Understanding the Swelling Behavior of Modified Nanoclay Filler

Article pubs.acs.org/JPCC

Understanding the Swelling Behavior of Modified Nanoclay Filler Particles in Water and Ethanol Sebastian Metz,*,† Richard L. Anderson,† Dawn L. Geatches,† James L. Suter,‡ Robert Lines,§ and H. Chris Greenwell*,∥ †

Hartree Centre, STFC Daresbury Laboratory, Scientific Computing Department, Sci-Tech Daresbury, Keckwick Lane, Daresbury, Warrington WA4 4AD, U.K. ‡ Centre for Computational Science, Department of Chemistry, University College London, 20 Gordon Street, London, U.K. § Sun Chemical Ltd., St. Mary Cray Technical Centre, Cray Avenue, St. Mary Cray, Orpington, Kent BR5 3PP, U.K. ∥ Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham DH1 3LE, U.K. S Supporting Information *

ABSTRACT: Clay−polymer nanocomposite materials have gained much attention owing to their low weight ratio of filler to reinforcement properties, delivering lightweight yet resilient materials with excellent barrier properties to gas diffusion. An important process in their production is clay exfoliation, as maximum reinforcement and improvement of barrier properties occur when the clay mineral platelets are fully separated and dispersed through the polymer matrix with a preferred orientation. In this study we examine clay swellingthe first step leading to exfoliationusing molecular dynamics to generate solvation energetics, swelling curves, and atomic density profiles of three types of clay mineralsmontmorillonite, vermiculite, and hectoritewith interlayer Na+ cations and/or three quaternary ammonium surfactants in water and ethanol. Analysis based on the provided simulations can help to distinguish between favorable and unfavorable swelling profiles of mineral/surfactant/solvent systems and therefore guide further research into this complex field.

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INTRODUCTION Since the early 1990s, when first introduced by Toyota, clayreinforced plastics, or clay−polymer nanocomposite (CPNC) materials, have attracted much attention owing to their superior performance and additional properties relative to conventionally filled (e.g., glass fiber) plastics. The term “nanocomposites” arises owing to the clay filler particles having at least one size domain in the nanometer region. Clay minerals are defined by Guggenheim and Martin as “phyllosilicate minerals and minerals which impart plasticity to clay and which harden upon drying or firing”.1 This typically refers to the fraction of soils with particle size of a layer thickness lower than 2 μm. As individual montmorillonite clay mineral layers are only 10 Å (1 nm) thick, these kind of clay minerals have a very high aspect (breadth to thickness) ratio. At relatively low filler loadings, polymers reinforced with clay minerals gave rise to similar mechanical properties for CPNCs as composites with much higher conventional filler loadings.2 Initially, it was considered that the nanoscale size of these clay minerals conferred unique properties to the CPNC material. However, in time it became apparent that the strongly bound polymer interfacial layers on the clay mineral allowed CPNC materials to be placed within conventional composite theory.3 In addition, owing to the preferred orientation and overlap of the highly anisotropic clay mineral platelets, CPNC materials were found to have far superior barrier properties to gas diffusion and better thermal stability compared to the pure polymers, or conventional composites, accelerating their deployment in © XXXX American Chemical Society

packaging applications. The dramatically increased usage of plastic-based packaging materials over the last few decades together with future consumption predicted to increase even further4 have served to stimulate the industry to provide new, more efficient barrier solutions5−7 with the potential to further extend the usage of CPNC materials. The main obstacles to widespread take-up of CPNC materials so far have been associated with processing and fully dispersing clay minerals in the polymer matrix, without aggregation occurring. A straightforward way to achieve this is to exfoliate (i.e., to induce swelling and dispersion of) the clay layers in a solvent beforehand. This is especially useful in the case of rapid printing of polymer coatings where volatile solvents are employed to allow coatings to be deposited at high speed. If the clay minerals cannot be readily dispersed, and kept dispersed, clay particle aggregation will occur resulting in inferior properties. In order to understand the swelling processes leading to dispersion, it is advantageous to understand the structure of clay minerals. Clay minerals are naturally occurring, abundant aluminosilicate minerals widely formed in nature by the weathering and decomposition of rocks. They are layered materials known as “phyllosilicates”;8 these types of aluminosilicates are TOT 2:1 clay minerals, meaning that each clay layer consists of two Received: December 9, 2014 Revised: May 5, 2015

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DOI: 10.1021/jp512257z J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. System setup consisting of two layers of clay minerals (here hectorite), depicted as spheres and two interlayer regions filled with Na+ ions (dark blue spheres) and solvent (here ethanol), depicted as lines. The solid lines of the simulation box define the periodic boundaries, and the dashed line on the right indicates the fact that we show only half of the simulation cell in that direction here. A full system of the same clay mineral is shown in Figure 6; both figures were generated with VMD.14

of clay mineral particles such that a tortuous and extended path is presented to the diffusing gas,17,18 and consequently, the chances of interacting physically or chemically with the clay mineral particle are optimal. A key challenge to the inclusion of organoclay nanofillers in a wide range of polymer systems is the understanding of the factors controlling the dispersion of organoclay minerals within nonaqueous organic solvent systems. Extensive research has been undertaken to understand the swelling properties of clay minerals when contacted with water or aqueous brine/chemical solutions (see Anderson et al.19 and references therein, as well as Fripiat et al. 20 ) or for the treatment of natural (i.e., nonfunctionalized) clay minerals in a variety of solvent systems. These have included studies to understand the specific effect of the solvent’s dielectric constant on swelling of montmorillonite,21,22 the swelling of clay minerals in water/dimethlysulfoxide mixtures,23 the swelling of montmorillonite (MMT)−Na systems in ethanol24−30 and of formamide-treated MMT in polar organic solvents,31 as well as ethylene glycol, now in commonplace use for X-ray identification of clay minerals.32 Converse to the trends in most of these studies, Berkheiser and Mortland found little correlation of swelling with either solvent parameters or dielectric constants in their study of highly charged clay minerals substituted with various cations.30 Despite sustained experimental research effort in the field of organoclay polymer composite systems,3,33 and the established use of organoclays as rheology modifiers in oil-based fluids for oil and gas drilling operations,34 a full understanding of the fundamental thermodynamic controls on organoclay dispersion has not yet been achieved. Of particular interest in this area are the experimental studies of Burgentzlé et al.35 and Tran et al.,36 studying the structure and swelling behavior of MMT and quaternary ammonium cations pertinent to the nanoclay systems simulated in this study, and of Slade and Gates,37−40 studying the effects of different clay mineral types, the presence of cointercalated ion pairs and solvent on swelling, and the interlayer structure in organoclays. Before dispersion can be addressed, it is important to understand that the preceding stage of organoclay swelling and computational simulations can help to gain a detailed understanding of the structural changes taking place in this process. From this perspective, Tambach and co-workers41 and Hackett et al.42 have examined changes in intercalated surfactant, and

