Theoretical Study of the Roles of Na+ and Water on the Adsorption of

Article pubs.acs.org/JPCC

Theoretical Study of the Roles of Na+ and Water on the Adsorption of Formamide on Kaolinite Surfaces Andrea Michalkova Scott,†,‡ M. Michele Dawley,§ Thomas M. Orlando,§ Frances C. Hill,‡ and Jerzy Leszczynski*,†,‡ †

Interdisciplinary Nanotoxicity Center, Department of Chemistry, Jackson State University, 1400 Lynch Street, P.O. Box 17910, Jackson, Mississippi 39217, United States ‡ U.S. Army Engineer Research and Development Center (ERDC), Vicksburg, Mississippi 39180, United States § School of Chemistry and Biochemistry, Georgia Institute of Technology, 901 Atlantic Drive, Room G209C, Atlanta, Georgia 30332-0400, United States ABSTRACT: The adsorption of formamide (FA) on kaolinite surfaces and the roles of Na+ and water on the adsorption were investigated theoretically using a density functional theory. The calculations reveal that adsorbed FA is able to form stable complexes with the tetrahedral and octahedral surfaces of kaolinite. The octahedral surface possesses a larger binding affinity toward FA than the tetrahedral site of kaolinite partially due to the presence of the surface hydroxyl groups that are more active in the intermolecular interactions than the basal oxygen atoms of the siloxane or tetrahedral sites. The calculated (basis set superposition error corrected) binding energies of FA on the kaolinite octahedral and tetrahedral surfaces are −14.8 and −13.7 kcal/mol, respectively. FA also forms slightly less stable complexes with the negatively charged kaolinite fragments than with neutral ones. The addition of a sodium cation plays a key role in this adsorption, while addition of one water molecule affects the binding strength insignificantly. A chemical reaction occurs involving the Na octahedral surface complex, during which hydrogen is removed from FA and binds with the surface oxygen. Full solvation decreases the adsorption affinity of FA toward both kaolinite surfaces. Estimated Gibbs free energies indicate that the adsorption of FA on most of the studied kaolinite clusters is thermodynamically feasible from both gas phase and water solution.

1. INTRODUCTION Mineral surfaces have been suggested as substrates that support the catalytic assembly of organic and biochemical molecules and thus may play a major role in the origin of life.1 Specifically, they are believed to have the appropriate characteristics to harbor precursor organic molecules for the synthesis of important biomolecules, e.g., concentrating possible reactants and products. Moreover, their involvement as catalysts in the molecular evolution (including adsorption, concentration, and prebiotic catalysis of environmental substances) has also been proposed.1 In this study of interactions between model surfaces and organic molecules, we started from a simple precursor molecule, formamide (hereafter denoted as FA), since it can serve as a highly versatile building block. FA is one of the simplest molecules containing the four most common elements of the universe (H, C, O, and N). FA is a small, highly polar molecule that readily intercalates kaolinite.2,3 It has been shown that hot FA in the presence of various catalysts (i.e., TiO2, zeolites, common clays, kaolin, olivines, and phosphate minerals)4 reacts to form a large variety of nucleic bases. The richest yields were obtained in the presence of clays, TiO2, and cosmic dust analogues.5 In this paper we focus on the interactions of FA with the clay kaolinite (Al2Si2O5(OH)4), which has a 1:1 dioctahedral structure.6−8 An individual layer consists of two connected © 2012 American Chemical Society

sheets: a tetrahedral sheet formed from SiO4 tetrahedra sharing corners and an octahedral sheet consisting of AlO6 octahedra sharing edges. Both sheets share a common plane of apical oxygen atoms. Layers are kept together via hydrogen bridges between surface hydroxyl groups on the aluminum−oxygen side and the basal oxygen atoms on the siloxane side. Minerals of the kaolinite group are very often used for the preparation of intercalated materials. A review was recently published which provides a summary of the most recent achievements in the study of kaolinite organo complexes (among them kaolinite− FA) using vibrational spectroscopy combined with X-ray powder diffraction and thermal analysis.9 FA is known to adsorb by forming hydrogen bonds (as shown experimentally10−15 and theoretically16−19) between the carbonyl group and nitrogen of FA and inner surface hydroxyls of the kaolinite group.15 These hydrogen bonds were shown to be quite strong.16 Moreover, FA hydrogen atoms (from NH2 or CH groups) can also form relatively weak H-bonds with the siloxane surface as has been calculated (PM3 and PBE/6-31G) for intercalated FA−kaolinite.19 A two-stage intercalation was shown to increase the strength of hydrogen bonding between Received: May 9, 2012 Revised: October 12, 2012 Published: October 17, 2012 23992

dx.doi.org/10.1021/jp3045324 | J. Phys. Chem. C 2012, 116, 23992−24005

The Journal of Physical Chemistry C

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the inner surface hydroxyls of kaolinite and FA.20 Edge sites are dominant in the reaction and basal surfaces remain unaltered.21 Moreover, minerals of the kaolinite group can contain the isomorphic substitution of some Mg2+ for Al3+ ions in the octahedral sheet and/or some Al3+ for Si4+ species in the tetrahedral sheet.22 This can cause the presence of local charge defects23 or permanent negative charges and lower the pH of the system. The pH was shown to affect significantly the adsorption of nucleic acids on clay mineral surfaces.24,25 Maximum adsorption was found at lower pH (7). The negative mineral charges can be compensated by exchangeable cations. One of the most common interlayer cations of kaolinite is the sodium cation.26 Since even the simplest interaction is not fully understood, we concentrate on understanding the molecular adsorption and potential dissociative adsorption energetics and structures. Thus, our theoretical study focuses on the interactions of FA with (1) pure kaolinite, (2) negatively charged kaolinite fragments, and (3) kaolinite in the presence of a sodium cation. It has also been suggested that prebiotic evolution from the one carbon atom compound (H2NCOH) to phosphorylated nucleosides (i.e., along the path of increasing chemical complexity) had to shift to water-based processes.4 Therefore, the adsorption of FA on kaolinite in the presence of water has also been examined in this work. Both the octahedral and tetrahedral sheets of the kaolinite layered structures have been investigated.

