Monomer Adsorption on Kaolinite - American Chemical Society

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Monomer Adsorption on Kaolinite: Modeling the Essential Ingredients Dawn L. Geatches,*,† Alain Jacquet,‡ Stewart J. Clark,† and H. Christopher Greenwell§ †

Physics Department, Durham University, South Road, Durham DH1 3LE, U.K. Lafarge - Centre de Recherche, 95 Rue du Montmurier, BP 15, 38291 Saint Quentin Fallavier, France § Earth Sciences Department, Durham University, South Road, Durham DH1 3LE, U.K. ‡

ABSTRACT: In this study we investigate the fundamentals of a problem pertinent to the cement and concrete manufacturing industries, where clay minerals are pollutants of sands due to their capacity to adsorb additives designed to improve concrete workability. In this density functional theory (DFT) investigation we examine the adsorption of a selection of organic monomers, (e.g., CH3CH2CHOHCH3 and (CH3)3N+CH2CHOHCH2CH3,Cl−) on kaolinite (Al2Si2O5(OH)4) to determine the nature of the basal surface/monomer interactions and, also, to determine whether the presence of an additional clay layer and separately water changes the nature of these interactions. We gauge these effects by examining their formation energies, structural configurations post relaxation, Mulliken charges, and molecular orbitals occupancies. The results show that interactions are predominantly electrostatic for charged monomers and H-bonding for noncharged and also that increasing the complexity of these systems does not change the nature of these interactions, but that it does change the strength of them as well as the potential chemical reactivity of these clay/monomer environments.

1. INTRODUCTION Wider access to supercomputers and increasingly efficient computational algorithms have increased the application of quantum mechanical methods such as density functional theory (DFT), to materials far removed from traditional condensed matter origins, and this usually involves models of greater size and complexity than, for example, simple crystals of silicon for which the computational method was originally developed. Although we can now examine relatively large models using DFT (up to a few thousand atoms), this is not usually practical for routine investigations where time restrictions can limit the models to hundreds of atoms. This size restriction, which is necessarily a restriction on system complexity, requires the researcher to identify the most important aspects of their investigation and to focus on these, while assuming that if the model contains the essential ingredients, then the fundamental interactions will be captured and identified. In this study we investigate whether simple models can capture the fundamental interactions between organic monomers and the clay mineral, kaolinite, where these interactions are a costly part of concrete manufacture. © 2012 American Chemical Society

The clay mineral kaolinite is one of the most abundant clay minerals in the Earth’s crust1 and is used in many different industries from the production of ceramics, drilling fluids and plastics to chemical carriers and catalysts.2 In the cement and concrete manufacturing industry, kaolinite is one of the clay mineral pollutants of the sands used to make concrete.3 Kaolinite is a “pollutant” in part because of its capacity to adsorb the superplasticising additive from the cement mix, thus reducing concrete workability and increasing production costs. To counteract this effect, various organic, cationic polymers are added to the sands as these interact with the clays and are preferentially adsorbed over the superplasticiser, thus rendering the clays inert. The nature of the interactions between the organic polymers and clay minerals is not fully understood, with the implication that if it were, greater control over the effect of the clays on the production of concrete could be attained. Received: June 28, 2012 Revised: September 22, 2012 Published: September 27, 2012 22365

dx.doi.org/10.1021/jp306371m | J. Phys. Chem. C 2012, 116, 22365−22374

The Journal of Physical Chemistry C

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whether using a background charge rather than a chloride ion has any impact on the results. The polymers are prepared in water when used as inerting agents and applied to sand, and when adsorption is tested experimentally, the polymers and clay are mixed together in water. The second modeling complexification is therefore the addition of water to the models to determine whether its inclusion affects the electronic structure of the interactions between the monomer and kaolinite surfaces. There are a number of ab initio studies examining kaolinite from the determination of its structure4−7 to its interactions with various molecules such as water and organic polymers.8−11 Our study is adding to this first-principles work by answering the question “How fundamental are fundamental interactions?”, and we do so by comparing three monomers and their relative adsorptions on both basal surfaces of kaolinite, in the presence of chloride and, separately, background charge, and then we add a second layer of kaolinite to one of the monomer models and, separately, fill the vacuum space with water to determine whether these larger and more complex models fundamentally alter the nature of the interactions between the monomer and kaolinite.

