Inelastic Neutron Scattering and Molecular Simulation of theDynamics of Interlayer Water in Smectite Clay MineralsRandall T. Cygan,*,† Luke L. Daemen,‡ Anastasia G. Ilgen,† James L. Krumhansl,† and Tina M. Nenoff†

†Sandia National Laboratories, Albuquerque, New Mexico 87185, United States‡Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

ABSTRACT: The study of mineral−water interfaces is of great importance to a variety ofapplications including oil and gas extraction, gas subsurface storage, environmental contaminanttreatment, and nuclear waste repositories. Understanding the fundamentals of that interface is keyto the success of those applications. Confinement of water in the interlayer of smectite clayminerals provides a unique environment to examine the interactions among water molecules,interlayer cations, and clay mineral surfaces. Smectite minerals are characterized by a relativelylow layer charge that allows the clay to swell with increasing water content. Montmorillonite andbeidellite varieties of smectite were investigated to compare the impact of the location of layercharge on the interlayer structure and dynamics. Inelastic neutron scattering of hydrated anddehydrated cation-exchanged smectites was used to probe the dynamics of the interlayer water(200−900 cm−1 spectral region) and identify the shift in the librational edge as a function of theinterlayer cation. Molecular dynamics simulations of equivalent phases and power spectra, derivedfrom the resulting molecular trajectories, indicate a general shift in the librational behavior withinterlayer cation that is generally consistent with the neutron scattering results for the monolayerhydrates. Both neutron scattering and power spectra exhibit librational structures affected by the location of layer charge and bythe charge of the interlayer cation. Divalent cations (Ba2+ and Mg2+) characterized by large hydration enthalpies typically exhibitmultiple broad librational peaks compared to monovalent cations (Cs+ and Na+), which have relatively small hydrationenthalpies.

■ INTRODUCTIONThe structure and dynamics of water and aqueous species in clayminerals and related nanoporous materials are typicallycontrolled by the mineral structure, composition, chargedistribution, protonation state, and related properties of themineral−water interfaces. Surface chemistry and the reactivity ofclay minerals can influence mineral−water interfaces and controladsorption, dissolution, precipitation, solute and water diffusion,nucleation, and growth phenomena in the environment.1−5

Mineral nucleation and growth processes are of particularinterest because of their impact on mesoscopic and macroscopicrock properties, such as permeability, porosity, wettability, andmechanical strength. Clay minerals and their interfacialchemistry are critical in the extraction of gas and oil fromunconventional geological reservoirs such as shale rocks,6−8 inunderstanding the geochemistry of CO2 storage and sequestra-tion in the subsurface,9−12 and in the performance of geologicnuclear waste repositories.13−16 Moreover, the clay interlayerpresents a unique structural environment for examining the roleof confinement on water structure, water−cation dynamics andtransport, and how interlayer cations can further impact thatbehavior.Molecular details of geochemical processes involving clay

minerals are typically beyond the sensitivity of most experimentaland analytical techniques, especially for evaluating waterstructure and behavior. Neutron scattering methods, however,with high sensitivity to hydrogen-containing materials, provide a

unique probe of smectite minerals and their hydrated interlayerenvironment. Inelastic neutron scattering (INS), in particular,has been used to investigate interlayer water molecules, hydroxylgroups, and hydrogen bonding in a variety of phyllosilicates andclay minerals, including kaolinite,17,18 muscovite,19 vermicu-lite,19,20 palygorskite,21 and sepiolite.21 Neutron scatteringmethods have also been used to derive diffusion rates ofinterlayer water in smectite minerals.22−24 Several of the previousstudies have focused on the vibrational spectroscopy associatedwith the neutron scattering experiments, evaluating the mid-infrared region where the librational, or rotational, responseassociated with water dynamics occurs.Vibrational spectroscopy, in general, is an analytical technique

that provides information about molecular structure, chemicalbonding, and intermolecular interactions of materials. Infraredabsorption and Raman spectroscopies are well-known examplesof this widely used form of spectroscopy.25,26 With thedevelopment of advanced instrumental analytical facilities,spectroscopies that use neutrons rather than photons as aprobe of molecular vibrations are more widely available. Thisapproach has several advantages over optical spectroscopy,including high sensitivity to hydrogen, absence of selection rules,ease of computation of the vibrational spectrum, isotopic

Received: September 10, 2015Revised: November 13, 2015Published: November 16, 2015

Article

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sensitivity, no energy deposition in sample, and high neutronpenetrability through bulky sample environment.27

Computational chemistry methods, including moleculardynamics simulation, provide a unique complementary probeof the clay mineral−water interface to support neutron scatteringanalysis. Molecular simulation offers details of the structure,energetics, and dynamics of the interface, including the mineralsubstrate and adsorbates, solutes, and water molecules of theaqueous phase.28−36 Molecular dynamics simulations provideanalysis of time-dependent processes ranging from femto- topico- to nanoseconds and for spatial scales from Ångstroms totens of nanometers. In some instances, the molecular-scaleresults can influence the interpretation of geological eventsassociated with meters to kilometers and millions of years.However, multiscale modeling and upscaling methods remain achallenge for many geological applications.37−40

Two clay minerals of interest are montmorillonite andbeidellite, model smectite minerals which enable the study ofconfined cations and water molecules in their respectiveinterlayers. Montmorillonite has an idealized formula ofNa0.75(Al3.25,Mg0.75)Si8O20(OH)2·nH2O with layer charge lo-cated in the octahedral sheet created by the heterovalentsubstitution of Mg2+ for Al3+. Beidellite, where the charge isproduced by the substitution of Al3+ for Si4+ in the tetrahedralsheet, has a composition of Na0.75Al4(Si7.25Al0.25)O20(OH)4·nH2O. Both clay minerals possess a layer charge of approximately−0.75 per O20(OH)4, a relatively low layer charge representativeof a smectite clay that can swell with increasing relative humidity.In the present study, we apply inelastic neutron scattering and

vibrational spectroscopy to examine ion-exchanged smectiteminerals and compare different hydration environments of theclay interlayer. Montmorillonite and beidellite clay minerals areinvestigated to provide a comparison of the effect of chargelocation in the clay layer and its impact on water dynamics.Molecular dynamics simulations and power spectra derived fromatomic trajectories are used to interpret the experimentalvibrational data, specifically to better understand the waterdynamics and the influence of the interlayer cation on thelibrational region of the spectra.

