The role of fluid polarity in the swelling of sodium-montmorillonite clay:A molecular dynamics and Fourier transform infrared spectroscopy study

Dinesh R.Katti,Keshab B.Thapa,Kalpana S.Katti

Department of Civil and Environmental Engineering,North Dakota State University,Fargo,ND,58108,USA

Keywords:Swelling clays Montmorillonite Fourier transform infrared(FTIR)technique Molecular dynamics(MD)Organic fluids Polarity

A B S T R A C T Swelling clays are found extensively in various parts of the world,and sodium-montmorillonite(Na-MMT)is the main constituent of an expansive clay mineral.In this work,the swelling behavior of Na-MMT clay with a wide range of organic fluids,high polar through low polar fluids,is studied using a combination of Fourier transform infrared(FTIR)technique and molecular dynamics(MD)simulations.The construction of the representative clay- fluid models is carried out,and the nature of nonbonded interactions between clay and fluids is studied using MD.Our FTIR and MD simulations results suggest the significant nonbonded interactions between Na-MMT clay and polar fluids,such as form amide and water.The nonbonded interactions of Na-MMT with methanol and acetone are significantly less than those in Na-MMT with polar fluids.The interactions of the fluids with various entities of the clay such as Si-O,Fe-OH,Mg-OH,and Al-OH captured via the spectroscopy experiments and modeling provide a finer understanding of the interactions and their contributions to swelling.The MD simulations are able to capture the band shifts observed in the spectra obtained in the spectroscopy experiments.This work also captures the conformations of interlayer sodium ions with form amide,water,methanol,and acetone during swelling.These nonbonded interactions provide insight into the molecular mechanism that the polarity of fluids plays an important role in the initiation of interlayer swelling,alteration in the orientations,and evolution of microstructure of swelling clays at the molecular scale.

1.Introduction

Understanding of the expansive behavior of swelling clays is of significant importance in geotechnical and geoenvironmental applications,in petroleum and industrial engineering,and for the design of polymer-clay-nanocomposites.The volume of swelling clayincreases when it interacts with water,resulting in a significant increase in swelling and swelling pressure.The infrastructure such as buildings,roads,retaining walls,dams,and irrigation canals are prone to damage from swelling pressure(Chen,1988;Rao et al.,1988;Katti et al.,2002).However,this type of clay has also been used for land fill liners(Kayabali,1997),borehole stabilization when drilling mud(Murray,1999),enhancing the material properties in clay-nanocomposites(Sikdar et al.,2008a),biomedical application(Ambre et al.,2013),and modifying asphalt in pavement construction(Abdelrahman et al.,2014).

Sodium-montmorillonite(Na-MMT),in many cases,is the main component of swelling clay mineral found in these clays.The mineral consists of a tetrahedral-octahedral-tetrahedral(T-O-T)structure where octahedral clay sheet is sandwiched between two tetrahedral clay sheets(Mitchell and Soga,2005).Na-MMT clay is used extensively as a barrier material in geotechnical and geoenvironmental engineering due to its high surface area and low hydraulic conductivity.Hence,understanding the swelling mechanisms and evaluating the interactions occurring on the molecular scale of Na-MMT clay with various organic fluids are crucial for predicting swelling clay response,designing land fill clay liners,and avoiding enormous damages caused by swelling clays.Land fill leachate includes a wide range of organic fluids.The United States Environmental Protection Agency(US-EPA)has labeled fluids such as toluene,trichloroethylene(TCE),acetone,methanol,and formamide among other fluids as toxic and dangerous to health.The interactions between Na-MMT clay and organic fluids are very important in determining the appropriate use of clays in the land fill liners.In swelling clays,swelling is categorized as inner crystalline swelling and osmotic swelling(Madsen and Muller-Vonmoos,1989).The inner crystalline is due to initial hydration of exchangeable interlayer cations of dry clays when they come in contact with an aqueous phase such as water.The clay-water interactions have been studied using diffuse double layer theory(Jo et al.,2001).However,the interlayer hydration of swelling clay was not described accurately by these theories.

