Dolomite Surface - ACS Publications - American Chemical Society

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Adsorption of Molecules onto (101̅4) Dolomite Surface: An Application of Computational Studies for Microcalorimetry Elizabeth Escamilla-Roa,*,† C. Ignacio Sainz-Díaz,‡ F. Javier Huertas,‡ and Alfonso Hernández-Laguna‡ †

Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain Instituto Andaluz de Ciencias de la Tierra (CSIC-Universidad de Granada), Av. Las Palmeras 4, 18100 Armilla, Granada, Spain

‡

ABSTRACT: Dolomite (CaMg(CO3)2) is a sedimentary mineral that is found in the majority of natural carbonate rocks. Dolomite has been used in the treatment of polluting agents, and as a potential adsorbent of CO2. Dolomite is usually associated with oil deposits. Secondary oil recovery is a problem related to the adsorption processes of hydrocarbons onto rocks covering the rock surface as a film. The aim of the present study is to investigate the adsorption processes of water and several organic molecules (hexane, cyclohexane, cyclohexene, and benzene) onto the most stable (101̅4) dolomite surface by means of Density Functional Theory (DFT) calculations for microcalorimetry applications. One molecule was adsorbed per surface, and different adsorption configurations were explored. Adsorption energy (AE) of water was 16.27 kcal/mol, whereas the AE of the different organic compounds investigated ranged between 3.14 and 5.00 kcal/mol, being much lower than water due to the lack of H bonding and electrostatic interactions that are present in the adsorption of water. Surface tension (σ) was calculated in all adsorption complexes. The difference of σ between the pristine surface and the adsorption complexes was correlated with the experimental enthalpy of immersion, finding a good agreement.

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been related to carbonates minerals, such as dolomite.8−10 Secondary oil recovery is strongly related to the adsorption processes of oil molecules onto rock; these molecules cover the rock surface and form oil films, and in some cases they form jams in oil drillings. The volume of oil film may account for up to 20% of the total pore volume and is an important part of the residual oil.11 The understanding of interactions of the chemical components of crude oil with hydrophilic surfaces of rocks is important for the secondary oil recovery techniques and to eliminate jams in drilling. For these purposes, the knowledge of interfacial phenomena is very important; nonetheless, they are very complex because they depend on the chemical composition of oil, the physical and chemical properties of the surfaces, and the heterogeneity of the minerals. When a rock is fully covered with crude oil, which is immiscible with water, the process of water penetration in the rock, named displacement, depends on pore size and the affinity of water and oil to the porous rocks saturated with crude oil.12−16 The interactions of the chemical components of crude oil with the surface of the rocks have been investigated by means of calorimetric techniques measuring the immersion enthalpy of organic molecules on several mineral surfaces.17,18 The surface structure, features, and stability of dry and hydrated dolomite surface were investigated with experimental19 and computer modeling methods.5,20−24 Theoretical calculations explored different surfaces of this mineral finding

INTRODUCTION Carbonates are a type of sedimentary rocks extensively present in the Earth’s crust. They are comprised of carbonate minerals such as calcite (CaCO3), dolomite (CaMg(CO3)2), magnesite (MgCO3), and siderite (FeCO3). Calcite and dolomite account for more than 90% of the natural carbonates in rocks.1 The origin of dolomite in sedimentary conditions is not well-known yet. Several experimental and theoretical studies about the growth mechanism of dolomite under sedimentary conditions have been reported.2 A biomineralogical origin has been proposed, where a bacterial activity can transform the mixed carbonate mineralogy to dolomite and probably the interactions of organic molecules onto mineral surface can be important.3 The structure of dolomite is taken from the calcite group structure, with the amounts of calcium and magnesium being similar. The general formula of this group is AB(CO3)2, where A can be calcium, barium, and/or strontium and B can be iron, magnesium, zinc, and/or manganese.4 Magnesite and dolomite are two salt-type minerals with crystal structure and surface properties similar to those of a natural hydrophilic material. The dolomite crystal structure is similar to calcite (CaCO3) and magnesite (MgCO3), but with the loss of the c-glide.5 It comprises alternating planes of Mg2+ and Ca2+ ions, separated by carbonate planes in the c direction that result in a hexagonal lattice belonging to the R3̅ space group, with the lattice parameters a = b = 4.81 Å and c = 16.01 Å.6 The dolomite has been used in the treatment of polluting agents for removal of acidic gaseous pollutants in the removal of copper(II) from aqueous solution and as a potential sorbent of CO2.7 On the other hand, some deposits of petroleum have © 2013 American Chemical Society

