Molecular Forces Governing Shear and Tensile Failure in Clay-Dye

Article pubs.acs.org/cm

Molecular Forces Governing Shear and Tensile Failure in Clay-Dye Hybrid Materials Eduardo Duque-Redondo, Hegoi Manzano,* Nerea Epelde-Elezcano, Virginia Martínez-Martínez, and Iñigo López-Arbeloa Molecular Spectroscopy Laboratory, Department of Physical Chemistry, University of the Basque Country UPV/EHU, Aptdo. 664, 48080 Bilbao, Spain S Supporting Information *

ABSTRACT: Hybrid materials based on photoactive molecules confined into nanostructured substrates are very promising for technological applications. However, little is known about the impact of organic dyes on the mechanical properties of the substrate, a key aspect for their practical implementation. In this work, we use atomistic simulation methods to investigate the mechanical properties of two hybrid systems consisting on a clay matrix (laponite) loaded with two different cationic dyes (LDS-722 and pyronin Y). We applied tensile and shear deformations to the layered hybrid materials and characterize the key mechanism triggering their failure. It has been observed that the water and dye molecules located in the interlaminar spaces are those involved in the deformation processes, while the structure of the laponite layers does not change. Furthermore, it has been also found that the incorporation of dye molecules modifies the hydrogenbonding network of water in the interlaminar space, worsening the mechanical properties of the hybrids with respect to the clay. The information obtained by molecular simulation help us to assess the mechanical behavior of these materials, and to design materials with tailored strength.

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INTRODUCTION Design of new photoactive materials has a great scientific and technological interest. Hybrid systems based on molecules confined in nanostructured substrates is a particularly promising field. The possibility of combining different types of organic dyes and inorganic matrices enables the creation of photoactive systems with tailored properties, where the synergy between the dye and the matrix results in materials combining the optical and spectroscopic properties of the former with the mechanical, structural, and thermal properties of the latter.1 The versatility of such systems makes them suitable for wide variety of applications, ranging from biological and chemical sensors2,3 to water purification,4 to solid-state active media for laser,5 to phototherapy treatments against cancer.6 A particular advantage of these systems is the selforganization of the host molecules in the nanostructure. For instance, it is possible to achieve a macroscopic order of organic dyes confined in nanoclay thin films: on the one hand, the dye locates in preferential orientation within the clay interlaminar space due to specific host−guest interactions. On the other hand, the clay nanoparticles lay parallel to the substrate, because of their platelike morphology. Such spatial homogeneization of the dye orientation may give rise to nonlinear optics (NLO) effects, with applications as waveguides,7 second harmonic generators,8 or dichroic crystals.9 Furthermore, dye encapsulation inside clays improves the thermostability and can © 2014 American Chemical Society

reduce the nonradiative deactivation mechanisms of chromophores.10 Nevertheless, the implementation of these hybrid systems presents practical challenges, including possible adhesion problems between clay sheets due to the incorporation of the dye. It is well-known that the absorption of organic molecules and water induces an expansion of the clay interlaminar space, decreasing the cohesion, and leading to delamination or exfoliation in extreme cases.11 However, the situation is not always so dramatic, and several hybrid systems with polymers,12 vitamins,13 or organometallic complexes14 intercalated between clay sheets have shown a good cohesion between layers, despite the interlaminar space expansion. To the author’s knowledge, nothing has been reported about the influence of cationic dyes on the cohesion between clay sheets and mechanical properties of clays in general, a key aspect for the practical implementation of the hybrid NLO material. The objective of this paper is to evaluate the impact of photoactive molecules confinement on the mechanical properties of clays using atomistic simulation. We investigated the fundamental molecular forces that trigger the mechanical failure and govern the adhesion between clay layers. In particular, we paid attention to two hybrid materials that have been Received: February 24, 2014 Revised: July 10, 2014 Published: July 14, 2014 4338

dx.doi.org/10.1021/cm500661d | Chem. Mater. 2014, 26, 4338−4345

Chemistry of Materials

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experimentally produced and present promising performance to develop NLO devices. Both use laponite clay as the host matrix and incorporate different cationic dyes, styryl LDS-722 and pyronin Y, chosen because of their interesting photophysical properties. We applied tensile and shear deformations to the hybrid systems to study their strength and analyze the failure processes at molecular scale. The knowledge of their fundamental properties will give us valuable information for the design of materials with enhanced mechanical resistance.

