Distinctive Diffusion Regimes of Organic Molecules in Clays: (De

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Distinctive Diffusion Regimes of Organic Molecules in Clays: (De)Coupled Motion with Water Eduardo Duque-Redondo,*,† Iñigo Loṕ ez-Arbeloa,† and Hegoi Manzano‡ †

Molecular Spectroscopy Laboratory, Department of Physical Chemistry, and ‡Department of Condensed Matter Physics, University of the Basque Country UPV/EHU, Aptdo. 664, 48080 Bilbao, Spain

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S Supporting Information *

ABSTRACT: The combination of organic and inorganic components enables the development of new generation of materials with innovative applications in many fields. Clays are commonly used to confine different ions and molecules because they act as an engineering barrier, avoiding the release of the entrapped species. In this work, we have employed molecular dynamics simulations to study the diffusivity of a variety of organic molecules enclosed in the interlaminar space of clays. We have found that the diffusivity of confined organic molecules follows a universal trend, exhibiting low diffusion coefficients at low water contents, which are radically increased at a certain water content. This onset of diffusion is because of the rearrangement of the water molecules from monolayer to bilayer. At that point, the motion of the water/guest molecule clusters is decoupled, leading to the sharp increase of the diffusion coefficients.

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mechanical performance.19−21 Many studies are focused on the adsorption mechanisms and interactions between these molecules and the clay surfaces, whereas the diffusivity throughout the interlaminar space is not investigated.22−24 Likewise, there are some studies related to the adsorption and binding mechanisms of dye molecules to clay surfaces.25−28 However, these studies are not devoted to the diffusive behavior of organic molecules within clay minerals. In this paper, we have employed molecular dynamics (MD) simulations to study the dynamics and diffusivity of organic guest molecules confined in the interlaminar space of clays. For that purpose, we built hybrid systems based on two common smectite clays, Laponite, and saponite, that accommodate in their interlaminar space, different organic molecules and a variable water content, from 1.25 to 10 Mwater/Mclay. We have chosen a variety of guest molecules with structural dissimilarities, as well as different sizes and charges. These differences allow drawing a global diffusive behavior for organic molecules embedded in clays, providing new insights into the diffusion mechanisms of adsorbed organic molecules.

INTRODUCTION The design of new materials based on organic−inorganic complexes has demonstrated an outstanding potential for novel and improved applications in many areas of material science, medicine, energy, optics, electronics, or environmental protection.1−5 Among the inorganic substrates, clays show great versatility because of their swelling properties and high cation adsorption capacity, which provide the possibility of confining a large number of species in their interlaminar spaces. Nevertheless, to ensure a good performance of the organic-clay systems it is necessary to maintain a constant concentration of the organic compounds during its operative life, avoiding leaking processes that can affect to the global workability. Moreover, clays have low hydraulic permeability in a saturated state, acting as a barrier avoiding the migration of the confined species and providing a very efficient isolation.6,7 Because of these properties, clay materials are commonly used to deep storage of radioactive waste. The diffusion of the confined radionuclides within clay minerals may result in their eventual release. Therefore, the ionic diffusion in clays has been extensively studied, especially for alkali and alkaline earth metals.8−13 These studies point out that the ionic diffusivity is strongly influenced by the water content of the clay minerals, related to the relative humidity and shrink-swell capacity. In general, the ionic diffusivity increases with the water content,11,14,15 but there are other factors such as the ionic size and charge, the hydration energy or the distribution of the charge in the clay minerals that affect the diffusion.11,12,16−18 In contrast, the diffusivity of organic molecules, as the ones analyzed in this study, has not been analyzed in depth. For instance, polymeric species are commonly introduced into the interlaminar spaces of clay minerals in order to enhance their © XXXX American Chemical Society

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COMPUTATIONAL DETAILS As the starting point, we employed the structures of hectorite (natural analogous of synthetic Laponite) and saponite described by Breu29 and Rayner,30 respectively. We modified the chemistry to obtain a stoichiometry in Laponite of [Na 0.83 ][Mg 5.17 Li 0.83 ][Si 8 ]O 20 (OH) 4 and [Na 0.83 ][Mg 6 ][Si7.17Al0.83]O20(OH)4 for saponite. The simulation cell Received: October 2, 2018 Revised: December 19, 2018 Published: December 20, 2018 A