Received: May 7, 2013 Revised: July 19, 2013 Published: July 26, 2013 17583

dx.doi.org/10.1021/jp404529e | J. Phys. Chem. C 2013, 117, 17583−17590

The Journal of Physical Chemistry C

Article

⎡ ∂(σS − σSL) ⎤ −ΔHimm = σS − σSL − T ⎢ ⎥ ⎣ ⎦p ∂T

that the most stable surface of dolomite is the (1014̅ ) one.19,20 Fenter et al.19 explored experimentally the reactivity of water of this surface describing the structure and disposition of water molecules on the surface. However, the experimental exploration of the nanostructure of liquids in the interface with the mineral solid surface is not easy, and computational atomistic studies can be useful for this investigation. Other theoretical calculations studied the adsorption and reactivity of water on this dolomite surface describing the geometrical features of hydrated surface. Most of the calculations were based on empirical interatomic potentials, and several differences were found in the determination of the energy surface for the dry and hydrated dolomite surfaces.21−23 This research requires large and complex systems for describing the liquid− solid interface, and it is a challenge to explore it at first principles level. Few ab initio quantum mechanical studies have been used for the study of adsorption on the surface/water interface of TiO225,26 and other minerals.26,27 The aim of the present study is to investigate, by means of computational studies based on the Density Functional Theory (DFT), adsorption processes of several organic molecules coming from petroleum with the most stable surface of dolomite, (101̅4), through the following parameters: adsorption energy, surface energy, surface tensions, and enthalpies of interface solid−gas. These molecules can be considered as models of interaction of organic molecules on the surface of dolomite, and our results can help to interpret experimental results obtained by microcalorimetry and others based on the interactions between minerals and organic molecules to improve oil recovery.

The surface energy per area unit (γ) was calculated as a measure of the thermodynamic stability of the surfaces as follows:25 γ=

(4)

G cleansurface=E surface − Ebulk

(5)

surface

where E is the total energy of the relaxed surface, and Ebulk refers to the total energy of a bulk crystal with equal number of atoms and chemical species to the pristine surface. In the adsorption processes, the geometry of the adsorbate molecule (water, benzene, hexane, cyclohexane, and cyclohexene) was optimized alone within a large periodical box with the same size of the adsorption complex periodic system at constant volume. This molecule was also optimized with the periodical surface of mineral at constant volume with the same crystal cell parameters. Analogously, in the surface with adsorbate, the surface energy Gad_surface will be: G ad_surface=E surface + adsorbate − Ebulk − E adsorbate

(6)