case of the tetrahedral systems, the FA molecule was initially placed in an almost parallel orientation with respect to the surface. It has been previously reported that FA favors this orientation when intercalated in kaolinite, as calculated at the PBE level.19 The ab initio molecular orbital calculations were performed with the Gaussian 09 program package31 using density functional theory32 and the M05-2X functional33−35 in conjunction with the 6-31G(d) basis set.36 The M05-2X functional was chosen because it shows an acceptable performance for the main group thermochemistry and noncovalent interactions. The minimum energy geometries were determined to be the true minima by the absence of the imaginary frequencies in the calculated vibrational spectra. The same methodology is used to calculate the infrared spectra of FA−kaolinite systems, and the theoretical results are compared with experimental data in our companion paper.37 The interaction energy for each system was calculated as the difference between the total electronic energy of the whole system and the sum of total electronic energies of both subsystems. Calculated interaction energies were corrected for the basis set superposition error (BSSE) using the counterpoise method.38 The adsorption energies of all studied FA−kaolinite systems were also calculated at the M05-2X/6-31G(d,p) level of theory to test the effect of a larger basis set on the FA− kaolinite interactions. COSMO (COnductor-like Screening MOdel)39 was used to obtain interactions of the system with a solvent where the solvent is treated as a dielectric continuum with a permittivity, ε, surrounding the solute molecules outside a molecular cavity. The COSMO method is more accurate for solvents with a higher permittivity because a solvent with infinite permittivity behaves like an ideal conductor. With water (ε ≈ 78.4), a very good accuracy is achieved. For all studied adsorption systems of FA on kaolinite, the adsorption enthalpy (ΔH) and Gibbs free energy (ΔG) were calculated at a temperature of 110 K to characterize the thermodynamics of FA interactions with the kaolinite surfaces. The entropy S(T) values were calculated using the rigid rotor− harmonic oscillator−ideal gas approximation based on the vibrational frequencies at 110 K. The existence and characteristics of the intermolecular interactions were studied using Bader’s “atoms in molecules” approach (AIM).40 This approach provides a mathematical occurrence criterion of a (3,−1) critical point of the electron density distribution between two atomic centers to indicate the presence of a stabilizing interaction. This generally indicates the existence of a chemical bond.41,42 The charge density (ρ(r)) and the Laplacian of the charge density (∇2ρ(r)) at such points were also calculated in this study. Depending on the values obtained from the total electron density distribution, various types of interaction can be distinguished using these parameters. A small value for ρ(r) at a critical point and a large, positive value for ∇2ρ(r) at a critical point are typically observed for ionic, van der Waals, and hydrogen bonds. On the other hand, covalent, dative, or metallic bonds are identified with large values of ρ(r) and large, negative ∇2ρ(r) values at (3,−1) critical points. The basal planes of kaolinite are believed to carry a permanent negative structural charge due to the isomorphic substitution of the central Si4+ by Al3+ ions in the tetrahedral sheet in the crystal lattice.43 Moreover, substitution of Al3+ by Mg2+ was experimentally shown to occur in the octahedral

2. MODELS AND METHODS The cluster model of the single kaolinite layer was prepared using the experimental crystal structure data.27 Each cluster was constructed as a cutoff from the periodic structure of this mineral. Both types of clay surfaces were simulated. The model consists of one tetrahedral ring of the siloxane sheet consisting of six SiO4 tetrahedra, which share corners, and one octahedral ring of the aluminum−oxygen sheet formed of six AlO6 octahedra sharing edges. The positions of hydrogen atoms in the starting geometry for optimization were estimated. All surface hydroxyl groups linked to the aluminum atoms are also included in the model. Dangling bonds of the clusters were saturated by hydrogen atoms to keep the model electroneutral. This method for termination of the missing bonds was shown to be the most efficient in several theoretical studies of adsorption on cluster models of clay minerals.28−30 The chemical formula of the neutral cluster is Al6Si6O36H30, and we refer to the octahedral or tetrahedral clusters as K(o) or K(t), respectively. Initially, FA was placed above the middle of the ditrigonal and octahedral cavities of the mineral fragment. Several different orientations of FA toward both surfaces were tested using smaller kaolinite models to investigate the most favorable orientation (parallel, perpendicular, by the carbonyl oxygen atom, or by the FA amide or C−H group). Based on the obtained results using small kaolinite models and previously published studies,16,19 the following initial geometries for our studied systems of FA on K(o) and K(t) were used to proceed with further calculations. In the octahedral system, the FA molecule was oriented almost perpendicular to the carbonyl oxygen atom and one hydrogen atom of the NH2 group facing the surface of the small kaolinite model. This orientation was shown to be favorable for FA with respect to the plane of the octahedral sheet during adsorption and intercalation in dickite16 and kaolinite calculated at the PM3 level.19 In the 23993



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Figure 1. Top and side views of the optimized structure of formamide (FA) adsorbed on cation free neutral nonhydrated octahedral (a, K(o)−FA) and tetrahedral (b, K(t)−FA) surfaces of kaolinite mineral (M05-2X/6-31G(d)).

sheet of kaolinite.44 Thus, substitution of Si4+ by Al3+ in the tetrahedral fragment and Al3+ by Mg2+ in the octahedral fragment, which causes the kaolinite fragment to become negative (having 1− charge), was also modeled. Negatively charged octahedral and tetrahedral fragments of kaolinite will be denoted K(t−) and K(o−). In the presence of a sufficient amount of Na+, the surface charge of the mineral can be neutralized, making negatively charged mineral surfaces good adsorbents.45 The cation’s nearest environment contains one magnesium substitution in the octahedral sheet46,47 or one aluminum substitution in the tetrahedral sheet.48 Thus, in our study after substitution in the sheets, the model was kept electroneutral by the addition of a sodium cation (one of the most common exchangeable cations of kaolinite26), which compensated a single (1−) negative surface charge. The nonhydrated systems of FA adsorbed on octahedral and tetrahedral surfaces of kaolinite will be denoted K(o)Na−FA and K(t)Na−FA. Special attention was paid to the initial positions of Na+ and water molecule in the FA−kaolinite systems. In a way similar to that described above, based on the investigations of different orientations of FA adsorbed with water and a sodium cation on smaller kaolinite models and based on other theoretical and experimental studies cited below, initial structures of these adsorbates on K(o) and K(t) were prepared. The cation was positioned above the middle of the ditrigonal cavity. This position was also calculated to be the most favorable on various clay mineral models.46,48 In the case of tetrahedral system with sodium cation, the angle between the molecular plane of FA and the surface was about 30°. This orientation of FA with

respect to the plane of the silicate sheet was determined experimentally with IR spectroscopy for FA intercalated in Namontmorillonite.49 Adsorption and intercalation of one water molecule in kaolinite have been already studied theoretically.50,51 The preferable position and orientation of the water molecule were governed by the formation of two hydrogen bonds (one with an oxygen atom and one with the surface OH group on the octahedral surface and both with the basal oxygen atoms on the siloxane surface). For hydrated systems with a sodium cation, it has been shown that water adsorbs most preferably via a hydrogen bond to one of the surface oxygen atoms while oriented by its oxygen atom toward Na+.47 These findings agree with our results for small kaolinite models. Thus, such initial structures of water and Na+ placed close to FA on the kaolinite surface were applied in calculations presented in this paper. The models were allowed to fully optimize, and the most stable structures were further analyzed. The models of FA adsorbed on the hydrated octahedral and tetrahedral surface of kaolinite without sodium cation will be denoted K(o)W−FA and K(t)W−FA, and those with sodium cation will be denoted K(o)NaW−FA and K(t)NaW−FA, respectively. The oxygen atoms of the surface hydroxyls of the octahedral site will be denoted Os. We will also use the following notations: Ob for the basal oxygen atoms of the tetrahedral site, and Ow and Hw for the oxygen and hydrogen atoms of water, respectively. 23994



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Table 1. Calculated M05-2X/6-31G(d) H···Y and X···Y (in Parentheses) Distances (Å), X−H···Y Angles (deg), and Electron Density Characteristics (ρ (au) and ∇2ρ (au)) for Nonhydrated K−FA Complexes from Gas Phase K(o)−FA HB1 HB2 HB3 HB4

H···Y (X···Y)

X-H···Y

1.851 (2.863) 2.347 (3.222) 1.880 (2.804) 2.1334 (2.987)

167.5 149.2 158.3 146.5 H···Y (X···Y)

Na···N (Na···(O)C) HB1 HB2 HB3 HB4 H···Y (X···Y) HB1 HB2 HB3 HB4

1.685 2.507 1.999 2.072

(2.717) (3.383) (2.931) (2.994)

2.375 2.256 1.806 1.654 1.804

K(t)−FA ρ 0.033 32 0.012 43 0.030 61 0.015 24 K(o)Na−FA

X−H···Y

− (3.116) 147.8 (2.714) 153.1 (2.635) 166.0 (2.708) 151.7 K(o−)−FA

∇2ρ

H···Y (X···Y)