■ METHODSClayMaterials.Claymineral samples were obtained from the

Source Clays Repository operated by the Clay MineralsSociety.41 Texas montmorillonite (STx-1) and Idaho beidellite(SBId-1) were selected as representative dioctahedral smectiteshaving similar layer charge but with charge localized to either theoctahedral sheet (STx-1) or the tetrahedral sheet (SBId-1).Approximate chemical formula are M+

0.66(Al3.12,Fe3+

0.12,Mg0.71)-S i 8O 2 0 (OH) 4 · nH2O f o r mo n tmo r i l l o n i t e a n dM+

0.67(Al3.77,Fe3+

0.11,Mg0.21) (Si7.27Al0.73)O20(OH)4·nH2O forbeidellite, representing stoichiometries derived from multiplecharacterization studies,41−44 and where M+ represents theinterlayer cations which for these samples is primarily Ca2+ withsome Na+, K+, and Mg2+. The variable interlayer water content isrepresented in the formula by nH2O and is dependent on therelative humidity. Except for the location of the layer charge,these samples have similar charge characteristics that ultimatelyimpact their respective interlayer environments (Figure 1). Forexample, Idaho beidellite, having layer charge localized in thetetrahedral sheet where it would be closer to the exchangeableinterlayer cations, is expected to have a significant influence onthe cations and the interlayer water structure.

Cation-Exchange Experiments. Cation exchange wasachieved by equilibrating clay mineral samples in 0.5 M saltsolutions of selected cations followed by multiple rinses withdeionized water to remove unwanted cations (primarily Ca2+)from the samples.45 Specifically, 10 g of each clay mineral wasplaced in a 125 mL bottle with 50 mL of water, after whichenough sodium, magnesium, cesium, or barium nitrate wasadded to bring the metal ion concentrations to 0.5 M. Theselection of cations represents a range of hydration enthalpies,related to cation size and charge, which ultimately impacts theinteraction of the cation with interlayer water molecules and claysurfaces.The slurry was allowed to equilibrate at room temperature for

approximately 12 h, after which an additional 50 mL of deionizedwater was added to each bottle to begin rinsing the clay. The clayminerals were allowed to settle (occupying 20−40 mL of space),and then supernatant salt solution was decanted off the top(approximately 70 mL of clay-free water). Centrifuging ratherthan gravity settling was used for processing the Na-exchangedclay samples. Additional deionized water rinse cycles were carriedout about 3−4 times until the clay sample started to peptize,preventing settling and making further rinsing unproductive.Lastly, the final volume of the slurry was adjusted back to 50 mL,and sufficient salt was added to bring the metal cationconcentration back up to 0.5 M. The samples equilibrated forapproximately 12 h again, and the rinsing process was resumed.In all, seven exchange cycles were carried out.Once the exchange cycles were completed, the clays were

subjected to six wash cycles by repeatedly filling the bottles withdeionized water and then decanting off the clear fluid after theclay settled. Finally, the essentially salt-free clay slurries werepoured into shallow plastic pans and allowed to dry. At the

Figure 1. Schematic and molecular models showing the differencesbetween montmorillonite and beidellite for charge substitutionlocations in the clay layer. Molecular models include a single interlayerNa+ (green) that compensates for the negative charge on the clay layercreated by the substitution of a lower charged cation, Mg2+ in theoctahedral (O) sheet or Al3+ in the tetrahedral (T) sheet. Projected viewfor the molecular models is along the a axis. Interlayer water is notshown in the models. Mg is blue, Al is magenta, Si is yellow, O is red, andH is white.

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conclusion of the drying process none of the pans showed visiblesalt crusts, indicating that the salts had successfully been washedout of the slurry. X-ray fluorescence analysis comparing pre- andpostexchange samples confirmed that the process had success-fully replaced the original exchangeable cations with the targetcations.X-ray Diffraction. Oriented samples for X-ray diffraction

(XRD) analysis were prepared for each of the four cation-exchanged montmorillonite and beidellite minerals. Wet claymineral slurries were deposited on XRD sample holders andallowed to dry overnight. Samples were then examined by X-raydiffraction using a Bruker D8 Advance system to confirm purityand basal (001) d spacings of the clay minerals. Diffraction scansfrom 2° to 82° 2θ were obtained using Cu Kα X-rays (λ = 1.5406Å) at a scan rate of 0.02° 2θ/s. A diffraction scan with a quartzstandard was initially used to calibrate and confirm the 2θ valuesand corresponding d spacings. Instrumental error for XRDmeasurements in the 2−10° 2θ region, based on 10 repetitivescans of a smectite mineral, is 0.04° 2θ.Inelastic Neutron Scattering. The vibrational response of

the cation-exchanged montmorillonite and beidellite sampleswas investigated by inelastic neutron scattering methods at oneof the following neutron scattering facilities. Samples involvingCs-, Mg-, and Ba-exchanged smectites were analyzed at the LosAlamos Neutron Science Center (LANSCE), while samples ofNa-exchanged smectites were examined at the SpallationNeutron Source (SNS) located at Oak Ridge NationalLaboratory.The filter difference spectrometer (FDS) at LANSCE was

used first for vibrational spectroscopy by incoherent inelasticneutron scattering for analysis of the clay samples. FDS isdesigned for high count rates using a large solid-angle detectorsubtending 3 steradians.46 Approximately 1 g of each sample wasloaded in cylindrical aluminum cans (20 mm diameter and 100mm height) while in a glovebox with a helium atmosphere.Samples were cooled to 10 K in a top-loading closed-cyclerefrigerator prior to the collection of scattering counts. Spectrawere detected by neutron energy loss using beryllium filters andtime-of-flight analysis for up to energy transfer values up to 500meV, corresponding to about 4000 cm−1. Data collectionoccurred over a 1 h time period, and the spectrum for theempty sample holder was subtracted from the data collected forthe clay samples. Additional INS spectra were collected fordehydrated samples for each of the clay samples by approximate12 h treatment at 115 °C and under vacuum. Removal ofinterlayer water from the samples was confirmed by mass andthermal gravimetric analyses.41,47 These spectra were used insubsequent data analysis to remove contributing signal from theclay layers and obtain an INS spectra representing solely thewater dynamics associated with the clay interlayer. All INSspectra were collected for samples equilibrated at 10 K.The Na-exchanged clay samples examined at SNS were