tetrahedral (T) sheets sandwiching a central octahedral (O) sheet. The interlayer spaces between the stacked layers are occupied by cations that compensate the negative charge originating from isomorphic substitutions within the clay layers (see Figure 1). The variety of compositions in clay minerals, the interlayer cations and the polymorphism of the structure of stacked layers, simultaneously endow clay minerals as a group with industrially exploitable properties and make characterization challenging.9 Many applications make use of the high surface area that clay layers have and/or the cation exchange properties of the mineral. The interlayer cations and twodimensional nature of the clay mineral platelets are responsible for the unique swelling and ion exchange properties of clay minerals in contact with water.10 In their naturally occurring state most clay minerals contain small inorganic cations, which render the surface hydrophilic and make it difficult to fully exfoliate many clay nanoparticles, depending upon the nature of the solvent. As such, clay minerals can therefore be modified with cationic organic surfactant molecules,11 such as quarternary ammonium cations,12,13 to displace organophobic cations and promote exfoliation in organic solvents. Conceptually, the exfoliation of clay minerals is a two-step process whereby, upon contact with solvent, the clay minerals undergo first swelling and second, once interfacial energies have decreased sufficiently, dispersion. At a molecular level, clay swelling is characterized by the ingress of solvent molecules into the interlayer space, such that the cations in the interlayer region and the clay mineral surfaces become solvated.15 While this allows adjacent clay layers to spatially separate, it does not necessarily mean that the clay layers will exfoliate, owing to the remaining attraction between the solvated cations and the negatively charged clay layers and other short-range interactions. For some clay mineral/solvent combinations, enough solvent molecules can enter the interlayer region, such that the layers are widely separated and finally are able to move independently; for others additional mechanical or ultrasonic force is needed to achieve dispersion and exfoliation of the solvated clay mineral layers. The exfoliation process is particularly pertinent to the polymer film industry where the polymer−clay film barrier properties depend on optimal dispersion and arrangement of clay mineral particles.16 The suggested optimal end arrangement is one where all clay mineral particles are oriented parallel (inplane) to the polymer film, with overlap between adjacent layers B


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Figure 2. Three quaternary ammonium cations used in the simulations. QA and QB are different representations of a mix of quaternary ammonium cations used in commercially available systems. The key difference between the structures is the size of the polar N+ headgroup (QA > QB > QC) and the length of the polyether side chain.

Osman et al.43 have studied alkyl chains assembled on mica using combined experimental and molecular dynamics (MD) simulations. It should also be noted that the majority of simulation studies looks at the structure and dynamics of the water/solvent or stable solvation states, rather than the propensity for a system to exfoliate. For example, Liu et al. have studied alkylammonium smectites and their interactions with water.44 In recent work, MD has been used to understand the effect of water on the structure of surfactant-intercalated clay minerals, in particular the configuration of alkyl chains and the competition of water for the polar head groups and clay mineral surface sites.45 Early studies using molecular modeling to study the interlayer structure and relative swelling of hydrated clay minerals used Monte Carlo simulations, where, for fixed interlayer heights and compositions, water can be used to populate the interlayer from which relative energetics and chemical potentials for the processes could be studied.46,47 Though these early simulations were able to show a good match with the d-spacing (i.e., distance between the midplane of two adjacent clay mineral layers) vs water content when compared to the experimental hydration curves, predicting the d-spacing to which a given smectite mineral might swell before dispersion occurred was not possible. Generally, a limitation arises owing to the models being based mainly on energetics and with no consideration of relative external energy of the solution, which has been addressed in later work, vide inf ra, though attempts have been made to use grand canonical simulations to understand clay swelling in terms of free energy changes.48,49 Furthermore, the swelling process depends on factors such as the relative humidity, prior stress, and/or heating of the clay mineral,50−52 all of which are unaccounted for by the actual computational simulations based on (to some degree) idealized model systems. Using equilibrium molecular dynamics allows the time-wise evolution of the system of interest to be studied and processes such as cation hydration and the formation of inner/outer sphere complexes to be understood. For example, Kalinichev and co-workers used MD simulations to develop a method to calculate the stable hydration energy of layered double hydroxide layered minerals.53,54 This method was further developed for applying MD to investigate the swelling behavior of clay minerals found in drilling muds.19,55−57 Only relatively recently have molecular level simulations, run on high performance computing facilities, started to address these phenomena such that reasonable predictions can be made based on accurate, atomistic clay mineral models and their adsorbed/intercalated species.58,59

Molecular simulation of clay minerals has also been employed to gain insight in other areas; for example, montmorillonite/ clay−polymer nanocomposites have been studied with both multi-scale60 and MD simulation.57 Further MD simulations investigated water at montmorillonite surfaces,61 as well as water hydrating the interlayers of montmorillonite thus causing swelling.62−66 The mechanical properties of clay sheets have been investigated,67−69 as well as the immobilization of clay particles in polymers.70 In the context of understanding the dispersion of clay particles, the molecular simulation work of Heinz and co-workers has looked at the interlayer interfacial energy as a function of cation type.71−74 Further examples of the application of computer simulations to clay minerals in the context of materials chemistry can be found in the reviews of Cygan et al.75 and Greenwell et al.58 While alkylammonium-intercalated clay minerals44,76,77 and several nanocomposite systems45,70 as well as the interaction of ethanol on different inorganic surfaces78−83 have been studied using different computational methods, it is noteworthy that no molecular simulation studies, to our knowledge, specifically addressed the swelling of clay minerals in ethanol nor the specific combination of clay mineral, surfactant, and solvents pertinent to the use of CPNCs by industry. Consequently, in the present manuscript we apply our recently described hydration energy calculation methodology55 used for organo-modified clay minerals in aqueous systems, to look at the solvation energy of montmorillonite, vermiculite, and hectorite, with two types of solvent, water and ethanol; Na + cations; and three types of surfactants, quaternary ammonium cations QA, QB, and QC (see Figure 2). This MD study aims to provide some much needed insight into the first swelling stage of clay mineral exfoliation by rationalizing the behavior with respect to clay swelling of different clay minerals/solvents/surfactants combinations and identifying promising prototyping systems, e.g., for rapid printing techniques.

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COMPUTATIONAL DETAILS Computer simulations of surfactant-functionalized clay minerals are typically carried out using atomistic, force-field-based methods as more detailed quantum mechanical simulations are computationally prohibitive for systems of a size necessary to represent these systems. The ClayFF force field of Cygan et al.75 has been shown to be particularly successful in the simulation of clay mineral systems44,69,84 and was applied throughout this study to simulate montmorillonite, vermiculite, and hectorite. For water the SPC water model of Berendsen et al.85 was used. For the ethanol and the QA−C molecules, the bonding parameters C