In our previous work,swelling pressure of saturated bentonite clay at a predetermined swelling level has been studied experimentally and showed that clay particles break down into a smaller size with increase in swelling level and reduction of swelling pressure(Katti and Shanmugasundaram,2001).Fourier transform infrared(FTIR)spectroscopy is a nondestructive technique(Maria Gomez-Caravaca et al.,2013)and used for the analysis of clayfluid interactions.Extensive studies on Na-MMT clay with water have been carried out using FTIR technique(Katti and Katti,2006),and the disorientation of clay sheets increases due to clay-water interactions with increasing swelling magnitude.In our prior work,the hydraulic conductivity of the Na-MMT clay interlayer region is studied using polarized FTIR spectroscopy.In addition,the change in Si-O stretching band in tetrahedron clay sheets,O-H stretching of a structural hydroxyl group,and H-O-H bending vibration band of bulk water at the molecular scale showed a significant interaction between clay and water.Extensive experimental studies have been carried out on Na-MMTclay with high polar fluids,formamide and water,medium polar fluids,methanol,and low polar fluids,acetone,chloroform,trichloroethylene,and toluene(Amarasinghe et al.,2009,2012).The interactions between Na-MMT and organic fluids are nonbonded in nature:the nonbonded interactions are very high for polar fluids and almost negligible for low polar fluids.Therefore,the clay- fluids molecular interactions control the evolution of the macroscopic structure of Na-MMT clay.The macroscale mechanical properties,such as hydraulic conductivity,consolidation,and swelling pressure,of Na-MMT clay with these fluids,are studied.The hydraulic conductivity decreases dramatically with the increase in polarity of fluids(Amarasinghe et al.,2012).Furthermore,the initial swelling of montmorillonite clay depends on the type of interlayer cations and hydration energy of interlayer cations(Norrish,1954),and the clay particles breakdown due to a significant cation- fluid hydration energy.

In addition to these experimental studies,the computational techniques have been used to investigate the behavior of swelling clays at the molecular level.Molecular dynamics(MD),Monte Carlo(MC),and discrete element method(DEM)have been carried out to study the interaction between clay and water(Delville,1991;Boek et al.,1995;Chang et al.,1995;Karaborni et al.,1996;Anandarajah,1997;Teppen et al.,1997;Shroll and Smith,1999;Katti et al.,2005a,2005b,2007,2009a,2015;Schmidt et al.,2005;Pradhan et al.,2015;Kadoura et al.,2017).In our previous work,the mechanical behavior of dry and hydrated Na-MMT clay interlayer and clay-water interactions are studied using steered MD(SMD),and it has been observed that the solvation of interlayer cations and clay sheets play a significant role in the swelling properties of Na-MMT clay(Schmidt et al.,2005;Katti et al.,2007;Greathouse et al.,2015;Makaremi et al.,2015;Sun et al.,2015).DEM studies showed that particle subdivision of Na-MMT clay causes an increase in swelling pressure(Katti et al.,2009a).The attractive interactions between sodium and clay hold the clay sheets together in dry clay;however,when water is introduced,the attractive interactions between sodium and water result in the formation of solvation shell around the cations,resulting in decreased attractive interactions between clay sheets and sodium ion,in turn increasing the crystalline swelling and with increased interlayer hydration,causing exfoliation and particle breakdown of swelling clay(Katti et al.,2015;Pradhan et al.,2015).The initiation of the crystalline swelling mechanism of Na-MMT clay has been carried out using MD simulations.Interlayer sodium cations attract the water molecules into the interlayer,and there is also a significant attractive nonbonded interaction between them,which initiates interlayer swelling(Katti et al.,2015).Furthermore,interactions of Na-MMT with formamide,water,methanol,acetone,and toluene have been quantitatively studied in a previous work.We have observed the significantly higher nonbonded interactions between clay and polar fluids than that between clay and low polar fluids(Katti et al.,2017).Previous studies on clay- fluid interactions have been conducted either experimentally orcomputationally but not concurrently on the same system.In the current work,we attempt to link interaction energies from MD simulations to observed changes in the experimentally obtained FTIR spectroscopy spectra.The molecular models used in the study resemble the clay used in the experiments.Also,by conducting MD studies on the observedd-spacing values of samples used in FTIR studies,the fluid content in the interlayers in MD and FTIR experiments is similar providing an insight into molecular mechanisms.This current study presents our FTIR and MD simulation results on Na-MMTclay with a wide range of organic fluids and provides insight into nonbonded interactions quantitatively at the molecular scale.In addition,the conformations of each organic fluid in close proximity to interlayer sodium cations are presented.