surface+adsorbate

where E is the total energy of the relaxed surface with the adsorbate, Ebulk refers to the energy of the bulk crystal with equal number of atoms and chemical species to the surface model, and Eadsorbate is the energy of the adsorbate molecule. Different adsorption configurations of the organic molecules on the surface were tested to determine the effect of the adsorption process on surface tension (σ). To calculate σ, we applied a two-dimensional dilation to the slab in the plane of the surface. The ΔG is calculated for a set of area dilations (0.0−1.5% of the total area), and all atomic positions of each slab dilated were optimized. For any area dilation, we calculate the new u and v surface lattice parameters that correspond to ΔA, considering a constant u/v ratio. By plotting the new energies ΔG with respect to ΔA, the surface tension was estimated from the slope of the linear least-squares fitting with the intercept through zero (considering that the change in free energy is zero for an unstressed area). This methodology has been used in previous studies of morphology on TiO225 and the adsorption process of several molecules onto TiO2 surfaces.25,32,33 Finally, we have calculated the enthalpies using the equation of immersion enthalpy (ΔHimm) used in adsorption experiments (eq 3).17,29,30 In our calculations, temperature (T) is a constant value (third term equal to zero). Besides, we calculate these systems in vapor phase because we cannot simulate the liquid phases. Next, we calculate the solid−vapor interface as an approximation to ΔHimm (eq 3) for relative comparison purposes.

METHODOLOGY Enthalpy of Immersion. Experimentally, the adsorption process of molecules on mineral surfaces has been studied by thermodynamic methods, such as the surface Gibbs free energy of solids,28 and can have a strong relation with chemical modifications that occur in the surface due to interfacial processes, such as a new bond created in the chemisorption or hydrogen bonds in physisorption processes. From experiments using immersion microcalorimetry,17,29,30 the surface Gibbs free energy and immersion enthalpy can be calculated. The enthalpy of immersion (ΔHimm) measures the small heat quantities that evolve when a dry solid is immersed in a liquid.31 In these microcalorimetry experiments, the bare surface, a pure liquid (or a mixture), and its vapor (or vacuum)17,29,30 are considered to be a good approximation for estimating the interactions between a solid and a liquid. The energy of adhesion Wadh31 can be expressed as a function of surface tensions: (1)

The term σ is defined as the surface tension of the solid (σS), liquid−vapor (σLV), and solid−liquid (σSL) interface. At constant volume (V) and pressure (P), the expression of σ as a function of the Gibbs free energy variation for adsorption is: σ = ΔG /ΔA

G A

In our DFT calculation conditions (0 K), the free energy of the slab surface can be approached to:25

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Wadh = σS + σLV + σSL

(3)

(2)

−ΔHtheoretical = σS − σSV

where σ is the surface tension, ΔG is the variation of free energy (G) of the bare slab surface or with organic molecule, and ΔA is the variation of surface area (A). Previous experimental work assumed that the free surface energy is equivalent to the surface tension17 and the ΔHimm was related to surface tensions:

(7)

Computational Methods. Geometric parameters and total energy of dolomite bulk, free surface, and organic molecules isolated and adsorbed onto dolomite surface were calculated using DFT with periodical boundary conditions to simulate a periodical crystal lattice. Given the fast development of dispersion methods within DFT and the lack of a general 17584



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assessment on their beneficial effects,34,35 we decided to rely upon standard GGA functional, for which performances and limitations are well established. We used the generalized gradient approximation (GGA) with RPBE exchange correlation functional.36 All electronic calculations were performed with a double-ξ basis set with polarization functions for all atoms and semicore pseudopotentials (DSPP). Previous authors37 have confirmed that the magnitude of BSSE in the numerical basis set in DMol3 is very small and negligible as expected in comparison with the Gaussian basis set. Therefore, we have not considered the use of counter-poise method to calculate the BSSE due to the high computational cost. This approach was considered previously with satisfactory results.32,38 The convergence criterion for the self-consistent field was 1 × 10−6 hartrees. To improve the SCF convergence, a smearing of 0.005 hartree was used in all calculations. The calculations were performed with different k-points sampling within the Brillouin zone. The geometries of the surfaces and adsorbed systems were optimized to reach a maximum force of 0.002 hartree Å−1, and a maximum displacement of 0.005 Å between the optimization cycles. To explore the weak van der Waals interactions in the adsorption complexes, corrections with the semiempirical formalism of Grimme39 for the weak dispersion forces were included in some calculations for comparative purposes. All DFT calculations were performed with the Dmol3 program40 within the Materials Studio (MS) package.41

