On Stability of Tetrabutylphosphonium Beidellite Organoclay - The

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

On Stability of Tetrabutylphosphonium Beidellite Organoclay Eva Scholtzova, Lubos Jankovic, and Daniel Tunega J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01042 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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The Journal of Physical Chemistry

On Stability of Tetrabutylphosphonium Beidellite Organoclay Eva Scholtzová1,*‡, Ľuboš Jankovič,1‡ and Daniel Tunega2,*‡ 1

Institute of Inorganic Chemistry of Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava,Slovakia, 84536

2

Universität für Bodenkultur, Institut für Bodenforschung, Peter-Jordan-Strasse 82, Wien, Austria,A-1190,

*Address correspondence to [email protected], [email protected]

1 Environment ACS Paragon Plus

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

Organoclays originating by interaction of clays with organic cations/molecules are important in the industrial processes for their physicochemical properties. The beidellite (Bd) mineral is an interesting material e.g. as a potential sorbent of waste and pesticides. For this reason, its behavior at the atomic scale is of great interest. The DFT method is used at the research of the stability of Bd intercalated with tetrabutylphosphonium cation (TBP). The interlayer distances (d001) of proposed models agree well with the experimental values. The ab initio molecular dynamics (AIMD) is successfully employed in a detailed explanation of the experimental FTIR spectrum of the TBP-Bd which has a complicated structure with overlapped bands. Detailed analysis of the present hydrogen bonds and calculated intercalation energies show that the TBPBd is a highly stable intercalate. Its stability decreases with growing amount of water, but the high stability is still preserved (–87.5 kJ mol-1).


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INTRODUCTION Clays belong to important minerals in many diverse uses. Clay minerals have a layered structure formed by tetrahedral (Si/AlO4) and octahedral (Al/Mg/FeO6) sheets connected together through apical oxygen atoms.1 In an ideal composition clay layers are neutral. If central cations in the octahedral or tetrahedral sheets are substituted by a cation with a different formal charge layers become negatively charged. To keep such layers together and neutralize the negative layer charge, in the interlayer space are compensating cations that can be also hydrated. One way of classifying clays is related to the type of cations present in the octahedral or tetrahedral sheet and the size of the negative layer charge2. One of the common clay families are smectites with a relatively low layer charge density. One of the most studied smectite is montmorillonite (Mnt) and its net negative surface charge originates from isomorphic substitution of Mg2+ for Al3+ in the silicate octahedral sheets.3-4 In contrast to montmorillonite, a layer charge of beidellite (Bd) has origin in the tetrahedral substitution of Si4+ by Al3+.5 Clays are minerals that have unique physical and chemical properties. They are formed by very small crystals of poor quality, usually in the form of hexagonal platelets, which agglutinates to form conglomerates. When they are hydrated, separation of lamellar layers and intercalation of ions and molecules can happened.6 Interactions between clay minerals and organic molecules are important in numerous industrial processes; therefore, they are interesting for academic studies.7 Owing to the hydrophilic character of smectite surfaces, natural smectites are not effective sorbents for nonionic and non-polar organic compounds from water even if clay has a high specific surface area. Reactions of ion exchange



may greatly modify the surface properties of natural smectites. When, for example, organic cations of formula (CH3)3NR+, where R is an alkyl hydrocarbon, replace natural inorganic cations (e.g. Na+, K+, Mg2+, or Ca2+) in the smectite structure, the surface properties of new hybrid material can change from hydrophilic to hydrophobic.8 Thus, such prepared organoclay materials with fixed organic cations on the surface or in the interlayer space of smectite can be effectively used in removing of the nonionic and non-polar organic compounds from water. The modified clays are usually prepared using quaternary ammonium cations of a general formula: [(CH3)3NR]+ or R4N+ where R is an alkyl chain. The physical properties of modified clays (e.g. adsorption capacity, hydrophobicity/hydrophilicity) depend on the molecular size and type of the groups R.9 Modification of clay minerals by organic cations is a subject of a great interest because of possibility to develop new materials with the specific properties for new technological applications e.g. polymer-clay nanocomposites, active sorbents, drug release retardation from biocomposite hydrogels, storage of radioactive waste reinforcement of antimicrobial paper packaging improving its tensile strength etc.10 A lot of experimental studies (e.g. XRD11, FTIR12-13, or NMR14) have been done to understand the structure, properties, stability, and formation of intercalated clays. The results showed that intercalation of smectites may be linked to the type and level of isomorphous replacements, the property of their associated exchangeable cations, as well as the nature of ambient aqueous solutions. However, there is not much understanding of specific molecular mechanisms characterizing organocation-clay interactions. Molecular simulations based on the methods of computational chemistry can provide detailed description of chemical or physical processes of clays and their intercalates.15 Though the usual methods are classical molecular dynamics and Monte Carlo simulations to investigate processes such as the diffusion of water and cations on the surface or in the interlayer


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space of clay minerals,16-17 the quantum chemical approach is more used to understand the atomic structures and electronic properties when studying the molecular adsorption on clay surfaces.18-20 There are various quantum chemical methods with varying degrees of sophistication, including semi-empirical method and density functional theory (DFT). Specifically, DFT methods were used in studies of clay intercalates.21-25 DFT was also used in the study of organoclays containing small organic alkylamonnium cations.26-27 In a recent time, closer attention is also paid to phosphonium-based organic cations that could be used to prepare organoclays with improved properties and higher stability in comparison to organoclays containing conventional alkylammonium cations. The recent studies have shown that phosphonium cations have a high potential for technological use due to their very good physical-chemical properties, e.g. better thermal stability than organoclays prepared with alkylammonium cations.28-33 In the present work we present a study of structural and spectroscopic characteristics of beidellite mineral intercalated with tetrabutylphosphonium cation (TBP) using combined experimental and modeling approach. Beidellite was selected as a smectite with predominant isomorphic substitutions in tetrahedral sheets in contrast to montmorillonite, in which octahedral substitutions are dominant. It is expected that different charge distribution can improve a stability of prepared organoclays from beidellite. Moreover, montmorillonite is much more frequently used in the preparation of organoclays than beidellite. Only a few studies on organically modified beidellite with alkylammonium cations exist34-35 and no report was found on beidellite intercalated with phosphonium organocations. Thus, this study provides a first insight into the understanding of the structure and properties of organoclays based on beidellite clay intercalated with alkylphosphonium cations.



