ammonium groups, which were obtained by reacting N,N-diethylethylenediamine with P(S)Cl2 terminal groups. Schematic representations of the cationic dendrimers, named GC1 (first generation) and GC2 (second generation), are shown in Fig. 1. The hydrodynamic radii of GC1 and GC2, measured with the

Fig. 2. Adsorption isotherms of the dendrimers studied on Na+-saturated clays. T = 21 ± 1 °C, pH = 6.5.

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Fig. 3. 23 Na solid-state NMR spectra of Na-clays, GC1-clays and GC2-clays. Na+-AT (a); GC2-AT (b); GC1-AT (c). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′).

dynamic light scattering technique (Hameau et al., 2015), were estimated to be 1.1 and 1.5 nm respectively.

3. Experimental procedures 3.1. Adsorption of the dendrimers onto Na+-saturated clays Aqueous dispersions composed of Na+-clays (3–32 g/L) and dendrimers (0.5–8 g/L) were magnetically agitated for 24 h. The temperature and pH were naturally maintained constant. The mixtures were centrifuged at 10,000 rpm, and the separated supernatants were freeze-dried for 66 h, enough time to obtain free-water dendrimers.

3.2. Adsorption of methylene blue on organoclays and Na+-clays For the plot of the adsorption isotherms, the dispersions composed of MB (10 − 3 -8 × 10 − 2 mmol/L) and Na +-clays or organoclays (0.5 g/L) were stirred (250 rpm) for 4 h, exceeding largely the equilibrium time. The temperatures tested were 298, 308 and 318 K, and pH was kept to be 4. The mixtures were centrifuged at 4000 rpm, and the supernatant was separated. The absorbance of the supernatant was measured with a LAMBDA 750 UV/ Vis/NIR spectrophotometer. The wavelength was varied in the range of 400–800 nm. The maximum absorbance for MB occurred at 664 nm. The concentration of MB (Ce; mmol/L) in the supernatant was deduced from the plot of the Beer-Lambert law.

Fig. 4. Portions of the X-ray diffraction patterns of the organoclays. Na+-AT (a); GC2-AT (b); GC1-AT (c). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′).

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Fig. 5. HR-TEM micrographs of Na+-clay and GC1-based organoclays. Na+-AT (a); GC1-AT (b). Na+-RH (a′); GC1-RH (b′).

The amount of the adsorbed MB per weight of adsorbent (qe; mmol/g) was evaluated according to the relation: qe = (Co − Ce).V/m Co is the initial concentration of MB (mmol/L); V: the volume of solution (L); m: the weight (g) of the adsorbent used. The X-ray diffraction (XRD) analysis of dried powders of the samples studied were realized with a X'Pert-PRO diffractometer equipped with a graphite monochromator and a copper anode (wavelength = 1.5406 Å), and operating in the following conditions: generator setting: 40 mA, 45 kV; step scanning: 0.017°; scan step time: 400.05 s. The thermal analysis of the organoclays was realized with a SETARAM 92-16.18 apparatus functioning under air atmosphere at 10 °C/min. The spectra of the solid-state nuclear magnetic resonance (NMR) were recorded at ambient temperature (21 °C) on a BRUKER Avance III 400 WB (9.4 T) apparatus. The working frequencies were 8 kHz for 31 P, 23 Na and 29 Si, and 9 kHz for 27Al. The magic angle spinning (MAS) probe was fixed at 4 mm, and the adopted recycle delayers were 2, 3, 10 and 30 s for 23 Na, 27 Al, 31 P and 29 Si. For 23 Na analysis, a solution of NaCl (1 M) was taken as a reference. The references used for 29Si, 27Al, and 31P were tetramethylsilane (TMS), NaCl diluted aqueous solution containing Al(H2 O) 36 + (1 M), and phosphoric acid solution (85%) respectively. The high resolution examinations were carried out with a JEOL JEM 2100F (200 kV) transmission electron microscope (TEM) (0.23 nm resolution). For this purpose, a droplet of a homogeneous dispersion consisting of 0.25 mg of organoclays or Na +-clays and 5 mL of ethanol was placed on a copper grid, which was previously coated with a collodion film and a thin layer of carbon. The supported droplet was gently dried in a vacuum chamber.

Fig. 6. Thermogravimetry curves of Na+-clays and organoclays. Na+-AT (a); GC2-AT (b); GC1-AT (c). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′).