In this study we use the ab initio method, density functional theory (DFT) to examine the interaction between three organic monomers and kaolinite, as the first step on the path toward improving the efficiency of the superplasticizer. The long-term aim is to produce large, realistic models of clay/additive environments, and we want to determine at this early stage whether we can gain useful information from very simple models. In a first attempt to understand the interaction between the clay and the polymers, we are investigating the interaction between kaolinite and monomers of a similar conformation to the polymer units, in a basic monomer/clay model. We then create two more complex environments with one of the monomers to determine how this affects the interactions seen in the simple models. The purpose of this investigation is 2-fold: first, to understand the nature of the interactions between the polymers and clay mineral and, second, to determine whether we have captured the essential aspects of the systems in the simple models, by examining the changes that occur when we make the systems more complex. Although clay minerals are not homogeneous materials, we create a unit cell representing a homogeneous sample of kaolinite. This is the first major modeling simplification and is justified as we know that the model we create is representative of at least a portion of kaolinite, and hence there will be a comparable environment existing within a sample of kaolinite. The second simplification is the employment of monomers compared to the polymers used in industry; this is justified because we are investigating the potential for chemical reactions, and hence can assume that this must occur at the molecular scale of monomers, so that any chemical reaction seen between the monomers and clay mineral can also occur between the polymers and clay mineral, regardless of any other interaction that might occur. Kaolinite (Al2Si2O5(OH)4) is a 1:1 dioctahedral clay mineral comprising a tetrahedral SiO4, and an octahedral AlO6 sheet, which creates two different basal surfaces. The SiO4 is the siloxane surface and the AlO6 surface is terminated by hydrogens and described in this study as the “hydroxyl” surface. In nature kaolinite is never found as a single, two-sheet layer, but rather as a multilayered stack or plaque, where the edges of the plaque form two further types of potentially reactive surfaces, [010] and [110]. In this study we have chosen to concentrate on both basal surfaces, with a future article detailing the edge-model investigations. The importance of reproducing a stack of kaolinite rather than a single layer is examined by creating two-layered models, and this comprises the first of the modeling complexifications. To obtain information about the adsorption of inerting agents on kaolinite, we compare the interactions of three monomers at close proximity to both basal surfaces. Two of the monomers contain quaternary ammonium, the polymers of which are synthesized using a salt such as sodium chloride, implying that N+ exists in the presence of an anion; hence we include chloride in our models to determine the role this plays in adsorption. However, including charged species such as the cationic monomer and chloride ion can be problematic for periodic DFT methods, as they can cause the introduction of artifactual electrostatic interactions between periodic images. The ultimate aim would be to model polymers and hence a correspondingly large number of chloride ions, which could be represented by the addition of a background charge (a technique often used in cluster model studies) to reduce the computational expense. Consequently, in this study we test

2. COMPUTATIONAL DETAILS AND MODELS 2.1. Computational Details. All calculations were carried out with the CASTEP12 code, using a planewave basis set within the DFT formalism.13−15 Convergence testing showed that a planewave basis represented by a kinetic energy cutoff of 650 eV gave an energy difference in total energies of less than 1.3 meV per unit cell for higher cutoffs. The Brillouin zone integrations were performed on a grid containing two k-points giving an energy difference between one, two, four and five kpoints within the error bound just described. We used the generalized gradient approximation (GGA) density functional, specifically Perdew, Burke, and Ernzerhof (PBE)16 as this usually describes molecular bonding to a greater accuracy than does the local density approximation (LDA). PBE norm-conserving pseudopotentials were used as these are consistent with the PBE exchange−correlation functional (Table 1) and enable the calculation of spectroscopic data Table 1. Electronic Structure of the Norm-Conserving Pseudopotentials (GGA-PBE) element

configuration

Al C H N O Si Cl

3s23p1 2s22p2 1s1 2s22p3 2s22p4 3s23p2 3s23p5

should further studies require this. The (geometry) optimizer was Broyden−Fletcher−Goldfarb−Shanno (BFGS)17 and the electronic method was ensemble density functional theory (EDFT).18 Further convergence details per BFGS iteration are as follows: energy change per ion, dE/ion, 2 × 10−5 eV; electronic energy tolerance, 10−6 eV; maximum force, |F|max, 0.05 eV/Å; change in displacement, |dR| 0.002 Å. All calculations were non spin polarized. van der Waals forces are one of the important forces in producing the layered structure of clay minerals,19 and their 22366