2.Materials

Na-MMT(SWy-2,Crook County,Wyoming,USA)clay was acquired from Clay Minerals Repository at the University of Missouri,Columbia,Missouri,USA.The cationic exchange capacityof this clay is about 76.4 meq/100 g.The fluids used were 90%-100%purity formamide and 99.9%purity acetone obtained from Mallinckrodt Baker Inc.,New Jersey and 99.9%purity methanol obtained from Alfa Aesar,Massachusetts.The deionized water used was obtained from our laboratory at North Dakota State University.The fluids used in this study range from high dielectric constant to low dielectric constant reflecting high polar to low polar fluids.Formamide and water have high dielectric constants of 110 and 80,respectively,methanol has a medium dielectric constant of 33,and acetone has a low dielectric constant of 20.

3.Fourier transform infrared technique

A comprehensive infrared spectroscopic study on Na-MMT clay and organic solvents samples was described in our previous work(Amarasinghe et al.,2009).Briefly,Na-MMT was first ground and passed through a No.325 sieve(45μm mesh).The consistent moisture content was acquired by drying clay in an oven at 50°C temperature for 24 h prior to each sample preparation.The thin layer of Na-MMT was placed on the gold-coated metal substrate with a glass slide,and fluid was added to the surface of the sample.For transmission FTIR spectroscopy study,the powder samples were gently compressed against a silicon window with a glass side,and then the fluids were added to the surface of the clay.Another silicon window was used to sandwich the sample by gently pressing them together to make a thin layer of sample between silicon windows.The homogeneous samples for formamide and water were prepared by mixing clay with fluids in a porcelain dish to obtain a thin layer of slurry.The sample was smeared on a silicon window,and coated with gold for transmission and reflectance FTIR experiments,respectively.Data acquisition was accomplished immediately after the samples were wetted.The FTIR spectroscopy experiments were performed using a Nicolet 850 FTIR spectrometer with KBr beam splitter in the range of 4000-400 cm-1at a spectral resolution of 4 cm-1.Awiregrid polarizer was used to yield a p-polarized infrared(IR)beam,and angle of incidence was at 45°for the reflectance experiment,and the gold-coated metal substrate was used for obtaining background spectra.The clean silicon windows and gold-coated metal substrate were used,respectively,for background spectra in transmission and reflectance experiments.

4.Model construction

The chemical formula of the Na-MMT SWy-2 unit cell is NaSi16(Al6FeMg)O20(OH)4.The initial coordinates were obtained from the model proposed in the literature(Skipperet al.,1995).Also,the atomic charges were obtained from the literature(Teppen et al.,1997).The Na-MMT 4×2 model was initially constructed(Schmidt et al.,2005)in our previous studies,and the dimensions of unit cell were 5.28 ? × 9.14 ? × 6.56 ?.The structural charges in the molecular models of clay sheets are consistent with those in the experiments.The reported chemical formula for SWy-2 clay used in the experiments(van Olphen and Fritpiat,1979)is(Ca0.12Na0.32K0.05)[Al3.01Fe(III)0.41Mn0.01Mg0.54Ti0.02][Si7.98Al0.02]O20(OH)4.The chemical formula used for the clay models is a simplified version NaSi16(Al6FeMg)O20(OH)4.Almost all of the unbalanced charge in the clay sheet comes from isomorphous substitution and is reported as 0.53e.In our models,the charge due to isomorphous substitution in the octahedral sheet is 0.5e per unit cell(Katti et al.,2007).This model has been extensively used for clay- fluid interactions(Katti et al.,2005a,2007,2015;Pradhan et al.,2015)using CHARMM force field parameters(Katti et al.,2005a,2005b,2007).The Na-MMT clay layers have a T-O-T structure,and each octahedral clay sheet is sandwiched between tetrahedral clay sheets.In our work,the clay model has 6×3 unit cells,and the dimensions of the unit cell are 31.68 ? × 27.44 ? × 24.16 ?.The molecular model consists of 6 unit cells inX-direction and 3 unit cells inY-direction,as shown in Fig.1.A detailed explanation of the model construction is described in our previous work to study polymer clay nanocomposites(Sikdar et al.,2006).

The negative charge was developed on the individual claysheets due to isomorphic substitution in the octahedral sheet.In the 6×3 model,nine aluminum cations(Al3+)are substituted by nine iron cations(Fe3+),and nine aluminum cations(Al3+)are substituted by nine magnesium cations(Mg2+).Thus,nine sodium cations(Na+)were introduced in the interlayer to balance the negative charge(-0.5e per unit cell)of the clay sheets.In this study,the molecular weight of 6×3 unit Na-MMTclay is 13,414 g/mol,and hence 1 mol of clay contains 13,414 g of clay.Fig.2a-d shows the molecular structures of acetone,methanol,water,and formamide.