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4. Results and discussion 4.1. Characterisation of the organoclays The equilibrium isotherms of the adsorption of GC1 and GC2 on both clays showed the curves of L-shape (Fig. 2), expressing thus a high

affinity of the dendrimers for the active sites of Na+-clay minerals. Considering the CEC of the studied clays (Table 1), the maximum amounts of GC1 retained by AT and RH were estimated to be 30%CEC and 100%CEC respectively. In the case of the adsorption of the second generation dendrimer, the clay loadings were found to be 74%CEC and 100%CEC. Apparently, the cation change was more favoured for

Fig. 7. Solid-state nuclear magnetic resonance spectra of some nucleus of Na+-RH, RH-based organoclays and dendrimers (GC1 and GC2). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′). 29Si (A); 27 Al (B); 1H (C). Po is the inner phosphorus of the dendrimers.

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stevensite, which is a trioctahedral clay mineral. Due to the adsorption of the dendrimers on both Na+-clay minerals, the 23Na NMR peaks at 7.5 and 8.0 ppm vanished (Fig. 3). This fact was linked to the desorption of Na+ ions from the free surfaces of montmorillonite and stevensite (Strankowska et al., 2011). The NMR signals at − 5.2 and − 5.5 ppm, which were related to the Na+ ions found within the interlayer space (Strankowska et al., 2011), showed an upfield shift. This shielding effect could be related to the reorganization of Na+ ions within the interlayer space due mainly to their interaction with intercalated dendrimers. The exchange of Na+ ions located at free surfaces of montmorillonite and stevensite by GC1 and GC2 was possible because of the high polarizing power (Z/R, Z: cation valency; R: cation radius) of the dendrimers (10.9 and 16.0 nm−1 for GC1 and GC2 respectively). In the light of the above results, dendrimers was adsorbed on clay minerals by cation exchange. They were placed at surfaces of the clay minerals, and went into the interlayer spaces as well. In relation with the latter observation, the XRD analysis showed that both clays expanded in the contact of the dendrimers, but the basal spacings (d001) was larger with GC1 (Fig. 4). The spacings evolved from 1.34 nm to 1.83 nm for AT and from 1.4 nm to 2.02 nm for RH. These results supported the exchange mechanism, and therefore dendrimers intercalation. Taking into consideration the shape and the position of the XRD reflections, the organoclays were composed of intercalated nanocomposites, and the intercalation was more obvious for GC1. The HR-TEM observations of the organoclays supported the latter results, since packets of ordered layers with basal spacings (1.54–2.26 nm) exceeding d001 measured for Na-AT and Na-RH were identified (Fig. 5). In this respect, it may be noted that the value of the basal distance (d001) from XRD represents the average of the actual basal spacings (Sun et al., 2013). Bearing in mind the measured d001 of the organoclays and the basal spacings of dehydrated montmorillonite and stevensite (1 nm), the inserted dendrimers lost their original size. For both clay minerals used, the size of the confined GC1 and GC2 species seemed to be reduced by about 70% and 90% respectively. Based on the weight losses linked to the departure of physisorbed water in the range of 30–169 °C (Fig. 6), the prepared organoclays were less hydrophilic in comparison to the raw clays. This was supported by FT-IR examinations (not shown) since the intensities of the characteristic bands of adsorbed water (3381 cm− 1 and 1635 cm−1) evolved in the order: Na+-clay N GC2-clay N GC1-clay. Indeed, organoclays are hydrophobic materials (Yariv and Cross, 2002). Considering once again the thermal curves of Fig. 6, the weight loss occurring at 237–295 °C was ascribed to a partial decomposition of the dendrimers (about 50% and 60% of the maximum uptake amounts of GC1 and GC2 respectively). The residual amounts of the dendrimers started reacting at about 670 °C as the dehydroxylation process of the clay minerals was achieved. It should be noted that in the presence of dendrimers, the dehydroxylation process of montmorillonite started to happen at high temperature (500 °C instead of 400 °C). Considering the maximum uptake amounts of GC1 and GC2, and the weight related to the dendrimers losses, the relative quantities of GC1 and GC2 species within the interlayer space of montmorillonite were estimated to be 50% and 40% respectively. For stevensite-based organoclays, the relative amount of the dendrimers within the interlamellar spacing was estimated to be 61%. Considering for instance the NMR spectra of Na+-RH and RH-based organoclays (Fig. 7), the electronic environment of Al3+ within the octahedral sheet (AlVI) (Cadars et al., 2012) was not affected as a result of the adsorption of the dendrimers. In contrast, the electrons surrounding Al3+ ions within the tetrahedral sheet (AlIV) was slightly disturbed (Δδ = 0.6 ppm). A slight disturbance of the electronic environment of Si (SiIV) (Cadars et al., 2012) was observed particularly in the case of GC1-RH. For the latter organoclay, a marked deshielding was recorded for the proton of the hydroxyl group in the structure of stevensite. This effect did not happen for GC2-RH. These results suggested that