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RESULTS AND DISCUSSION Structure of Bulk and Clean Surface (101̅4). The bulk structure of the optimized dolomite crystal (Figure 1a) has the cell parameters a = b = 4.92 Å, c = 16.63 Å, α = β = 90°, and γ = 120°, close to the experimental values (a = b = 4.81 Å, c = 16.01 Å, α = β = 90°, and γ = 120°)42 with only slight deviations from experimental parameters (2.42% and 3.88% for a = b, and c, respectively). The average C−O, Ca−O, and Mg−O bond lengths calculated by radial distribution function are 1.30, 2.46, and 2.15 Å, respectively, which is consistent with the experimental values of 1.29, 2.38, and 2.08 Å, respectively.42,43 Nevertheless, the calculated values are slightly longer than the experimental ones. Both cations are 6-fold coordinated and O atoms are 3-fold (3f) coordinated forming regular octahedra with the same M−O distance for each cation. This bulk crystal structure of dolomite shows CO32− layers sandwiched by Ca2+ and Mg2+ layers (Figure 1a). Previous theoretical and experimental works confirmed that the (101̅4) surface is the most stable.20,44 This surface was created from the optimized bulk crystal by cleaving the optimized bulk structure along the (101̅4) plane. One 2-D 1 × 2 × 1 supercell slab was generated, which contains four layers of CO32− with ions of Ca2+ and Mg2+ (slab model). In this case, the slab has a vacuum space 14 Å over the surface. Taking into account the stacking sequence of these (101̅4) planes (Figure 1b), we have considered the surface termination where the carbonate groups remain complete and the O, Ca, and Mg atoms are exposed to the adsorption (Figure 1c). This (1014̅ ) surface slab was fully optimized, resulting in the crystal surface parameters u = 8.05 Å and v = 9.96 Å. One section of this minimized slab containing two CO32− layers was afterward optimized maintaining the deeper layers fixed. This optimized model was used for the adsorption studies. The cleavage of this surface produces an under-coordination of 5-fold (5f) for Mg and Ca cations, creating a slight distortion of the Mg−O and Ca−O bond

Figure 1. Bulk crystal structure of dolomite (view of a 3 × 3 × 1 supercell) (a), a series of parallel (101̅4) dolomite planes (b), and our clean (101̅4) dolomite surface (c). The O, C, Mg, and Ca atoms are displayed as red, gray, yellow, and green spheres, respectively.

lengths of the surface with respect to the crystal lattice. Hence, the O atoms have different coordinations. The topmost O atoms (Ot) have a 2-fold (2f) coordination. The second layer 17585



