Infrared Signature of the Cation−π Interaction between Calcite andAromatic HydrocarbonsHaitao Wang,† Daniel J. Grant,‡ Peter C. Burns,†,§ and Chongzheng Na*,†

†Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, NotreDame, Indiana 46556, United States‡Department of Chemistry, University of Minnesota, 207 Pleasant Street Southeast, Minneapolis, Minnesota 55455, United States§Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556,United States

*S Supporting Information

ABSTRACT: The cation−π interaction is proposed as animportant mechanism for the adsorption of aromatic hydro-carbons having non-zero quadrupole moments by mineralsurfaces. Direct evidence supporting such a mechanism is,however, limited. Using the model mineral calcite, we probethe cation−π interaction with adsorbed benzene, toluene, andethylbenzene (BTE) molecules using attenuated totalreflectance Fourier transform infrared spectroscopy. We showthat the presence of calcite increases the energy required toexcite the synchronized bending of aromatic C−H bonds ofBTE molecules. The unique conformation of this vibrational mode indicates that the planar aromatic rings of BTE molecules areconstrained in a tilted face-down position by the cation−π interaction, as further confirmed by density functional theorycalculations. Our results suggest that the shift of the excitation energy of the aromatic C−H bending may be used as an infraredsignature for the cation−π interaction occurring on mineral surfaces.

1. INTRODUCTION

The cation−π interaction can arise from the electrostatic forcebetween a positively charged cation and the quadrupolemoment of a neutral molecule with additional contributionsfrom polarization, induction, exchange, and dispersion forces.1,2

Theoretical calculations suggest that the enthalpy of thecation−π interaction can be as high as 80 kJ mol−1, which isapproximately 1/5 the enthalpy of a covalent bond and 5 timesthat of a hydrogen bond.3,4 Given the strength of the cation−πinteraction, it is well-recognized as an important mechanism forcontrolling intermolecular processes, such as biomolecule self-assembly,5 ligand−protein recognition,6 organic synthesis,7 andselectivity of ion channels.8 In comparison, only limited effortshave been devoted to elucidating the mechanism of thecation−π interaction in surface-related processes.9−13

On a surface, the cation−π interaction can be envisioned tooccur over the positively charged surface cations and, thus, hasbeen proposed as an important mechanism responsible for theadsorption of aromatic hydrocarbons on mineral surfaces.9,10

Direct evidence supporting the cation−π interaction on amineral surface is currently limited to measurements madeusing deuterium nuclear magnetic resonance (2H NMR)spectroscopy.11 The pioneering application of 2H NMR showedthat the quadrupole interaction of deuteriated aromatichydrocarbons with an external magnetic field decreased uponadsorption.12 The extent of the decrease varied with the changeof surface cations, following a trend consistent with that of

partitioning coefficients deduced from sorption isotherms.13

Obviously, complementary analytical techniques are in need tofurther advance the knowledge of the cation−π interactioninvolving mineral surfaces.Fourier transform infrared spectroscopy (FTIR) is a sensitive

analytical technique that has been used to study the interactionsof aromatic hydrocarbons with metals,14 metal oxides,15

aluminosilicates,16 and silicates.17 To probe molecules adsorbedon a solid surface, a sampling technique called attenuated totalreflection (ATR) is often used in conjunction with FTIR.18,19

In ATR−FTIR,20,21 a powder of the solid is pressed on a crystalwith a high reflective index serving as a conduit to direct theexcitation light by total internal reflection. The light creates anevanescent wave extending out of the crystal and into thepowder. The absorption of the excitation light by the powderprovides structural information pertaining to the moleculesadsorbed on the powder surface.22 For surface-adsorbedaromatic hydrocarbons, as compared to their counterparts ina liquid, interactions such as the cation−π interaction with thesurface can vary the energies required for exciting certainmodes of molecular vibration.23 However, an infrared (IR)signature distinctively linking ATR−FTIR measurement andthe cation−π interaction has not been identified.