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model and counterions into the interlayer spacing. The montmorillonite systems were then further replicated to create the full clay mineral system as described shortly. The same protocol has previously been used for other simulations of montmorillonite.67 The distribution of the isomorphic substitution (defect) sites in the tetrahedral sheets and the octahedral sheet of the Wyoming clay mineral was chosen following the procedure described by Heinz et al.96,97 This procedure is based on 29Si and 27 Al solid-state nuclear magnetic resonance (NMR) measurements. One consequence is that direct oxygen atom bridges between sites of isomorphic substitution are avoided, such as Al− O−Al in the tetrahedral sheet and Mg−O−Mg in the octahedral sheet. QA and QB are representatives of the mixture of alkyl methyl dipolyoxyethylene ammonium cations first used by Sekimoto et al.98 and are used in the interlayer space of commercial clay mineral systems.99 QC finally represents the tallow-based ammonium cation used in the experimental, commercial Cloisite B30 system. The fully dehydrated MMT-Na model above was replicated to correspond to an equilibrated simulation cell of Lx = 83.2 Å, Ly = 71.9 Å, and Lz = 18.7 Å, including 144 Na, 896 Al, 136 Mg, 2040 Si, 6144 O, and 1024 H atoms, totaling 10 384 atoms. The Lz value taken at the end of the MD simulation translates into a dehydrated d-spacing of 9.35 Å (compared to an experimental value100 of 9.69 Å), confirming that the force fields we used are very reliable. The difference between experiment and simulation is most likely due to the fully ionic force field, which can have a tendency to overestimate surface charges.101 Vermiculite (VMT). To represent the experimental VMT-Na linked to the commercial Microlite 903 system, a Vermiculite system of the formula [Na1.333]inter[Mg6]oct[Si6.667Al1.333]tetO20(OH)4 representing a CEC of 169 mequiv/100 g was used. The vermiculite system was created following a procedure of Skipper et al.95 by modifying the composition of a trioctahedral, uncharged (talc) clay mineral. Isomorphic charge substitutions were placed in the talc clay mineral framework such that the final structure of the vermiculite is that specified above. For the vermiculite structure, only isomorphic substitution sites in the tetrahedral sheet were considered. The layer charge of 1.33, a value in the middle of the defining range for a vermiculite (1.2 < x < 1.7), see Skipper et al.,95 is equivalent to a CEC of 169 mequiv/ 100 g and therefore, in agreement with the literature,102 is much higher in charge than the other two clay mineral systems. The vermiculite structure, which is similar to MMT, is based on a single crystal refinement103 and has been adapted by isotopic substitutions in the tetrahedral sheet. The initial, fully dehydrated model above was replicated to correspond to a simulation cell of Lx = 62.9 Å, Ly = 109.0 Å, and Lz = 19.2 Å, containing 384 Na, 1728 Mg, 1920 Si, 384 Al, 6912 O, and 1152 H atoms, totaling 12 480 atoms. The Lz value at the end of the equilibration system without solvent loading translates into a dehydrated d-spacing of 9.60 Å. This compares favorably with a d-spacing of 9.6 Å reported for dehydrated Na-vermiculite.104 However, it should be noted that due to variable composition and structure experimental results might not correspond exactly to the system being modeled. Replacement of the Na+ cations with QA−C allows us to investigate the influence of the different clay minerals on the behavior of the quaternary cations. Hectorite (HCT). Laponite is a synthetic smectite clay that resembles the naturally occurring clay hectorite in structure and

(bonds, angles, and dihedrals) were taken from the CVFF force field,86 which has the same functional form as the ClayFF force field, and the nonbonding interactions (charges and van der Waals parameters) were well and consistently parameterized within the CHARMM force field,87−90 such that we adapted them for this study. Further details and a validation of the ethanol parameters are provided in the Supporting Information (SI). The combination of ClayFF and CVFF parameters has been successfully used to simulate alkylammonium-intercalated clay minerals,44,76,77 in addition to several nanocomposite systems.45,70 MD simulations were performed using the large-scale atomic/ molecular massively parallel simulator (LAMMPS).91 A time step of 1.0 fs per iteration was used for all simulations, and a total data production time per simulation (i.e., after system equilibration phases were carried out) was 2.0 ns. Final results are reported as an average over the final 0.5 ns of simulation. Atomic positions were recorded every 10.0 ps of simulation time during these production phase runs. Simulations were carried out using an NPT isothermal−isobaric ensemble with a Nosé− Hoover thermostat to regulate the temperature and pressure. The pressure was kept at 1 atm with a damping parameter of 0.5 ps to regulate pressure control, and the temperature was set to 300 K. Electrostatic interactions were simulated using the particle−particle particle-mesh (P3M) solver92 with a precision value of 0.001 and a grid order of 4. The Coulombic sum and the Lennard-Jones potential were assigned a cutoff of 9 and 10 Å, respectively. The extra skin distance for building neighbor lists was set to 2 Å, while the simulation cell was constrained to be rectilinear in shape (α = β = γ = 90°). Further details of the model parameters can be found in the SI. In order to calculate bulk energy values for ethanol and water, simulations were carried out for pure water and pure ethanol. Associated simulation boxes contained approximately 35 000 atoms. Simulations were performed in an identical fashion to the method for the clay mineral systems. The obtained energies for the bulk water and ethanol were averaged and included in the swelling diagrams. System Setup. All three systems simulated within this study are 2:1 TOT smectites with different isotopic substitutions in the octahedral and/or the tetrahedral sheets representing two clay minerals with charges in both sheets that vary in their overall layer charge and one clay mineral that has only octahedral sheet charges. This range of clay minerals provides insight into the effects of the strength and location of charges on the solvent and surfactant molecules, and they also closely represent experimental clay mineral systems. Montmorillonite (MMT). The used model system is based on a Wyoming (SWy-2) montmorillonite and has the formula [Na0.56]inter[Al3.46875Mg0.53125]oct[Si7.96875Al0.03235]tetO20(OH)4. With a cation exchange capacity (CEC) of 77 mequiv per 100 g of clay mineral (77 mequiv/100 g) we are very close to the experimental value of 76.4 mequiv/100 g for MMT-Na (Wyoming) SWy-1(SWy-2).93 Though this is at the lower end of CECs encountered in montmorillonite varieties,94 the Wyoming montmorillonite provides a useful comparator intermediate between the CEC of the hectorite and vermiculite models, thus allowing the effect of a range of surface charge to be probed. The Wyoming montmorillonite clay mineral system was created following the procedure of Skipper et al.,95 adding isomorphic (defect) sites to the clay mineral framework of an uncharged, dioctahedral pyrophyllite clay mineral unit cellD


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Figure 3. Swelling energetics for the three different Na+−clay minerals using water (left column) and ethanol (right column) as solvent. The black curves represent the d-spacing (left axis), the red curve the solvation energy (right axis), and the dashed line the averaged energy of the bulk solvent. The vertical gray lines indicate the inflection points in the d-spacing curve, associated with the formation of a new layer.

composition.8 A hectorite clay system based on previous work of Carvalho and Skipper 1 0 5 of the formula [Na0.5]inter[Mg5.5Li0.5]oct[Si8]tetO20(OH)4 representing a CEC of 66 mequiv/100 g (the lowest CEC of all the model clay minerals investigated in this work) was used. This CEC is typical for a hectorite/laponite clay and is in good agreement with experiment.106 Unlike the typical montmorillonite and vermiculite models, hectorite contains no tetrahedral sheet substitutions within the clay mineral layers. The fully dehydrated Laponite model above was replicated to correspond to a simulation cell of Lx = 209.5 Å, Ly = 181.3 Å, and

Lz = 18.6 Å with 800 Na, 8800 Mg, 800 Li, 12 800 Si, 38 400 O, and 6400 H atoms, totaling 68 000 atoms. The Lz value at the end of the equilibration simulation without solvent loading translates into a dehydrated d-spacing of 9.30 Å. While this is slightly lower than the experimental value of 9.67 Å for a similar system,107 it is in very good agreement with previous simulations.105 Due to the close similarities between the MMT and the HCT systems found when investigating swelling in pure, Na+ clay minerals, no HCT systems with quaternary ammonium ions were investigated. In this study, periodic models with two interlayer regions were used. Three-dimensional periodic boundary conditions were E