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adsorption energy (16.27 kcal/mol) than the organics, due to the stronger interaction with mineral surface, where the water O atom (Ow) is coordinated to the surface Mg cation at 2.18 Å and the water H atoms show hydrogen bonding with the surface O atoms at 2.15−2.31 Å. This adsorption energy is higher, due to the hydrophilic character of dolomite, than that obtained experimentally in the water adsorption on mica surfaces 13 kcal/mol.45 In the optimized adsorption complex, the water molecule adopts a parallel disposition (Figure 2a) with respect to the surface according to previous theoretical results.23,44 However, these previous calculations were based on empirical interatomic potentials and showed the water adsorbed at longer distances to the surface (Mg···Ow = 3.02 Å) than our results. Our results are closer to experimental studies of the water−dolomite interface where a distance of water molecule layer to the metal layer of dolomite surface was found to be about 2.26 Å.19 Considering the zero-point energy (ZPE) correction for the water adsorption complexes and clean surface and water molecule, ΔZPE = 2.37 kcal/mol. Hence, the adsorption energy will be −13.90 kcal/mol, consistent with previous studies of water adsorption on the TiO2 surface.26 On the other hand, to explore the application of other methods that can describe the low dispersion interactions of the adsorption processes, we applied the semiempirical Grimme dispersion correction for a single-point energy PBE/GGA DFT calculation (DFT-D)39 on the structures optimized previously above, following the procedure of previous studies about adsorption on other metal oxides,46 and the adsorption energy was slightly higher than without this correction (18.45 kcal/ mol). Besides, we explored another approach to include the dispersion correction by fully optimizing adsorption complex and reactant with PBE/GGA + Grimme correction; however, this yielded too high adsorption energy (27.9 kcal/mol). This means that this Grimme dispersion correction overestimates the weak interactions in our system according to previous works.26 The adsorption energies of alkane adsorbates were drastically lower than that of water, being 3.14 kcal/mol for hexane and 3.50 kcal/mol for cyclohexane. These molecules are more separated from the surface than water with some H atoms oriented to the O atoms of the top-surface at a CH···O distance of 3.2−4.2 Å (Figure 2b and c). The adsorbates with π CC bonds show a slightly higher adsorption energy (5.00 kcal/mol for cyclohexene) than alkanes. In this complex, the π bond is oriented to the Mg atom of the top-surface, and the H atoms are oriented to the O atoms of the top-surface at a CH···O distance of 2.7−3.2 Å (Figure 2d). In the case of benzene, two initial adsorption configurations with respect to the surface were considered for the adsorption, parallel and perpendicular to the surface, yielding similar adsorption energies (3.5 and 4.6 kcal/mol, respectively). Hence, we consider the average values of both configurations assuming a similar probability for both in a Brownian movement of the molecule with respect to the surface during the adsorption. These adsorption energies of organic molecules on a dolomite surface are in the range of weak interactions between organics without polar functional groups and mineral surfaces.47 The application of the semiempirical Grimme dispersion correction for single-point total energy PBE/GGA DFT calculations (DFT-D) on the structures optimized previously above yielded too high adsorption energies (9.03, 8.18, and 12.39 kcal/mol for hexane, cyclohexane, and cyclohexene, respectively) for such weak interactions. On the other hand,

of O atoms is in the same level of the surface cations with slight deviations maintaining a 3f coordination (with one C atom and two cations), and we named them as basal O atoms (Ob). The third level of the O atoms is less exposed to the surface in the inner part of the surface cations octahedra, and we named them as apical O atoms (Oa). In Table 1, we show the main M−O Table 1. Main Interatomic Distances (in Å) of the Clean Surface and Bulk of Optimized Crystal Structure of Dolomite distancea

Mg−O

Ca−O

M−Ot M−Ob M−Oa M−Obulk

2.10 2.17 2.08 2.15

2.41 2.51 2.38 2.46

M is the five-coordinated cation close to the surface; Ot is the O atom placed in the topmost position of the surface; M−Ob is the average value of the distances between the cations of the surface and the basal O atoms; Oa is the O atom placed in the apical position of the partial octahedral of the cations from the first layer of the surface; and M−Obulk means the M−O bond lengths of the cation octahedra in the bulk. a

interatomic distances of the surface and bulk of dolomite. After formation and optimization of the surface, the Mg and Ca atoms have different bond distances with oxygen atoms, whereas in bulk there is only one value of M−O bond distance for each cation from the regular octahedra. The Mg−O distance with the oxygen located at the topmost position (Ot) of the surface is shorter than in the bulk due to the low-coordination of this oxygen. On the other hand, the average value of the Mg−O bond length with the basal oxygens (Ob) is similar to that of the bulk. The Mg−O bond length with the apical Oa atom is shorter than in the bulk due to the low coordination of this Mg cation. This behavior is also observed in the Ca−O bond distances. The surface energy was 0.86 J/m2, which is consistent with the previous theoretical values of 0.64 J/m2.5 Adsorption of Small Molecules onto (101̅4) Dolomite Surfaces. We modeled the adsorption of organic (benzene, hexane, cyclohexane, cyclohexene) and water molecules with the (101̅4) dolomite surface, which can be representative of the petroleum/water system (Figure 2). The adsorption of one molecule onto the surface was modeled, simulating the interaction of a monolayer of organic molecule with the surface, which means coverage of one molecule per surface area (1.25 mol/nm2). In the starting configuration, the adsorbate was placed on the center of the supercell at 4 Å over the top surface exploring different initial dispositions of the adsorbate with respect to the surface. The geometry of each molecule along with the surface mineral was optimized following the same procedure applied to the clean surfaces. The adsorption energy was calculated as: Eads = −(Emol/surface − E cleansurface − Emol) mol/surface