X-H···Y

0.103 62 0.040 55 0.098 14 0.058 19

2.409 (3.311) 2.275 (3.014) 2.034 (2.694) −

148.2 128.9 123.4 −

ρ

∇2ρ

0.009 37 0.014 01 0.020 83 − K(t)Na−FA

0.037 92 0.052 07 0.090 01 −

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

0.022 68 0.017 44 0.037 56 0.051 00 0.032 49

0.124 95 0.050 72 0.119 03 0.156 38 0.120 63

2.223 1.915 (2.872) 2.626 (3.200) − −

− 154.6 116.1 − − K(t−)−FA

0.024 27 0.027 31 0.008 14 − −

0.166 13 0.093 57 0.030 34 − −

X−H···Y

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

169.6 149.9 160.1 158.1

0.048 89 0.008 41 0.023 36 0.018 76

0.151 70 0.031 68 0.074 91 0.064 75

2.442 (3.060) − − −

118.6 − − −

0.010 15 − − −

0.039 35 − − −

3. RESULTS AND DISCUSSION 3.1. Intermolecular Interactions. 3.1.1. Nonhydrated Complexes. The structures of FA adsorbed on the nonhydrated Na+ free tetrahedral (K(t)−FA) and nonhydrated Na+ free octahedral (K(o)−FA) surface of kaolinite are depicted in Figures 1−3. The geometrical and topological characteristics of the intermolecular interactions formed between FA and the mineral surfaces are given in Table 1. For adsorption on the octahedral surface (Figure 1a), the target molecule is adsorbed almost above the middle of the sixmember ring comprised of octahedra. Two hydrogen bonds are created between the molecular amino group and surface hydroxyls (HB1 and HB2 in Table 1 and Figure 1a). These are regular N−H···Os and N···H−Os type H-bonds, in which the FA nitrogen or surface oxygen behave as proton-acceptors. N− H···Os is stronger (H···Os is 1.85 Å with ρ and ∇2ρ values of 0.03 and 0.1 au, respectively) than a regular O−H···O H-bond, but the N···H−Os is much weaker (a H···O distance of 2.3 Å and smaller ρ and ∇2ρ values of 0.01 and 0.04 au, respectively). The carbonyl oxygen of FA plays a key role in the interactions with the octahedral surface. The adsorption is governed by the formation of hydrogen bonds between the FA carbonyl oxygen (proton acceptor) with the protons of the surface hydroxyl groups (the Os−H···O(C) H-bonds, given as HB3 and HB4 in Table 1 and Figure 1a). Distances between the carbonyl oxygen and proton donors of the surface hydroxyls are about 1.9 and 2.1 Å with ρ and ∇2ρ values of 0.015−0.03 and 0.058− 0.098 au, respectively. This demonstrates larger bond strengths when compared with HB2. Similar positions and orientations of FA were revealed in theoretical16 and experimental studies52,53 of intercalation and adsorption of FA on dickite, where the carbonyl oxygen atom forms three H-bonds with the surface hydroxyl groups. This finding is also supported by the experimental results of Lipsicas and co-workers for intercalated FA−kaolinite54 where the inner surface hydroxyls were suggested to bond with the carbonyl oxygen. The same intermolecular interactions between FA and dickite were revealed in our previous computational study17 and in several experimental studies,13,14 where in addition to

three O−H···O H-bonds, one more is created between the amide N and hydroxyl of dickite. The NMR study confirms hydrogen-bonding interactions between the amide protons and the kaolinite host in kaolinite−FA.52,53 Adsorption of FA on the tetrahedral surface is quite different. The C−N bond of FA is almost parallel with the plane of the basal oxygen atoms (Ob). The NH2 group of FA forms two N− H···Ob H-bonds (denoted as HB1 and HB2 in Figure 1b and Table 1) with the surface oxygen atoms. Here the NH2 group acts as a proton donor and Ob acts as a proton acceptor. This observation agrees well with conclusions of the NMR experiments of FA−kaolinite intercalation,52,53 which indicates that both amide protons weakly interact with the silica sheet of the adjacent layer. In contrast, it was shown both theoretically22,23 and experimentally19,20 that in intercalated FA− dickite the target molecule interacts with the adjacent silica sheet through one N−H···Ob. This was revealed to be also true in another experimental study of FA intercalated in kaolinite,54 where the FA hydrogen was assumed to donate a hydrogen bond to the basal oxygens of the adjacent kaolinite surface. The carbonyl oxygen creates one H-bond (denoted HB3 in Figure 1b and Table 1) with the OH group at the edge of the mineral fragment. This H-bridge is most likely due to the edge effect of the cluster model used. Therefore, we suggest that this type of interaction does not exist in the real system (it will not be considered in further analysis). HB1 and HB2 are much weaker than N−H···Os in K(o)−FA as can be seen from the long H···O distances and small electron density and Laplacian of the electron density values (see Table 1). The larger ρ and ∇2ρ values in the K(o)−FA correspond with larger adsorption strength of the octahedral complexes compared with K(t)−FA. This implies that FA is more strongly adsorbed on the aluminum−oxygen (octahedral) than on the siloxane (tetrahedral) terminated sheet. 3.1.1.1. Influence of Surface Charge. In order to investigate the differences in the interactions between FA and the kaolinite surfaces due to the presence of different surface charges, calculations of FA adsorbed on a (1−) charged kaolinite fragment were carried out. Thus, new models were prepared to 23995



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Figure 2. Top and side views of the optimized structure of formamide (FA) adsorbed on cation free (1−) negative nonhydrated octahedral (a, K(o−)−FA) and tetrahedral (b, K(t−)−FA) surfaces of kaolinite mineral (M05-2X/6-31G(d)).

is similar to its position in K(t)−FA, but the orientation and intermolecular interactions slightly differ. FA is oriented in such a way that the C−N group points directly toward the place of substitution. The NH2 group behaves again as a proton donor similar to K(t)−FA, but it forms only one weak N−H···Ob Hbond (HB1 in Table 1 and Figure 2b) with the basal oxygen atom bonded directly with the substituted Al3+. The AIM analysis shows this interaction in the negatively charged complex (2.44 Å) to be as strong as HB1 in the neutral one. Also, the ρ and ∇2ρ values are very similar for this H-bond in both complexes (about 0.01 and 0.04 au, respectively). Therefore, we conclude that the presence of a negative charge leads to a small decrease of the adsorption affinity of the kaolinite mineral toward FA. This effect is slightly more significant for the tetrahedral site than for the octahedral site. In this particular case, adsorption of small organic molecules such as FA may be less efficient on negatively charged kaolinite fragments. 3.1.1.2. Sodium Cation Complexes. The presence of a sodium cation drastically affects the adsorption of FA on both kaolinite surfaces (see Figure 3). A sodium cation is located close to the place of substitution and remains in the vicinity of several surface oxygen atoms. This agrees with previous findings on the exchangeable cations, which were found to appear mostly on the edges and hydroxyl faces.55 Such positions were suggested to account for the ionization magnitude of kaolinite and cation adsorption with pH.43,56 As described in section 3.1.1.1, the influence of a negative surface charge in K(o−)−FA and K(t−)−FA causes only slight changes in the FA adsorption. However, the situation is more