processed similarly for neutron data collection as was done forthe Cs-, Mg-, and Ba-exchanged samples analyzed at LANSCEusing the FDS instrument. INS spectra were collected up toenergy transfer values of 1000meV, corresponding to about 8000cm−1. Vibrational spectroscopy with the VISION instrument atSNS is optimized to characterize molecular vibrations in a widerange of crystalline and disordered materials over a broad energyrange (greater than 5 meV to less than 600 meV). An invertedgeometry for VISION offers enhanced performance by couplinga white beam of incident neutrons with two banks of sevenanalyzer modules, equipped with curved pyrolytic graphite

crystal analyzer arrays that focus neutrons on a series of smalldetectors and offering improved signal-to-noise ratio.

Molecular Simulation.Montmorillonite and beidellite wereinvestigated as model smectite minerals to examine theinteractions of confined cations and water molecules in theirrespective interlayers. The initial dry montmorillonite model wasbuilt from an orthogonalized (originally monoclinic) unit cellwith an anhydrous stoichiometry of Na0.75(Al3.25,Mg0.75)-Si8O20(OH)2 with layer charge located in the octahedral sheetcreated by the heterovalent substitution of Mg2+ for Al3+. Theinitial dry beidellite model was derived from a unit cellcomposition of Na0.75Al4(Si7.25Al0.25)O20(OH)4 where the chargeis produced by the substitution of Al3+ for Si4+ in the tetrahedralsheet. Both molecular structures possess a layer charge of −0.75per O20(OH)4, similar to the natural clay materials used in theINS investigation.Each claymodel, having P1 symmetry, was converted to a 2× 2

× 1 cell with an expanded interlayer for the introduction of watermolecules and to manually reduce the ordering associated withthe substitution sites. The molecular models were then furtherexpanded to 8 × 6 × 4 supercells comprised of approximately10 000−12 000 atoms and having full periodic boundaries andfour hydrated interlayer regions. Supercell dimensions areapproximately 42 Å × 54 Å × 50 Å for the monolayer hydratemodels. Two models for each smectite phase were prepared torepresent a monolayer hydrate of 3.75 H2O per O20(OH)4 and abilayer hydrate of 6.75 H2O per O20(OH)4. Equivalentmontmorillonite and beidellite models for the other endmemberinterlayer compositions were prepared by replacing the Na-basedmodels with Cs+ or with Mg2+ or Ba2+ having one-half thenumber of cations to preserve charge neutrality. Figure 2provides the fully developed supercell used in the molecularsimulations for the monolayer hydrate of Na-montmorillonite. Inaddition to the smectite clay models described earlier, a hydratedpyrophyllite modelwith no layer substations and no layerchargewas developed. Pyrophyllite48 (AlSi2O5(OH)) is not aswelling clay, and therefore, this model is essentially a virtual

Figure 2. Molecular model of a monolayer hydrate of Na−montmorillonite showing a periodic simulation cell comprised of fourclay layers and four interlayers. Equilibrated structure was obtained after1.5 ns of molecular dynamics simulation using an NPT ensemble whichallows for variation in cell angles and lengths. Same color scheme as inFigure 1.

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phase for comparing its water behavior with that of the variousmontmorillonite and beidellite models.Interatomic potentials associated with the Clayff force field49

were assigned to every atom for each of the montmorillonite andbeidellite models. Clayff is a fully flexible force field that has beensuccessfully applied to many oxide, hydroxide, and clay mineralsystems of environmental interest.28,35,50−71 Clayff is primarily anonbonded energy force field, with harmonic bond terms usedonly for hydroxyl and water molecule bonds. All other atoms arestructurally represented by nonbonded interactions involvingvan der Waals and electrostatic terms. Force field parameters forthe aqueous cations are taken from Clayff which includes valuesfor Na+,72 Cs+,73 Mg2+,74 and Ba2+.74 and collectively tabulated inOckwig et al.75

Molecular dynamics (MD) simulations were performed usingthe Forcite classical molecular mechanics program as imple-mented in the Materials Studio suite of molecular modelingsoftware.76 A real-space cutoff of 10.0 Å was used for short-rangeinteractions, and long-range electrostatics were calculated withan Ewald summation algorithm at a precision of 1.0 × 10−4. Atime step of 1 fs was used for both long-range and short-rangeinteractions. All atoms were allowed to freely translate and cellparameters vary during the course of the MD simulation.Although simulation supercells were initially constructed to beorthogonal; during long MD simulation times the hydrated layerstructure is prone to shearing along the clay interlayers resultingin a nonorthogonal cell (cf. Figure 2). Without constrainingselected atoms during the simulation which would prevent suchdeformation, the equilibrated simulation cells still effectivelyrepresent the bulk structure and properties of the clay minerals.Thermodynamic ensembles were created in a process similar

to that used in recent MD simulation studies of bulk clayminerals.21,33,70,77 Molecular dynamics simulations were per-formed using the Nose−Hoover thermostat implemented in theForcite code. The thermostat relaxation time was 0.1 ps. Theinitial configuration was equilibrated with a 50 ps NVE(microconical ensemble where N is the number of atoms, V isconstant volume, and E is constant energy) simulation at atemperature of 300 K to allow atoms to relax from the initiallattice configuration and then further equilibrated with an NVT(canonical) simulation for an additional 50 ps where T is thetemperature. The supercell was then fully equilibrated for 1.5 ns(106 time steps) using an NPT ensemble (isothermal−isobaricensemble) where P is the pressure of 0.1 MPa to representsurface environmental conditions. For the NPT simulations, aBerendsen barostat78 was used with a half-life decay time of 0.1ps. A restart trajectory file was saved every 50 ps of the simulationallowing for monitoring potential and kinetic energies, temper-ature, pressure, atomic velocities, and structural properties of theclay model. The final equilibrated structure and associatedatomic velocities were subsequently used for the start of a final 40ps NPT simulation in which the trajectory file included atomiccoordinates and velocities collected every 4 fs. This frequentcollection ensures the capture of the relatively fast motions ofhydrogen associated with water molecules and hydroxyl groups.It provides 10 000 atomic structures and corresponding atomicvelocities for the power spectrum calculation.The trajectories derived from the final NPT simulations were

used to calculate power spectra for each of the cation endmemberclay models. Such theoretical vibrational spectra are important ininterpreting experimental infrared and Raman spectra andhelping to understand the structure and dynamics of clayminerals and other environmental phases.79 The power spectra

were calculated as Fourier transformations of the velocityautocorrelation functions (VACF) using the trajectoriesobtained from the models.80 The VACF for each atom type isgiven by