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we base our conclusion on analysis carried out related to our results for the well-understood Na+−clay systems. The pure Na+−clay minerals all showed some distinct stepwise increases in d-spacing, which were used to identify mono- to quadrilayer solvent occupation in the interlayer of the clay minerals, and these regions are identified on the respective diagrams in Figure 3. Additional evidence can be taken from the solvent occupation as identified from the 1D atom density plots for these systems (see Figure 4). Solvation in Water. This section directs the reader to the dspacing and solvation energetics in Figure 3 and the 1D atom density plots shown in Figure 4. Montmorillonite. Initially, at zero solvent loading the Na+ ions form a bilayer in the interlayer space, as shown in Figure 4. As water is added the d-spacing increases, and the water molecules start solvating the Na+ ions, moving them closer into the middle of the interlayer region, in good agreement with the experimental results of Sposito et al.108 and previous simulation work.109 After formation of the water monolayer the d-spacing increases more gradually until a water bilayer starts to form at about 125 mg/g clay mineral. This water bilayer allows further solvation, with the Na+ ions concentrated in the middle of the interlayer space, sandwiched between two layers of oxygen atoms. The formation of a water trilayer has begun by 300 mg/g clay mineral, and the Na+ ions rearrange into two layers (again located between the oxygen atoms). The trilayer continues to grow without an abrupt change in d-spacing, and at 400 mg/g clay mineral the Na+ ions are also forming a trilayer, indicating that the water trilayer is about to separate into a quadrilayer. The solvation energetics show there is a small energy barrier (∼5 kJ mol−1) to solvation up to a trilayer. At this point the energy of bulk water is exceeded, and swelling will cease.55 It is worth noticing that the d-spacing for zero, one, and two solvent layer(s) of 9.35, 12.42, and 16.05 Å coincide well with the experimental values of 9.5−9.8 Å, 12.0−12.7 Å, and 15.0−15.6 Å, as collated by Karaborni et al.110 Vermiculite. The d-spacing changes of VMT differ from MMT in that the transitions are initially less abrupt, which could be because the higher negative charge of VMT requires more counterbalancing Na+ ions or due to the screening effect of the water molecules. The water monolayer and the Na+ distribution for the 100 mg/g clay loading show the same central oxygen peak as MMT (but stronger hydrogen bond interaction with the clay surface) which can be well rationalized from the crystal structure of VMT-Ba.111 The interlayer structure for the 200 mg/g clay loading, where the Na+ cations in the center of the interlayer region are sandwiched by one layer of water molecules on each side, directly coincides with experimental evidence.112,113 There is, in general, more structuring of solvent and Na+ ion concentrations in VMT than MMT, as exemplified by the formation of a water quadrilayer by 400 mg/g clay mineral and five (mostly) distinct regions of Na+ ions. Also, similar to MMT, the energy of bulk water is overreached by about 5 kJ mol−1 between the formation of a trilayer from a water bilayer (300 mg/ g clay mineral). The solvation energetics (Figure 3) indicate that hydration of VMT is more favorable than that of MMT, which is due to the higher clay layer charges and hence electrostatically repulsive forces between the adjacent clay layers in VMT. The effect of the higher charges in VMT leads to more pronounced structuring of water and Na+ ion concentrations, and although the energy of bulk water is reached this is not reflected in the solvent structure; i.e., within the interlayer space the energy of water corresponding to the energy of bulk water

applied to all models to represent the bulk material. The workflow for each clay mineral/cation/solvent combination consists of a series of Perl scripts, which automate the insertion of solvent and surfactant molecules, further details of which can be found in the SI. Analytical Methods. The purpose of this investigation is to probe the swelling behavior in water and ethanol of three differently charged clay minerals in their pure state, i.e., containing only intercalated Na+ ions, and in their intercalatedsurfactant state. Swelling trends in the clay mineral/surfactant systems are analyzed with reference to swelling in the pure state, where the analysis involves examining the interlayer structural trends apparent in the 1-dimensional (1D) atom density plots at zero to 400 mg/g clay mineral solvent loading in steps of 100 mg/g clay mineral, changes in d-spacing as the clay minerals swell, and calculation of the solvation energetics, further details of which can be found in the SI. To gain further insight into the interlayer structure at the higher levels of swelling, the 1D atom density plots are analyzed at a solvent loading of 450 mg/g clay mineral and are described in the SI.

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RESULTS

General Solvation Process. In all solvent-free clay mineral systems presented in this study the clay layers are separated only by interlayer cations (Na+ or QA−C). Initially, solvent molecules will fill up the available space and solvate the cations and the clay mineral surfaces until all space is filled by a solvent monolayer. For this process, the interlayer has to increase for the first solvent molecules but does not change during the completion of the monolayer. To form a new layer, more space needs to be generated in the interlayer region, usually corresponding to a sharp increase in the d-spacing. The additional space is then filled with solvent molecules with again no concurrent change in dspacing until another new layer has to be formed and the pattern is repeated. With increasing numbers of layers, the steps in dspacing become less distinct. The d-spacings for all Na+−clay minerals are shown in Figure 3 along with the accompanying solvation energetics. The d-spacings at which the bulk solvent energies are reached for all systems covered in this work are recorded in Table 1 together with the corresponding solvent loadings. For the purposes of comparison between the different systems, we assume that a larger solvent loading (and hence a greater degree of swelling) at which the energy of bulk solvent is reached and a small energy barrier to initial solvation are indicators of a capacity to swell further and disperse. In general, Table 1. Amount of solvent required for the solvation energy to match the bulk solvent energy (SolvEq). This equates to the expected degree of swelling within the system. The d-spacing of the interlayer at which this occurs is also given (deq) water solvent

ethanol solvent

model

solvEq (mg/g)

deq (Å)

solvEq (mg/g)

deq (Å)

MMT-Na VMT-Na HCT-Na MMT-QA MMT-QB MMT-QC VMT-QA VMT-QB VMT-QC

305 260 140 410 275 410 >500 300 >500

17.5 15 13 34 30 28 >52 48 >42

0 0 0 >500 415 >500 365 250 365

9.5 9.5 9.5 >40 >37.5 >35 52.5 49 42 F


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Figure 4. 1D plots for the three Na+−clay minerals at different solvent (water) loadings. Images for the same clay mineral use the same scale on the y-axis, and each tick on the x-axis indicates a distance of 5 Å. The dashed lines in the unsolvated cases indicate the clay surface.

Figure 5. 1D plots for the three Na+−clay minerals at different solvent (ethanol) loadings. Images for the same clay mineral use the same scale on the yaxis, and each tick on the x-axis indicates a distance of 5 Å. The dashed lines in the unsolvated cases indicate the clay surface.

and exceeded already at the formation of a bilayer at about 145 mg/g clay mineral, although, as in the case of MMT, the overshoot is small. The similarities in the results of MMT and HCT suggest that the location of the charge in the clay layers, i.e., whether it is in the tetrahedral or octahedral sheets or both, has no effect on the structure of the interlayer water molecules or the location of the Na+ ions. The greater structuring of both the water and Na+ ions seen in the VMT results suggests that the higher charge of the clay layers is responsible for this effect.

does not reflect a disordered bulk structure but rather an emerging triple layer. As for MMT, the d-spacing for zero, one, and two solvent layer(s) for VMT of 9.58, 12.32, and 15.01 Å coincides well with experimental results of 9.82, 11.8, and 14.8 Å, respectively, for a similar VMT system.114 Hectorite. The hydration of HCT is very similar to that of MMT with respect to the structure of the concentrations of Na+ ions and the changes in d-spacing. The main difference lies in the solvation energetics where the energy of bulk water is reached G


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Figure 6. Structure that demonstrates the transition from a bilayer to a trilayer in hectorite and local differences in the layer structure.