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ATOMISTIC MODEL AND SIMULATION DETAILS Building Realistic Hybrid Materials. In this work, we evaluate the mechanical properties of materials for NLO that we prepared in our laboratory. Therefore, the atomic scale models should be as realistic as possible, matching the structure and composition from the characterization of the hybrid materials. The substrate, laponite, is a commercial smectite clay with stoichiometric formula [Na+0.6][Mg5.4Li0.6][Si8]O20(OH)4. The lamellas are formed by a central layer of octahedrally coordinated Mg, partially replaced by Li atoms, between two tetrahedral layers of silicate. The substitution generates a negative charge excess, which is compensated by exchangeable cations in the interlaminar space.15 We used, as a starting point, the crystal structure of the natural analogue of laponite, hectorite,16 resolved by Brau et al.,17 and modified it to match the commercial composition.18 First, we substitute the F− anions in octahedral layer by hydroxyl groups, placing the O atoms in the position occupied by the F atoms and H atoms oriented toward the tetrahedral layer. Second, we performed an isomorphic substitution of 8.3% of the Mg atoms by Li atoms in random positions of the octahedral layer. Finally, the Cs+ cations in the interlaminar space were replaced by Na+, reaching the experimental stoichiometry. The organic dyes are LDS-722 and pyronin Y. Both are cationic dyes with fluorescents properties and present a planar structure with delocalized charge. LDS-722 is a member of styryl dyes, with a push−pull structure that confers a high Stokes shift and response to linearly polarized light. It exhibits high absorption in the blue region, at 494 nm, and fluorescent emission in the red zone with its maximum at 702 nm. Pyronin Y is a xanthene-type dye, with a strong absorption in green zone of the spectrum, centered at 547 nm, an fluorescent emission in the greenish-yellow region (at 568 nm), and very interesting properties when J aggregates form.19 We built the dye structures, shown in Figure 1 (shown with more detail in the Supporting Information (SI)), using the Avogadro builder,20 and relaxed them to a local energy minimum using the density functional theory (DFT) method, as implemented in Gaussian09.21 The B3LYP22 hybrid exchange-correlation functional and a triple-ζ polarized basis set with diffuse functions 6-311+G(d,p) were employed. In order to simulate the localization of the positive charge when dyes are in anionic media, a F atom was placed close to one ammonium N atom in pyronin Y. The atomic charges were computed from the electrostatic potential using the ChelpG scheme,23 and used for the subsequent empirical potential simulations (see the SI for more details). We built a simulation box, which consisted of a laponite 4 × 2 × 2 supercell, and set the amount of each dye intercalated between the laponite layers by looking at the experimental cationic exchange (CEC). The elemental analysis determined that pyronin Y reaches the saturation at ∼20% CEC, while the saturation for LDS-722 is reached at ∼45% CEC. We

Figure 1. (a) Structure of the laponite viewed along the ycrystallographic axis. Silica tetrahedra are colored yellow, O atoms in red, H atoms in white, Mg atoms in blue, Li atoms in green, and Na atoms in orange. (b) Structure of LDS-722 and (c) pyronin Y. C atoms are colored black, O atoms are shown in red, H atoms are shown in white, and N atoms are shown in blue.

substituted Na+ ions by dye molecules to match these maximum CEC values, keeping the molecules as homogeneously distributed as possible. We measured the water content in each hybrid system and in the dye-free laponite under room-temperature conditions by thermogravimetric analysis (TGA). Their values are presented in Table 1. Henceforth, the system without dyes is named L16, Table 1. Water Weight Percentage, Maximum CationExchange Capacity, and Basal Distance (d001) for the Studied Systems basal spacing, d001 (Å) system

H2O (wt %)

maximum cation-exchange capacity, CEC %

simulated

experiment

L16 L10-PY20 L5-LDS45

16 10 5

20 45

13.5 12.3 15.3

12.9 12.6 15.0

L10-PY20 the system with pyronin Y, and L5-LDS45 the system with LDS-722. In this nomenclature, L refers to the laponite and the first number to water content in weight percentage, followed by the incorporated dye with its corresponding CEC value. Atomistic Simulations. The molecular dynamics (MD) simulations were performed using LAMMPS.24 The dye molecules were described using the CHARMM22 force field,25,26 and laponite was simulated using CLAYFF.27 The organic and inorganic components interact via electrostatic and dispersive forces, the latter of which being computed by a geometric combination of the Lennard-Jones parameters of each atomic pair. The combination of these force fields has been previously used to model hybrid systems with accurate results.28−30 Periodic boundary conditions were applied in all three directions, and the Ewald31 method was used to compute the long-range Coulombic interactions. First, we minimized the energy of the hybrid systems relaxing both the simulation box and the atomic positions. Then, molecular dynamics in the canonical ensemble (NVT) was performed at atmospheric pressure and heated from 1 K to 300 K within 10 ps. The equations of motion were integrated using 4339