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i.e., monomer−monomer, monomer−chloride, and chloride− chloride, showed that the total energy increased depending on the model and the direction of increased cell size. The largest increase in total energy was for the monomer + charge models (i.e., no chloride ions) on increasing the b-length of the unit cell (quantitative results not shown here), exemplifying the problem associated with charged species in periodic DFT. This increase in total energy occurred because the screening density of the background charge decreased when the b-length was doubled; thus the positively charged monomers were able to interact more strongly. We found a similar effect due to a decrease in background charge density in a previous study.34 As increasing the cell size in the x and y directions did not reduce electrostatic interactions, the cell size described at the beginning of this section and shown in Figure 1 was considered

inclusion in DFT calculations has been shown to improve agreement between calculated lattice parameters and experimental values.20 They also appear to play an important role in the computational representation of water and have been shown to improve the diffusivity of water in molecular dynamics simulations; see Wang et al.21 and Jonchiere et al.22 and the references within. However, other studies show that determining the most appropriate parameters to calculate van der Waals forces is somewhat of an art in itself and is system dependent.23,24 We have chosen to ignore van der Waals effects in this study for two reasons; first, kaolinite does not have an interlayer space, and second, we are not considering the dynamics of water in this study, only its screening effects and how this might change the interaction of the monomers with the clay surface. Wang et al.21 report that “The current DFT description of liquid water overestimates intermolecular bonding...”, which suggests that models without van der Waals interactions will have a greater screening effect than when they are included, and hence a limiting case is tested in our study. 2.2. Models. A double unit cell (along the b-length) of kaolinite was made according to the template provided in ref 25, and after initial relaxation, the c-length was expanded to 17.00 Å, as shown in Figure 4 to accommodate (separately) three organic monomers [2-hydroxy-N,N,N-trimethylbutan-1amminium chloride, (CH3)3N+CH2CHOHCH2CH3,Cl−); N,N,N-trimethylbutan-1-amminium chloride, (CH3)3N+(CH2)3CH3,Cl−); 2-butanol, CH3CH2CHOHCH3 (labeled A, B, and C, respectively)] and to ensure no interlayer space was created, as this does not occur in kaolinite. The parameters of the relaxed structure agree with experimental data.26 The siloxane surface of kaolinite has a relatively positive charge compared to the relatively negative charge of the hydroxyl surface. This charge difference creates an artifactual electrostatic field between periodic images, which we addressed by fixing all lattice parameters and allowing only the atomic positions to subsequently relax, which enabled the comparison of like-with-like systems potentially in the presence of monopoles, dipoles and quadrupoles. Monomers A and B contain quaternary nitrogen atoms, the corresponding polymers of which are produced using a chloride or bromide salt; therefore, the polymers cannot exist in the absence of a chloride or bromide anion. In periodic models a charged ion (in this study it is both chloride and the monomers themselves) can interact with its periodic image electrostatically, just as the differently charged basal surfaces of the same clay layer can interact. Counteracting the electrostatic fields arising from this artifactual interaction has proven to be one of the more problematic aspects of using periodic, computational methods. There are various techniques used to combat this effect,27−32 as well as the method of the application of background charge. In our study, as an alternative to including the anion itself, it is possible to represent the removal of an electron from the monomer by the addition of a uniform background charge.33 This mimics the effect of the chloride with respect to the quaternary nitrogen of monomers A and B, where a chloride accepts one electron from one monomer. The models with a charge rather than a chloride ion are referred to as “no-chloride” or “charge” from this point onward. Further consideration was given to the optimal cell sizes for the (positively charged) monomers and the (negatively charged) chloride ions. Convergence testing of the unit cell sizes with regard to minimizing the periodic image interactions,

Figure 1. Single cell of kaolinite (with twice the unit b-length) and its dimensions in angstroms and degrees. Three organic monomers similar in conformation to the inerting agents mentioned in the Introduction. Color scheme (used for all figures): white, hydrogen; red, oxygen; pink, aluminum; yellow, silicon; gray, carbon; blue, nitrogen; purple, chloride.