As for the other systems, the d-spacing for zero, one, and two solvent layer(s) for HCT of 9.29, 12.73, and 16.31 Å coincides well with experimental results of 9.67, 12.45, and 15.10 Å, respectively, for a similar HCT system,107 and the results are simulated by Morrow et al.115 and Bowers et al.116 Solvation in Ethanol. This section directs the reader to the d-spacing and solvation energetics in Figure 3 and the 1D atom density plots shown in Figure 5. In all cases the initial solvation of the dry clay mineral with ethanol is energetically unfavorable, as shown by the high solvation energetics compared to bulk ethanol (Figure 3), which at zero solvent loading would have points of origin off the scale of the graphs shown. Montmorillonite. Our simulations show that formation of an ethanol monolayer, while energetically favorable as shown by the solvation energy, is associated with a large energy barrier for moving the first ethanol molecules into the interlayer region (see Figure 3). Transition from the monolayer to the bilayer is associated with a barrier but according to our simulations might still be possible. These findings, i.e., the formation of mono- and bilayer, are in agreement with experimental findings for MMT− Na systems.24−29 The d-spacing changes are more abrupt than in the case of hydration, and stepwise swelling behavior is much more evident. For zero, one, and two solvent layer(s) in MMT, the d-spacing is simulated to be 9.30, 13.39, and 17.27 Å, which is in good agreement with experimental results of 9.5−9.8 Å, 12.8− 13.5 Å, and 16.8−17.4 Å, respectively, for similar MMT systems.24−30,110 The Na+ ions also behave differently in ethanol than in water, as they aggregate at each clay surface rather than dispersing in the solvent. The backbone of the ethanol molecules forms distinct mono-, bi-, and trilayers on increasing solvation, aligning themselves parallel (rather than perpendicular) to the clay surfaces, as postulated by Dowdy and Mortland based on results from MMT-Cu.24 The ethanol molecules initially lie in the middle of the interlayer space (100 mg/g clay mineral), then split up to form two layers with the OH groups pointing toward the clay mineral surface (200 mg/g clay mineral), and then repopulate the middle of the interlayer space by forming a third layer (300−400 mg/g clay mineral) maintaining a high degree of structure throughout solvation. Also with increasing solvation the Na+ ions remain localized at the clay surfaces; that is, the interlayer space fills with ethanol molecules, causing physical separation of the clay mineral layers. Vermiculite. Similar to the MMT model the d-spacing changes are more abrupt than in the case of hydration. Formation of an ethanol monolayer is associated with an even larger energy barrier for moving the first ethanol molecules into the interlayer region and is energetically less favorable than for MMT as shown by the solvation energy in Figure 3. Once the first solvent layer is formed, according to the solvation energies, VMT should solvate up to a trilayer. For zero, one, and two solvent layer(s) in VMT, the d-spacing is simulated to be 9.60, 13.20, and 17.89 Å, respectively. To the best of our knowledge, there are no

experimental results available with which to compare the simulated values given above. The structure of the interlayer during solvation is also quite different in VMT, where, from 200 mg/g clay mineral, the Na+ ions are more strongly localized at the clay surfaces and are barely present in the interlayer space. The orientation of the ethanol molecules is also more diverse in VMT, as can be seen from the deviation between the C2 and the C3 curve in Figure 5. Especially the results for 300 mg/g clay mineral indicate a higher alignment angle relative to the surface with the hydroxyl groups being consistently concentrated at the clay mineral surfaces, forming stronger hydrogen bonds to the higher (compared to MMT and HCT) charged clay mineral surfaces. At 400 mg/g clay mineral the 1D profile becomes asymmetric, which coincides with the transition from a tri- to a quadrilayer of ethanol. A similar effect is illustrated in Figure 6 for HCT. Hectorite. Solvation of HCT in ethanol closely resembles that of MMT in d-spacing, solvation energetics, and structure of ethanol, with the major difference being that in HCT the Na+ ions have a stronger tendency to populate the interlayer space rather than remain concentrated at the clay mineral surfaces. Also the H of the hydroxyl groups of ethanol lies closer to the clay mineral surfaces than the Na+ ions throughout increasing solvation, which is the opposite of their respective orders seen in MMT. The parallel alignment of the backbone of the ethanol molecules to the clay mineral surfaces is very similar in MMT and HCT up to 200 mg/g clay mineral, beyond which asymmetry appears in the HCT 1D plots. This asymmetry originates in a mixed bi-/trilayer structure (see Figure 6), which explicitly shows the transition between the differently layered systems, corresponding to points of inflection in the d-spacing. It is noteworthy that these structures can only be observed in large enough systems as they require some flexibility in the clay mineral layer, which cannot be achieved in small systems, even when they are periodic.67 For zero, one, and two solvent layer(s) in HCT, the d-spacing is simulated to be 9.27, 13.32, and 17.22 Å, respectively. To the best of our knowledge, there are no experimental results available with which to compare these simulated values. In summary, the differences between MMT and HCT in the location of the Na+ ions suggest that in ethanol the location of charge in the tetrahedral sheets of the clay mineral layers is necessary to order the Na+ ions. Increasing the charge in both the tetrahedral and octahedral sheets (as in the case of VMT) enables the ordering of both Na+ ions and ethanol molecules and in particular the hydroxyl groups of ethanol at the expense of ethanol’s otherwise perfect parallel alignment to the clay mineral surfaces. The VMT ethanol system differs from both MMT and HCT ethanol systems in that the Na+ ions are more localized at the clay mineral surfaces, as expected due to the higher surface charge. Also, although the ethanol molecules are oriented into four layers there is an increase in density of the OH group in the midplane, H


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Figure 7. 1D plots for the two QA−clay minerals at different solvent loadings. Images for the same clay mineral/solvent system use the same scale on the y-axis, and each tick on the x-axis indicates a distance of 10 Å. C(CH3,QA) denotes the end group of the hydrophobic tail, O(QA) the oxygen atoms of the ether groups, O(W) the oxygen atom of the water molecules, and C(CH3,E) and C(CH2,E) the two different carbon atoms in ethanol.

Figure 8. Swelling energetics for the three different QA−clay minerals using water (left column) and ethanol (right column) as solvent. The black curves represent the d-spacing (left axis), the red curve the solvation energy (right axis), and the dashed line the averaged energy of the bulk solvent. As for most other Q−clay minerals there are no well-defined inflection points.

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ethanol’s C3 and C2 carbons atoms (see Figure 7). The extension of the ether chains of QA into the interlayer space barely changes on solvation; therefore, with increasing solvent loading, the overlap of the polyether groups anchored at two different surfaces decreases. As for the water case, there remains a significant amount of ether groups at the clay surface, independent of the solvent loading. The similarities in the solvent structures are mirrored in the d-spacings and solvation energetics (Figure 8) where the former show linear, monotonic increases and the latter oscillate with small amplitudes toward a constant. The bulk energy of water is reached at about 400 mg/g clay mineral, but that of ethanol is not reached within this study. Compared to the MMT-Na models with water and ethanol, QA has slightly increased the initial energy barrier to hydration and reduced it to further hydration and considerably reduced the energetic barriers to ethanol solvation. Vermiculite Solvation in Water and Ethanol. Due to the increased number of QA molecules in VMT-QA compared to MMT-QA, a much higher d-spacing is observed and the chains aggregate in a paraffin-like bilayer with a significant amount of bent (gauche) bonds; however, due to the fact that the polyether side chains are not long enough, there is hardly any interdigitation of the polyether side chains linked to the two opposing surfaces. Upon solvation, the VMT-QA water and ethanol 1D plots show some distinct similarities (Figure 7): in both solvents the structure of QA is very similar and between 100 and 400 mg/g clay mineral barely changes on increasing solvation. For water as well as ethanol, the first distinctive layer established is the surface layer. As hydration continues the surface coverage barely changes in favor of the growth of the central interlayer region. Ethanol behaves similarly except that for 400 mg/g clay mineral, and the interlayer region starts forming a bilayer of ethanol in the middle of the interlayer with the ethanol molecules oriented parallel to the clay surfaces. There is a noteworthy, unique feature in the 1D plots (Figure 7): while the surface for all dry clay minerals shows a distinctive peak for the QA-ether groups, this is replaced by water in VMT-QA, but contained in all other QA systems. This shows the very strong stabilization effect of the negatively charged clay surface with water as solvent, an effect which is essentially non-existent with ethanol. The solvation energetics (see Figure 8) differ at low solvent loadings, where more energy is gained for water solvation than ethanol solvation, although the solvation energies for both solvents flatten off at about 250 mg/g clay mineral of solvent with small amplitude oscillations. The bulk energy of ethanol is reached at about 355 mg/g clay mineral, but that of water is not reached within this study. Compared to the VMT-Na models in water and ethanol, QA makes solvation for both solvents considerably more favorable: For water, the energy gain upon generation of the first surface layer is much more favorable, and the value for bulk solvent is no longer reached within the simulations carried out. For ethanol, the very large energy barrier associated with formation of the first solvent layer in VMT-Na vanishes, whereas a swelling barrier at 355 mg/g clay mineral might still prevent complete exfoliation. Surfactant QB. Montmorillonite Solvation in Water and Ethanol. In the unloaded systems, the interlayer structure of MMT-QB, despite the differences in the headgroup and the different length of the polyether side chains, is very similar to that of MMT-QA. The side chains order in a pseudotrimolecular layer structure, and in the central layer, interdigitation of the polyether side chains linked to the two opposing surfaces is observed. Also