Fig.1.Molecular model of Na-MMT showing clay sheets,Na cations,and d-spacing.

The Na-MMT and fluids models were developed using Material Studio?and PSFGen plug-in of visual MD software(VMD 1.9.2)(Humphrey et al.,1996).The force field parameters for formamide,methanol,and acetone were obtained from CHARMM GUI Archive-CHARMM Small Molecule(MacKerell et al.,1998),and the water molecule was Transferable Intermolecular Potential 3 Point(TIP3P)(Jorgensen et al.,1983).FTIR spectroscopy experiments were conducted on samples created by clay slurries of clay+acetone and clay+methanol.In addition,samples were prepared with clay mixed with predetermined amount of water and clay mixed with predetermined amount of formamide.Under the same condition,XRD experiments were conducted on the samples to evaluatedspacing.Inverse calculations were conducted to evaluate the amount of fluid in the interlayer by comparingd-spacing obtained from MD simulations withd-spacing values found from XRD experiments.The molecular models with the computed amounts of fluid molecules in the interlayer were used to evaluate the interaction energies.Thedspacing values for the molecular models with 10%acetone,20%methanol,30%water,and 30%formamide matched with thedspacing results from the XRD experiments.In order to compare FTIR spectroscopy results with the MD interaction energy results,the representative clay models consist of 10%acetone,20%methanol,30%water,and 30%formamide in the interlayer(Amarasinghe et al.,2009,2012;Katti et al.,2009b,2017).The detailed procedure of the model construction was described in our previous work(Katti et al.,2017).CHARMM force field parameters have been used for both organic fluids and Na-MMT clay(Katti et al.,2005b,2007;Schmidt et al.,2005).The CHARMM parameters have been found and validated by the authors.Based on the molecular weight of individual molecules,10% fluid content is equivalent to 24 molecules of acetone,40 molecules of methanol,64 molecules of water,and 48 molecules of formamide.In addition,20%methanol has80methanol molecules,30%water has 216 water molecules,and 30%formamide has 90 formamide molecules in the clay interlayer.

5.Simulation details

Fig.2.The molecular structure of(a)Acetone,(b)Methanol,(c)Water,and(d)Formamide.

Fig.3.IR spectra for(a)Mixture of Na-MMT and formamide in the energy range of 800-1275 cm-1,(b)Mixture of Na-MMT and water in the energy range of 750-1250 cm-1,(c)Mixture of Na-MMT and methanol in the energy range of 675-1325 cm-1,and(d)Mixture of Na-MMT and acetone in the range of 775-1200 cm-1.

MD simulations and visual MD software(VMD 1.9.2)were used for the molecular modeling of Na-MMT clay with different organic fluids.In MD,nanoscale molecular dynamics software(NAMD 2.9)was used,which was developed by Theoretical and Computational Biophysics Group,Beckman Institute,the University of Illinois at Urbana-Champaign(Phillips et al.,2005).All interaction energies were computed using VMD,which was compatible with the CHARMM force field(Brooks et al.,1983).The simulations were run using 2.66 GHz Intel Xenon X5550 processor and 127 nodes,each node consisting of 8 processor cores,at the Center for Computationally Assisted Science and Technology(CCAST)at North Dakota State University.

The conjugate method was carried out for the minimization of all models.The temperature of 0 K and the pressure of 0 atm are used during minimization.All the simulations were run at room temperature and atmospheric pressure,resulting in the isothermal-isobaric ensemble.The temperature was raised in three steps,i.e.0 K through 300 K,with an increment of 100 K.The pressure was raised in four steps,i.e.0 kPa through 101 kPa(1 atm),with an increment of 25 kPa while keeping the temperature constant at 300 K.The pressure was controlled by the method of Langevin piston Nose-Hoover(Feller et al.,1995)and temperature by Langevin dynamics.