DOI: 10.1021/acs.jpcc.8b09639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C consisted of 24 unit cells, with a size about 2.1 × 1.8 nm2 in the basal plane and 3.4 nm in the perpendicular direction. The clay layers exhibit a net negative charge as a consequence of the replacement of Mg2+ by Li+ in the octahedral sheets of Laponite and Si4+ by Al3+ in the tetrahedral sheets of saponite. It should be noted that, although the isomorphic substitutions are located randomly within the clay sheets, the distribution of the Al3+ is similar in both tetrahedral sheets of each layer and is constrained by the Lowenstein rule. The excess negative charge is compensated with Na+ exchangeable cations to maintain the electroneutrality of the system. The organic molecules were built using Avogadro and relaxed using density functional theory B3LYP/6-311+G(d,p) simulations as implemented in Gaussian.31 The charges were computed using the electrostatic potential fitting method32 (see Tables S1−S6). Among the organic molecules under study (see Figure 1a), there are cationic dyes, such as Pyronin-Y and LDS-722, both

interactions in clays, organic, and water molecules, respectively. The combination of those force fields has been previously used and proved appropriate to model hybrid systems with clays, water, and organic molecules.38−42 All the simulated systems were initially minimized, relaxing both the box parameters and the atomic positions. Afterwards, MD simulations in the canonical ensemble (NVT) were performed at 300 K during 250 ps. It was used a thermostat coupling constant of 0.1 ps and a time step of 1 fs, using Verlet algorithm43 for the integration of the equations of motion. Then, the atomic positions and volume were equilibrated under room conditions (300 K and 1 atm) turning to the isobaric−isothermal ensemble (NPT) during 1 ns, using a barostat and thermostat coupling constants of 1 and 0.1 ps. Finally, a canonical ensemble at 300 K with a thermostat coupling constant of 0.1 ps was carried out during 0.1 μs for obtaining reliable computed diffusion constants.

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RESULTS AND DISCUSSION Basal Distances. After the equilibration of the systems, we have analyzed first the variation of the basal distances as the water content increases. Figure 2 shows that the expansion

Figure 1. Representation of the guest molecules: (a.1) Pyronin-Y, (a.2) LDS-722, (a.3) benzo(a)pyrene, (a.4) N,N-dimethyldodecylamine, (a.5) N,N-dimethyloctadecylamine and (a.6) N,N,N-trimethyloctadecylammonium; and the clays: (b.1) Laponite and (b.2) saponite. In the organic molecules, the black, blue, red and white balls represent C, N, O, and H atoms. In the clay structures, the yellow and orange tetrahedrons correspond to silicate and aluminate chains, whereas the blue and purple octahedrons represent the Mg and Li sheets.

with excellent fluorescent properties;2 commonly-used surfactants of different length, and charge, as N,N-dimethyloctadecylamine, N,N-dimethyldodecylamine, and N,N,N-trimethyloctadecylammonium; and as benzo(a)pyrene, a carcinogen aromatic polycyclic molecule.33 On the other hand, the two smectite clays selected (see Figure 1b) differ in the charge distribution in their sheets. Laponite presents an aliovalent substitution in the octahedral sheet, replacing Mg2+ partially by Li+ atoms, originating a negative charge excess in this inner sheet. In contrast, saponite has a partial substitution of Si4+ by Al3+ in the tetrahedral sheet, leading to a negative charge in the external sheets. In this way, these two clays enable the evaluation of the effect of the charge distribution in the diffusion properties. Besides that, increasing water content allows to ascertain the role of water in the dynamics of the guest molecules. The relaxed organic molecules were inserted into the interlaminar spaces of Laponite and saponite. For each organic molecule, we performed 8 simulations with different water contents from 1.25 to 10 Mwater/Mclay increasing in regular amounts of 1.25 Mwater/Mclay. A total of 96 simulations were carried out, measuring the interlaminar spaces and computing the diffusion coefficients of the guest molecules for each of them. LAMMPS simulation package34 was used to carry out the MD simulations. ClayFF,35 CHARMM36 and SPC37 force fields were used to describe the bonded and nonbonded

Figure 2. Basal distance evolution as the water content increases for the different studied systems based on (a) Laponite and (b) saponite.

produced by water is uniform for all the systems, independently of the organic molecule incorporated. The expansion of the interlaminar space of both clays is about 4.5 Å from the lowest water content, 1.25 Mwater/Mclay, to the highest one, 10 Mwater/Mclay. However, the widening of the interlaminar space is not linear with the water addition. There is an abrupt expansion in both clays from 5 to 6.5 Mwater/Mclay. This pronounced swelling has been attributed to a rearrangement process of the water molecules located in the interlaminar space of the clays as the water content increases.44,45 In order to verify that the sharp increase in the basal distance is caused by a water rearrangement, we have computed the density profiles of interlayer spaces. Density Profiles. The density profiles enable the study of the water structure in the interlaminar space and its evolution as the water content grows. Likewise, this tool is very useful to evaluate the distribution of the guest organic molecules in the clay slit pores and how they are affected by the water content. The distribution of the water and organic molecules in the interlaminar space for Laponite and saponite at different water contents is shown in Figure 3. B

DOI: 10.1021/acs.jpcc.8b09639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. Density profiles of the interlaminar spaces of (a) Laponite and (b) saponite that incorporate Pyronin-Y as function of z distance for (1) 1, (2) 1.5, and (3) 2 water layers. In orange are shown the profile of Pyronin-Y molecules, the water oxygen and hydrogen atoms are in green and blue. The zero corresponds to the center of the interlaminar space.