contrasting with the (slight) decrease in this region seen in MMT and HCT. These findings are to some extent different from previously reported simulation studies of ethanol interacting with inorganic surfaces, e.g., calcite,78,80,81,83 alumina,79 Al(OH)3 (representing gibbsite82), and FeOOH (representing lepidocrocite82). While these studies find ordering effects of the ethanol molecules on the surfaces, only the formation of a single layer, followed by a “depletion” layer (where the ethanol occupation is essentially zero) and then the bulk region are reported. In our simulations, we find strong ordering up to a quadrilayer, clearly separated by depletion layers. This can be assigned to the fact that the previous studies included only one surface, whereas in our study the ethanol molecules are sandwiched between two surfaces, leading to a stronger ordering. In addition, the preferred orientation of the ethanol molecule relative to the surface turns out to be different from the previously reported studies. For calcite and alumina, the ethanol molecules mainly orient themselves perpendicular to the surface.78−81,83 Although for Al(OH)3 the preferred orientation is either around 0° or 60−70°82 and for FeOOH it is between 30−60°,82 we find a clear preference for the ethanol molecules to align parallel to the surface. These findings are in agreement with the findings of Wang et al.117 suggesting that the dipole length between the oppositely charged surface atoms is responsible for this alignment, even if the surface itself is uncharged. Accordingly, they found parallel orientation for short dipole lengths (1.62 Å) but perpendicular orientation for long dipole lengths (2.82 Å), both with charges of q = 1.0. In the present case, with a typical dipole length of about 1.5−1.6 Å, this should induce parallel ordering of ethanol at the clay mineral surface, which is exactly what was observed in this study, confirming experimental findings.24,36 The overall similarity of results between MMT and HCT obviates the need to continue with both clay minerals; hence, from this point onward only MMT and VMT are considered. Although all three clay minerals swell easily in water, they all have energy barriers to swelling in ethanol, and given the importance of this solvent to the polymer industry the addition of a surfactant to reduce this initial energy barrier is a possible solution to overcome the energy barrier to swelling and dispersion. Consequently, the following results report the effect of adding three different surfactantsQA, QB, and QCand comparing the solvation of MMT and VMT in water and ethanol. Surfactant QA. Montmorillonite Solvation in Water and Ethanol. At zero solvent loading (see Figure 7) the nitrogen (N) of QA resides at the clay surfaces with the ether chains partly covering the polar clay surface and partly extending into the interlayer. The CH3 groups of the hydrophobic side chains are mainly located in the interlayer region. Although Lagaly’s classification model33 was originally set up for alkylamines with only one hydrocarbon side chain, it can be used to identify the chain aggregations as pseudotrimolecular layers. As hydration proceeds, the N remains anchored at the clay surfaces, and solvent molecules also position themselves to lie close to the surfaces, with a clearly visible surface layer of water at 100 mg/g clay mineral and an increasing amount of water molecules in the interlayer region, forming up to four (at 400 mg/g clay mineral) additional, but less pronounced, layers. Ethanol behaves similarly with distinct alignment of ethanol molecules at the clay mineral surfaces and the growth of multilayers on increased solvation. As for the Na+−clay minerals, the ethanol molecules align parallel to the clay surfaces, resulting in essentially identical plots of J


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Figure 9. 1D plots for the two QB−clay minerals at different solvent loadings. Images for the same clay mineral/solvent system use the same scale on the y-axis: each tick on the x-axis indicates a distance of 10 Å. C(CH3,QB) denotes the end group of the hydrophobic tail, O(QB) the oxygen atoms of the ether groups, O(W) the oxygen atom of the water molecules, and C(CH3,E) and C(CH2,E) the two different carbon atoms in ethanol.

Figure 10. Swelling energetics for the three different QB−clay minerals using water (left column) and ethanol (right column) as solvent. The black curves represent the d-spacing (left axis), the red curve the solvation energy (right axis), and the dashed line the averaged energy of the bulk solvent.

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Figure 11. 1D plots for the two QC−clay minerals at different solvent loadings. Images for the same clay mineral/solvent system use the same scale on the y-axis, and each tick on the x-axis indicates a distance of 10 Å. C(CH3,QC) denotes the end group of the hydrophobic tail, OH(QC) the oxygen atoms of the alcoholic end groups, O(W) the oxygen atom of the water molecules, and C(CH3,E) and C(CH2,E) the two different carbon atoms in ethanol.

Figure 12. Swelling energetics for the two different QC−clay minerals using water (left column) and ethanol (right column) as solvent. The black curves represent the d-spacing (left axis), the red curve the solvation energy (right axis), and the dashed line the averaged energy of the bulk solvent.