METHODS Experimental details The smectite used in this study was reference beidellite (SBId-1) obtained from the Clay Minerals Society, Source Clay Repository (Department of Agronomy, Purdue University, West Lafayette, IN, USA). The structural formula (Si7.148Al0.852)(Al3.624Mg0.18Fe(III)0.224)O20 (OH)4 M+0.948 of the SBId-1 beidellite reported by Gailhanou et al. (2012)36 showed that the main isomorphic substitutions were located in the tetrahedral sheet (more than 80%). A size fraction of less than 1.0 µm (i.e., equivalent diameter) was collected using the method described in Ref. 37. For the preparation of tetrabutylphosphonium-beidellite sample (TBP-Bd), the solvent intercalation process was used. One gram of sodium beidellite (Na-Bd) was added to 100 mL of distilled water and the suspension was stirred for 24 h at room temperature to ensure the Na-Bd was adequately dispersed. Subsequently, a calculated volume of a 1% aqueous solution of tetrabutylphosphonium bromide was slowly added (1 mL min-1) to the suspension and the mixture was vigorously stirred for 24 h at laboratory temperature (~25°C). The total amount of the intercalated TBP corresponded to 25 % of the CEC of the beidellite sample. After the reaction, the suspension was centrifuged and the final product was washed by centrifugation five times with 250 cm3 of distilled water to remove excess amounts of water-soluble tetrabutylphosphonium and inorganic (NaBr) salts. The cation exchange capacity (CEC) of the purified sodium beidellite was 0.91 mmol/g evaluated by using the Cu-trien method.38 The pure beidellite and organoclay samples were characterized by powder XRD, FT-IR, and thermal analysis.X-ray powder diffraction data were collected on a D8 Advance Bruker


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diffractometer using Cu Kα (40 kV, 40 mA, λ = 1.541 78 Å) radiation and a secondary beam graphite monochromator. Diffraction patterns were collected in the 1 to 10° 2Θ range in steps of 0.02° 2Θ and with 2 s counting time per step. XRD results were used for the determination of the changes in the basal spacing (d001) after intercalation. The infrared spectra were obtained on a Nicolet 6700 FTIR spectrometer from Thermo Scientific by co-addition of 128 scans at a resolution of 4 cm-1. The KBr pressed disk technique (1 mg of sample and 200 mg KBr) and Smart Diffuse Reflectance Accessory were used to measure the spectra in the mid-IR (MIR, 4000–400 cm-1) region. Spectra manipulations were performed using the OMNIC™ software package from Thermo Scientific. The thermogravimetric analyses (TGA) were recorded using a NETZSCH STA409PC microanalyzer, with heating rate of 10ºC/min. A 50 mg sample was used for each measurement. The derivative of the TG curve (DTG) gives the weight loss, which was assigned to the loss of adsorbed & interlayer water (between 20 and 200ºC). Determined content of hydrated water was 12.1 and 7.9 wt.% for Na-Bd and TBP-Bd, respectively. Computational details Computational method Ab initio molecular dynamics calculations within the frame of Kohn−Sham electron density functional theory were performed using the Vienna ab initio simulation package (VASP).39-40 The exchange-correlation energy was expressed in the frame of the generalized gradient approximation (GGA) using the functional proposed by Perdew, Burke, and Ernzerhof (PBE).41 The electron−ion interactions were described using the projector-augmented-wave (PAW)



method42-43 in a plane wave basis set with an energy cut-off of 400 eV and with the required convergence in total energy of 10−4 eV. Brillouin-zone sampling was restricted to the gamma point only because of the large computational cells. The Verlet velocity algorithm44 with a time step of 1 fs was chosen for a numerical solution of equations of motion. In the initial thermal equilibration phase of the dynamics, the finite temperature calculations were performed on a canonical ensemble applying the Nosé−Hoover thermostat45 at 300 K with a simulation time of at least 10 ps. The equilibration was controlled through the temporal evolution of parameters such as temperature and potential energy of the system. After equilibration, the system was changed to the microcanonical (NVE) ensemble to obtain power spectra (PS). For the NVE ensemble, the total length of the MD run was 10 ps. Based on these dynamics runs, PS were computed by a Fourier transformation of the velocity autocorrelation functions. Additionally, pair distribution functions (PDFs) were calculated for selected atomic types. Computational models Clay minerals have, in general, a variable structure due to isomorphic substitutions of the central cations in the tetrahedral and/or octahedral sheets, respectively. Usually the structure of clays requires a certain level of simplification when constructing a model for use in molecular simulations, especially when quantum chemical methods are involved and complete structural data do not exist. Similar situation was with a natural beidellite (Na-Bd) used in the sample preparation. Therefore, the model of montmorillonite (Mnt) structure from the previous study of Mnt intercalated with tetrabutylphosphonium cation27 was used as a base for the preparation of Na-Bd and TBP-Bd models, respectively. It means that one Si4+/Al3+ substitution was created in the tetrahedral sheet and the original Mg2+ substitution in the octahedral sheet was changed back to Al3+ cation. Owing to the size of the organic cation, computational cell for the TBP-Bd models


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was extended to size of 4a2bc of the Na-Bd elementary cell46 with final lattice vectors a= 20.966 Å, b= 18.176 Å, and c= 20.7 Å, respectively. First, four Na-Bd models with a different mutual position of the substituted tetrahedron and hydrated sodium cation were proposed. Models Nn1 and Nn2 (Figures 1a, b) had Si4+/Al3+ substitution in the proximity of the hydrated sodium cation (1– top tetrahedral sheet, 2 – bottom tetrahedral sheet) whereas in the models Nf1 and Nf2 (Figures 1c, d) the substitution was farer from the Na+. The Na+ hydration was represented by 6 water molecules in octahedral coordination (it means ~2 wt% of water content). Further, Na-Bd models with multiple substitutions in the tetrahedral sheet were constructed, particularly models N2, N3, and N4 having two, three, and four tetrahedral substitutions and corresponding number of hydrated sodium cations in the interlayer space (water content is 3.7, 5.6, and 7.4 wt%). N2N4 models are displayed in Supporting Material (SM), Figure S1. The N4 model with a summary formula of (Si7.5Al0.5)(Al4)O20(OH)4Na0.5+(H2O)3 has a similar composition as the real beidellite sample used in the experiment. In the next step eight models of TBP-Bd with different mutual position of TBP cation and substituted tetrahedron (without hydrated sodium cation) were proposed. Four models were derived from Na-Bd models by substitution of hydrated sodium cation with TBP cation (P1, P2, P3 and P4). The rest four models (P5, P6, P7 and P8) were prepared by moving the TBP cation from the centre of the computational cell in P1-P4 models closer to the side of the computational cell. All eight models (P1-P8) are displayed in Figure S2 of SM. Finally, TBP-Bd models with multiple substitutions in the Bd layer were constructed (Figure S3 in SM). Three models with different substitutions in the tetrahedral sheet were proposed: P1N1 – two

Si4+/Al3+

substitutions,

P1N2

–

three

Si4+/Al3+

substitutions,

P1N3

–

four

Si4+/Al3+substitutions in the tetrahedral sheet (Ni – number of hydrated sodium cations together with one TBP (P1) cation in the interlayer space for a layer charge compensation).