At low water contents, a perfect water monolayer is observed up to 3.1 Mwater/Mclay. Above this limit, the water monolayer evolves to a bilayer, with intermediate distributions, reaching a perfect water bilayer at water contents about 5.6 Mwater/Mclay. It is remarkable that the incorporation of the organic molecules do not modify substantially the water arrangement reported for unmodified clays.46,47 The monolayer and bilayer distribution are stable hydration states and consequently water molecules tend to maintain those distributions.47 In a monolayer distribution, water molecules are able to establish hydrogen bonds with the two clay surfaces that define the interlaminar space, while they can only form hydrogen bonds with only one surface in a bilayer distribution. Thus, as the water content increases, these molecules tend to maintain the monolayer distribution, although distorted, until it is more favorable the organization in a bilayer. This transition from monolayer to bilayer causes the abrupt rise in the basal distance. The water arrangement determines the distribution of the guest molecules in the interlaminar space. At low water concentrations, the organic molecules are aligned in the middle of the pore with the water molecules. However, when the pore is expanded because of water incorporation, the organic compounds tend to stay close to the clay sheets because of its hydrophobicity as can be seen in Figure 3. This behavior is observed in both clays and for all the studied molecules. Diffusion Coefficients. We have computed the mean square displacement in order to obtain the diffusion coefficients of the species confined in the clays. The plot of the evolution of the diffusion coefficients as the water content increases, Figure 4, clearly shows that the water arrangement has a significant impact. The diffusivity of the organic molecules in the water monolayer regime is very low, but their diffusion coefficients are gradually increased as the water content grows. When the water bilayer regime is reached, there is a sharp increase of the diffusion coefficients, up to 2 orders of magnitude higher than in the monolayer regime. It is worth highlighting that the guest molecules confined in our systems exhibit a universal behavior in their diffusivity, independently of their size, charge or structure flexibility, and it is also found in both clays, Laponite, and saponite. The diffusion coefficients of all the organic molecules are in the same range, with small variations because of their different size and structure. In Laponite, it is possible to establish an

Figure 4. Evolution of the diffusion coefficients of the organic molecules under study at different Mwater/Mclay in (a) Laponite and (b) saponite. Comparison of the diffusion coefficients for (c) a neutral molecule, N,N-dimethyloctadecylamine and (d) a charged one, LDS722 in Laponite (blue lines) and saponite (red lines).

inverse relationship between diffusion coefficients and molecular weight and/or volume: the higher the size of the organic molecule, the lower its diffusivity. In saponite, this relationship is not as clear as in Laponite. This may be due to the different distribution of the negative charge in the layers of those clays. In saponite, the negative charge is located in the tetrahedral sheets, the external ones, whereas in Laponite the charge is in the internal sheet, in the octahedral sheet. In order to evaluate the effect of the charge distribution in the clay sheets we have compared the evolution of the diffusion coefficients of the guest molecules in Laponite and saponite. It can be seen in Figure 4c that the location of the charge in the clays has no noticeable impact on the diffusivity of N,Ndimethyloctadecylamine, a neutral molecule. In contrast, this C

DOI: 10.1021/acs.jpcc.8b09639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figures 6 and S7−S11). At low water contents, water and organic molecules behave as a cluster, exhibiting a coupled

effect is remarkable in case of LDS-722, a charged molecule, as it is shown in Figure 4d. Neutral molecules do not establish intense electrostatic interactions with the charged surfaces of saponite. However, the electrostatic interactions between charged molecules and surfaces are particularly intense and consequently, reduce the mobility of charged species.41 Thus, the strong interactions between the charged LDS-722 and the charged surface of saponite cause a decrease of diffusion coefficients in comparison with Laponite. It might be thought that the abrupt expansion of the interlaminar space in the transition from water monolayer to bilayer may favor the motion of the guest molecules not only in the parallel plane to the clay sheets, but also in the perpendicular direction to them, hindered at low water contents. However, the analysis of the diffusivity decomposed in the x, y, and z-axis shows that the motion of the guest molecules is scarcely increased in the perpendicular direction to the clay sheets. For instance, the diffusion in z direction of benzo(a)pyrene (Figure 5a) does not change significantly as