∑ ν ν= ·=

C tN

t( ) 1 [ (0) ( )]j

N

j j1 (1)

where C(t) is the normalized summation of the dot products ofatomic velocity vj(t) relative to an initial velocity vj(0). Thesummation is over the number of atomsN of a given atom type orfor an entire set of atoms (total VACF) and is performed using acenter-of-mass frame. The frequency-based power spectrum canbe derived by squaring the Fourier-transformed VACF. Atrajectory sampling period of 4 fs ensures that vibrationalfrequencies up to 4166 cm−1 will be surveyed, and a correlationwindow of 6 ps provides a frequency resolution of approximately2.8 cm−1. Librational motions, bending modes, and stretchmodes of water and hydroxyl groups can be calculated underthese settings. All power spectra are based onMD trajectories forthe atomic velocities of the hydrogen atoms of the watermolecules collected for simulations at 300 K. This approachallows for the direct filtering of atomic contributions from theclay layers and only maintaining contributions from interlayerwater. A similar computational procedure was used by Praprotnikand co-workers81 to calculate the temperature dependence ofpower spectra for bulk water. Similarly, Deshmukh and co-workers82 simulated variable water content between sheets ofgraphene and calculated power spectra to compare differences inlibrational behavior for several expanded interlayers.

■ RESULTS AND DISCUSSIONClay Mineral Structure. As low-charged layered phyllosili-

cates, the structures of the cation-exchanged claymineral samplesare most effectively described by their basal (001) d spacingswhich are descriptive of the expanded dimensions of theinterlayer region. Figure 3 provides the low-angle (4−10° 2θ)

Figure 3. X-ray diffraction patterns and d spacings for the basal (001)peak of the cation-exchanged montmorillonite samples.

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X-ray diffraction patterns for each of the cation-exchangedmontmorillonite samples. The mean d spacings vary from 11.9to13.7 Å and indicate the occurrence of primarily monolayerhydrates for these samples under the relative humidity(approximately 25% RH) at the time of analysis. In general,depending upon interlayer cation and layer charge, anhydroussmectites exhibit basal d spacings of about 9.5−10 and 11.5−13.5Å for monolayer hydrates at such low relative humidities.83 Atmoderate to high humidities (or partial water pressures)smectites expand to bilayer hydrates characterized by d spacingsof about 14−16 Å for most common interlayer cations.The broad and asymmetric basal diffraction peaks in Figure 3

suggest the occurrence of some degree of mixed layer hydratestates in the cation-exchanged samples,84 although thedistribution is clearly dominated by basal d spacingsrepresentative of the monolayer hydrate. Similar mean basal dspacings are observed for the cation-exchanged beidellitesamples. These results are summarized for untreated andcation-exchanged clay samples in Table 1. The larger d spacings

(13.6 and 13.7 Å) observed for the Mg-exchanged smectitescompared to the other compositions is indicative of the fullhydration spheres of Mg2+ that predominate in the clay interlayerrelative to the other cations that typically shed water molecules toadsorb directly to the clay surface.33

For comparison to the XRD results, basal d spacings werederived from the NPT-equilibrated MD trajectories (40 pssimulations) obtained for monolayer and bilayer models for eachof the smectite molecular models. Table 2 presents a summary ofthe values for montmorillonite, beidellite, and a virtualpyrophyllite model to represent an uncharged layer structure.Results for two dry clay models (pyrophyllite and Ba-exchangedmontmorillonite) are included for comparison. The basal dspacings vary from 9.24 to10.14 Å for the dry state, about 12.25to 12.51 Å for the monolayer, and 14.23 to 14.89 Å for the bilayerdepending on the interlayer cation and the layer charge location.The expansion of approximately 2.9 Å for each successive waterlayer in pyrophyllite models is consistent with the size of a watermolecule. The basal spacing for the dry pyrophyllite model is inagreement with the experimental refinement for single-rystal

pyrophyllite.48 There is some variation in the layer expansionbetween montmorillonite and beidellite depending upon thetype of interlayer cation and how it resides in the interlayer: fullyhydrated, adsorbed to clay surface, or some combination.Montmorillonite models exhibit smaller basal spacings for the

monolayer compared to beidellite but have larger basal spacingsthan beidellite for the bilayer models. The calculated basal dspacings for the clay models are consistent with the XRD valuesfor the natural clay phases and confirm the dominance of themonolayer hydrate in the real samples. However, the occurrenceof mixed layers of water in natural samples and the largeuncertainties in analyzing the distribution of basal d spacings forbroad diffraction peaks85,86 make it difficult for a direct anddetailed comparison of XRD basal spacings with the MD results.