Figure 1. a) Nn1, b) Nn2, c) Nf1 and d) Nf2 models. Starting position of hydrated sodium cation – up, optimized – down, AlO4 tetrahedron – in cyan. The stability of the TBP-Bd models was derived on the base of the intercalation energy calculated for a general intercalation reaction scheme: {Na[(H2O)6]+}x-Bd + TBP+→{Na[(H2O)6]+}(x-1)TBP-Bd + Na[(H2O)6]+ (1) where Na[H2O]6+-Bd represents a particular Na-Bd structure, Na[(H2O)6]+ is hydrated sodium cation coordinated with six water molecules, x is a number of hydrated sodium cations, TBP+ is tetrabutylphosphonium cation, and {Na[(H2O)6]+}(x-1) represents TBP-Bd structures with


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hydrated sodium cations. Individual Na[(H2O)6]+ and TBP+ cations are optimized as isolated entities in the particular computational cell of the corresponding Na/TBP-Bd structure. RESULTS AND DISCUSSION Determination of the d001 parameter Intercalation of beidellite with the TBP organic surfactant was examined by means of XRD method to determine the changes of the basal spacing. Figure S4 in the Supporting Material collects diffraction patterns of the pure beidellite and TBP-Bd intercalate (only peaks corresponding to the d001 diffraction are shown). Upon surfactant intercalation (25 % of CEC) into beidellite via ion exchange reaction, the hydrated Na+ cations in the interlayer spaces were replaced by larger tetrabutylphosphonium ions what resulted to the d001 change. The pure Na-Bd sample had a basal spacing of 12.4 Å, whereas a value of 15.8 Å was determined for the TBP-Bd sample. The increase in the basal spacing indicates that the organic modifier has diffused into the interlayer galleries between beidellite layers and modification was effectively accomplished. Single peak in the diffraction pattern corresponding to 15.8 Å indicates that the intercalated TBP cations are regularly distributed in the interlayer space of beidellite microcrystals. Generally, the arrangement of the cations in the interlayer space depends on the packing density, temperature, chain length, and on the concentration of the organic cations in the solution used in the organoclay preparation.25 It is supposed that the organic cations with the shorter alkyl chains prefer monolayer arrangement. Tetrabutylphosphonium cation belongs to this type of the organic cations and the determined d001 value of 15.8 Å indicates that the cations in the TBP-Bd organoclay are in a monolayer arrangement. This assumption was used in the constructions of the TBP-Bd structural models.



Na[(H2O)6]x+-Bd models The four models of Na[(H2O)6]+-Bd structure were prepared with a different mutual position of the substituted tetrahedron and hydrated Na+ cation (Figures 1 a-d, upper – initial position, bottom – optimized structures). In the optimized structures of models Nn1 and Nn2 the Na[(H2O)6]+ cation, originally positioned in a center of the ditrigonal hole (projected view) and in a proximity of the tetrahedral Si4+/Al3+ substitution, moved closer to the substitution (Figures 1a-d) with Na•••Al optimized distances of about 3.5 and 5.5 Å (Table 1, Figure 2). Similarly, also in the models Nf1 and Nf2, the hydrated Na+ cation moved from a center of the ditrigonal hole to any tetrahedral unit of the tetrahedral sheet with Na•••Al optimized distances of about 7 Å (Table 1, Figure 2). The localization of the interlayer cation in the proximity of the tetrahedral site with the Si4+/Al3+ substitution in the beidellite layer is due to Coulombic interactions with a negative layer charge localized on surface oxygen atoms bound directly to the substitution. It is confirmed by the fact that the model Nn1 with the shortest Na•••Al distance is energetically most stable (Table 1).

Figure 2. Number of water layers, Na•••Al and d001distances in Na-Bd models with one hydrated sodium cation.


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Table 1.The interlayer, bond and hydrogen bond distances, and total energy in models of Na-Bd. For N2, N3 and N4 models the maximal shortening is presented. model

d001

total energy

r(Na•••Al)

Ow–H•••Ob

Ow–H•••Ow

Nf1

12.44

-2444.78045

7.137

1.884;2.773;2.873

1.744;1.837;2.012

Nf2

12.27

-2444.70000

6.944

1.840;2.649;2.897

1.734;1.831;2.877

Nn1

11.89

-2445.18363

3.489

1.927;2.597;2.873

1.759;1.997;2.379

Nn2

12.03

-2444.93036

5.583

1.840;2.700;2.872

1.791;2.031;2.247

EXP

12.4

N2

14.20

-2520.01723

3.012

1.826;2.521;2.911

1.829;1.919;2.659

N3

14.32

-2625.29889

5.154

1.820;2.527;2.917

1.866;2.359;2.898

N4

14.95

-2714.34293

5.052

1.826;2.639;2.917

1.809;2.345;2.902

d001- interlayer distances, r(Na•••Al) bond distance and hydrogen bond distances (min,median, max) [Å]; total energy [eV]

In case of montmorillonites with layer charge coming from octahedral substitutions our previous study showed that interlayer cations are localized preferably in a center of the ditrigonal hole (projected view) as excess negative layer charge is delocalized over all surface oxygen atoms.26 In the models Nn1 and Nn2, in addition to the movement of the cation towards the substitution, the Na+ coordination changed during the geometry optimization, too. Two water molecules released the coordination shell getting Na•••Ow distance up to ~4.5 Å. The rest four H2O molecules rearranged to a planar configuration with Na•••Ow distances of ~2.3 Å (Figures 1a and 1b). Thus, in these two models water has a monolayer arrangement what is also reflected in smaller d001 values in comparison to the models Nf1 and Nf2 (Table 1, Figures 2). In those two models, water molecules preserved a deformed octahedral coordination of Na+ with Na•••Ow distances of about ~3.7 and/or ~2.3 Å. forming a bilayer water arrangement in the interlayer