Figure 6. Combined distribution functions of displacement and lifetime for water-organic molecules clusters at (a) low and (b) high water content for the LDS molecules adsorbed in Laponite.

motion up to 3.5 ns in case of water−LDS clusters, as can be seen in Figure 6a. They cover small distances of about 1 nm because of the low diffusion coefficients. On the other hand, at higher water contents the LDS and water molecules remain together less than 0.5 ns (Figure 6b), seven times less than at low water contents, although they cover together higher distances. The same behavior has been found for the clusters with the other organic molecules analyzed in this study (see Figures S7−S11). The high lifetimes indicate that the motion of water and organic molecules is coupled at low water contents, but as the water content is increased, the motion of the guest molecules is decoupled from the motion of water, enabling greater freedom of those molecules. This results in the increase of the diffusion coefficients of both water and organic molecules.

Figure 5. (a) Benzo(a)pyrene motion with respect to the z-axis, perpendicular to the clay sheets. Green and red colors are referred to low and high water contents, respectively. (b) Decomposed diffusion coefficients in x, y and z-axis for benzo(a)pyrene molecules in Laponite.

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CONCLUSIONS In this work, we have employed MD simulations to study at atomic level the processes that govern the diffusivity of the organic molecules confined in clay slit pores. For that purpose, we built systems based on Laponite and saponite clays with a variety of organic molecules, ranging from dyes, surfactants to aromatic polycyclic compounds. The different structure, flexibility, charge, and size of the organic compounds under study allow to define a common diffusive behavior for guest molecules occluded in clays as a function of the water content. It is remarkable that we have found a universal diffusive trend for the confined organic molecules. At low water contents, the diffusion coefficients of these species are very low, but increase progressively as water is added. As with the basal distances, the diffusion coefficients increase drastically when the bilayer regime is reached. The analysis of the diffusion in each direction has shown that only the diffusion in the parallel plane to the clay contributes significantly to the global diffusivity. The analysis of the lifetimes and distances covered by water-organic compounds clusters indicates that their motion is coupled only at low water content, in the monolayer regime, but is decoupled at higher water contents, in bilayer regime. The evolution from a jammed stated when the interlayer molecules’ motion is coupled to a loose state when it is decoupled might be the ultimate reason of the increase of the diffusion coefficients. The results of this work shed light on the diffusion of organic molecules confined in clays. The universal trend found

the interlaminar space is expanded, while the diffusion parallel to the clay sheets increases gradually. This results in diffusivities 3 orders of magnitude lower in the perpendicular direction to the clay sheets than in the parallel ones. Nevertheless, at high water contents, it has been observed zigzagging motion in the z-axis of the guest molecules, whereas at low water contents, the motion in that direction is limited, as can be seen in Figure 5b. The jumps from the proximities of one clay layer to the other are because of hydrophobic character of the guest molecules. However, these jumps barely contribute to the diffusivity because, despite the expansion of the interlaminar space since the distances that the molecules travel in the z-direction are much smaller than the distances covered in the xy plane. (De)Coupled Water/Guest Molecule Motion. At low water content, water and organic molecules should flow in the same plane because they are aligned in the center of the pore (see Figure 3), while at high water content, water can flow also below or above the guest molecules. We hypothesized that such a degree of freedom could be the origin of the diffusion increase. We have computed the combined distribution function of displacement and lifetime for water-organic molecules clusters in order to evaluate if water and guest molecules move coupled or not. The diffusive behavior for these clusters shows significant differences at low and high water content for all the analyzed organic molecules (see D

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in the diffusive behavior of these molecules could be possibly extrapolated to organic molecules enclosed in other hydrophilic laminar systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09639.

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Partial charges of the organic molecules under study. Water−organic molecules combined distribution functions of displacement and lifetime at low and high water content. Average diffusion coefficients of Na ions and water molecules at different Mwater/Mclay rates. Na+ hydration shell characterization. Impact of extra organic molecule addition (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +34 946017935. ORCID

Eduardo Duque-Redondo: 0000-0002-1890-4687 Hegoi Manzano: 0000-0001-7992-2718 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “Departamento de Educación, ́ Lingüistica ́ Politica y Cultura del Gobierno Vasco” (IT912-16) and the ELKARTEK program. E.D.-R. acknowledge the “Contrato de Personal Investigador Predoctoral en Formación” from the UPV/EHU. The authors thank for technical and human support provided by IZO-SGI SGIker of UPV/EHU and European funding (ERDF and ESF), as well as the i2basque computing resources.

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DOI: 10.1021/acs.jpcc.8b09639 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.8b09639 J. Phys. Chem. C XXXX, XXX, XXX−XXX