to be optimal within our method of like-with-like comparisons; that is, by reproducing identical environments in which only one variable changes (the monomers), we are effectively canceling-out the effect of the electrostatic interactions. We consider the effect of these modeling artifacts further in section 2. To compare the interaction of the three monomers with the basal surfaces of kaolinite, we created ten models in the following combinations: A and B with and without chloride and adjacent to the hydroxyl and siloxane surfaces, and the latter two scenarios with monomer C, as exemplified in Figure 2. The next consideration was the presence of two layers of kaolinite compared to one, that is, if the electronic structure of the clay/monomer system is the same in a two-layer model as in a single-layer model then we can say that although in nature kaolinite always exists as a multilayer stack, in quantum mechanical modeling terms, one layer is sufficient. There are several theoretical studies into the orientations of two layers of kaolinite with respect to each other and how this changes under pressure35−37 and the effect of the presence of stacking defects;38 and the conclusion drawn from these studies is the potential complexity of a simple two-stack layer of kaolinite. In the interests of simplicity, we have chosen to reproduce the second layer adjacent to the first, without any translations or rotations, as the purpose of this study is to determine whether the presence of the second layer makes any difference to the nature of the monomer/surface interactions, and not to 22367



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In all of the modeling scenarios, all of the atomic positions were allowed to relax under the convergence criteria previously detailed, and then the Fermi level orbitals, corresponding to the highest occupied molecular orbitals (HOMO), and the lowest conduction band or the lowest unoccupied molecular orbitals (LUMO) were visualized, as these indicate where electron excitations would occur, i.e., from the HOMO to the LUMO. This is one of the methods of analysis employed in this investigation to determine the effects of the modeling constraints and in this case reports the chemical reactivity within the clay/monomer systems. We also examined the formation energies to determine relative adsorption of the monomers, Mulliken charges to examine the nature of the monomer/clay interactions, and the geometrical structure of the relaxed models to determine the relative orientation of the monomers with respect to the clay surfaces as this provides further information regarding their adsorption. These results can be found in the following section.

3. RESULTS AND DISCUSSION There are several facets to the results we obtained: the comparison of the relaxed configurations of the clay/monomer interactions and the identification of the nature of these interactions; the effect of the presence of chloride compared to charge; the effect of the proximity of the monomers to the two different clay surfaces; the effect of including a second layer of kaolinite, and separately, the effect of the inclusion of water. 3.1. Formation Energies. Formation energies indicate the relative strength of the interactions between the monomers and the surfaces and hence describe one aspect of their relative adsorption. In this study, “formation energy” is defined as the difference between the relaxed, full system and the sum of the energies comprising the parts of this relaxed model; e.g., for model A + 2 layers, the parts constitute separate models of the two clay layers and a model of monomer A both with and without chloride. As described in section 2.2, convergence testing of unit cell sizes for the monomers-plus-charge and the monomers-plus-chloride showed a direction-dependent increase in total energy for both the monomers-plus-charge and the monomers-plus-chloride models. These calculations were performed without the clay layer and hence are similar to the no-clay component models used to compute formation energies. The largest increase in energy on an increase in cell size (in the y-direction) was for the monomers-plus-charge, due to the reduced screening of the artifactual electrostatic field between periodic images of the positively charged monomers. This explains why the formation energies (Table 2) for the nochloride models are larger than those for the chloride models. The component models used to calculate the formation energies of the no-chloride models have stronger electrostatic

Figure 2. Examples of the models created: top row, monomers A and B adjacent to the hydroxyl surface with chloride (purple) and without; bottom row, monomer C adjacent to the hydroxyl and siloxane surfaces, respectively.

determine absolutely the structure of a two-layer stack of kaolinite. After two layers of kaolinite were relaxed, the c-length was increased to create the same distance between periodic surfaces as in the single-layer models and then four models were created using monomer A, with and without chloride and adjacent to the hydroxyl and siloxane surfaces, as shown in Figure 3. The two-layer models are referred to from this point onward as “A + 2 layers”.

Figure 3. Monomer A with chloride adjacent to the hydroxyl surface of a two-layer model of kaolinite (dimensions in angstroms and degrees).