model the kaolinite fragment with permanent negative charge and to investigate the influence of negative charge formed in different kaolinite sheets on the FA adsorption. These models contain substitution in the octahedral sheet (one Al3+ was replaced by Mg2+) and in the tetrahedral sheet (one Si4+ was replaced by Al3+), and they are denoted as K(o−)−FA and K(t−)−FA in the text, tables, and figures. The optimized structures of FA interacting with such mineral surfaces are illustrated in Figure 2. One noticeable change of the FA adsorption due to the presence of the negative surface charge is that in both the K(t−)−FA and K(o−)−FA complexes FA is transferred close to the place of the isomorphic substitution. However, no significant changes in the intermolecular interactions of K(o−)−FA are observed despite a strong influence of the negatively charged octahedral surface of kaolinite. FA interacts in the same way with this type of surface as in K(o)−FA (the same orientation of FA and the formation of four H-bonds with the surface OH groups close to the place of substitution). Only HB1 in K(o−)−FA, which is of N−H···Os type (see Table 1 and Figure 2a), is slightly stronger than in neutral K(o)−FA (characterized by a shorter H···Os distance (1.685 Å) and larger ρ and ∇2ρ values (0.05 and 0.15 au, respectively)). The rest of the H-bonds in the charged complex, N···H−Os (HB2) and Os−H···O(C) (HB3 and HB4), are slightly longer (about 0.1 Å) with smaller ρ and ∇2ρ values (about 0.005 and 0.01−0.02 au, respectively) than in K(o)−FA. The influence of the negative surface charge is slightly larger in the case of adsorption of FA on the tetrahedral kaolinite surface than for K(o−)−FA. The location of FA in K(t−)−FA 23996



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Figure 3. Top and side views of the optimized structure of formamide (FA) adsorbed on nonhydrated octahedral (a, K(o)Na−FA) and tetrahedral (b, K(t)Na−FA) surfaces of kaolinite mineral in the presence of a sodium cation (M05-2X/6-31G(d)).

K(o)Na−FA. HB3 is the strongest among all H-bonds in all studied complexes characterized by 1.65 Å H···O distance, and this bond has the largest values of the electron density and the Laplacian of the electron density (0.05 and 0.16 au). In K(t)Na−FA, a sodium cation was found to interact strongly with FA as well as with the model surface of kaolinite. On the tetrahedral surface, Na+ is coordinated by three surface oxygen atoms (with Na···Ob distances of 2.28−2.44 Å) and the carbonyl oxygen of FA (with Na···(O)C distance of 2.223 Å). This leads to a change of the position and orientation of FA toward this surface. The C−N is oriented toward the Ob atom, and the FA molecule is almost parallel with the surface plane. This agrees well with orientation of FA with respect to the plane of the silicate sheet found experimentally for FA intercalated in Na-montmorillonite,49 where the angle between the molecular plane of FA and the surface was about 33°. The hydrogen atoms of the NH2 group are pointed out in the direction of the two surface oxygen atoms. They form two N− H···Ob H-bonds (denoted as HB1 and HB2 in Table 1 and Figure 3b). HB1 is stronger (H···O is 1.92 Å, and ρ and ∇2ρ values are 0.027 and 0.094 au, respectively) and HB2 is slightly weaker (H···O is 2.6 Å, and ρ and ∇2ρ are 0.008 and 0.03 au, respectively) than the N−H···Ob bonds formed in the cation free K(t)−FA system. 3.1.2. Hydrated Complexes. The addition of a water molecule causes the structure of adsorbed FA (K(o)W−FA and K(t)W−FA) to change only slightly compared with the

pronounced when sodium cation is added into the K(o)−FA system. The K(o)Na surface model, due to the presence of both the negatively charged surface oxygen atoms and sodium cation, possesses basic properties. Thus, FA in such an environment reacts somewhat like a simple acid in which the amide group is deprotonated. The removed H+ is transferred close to the negatively charged surface oxygen of the mineral fragment, and it forms an O−H chemical bond. This H+ cation is replaced in FA by sodium cation. FA was also found to react with exchangeable Na+ cation in the FA−Na-montmorillonite intercalate.49 However, due to the level of approximation used (cluster model of kaolinite, 6-31G(d) basis set), the behavior of FA on the K(o)Na surface described above can be an artifact of the approaches applied. Thus, further investigations need to be performed using plane wave methods and models with inclusion of periodic boundary conditions to confirm the above adsorption of FA on the negatively charged octahedral surface of kaolinite with a sodium cation. Na+ forms an ionic bond with the nitrogen atom of FA where the N···Na distance is 2.4 Å (see Table 1). One N···H−Os (HB1 in Table 1 and Figure 3a) and three Os−H···O(C) Hbonds (HB2−HB4 formed between the carbonyl oxygen and surface hydroxyl groups) provide additional stabilization of the complex. These Os−H···O H-bonds (HB2 and HB3) are quite strong compared with N···H−Os (HB1, N···Os distance is 3.1 Å shown in parentheses for K(o)Na−FA in Table 1). The Na···N and HB3 interactions govern the FA adsorption in 23997



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Table 2. Calculated M05-2X/6-31G(d) H···Y and X···Y (in Parentheses) Distances (Å), X−H···Y Angles (deg), and Electron Density Characteristics ρ (au) and ∇2ρ (au) for Hydrated K−FA Complexes from Gas Phase K(o)W−FA H···Y (X···Y) HB1 HB2 HB3 HB4

Na···O(C) HB1 HB2 HB3 Na···Ow

2.259 1.862 1.861 2.064

(3.148) (2.862) (2.818) (2.854)

150.8 164.0 168.2 137.6

K(t)W−FA ρ

X−H···Y

0.015 09 0.032 73 0.031 48 0.018 47 K(o)NaW−FA

∇2ρ

H···Y (X···Y)

0.047 01 0.102 07 0.099 89 0.070 32

2.378 (3.068) 2.153 (2.836) 1.973 (2.926) −

X−H···Y

ρ

∇2ρ

124.4 0.010 88 122.9 0.018 07 154.8 0.029 28 − − K(t)NaW−FA

0.042 57 0.067 42 0.088 39 −

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

2.290 1.734 (2.774) 2.358 (3.105) 2.353 (2.950) 2.359

− 173.6 134.1 119.3 −

0.019 30 0.044 48 0.012 67 0.013 20 0.019 64

0.129 10 0.132 04 0.044 89 0.048 95 0.114 67

2.218 1.872 (2.896) − − 2.250

− 177.1 − − −

0.024 14 0.032 16 − − 0.026 15

0.168 29 0.098 63 − − 0.165 99

Figure 4. Top and side views of the optimized structure of formamide (FA) adsorbed on cation free neutral hydrated octahedral (a, K(o)W−FA) and tetrahedral (b, K(t)W−FA) surfaces of kaolinite mineral (M05-2X/6-31G(d)).

for one Os−H···O H-bond in K(o)W−FA (1.86 Å), which are consistent with the largest ρ and ∇2ρ values among all of the Hbonds in these two complexes (0.03 and 0.1 au, respectively). Os−H···O(C), which occurs in K(t)−FA (HB3), is not formed in K(t)W−FA. This suggests that such a bond does not exist in the real adsorption system of FA on the tetrahedral kaolinite surface and that water may destabilize this adsorption complex.

nonhydrated complexes. In other words, the result is nearly the same position and orientation of FA. Moreover, the same number and character of H-bonds with the surfaces is seen in K(o)W−FA and K(t)W−FA as in nonhydrated systems. In the case of K(o)W−FA, one N−H···Os, one N···H−Os (HB1 and HB2, respectively, in Table 2 and Figure 4a), and two O−H···O (HB3 and HB4) H-bonds are created. For K(t)W−FA, two N− H···Ob H-bonds (HB1 and HB2 in Table 2 and Figure 4b) are created. The H···O distances are the shortest for N−H···Os and 23998