space (Figures 1c, 1d, 2). The d001 values for models Nf1 and Nf2 are larger than in case of models Nn1 and Nn2 and are closer to experimental value of 12.4 Å (Table 1). The d001 value is very sensitive to water amount in the interlayer space. The Nf1 model can correspond to dried Bd structure with a theoretical water content of 1.9 wt%. However, the experimental sample with d001 ~12.4 Å contains larger amount of water (12.1 wt%). Moreover, models Nf1, 2 and Nn1, 2 contain lower concentration of the tetrahedral substitution than in the experimental structure. The d001 values of the models with multiple substitutions in the tetrahedral sheet (N2, N3 and N4) increased to values of ~14 Å in comparison to the models with only one substitution (Table 1). Also in these structures some hydrated cations moved towards to the substituted centers during the structure optimization (Figure S1). The closest Na•••Al distances are collected in Table 1. The bilayer water arrangement and octahedral coordination of Na cations stayed preserved in all models with multiple substitutions. Theoretical water amount in the N4 model, which is the closest to the experimental structure, is 7.4 wt% (experimental sample – 7.8 wt%). Hydrogen bonds The analysis of hydrogen bonds in the respective models of Na-Bd structure has shown that two types exist in the interlayer space – between water molecules themselves (Ow–H•••Ow) and between water molecules and basal surface oxygen atoms (Ow–H•••Ob), respectively. According to the acceptor ••• hydrogen (A•••H) distances the Ow–H•••Ow hydrogen bonds are of moderateto-weak strength and are stronger than the Ow–H•••Ob hydrogen bonds (Table 1). Generally, in the N2-N4 models (models with multiple substitutions in the tetrahedral sheet) both Ow–H•••Ow and Ow–H•••Ob hydrogen bonds are slightly weaker than in the models with one tetrahedral substitution (longer interatomic distances) what is reflected in larger d001 values


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(Table 1). In addition, also longer Na•••Ow distances in the N2-N4 models than in the models with one substitution contribute to the larger interlayer space. TBP-Bd models In the next step, the structures of eight models of TBP-Bd organoclays without presence of water in the interlayer space were optimized (Figure 3). The obtained d001 values vary in a range

Figure 3. Optimized position of the TBP cation in P1 model as example. of 14.41-14.90 Å (Table 2). During the geometry optimization the organic cation changed its position (originally localized with a phosphonium head above the center of the ditrigonal hole), similarly as in a case of Na-Bd models.



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Table 2. The interlayer, bond and hydrogen bond distances, and intercalation energy in the TBPBd models. model

d001

P•••Al

∆(P•••Al)

∆Eint

C–H•••O

P1

14.74

5.121

5.807

-150.51

2.419; 2.695;2.901

P2

14.41

10.933

3.787

-129.95

2.316;2.767;2.901

P3

14.90

7.613

0.172

-96.45

2.552;2.776;2.919

P4

14.76

5.118

3.211

-121.01

2.173;2.762;2.901

P5

14.60

5.009

2.183

-147.856

2.34;2.742;2.891

P6

14.65

4.75

3.579

-107.725

2.186;2.751;2.911

P7

14.78

7.658

3.51

-101.867

2.449;2.765;2.917

P8

14.74

10.301

2.724

-130.345

2.433;2.675;2.880

d001-interlayer distances, ∆(P•••Al) - changes in the P•••Al distances after optimization, and hydrogen bond distances (min, median, max) [Å]; intercalation energy - ∆Eint [kJ mol-1] In some of the optimized structures, the center of the phosphonium head moved relatively close to the tetrahedral substitution of the ditrigonal ring (P•••Al of ~5 Å, see also P4 model in Figure 4). The P•••Al distances and their changes with respect to the initial configuration are collected in Table 2. Intercalation energies calculated according Eq. 1 (∆Eint in Table 2) showed that the phosphonium-based organoclays are very stable. The most stable P1 model reached the intercalation energy of -150 kJ mol-1. There is no evident correlation between the P•••Al distance and the intercalation energy. Thus, not only Coulombic interactions between the substitution and the positively charged phosphonium head contribute to the stabilization of the phosphonium cation but also other factors can play a role, e.g. weak dispersion interactions between butyl chains and the surface oxygen atoms of the beidellite layers. During the optimization butyl chains changed their configurations from originally regular star structure with the straight chains to the quasi-planar structure with deformed CC backbone (Figure 4 for P4 model as example).


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Figure 4. Optimized curved structure of butyl chains of TBP cation in P4 model as example. Similar quasi-planar structure was also observed in modeling of tetrabutyl ammonium cations intercalated in montmorillonite.26, 47 Hydrogen bonds and intercalation energies The randomly chosen substitutions in the tetrahedral sheet of clay did not have an essential influence on the stability of the TBP-Bd organoclay models. The intercalation energies (Table 2) vary in a range from -150.51 to -96.45 kJ mol-1 and demonstrate a better stability of all eight TBP-Bd models comparing to the model of TBP cation intercalated in montmorillonite (-72,20 kJ mol-1).27 The TBP cation has stronger Coulombic interactions with the beidellite layers through excess negative layer charge localized on the surface basal oxygen atoms bound to the isomorphic substitution than TBP cation interaction with a delocalized charge in montmorillonite layers. Additional stabilization comes from hydrogen bonds formed between CHx (x= 2, 3) groups of the butyl chains and the surface basal oxygen atoms (C–H•••Ob), respectively. These hydrogen bonds are weak48-49 with H•••O distances of about 2.7 Å (Table 2). Although similar hydrogen bonds in the TBP-Mnt models are slightly stronger (2.55;2.82;2.89).27 TBP-Bd models are significantly more stable because of stronger Coulombic interactions.



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{Na[(H2O)6]+}x-1TBP-Bd models The third set studied comprises intercalated TBP-Bd models with a presence of hydrated sodium cations. These models were selected because in the real organoclay samples residual hydrated cations might remain in the interlayer space.50 Particularly, in our TBP-Bd samples water content was about 5.4 wt%. The optimized interlayer parameter d001 of models P1N1, P1N2, and P1N3 is collected in Table 3.