The final modeling consideration was the inclusion of water and its effect on the electronic structure of the interacting clay/ monomer system. The polymers exist in solution so it is logical to assume that any “realistic” model should include water molecules. To test the effect of the presence of water, we created four models using monomer A, with and without chloride adjacent to the hydroxyl and siloxane surfaces. The water was assumed to fill completely the vacuum space, which in the case of the chloride models amounted to 25 molecules and without chloride, 26, reproducing a density of water of 0.8 g cm−3, which is about 80% of the density of water at 300 K. An exact reproduction of the density of liquid water is difficult in the vacuum space of our models and, as previously described, is more important when the diffusive properties of liquid water are modeled. The water models are referred to as “A + water” from this point onward.

Table 2. Formation Energies (eV) of the Models, Where “Formation Energy” Is As Described in the Main Texta hydroxyl

a

22368

siloxane

model

chloride

no chloride

chloride

no chloride

A B C A + 2 layers A + water

1.444 1.323 N/A 1.452 5.046

4.472 4.499 0.331 6.728 12.466

1.650 1.592 N/A 2.459 6.257

5.215 5.244 0.108 7.010 11.268

All values are negative. dx.doi.org/10.1021/jp306371m | J. Phys. Chem. C 2012, 116, 22365−22374


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Figure 4. Configurations after relaxation of all atoms. Paler blue dashed lines are hydrogen bonds ≤2.5 Å. Rows A−C refer to the monomers.

interactions than the component models of the (neutral group of) monomer-plus-chloride in the chloride models; hence there is more contribution to the formation energy from the reduction in the presence of artifactual electrostatic fields in the no-chloride than the chloride models. The formation energy results are not quantitative, rather qualitative and comparative, so the differences in magnitude between the chloride and charge models are not significant beyond informing the artifactual electrostatic interactions between charged species. Although we cannot compare between chloride/no-chloride scenarios, we can compare some of the results within each of these environments. Looking down the columns of Table 2 reveals the same trends, that monomers A and B have approximately the same formation energies and that this is approximately 15% greater on the siloxane surface of the single-layer models, for both the chloride and charge-only models. For monomer C the formation energy is an order of magnitude smaller than that for monomers A and B and is 3 times greater on the hydroxyl surface than the siloxane. The models containing two layers of kaolinite show the same trend as those with a single layer, but the difference in formation energies of the chloride models with respect to the surfaces is greater; i.e., the formation energy for the siloxane surface is 70% greater than that for the hydroxyl surface. The adsorption energies of the water models are in most cases nearly triple those of the comparable, water-free systems, which is due to the screening effect of the water molecules reducing the electrostatic interactions of periodic images of the charged species. The water models show a reduced formation energy in the presence of chloride compared to charge,

mirroring the same trends seen in the water-free and two-layer systems. Furthermore, the siloxane surface has the greater formation energy in the chloride models (by approximately 24%), whereas the hydroxyl surface has the greater energy in the charge models (by approximately 10%). The formation energies indicate that monomers A and B are more strongly adsorbed than monomer C, and adsorption is preferred on the siloxane surface for monomers A and B and the hydroxyl surface for monomer C. The presence of chloride has a modeling effect rather than a physical effect on this adsorption. The presence of a second clay layer appears to increase the strength of adsorption of monomer A on the siloxane surface, and the presence of water at least doubles the strength of adsorption in all scenarios, with the siloxane surface preferred only in the chloride models. 3.2. Structure. Examining the relaxed conformations provides further information regarding adsorption, indicating how the monomers orient themselves with respect to the two basal surfaces. Figure 4 shows the relaxed conformations of monomers A and B are very similar on each of the two basal surfaces and in the presence and absence of a chloride anion. Both monomers lie horizontal to the Al−OH surface in the presence and absence of chloride, at N+-to-surface distances: A 4.4 Å and B 4.6 Å in the presence of chloride (Table 3) and A, B 4.9 Å in the absence of chloride. On the Si−O surface A and B lie more vertically to the surface in the presence of chloride at A 6.6 Å and B 6.4 Å and more horizontally in the absence of chloride at A 4.9 Å and B 4.7 Å. Monomer C lies horizontally to both surfaces, at C-(of C−OH)-to-surface distances of 3.9 and 5.5 Å to the hydroxyl and siloxane surfaces, respectively, and a hydrogen bond has formed where the OH group is oriented 22369



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monomers closer to the hydroxyl surface and in the case of monomer A, there is the possibility of a hydrogen bond forming between its lateral OH group and the siloxane surface, orienting the monomer to form a pillar between the two surfaces. In all cases there has been no bond breakage nor reformation, which suggests that the interactions between the monomers and kaolinite are all of the same type. The presence of the chloride compared to the addition of background charge has caused a difference in the orientations of monomers and A and B with respect to the siloxane surface. The chloride has a greater attraction for the hydroxyl than the siloxane surface, as can be seen by the hydrogen bonds forming between them, and this in turns attracts the N+ of monomer A and B.