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Figure 5. Top and side views of the optimized structure of formamide (FA) adsorbed on neutral hydrated octahedral (a, K(o)NaW−FA) and tetrahedral (b, K(t)NaW−FA) surfaces of kaolinite mineral in the presence of sodium cation (M05-2X/6-31G(d)).

inhibiting the interaction between FA and Na-kaolinite. Thus, the sodium cation, for which coordination with water molecules is more energetically favorable on this type of kaolinite surface compared with K(o)Na−FA, tends to favor the Na-kaolinite surface complex. For K(o)NaW−FA and K(t)NaW−FA in the presence of a sodium cation, both the FA and water oxygen atoms bind with Na+. See Figure 5. These interactions between the sodium cation and carbonyl oxygen of FA and water’s oxygen atoms possess similar geometrical and topological characteristics (Na···O equals 2.22−2.36 Å, ρ and ∇2ρ are 0.019−0.026 and 0.11−0.17 au, respectively). In the same way as in nonhydrated systems, Na···O(C) and HB1 (strong N−H···O H-bond with H···O distances of 1.7−1.87 Å and ρ and ∇2ρ values of 0.03−0.04 and 0.1−0.13 au, respectively) are mainly responsible for the FA stabilization in both K(o)NaW−FA and K(t)NaW−FA. In K(o)NaW−FA, the additional adsorption strength is due to the formation of one N···H−Os (HB2) and one (C)O···H−Os (HB3) H-bond (H···O distances are 2.35−2.36 Å and ρ and ∇2ρ amount to 0.013 and 0.04−0.05 au, respectively). The Na···O(C) and HB1 interactions are stronger than such interactions in nonhydrated Na+ systems. This can be seen from comparison of the geometrical and electron density characteristics presented in Tables 1 and 2. 3.1.3. Globally Solvated Complexes. The geometrical and topological characteristics of the intermolecular interactions

The most favorable configuration of water on the octahedral and tetrahedral surfaces varies. Water in K(o)W−FA interacts with the mineral fragment through an oxygen atom and protons, whereas the interaction on K(t)W−FA occurs only through protons. This means that there are two O−H···O Hbonds created. This binding is very similar to that found in other theoretical studies for an isolated water molecule adsorbed and intercalated in kaolinite.50,51 The formed Hbonds are stronger in K(o)W−FA (H···O distances are 1.68 and 1.81 Å) relative to K(t)W−FA (H···O equals 2.3−2.4 Å). Moreover, in K(t)W−FA, the oxygen atom from water creates a H-bond with the proton of the amino group of FA (HB3 in Table 2 and Figure 4b). FA is more strongly attracted to the water molecule through the formed H-bond (Ow···H(−N) distance is 1.97 Å) than to the Ob atoms of kaolinite; this interaction does not contribute to the stabilization of FA on the hydrated tetrahedral surface. The K(o)NaW−FA system possesses FA and one water molecule adsorbed on the negatively charged octahedral surface due to substitution of Al3+ by Mg2+, which is compensated for by a sodium cation. No deprotonation of FA (no reaction between FA and kaolinite surface) occurs in the system with added water, as observed in K(o)Na−FA, since the interactions of FA and water with the surface become competitive. See Figure 5. Water primarily interacts with the Na+ cation and FA in K(o)NaW−FA (causing higher stabilization of Na+), 23999



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Table 3. Calculated M05-2X/6-31G(d) H···Y and X···Y (in Parentheses) Distances (Å), X−H···Y Angles (deg), and Electron Density Characteristics (ρ (au) and ∇2ρ (au)) for Nonhydrated K−FA Complexes from Water Solution K(o)−FA H···Y (X···Y) HB1 HB2 HB3 HB4

1.973 3.080 1.779 2.047

(2.992) (3.746) (2.738) (2.890)

175.6 127.0 166.0 144.3

0.032 85 0.012 07 0.030 81 0.015 37 K(o)Na−FA

H···Y (X···Y) Na···N (Na···(O)C) HB1 HB2 HB3 HB4

HB1 HB2 HB3 HB4

K(t)−FA ρ

X−H···Y

2.439 2.046 1.640 1.788 1.747

H···Y (X···Y)

X−H···Y

0.105 59 0.040 26 0.096 42 0.057 84

2.701 (3.614) 2.260 (3.047) 2.006 (2.702) −

150.3 133.8 126.8 −

ρ

∇2ρ

0.009 43 0.014 13 0.020 87 − K(t)Na−FA

0.038 16 0.052 44 0.089 92 −

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

0.018 21 0.025 57 0.053 21 0.033 49 0.043 37

0.098 43 0.070 00 0.158 55 0.125 35 0.133 58

2.285 1.906 (2.927) − − −

− 176.6 − − − K(t−)−FA

0.020 24 0.027 67 − − −

0.133 99 0.092 03 − − −

X−H···Y

− (3.019) 176.3 (2.627) 166.6 (2.685) 149.8 (6.033) 156.3 K(o−)−FA

∇2 ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

1.836 (2.864) − 1.890 (2.803) 1.946 (2.898)

178.5 − 154.3 165.0

0.034 65 − 0.025 95 0.029 96

0.106 74 − 0.082 79 0.095 27

2.454 (3.388) − − −

153.0 − − −

0.008 40 − − −

0.035 17 − − −

Table 4. Calculated M05-2X/6-31G(d) H···Y and X···Y (in Parentheses) Distances (Å), X−H···Y Angles (deg), and Electron Density Characteristics ρ (au) and ∇2ρ (au) for Hydrated K−FA Complexes from Water Solution K(o)W−FA HB1 HB2 HB3 HB4

H···Y (X···Y)

X−H···Y

1.989 (3.008) − 1.776 (2.736) 1.999 (2.846)

175.4 − 166.3 144.7

Na···O(C) HB1 HB2 HB3 Na···Ow

K(t)W−FA ρ

0.02473 − 0.03880 0.02131 K(o)NaW−FA

2

∇ρ

H···Y (X···Y)

0.075 65 − 0.121 91 0.077 57

2.279 (5.391) 2.367 (2.905) 2.066 (2.825) −

X−H···Y

ρ

∇2ρ

141.9 0.013 03 112.2 0.013 60 129.9 0.022 06 − − K(t)NaW−FA

0.047 56 0.054 24 0.072 43 −

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

H···Y (X···Y)

X−H···Y

ρ

∇2ρ

2.364 1.870 (2.900) 3.023 (3.773) 1.890 (2.789) 2.338

− 174.2 135.2 152.4

0.019 42 0.044 32 0.012 52 0.019 42 0.019 77

0.129 52 0.134 74 0.044 58 0.129 52 0.115 12

2.256 1.851 (2.880) − − 2.289

− 175.0 − − −

0.024 29 0.032 56 − − 0.025 99

0.168 71 0.098 69 − − 0.165 37

Table 5. Calculated (M05-2X/6-31G(d) and M05-2X/6-31G(d,p)) Corrected Adsorption Energies (BSSE Correction in Parentheses), and Enthalpies and Gibbs Free Energies from Gas Phase (ΔEads, ΔHads, ΔGads) and Solution (ΔEsol, ΔHsol, ΔGsol) [kcal/mol] ΔEads

ΔEsol

system

6-31G(d)

6-31G(d,p)

6-31G(d)

6-31G(d,p)

ΔHads

ΔHsol

ΔGads

ΔGsol

K(o)−FA K(t)−FA K(o)W−FA K(t)W−FA K(o)Na−FA K(t)Na−FA K(o)NaW−FA K(t)NaW−FA K(o−)−FA K(t−)−FA