Table 3. The interlayer and hydrogen bond distances, and intercalation energy in the TBP-Bd models with hydrated sodium cations in the interlayer space. model

d001

∆Eint

Ow–H•••Ob

Ow–H•••Ow

C–H•••Ow

C–H•••Ob

P1N1

14.78

-144.39

1.732;2.108;2.907

1.806;2.032;2.894

2.588;3.260;3.322

2.382;2.712;2.906

P1N2

15.18

-100.95

1.828;2.301;2.895

1.711;1.809;2.896

2.894;2.901;2.954

2.145;2.766;2.916

P1N3

15.75

-87.50

1.728;2.681;2.917

1.710;1.812;2.919

2.282;2.699;3.019

2.486;2.770;2.915

EXP

15.8

d001- interlayer and hydrogen bond distances (min, median, max) [Å]; ∆Eint -intercalation energy [kJmol-1] The d001 increases with the increasing number of [Na(H2O)6]+ cations getting a good agreement with an experimental value of 15.8 Å (Table 3). The P1N3 model reached the closest value to the experimental data having a theoretical water content of 5.4%. Also its chemical composition is similar to the composition of beidellite used in the experiment. Thus, the P1N3 model can be considered as suitable for explanation how TBP cation is intercalated in the interlayer space. Hydrogen bonds and intercalation energies The analysis showed that in the P1Nx (x=1-3) models four types of hydrogen bonds exist – two of the weak strength, C–H•••Ob and C–H•••Ow hydrogen bonds, and also moderate-to weak Ow–


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H•••Ob and Ow–H•••Ow hydrogen bonds formed by water molecules presented in the interlayer space (Figure 5, Table 3). The strength of hydrogen bonds formed between basal oxygen atoms and TBP and/or water molecules decreases with increasing number of water molecules in the respective models (Table 3) what is also reflected in the predicted exchange intercalation energies calculated according the reaction scheme (Eq. 1).

Figure 5. Hydrogen bonds in the P1N3 model (yellow dash - Ow–H•••Ob, blue dash - Ow– H•••Ow, mangenta dash - C–H•••Ow and green dash - C–H•••Ob). Vibrational modes Na-Bd From four Na-Bd structural models ab initio molecular dynamics was performed for the N4 model to get power spectrum (PS). The calculated spectrum is presented together with the experimental FTIR spectrum for the Na-Bd sample in Figure 6. Generally, a good correspondence is observed between both spectra. The bands at 3812, 3756, and 3636 cm-1 in the region with the highest energy in the calculated PS have analogy with the bands observed at 3700, 3655 and 3620 cm-1 in the experimental IR spectrum and are attributed to the stretching vibrations of hydroxyl groups of the beidellite structure. Farmer and Russell presented two



broadened bands at 3660 cm-1 and 3630 cm-1 with remark that these bands must arose from two types of OH groups, differing, perhaps, with respect to the site with Si/Al substitution in neighboring tetrahedral.51 Broadened bands in a range of ~ 3550-3200 cm-1 in both spectra belong to the OH stretching vibrations of water molecules in the interlayer space of the Na-Bd structure. A peak at 1620 cm-1 in the calculated PS has a correspondence with peak of low intensity at 1632 cm-1 in the FTIR spectrum and is assigned to the HOH bending modes of water molecules.

Figure 6. Experimental IR and calculated power spectra (N4 model) of Na-Bd. Also Grundgeiger et al. assigned a band at 3621 cm-1 to the stretching vibrations of the OH groups in the Bd structure, the broad band 3550-3200 cm-1 to the water OH stretching vibrations, and small sharp band at 1635 cm-1 to the water bending vibrations.34 Detailed view at the calculated PS in a range 1200 – 400 cm-1 helped to interpret experimental spectrum finding individual contributions and order of Al–O–Al, Al-OH-Al, and Al–O–Si units of the tetrahedral and octahedral sheets, respectively. Calculated projected vibrational density of states (PVDOS) clearly distinguished the type of the individual vibrations in this field of spectrum and together with the FTIR spectrum is shown in Figure 7. The Al-O/Si-O stretching modes of the Al–O–Al,


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Al-OH-Al, and Al–O–Si units are well separated in the PVDOS showing the highest frequencies for the Al–O–Si units (1058 and 1000 cm-1) that can correspond to bands at 1117 and 1038 cm-1 in the experimental IR spectrum (Figure 7). Similar assignment of the mixed stretching vibrations of the Si–O–Si and Si–O–M (M= Al, Mg, Fe) bridges was done by Karakassides et al. 52

(including authors cited therein) in a region 1200–950 cm-1 of the Li+ montmorillonite. In a

range of 850-650 cm-1 Farmer and Russell51 assigned bands to the bending vibrations of the Si– O–Si and Si–O–Al bridges mixed with the stretching Al-O modes of the Al-OH-Al groups. Typical beidellite bands (925, 534, 474 and 418 cm-1) visible in the experimental spectrum were confirmed by AIMD calculations (Figures 6, 7) very clearly showing the presence mainly of the Al-OH-Al bending vibrations in these bands. These bands were also detected by Rusell et al.53 at 935, 555, 470 and 416 cm-1, which were shifted to 705, 530, 466 and 330 cm-1 in the deuterated form of beidellite sample. Projected power spectrum of Al-O-Al vibrations (Figure7) revealed that their main contributions could be to the experimental bands at 805, 708, 565 (sh), 534, 474 and 418 cm-1 in the FTIR spectrum. In this low frequency region (below 1000 cm-1) numerous overlapping vibrations of different structural units exist and their complete interpretation is very difficult. The calculated projected PS clearly showed how and which structural units can contribute to the individual bands.

TBP – Bd FTIR spectrum of the TBP-Bd sample is presented together with the FTIR spectrum of the natural beidellite in Figure 8. Evidently, both spectra have many similar features specifically in



ranges below 1200 cm-1 and above 3000 cm-1. Spectral bands in these regions have an origin in

Figure 7 Contributions of calculated projected power spectra of respective vibrations (N4 model) compared to the experimental FTIR of Na-Bd. the Na-beidellite structure as discussed above. The broad band in the FTIR spectrum of the TBPBd in a range 3500 – 3200 cm-1 assigned to the OH stretching vibrations of water molecules is well distinguished giving an evidence of the presence of structural water (7.9%) in the TBP- Bd intercalate. Thus, this finding supports to use in the molecular simulations TBP-Bd intercalate models that contain in the structure also hydrated Na+ cations (models P1N1-3). From those models the power spectrum was calculated for the P1N3 model and is shown together with the FTIR spectrum of the TBP-Bd sample in Figure 9. The calculated bands belonging to the


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stretching OH modes of water molecules are more structured comparing to the PS of the N4 model of the Na-Bd intercalate (Figure 6) because of structural changes in the arrangement of water molecules. The bands in a range 1750 – 1900 cm-1 belongs the HOH bending vibrations (Figures 8 and 9). In the FTIR spectrum of the TBP-Bd sample new bands arise in ranges 2500– 3200 cm-1 and 1240–1500 cm-1, respectively (Figure 9).

Figure 8. Experimental FTIR spectra of Na-Bd and TBP-Bd samples.