Inelastic Neutron Scattering. Inelastic neutron scatteringis a highly specialized instrumental method that is especiallysuited for the analysis of hydrogen-containing components incomplex materials. It is particularly sensitive to the intra-molecular motions of libration (rotation) for water, hydroxyl,and related species that are observed at low frequencies of 300−1100 cm−1 in the INS spectrum, while contributions from non-hydrogen species are minimal. Librational modes result frommolecular motions with nonzero angular moments, whichproduce three distinct modes for molecular water: rocking,wagging, and twisting, which occur in the range of 500−1000cm−1. Librational modes for water are typically broad and oftencombine into a single massive spectral feature, rendering thediscrete rotational modes indistinguishable from each other.87,88

Librational modes are highly dependent on the local environ-ment, position, and ordering of water.89−91 Sharp librationalfeatures in a spectrum can occur for a hydrogen-containingcomponent that is very highly ordered.75,92 Because mostlibrational spectra exhibit librational modes that are extremelybroad, occurring over 500 cm−1 in width, they are difficult tointerpret. The absolute assignment of discrete rocking, wagging,and twisting modes is a difficult if not an impossible task.Nonetheless, it is well documented that librational frequenciesincrease and peak widths decrease as hydrogen bonding,electrostatics, and steric restrictions to the rotation areincreased.75,87,93 The onset of librational features, commonlyreferred to as the librational edge, is the only definitivespectroscopic assignment reported in INS experiments. Addi-tionally, librational dynamics are amenable to quantitativeprediction due the relative simplicity of neutron−nucleusinteractions.94−96 A powerful analytical combination is the useof molecular simulation studies to calculate power spectra fromthe analysis of atomic trajectories using molecular dynamicssimulations to better understand librational motions of water inconfinement.21,75,82,93,97

Table 1. Basal (001) d Spacings for Untreated and Cation-Exchanged Monohydrate Smectite Samples

montmorillonite (STx-1) (Å) beidellite (BId-1) (Å)

untreated 12.6 12.8Na exchanged 12.0 12.2Mg exchanged 13.7 13.6Cs exchanged 12.3 12.2Ba exchanged 12.0 11.9

Table 2. Basal (001) d Spacings Derived from Equilibrated MD Simulations

montmorillonite (Å) beidellite (Å) pyrophyllitea (Å)

dry monolayer hydrate bilayer hydrate dry monolayer hydrate bilayer hydrate dry monolayer hydrate bilayer hydrate

Na 12.250(2) 14.887(2) 12.509(2) 14.224(2)Cs 12.253(2) 14.892(2) 12.509(2) 14.233(2)Mg 12.265(2) 14.886(2) 12.514(2) 14.224(2)Ba 10.135(2) 12.290(3) 14.889(2) 12.504(2) 14.217(2)

9.238(3)b 12.220(2) 15.104(2)

aMonolayer and bilayer hydrate models of pyrophyllite are virtual phases and do not occur in nature. bExperimental value of 9.144 Å for single-crystal X-ray structure48

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Librational spectra collected at 10 K for cation-exchangedmontmorillonite and beidellite obtained from inelastic neutronscattering experiments are presented, respectively, in Figures 4and 5. Spectra for the equivalent dehydrated samples and theresulting difference spectra are also presented for each cation-exchanged clay sample. INS data are presented only for the 200−1600 cm−1 frequency range to better visualize the librationalmotions for the hydrogen components of the clay and interlayer.The librational edge, which encompasses the rock, wag, and twistmotions of water, ranges from about 200 to 900 cm−1 for thesesamples. The strong peaks at 800−900 cm−1 found in bothhydrated and dehydrated spectra for all samples are associatedwith the deformation motions of the hydroxyl groups of thesmectite octahedral sheet: Al−OH (900 cm−1) and Mg−OH(850 cm−1) in montmorillonite and only Al−OH (900 cm−1) inbeidellite.98,99 The INS difference spectrum (Figure 6) effectivelyremoves these clay-based librations and most of the backgroundsignal leaving only the librational contributions from theinterlayer water.In general, the librational edge of the smectite samples is

represented by the INS difference spectra. It is significantlybroad, to some extent noisy, and consequently difficult to resolveinto component librational motions. However, there is a shift ofthe low-energy (i.e., low-frequency) edge of the librations that isdependent on the interlayer cation. Cesium- and Ba-exchangedsamples of both montmorillonite and beidellite exhibit librationsat energies as low as 250 cm−1, whereas Mg-exchanged samplesfor both phases show a shift of the low-energy edge to higher

frequencies (325−350 cm−1), indicating a more constrainedenvironment for the water associated with Mg2+ in the interlayer.Sodium-exchanged samples exhibit a less-developed low-energyedge from 275 to 350 cm−1, suggesting a different distribution oflibrational motions or a modified disorder state with multipleenvironments for the interlayer waters.The variation in the librational edge structure is highlighted in

the direct comparison of INS difference spectra presented inFigure 6 for montmorillonite and beidellite samples. Thelibrational motions of water molecules in the interlayer areimpacted by the interlayer cation; that cation can be eithersolvated by the interlayer water or adsorbed directly to the claysurface (ultimately with less impact on the water dynamics). Thelow-energy INS edge for Na- and Mg-exchanged samples ofmontmorillonite are shifted to higher frequencies by about 50−75 cm−1 relative to the Cs-exchanged sample. Relative shifts inthe INS edge structure for the beidellite samples are similar butvary by about 75−100 cm−1 relative to the Cs-exchanged sample.Cesium- and Ba-beidellite samples exhibit a librational edgesignificantly lower in energy than the other cation-exchangedsamples.These INS results indicate that both confinement and cation

type modify the librational behavior of water. Infrared measure-ments of bulk aqueous solutions containing monovalent cationsindicate that larger cations have greater impact on the librationalspectra of water.100 Alternatively, this can be interpreted as howstrongly the water molecules are bound to the cationhydrationenthalpywhich is related to cation charge and size. Hydration

Figure 4. Inelastic neutron scattering vibrational spectra collected at 10 K for Na-, Cs-, Mg-, and Ba-montmorillonite samples for hydrated (red) anddehydrated (green) states and the resulting difference (blue) spectrum.

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enthalpies and ionic radii for each of the interlayer cations areprovided in Table 3.101 The increasing trend of hydration

enthalpy for the cations follows the trend of decreasing cationsize and increasing charge, although secondary effects of

Figure 5. Inelastic neutron scattering vibrational spectra collected at 10 K for Na-, Cs-, Mg-, and Ba-beidellite samples for hydrated (red) and dehydrated(green) states and the resulting difference (blue) spectrum.

Figure 6. Librational region of INS difference spectra (between hydrated and dehydrated samples) collected at 10 K for Na-, Cs-, Mg-, and Ba-montmorillonite (left) and Na-, Cs-, Mg-, and Ba-beidellite (right). Color arrows indicate approximate location of librational edge.