Received: February 14, 2015Revised: May 13, 2015Published: May 14, 2015

Article

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© 2015 American Chemical Society 5820 DOI: 10.1021/acs.langmuir.5b00610Langmuir 2015, 31, 5820−5826

Here, we report the characterization of cation−π interactionsbetween benzene, toluene, and ethylbenzene (BTE) com-pounds and calcite (CaCO3) using ATR−FTIR. Calcite is amodel rhombohedral carbonate mineral that is the maincomponent of oil-trapping chalk formations and a majorcomponent of soils, sediments, and atmospheric dust particles.BTE compounds are important constituents of organic solventsand liquid fuels that are widely used in industrial processes andconsumer products. We show that the adsorption of BTEmolecules on calcite results in a significant increase of theenergy required to excite the out-of-plane aromatic C−Hvibration (δC−H), providing a prominent analytical signature forprobing the cation−π interaction on mineral surfaces. Theconnection between δC−H vibration and the cation−πinteraction is further supported by density functional theory(DFT) calculations.

2. EXPERIMENTAL SECTIONHigh-quality calcite was purchased from Ward’s Science (Rochester,NY). The structure of the calcite was verified using X-ray diffraction(Bruker Advance). The elemental impurities in calcite weredetermined to be less than 0.19% (see Table S1 of the SupportingInformation) by inductively coupled plasma mass spectrometry(Thermo Scientific Element 2) after digestion in concentrated nitricacid. Calcite was ground into a powder using a mortar and a pestle.

The powder was sifted using standard sieves (270 and 325) to selectfor particles having diameters from 45 to 53 μm [nominal diameter of49 (±4) μm]. Reagent-grade liquids of BTE were purchased fromSigma-Aldrich (>99.99%). The liquids were dried using a 4 Åmolecular sieve (Fisher Scientific), which reduced the water contentbelow 5 ppb, as measured by the Karl Fischer titration (Aqua Counter300).

Before mixing with BTE, the calcite powder was baked at 300 °C for24 h to remove water from the surface of the particles withoutchanging their structure and chemistry.24 The dehydrated powder wascooled to room temperature in a vacuum desiccator (Bel-Art) to avoidinteractions with water vapor before mixing with BTE compounds. Tofacilitate the deconvolution of the IR spectrum, the dehydratedpowder was rehydrated in humid air overnight to saturate the calcitesurface with water molecules. This was performed by spreading thepowder evenly at the bottom of a sealed chamber that contained asmall beaker of deionized (DI) water.

The dehydrated calcite powder (400 mg) was mixed with 100 μL ofBTE liquid in a dry glass vial. The vial was then sealed and placed in anoven at 60 °C for 1.5 h. The elevated temperature facilitated thedispersion of BTE molecules and created a uniform surface coverage ofBTE molecules on the calcite particles. The rehydrated powder wasmixed with the BTE liquid at room temperature to prevent drying ofthe hydrated calcite.

To probe the cation−π interaction at the calcite−BTE interface,approximately 35 mg of the BTE-wetted powder was placed on thediamond crystal element of a FTIR spectrometer equipped with an

Figure 1. IR spectra of (a) calcite, (b, d, and f) liquids of BTE, and (c, e, and g) calcite surfaces absorbed with BTE molecules. Each spectrum ispresented with a full scan from 400 to 4000 cm−1 on the left and an expanded view from 640 to 770 cm−1 on the right. The wavenumber range forthe right panels is highlighted on the left panels in gray. The bands of the aromatic C−H out-of-plane bending of BTE molecules are fitted withLorentzian models. On the basis of the shifts, the Lorentzian fits are marked as I for the first layer of surface-bound molecules and E for the extendedlayers of surface-bound molecules, as compared to L for the molecules in liquids. The absorbance is scaled, so that the spectra could be plottedtogether. The inset structures represent the synchronized out-of-plane aromatic C−H bending modes (⊕, C; ⊖, H; and X = CH3 for toluene andC2H5 for ethylbenzene).