Figure 9. Experimental FTIR and calculated power spectra (P1N3 model) of TBP-Bd. They have an origin in the presence of the TBP cations in the sample and represent CH stretching (higher frequency range) and bending (lower frequency range) modes of the CH3 23 Environment ACS Paragon Plus


and/or CH2 units, respectively (Figure 9). In the FTIR spectrum bands of the CH bending modes are of low intensity (Figure 8) whereas in the calculated PS are well resolved. The CHx bands are similar to the bands assigned to the tetrabutyl ammonium cation intercalated in montmorillonite (TBA-Mnt)32. Two high frequency bands at 2962, 2934 cm-1 could be assigned to the asymmetric CH stretching modes whereas the band at 2872 cm-1 can represent symmetric CH stretching vibrations, in analogy to the interpretation of the FTIR spectrum of TBA-Mnt.32 Calculated projected power spectra allowed distinguishing of contributions from CH3- and CH2units. Each projected PS showed two peaks (Figure 10) and from the positions of respective bands it can be concluded that the CH3- groups have the asymmetric band shifted to the higher frequency than the CH2- groups by about 22 cm-1. The energies of the symmetric stretching vibrations (lower frequency at ~3000 cm-1 in the PS) are almost the same for both CH units. The intensities of the projected bands in Figure 10 were enlarged for clarity. The spectrum in the region below 1500 cm-1 is more complicated because of many overlapped vibrations of different structural units existing in the TBP-Bd intercalate and mixing and overlapping of different types of vibrations (e.g. stretching of C-P bond, C-H bending modes, etc.). The analysis of the eigenvectors of the calculated modes allowed describing this region in detail. The asymmetric CH bending vibrations were detected in a region from 1348 to 1495 cm-1 in the calculated spectrum that corresponds to a region from 1366 to 1482 cm-1 in the FTIR spectrum (Figure 8). The symmetric CH bending vibrations act in a region 1242 - 1349 cm-1 (calculated PS) and corresponding bands with low intensity could be attributed to a range between 1260 and 1365 cm-1 in the experimental spectrum.


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Figure 10. Projected stretching VDOS of the CH3- and CH2- groups and the total power spectrum of the TBP-Bd (P1N3 model). PVDOSs are 10x scaled for a better resolution. The projected PS revealed the asymmetric C-P stretching vibration at 1266 cm-1 in the calculated PS. In the experimental spectrum the C-P vibrations are badly distinguished as they overlap with Si-O and Al-O stretching modes. A small shoulder at 1112 cm-1 appeared with an intensive band with maximum at 1034 cm-1 could be assigned to the C-P modes. Calculated typical beidellite band at 917 cm-1 for AlOHAl bending vibrations corresponded with band at 925 cm-1 in FTIR spectrum. The symmetric C-P stretching modes have lower frequencies than asymmetric and from the calculated projected PS corresponding band is detected at 888 cm-1 which overlaps with the Al-O-Si bands. The assigned band could have a correspondence with the band at 860 cm-1 (sh) in the FTIR spectrum.



CONCLUSIONS Beidellite intercalated with tetrabutyl phosphonium cations was studied both experimentally and theoretically. DFT-based calculations were used to explain details of the structure, the mechanism of intercalation, and the stability of the TBP-Bd intercalate. Ab initio molecular dynamics simulation was applied to predict vibrational density of states that helped in the interpretation of the FTIR spectra. Good correspondence between calculated and experimental d001 values was found for Na-Bd models and beidellite sample. It was found that the negative layer charge localized on the surface oxygen atoms bound the Al3+ substitution in the tetrahedral sheet stabilizes the hydrated interlayer cation trough the Coulombic interactions. In addition, the stabilization is also enhanced by the hydrogen bonds formed between water molecules and the surface oxygen atoms of the beidellite layers. Analysis of the hydrogen bonds in the beidellite interlayer showed that they are from weak to moderate strength. It was observed that in the TBP-Bd intercalates phosphonium cation changed their configuration to flatty arrangement in the interlayer space. The calculations showed that the certain water content had to be considered in the model construction. The P1N3 model, which contains hydrated Na+ cations, showed a good agreement in d001 spacing with experimental value of the TBP-Bd sample. Calculated intercalation energies evidenced that the organoclays with the phosphonium cations are significantly more stable than the corresponding intercalates with alkylammonium cations. The stability of the TBP-Bd models has the origin in the Coulombic interactions between the positively charged phosphonium head and the negative charge of the beidellite layer localized at the Si4+/Al3+ substitution, similarly as in case of the Na-Bd models.


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The stability is further enhanced through the dispersion interactions (weak hydrogen bonds) of the butyl chains and basal surface oxygen atoms of the beidellite layers. FTIR spectra of both Na-Bd and TBP-Bd samples were interpreted with the help of the analyzed vibrational density of states obtained from the AIMD calculations. Application of the projected VDOS allowed a detailed interpretation of the vibrational modes of the structural units of the beidellite layers (e.g. Al–O–Si, Al–O–Al, and Al-OH-Al) and/or TBP cations (e.g. P-C and CH stretching modes). ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Figure S1. Initial Na-Bd models with multiple substitutions. (TIF) Figure S2. Initial TBP-Bd models without water content in the interlayer space. (TIF) Figure S3. Initial TBP-Bd models with multiple substitutions in the Bd layer. (TIF) Figure S4. X-ray diffraction patterns of Na-Bd and TBP-Bd samples (TIF).

AUTHOR INFORMATION Corresponding Authors *tel.: +421 2 59410457, e-mail: [email protected]; *tel. +43 1 4765491148, e-mail: [email protected] Author Contributions



The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.‡ These authors contributed equally. Funding Sources

Authors ES and ĽJ received funding from Slovak Grant Agency VEGA (Grant 2/0141/17) and Slovak Research and Development Agency (APVV-15-0741 and APVV-15-0347). Author DT received funding from Austrian Grant Agency (FWF), project No. I3263-N34.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT ES and LJ are grateful for the financial support by the Slovak Grant Agency VEGA (Grant 2/0141/17) and Slovak Research and Development Agency (APVV-15-0741 and APVV-150347). DT thanks to the support by the Austrian Grant Agency (FWF), project No. I3263-N34. The authors wish to express their thanks to Dr. H. Pálková for IR spectra measurements. REFERENCES 1. Grim, R., Clay Mineralogy, 2nd Ed.; McGraw-Hill Book Co.: New York, 1968. 2. Klein , C.; Hurlbut, C., Systematic Mineralogy Part Iv: Silicates. Manual of Mineralogy, 20th Ed. ; John Wiley & Sons: New York, , 1985. 3. Schultz, D., An Introduction to Soil Mineralogy. In Minerals in Soil Environments;. Dixon, J., Weed, S., Eds.; Soil Science Society of America: Madison, WI: 1989; pp 1-34. 4. Amonette, J. E.; Zelazny, L. W.; Eds. Origin and Mineralogy of Clays: Clays and the Environment; Springer-Verlag: New York, NY, 1995. 5. Post, J. L.; Cupp, B. L.; Madsen, F. T., Beidellite and Associated Clays from Delamar Mine and Florida Mountain Area, Idaho. . Clays and Clay Minerals 1997, 45, 240-250. 6. Bergaya, F.; Theng, B.; Lagaly, G.; Handbook of clay science. Elsevier, Amsterdam, 2006. 7. Teng, B. K. G., The Chemistry of Clay-Organic Reactions; Adam Hilger, London, 1974.