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electronic polarization and other mechanisms will impact thesevalues. To a first order, the cation hydration energy will controlwhether the interlayer cation prefers being solvated by water orbeing adsorbed onto the clay surface. The binding energy of thecation to the surface will be primarily electrostatically controlled.It is therefore not surprising that Cs+, having the lowest hydrationenthalpy, is expected to preferentially adsorb to the clay surfacerather than remain solvated by water molecules in the interlayer.In contrast, Mg2+ with the highest hydration enthalpy remainssolvated by water in the interlayer and does not adsorb to thesurface. In general, Na+ and Ba2+ are expected to exhibitintermediate behavior with possibly both mechanisms. None-theless, the significance of electrostatic interactions of cationswith the clay surface, especially for the near-surface charged sitesin beidellite, can have a dramatic impact on the interlayerdynamics.The contrast in the INS difference spectra for montmorillonite

and beidellite samples involving the same cation, as observed forthe Na- and Ba-exchanged samples, is striking, most likely due tothe difference in charge location for each smectite (octahedralversus tetrahedral charge site; see Figure 1). The INS differencespectra of Figure 6 probably represent the predominance ofmonolayer hydrates for each smectite phase based on theambient humidity associated with the INS experiments, and thebasal d spacing results from XRD analysis. However, bilayerhydrates of these phases are expected to impact the librationaledge in the INS spectra due to the potential for enhanced cationsolvation with the increased interlayer water content. Lastly,there is the possibility that valence for the interlayer cations andthe corresponding difference in cation concentration (i.e.,concentration of Cs+ and Na+ in the interlayer is twice theconcentration of Mg2+ and Ba2+) will lead to multipleenvironments for water in the interlayer.The INS difference spectrum for the Ba-montmorillonite

provides the best example among the clay spectra for a case ofordering in the water motions (Figure 7). Three Gaussian peakswere fit to the spectrum to determine the relative contributionsof rock, wag, and twist modes for water. No attempt was made toassign exact libration motion to each of the peaks due to thedifficulties in deconvolution of an extremely broad librationalpeak, a characteristic of water in clay interlayers. In rare cases forINS spectra of single-crystal hydrate samples it is possible toidentify and resolve the rocking mode of water, which occurs atthe lowest libration frequencies (approximately 640 cm−1)from an unresolved combination of wag and twist modes.104

Power Spectra. Power spectra derived from the moleculardynamics trajectories for the smectite models cover thefrequency range from 0 to about 4200 cm−1. An example of apower spectrum based on hydrogen velocities of interlayer wateris presented in Figure 8 for the monolayer hydrate of Ba-montmorillonite. The spectrum includes the far- and mid-infrared regions and low-frequency end of the near-infraredregion. The dominant absorption peak, which occurs at 3830

cm−1, represents the asymmetric stretch of the O−H bond, whilethe symmetric stretch for the same bond is found at 3740 cm−1.The confined nature of water in the clay interlayer resolves thesetwo peaks into a collectively formed broad peak for liquid waterand ice phases. Additionally, the difference in frequencies forthese modes compared to that for liquid water, eitherexperimental or simulated values, is affected by the localenvironment of the clay interlayer and the accuracy of theforce field parameters. The bendingmode for the interlayer wateroccurs at 1600 cm−1. Librational modes of rock, wag, and twistmotions for water occur from approximately 250−1000 cm−‑1 inthe near- and mid-infrared regions. The disorder state of water,especially in the clay interlayer, limits any useful discriminationamong the three libration modes. The small low-frequency (90cm−1) peak associated with the broad libration bandmost likely isassociated with bending and stretching motions of hydrogenbonds among water molecules.Figures 9 and 10 present the librational region (200−1000

cm−1) of the power spectra derived from the equilibrated MDsimulations for each of the interlayer cation smectite models forwater in both monolayer hydrate and bilayer hydrate systems,respectively. Power spectra were also derived from the atomictrajectories for the hydrated pyrophyllite phase to provide a

Table 3. Ionic Radii and Hydration Enthalpies Associatedwith Selected Alkali and Alkaline Earth Metal Cations

ionic radiusa (Å) −ΔHhydb (kJ/mol)

Na+ 1.16 418Cs+ 1.81 285Mg2+ 0.86 1941Ba2+ 1.49 1322

aOctahedral coordination.102,103 bReference 101.

Figure 7. Librational region of INS difference spectrum collected at 10 Kfor Ba-montmorillonite (blue) and corresponding deconvolution oflibrational modes (dashed).

Figure 8. Complete power spectrum for interlayer water of themonolayer hydrate of Ba-montmorillonite calculated at 300 K indicatingthe primary region for librational motion and the bend and stretchmodes of water.

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baseline for comparing water dynamics when layer charge isabsent in the clay mineral. In contrast to the experimental INSdifference spectra with librational edges and often a complexspectral structure, the power spectra for the interlayer waterexhibit a broad relatively disordered librational peak with noclearly defined low-energy edge. The rather small shoulder peaksat approximately 300 and 850 cm−1, most obvious for the Ba andMg spectra, suggest secondary contributions from two of therock−wag−twist librational motions and are also consistent withthe librational peaks exhibited by the INS spectra (see Figure 7).

The power spectra derived for the monolayer hydrate models(Figure 9) show a trend to higher frequency peaks as theenthalpies of hydration decrease for the smectite systems (i.e.,Mg2+ > Ba2+ > Na+ > Cs+). However, there is a marked variationwith interlayer cation and how the cation impacts the waterdynamics. Except for the Ba models, there is a significantdifference in the librational maxima between montmorilloniteand beidellite models, with the largest difference associated withthe Cs models; maxima occur at 375 and 525 cm−1, respectively.This 150 cm−1 difference is related to the stronger local binding

Figure 9. Librational region of power spectra at 300 K formonolayer hydrates of Cs-, Na-, Ba-, andMg-montmorillonite (left) andCs-, Na-, Ba-, andMg-beidellite (right). Corresponding power spectrum for a monolayer hydrated pyrophyllite model is included for each graph. Dashed trendlines suggest ageneral shift of peak maximum with interlayer cations ranked relative to their hydration enthalpy.