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ATR assembly (Bruker Tensor 27). The instrument was enclosed andoperated in a fume hood. IR spectra were acquired immediately afterthe powder was transferred from the sealed vial onto the diamondcrystal. The acquisition continued until the signals for surface-boundBTE molecules completely disappeared as a result of evaporation.Spectra ranging from 400 to 4000 cm−1 were acquired with aresolution of 1 cm−1. To identify the IR absorption bands unique tosurface-bound molecules, spectra were also obtained for pure BTEliquids and the pristine calcite powder for comparison. All spectra werereferenced to the bare diamond crystal.Geometry optimizations and single-point calculations were

performed using the Gaussian program package.25 To do so, a BTEmolecule was placed on top of the 101 4 cleavage surface of a calciteslab consisting of 18 CaCO3 units.

26 Calculations were performed withthe M06 functional and a 6-31G* basis set on all atoms.27,28 Fullgeometry optimizations of benzene, toluene, and ethylbenzene wereperformed in D6h, C1, and Cs symmetries, respectively. For the calcite−BTE structures [C6H6(CaCO3)n, C6H5(CH3)(CaCO3)n, andC6H5(C2H5)(CaCO3)n, where n = 18], constrained geometryoptimizations (with fixed terminal CO3) were performed withoutimposing any symmetry constraint. The calcite surface was held fixedwhile BTE geometries were fully optimized. Analytical harmonicfrequencies were calculated only for BTE compounds to ensure thateach was a minimum energy structure not characterized by anyimaginary frequency.

3. RESULTS

We probe the interaction of the calcite surface with BTE usingATR−FTIR. As shown in the left panels of Figure 1, dry calcitepowders treated with BTE molecules (c, e, and g) have IRabsorption bands attributable to both calcite (a) and BTEmolecules (b, d, and f). All of the vibrational modes of surface-bound BTE molecules can be readily identified by comparingthem to the vibrational modes of pure BTE liquids. Vibrationalmodes of pure BTE liquids are listed in Tables S2−S4 of theSupporting Information. Between BTE molecules in liquids andthose adsorbed on calcite, significant differences are found forthe out-of-plane bending of aromatic C−H bonds, δC−H. In thisvibrational mode, all of the aromatic C−H bonds (six forbenzene and five for toluene and ethylbenzene) bend togetherperpendicular to the aromatic ring (i.e., all H atoms go in andout of the paper together), as illustrated by the inset structures(X = CH3 for toluene and CH2CH3 for ethylbenzene).The δC−H vibrational bands for BTE compounds are located

between the wavenumbers of 640 and 770 cm−1. This range isshaded in gray in the left panels of Figure 1 and magnified inthe right panels. In the absence of calcite (panels b, d, and f ofFigure 1), the excitation of δC−H vibration requires light with a

wavenumber of κB = 667 cm−1 for liquid benzene, κT = 726cm−1 for liquid toluene, and κE = 744 cm−1 for liquidethylbenzene. These features do not overlap with anyvibrational absorption from calcite (Figure 1a) and, thus, arespecific for BTE compounds. Upon the adsorption of BTE bycalcite (panels c, e, and g of Figure 1), the wavenumbers ofδC−H vibration are shifted to greater values, indicating that thesimultaneous bending of aromatic C−H bonds are constrainedby the calcite surface. This suggests that the BTE molecules areadsorbed on the calcite surface in a face-down position. Thisorientation is equivalent to placing a calcium cation directlyabove the aromatic ring of a BTE molecule, favoring thecation−π interaction.Each of the δC−H bands associated with surface-bound BTE

molecules can be deconvoluted into two separate bands. Asshown in the right panel of panels c, e, and f of Figure 1, weinterpret the two deconvoluted bands as the absorption fromthe first (I) and extended (E) layers of adsorbed molecules.Least-squares regression using the Lorentzian function gives themean wavenumbers of κB(I) = 680 cm−1, κT(I) = 731 cm−1, andκE(I) = 745 cm−1 for the first layers of adsorbed molecules andκB(E) = 673 cm−1, κT(E) = 728 cm−1, and κE(E) = 744 cm−1 forthe extended layers. The devolution of δC−H bands into I and Elayers is supported by the changes of their intensities with time.As shown in Figure 2, the absorption intensities of all of the Ebands (normalized to the calcite band at 712 cm−1) decreasecontinuously as BTE molecules evaporate (at approximately 0.2g m−2 s−1).29 In comparison, the absorption intensities of all ofthe I bands remain stable until the end of measurement.The deconvulation of δC−H bands supports the hypothesis