All the simulations were run for 150,000 steps,which is equivalent to 75 ps(10-12s),with the time step of 0.5 fs(10-15s).The infinite number of clay sheets was mimicked by applying the period boundary conditions.Although the clay sheets were restrained inX-andY-directions,they were allowed to move in theZ-direction.All the organic fluids were free to move in all directions,mimicking experimental conditions.Thed-spacing of each modelwas measured by the distance between the corresponding surface oxygen atoms of clay sheets in theZ-direction.The interaction energies were computed considering the last 20 ps of the trajectory of simulation.Nonbonded interaction,such as electrostatic energy,was computed using particle mesh Ewald(PME)method.For all models,the van der Waals switch and cut off distances were 16 ? and 17 ?,respectively.

Table 1 FTIR band assignment for dry Na-MMT clay.

6.Results and discussion

6.1.Fourier transform infrared results

In our previous work,a detailed vibrational spectroscopic study on clay- fluid interactions was described using transmission and reflection FTIR experiments(Amarasinghe et al.,2009).The combined IR spectra for dry Na-MMT and Na-MMT with acetone,methanol,water,and formamide are shown in Fig.3a-d.

Table 1 shows the band assignments obtained from the literature(Amarasinghe et al.,2008;Ambre et al.,2011;Katti et al.,2006,2014).The band at 1118 cm-1in the IR spectra of dry Na-MMT is attributed to the Si-O stretching.It is observed in Fig.3a and b that the Si-O stretching band has a shift of 7 cm-1and 6 cm-1to lower energy for the clay-formamide complex and the clay-water complex,respectively.

The band shift observed in the spectra suggests the alteration in the orientation of silica tetrahedral region due to swelling characteristics,as the polar fluids enter into the clay interlayer galleries.The shift also signifies that there is nonbonded interaction between surface oxygen of clay and interlayer solvents;this phenomenon has been observed in prior work in the literature to study the polymer clay nanocomposites(Sikdar et al.,2008b),evolution of clay microstructures(Katti and Katti,2006;Amarasinghe et al.,2009,2012),and interaction between clay and organic matter(Katti et al.,2014).The band shifts are not significant in the Si-O region in the case of clay-methanol and clay-acetone complexes,which indicates that intermediate polar and low polar fluids have lower nonbonded interactions in Si-O region.Furthermore,Mg-OH deformation band at 846 cm-1and Al-OH deformation band at 918 cm-1of dry clay have no significant shifts in any clay fluids complexes,indicating insignificant nonbonded interactions in the octahedral clay sheets.Fe-OH deformation band of dry Na-MMT at 879 cm-1is shifted towards lower energy by 9 cm-1and 5 cm-1in the spectra of the clay-formamide complex and claywater complex,respectively.These shifts are significant due to no overlapping bands in IR spectra.From the spectra of clay-methanol and clay-acetone complexes,no significant band shifts are observed in the tetrahedral region,or octahedral region of clay sheets.The shifts towards either lower or higher energy observed in the Si-O stretching band and the Al-OH,Fe-OH,and Mg-OH deformation bands suggest that the molecular interactions between clay and polar fluids are significantly higher than those in clay and medium and low polar fluids.

6.2.Molecular dynamics simulations

The experimental d-spacing of dry Na-MMT clay is 9.85 ?(Amarasinghe et al.,2009).The MD simulation showed that the averaged-spacing of dry Na-MMT was 10.7 ?,resulting in the representative clay model for this study.The experimentaldspacing values of Na-MMT with acetone,methanol,water,and formamide were found to be 13.07 ?,17.14 ?,18.32 ?,and 18.99 ?,respectively(Fu et al.,1990;Amarasinghe et al.,2009;Sun et al.,2015;Katti et al.,2017).The correspondingd-spacing values of the representative clay- fluid models after simulations are presented in Table 2.Thed-spacing values of the clay model with 10%acetone,20%methanol,30%water,and 40%formamide were 13.38 ?,17.6 ?,17.89 ?,and 18.46 ?,respectively,indicating that the interlayer fluid content increased with increase in dielectric constant of fluids.These results are consistent with experimental results and can be related to the high swelling and swelling pressure,particle breakdown,low permeability,and low compression of the expansive clay in the presence of polar fluids(Amarasinghe et al.,2012).In this study,the swelling of the interlayer is associatedwith an increase in the distance between two clay sheets;however,the thickness of clay sheet remains constant.The clay- fluid molecular models consist of clay sheets,sodium ions,and interlayer fluids.The nonbonded interactions among these constituents provide insight into swelling behavior of the expansive clay.

Table 2 d-spacing of the clay- fluid complexes from simulation and experiment results.