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8. Smith, J. A.; Jaffe, P. R.; Chiou, C. T., Effect of 10 Quaternary Ammonium Cations on Tetrachloromethane Sorption to Clay from Water. Environmental Science & Technology 1990, 24, 1167-1172. 9. Gitipour, S.; Bowers, M. T.; Bodocsi, A., The Use of Modified Bentonite for Removal of Aromatic Organics from Contaminated Soil. Journal of Colloid and Interface Science 1997, 196, 191-198. 10. Kaufhold, S.; Pohlmann-Lortz, M.; Dohrmann, R.; Nuesch, R., About the Possible Upgrade of Bentonite with Respect to Iodide Retention Capacity. Applied Clay Science 2007, 35, 39-46. 11. Ferrage, E.; Lanson, B.; Michot, L. J.; Robert, J. L., Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-Ray Diffraction Profile Modeling. Journal of Physical Chemistry C 2010, 114, 4515-4526. 12. Guegan, R., Intercalation of a Nonionic Surfactant (C10e3) Bilayer into a NaMontmorillonite Clay. Langmuir 2010, 26, 19175-19180. 13. Kuligiewicz, A.; Derkowski, A.; Szczerba, M.; Gionis, V.; Chryssikos, G. D., Revisiting the Infrared Spectrum of the Water-Smectite Interface. Clays and Clay Minerals 2015, 63, 1529. 14. Kirkpatrick, R. J.; Kalinichev, A. G.; Bowers, G. M.; Yazaydin, A. O.; Krishnan, M.; Saharay, M.; Morrow, C. P., NMR and Computational Molecular Modeling Studies of Mineral Surfaces and Interlayer Galleries: A Review. American Mineralogist 2015, 100, 1341-1354. 15. Cygan, R. T.; Greathouse, J. A.; Heinz, H.; Kalinichev, A. G., Molecular Models and Simulations of Layered Materials. Journal of Materials Chemistry 2009, 19, 2470-2481. 16. Marry, V.; Rotenberg, B.; Turq, P., Structure and Dynamics of Water at a Clay Surface from Molecular Dynamics Simulation. Physical Chemistry Chemical Physics 2008, 10, 48024813. 17. Koteja, A.; Szczerba, M.; Matusik, J., Smectites Intercalated with Azobenzene and Aminoazobenzene: Structure Changes at Nanoscale Induced by Uv Light. Journal of Physics and Chemistry of Solids 2017, 111, 294-303. 18. Dawley, M. M.; Scott, A. M.; Hill, F. C.; Leszczynski, J.; Orlando, T. M., Adsorption of Formamide on Kaolinite Surfaces: A Combined Infrared Experimental and Theoretical Study. Journal of Physical Chemistry C 2012, 116, 23981-23991. 19. Scott, A. M.; Dawley, M. M.; Orlando, T. M.; Hill, F. C.; Leszczynski, J., Theoretical Study of the Roles of Na+ and Water on the Adsorption of Formamide on Kaolinite Surfaces. Journal of Physical Chemistry C 2012, 116, 23992-24005. 20. Peng, C. L.; Min, F. F.; Liu, L. Y.; Jun, C., The Adsorption of Caoh+ on (001) Basal and (010) Edge Surface of Na-Montmorillonite: A Dft Study. Surface and Interface Analysis 2017, 49, 267-277. 21. Michalkova, A.; Tunega, D., Kaolinite : Dimethylsulfoxide Intercalatea Theoretical Study. Journal of Physical Chemistry C 2007, 111, 11259-11266. 22. Michalkova, A.; Tunega, D.; Nagy, L. T., Theoretical Study of Interactions of Dickite and Kaolinite with Small Organic Molecules. Journal of Molecular Structure-Theochem 2002, 581, 37-49. 23. Emmerich, K.; Koeniger, F.; Kaden, H.; Thissen, P., Microscopic Structure and Properties of Discrete Water Layer in Na-Exchanged Montmorillonite. Journal of Colloid and Interface Science 2015, 448, 24-31.