that the first adsorption layers of BTE molecules interact withthe calcite surface strongly through the cation−π interaction.Under this interaction, the electron-poor surface calcium drawsπ electrons from the aromatic ring. The electron-withdrawingeffect extends to and strengthens the aromatic C−H bonds,resulting in an increase of excitation energy, as shown by anincreased wavenumber of the δC−H absorption band.37,38 Inaddition, the electron densities of aromatic rings are reduced,making it difficult for the C−H carbon atoms on the ring tofollow the motion of bonded hydrogen atoms through a partialtransition from the sp2 hybridization to the sp3 hybridization.30

In contrast, there is no cation−π interaction in the extendedadsorption layers. The relatively small amount of increase inexcitation energy may be attributed to the increase of the liquid

Figure 2. Change of the absorption intensity associated with the aromatic C−H out-of-plane bending with time for (a) benzene, (b) toluene, and (c)ethylbenzene. The normalized absorption, A, is obtained by dividing the band areas of the C−H out-of-plane bending for the first (I) and extended(E) layers of BTE molecules by the band areas of the calcite band between 700 and 720 cm−1. The solid line is an average of A for the first layer, andthe dashed curves are fitted to exponential functions.

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density next to an adsorbing surface,31 which hinders the δC−Hvibration by spatial blockage.To further validate our interpretation of the deconvolution of

δC−H vibrational bands, we modify calcite by saturating itssurface with water molecules before putting the calcite powderin contact with benzene. The surface modification is performedby leaving the calcite powder in an atmosphere with nearly100% relative humidity overnight. Under such conditions, morethan four layers of water molecules can condense on the calcitesurface.32−34 As shown in Figure 3a, the success of water

adsorption is confirmed by the observation of vibrational bandsbetween 3000 and 3700 cm−1 and at 1646 cm−1, which areassociated with the O−H stretching and bending of watermolecules, respectively.35 When the wet calcite is used toadsorb benzene, only one δC−H band is found at κB(E) = 673cm−1 associated with the extended adsorption layers. Theabsence of the signature band at κB(I) = 680 cm−1 is consistentwith the fact that the first layer is now occupied by watermolecules. These results are consistent with the reduction of

cation−π interaction energy upon the solvation of metal cationssuggested by ab initio calculations.36

The ATR−FTIR results presented above suggest that theshifts of δC−H bands can be used as an IR signature for probingthe cation−π interaction between mineral surfaces and aromatichydrocarbons. On calcite, the shift is most sensitive for theadsorption of benzene with a shift of ΔκB(I) = 13 cm−1, followedby a reasonable sensitivity for toluene of ΔκT(I) = 5 cm−1. Thesensitivity for ethylbenzene is the lowest among the three BTEcompounds, with ΔκE(I) = 1 cm−1. The reduction of ΔκT(I) andΔκE(I) compared to ΔκB(I) can be attributed to the substitutioneffect. In toluene and ethylbenzene, the substitution ofhydrogen by alkyls increases the aromatic π electron densitythrough the inductive electron-donating effect. The electron-donating effect of alkyl substitution balances the electron-withdrawing effect of the surface calcium, leading to theobserved reduction in the IR shifts of δC−H bands.To visualize the cation−π interaction, DFT calculations are

performed to verify the orientation and position of a BTEmolecule on the calcite surface. As shown in Figure 4, thecalcite surface is represented by a slab consisting of two layersof 3 × 3 CaCO3 units with an overall neutral charge. A BTEmolecule is then placed on top of the calcite slab for geometryoptimization. Calculations performed without structural con-straints and with full relaxation of the BTE−calcite complexeshave resulted in the dissociation of terminal CO3 units, possiblybecause of the limited size of the calcite slab. Because the calcitesurface is not expected to reconstruct significantly upon theadsorption of BTE molecules, the structure of the CaCO3 slabis constrained while the BTE molecules are being fullyoptimized.As shown in Figure 4, all three BTE molecules are similarly