−14.8 (5.5) −13.7 (5.4) −9.2 (6.4) −5.9 (9.1) −108.2 (15.2) −20.3 (5.9) −21.7 (5.9) −17.8 (5.0) −14.1 (4.7) −11.0 (7.8)

−14.8 (5.2) −14.0 (5.3) −9.4 (6.2) −6.2 (9.2) −108.5 (15.2) −20.6 (5.9) −21.3 (5.9) −18.0 (5.0) −13.7 (4.6) −11.6 (7.8)

−7.4 (5.5) −3.8 (5.4) −7.4 (6.4) −4.3 (9.1) −19.7 (15.2) −8.0 (5.9) −2.5 (5.9) −8.1 (5.0) −8.2 (4.7) −2.5 (7.8)

−7.9 (5.2) −4.3 (5.3) −7.9 (6.2) −4.4 (9.2) −20.3 (15.2) −8.3 (5.9) −1.9 (5.9) −8.6 (5.0) −9.1 (4.6) −3.3 (7.8)

−13.0 −11.9 −7.4 −4.5 −107.4 −19.1 −20.8 −15.5 −12.8 −9.0

−5.9 −3.0 −6.0 −2.3 −18.5 −6.6 −2.5 −6.9 −6.7 −1.0

−6.8 −6.6 −1.0 1.6 −101.8 −13.0 −15.2 −9.5 −7.3 −2.6

−5.6 −2.7 −5.3 −1.2 −49.4 −6.2 −1.8 −6.4 −6.4 −0.4

solution as in the gas phase occurs in all tetrahedral systems except for K(t)Na−FA, where FA prefers more a perpendicular orientation toward the surface instead of an almost parallel orientation. This causes the formation of one less H-bond. Small changes are found in the strength of intermolecular

formed between FA and the mineral surfaces from global solvation calculations are given in Tables 3 and 4. Calculations of all studied systems using a polarized continuum model show a small solvent effect on the geometry and binding of the optimized structures. The same formation of H-bonding in 24000



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that the overestimation of ΔEads due to formation of one additional H-bond with the edge OH group amounts to about 2 kcal/mol. Thus, we suggest that the strength of FA adsorption on the nonhydrated tetrahedral surface of kaolinite is slightly lower than revealed using the K(t)−FA model. Addition of a Na cation leads to an increase in binding strength, which corresponds with increased adsorption energies of −108.2 (K(o)Na−FA) and −20.3 kcal/mol (K(t)Na−FA). In the case of K(o)Na−FA, the large adsorption energy reflects a reaction in which FA is dehydrogenated. The removed H+ forms a chemical bond with the surface oxygen atom. This corresponds with the existence of an ionic bond between the Na+ cation and atomic nitrogen of FA. For more details about the reaction, see section 3.1.1.2. The presence of water was found to decrease the interaction energy, while the presence of Na+ leads to an increase of FA stability on the kaolinite surface. However, the effect of the presence of Na+ is much higher than that of the addition of water. This is indicated by a decrease of the ΔEads values by about 6 kcal/mol (−9.2 kcal/mol) for K(o)W−FA and by about 8 kcal/mol (−5.9 kcal/mol) for K(t)W−FA compared with the nonhydrated systems. On the other hand, the K(o)NaW−FA and K(t)NaW−FA complexes (characterized by ΔEads equal to −21.7 and −17.8 kcal/mol, respectively) are several times more stable than systems without a cation (about 2 times for the hydroxylated surface and about 3 times for the siloxane sheet). The adsorption energy values indicate that the orientation of water in K(o)W−FA and K(o)NaW−FA (formation of one O−H···Ow bond and one Ow−Hw···O bond) is more favorable than its orientation in K(t)W−FA and K(t)WNa−FA (two hydrogen atoms from water facing toward the surface). Experimentally, it has been shown that greater amounts of FA could be intercalated into kaolinite in the presence of water.58 Our calculations reveal the opposite trend for the particular models and methods used. Regarding the energetics in the hydrated complexes, the presence of one water molecule destabilizes FA adsorbed on the kaolinite surface (even in the K(t)NaW−FA complex where water forms H-bonds with the target molecule). This is the reason why in most of the systems addition of water leads to a decrease of the ΔEads value. It has been shown that the ΔEads values for adsorption of water molecule on a comparative surface of Na-smectite range from −14.3 to −10.5 kcal/mol.59 This shows the competing interaction between water and FA with these clay mineral surfaces. All of our tetrahedral complexes were found to be less stable than the octahedral systems (ΔEads values are in a range from −6 to −20 kcal/mol vs from −9 to −108 kcal/mol, respectively). This is consistent with the number of H-bonds between the adsorbate and surface, their strengths, and the presence of the surface OH groups. These surface OH groups are more active in intermolecular interactions than the basal oxygen atoms of the siloxane site. This finding agrees with the results of other theoretical studies of adsorption of small organic molecules on minerals of the kaolinite group,57,60,61 such as water60 and thymine57 (−13.1 and −37 kcal/mol vs −4.1 and −26 kcal/mol). The largest difference in the ΔEads values is obviously found for K(o)Na−FA and K(t)Na−FA due to a reaction between FA and the kaolinite surface in K(o)Na− FA. AIM analysis revealed that the presence of a negative charge in the selected position of the six-member octahedral or

interactions as can be seen from comparison of distances and electron density characteristics of globally solvated and nonsolvated models (see Tables 1−4). The presence of solvent results in elongation (0.1−0.3 Å) of the H···O distances of some of the H-bonds, in which the FA amide group and basal or water oxygen atom are involved. The Na···O distances also become slightly longer (0.1 Å). Geometry and bonding changes in the octahedral systems due to the global solvation are very similar as described above for the tetrahedral complexes. Small changes occur in the lengths of Na···O distances and H-bonds (0.1−0.7), and one less N···H−Os H-bond (HB2 in Tables and Figures) is formed between the FA nitrogen atom and a surface OH group due to the different orientation of one outer hydroxyl group in the K(o−)−FA and K(o)W−FA complexes. Moreover, in the two systems with one water molecule (K(o)NaW−FA and K(t)W− FA), global solvation leads to a different orientation of water and formation of only one H-bond with the surface instead of two as in the systems calculated in the gas phase. 3.2. Adsorption Energy. 3.2.1. Gas Phase. Table 5 displays the adsorption energies of all studied systems. These energies were obtained at the M05-2X/6-31G(d) level of theory. In the gas phase the BSSE corrected interaction energy values (ΔEads) for K(o)−FA and K(t)−FA are −14.8 and −13.7 kcal/mol, respectively. These ΔEads values are very close to the mean desorption activation energy determined from our experimental temperature programmed desorption (TPD) curves (11.7 ± 0.24 kcal/mol) that is discussed in more detail in our companion paper.37 Moreover, the calculated adsorption energy for the K(o)−FA system is similar to the interaction energy derived for FA adsorbed (−14.6 kcal/mol) and intercalated (−20.2 kcal/mol) on dickite (B3LYP/3-21G(d)).16 The coordinative orientation of the FA molecules affects largely the interaction strength with dickite17 and can decrease the adsorption energy by 5−22 kcal/mol. Prior works show that for water and acetic acid molecules interacting with the hydroxylated sheet of kaolinite the interaction energies (B3LYP/SVP) are −10.5 (this value is similar to the calculated ΔEads for K(o)−FA) and −70.1 kcal/mol. However, for the adsorption on the siloxane sheet the interaction energies are only −3.8 and −2.6 kcal/mol.51 Thymine and uracil adsorbed on the aluminum−oxygen side of the mineral of the kaolinite group (B3LYP/6-31G(d))57 interact more strongly with this type of surface than FA. The difference in ΔEads is ∼13 kcal/ mol for the tetrahedral surface and ∼21 kcal/mol for the octahedral surface. This indicates that adsorption of nucleobases is nearly twice as effective as adsorption of FA on the 1:1 clay mineral surfaces. As discussed in section 3.1.1, the HB3 bond in the K(t)−FA system is formed with the hydrogen atom of the edge OH group added to saturate the dangling bond of the tetrahedral site of the kaolinite cluster. This needs to be considered when describing the adsorption energy of K(t)−FA, which can be slightly higher and showing stronger FA adsorption on the tetrahedral surface due to the formation of this H-bond. Thus, an additional calculation of the K(t)−FA system was performed, in which challenging hydrogen atoms were kept oriented toward the surface. In K(t)−FA, FA is oriented in the same way toward the surface forming the same HB1 and HB2 as found after full optimization for K(t)−FA, with the exception of HB3. The BSSE corrected adsorption energy of the newly calculated K(t)−FA system is −12.0 kcal/mol. A comparison of this value with ΔEads of K(t)−FA (−13.7 kcal/mol) indicates 24001