24. Schampera, B.; Tunega, D.; Solc, R.; Woche, S. K.; Mikutta, R.; Wirth, R.; Dultz, S.; Guggenberger, G., External Surface Structure of Organoclays Analyzed by Transmission Electron Microscopy and X-Ray Photoelectron Spectroscopy in Combination with Molecular Dynamics Simulations. Journal of Colloid and Interface Science 2016, 478, 188-200. 25. Heinz, H.; Vaia, R. A.; Krishnamoorti, R.; Farmer, B. L., Self-Assembly of Alkylammonium Chains on Montmorillonite: Effect of Chain Length, Head Group Structure, and Cation Exchange Capacity. Chem. Mat. 2007, 19, 59-68. 26. Scholtzová, E.; Madejová, J.; Jankovič, L.; Tunega, D., Structural and Spectroscopic Characterization of Montmorillonite Intercalated with N-Butylammonium Cations (N = 1-4) – Modeling and Experimental Study. Clays and Clay Minerals 2016, 64, 399–410. 27. Scholtzová, E. In Problematic Parts of Ir Spectrum and Stability of Organoclays - DFT Study., BIT's 5th Annual Conference of AnalytiX-2017, Fukuoka, Japan, Fukuoka, Japan, 2017; p 117. 28. Pálková, H.; Zimowska, M.; Jankovič, L.; Sulikowski, B.; Serwicka, E. M.; Madejová, J., Thermal Stability of Tetrabutyl-Phosphonium and -Ammonium Exchanged Montmorillonite: Influence of Acid Treatment. Applied Clay Science 2017, 138, 63-73. 29. Alves, J. L.; Rosa, P.; Morales, A. R., A Comparative Study of Different Routes for the Modification of Montmorillonite with Ammonium and Phosphonium Salts. Applied Clay Science 2016, 132, 475-484. 30. Xie, W.; Xie, R. C.; Pan, W. P.; Hunter, D.; Koene, B.; Tan, L. S.; Vaia, R., Thermal Stability of Quaternary Phosphonium Modified Montmorillonites. Chem. Mat. 2002, 14, 48374845. 31. Madejová, J.; Pálková, H.; Jankovič, Ľ., Degradation of Surfactant-Modified Montmorillonites in Hcl. Materials Chemistry and Physics 2012, 134, 768-776. 32. Scholtzova, E.; Madejova, J.; Tunega, D., Structural Properties of Montmorillonite Intercalated with Tetraalkylammonium Cations-Computational and Experimental Study. Vibrational Spectroscopy 2014, 74, 120-126. 33. Scholtzová, E.; Tunega, D.; Madejová, J.; Pálková, H.; Komadel, P., Theoretical and Experimental Study of Montmorillonite Intercalated with Tetramethylammonium Cation. Vibrational Spectroscopy 2013, 66, 123-131. 34. Grundgeiger, E.; Lim, Y. H.; Frost, R. L.; Ayoko, G. A.; Xi, Y. F., Application of Organo-Beidellites for the Adsorption of Atrazine. Applied Clay Science 2015, 105, 252-258. 35. Rhouta, B.; Bouna, L.; Maury, F.; Senocq, F.; Lafont, M. C.; Jada, A.; Amjoud, M.; Daoudi, L., Surfactant-Modifications of Na+-Beidellite for the Preparation of TiO2-Bd Supported Photocatalysts: I-Organobeidellite Precursor for Nanocomposites. Applied Clay Science 2015, 115, 260-265. 36. Gailhanou, H., et al., Thermodynamic Properties of Illite, Smectite and Beidellite by Calorimetric Methods: Enthalpies of Formation, Heat Capacities, Entropies and Gibbs Free Energies of Formation. Geochimica Et Cosmochimica Acta 2012, 89, 279-301. 37. Robin, V.; Tertre, E.; Beaufort, D.; Regnault, O.; Sardini, P.; Descostes, M., Ion Exchange Reactions of Major Inorganic Cations (H+, Na+, Ca2+, Mg2+ and K+) on Beidellite: Experimental Results and New Thermodynamic Database. Toward a Better Prediction of Contaminant Mobility in Natural Environments. Applied Geochemistry 2015, 59, 74-84. 38. Ammann, L.; Bergaya, F.; Lagaly, G., Determination of the Cation Exchange Capacity of Clays with Copper Complexes Revisited. Clay Minerals 2005, 40, 441-453.


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39. Kresse, G.; Hafner, J., Ab-Initio Molecular-Dynamics for Open-Shell Transition-Metals. Physical Review B 1993, 48, 13115-13118. 40. Kresse, G.; Furthmuller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Physical Review B 1996, 54, 11169-11186. 41. Perdew, J. P.; Burke, K.; Wang, Y., Generalized Gradient Approximation for the Exchange-Correlation Hole of a Many-Electron System. Physical Review B 1996, 54, 1653316539. 42. Blochl, P. E., Projector Augmented-Wave Method. Physical Review B 1994, 50, 1795317979. 43. Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Physical Review B 1999, 59, 1758-1775. 44. Ferrario, M.; Ryckaert, J. P., Constant Pressure-Constant Temperature MolecularDynamics for Rigid and Partially Rigid Molecular-Systems. Molecular Physics 1985, 54, 587603. 45. Nosé, S., A Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511-519. 46. Beidellite, http://www.mindat.org/Min-604.Html (Accessed Oct 15, 2016). 47. Scholtzová, E.; Madejová, J.; Tunega, D., Structural Properties of Montmorillonite Intercalated with Tetraalkylammonium Cations—Computational and Experimental Study. Vibrational Spectroscopy 2014, 74, 120-126. 48. Castellano, R. K., Progress toward Understanding the Nature and Function of C-H Center Dot Center Dot Center Dot O Interactions. Current Organic Chemistry 2004, 8, 845-865. 49. Desiraju, G. R.; Steiner, T., The Weak Hydrogen Bond in Structural Chemistry and Biology 2nd ed.; Oxford University Press: Oxford, 2006. 50. Dweck, J.; Barreto, E. P.; Meth, S.; Buchler, P. M., Partially Exchanged Organophilic Bentonites. Journal of Thermal Analysis and Calorimetry 2011, 105, 907-913. 51. Farmer, V. C.; Russell, J. D., Infrared Absorption Spectrometry in Clay Studies. Spectrochim. Acta 1964, 20, 1149-1173. 52. Karakassides, M. A.; Gournis, D.; Petridis, D., An Infrared Reflectance Study of Si-O Vibrations in Thermally Treated Alkali-Saturated Montmorillonites. Clay Minerals 1999, 34, 429-438. 53. Russell, J. D.; Farmer, V. C.; Velde, B., Replacement of OH by OD in Layer Silicates, and Identification of the Vibrations of These Groups in Infra-Red Spectra. Mineralogical magazine 1970, 37, 869-879.



TOC Graphic


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Figure 1. a) Nn1, b) Nn2, c) Nf1 and d) Nf2 models. Starting position of hydrated sodium cation – up, optimized – down, AlO4 tetrahedron – in cyan.

ACS Paragon Plus Environment


Figure 2. Number of water layers, Na•••Al and d001 distances in Na-Bd models with one hydrated sodium cation. 202x142mm (300 x 300 DPI)


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Figure 3. Optimized position of the TBP cation in P1 model as example.



Figure 4. Optimized curved structure of butyl chains of TBP cation in P4 model as example.


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Figure 5. Hydrogen bonds in the P1N3 model (yellow dash - Ow–H•••Ob, blue dash - Ow–H•••Ow, mangenta dash - C–H•••Ow and green dash - C–H•••Ob).



Figure 6. Experimental IR and calculated power spectra (N4 model) of Na-Bd.


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Figure 7 Contributions of calculated projected power spectra of respective vibrations (N4 model) compared to the experimental FTIR of Na-Bd.



Figure 8. Experimental FTIR spectra of Na-Bd and TBP-Bd samples.


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Figure 9. Experimental FTIR and calculated power spectra (P1N3 model) of TBP-Bd.



Figure 10. Projected stretching VDOS of the CH3- and CH2- groups and the total power spectrum of the TBP-Bd (P1N3 model). PVDOSs are 10x scaled for a better resolution.


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On Stability of Tetrabutylphosphonium Beidellite Organoclay - The

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