arranged with their aromatic rings facing the exposed surfacecalcium cations. Each surface calcium ion is connected with fiveoxygen atoms from four surface carbonate ions and onecarbonate ion underneath; therefore, each of them has oneempty orbital from the 4s3d5 hybridization exposed at thesurface with 1/3 of an elementary charge. The aromatic rings ofBTE molecules are oriented perpendicular to the empty orbitalunder the cation−π interaction, so that their π electrons canenter the empty orbital of the surface calcium atom. Thearomatic rings are not centered over the calcium atoms but

Figure 3. IR spectra of (a) hydrated calcite and (b) benzene-coveredhydrated calcite. The gray bar marks the region corresponding to theband associated with the aromatic C−H out-of-plane bending ofbenzene. Each spectrum is presented with a full scan from 400 to 4000cm−1 on the left and an expanded view from 640 to 770 cm−1 on theright.

Figure 4. Positions and orientations of (a) benzene, (b) toluene, and (c) ethylbenzene on calcite optimized at the M06/6-31G* level. Eachoptimized calcite−BTE complex is shown with a side view on top and the top view at the bottom. Atoms are colored with gray for C, white for H,red for O, and green for Ca.

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rather are shifted toward the recessed surface oxygen atombonded to the calcium atom. The methyl group of toluene ispositioned next to the recessed surface oxygen and pointingtoward an adjacent calcium. The ethyl group of ethylbenzene isarranged similarly as the methyl group of toluene. Thegeometry parameters for the optimized BTE−calcite structuresare listed in Table S5 of the Supporting Information. Overall,the BTE molecules are adsorbed on the calcite surface in tiltedface-down positions.The total interaction energies with calcite are estimated to be

36.8, 48.5, and 54.8 kJ mol−1 for BTE from single-pointcalculations. Each of the interaction energies is expected toinclude contributions from electronic and dispersive inter-actions as well as from the interaction between the surfacecalcium cation and the quadrupole moment of the aromaticring. Using a simple classical model (see Note S1 of theSupporting Information),37 the cation−quadrupole energy maybe estimated from a function of the quadrupole moment Q of aBTE molecule, its distance d to the nearest surface calcium, andthe angle θ defining the position of calcium off the center of thearomatic ring

θ θ= − −E dQ

d( , )

48(3 cos 1)3

2

(1)

where Q = −8.099, −7.610, and −7.828 Buckingham (1Buckingham = 1 Debye Å = 3.34 × 10−40 C m) for benzene,toluene, and ethylbenzene, respectively,38 d ≈ 3.4 Å, and θ =25° (cf. Table S5 of the Supporting Information). Using thesevalues, we obtain EB = 15.0 kJ mol−1 for benzene, ET = 13.2 kJmol−1 for toluene, and EE = 13.9 kJ mol−1 for ethylbenzene,suggesting that the cation−quadrupole energy is similar for allthree BTE compounds. On a relative scale, the cation−quadrupole energy contributes 41, 27, and 25% of the totalinteraction energy with calcite for benzene, toluene, andethylbenzene, respectively. The decreasing trend is consistentwith the increasing interaction between the substituent groupand the surface as the substituent group increases in size.

4. DISCUSSIONATR−FTIR measurements presented above show that BTEmolecules placed next to the calcite surface are organizeddifferently from the molecules in bulk liquids, revealing surface-induced liquid restructuring under the cation−π interaction.Elucidating the mechanism of liquid restructuring near surfacesis fundamentally important to understand and modelheterogeneous systems involving the two phases. Previously,studies employing innovative analytical techniques haverevealed surface-induced restructuring of water,39 mercury,40

alkanes,41 alcohols,42 and ionic liquids.43 Through these studies,solvation force,39 hydrogen bonding,39,42 surface tension,40 vander Waals attraction,41 and electrostatic attraction43 have beenidentified as important surface−liquid interactions responsiblefor inducing liquid restructuring. Liquid restructuring under thecation−π interaction has not been previously proposed orobserved.The revelation of the cation−π interaction as an important