Figure 10. Librational region of power spectra at 300 K for bilayer hydrates of Cs-, Na-, Ba-, and Mg-montmorillonite (left) and Cs-, Na-, Ba-, and Mg-beidellite (right). Corresponding power spectrum for a bilayer hydrated pyrophyllite model is included for each graph. Dashed trendlines suggest ageneral shift of peak maximum with cations ranked relative to their hydration enthalpy.

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of Cs+ to the tetrahedral charge sites at the beidellite surface,allowing the interlayer waters to behave as those observed in thebaseline pyrophyllite model (both librational maxima at 525cm−1). In montmorillonite, Cs+ is adsorbed at the clay surface(due to the very lowCs+ hydration enthalpy) but the electrostaticbinding of Cs+ is weaker (relative to beidellite) due to theincreased distance to the charged octahedral sites of the clay. Thisallows the interlayer waters to have increased association with theinterlayer Cs+. Sodium and Ba models exhibit smaller differencesof about 25 cm−1 in their maxima, while the spectra for the Mgmodels suggest a broad bimodal distribution between lessrestricted water dynamics at 350 cm−1 and more restricted waterat 575 cm−1.Figure 9 also includes trendlines that suggest a shift of the peak

maxima with spectra for the various interlayer cation structuresordered relative to their hydration enthalpy: Cs+ with the lowestvalue (−285 kJ/mol) to Mg2+ with the highest (−1941 kJ/mol).101 The trendline for the montmorillonite models indicatesa significant increase in frequency (blue shift) for librationalpeaks with increasing cation hydration enthalpy, although thisresult is somewhat complicated by the Mg power spectrum (seebelow). In contrast, the trend for the beidellite models suggests aslight decrease in frequency (red shift) of the peak maxima withincreasing cation hydration enthalpy. This difference in libra-tional peak trends is linked to how water dynamics is impacted bycation−water interactions and whether the cations are adsorbeddirectly to the clay surface. X-ray diffraction analysis and recentMD simulations suggest an increase in cation adsorption

(especially for Cs+ and Na+) at the charged tetrahedral sites ofbeidellite compared to montmorillonite where cations occurmostly in the midplane of the interlayer.33,71,105

The divalent cations, especially Mg2+, exhibit power spectrawith possibly two primary librational contributions to the broadpeak, positioned at about 350 and 450 cm−1 for Mg-montmorillonite and 300 and 600 cm−1 for Mg-beidellite.Compared to the monovalent cation cases, these data suggesttwo primary environments for interlayer water motions resultingfrom the higher charge and lower concentration of the divalentcations in the interlayer: water associated more strongly with thecation and the comparatively free water in the interlayer,respectively. The third and smallest libration peak is observed athigher frequencies (830−875 cm−1) for most of the powerspectra. No effort has been made to assign specific rock−wag−twist modes to the calculated peaks (cf. Figure 7).Similar interpretations of water dynamics apply to the power

spectra obtained for the bilayer hydrate models (Figure 10),although the contrasts between montmorillonite and beidellitemodels are less pronounced. The baseline pyrophyllite modelwith two layers of interlayer water exhibits a maximum at 500cm−1 that indicates a less restricted water environment thanobserved for the monolayer hydrate. Rather than a mostly planararrangement of water molecules in the pyrophyllite interlayer,the bilayer hydrate exhibits a water−water interface in theinterlayer that has significant hydrogen bond development.Relative to the power spectra for the monolayer hydrates, thebilayer hydrate spectra for Na- and Cs-beidellite exhibit a

Figure 11. Librational region of power spectra (solid lines) at 300 K for monolayer hydrates of Na-, Cs-, Ba-, and Mg-montmorillonite (left) and Na-,Cs-, Mg-, and Ba-beidellite (right). Corresponding INS difference spectra are included (dotted lines). Dashed trendlines suggest a general shift of peakmaximum for the power spectra with cations ranked relative to their hydration enthalpy.

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librational maximum at higher energies compared to montmor-illonite, typically shifted by roughly 25 cm−1. Barium models areall quite similar no matter the water content or type of smectite.Librational maxima are typically within 25 cm−1 of each other butsuggest a somewhat less restricted environment for interlayerwater in the Ba-beidellite compared to Ba-montmorillonite.Power spectra for the Mg-based bilayer hydrates exhibit a weaklystructured librational signature differing from the broad diffusepeaks observed for the monolayer hydrate models. The broad-topped peak observed for both bilayer hydrates of Mg-montmorillonite and Mg-beidellite exhibit librational contribu-tions at about 350 and 550 cm−1 and a small shoulder peak at 900cm−1. This third librational peak is consistently blue shiftedrelative to the other cations for all Mg-smectite models no matterthe water content. In contrast to the monolayer hydrates, thetrendlines for the bilayer hydrate power spectra in Figure 10suggest only minor changes in water dynamics as a function ofinterlayer cation or location of the layer charge and with nosignificant difference between montmorillonite and beidellite.The expanded water content of the fully hydrated interlayer

systems can affect the arrangement of interlayer cations,potentially providing enough water and space to develop fullhydration spheres for cations with large hydration enthalpies(e.g., Mg2+). In general, the results for the bilayer hydrate modelsshow only minor effects in the power spectra associated with thisincreased interlayer water. It is likely that the power spectra forthe monolayer hydrates (Figure 9) are the better models forinterpreting the experimental INS spectra of this study. Theresults for the bilayer models can be used to provide guidance forpredicting the dynamics of water in more water-saturatedenvironments (i.e., high relative humidity). This is supported bythe basal d spacings, obtained by XRD and supported by the MDsimulations, which strongly suggest the experimental cation-exchanged smectites represent monolayer hydrate phases.It is instructive to combine the experimental INS difference