Table 3. Distances (Å) between Kaolinite Surfaces and the N+ of Monomers A and B and between Kaolinite Surfaces and the Carbon of the C−OH of Monomer C hydroxyl

siloxane

model

chloride

no chloride

chloride

no chloride


4.4 4.6 N/A 4.6 4.3

4.9 4.9 3.9 5.5 4.9

6.6 6.4 N/A 5.1 6.7

4.9 4.7 5.5 4.7 6.2

toward the hydroxyl surface, which contributes to the greater formation energy on this surface described in the previous section. In general, the presence of the chloride ions attracts the

Figure 5. Fermi level occupation in two-layer models of kaolinite with monomer A: highest occupied molecular orbitals (HOMO) (blue shells) and lowest unoccupied molecular orbitals (LUMO) (green shells). Top row shows closer proximity to the hydroxyl surface and the bottom row closer proximity to the siloxane surface. 22370



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Figure 6. Fermi level occupation in water models with monomer A: Highest occupied molecular orbitals (HOMO) (blue shells) and lowest unoccupied molecular orbitals (LUMO) (green shells). Top row shows closer proximity to the hydroxyl surface and the bottom row closer proximity to the siloxane surface.

The two-layer, monomer-A models (Figure 5) show similar monomer orientations in all scenarios to the single-layer models, with the exception of the siloxane-plus-chloride, where the monomer lies horizontally inclined rather than vertically toward the surface. The distances between the N+ of A and the hydroxyl surface are 4.6 Å compared to 4.4 Å (single-layer models) in the chloride scenarios and 5.5 Å compared to 4.9 Å (single-layer models) in the no-chloride systems. On the siloxane surface the N+-to-surface distances in the chloride models has decreased from 6.6 Å (single-layer models) to 5.1 Å, and in the charge models the reduction is smaller from 4.9 Å (single-layer models) to 4.7 Å. These results indicate that chloride has less of an effect in the two-layer models than in the single-layer systems. The water models (Figure 6) show the same orientations of the monomers with respect to the surfaces as the clay/ monomer-only models and a closer examination of adsorbateto-surface distances (as previously parametrized) are the same or within 0.1 Å of the non-water models with the exception of the siloxane, no-chloride model where the measured distance is now 6.2 Å compared to 4.9 Å in the non-water system. This suggests that the presence of the water molecules between the monomer and surface is screening the electrostatic interactions between them. These results show that, realistically (considering the twolayer models), the monomers lie similarly positioned at both surfaces, differing in proximity rather than orientation, and the presence of the chloride ion does not affect this orientation. Similarly, given that the snapshot models cannot include any

dynamics, the presence of water has no effect on the orientation of the monomers relative to the basal surfaces. 3.3. Mulliken Charges. A comparison of the Mulliken charges of the monomers indicates the extent of interaction of the monomer with its environment. Mulliken population analysis is particularly suitable for analyzing the results of computations performed using well-converged planewave basis sets, which are the basis set of choice in the CASTEP code.12 Furthermore, Mulliken charge analysis is a very quick, postprocessing step, and although the resulting charges are not valid as physical charges for the determination of, for example, the quantitative magnitude of chemical bonds,39 they do yield qualitative information when relative comparisons are made between like systems.40 This is exactly the case in this study, where in our systems we are considering the relative charges of the three monomers to describe the nature of the interaction between the monomers and surfaces. This is followed by considering how this charge changes for monomer A when the environment is made more complex, and hence whether the fundamental monomer/clay surface interaction has been captured by the most simple model. Further details concerning Mulliken population analysis and its implementation in CASTEP can be found in references.41−44 The Mulliken charges shown in Table 4 illustrate the electrostatic nature of the interactions between the monomers A and B and the surfaces of kaolinite. The addition of a positive background charge rather than the presence of the chloride ion causes an increase in the overall charge of the monomers with respect to these monomer-only models containing the chloride ion, of between 1015%. Looking down the columns, 22371