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Figure 2. View along the xz-plane of the (a) L16, (b) L10-PY20, and (c) L5-LDS45 systems. The color code for the atoms is the same as in Figure 1.

achieved between cationic exchange and the water expelled in the process. Shear and Tensile Tests. The evolution of tensile and shear stress under the applied strain are presented in Figures 3a

a Verlet algorithm with a time step of 1 fs and a thermostat coupling constant of 0.1 ps. After that, we turned to the isobaric−isothermal ensemble (NPT) to equilibrate the atomic positions and volume under room-temperature conditions for 10 ns with a barostat coupling constant of 1 ps. Finally, a canonical ensemble simulation was performed for 10 ns to average the system properties. The equilibrated systems represented the starting point for the mechanical deformations. We assume that the delamination may arise from a tensile and/ or shear stress. Hence, quasi-static tensile and shear deformations were applied to the simulated systems, relaxing the atomic positions after each displacement with a conjugate gradient energy minimization algorithm. It is known that clays are transversally isotropic materials with the weakest direction in perpendicular to the clay sheets.32 Therefore, the tensile deformations were applied in the perpendicular direction to the clay sheets and the shear deformations were applied in the xzdirection, parallel to the layers.

Figure 3. Stress−strain curves under (a) tensile and (b) shear strain. The black line corresponds to the dye-free clay, and the red and blue to Laponite loaded with pyronin Y and LDS-722 dyes, respectively.

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and 3b, respectively. The tensile curves present the typical shape of a ductile material, with an elastic regime followed by a plastic zone leading to failure. We can observe that the beginning of the elastic region is similar for the clay and the two hybrid systems. The Young modulus obtained from a linear fit up to 3% strain in perpendicular to the layers is 14.7 GPa for laponite, in good agreement with the experimental and theoretical values for smectite clays.34,35 The difference between the clay and the hybrid systems becomes significant in the plastic regime. Laponite’s tensile strength is considerably higher than the hybrid systems, which present the same maximum value. The mechanical response changes under shear strain. After the initial elastic region, at ∼10% strain, the three materials reach the plastic flow regime characterized moderate stress drops in cascades of successive stress relaxations. Laponite loaded with LDS-722 exhibits a worse mechanical behavior already in the elastic regime, which is maintained in the plastic region and translates into noticeable lower shear strength. By contrast, laponite loaded with pyronin Y presents properties similar to the dye-free clay in the entire stress−strain curve. Summarizing, the mechanical properties of the dye/laponite systems behave in a different way under tensile and shear strain. Furthermore, while both hybrid materials present a lower tensile strength, the one with pyronin shows similar strength to

RESULTS Dye Intercalation in the Interlayer Space. Figure 2 shows MD snapshots of the clay and the clay/dye hybrid systems after equilibration. The pyronin Y molecules lie parallel to the clay surface in the center of the interlaminar space. In contrast, LDS-722 dye cannot rest parallel to the surface, because of the high concentration of molecules. They are oriented with an average 30° tilt of its aromatic rings, with respect to the clay surfaces. The arrangement of water molecules and Na cations will be discussed later in detail. It can be seen that the incorporation of dyes in interlaminar space triggers a substantial modification of the basal spacing. The experimental33 and simulated basal distances, shown in Table 1, are in good agreement, which validates the conformation obtained from our simulations. It is interesting to note that the basal distance increases or decreases, depending on the combination of water and dye content. As the dye content increases, the amount of water decreases (they leave the clay together with Na cations during the cationic exchange) and both play an opposite role in the swelling and shrinkage of the interlaminar space: reducing the amount of water leads to shrinkage,15 while the exchange of Na cations for larger organic molecules induces swelling. Accordingly, the clay will swell or shrink, depending on the particular combination 4340



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the dye-free clay under shear. In the following subsections, we will explore the atomic scale mechanism and forces that explain such behavior. Failure Mechanism. In order to understand the mechanism that controls the strength and failure of the hybrid materials, the development of local deformations during stress relaxation was studied. For that end, we used the local strain tensor proposed by Shimizu et al.,36 which represents the displacement of a given atom, with respect to its neighbors. Figure 4 represents the spatial distribution of the local strain for

Figure 5. Density profile of O atoms (shown in red), H atoms (shown in blue), corresponding to the water molecules of the interlaminar space, and sodium atoms (shown in green). The zero corresponds to the center of the interlaminar space, and the clay surfaces are represented by the black lines.