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led to almost the same ΔEads and ΔEsol values when compared with the M05-2X/6-31G(d) level data. The difference in ΔEads is less than 0.4 kcal/mol, and the difference is less than 0.9 kcal/mol for ΔEsol. In the cases of K(o)−FA and K(t)−FA, the enthalpies and Gibbs free energies were also calculated at the M05-2X/6-31G(d,p) level. Similarly, the 6-31G(d) ΔHads and ΔGads values differ by less than 0.6 kcal/mol from the results calculated using the 6-31G(d,p) basis set. As expected, the difference is slightly larger for ΔHsol and ΔGsol (in the case of K(o)−FA it is 1.5−2 kcal/mol, but for K(t)−FA the difference is less than 0.9 kcal/mol). Despite these small differences, the trend in the stability of the calculated systems and our conclusions related to the thermodynamics of the adsorption remain the same. According to our best knowledge, there are no experimental adsorption enthalpy and Gibbs free energy values available for the FA−kaolinite system. Therefore, we can only compare our data with the binding energy obtained from our TPD experiments (11.7 ± 0.24 kcal/mol), with the previously published experimental enthalpy value for N-methyl-FA:kaolinite intercalates (−4.5 ± 0.5 kcal/mol),62 and with the theoretical interaction energy for FA adsorbed on dickite (−14.6 kcal/mol at the B3LYP/3-21G(d) level).16 The latter interaction energy value is suggested to be overestimated compared with the MP2/6-31G(d,p) results. All of our calculated adsorption energy values using the 6-31G(d) basis set are higher than the above listed experimental and other theoretical adsorption enthalpies. The ΔEads values obtained at the M05-2X/6-31G(d,p) level are slightly increased compared to the data calculated at the M05-2X/6-31G(d) level (by ∼1− 5%). This shows that application of the 6-31G(d) basis set leads to better agreement with already published interaction energy values. Therefore, this basis set was applied to the studied systems. 3.3. Enthalpy and Gibbs Free Energy. 3.3.1. Enthalpy. The enthalpy and Gibbs free energies for adsorption of FA on kaolinite fragments are given in Table 5. As is expected, the enthalpies are only slightly lower than the interaction energies due to thermal contributions (1−2 kcal/mol for all systems). ΔHads values reveal the same trend in the stabilization as that obtained from comparison of ΔEads values. The enthalpy of adsorption increases (in absolute values) in the following order: K(t)W−FA < K(o)W−FA < K(t−)−FA < K(t)−FA < K(o−)−FA < K(o)−FA < K(t)NaW−FA < K(t)Na−FA < K(o)NaW−FA < K(o)Na−FA. The ΔHads values depend only slightly on the presence of water, while Na+ substantially increases the adsorption strength of the kaolinite surface toward FA (in almost the same way as ΔEads). All of these values are larger than the enthalpy of adsorption of stearic acid (−2.9 kcal/mol) on kaolinite.63 The presence of water solvent changes the geometry slightly but significantly affects the thermodynamics of FA’s interaction with kaolinite. The adsorption becomes energetically much less favorable because of the interactions of FA with the solvent. In most cases, ΔHsol values are several times smaller than ΔHads values (the smallest ΔHsol values are obtained for K(t)−FA (−2.3 kcal/mol) and K(t−)−FA (−1.0 kcal/mol); see Table 5 for more details). The influence of water solution is smaller in the octahedral systems than in the tetrahedral complexes. The smallest hydration effect is found for K(o)W−FA (difference between ΔHads and ΔHsol is 1.4 kcal/mol, ∼19% of the ΔHads value) followed by K(t)W−FA (difference equals 2.2 kcal/ mol). For the rest of the systems, changes equal 6−89 kcal/

tetrahedral rings of kaolinite slightly destabilizes the target molecule. The charged complexes show decreased adsorption affinity for FA. This is confirmed by the ΔEads values for K(o−)−FA and K(t−)−FA. They are reduced by ∼0.7 and ∼2.7 kcal/mol, respectively (in absolute value), compared with the ΔEads values for K(o)−FA and K(t)−FA. This suggests that the presence of a negative surface charge can decrease the strength of FA adsorption on the employed kaolinite models. The influence is more significant for the case of the tetrahedral sheet than for the octahedral site. 3.2.2. Solution. The studied adsorption complexes were also evaluated in water solution, and the adsorption energies from solution (ΔEsol) for all studied systems are given in Table 5. The COSMO model was applied to compare two different methods of hydration (direct inclusion of a water molecule (microsolvation) with the global solvation approach, in which the solvent is treated as a continuum). The ΔEsol values are decreased by ∼10 kcal/mol due to the influence of solution for K(t)−FA and by ∼6−9 kcal/mol for K(o)−FA, K(o−)−FA, and K(t−)−FA compared with the ΔEads results. This difference is almost the same for K(t)NaW−FA (10 kcal/ mol), but it is only 2 kcal/mol for K(o)W−FA and K(t)W−FA. On the other hand, ΔEsol differs from the ΔEads value by 12 kcal/mol for K(t)Na−FA, by 19 kcal/mol for K(o)NaW−FA, and by more than 5 times for K(o)Na−FA. For the models used, this shows that an aqueous solution can significantly destabilize FA interacting with both mineral surfaces. The large difference in the ΔEsol (obtained using the COSMO method) and ΔEads values (obtained by addition of water molecule) can be partially due to how the COSMO approach treats the solution (it considers the whole system and subsystems interacting with a polarized model of water). Despite the large effect of hydration, all of the ΔEsol values were found to be negative. K(o)Na−FA is the strongest binding system (ΔEsol equals −19.7 kcal/mol) in which the reaction between FA and the kaolinite surface occurs. FA in aqueous solution is more strongly adsorbed on the octahedral charged kaolinite surface (ΔEsol equals −8.2 kcal/mol) than on neutral K(o) (ΔEsol is −7.4 kcal/mol). Among all physisorbed complexes, K(o−)−FA is the most stable in solution. The order of adsorption strength of FA with the tetrahedral mineral fragments in solution is almost the same compared with the gas phase results. The most stable tetrahedral systems from solution are K(t)NaW−FA and K(t)Na−FA with an energy difference of only 0.1 kcal/mol. This indicates that solution affects more strongly interactions of FA with the surface hydroxyls than with the basal oxygen atoms of kaolinite. A comparison of COSMO adsorption energies (ΔEsol) with the results from microsolvation (ΔEads values for complexes with one water molecule) confirms the trend of destabilization of FA interacting with the applied kaolinite models due to the presence of water. The solution effect for the nonhydrated systems is the largest for K(o)Na−FA (as discussed above) followed by K(t)Na−FA. In all of the systems, the hydration effect due to the COSMO approach application is several times larger than that due to microsolvation. This demonstrates that the calculations of hydration using both approaches are important to provide a clearer picture of the adsorption from water solution. The interaction energies of FA−kaolinite adsorption systems were also calculated using the M05-2X/6-31G(d,p) method to investigate the accuracy of M05-2X/6-31G(d) results. The values are given in Table 5. The application of a larger basis set 24002