spectra with the calculated power spectra of the monolayerhydrates to better assess the correspondence of the librationalregions to each of the interlayer cations. Figure 11 provides thiscomparison by overlaying the respective spectra from 200 to1000 cm−1 and includes the trendlines associated with peakmaxima derived from the analysis of the power spectra. Ingeneral, the power spectra derived from the MD simulations ofan idealized periodic structure of each model are relativelysmooth. The INS spectra are comparatively noisy as they areobtained for treated natural clay minerals having finite size andexternal basal and edge surfaces and which include instrumentnoise and related uncertainties. Furthermore, the INS spectra arecollected at 10 K, while the MD simulations are calculated torepresent conditions at 300 K; a recent MD study suggestedlimited differences in power spectra for clay mineral modelssimulated at 300 K and at such low temperatures.21 Nonetheless,the overall behavior of the experimental and calculated spectraare consistent for both smectites, with each exhibiting adominant and broad librational peak for each cation in therange from 400 to 600 cm−1. Three observations of thecorrelation of data in both INS spectra and power spectrainclude the following: (1) there is an increased ordering oflibrations for the divalent smectite phases, which is mostpronounced for the Ba-montmorillonite example; (2) INSspectra for the Mg2+ samples do not exhibit a substantial low-frequency libration that is observed in the corresponding powerspectra; and (3) the Na-montmorillonite INS spectrum shows ared shift of about 125 cm−1 relative to its corresponding power

spectrum. There are someminor differences between experimentand simulation for these examples which indicate the practicallimitations associated with sample treatment and the collectionof INS data or possibly the inaccuracy of the interatomicpotentials used in the MD simulations. Nonetheless, the generalqualitative agreement in the spectra appears to confirm that themodels are capturing, to a certain degree, the complex dynamicsof water molecules in these particular clay minerals.Figure 12 presents a summary of the INS and power spectra

for the monolayer hydrates based on a comparison of the

frequencies corresponding of the librational peak maxima as afunction of the cation hydration energy. Included in the figure arefrequencies for librational maxima of power spectra for bulkwater and interlayer water in pyrophyllite; both are associatedwith an enthalpy of −41.7 kJ/mol, which is the self-interactionenthalpy of water (i.e., hydrogen bond network).106 The trendsfor the maxima frequencies for both cation-exchangedmontmorillonite and beidellite samples generally increase withhydration enthalpy (absolute values) with the minor exception ofthe Na-montmorillonite sample. The power spectra derived forthe cation-exchanged montmorillonite models follow thisexperimental trend but are shifted to lower librationalfrequencies. The frequencies for the cation-exchanged beidellitemodels are similarly red shifted but exhibit a decrease withhydration enthalpy (absolute values) in contrast to theexperimental INS results. As noted previously, this differencein the power spectra can only be attributed to the location of thecharge sites in the beidellite models (i.e., in the tetrahedral sheetdirectly at the clay−water interface).

Figure 12. Summary of the frequencies for the librational peak maximafor INS difference spectra (squares) and power spectra (circles) as afunction of cation hydration enthalpy for monolayer hydrates of Na-,Cs-, Ba-, and Mg-montmorillonite (blue) and Na-, Cs-, Mg-, and Ba-beidellite (red). Maximum frequencies for the Mg power spectrarepresent the mean value of the two dominant peaks. Referencefrequencies for bulk water and interlayer water in pyrophyllite (light bluediamonds) represent power spectra for cation-free systems.

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■ CONCLUSIONSVibrational spectra of cation-exchanged smectite minerals fromINS experiments and power spectra from MD simulationsprovide molecular insights into the structure, dynamics, andinteractions of confined water and cations in the interlayers ofclay minerals. Combined with materials characterizationtechniques such as XRD, this study has provided insight intothe hydration environments of two smectite minerals (mont-morillonite and beidellite). We observe trends in the edge of thebroad absorption band for the disordered librational modes ofwater for the various smectite phases that are consistent with thehydration energies of the interlayer cation. In particular, the trendin INS librational spectra follow that of cation hydration enthalpy(Mg2+ > Ba2+ > Na+ > Cs+). The librational data for the Mg2+

samples are the most unique because of an apparent bimodalbehavior of water where the spectra exhibit two distinct waterenvironments for both clay minerals. Material characterizationand XRD analysis of the basal d spacings indicate the INSsamples exist as monolayer hydrates at the experimentalconditions.The power spectra, which do not exhibit clean librational

edges as observed in the inelastic neutron spectra, help in theinterpretation of the experimental librational spectra. Use of largesimulation cells to represent the bulk clay systems improvesstatistics and the ability to capture disorder states for thedynamical modes of water. Both montmorillonite and beidellitemodels exhibit power spectra, characterized primarily by a broadand disordered librational peak that varies with interlayer cationand which are generally consistent with the INS observations.Monolayer hydrates exhibit more sensitivity to interlayer cationcompared to the expanded bilayer hydrate models. Monolayerhydrate models for beidellite, where the layer charge isconcentrated in the tetrahedral sheet at the interlayer surface,exhibit a red shift in libration peak maximum with increasingcation hydration enthalpy, whereas a blue shift is observed for thelibrational peak for the corresponding montmorillonite modelswhere the charge is localized to the octahedral sheet of the claylayer and removed from the interlayer. As observed in the INSspectra, power spectra derived for both smectite modelsinvolving divalent cations (Ba2+ and Mg2+) indicate someordering of the water libration modes and the presence of twounique environments in the interlayer.As with the difference spectra derived from the INS

measurements, the molecular simulations similarly offer theability to manipulate atomic trajectories to differentiate amongthe multiple atomic interactions of a complex hydrated mineralsystem. Through a comparison of two similar swelling claymineralsmontmorillonite and beidellite, each with the samelayer charge but different charge locationit is possible toevaluate the impact of the clay on the structure and dynamics ofinterlayer water. A future challenge will be to explore, throughfurther spectroscopy and molecular simulation, the waterdynamics for more complex interlayer compositions and underthe pressures associated with geological processes.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: rtcygan@sandia.gov.Author ContributionsThe manuscript was written through contributions of all authors.All authors have given approval to the final version of themanuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study benefitted from numerous technical discussions withJeffery Greathouse. This work was supported by the U.S.Department of Energy, Office of Science, Office of Basic EnergySciences, Geosciences Research Program. The authors gratefullyacknowledge support by Oak Ridge National Laboratory toperform the neutron scattering measurements at the SpallationNeutron Source (SNS), Oak Ridge, USA (Project IPTS-13608).Sandia National Laboratories is a multiprogram laboratorymanaged and operated by Sandia Corp., a wholly ownedsubsidiary of Lockheed Martin Corp., for the U.S. Department ofEnergy’s National Nuclear Security Administration underContract DE-AC04-94AL85000.

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