VMD was used to compute the interaction energies quantitatively.Energies were computed by considering the interlayer fluids in the clay interlayer after simulation period of 75 ps.The snapshots of molecular models of Na-MMT with acetone,methanol,water,and formamide after simulation are shown in Fig.4a-d,respectively.The sum of the electrostatic energy and van der Waals energy is the total nonbonded energy.The electrostatic interaction energy is the function of the charge and distance between two sets of atoms,whereas van der Waals interaction energy depends only on the distance between two sets of atoms.The negative interaction energy represents the attractive interaction,and the positive interaction energy represents the repulsive interaction between two atoms.

Fig.5 shows the interaction energy between clay sheets in the dry state and various interlayer fluids(polar,medium polar,and low polar fluids).In the case of the dry interlayer,clay-clay interactions are the highest and predominated by van der Waals energy.The repulsive electrostatic interaction was due to negative charge in each clay sheet.When fluids are introduced to the interlayer,the attractive nonbonded interactions are decreased with increasing amount of interlayer fluids and polarity of fluids,and interactions are rapidly diminished almost to zero at 30%of formamide.As shown in Fig.6,the attractive nonbonded interactions between sodium and clay layers are the highest in the dry condition,and these attractive interactions are primarily electrostatic and hold the clay sheets together.

This observation has also been reported on Na-MMT swelling clay with increasing water content using MD(Pradhan et al.,2015).At equilibrium,sodium ions are found to be in close proximity to the clay sheets in the molecular model of Na-MMT with acetone and methanol,but sodium ions are located near the center of the interlayer in the molecular model of Na-MMT with water and formamide.The attractive interaction is also decreased with increasing polarity of fluids and fluid content.The electrostatic energy between sodium and clay sheets in dry Na-MMT clay is 1.5 times higher than that in Na-MMT with 10%acetone.It is almost the same for Na-MMT with 20%methanol,30%water,and 30%formamide,and the van der Waals energy is negligible.Fig.7 shows the attractive interactions between sodium ions and interlayer fluid molecules are the highest followed by clay-sodium interactions and then clay-clay interactions.

The electrostatic energy between sodium ions and 20%methanol was more than 2 times greater than that between sodium ions and 10%acetone.Similarly,the electrostatic energy between Na-MMT and 30%water was 1.5 times greater than that between Na-MMT and 20%methanol.The total nonbonded interaction energy between Na-MMT and 30%water is slightly greater than that between Na-MMT and 30%formamide.The nature of these interactions is predominantly(attractive)electrostatic whereas(repulsive)van der Waals interactions are negligible.Thus,sodium ions have significant interactions with polar fluids than those with low polar fluids;the solvation of swelling clay was initiated by hydration of interlayer sodium ions,resulting in the initial interlayer swelling.

Fig.4.Na-MMT models with different interlayer fluids after simulation up to 75 ps:(a)10%acetone,(b)20%methanol,(c)30%water,and(d)30%formamide.

Fig.5.Interaction energies between clay sheets for fluids with a wide range of dielectric constants.1 kcal/mol=4.184 kJ/mol.

Fig.8 shows the snapshots of the representative models of Na-MMT with 10%acetone,20%methanol,30%water,and 30%formamide.This figure shows the planar view of conformation of the interlayer fluids with sodium ions.It was found that the oxygen atoms were attracted and directed to the sodium ions,and hydrogen atoms were attracted to oxygen atoms,resulting in the formation of an organized pattern.It can be seen that two oxygen atoms and four oxygen atoms are in proximity to the sodium ions in case of Na-MMT with 10%acetone and Na-MMT with 20%methanol,respectively.In addition,the clustering and proximity of fluid molecules to the sodium ions increase dramatically as the fluid content and the polarity of fluids increased,and this formation resulted in the well-organized pattern for high polar fluids such as water and formamide.All these fluid molecules laid on the same plane as sodium ions.

Fig.6.Interaction energies between clay sheets and sodium ions for fluids with a wide range of dielectric constants.

Fig.7.Interaction energies between sodium ions and fluid molecules.