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GC1 species were hosted within the di-trigonal (hexagonal) cavities (about 0.44 nm width and 0.22 nm depth), and their external groups (Fig. 1) interacted with clay mineral hydroxyls. It should be noted that in the case of GC2-AT, the 1H NMR signal of water shifted to low frequencies. This indicated the presence of a shielding effect, which could be attributed to the reorganization of water molecules such as reported by Gougeon et al. (2006). Such a phenomenon seemed to be insignificant with GC1 species since the position of the signal of 1H (Cadars et al., 2012) remained unchanged. The difference might have a relation with the hydrophilicity of the dendrimers (GC1 seemed to be more hydrophobic). Referring to the positions of the 31P NMR signals of the dendrimers, P0 and P1 of GC1 experienced a slightly shielding as a result of adsorption. 4.2. Interactions between methylene blue and organoclays 4.2.1. Structural investigations The X-ray diffraction pattern of GC2-AT (Fig. 8b), which was into contact with MB, displayed additional faint reflections at ° 2θ =

Fig. 8. X-ray diffraction patterns of the adsorbents after contact with methylene blue. Na+AT (a); GC2-AT (b); GC1-AT (c). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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30.25 and 32.60. Extra reflections also manifested in the X-ray diffractogram of the complex GC1-RH-MB (Fig. 8b′). These unexpected X-ray reflections were probably linked to an ordered structure formed for instance within the interlayer space as a result of the interaction (hydrogen bond for instance) between MB species and dendrimer ions. Some extra reflections were also manifested in the X-ray diffraction pattern of the organoclay composed of methylamine and montmorillonite (Hajjaji and Beraa, 2015). They were associated to a long-range order of confined adsorbate species. The XRD analysis showed that the basal spacings of Na-AT and GC2AT were reduced by about 23%, when that of GC1-AT increased by 17%. This result suggested that a part of confined solvated sodium cations of Na-AT and the dendrimers of GC2-AT left the clay structure and/or were substituted by MB + species. The observed increase recorded for GC1-AT might be due to the insertion of MB cations within the interlayer space. This process is further discussed hereafter. The UV–visible spectra of the residues of Na+-saturated clays showed that MB was adsorbed on clay minerals particles mainly as MBH2 + and MB+ (Fig. 9). The monomer was adsorbed on external surface of aggregates (λ = 670 nm) as well as on planar surfaces (λ = 653 nm) (Schoonheydt and Heughbaert, 1992). Considering the intensities of the absorption bands, the amount of the protonated form of MB seemed to be identical for Na+-RH and RH-based organoclays, and the quantities of MB+ on external

surfaces of aggregates and on planar surfaces exceeded those of MBH2 + (wavelength of maximum absorbance: 755 nm). Moreover, the amounts of the latter MB derivative species were almost similar for the organoclays. Based on the intensities of the bands of Fig. 9, the quantities of MBH 2 + were very close for Na+-RH and RH-based organoclays. This observation supported the idea that MB protonation mainly occurred on active acid-sites of stevensite. The protonation process was almost negligible in the case of Na+-AT and GC1-AT. The analysis of the spectra also showed that adsorption of MB+ on planar and external surfaces of both clay minerals enhanced in the presence of the dendrimers. The examination of the UV–visible spectra of the supernatants of the dispersions of both Na+-clays and their basic organoclays revealed the co-presence of MB+, (MB+)2 and (MB+)3 (Fig. 10). The absence of MBH2 + supported well the fact that MB protonation happened at the surface of the clay minerals, as observed elsewhere (e.g., Hajjaji et al., 2001). The protonation of MB could be also accomplished within the interlayer space, especially in the contact with residual water molecule, which manifests a strong acidic character. This may be supported by the fact that in the presence of the hydrophobic dendrimers (GC1), MBH 2 + species were less abundant. The relative abundance of the dimer and the trimer species in the supernatants could be linked to the MB concentration (Bergmann and O'Konski, 1963).

Fig. 9. Absorption spectra of Na+-clays and clays-based organoclays after contact with MB.

Fig. 10. Absorption spectra of the supernatants of the dispersions of Na+-clays and their basic organoclays after adsorption of MB.