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caution since the approaches used (cluster model, M05-2X/631G(d) level, continuum solvent model) can cause some discrepancies in calculations of thermodynamic parameters. Moreover, the proposed correction in the calculation of the Gibbs free energy can be modified based on the type of system studied and approximation employed. Thus, further investigations need to be performed using more precise methods and models to obtain results of experimental accuracy.

mol. The largest decrease is calculated for K(o)Na−FA in which a chemical reaction between FA and kaolinite surface takes place. 3.3.2. Gibbs Free Energy. Contrary to the thermal contribution, the entropy effect was found to be significant. The TΔS values for all of the adsorption systems from the gas phase and water solution were shown to be significantly overestimated (from 10 to 14 kcal/mol). This reduces the adsorption ability of FA to the point that the adsorption became ineffective. This effect is generally larger for the octahedral complexes than for the tetrahedral complexes. Recently, a series of theoretical studies was published, in which the accuracy of prediction of Gibbs free energies for various complexes including adsorption systems was critically evaluated.64−67 For adsorption of benzene, polyaromatics, and nitroaromatics on soil surfaces65 an entropy contribution was also found to be significant at several different levels of theory similarly as in our study. Moreover, the solvent effect was overestimated since the adsorbent interacts only partially with the solution. This led to much higher Gibbs free energies of adsorption calculated using the M05-2X and M06-2X functionals. To our knowledge, the only available experimental value of the free energy of adsorption on kaolinite is for stearic acid, which is −4.9 kcal/mol.63 Therefore, as recommended for the functional and basis set used,64 the calculated entropy term (both the gas phase and solution results) and Gibbs free energy of adsorbent (solution results) were corrected by a 0.5 scaling factor to reduce the entropy and solvent effect contributions. The calculated Gibbs free energies of adsorption from gas phase and water solution are denoted as ΔGads and ΔGsol, and they are given in Table 5. The ΔGads and ΔGsol values obtained using the above-mentioned correction are negative for all studied systems except K(t)W−FA from the gas phase. This shows that all of the studied FA−kaolinite systems are sufficiently stabilized to make the interaction thermodynamically feasible from the gas phase (except K(t)W−FA) and water solution. This is consistent with the results of several experimental studies in which FA was found to be part of the interlayer space of minerals of the kaolinite group (dickite− and kaolinite−FA intercalates) and interacted very well with these surfaces.10−17 Moreover, it has been shown that FA is hydrophilic and should be hydrated relatively easily. As expected, the ΔGsol values are more negative for the octahedral systems (from −49 to −2 kcal/mol) than for the tetrahedral complexes (from −6 to −0.4 kcal/mol). This corresponds with the nature of the intermolecular interactions as discussed in section 3.1, which are stronger for the octahedral complexes than for the tetrahedral complexes. The largest ΔGsol value is obtained for K(o)Na−FA (−49.4 kcal/mol). This indicates that the presence of charge plays an important role in the thermodynamics of adsorption on the mineral surfaces (in our case, the presence of a negatively charged mineral fragment or positively charged exchangeable cation). The ΔGsol values are smaller (in absolute value) than the ΔGads values with the exception of K(o)W−FA and K(t)W− FA. Thus, the presence of solution influences these complexes more than the entropy effect. This can be attributed to the water−substrate interactions, which are of two kinds: those with the siloxane surface and those with oxygenated surface groups which can both include dispersive and repulsive contributions. In the case of adsorption of FA on K(t)W, ΔGads is slightly positive (1.6 kcal/mol). However, one needs to consider the calculated Gibbs free energy of adsorption with

4. CONCLUSION This study provides insight into the adsorption of formamide (FA) on kaolinite calculated for the tetrahedral and octahedral surfaces of kaolinite applying quantum chemical methods and the cluster approach. Two different situations were modeled: adsorption from the gas phase and water solution (modeled by the two approaches of microsolvation and continuum solvation). The effects of adding a sodium cation and the presence of a negative surface charge were investigated. The energetic and thermodynamic parameters of these adsorptions were also calculated. The main finding is a chemical reaction that occurs during the interactions of FA with a Na octahedral site of kaolinite. In this system one molecular hydrogen atom is removed from FA and forms a chemical bond with the surface oxygen. The amide nitrogen atom of FA forms an ionic bond with the sodium cation. The adsorption of FA on kaolinite depends mostly on its capability to form hydrogen bonds with the hydroxyl groups of the aluminum−oxygen surface or with the basal oxygen atoms of the siloxane site. However, in the presence of a sodium cation, FA···Na interactions govern the adsorption. FA is more strongly adsorbed on the aluminum−oxygen (octahedral) than the siloxane (tetrahedral) terminated sheet of kaolinite. The presence of Na+ causes a large increase of the adsorption affinity of the kaolinite fragment for FA. FA is the most stable on a nonhydrated Na surface with a large interaction energy (−108 kcal/mol) due to a reaction between FA and kaolinite surface. The presence of a negative surface charge leads to similar intermolecular interactions and a slightly smaller adsorption strength of FA toward both kaolinite surfaces. Microsolvation slightly affects the adsorption, while continuum solvation influences the adsorption of FA on both types of kaolinite surfaces to a much larger extent. Comparison of the BSSE corrected adsorption energies reveals the following order of stability of the studied FA−kaolinite surface complexes calculated at 110 K: Na octahedral nonhydrated > Na octahedral hydrated > Na tetrahedral nonhydrated > Na tetrahedral hydrated > octahedral > tetrahedral > octahedral hydrated > tetrahedral hydrated. On the basis of estimated Gibbs free energy values, we predict that the adsorption of FA from water solution is effective for all studied surfaces of kaolinite. Moreover, we suggest that FA adsorption is thermodynamically feasible from the gas phase for all of the octahedral and tetrahedral complexes of kaolinite except for the tetrahedral system with one water molecule. However, the levels of approximation used here (cluster model, M05-2X/6-31G(d) method, COSMO approach) can lead to certain inaccuracies in calculations of thermodynamic parameters. Therefore, more precise approaches need to be applied to investigate the FA−kaolinite adsorption phenomenon to obtain energy data of experimental accuracy and confirm the behavior of FA on kaolinite surfaces. 24003



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (601) 979-3482. Fax: (601) 979-7823. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by NSF and the NASA Astrobiology Program, under the NSF Center for Chemical Evolution, CHE-1004570. This work was also facilitated by the NSF Grant EXP-LA No. 0730186. Some of the data presented herein were also obtained from research conducted by A.M.S. and F.C.H. under the Environmental Quality Technology Program of the United States Army Corps of Engineers by the USAERDC. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents.

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Theoretical Study of the Roles of Na+ and Water on the Adsorption of

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