The nonbonded interaction energy of Si-O of tetrahedral clay sheets with acetone,methanol,water,and formamide is shown in Fig.9.The interaction energy between Si-O and fluids increased as the polarity of fluids increased.The repulsive electrostatic energy between Si-O and water is more than 5.5 times greater than that between Si-O and acetone.Although the total nonbonded interaction energy between Si-O and formamide is slightly greater than that between Si-O and methanol,the corresponding electrostatic energy is similar and attractive in nature.These interaction energies agreed with shifts of Si-O band in our FTIR experimental analysis(Fig.3).These shifts suggest strong nonbonded interactions between the surface oxygen of clay sheets and fluid molecules,and strong interactions may change the orientations of silica tetrahedral sheets of clay.

Fig.8.Snapshots displaying conformation of molecules of fluids in proximity to sodium ions:(a)Acetone,(b)Methanol,(c)Water,and(d)Formamide.Sodium ions are rendered in VDW format.

Fig.9.Interaction energy of Si-O with acetone,methanol,water,and formamide.

Fig.10 plots the variation of interaction energy between Mg-OH of octahedral clay sheet and fluids.The attractive nonbonded energy between Mg-OH and methanol is 3.5 times greater than that between Mg-OH and acetone,however,van der Waals energy is similar in both cases.The nonbonded interaction energy between Mg-OH and water is approximately the same for Mg-OH and formamide,and these attractive interactions are predominantly electrostatic in origin.Nevertheless,van der Waals energy is almost the same for both cases.

The nonbonded interaction energies of Al-OH and Fe-OH of octahedral clay sheet with fluids are shown in Figs.11 and 12.

Fig.10.Interaction energy of Mg-OH with acetone,methanol,water,and formamide.

Fig.11.Interaction energy of Al-OH with acetone,methanol,water,and formamide.

Attractive electrostatic interaction energy between Al-OH and water is 2 times greater than that between Al-OH and acetone and almost 1.5 times greater than that between Al-OH and methanol.On the other hand,the electrostatic interaction between Al-OH and formamide is repulsive in nature.The van der Waals energy remains almost the same for all cases.Similarly,the total nonbonded energy between Fe-OH and water is more than 3 times greater than that between Fe-OH and acetone and 1.5 times greater than that between Fe-OH and methanol,but the(repulsive)electrostatic interaction between Fe-OH and formamide is not significant compared to other fluids.These results were consistent with the FTIR studies,which indicate that the interaction between polar fluids and clay increases with increase in the polarity of fluids.Thus,the FTIR and MD simulations studies showed a significantly higher nonbonded interaction between clay and formamide and water than that between clay and methanol and acetone.In addition,simulation results indicated that clay and formamide interactions were predominant.The results also show that various entities of the clay sheet interact differently with the fluid molecules even for fluid molecules with similar dielectric constants.

Fig.12.Interaction energy of Fe-OH with acetone,methanol,water,and formamide.

Fig.13.Nonbonded interaction energies between clay sheets for 10%acetone,methanol,water,and formamide and corresponding d-spacing.The interaction energies shown are clay-clay,clay-sodium,sodium- fluids,and clay- fluids.

Fig.13 shows the evolution of nonbonded interaction energies between the various components of the clay with 10%of fluid content in the interlayer withd-spacing.Thed-spacing corresponding to 10%acetone,10%methanol,10%water,and 10%formamide was 13.38 ?,13.88 ?,13.57 ?,and 13.71 ?,respectively.The figure indicates that although the fluid content is the same,the molecular interactions in the interlayer are significantly different.Although the clay interlayer contained the same fluid content,each of the clay- fluid models had differentd-spacing values,and the clay-clay,clay-sodium,sodium- fluids,and clay- fluids interactions were also different,as shown in Fig.13.The nonbonded interactions between clay sheets decreased with increase indspacing.The 10%of interlayer methanol had the highestd-spacing,resulting in the lowest nonbonded interactions between clay sheets.With the increase ind-spacing,the clay-sodium attractive nonbonded interactions decreased,but the interaction between clay and formamide remained slightly higher than that between clay and water,as shown in Fig.13.The sodium- fluids attractive nonbonded interactions increased with increase ind-spacing;the sodium-methanol interactions were more than 1.5 times greater than the sodium-acetone interactions;and the sodium-water interactions were slightly greater than the sodium-methanol and sodium-formamide interactions.The clay- fluids attractive nonbonded interactions increased with increase ind-spacing,but the interactions of water with clay layers had the lowest energy.Also,the interactions of methanol with clay layers were 3 times greater than that of clay layers with acetone and slightly less than that of clay layers with formamide,as shown in Fig.13.The clayclay attractive interactions were predominated by van der Waals energy for dry as well as clay with 10% fluids content,but the attractive clay-sodium,sodium- fluids,and clay- fluids interactions were primarily electrostatic in nature,as shown in Table 3.