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QC does not possess any ether groups but instead two short alcoholic groups, which can interact to different degrees with the clay surface and across the interlayer regiondepending on the solvent loading. During the early solvation stages, these side chains interact across the interlayer region, as visible most for the dry clay mineral (see Figure 11). This interaction remains intact upon addition of water or ethanol solvent molecules, which first form a surface layer. With increased solvent loading (>200 mg/g clay mineral), the d-spacing increases, and the OH groups of QC form two (for water) or three (for ethanol) distinctive peaks. Noteworthy is that the additional peak in the ethanol case appears closer to the clay surface than the nitrogen; i.e., the ethanol is solvating the cationic headgroup and forms H-bonds with the oxygen atoms of the mineral surface. The water molecules order themselves much in the way they do in QB hydration with more residing in the interlayer space especially during early hydration. Ethanol differs in that a bilayer is formed at 200 mg/g clay mineral and only starts to form a trilayer at 400 mg/g clay mineral in contrast to QB’s quadrilayer at the same loading. The d-spacings are less linear than those seen in the MMT, QA, and QB systems, with the ethanol showing more of a stepwise behavior during early solvation (see Figure 12). It is noteworthy that in the unsolvated case the simulated value of 18.7 Å matches well with the experimental value of 18.5 Å given in the literature.35,36 The hydration energetics has a steeper initial step and then tends toward a constant value, achieving the energy of bulk water at 400 mg/g clay mineral. The ethanol solvation energetics shows larger amplitude oscillations than those seen in the MMT-QA system also with an initially steeper step, although overall tending toward a constant and not reaching the energy of bulk ethanol. Compared to the MMT-Na system with water, QC has increased the energy gain, especially for formation of the first surface layer, and the loading at which the energy of bulk water is reached has been increased. Relative to the MMT-Na system with ethanol, the initial energy barrier to solvation has been replaced by a favorable formation of the first solvent surface layer, with the bulk energy not being reached within our simulations. This is in direct contrast to the experimental results of the solvation of similar MMT systems (an MMT-QC with a CEC of 90 mequiv/100 g clay) in organic solvents35,36 that found no swelling of the nanoclays occurred in alcohols with an alkyl chain length shorter than that in pentanol. The possible reasons for the differences between the simulated and experimental results are examined in the Discussion section. Vermiculite Solvation in Water and Ethanol. Due to the much higher number of QC molecules compared to the low charge MMT, no lateral bilayer, but a paraffin-like monolayer with a high degree of interdigitation of the hydrophobic side chains, incorporating both linear (all trans) and gauche conformations, is formed in VMT. From zero solvent loading to 400 mg/g clay mineral the structure of QC barely changes, with the surfactant’s headgroup being located at the clay surfaces throughout solvation. However, with increasing solvent loading, the degree of interdigitation of the side chains lowers, forming a solvent intermingled paraffin-like bilayer at a loading of 400 mg/ g clay mineral. The water structure follows the same pattern as for QA and QB; i.e., it is mostly concentrated at the clay surfaces beginning to occupy the interlayer between 200 and 300 mg/g clay mineral. In the VMT-QC system with water, however, there is no depletion region for water between the surface layer and the remaining water, the latter not showing any sign of structuring.

upon solvation in water and ethanol the general structure of QB in MMT remains very similar to that of QA (see Figure 9) with the main difference seen as stronger structuring of the ether side chains which is already observed in the dry clay minerals. As solvation increases the number of distinct, interlayer ether regions grows. Both solvents repeat the behavior seen in MMTQA, which is borne out by the similarities in their solvation energetics and d-spacings (Figure 10). The difference in structure between QA and QB is the rearrangement of the length of the ether side chain from a symmetrical four ether groups per side chain to an asymmetrical eight ether group in one side chain and no ether group in the other side chain, which enables QB to become more responsive to the solvent. An interesting difference between the two solvents is the orientation of the CH3 group of the hydrophobic side chain, which has a higher probability to be in the central interlayer region and to overlap there during the ethanol solvation. Compared to the MMT-QA models with water and ethanol, QB shows the same trends and overall behavior, only with slightly less favorable solvation energetics. On the one hand, this is reassuring, as QA and QB are two different representations of the same experimental quaternary cation. On the other hand, this might indicate that the exact solvation energetics depend on the actual fabrication batch of the organoclay and the relative amounts of QA and QB type surfactants incorporated in the clay mineral. Vermiculite Solvation in Water and Ethanol. In the unloaded systems, the interlayer structure of VMT-QB, despite the differences in the headgroup and the different length of the polyether side chains, is very similar to that of VMT-QA. The side chains order in a paraffin-like bilayer structure with a significant amount of gauche bonds and some interdigitation of the polyether side chains linked to the two opposing surfaces. While the nitrogen atom of QB is similarly anchored at the clay surfaces as for the QA systems, the ether side chains are more evenly distributed throughout the interlayer region rather than concentrated toward the clay surfaces. The near-even distribution of QB’s ether side chains persists on increasing solvation. The structure of the solvents is very similar to those in the QA models except for the delayed growth of the interlayer region of water molecules. In contrast to the MMT cases, the CH3 group of the hydrophobic side chains shows overlap for zero solvent loading but loses this overlap with solvation due to the d-spacing exceeding the chain length. The solvation energetics and dspacing (see Figure 10) are very similar to those of QA, although the bulk energy of water is reached at about 300 mg/g clay mineral and that of bulk ethanol at about 420 mg/g clay mineral. Compared to the MMT-QB systems the structure of QB in VMT is less affected by the solvent on increasing solvation. Compared to the VMT-Na models in water and ethanol, QB makes solvation for both solvents considerably more favorable: for water, the energy gain upon generation of the first surface layer is much more favorable, and the value for bulk solvent reached at a higher solvent loading generates a lower barrier. For ethanol, the high energy barrier associated with formation of the first solvent layer in VMT-Na vanishes, whereas swelling might face small barriers at several points of the swelling process. Surfactant QC. Montmorillonite Solvation in Water and Ethanol. In the unloaded systems of MMT-QC, in contrast to the MMT-QA and MMT-QC system, the surfactant molecules form a lateral bilayer in the interlayer region with a low amount of interdigitation of the two layers. As for QA and QB, the N of QC lies in close proximity to the clay surfaces throughout solvation. M


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imental findings therefore are expected to provide slightly different results than our studies, as HDMTA has a smaller and more polar headgroup and a significantly lower molecular size (HDMTA: NC19H42, QA/B: NC33H70O10, QC: NC22H50O2) than any of the surfactants presented here. Nevertheless, we attempt to correlate our observations with the existing literature on these related compounds. Slade and Gates have studied a number of HDMTA organosmectites, prepared in ethanol and water solvents and subsequently subjected to swelling in toluene.37 The closest clay minerals studied with respect to the present study were the SWy-1 montmorillonite (CEC of 76 mequiv/100 g clay) and the Llano Stop 10 vermiculite (CEC 150 mequiv/100 g clay). The reported d-spacings for the HDMTA variants of these clay minerals (water/ethanol preparations) were 20.1, 17.6, and 29.6 Å, respectively.37 Where the present study only examines the organo-montmorillonite and organo-vermiculite, the simulated nonsolvated d-spacings are (expectedly) in excess of the values recorded for the HDTMA systems, with surfactants QA and QB having d-spacings in excess of 23.6 Å in a nonsolvated state and QC, with a lower molecular size than QA/B, having a d-spacing of ca. 18.7 Å. These are in reasonable agreement with the experimental, observed d-spacings of Slade and Gates, indicating that with an increasing number of surfactant side chains interdigitation is less likely to occur.37 This can be contrasted with surfactant QC having simulated d-spacings of 35 Å at 500 mg ethanol/g clay mineral loading for montmorillonite and 42 Å at the simulated swelling maxima for vermiculite or surfactant QA/ QB having simulated swelling maxima in ethanol of 38 Å for montmorillonite and in excess of 50 Å for vermiculite. It should be noted that in the case of toluene adsorption the maxima observed were in the region of 50 mg/g clay,37 somewhat short of the simulated ethanol contents. Slade and Gates also studied the ordering of the HDTMA in the interlayer in the presence of cointercalated counterions,38,40 though this is not comparable to the present simulations where no counteranions are included. Interestingly, the studies showed that the use of toluene vapor yielded a different interlayer arrangement from the liquid.38 In a further study, with a more bulky surfactant molecule, Gates studied the swelling of a number of smectites intercalated with benzyloctadecyl-dimethylammonium (BODMA: NC27H50) cations.39 The author notes that organoclays seem to have a greater propensity for swelling in organic solvents with lower dielectric constants than is encountered in inorganic smectites in water, as we also observe in the present study. Through varying the amount of ethanol in water, from wholly water through to wholly ethanol, the study explored how the solvent mix affected swelling with, by way of example, Upton BODMA-montmorillonite, swelling from 20 Å in water through ca. 34 Å in ethanol. This shows qualitative agreement with the observation that in the calculated solvation energetics in water the surfactant systems quickly reach the bulk solvent energy, even at low solvent loadings, whereas the ethanol system, if we look at surfactant QC, approaches 35 Å. The results of the simulated systems presented here agree with experimental findings in their individual components, i.e., the structure and geometry of the pure clays, the structure and energetics of ethanol, and the structure of the organic cations, all of which validates the force field parameters used in the simulations. Specifically, the d-spacing of the MMT-Q C simulated to be 18.7 Å for the unsolvated case is in excellent agreement with the only available experimental value35,36 of 18.5 Å for the nanoclays studied in this work. The apparent