mechanism of surface−liquid interaction may help betterunderstand the interfacial processes involved in oil recoveryfrom carbonate reservoirs, where 60% of the world’s oil isstored.44 Clean carbonate minerals have hydrophilic surfacesthat should not interact strongly with hydrophobic liquids, suchas crude oil. However, conventional technologies, such as waterflooding, can only recover 30% of the oil trapped in carbonate

reservoirs.45,46 The low recovery efficiency is attributed to thepreferred wetting of carbonate surfaces in reservoir pores by oil,which prevents the displacement of oil by water under capillarypressure and buoyancy through a process called spontaneousimbibition.47 In the past, the adsorption of amphiphilicnaphthenates by carbonate surfaces has been proposed as themechanism responsible for surface hydrophobicity in carbonatereservoirs.48 The strong cation−π interaction shown heresuggests that neutral aromatic hydrocarbon molecules, whichaccount for up to 30% of crude oil,49 can also contribute tocarbonate surface hydrophobicity.In addition to oil recovery, the cation−π interaction may also

have important implications to the transport and trans-formation of aromatic hydrocarbons in the environment.Aromatic hydrocarbons, such as BTE compounds, can entersoils and sediments by leakage or spills at facilities forproducing, delivering, and distributing oil and oil products.50

The leakage and spill of oil and oil products can also releaseBTE compounds, given their high volatility, into theatmosphere, where they can interact with carbonate-containingaerosol particles originated from dust storms.51−54 Our resultssuggest that the cation−π interaction is likely to occur underlow relative humidity in soils and the atmosphere, where themineral surface is not completely covered by water. For calcite,the incomplete coverage of the cleavage (101 4) surface occursunder relative humidities below 50%,32−34 a condition that isnot difficult to encounter.The simple DFT calculations performed with a single BTE

molecule and a slab of 18 CaCO3 units show that BTEmolecules are organized in tilted face-down positions next tothe calcite surface as a result of the rugged organization ofsurface calcium and carbonate. The tilted orientation of BTEmolecules on calcite is similar to that of aromatic methyleneblue adsorbed on mica.55 Preliminary analysis of energeticssuggests that direct cation−quadrupole interactions contributesignificantly to the total interaction energies for all three BTEmolecules adsorbed on calcite. Further elucidation of the natureof the cation−π interaction involving mineral surfaces requiresthe use of molecular models better than the model that we haveused to represent the BTE−calcite system.

5. SUMMARY

Using calcite and BTE compounds as model systems, we haveshown that the strong cation−π interaction between a mineralsurface and a neutral molecule can lead to an appreciableincrease of energy required for the excitation of the out-of-plane aromatic C−H vibration. The increase of excitationenergy can be probed using ATR−FTIR, providing a uniquesignature for recognizing the previously underappreciated non-covalent interaction at the mineral surface. We show that the IRsignature is most sensitive for unsubstituted aromatic rings.Alkyl substitution reduces IR sensitivity because of its inductiveelectron-donating effect.

■ ASSOCIATED CONTENT

*S Supporting InformationImpurities in calcite (Table S1), vibrational modes of benzene(Table S2), vibrational modes of toluene (Table S3),vibrational modes of ethylbenzene (Table S4), selectedgeometry parameters of calcite−BTE complexes (Table S5),and estimation of the energy of the cation−quadrupoleinteraction by classical electrostatics (Note S1). The Support-

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ing Information is available free of charge on the ACSPublications website at DOI: 10.1021/acs.langmuir.5b00610.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: chongzheng.na@gmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSChongzheng Na acknowledges financial support from thedonors of the American Chemical Society Petroleum ResearchFund, National Science Foundation Environmental EngineeringProgram, and Notre Dame Sustainable Energy Initiative. Theauthors thank Notre Dame Center for Environmental Scienceand Technology for analytical assistance and Jian Lin forperforming ATR−FTIR experiments.

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