Fig.14 shows the conformation for each of four interlayer fluids with sodium ions.The acetone,methanol,water,and formamide molecules formed a well-organized pattern in proximity to sodium ions,and the oxygen atoms were also attracted and directed to the sodium ions.Although the fluid molecules tend to cluster around the sodium ions,the distribution of fluid molecules in the interlayer over the plane parallel to the clay surfaces shows conformational differences.This is particularly apparent with methanol and water,where the distribution of the fluid molecules is not as uniform as acetone and formamide.The most difference is observed in the case of water due to that the water molecules are aligned along theY-axis of the model.The size of the molecules,molecular charge distribution and the number of molecules seem to affect the molecular distribution of fluid molecules in the interlayer.The water molecule is the smallest among the four fluids,resulting in the lowest molecular weight as well.Thus,the number of water molecules is higher than those of other three fluids.In other words,10%fluid content is equivalent to 24 molecules of acetone,40 molecules of methanol,64 molecules of water,and 48 molecules of formamide.As shown in Fig.13,the sodium-water attractive interactions are significantly larger than clay-formamide interactions,some of which may be attributed to more clustering of water molecules with sodium ions(Fig.14).The figure also shows larger clay-formamide interactions compared to clay-water interactions,which may be because the formamide molecules are more uniformly distributed over the entireplane parallel to the claysurface.The relationship between the fluid molecular conformations and effect on interaction energies is qualitative based on the snapshots and more work will be needed to introduce the effects of size and conformations in addition to fluid polarity on clay swelling.

Table 3 Interaction energies of clay-clay,sodium- fluids,clay-sodium,and clay fluids for dry Na-MMT and Na-MMT with 10%acetone,10%methanol,10%water,and10%formamide.

Fig.14.Planar view showing the conformation of molecules of fluids in proximity to sodium ions:(a)10%acetone,(b)10%methanol,(c)10%water,and(d)10%formamide.Fluids molecules and sodium ions are rendered in CPK and VDW formats,respectively.

7.Conclusions

In this work,FTIR and MD simulations were carried out to investigate the swelling behavior of Na-MMT clay mineral with a wide range of organic fluids:formamide,water,methanol,and acetone.The shifts in the Si-O stretching band and Fe-OH deformation bands were observed on the FTIR spectra of Na-MMT clay with formamide and water showing larger band shifts than clay with methanol and acetone.This result indicates that there are significant nonbonded interactions between clay and polar fluids,resulting in the change in orientation of tetrahedral and octahedral clay sheets.The molecular models of Na-MMTclay with fluids were developed,and the representative clay- fluids models were validated by comparing MD simulationd-spacing with the experimental results.The MD simulations were used to provide insight into conformation and quantitative nonbonded interactions of clay with formamide,water,methanol,and acetone at the molecular level.The results of our simulations indicate that the attractive nonbonded interactions,which are primarily electrostatic in nature,between clay and formamide and water,are significantly higher than those between clay and methanol and acetone.These results are consistent with the shifts of bands in FTIR studies.Furthermore,the nonbonded interactions play a significant role in the conformations of fluid molecules in proximity to sodium ions in the interlayer.A planar view of the conformations shows a well organized pattern as the amount and polarity of interlayer fluids increase.The modeling study with ten weight percent of the four fluids in the interlayer gallery showed that the molecular interactions between claysheets,sodium ions and fluid molecules are significantly different.Thed-spacings in all four cases were different and ranged from 13.38 ? to 13.88 ?.The clay-clay interactions decreased with increasingd-spacing.The sodium- fluid and clay- fluid interactions appear to relate to the size,number of fluid molecules and the physical distribution of the molecules in the interlayer.These studies provide an insight into the molecular mechanism and indicate that the polarity of fluids plays a significant role in the interlayer swelling,alternation in the orientations,and evolution of microstructure of swelling clays at the molecular scale.

Conflict of interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant support for this work that could have influenced its outcome.

Acknowledgments

The authors acknowledge the support of USDoT,Mountain Plains Consortium,UGPTI under grant No.#69A3551747108.The authors also acknowledge Computationally Assisted Science and Technology(CCAST)for providing computational resources at North Dakota State University.