While for ethanol formation of a surface layer at low solvent loading is observed, so is a stronger tendency to occupy the interlayer region, with ethanol forming six distinct layers in addition to the surface layers. The d-spacings and hydration energetics are similar to those of QA and QB with a linear, monotonic increase, and the energy of bulk water is reached at a solvent loading of 370 mg/g clay mineral. The solvation energetics of ethanol has a steeper gradient than those of QA and QB, reaching the energy of bulk ethanol at 360 mg/g clay mineral. QC in VMT shows a much sharper peak for N and the alcoholic OH groups than QC in MMT; both water and ethanol have a much stronger tendency to localize at the clay surfaces of VMT than in MMT, and ethanol is highly structured in both clay minerals up to high solvent loadings. Compared to the VMT-Na system with water, QC has significantly increased the energy gained upon initial swelling and at the same time increased the loading at which the energy of bulk water is reached. Relative to the VMT-Na system with ethanol, the initial energy barrier to solvation has been erased, but there is still an overall energy barrier around 370 mg/g clay mineral which has to be overcome to achieve exfoliation.

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DISCUSSION In considering the solvation of a clay mineral system, the formation of the initial monolayer of solvent coverage of the clay surfaces should be energetically favorable if the clay minerals are going to swell readily. Our simulated results showed that in all three homo-Na+−clay minerals in water monolayer coverage was favorable and therefore does not prevent swelling at this early stage. In contrast to this finding for water, the formation of the first monolayer of ethanol is associated with a substantial barrier. With respect to the nonfunctionalized clays, the data for montmorillonite with water have been published in a number of studies, and agreement with experiment is summarized in the work of Suter et al.55 The use of ethanol with low and high charge MMT-Na systems has been studied experimentally by several groups, who showed the stable swelling state for these systems for zero, one, and two solvent layer(s) in MMT lay at 9.5−9.8, 12.8−13.5, and 16.8−17.4 Å, respectively.24−30,110 The simulated d-spacing of 9.30, 13.39, and 17.27 Å, respectively, in this study is in good agreement with these results. Exchange of Na+ by any of the three quaternary cations (QA, QB, or QC) improved the solvation energetics of the first layer: in the case of water, the solvation energy drops to 12 kJ/mol (in the case of VMT) and significantly reduces the swelling barrier to be overcome or even completely removes it. In the case of ethanol, the solvation energy of the first solvent layer is greatly reduced and renders swelling energetically favorable. The influence on the swelling barriers is strongly dependent on the clay mineral system: while there is a significant impact on the swelling barriers found for the MMT system, for which the large barriers for MMT-Na essentially disappear (a small barrier remains only for MMT-QB), the influence for the VMT systems is less pronounced. Although the swelling energetics are calculated to be much smoother than for VMT-Na, small swelling barriers remain for all the VMT-Q systems. Comparing these simulation results with experimental studies is inherently difficult, as clay swelling properties show a significant dependency on the specific nature of the quarternary ammonium cations and the clay minerals. The most thorough studies available are for clay minerals intercalated with hexadecyltrimethylammonium (HDMTA).37,38,40 These experN


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nanoclay polymer systems efficiently designed and fit-forpurpose. To this end, atomic force microscopy is being explored both to establish the point at which swelling ceases in the presence of solvents and also to attempt to measure the energy required to exfoliate one clay layer from the next.

discrepancysimulated results showing that the MMT-QC system swells in ethanol, whereas the experimental results show it does notcan be attributed to the fact that the macroscopic dynamics of swelling have not been simulated, and it is this phenomenon that is measured experimentally, as discussed by the authors of these studies. The experimental results of Burgentzlé et al.35 lead to the conclusion that macroscopic swelling leading to dispersion does not solely correspond to intercalation of solvent molecules between individual clay layers; rather, swelling without dispersion can occur in the absence of intercalation, when the solvent molecules occupy the spaces between clay particles (or tactoids) to form gel-like structures. Furthermore, the work of Tran et al.36 determines ethanol molecules to have both a relatively high polarity and hydrogen bonding, leading to the formation of hydrogen bonds with the edges of clay particles that then remain intact and thus not intercalated. To achieve swelling leading to dispersion, there must be both intercalation as well as separation of the tactoids, which is governed by the polarity of the solvent molecules as well as the surface energies of the solvents and the organoclays. These properties lead to the experimentally observed dynamic behavior that is not accounted for in our simulations. To gain further insight into the dynamics of swelling requires modeling techniques such as kinetic Monte Carlo or Grand Canonical Monte Carlo, and a deeper understanding of the atomistic origins of the mechanisms controlling clay swelling requires a more detailed examination of the electrostatics, which would be accessible using quantum mechanics on smaller systems. However, for the clay mineral/surfactant/solvent systems investigated we have identified the controlling factors leading to enhanced swelling and potentially dispersion and exfoliation in water and ethanolnamely, a moderate to high charged clay mineral (MMT/VMT) with alkylammonium surfactants containing ether side chains.

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ASSOCIATED CONTENT

* Supporting Information S

Details of the computational method (force field parameters), the system setup, automation details, the solvation energetics concept, and additional results on MMT-QA with higher ethanol loading. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jp512257z.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge use of Hartree Centre resources in this work. The Science and Technology Facilities Council (STFC) Hartree Centre is a research collaborator in association with IBM providing High Performance Computing platforms funded by the UK’s investment in e-Infrastructure. The Centre aims to develop and demonstrate next generation software, optimized to take advantage of the move towards exa-scale computing. We acknowledge funding by Sun Chemical Ltd and the Molecular Engineering Translational Research Centre (METRC) grant M1205 “Understanding Nano-Clay Filler Swelling In Solvent Systems”. H.C.G. also thanks the Royal Society for the award of an Industry Fellowship.

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CONCLUSION AND OUTLOOK The development of CPNC materials has been hampered by challenges associated with processing these materials, in particular the full dispersion of clay mineral particles within a polymer matrix. The exfoliation of the nanoparticles in a solvent is preceded by a swelling step, in which modified clay minerals are exposed to solvent to reduce interfacial tension between adjacent clay layers. Here, we study for the first time the swelling behavior and properties of a range of commercially relevant functionalized clay minerals in water and ethanol. In total we studied montmorillonite, vermiculite, and hectorite in both water and ethanol with interlayer Na+ cations and three different quaternary ammonium surfactants. By measuring the solvation energetics, basal d-spacing, and 1D atom density profiles, we can identify systems that, within the limitations of our study, appear conducive to exfoliation, namely, MMT-QA/C in ethanol and VMT-QA/C in water. By using molecular dynamics simulations we have gained insight into the swelling of clay mineral/surfactant/solvent systems and have qualitatively evaluated the energetics, which can be related to structural and compositional differences between the organoclays. Future computational work will involve examining the second stage of exfoliation, i.e., dispersion by incorporating the effects of shear in LAMMPS as demonstrated by Heinz and coworkers,73,96 with the ultimate aim of simulating exfoliation of clay minerals in different solvents. Needless to say, more experimental data will be required, as this will lead to greater synergy between the two methodologies and therefore to

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