cleansurface

(8)

mol

where E ,E , and E are the total energies of the dolomite (101̅4) surface with the adsorbate on the surface, the bare dolomite surface, and one isolated molecule of adsorbate, respectively. All organic molecules show low adsorption energies (Table 2) due to the weak interactions existing between surface and adsorbate. However, the water molecule yields a higher 17586



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Figure 2. The most stable positions of the adsorption complexes onto mineral surface: water (a), hexane (b), cyclohexane (c), cyclohexene (d), and benzene (e).

Table 2. Adsorption Energy, Surface Energy (γ), Surface Tension (σ), and Immersion Enthalpy (ΔHimm) of Several Adsorbates onto the Dolomite Surface As Compared to Experimental Data17 surface + adsorbates

adsorption energy (kcal/mol)

γ (J/m2)

σ (J/m2)

ΔHimm,theor (J/m2)

ΔHimm,exp (J/m2)

clean surface water cyclohexene benzene cyclohexane hexane

16.27 5.00 4.05 3.50 3.14

0.86 0.72 0.82 0.82 0.83 0.83

5.93 4.36 5.23 5.41 5.67 5.62

1.57 0.69 0.52 0.25 0.30

1.25 0.60 0.45 0.17 0.30

adsorption on mineral surface studies,26 and this semiempirical correction does not reproduce well the weak interactions on our dolomite surface; similar problems were found recently in benchmark studies in gas−water systems.34,35 Nevertheless, we find a linear relationship between the adsorption energy (with Grimme correction) and the σ (Figure 3b), indicating that the tendency between adsorbates is similar to that without this

optimizing the adsorption complexes and reactants with PBE/ GGA+Grimme correction, we obtained adsorption energy values (11.52, 14.04, 17.3, and 20.6 kcal/mol for hexane, cyclohexane, benzene, and cyclohexene, respectively). Like in the case of water adsorption, all values with this correction are too high. Hence, this dispersion correction overestimates the effect of van der Waals interaction as in previous organic 17587



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We calculated the surface tension following the procedure described above in the Methodology section (eq 2). For all cases, a linear relationship was observed between the variation of surface energy and the variation of area obtaining a good correlation degree (R = 0.9986, 0.9763, 0.9895, 0.9989, 0.9980, and 0.9842, for clean surface, and water, cyclohexene, cyclohexane, hexane, and benzene adsorption complexes, respectively). From the slopes of these correlations, the surface tension values were obtained (Table 2). We observed that in general there is an inverse relationship between adsorption energy and surface tensions (σ) (Figure 3). When the adsorbate is added to the mineral surface, the energy surface decreases, and the relaxation degree in the top layer and the layers beneath will have less need to reorganize and hence they will support less stress and σ will be lower. Within the organic molecules series, a linear relationship adsorption energy versus σ is observed, distinguishing the alkanes from the alkenes in two zones (Figure 3). This can be explained by the interactions of the π electrons of alkenes with the surface by electrostatic interactions with the cations exposed in the surface. Taking into account this linear correlation, zero adsorption energy yields a σ value greater than that of a clean surface. This can be due to the lack of describing the weak dispersion interactions of the method used. However, including the water adsorption (Figure 3), the intercept of the linear correlation is σ = 5.86 J/m2, which is close to that of a clean surface (Table 2). This fact can validate the theoretical approach used in our calculations to describe our system, where different interactions are included (weak and strong forces). On the other hand, we observed that the effect with water is greater than that with organics, due to the hydrophilic behavior of the surface, where the surface interactions (hydrogen bonds, strong electrostatic forces) are very different from the organics. Probably other organics with polar functional groups will have an intermediate behavior between water and our organic series. From these σ values, the immersion enthalpy was calculated following eq 7, consistent with the experimental data (Table 2). A linear relationship is observed between the calculated adsorption energy and immersion enthalpy for organics adsorption (Figure 4). We observe an increase of the immersion enthalpy with the adsorption energy. Therefore, the calculation of adsorption energy can give us one fast approximation about the microcalorimetric measurements of the immersion enthalpies. On the other hand, comparing the calculated immersion enthalpy values with the experimental ones, a good linear correlation is found (R = 0.99616) (Figure 5). This means that the variation of macroscopic calorimetric properties can be explained by the corresponding variation in the interatomic interactions between adsorbate and mineral surface.