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screening the monomers from the clay surfaces, by redistributing charge from the monomers to the water molecules. These results show that the principal interaction between monomers A and B and the clay surfaces is electrostatic, which agrees with the findings of experimental studies on charged polymers.45,46 There is further agreement with experiment that noncharged polymers (comparable to monomer C) interact via H-bonding with the surfaces, as can be seen in Figure 4.47 The nature of this interaction (for charged monomers) remains the same when the systems are made more complex, although the magnitude of the interaction changes. Therefore, it can be said that the fundamental nature of the interaction between the monomers and clay surface has been captured in the simple, single-layer monomer models. 3.4. HOMO and LUMO. Examining the HOMO and LUMO occupancies provides information about the potential reactivity of a system, as a chemical reaction comprises excitation of electrons from their Fermi levels to the lowest conduction band (which is the equivalent of a HOMO/LUMO transition), and it is interesting to determine if the complexities we have added change the chemical reactivity of the systems. As can be seen in Figure 7, the Fermi level electron and LUMO occupation of monomers A and B, single-layer models are very similar, and show that in the chloride models, an application of energy to these systems would result in the electrons of the chloride ion being excited to occupy the energy levels (LUMO) primarily of the monomers. In the models without chloride, the potentially reactive electrons occupy the clay layer, and excitation would again promote them to the LUMO of the

Table 4. Mulliken Charges (e) of the Monomers PostRelaxation hydroxyl

siloxane

model

chloride

no chloride

chloride

no chloride


0.86 0.83 N/A 0.85 0.64

0.98 0.95 0.01 0.91 0.67

0.85 0.85 N/A 0.90 0.71

0.93 0.93 −0.01 0.86 0.79

monomers A and B have approximately the same relative charges as each other on both surfaces with and without chloride. Monomer C has almost 1% of the relative charge of monomers A and B, suggesting there is comparatively no electrostatic attraction between it and the clay surfaces, which agrees with the smaller formation energies seen for monomer C (Table 2). The presence of a second layer of kaolinite reduces the differences between the chloride and charge models but the relative charges are very similar to those of the single-layer models. This is in contrast to the water models where for the hydroxyl surface models there has been a decrease in relative charge compared to the single-layer models of between 25 and 30%, whereas for the siloxane surface models the same comparative decrease is about 15%. This decrease in monomer charge corresponds to reported findings of the “... charge being taken up by the liquid water ...”.24 The water is effectively

Figure 7. Fermi level occupation: highest occupied molecular orbitals (HOMO) (blue shells) and lowest unoccupied molecular orbitals (LUMO) (green shells). Paler blue dashed lines are hydrogen bonds ≤2.5 Å. Rows A−C refer to the monomers. 22372



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The Mulliken analysis of section 3.3 shows that the addition of a second layer does not change the nature of the electrostatic interactions between monomer A and the basal surfaces of kaolinite. However, the analysis in this section shows that the addition of a second clay layer reduces the effect of the presence of the chloride ions, both on the orientation of the monomers and on the HOMO/LUMO occupation. In twolayer models, the chloride ions would no longer be involved in electron excitation, and there is also a reduction of the LUMO occupation of the monomers. Therefore, the catalytic nature of the clay layer has changed, from the single-layer, chloride models where the chloride is the electron donor to the doublelayer models (both chloride and charge) where the clay layer is the electron donor. The addition of water replaces LUMO occupation of the monomers by HOMO occupation, thus changing the reactivity of the water/monomer/clay systems. Therefore, increasing the complexity of these systems has changed the potential chemical reactivity of these clay/ monomer models.