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Fig. 11. 29Si and 27Al solid-state NMR spectra of Na+-clays, GC1-clays and GC2-clays. Na+-AT (a); GC2-AT (b); GC1-AT (c). Na+-RH (a′); GC2-RH (b′); GC1-RH (c′).

The solid-state NMR analysis (Fig. 11) showed that due to the interaction of the adsorbents studied with MB, AlIV and AlVI were the subject of shielding. The intensity of shielding was different according to the nature of the clay minerals and Al location: For Na+-RH and RH-based organoclays, the significant shifting was observed for AlIV. However, in the case of Na +-AT and AT-dendrimers, the main shifting was recorded for AlVI. As a substituent of Si, AlIV is encountered in the tetrahedral sites, which were located at the outer parts of the layers of montmorillonite and stevensite. Because of this geometrical configuration, AlIV ions interacted with MB+ adsorbed on clay minerals surfaces. It is worth noting that in the presence of MB, 29Si- and 27Al-NMR spectra of GC1-RH showed additional peaks at −111.1 and 55. 6 ppm, respectively. Referring to Cong and Kirkpatrick (1996), and Sousa et al.

(2015), the occurrence of the former peak revealed the absence of Si4 + substitution by Al3 +. Thus, due to the presence of MB, Al3+ left the tetrahedral sheet, likely for forming complexes with free MB species. The peak at 55.6 ppm could be assigned to the displaced Al3+, i.e., Al3+ of the plausible complex. 4.2.2. Thermodynamic data As can be deduced from Fig. 12, the adsorption of MB on both Na+-saturated clays and their based-organoclays occurred spontaneously (− 11.3 b Δ G° b − 0.9 kJ/mol; Δ G° is the Gibbs free energy). The spontaneity of the process evolved as follow: GC1-Clay N GC2Clay N Na+- clay minerals, and it was more pronounced for Na+RH and RH-based organoclays. The latter observation might have a link with the active sites involving Al IV, as revealed by NMR

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5. Conclusion The synthesized phosphorus-based dendrimers had high affinity for stevensite and montmorillonite, and adsorbed by cation exchange. They were adsorbed on sites at the free surfaces and within the interlayer spaces of the structure of the clay minerals. Due to the adsorption of the dendrimers, clays became hydrophobic and temperature of the dehydroxylation of the clay minerals increased. The adsorption of MB on the organoclays occurred spontaneously, and took place at external surfaces and within the interlayer spaces of the clay minerals. The adsorbed species were identified to MB+ and MBH2 +. The MB protonation seemed to form mainly on the clay minerals surfaces. The adsorption of MB on some organoclays resulted in the formation of an ordered structure and the expelling of Al3+ found in the tetrahedral sheet as a substituent of Si4 +. MB Adsorption took place by chemisorption as well as by physisorption. These processes involved the interactions of the octahedral and tetrahedral Al3+ ions. Acknowledgements The authors gratefully thank CNRS (France), CNRST (Morocco), Campus-France (France) and the LIA LCMMF (LIA/LCMMF-163/2011) for their financial support. References

Fig. 12. Variation of the Gibbs free energy of the adsorption of MB on studied adsorbents versus temperature.

analysis. The use of the plots of Fig. 12, and the thermodynamic relation: Δ G° = Δ H° − T Δ S° (Δ H°: heat; Δ S°: entropy) showed that the heat of adsorption was positive (Table 2). Thus, the adsorption process of MB on the studied adsorbents was an endothermic process. As a matter of fact, adsorption processes are in most cases exothermic. But referring to the results of the study of Rytwo and RuizHitzky (2003), the adsorption of methylene blue on montmorillonite was endothermic as the uptake amounts of MB were below 73% of the CEC. Based on the values of Δ H° (Table 2), the uptake of MB by Na +-AT and GC1-AT was realized by developing strong bonds (chemisorption; Δ H° N 40 kJ/mol (Winterbottom and King, 1999)). In contrast, the adsorption of MB onto Na+-RH, RH-based organoclays and GC2-AT took place by weak bonds (physisorption). The positive values of Δ S° (Table 2) expressed an increase in the disorder of the system.

Table 2 Enthalpy and entropy of the adsorption of MB species on Na+-clays and their organoclays. 298 ≤ T ≤ 318 K.

+

Na -AT GC1-AT GC2-AT Na+-RH GC1-RH GC2-RH

ΔH° (kJ/mol)

ΔS° (kJ/mol.K)

41.213 47.235 27.591 21.822 4.860 17.461

0.142 0.167 0.099 0.101 0.050 0.089

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