Figure 3. Relationship of surface tension (σ) with respect to adsorption energy calculated with RPBE (a) and PBE + Grimme correction (b). The labels chexa, hexa, ben, and chexe mean cyclohexane, hexane, benzene, and cyclohexene, respectively.

correction. On the other hand, the interactions existing between the atoms of the solid surface and bulk are not weak, and the contribution of the dispersion forces is very small. Hence, all calculations of the variation of surface tension and the immersion enthalpy are related to the solid surface, and this dispersion correction cannot be applied properly. Indeed, we have calculated the surface tensions including the Grimme correction, obtaining values of immersion enthalpy completely far from the experimental (0.26 and 2.05 J/m2 for ΔHimm of water and hexane, respectively). To study the effect of adsorbed molecules onto the surface, eqs 4, 5, and 6 were used to calculate the surface energy (γ) for the clean surface and the surface with adsorbates. We observed that the surface energy decreases more with respect to the clean surface when the adsorbate is strongly adsorbed, such as with the water molecule (Table 2). The energy surface of the complex with organic molecules is similar for all adsorbates. This means that the adsorbate stabilizes the mineral surface decreasing γ. In the case of water adsorption, the water oxygen is coordinated with the cation exposed to the surface forming a more stable octahedral cation shell, stabilizating the surface and decreasing the surface energy.

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CONCLUSIONS We have presented a theoretical study of the adsorption process of water and several organic molecules onto the (101̅4) dolomite surface. Our calculations have corroborated previous experimental results of the determination of a distance of 2.26 Å between water layer and dolomite surface. Organic molecules are adsorbed to mineral surface through a weak interaction; only the water molecule is more strongly adsorbed as a result of the hydrophilic character of this mineral. Several approximations for describing dispersion forces have been reported, and no clear preference can be chosen. A benchmark study of all 17588



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the behavior of organic adsorbates with polar functional groups in their adsorption on dolomite surface. This behavior will be intermediate between our organics series and water. This theoretical approach can be useful for experimental adsorption investigations, and it can be applied to any interactions of molecules onto other surfaces of dolomite and other minerals or solids. This study can be interesting for environmental, surface, and nanotechnology research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are especially thankful to Dr. D. Costa for her useful discussions, and to the “Centro Técnico de Informática” of CSIC, and the “Centro de Supercomputación de la Universidad de Granada” for allowing the use of its computational facilities, to the Andalusia Government PAIDI group RNM363, and contract FQM-4555 (Proyecto de Excelencia, Junta de ́ to F. Moreno for his help, and to Spanish Agency Andalucia), CGL2010-20748-C02-01 project for financial support. E.E.-R. is thankful to the Institute of Science and Technology of Mexico City for financial support and D. Rafael Esteso and D. Rafael Bellver for their help with the graphics.