monomers. This pattern is repeated in both basal surface scenarios and corresponds with the results of another DFT/ kaolinite (cluster model) study.48 For monomer C there are marked differences between the two kaolinite surfaces. In the hydroxyl surface model the HOMO and LUMO occupy the hydroxyl surface with some involvement of the LUMO on the OH group of monomer C. The siloxane surface model shows occupation of the LUMO on the hydrogens of the hydroxyl surface and complete occupation of the monomer by the HOMO. These two results suggest that in the hydroxyl surface models, electrons would be excited from one area of the clay to another area of the clay, with some involvement of the OH group of monomer C, and that in the siloxane surface models, electrons would be excited from the monomer to the hydroxyl surface of the clay. These results are not identical, illustrating an effect depending on proximity to the two surfaces and a mode of interaction between the monomer and surface different to that between monomers A and B and the surfaces. One apparent difference in the monomer C models is that a hydrogen bond has formed between monomer C and the hydroxyl surface, which perhaps stabilizes the monomer insofar as it does not contain the HOMO, but only a small fraction of LUMO. Where there is no hydrogen bond between monomer C and the siloxane surface, the monomer is relatively unstable and contains the HOMO, indicative of potential reactivity. The effect of making the model complex by the addition of a second clay layer, as shown in Figure 5 results in the Fermi level electrons occupying the lower clay layer in each system, and on excitation these would be promoted to the LUMO of the monomer and the hydroxyl surfaces except in the chloride/ siloxane model where there is relatively no LUMO occupation of the clay surface. These no-chloride results are the same as those for the single-layer models, with more LUMO involvement of monomer A in the latter case, and more LUMO involvement of the clay in the double layer models. The chloride results differ insofar as there is no HOMO occupation of chloride in the double layer models; therefore, excitation would occur from the clay layers to the LUMO of the clay and monomer with no involvement of the chloride ion. The addition of water, as illustrated in Figure 6 shows that in the hydroxyl surface models, the frontier electrons occupy the monomers plus a few of the water molecules. In the siloxane surface models, Fermi level occupation is primarily of a few of the water molecules with some involvement of the hydroxyl surface in the chloride model, and a few of the water molecules plus the monomer in the charge model. Electron excitation would be from the monomer to water molecules in both hydroxyl models; from water-plus-clay to water in the siloxane/ chloride model, and from water-plus-monomer to water-plusclay in the siloxane/charge model. This is different from the single-layer models where excitation was primarily to the monomer and not f rom the monomer as in these water models. The water results might not be as accurate as they could be if a more suitable density functional (i.e., more suitable for water) had been used, but the average number of hydrogen bonds is about 3.5, which is within 10% of the number reported in a bulk water study.22 In the work cited, which used the BLYP-D3 functional,49 3.8 hydrogen bonds per water molecule were seen to create a softer water structure representative of ambient water, which suggests the water we have modeled is not overstructured and not too dense, indicating that the resulting electronic structure of these water models is reasonable.

4. CONCLUSION By analyzing the formation energies, Mulliken charges, relaxed structures and orbital occupation of monomer/clay systems under the modeling regimes of the addition of chloride and charge, the proximity of the monomers to the two kaolinite basal surfaces, the addition of a second layer of clay, and the addition of water, we are able to state the following: (i) Quaternary ammonium, charged monomers A and B interact electrostatically with both clay surfaces, and the nature of this interaction does not change on complexification by the addition of a second clay layer or water. For the uncharged monomer C, there is interaction via H-bonding and relatively no electrostatic interaction with the basal surfaces. (ii) The addition of chloride rather than charge affects the orientation of the monomer on the surfaces and reduces the effects of artifactual electrostatic interactions by creating a neutral group in combination with the positively charged cation. (iii) The addition of a second clay layer ameliorates the structural effect of the chloride ion and changes the catalytic nature of the clay and hence the chemistry of reactivity when compared to the single-layer models. (iv) Addition of water has no effect on the orientation of the monomer both with and without chloride, but it completely changes the chemistry of the reactivity. Therefore, we can conclude that the basic, monomer/ clay models do comprise the essential ingredients required to determine the nature of the interactions between our organic monomers and clay surfaces and, furthermore, that the composition of the essential ingredients themselves depends on the question being asked of the system in focus.

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The authors declare no competing financial interest. 22373



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ACKNOWLEDGMENTS This work made use of the facilities of HECToR, the U.K.’s national high-performance computing service, which is provided by UoE HPCx Ltd. at the University of Edinburgh, Cray Inc., and NAG Ltd. and funded by the Office of Science and Technology through EPSRC’s High End Computing Programme, grant number EP(F037481/1). Thanks to the Lafarge Centre de Recherche (France) for providing their support with this project, and also to EPSRC and Durham University’s Knowledge Transfer Scheme for funding the work of D Geatches.

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Monomer Adsorption on Kaolinite - American Chemical Society

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