Figure 4. Relationship of adsorption energy with respect to the calculated immersion enthalpy of organic molecules. The labels chexa, hexa, ben, and chexe mean cyclohexane, hexane, benzene, and cyclohexene, respectively.

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REFERENCES

(1) Reeder, R. J. Crystal Chemistry of the Rhombohedral Carbonates. Rev. Mineral. Geochem. 1983, 11, 1−47. (2) Krauskopf, K. B. Introduction to Geochemistry; McGraw-Hill: New York, 1967. (3) Sánchez-Román, M.; Vasconcelos, C.; Schmid, T.; Dittrich, M.; McKenzie, J. A.; Zenobi, R.; Rivadeneyra, M. A. Aerobic Microbial Dolomite at the Nanometer Scale: Implications for the Geologic Record. Geology 2008, 36, 879−882. (4) Boynton, R. S. Chemistry and Technology of Lime and Limestone; Wiley: New York, 1980. (5) de Leeuw, N. H. Surface Structures, Stabilities, and Growth of Magnesian Calcites: A Computational Investigation from the Perspective of Dolomite Formation. Am. Mineral. 2002, 87, 679−689. (6) Ross, L. N.; Reeder, J. R. High-Pressure Structural Study of Dolomite and Ankerite. Am. Mineral. 1992, 77, 412−421. (7) Readman, J. E.; Blom, R. The Use of in Situ Powder X-Ray Diffraction in the Investigation of Dolomite as a Potential Reversible High-Temperature CO2 Sorbent. Phys. Chem. Chem. Phys. 2005, 7, 1214−1219. (8) Gregg, J. M. Basin Fluid Flow, Base-Metal Sulphide Mineralization and the Development of Dolomite Petroleum Reservoirs; Geological Society, Special Publications: London, 2004; Vol. 235, pp 157−175. (9) Clark, D.; Heaviside, J.; Habib, K. Reservoir Properties of Arab Carbonates, Al Rayyan Field, Offshore Qatar; Geological Society, Special Publications: London, 2004; Vol. 235, pp 193−232. (10) Burchette, T. P. Carbonate Rocks and Petroleum Reservoirs: A Geological Perspective from the Industry; Geological Society, Special Publications: London, 2012; Vol. 370, pp 17−37. (11) Lu, G.; Zhang, X.; Shao, C.; Yang, H. Molecular Dynamics Simulation of Adsorption of an Oil-Water-Surfactant Mixture on Calcite Surface. Pet. Sci. 2009, 6, 76−81. (12) Cuiec, L. Mouillabilité et Réservoirs Pétroliers. Oil Gas Sci. Technol. 1986, 41, 487−509. (13) Cuiec, M. L. Recommandations Pour la Détermination de la Mouillabilité d’un Échantillonde Roche Réservoir. Oil Gas Sci. Technol. 1978, 33, 907−914.

Figure 5. Comparison of calculated and experimental values of immersion enthalpy (ΔHimm). The labels chexa, hexa, ben, and chexe mean cyclohexane, hexane, benzene, and cyclohexene, respectively.

different methods for dispersion interaction correction for our systems is out of the scope of this work. Nevertheless, a similar study will be interesting for future investigations in surface science, because it will help to choose an appropriate approximation for describing H bonds and weak electrostatic and van der Waals interactions on surface adsorption processes. Therefore, further calculations with other methods for describing dispersion forces contribution, as a benchmark study, in our systems should be explored. With respect to the effect of the adsorption process on the surface, our theoretical immersion enthalpies on the solid−gas interface are consistent with the experimental values of immersion enthalpy. This validates our approach of atomistic calculation at quantum mechanical level along with the calculation of the immersion enthalpy from surface tensions for comparison with experimental microcalorimetry studies of immersion enthalpies. Our calculations can predict qualitatively 17589



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Dolomite Surface - ACS Publications - American Chemical Society

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