theoretically related to the elastic properties of clays.32,41 The arrangement of water molecules in the interlaminar space forming a monolayer and a bilayer are energetically stable situations and match peaks of maximum elastic properties. The transition states between them, when the layers are not properly formed, are energetically unfavorable, and the clays exhibit worse elastic properties.41 In our case, the density profiles for L16 and the L5-LDS45 system show a bilayer distribution. While in L16, it corresponds to an energetically stable state, the L5-LDS45 system presents a poorly organized bilayer. The amount of water in the interlaminar space of the LDS system is not enough to form even a monolayer in a dye-free clay,41 yet the interlaminar space expansion due to the presence of LDS molecules forces the water distribution to arrange in an unfavorable distribution. By contrast, the profile of L10-PY20 corresponds to a monolayer ordering of water molecules. The absorption of pyronin reduces the amount of water with respect to the dye-free clay, reaching the number of molecules appropriate to build a well-formed monolayer. In all of these systems, Na+ cations are located in the center of the interlaminar space. We have delved into the arrangement of these cations and its coordination shells. In all of the studied systems, the radial distribution function (RDF) between a Na+ ion and the O atoms of water shows a maximum at 2.35 Å, which matches with the average distances found experimentally for the sodium in bulk water.42 The sodium coordination number (CN) in the first shell is 5.5 for the dye-free system (5 water molecules and 0.5 surface oxygen), with a well-defined 3D shell surrounding the cation. The CN in the case of the L10-PY20 system is again 5.5, yet the structure is different, as a consequence of the water distribution in a monolayer: the cation is coordinated to 4 water molecules in the equatorial plane, while, in the apical directions, it is coordinated directly to 1.5 O atoms of the clay surface. The difference, arising for the bilayer and monolayer distribution of water, can be clearly visualized in the spatial distribution functions for water molecules included in Figure 6. In the system that incorporates LDS, the CN is smaller, ∼4.25. The spatial distribution function of water around the cation shows an incomplete coordination shell, because of the small amount of water available. Hydrogen-Bond Network Characterization. The diverse amount and distribution of water in the interlaminar space will influence the hydrogen-bond network having an impact on the layer cohesion. Therefore, we have characterized the connectivity and strength of the hydrogen bonds. There is no unique criterion to define a hydrogen bond. For bulk water, a geometric norm commonly employed considers hydrogen bonds as those with the distance between the O atoms of the

Figure 4. (a) Normalized spatial distribution of the local strain at 25% tensile strain and (b) at 30% shear strain, with respect to the equilibrium configuration of the dye-free clay. The color code assigns bluish colors to less deformed atoms and reddish colors to more deformed atoms. The strain tensor was computed with OVITO,40 considering the neighboring atoms within a distance of 4.5 Å.

the dye-free laponite. The atoms that suffer higher deformations are those located in the interlaminar space, in either tensile and shear strain modes. This indicates that stress is dissipated by relaxation of the molecules in the interlaminar space, while the clay sheets do not participate in the process. Under shear strain, the relaxation takes place by a “stick−slip” mechanism, by sliding of the laponite sheets over each other, without internal deformation of these sheets, a mechanism already reported for other clays 37 and other layered materials.38,39 The absorption of dye molecules does not alter the spatial distribution of the local strain (see the SI). The fact that only the atoms in the interlaminar spaces develop local deformations is due to the nature of the chemical forces. The Si−O covalent bonds in the layers are considerably stronger than the Coulombic and dispersive interactions that hold the sheets together, so the system fails at the interlaminar space. It is interesting to note that the mechanical properties under tensile and shear stresses depend merely on the cohesion between layers. Therefore, we will focus now on characterizing the forces and the structure of water molecules and cations in the interlaminar space. Molecular Arrangement in the Interlaminar Space. Before analyzing the forces acting between the clay layers, we studied the molecular arrangement in the interlaminar space. For instance, the molecular density profiles in the perpendicular direction to the clay layers (Figure 5) allow us to characterize the water distribution, which has been experimentally and 4341



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Figure 7. Distances and angles combined distribution function of hydrogen bonding between water molecules (above) and between water molecules and oxygen atoms of the clay surface (below). The maps were generated using TRAVIS.43

Figure 6. Radial and spatial distribution function between the oxygen atoms in water and clay surface and the interlaminar Na cations (a) L16, (b) L10-PY20, and (c) L5-LDS45 systems. The Na+ ion is represented in orange and the spatial distribution of the oxygen atoms in blue. The spatial distribution function was computed with TRAVIS43 for a probability of 15%.

hydrogen donor and acceptor molecules is shorter than 3.5 Å, and the angle formed between the vector connecting those O atoms and vector connecting the O and H atoms of the donor molecule is lower than 30°.44 However, the confinement and the hydrophilicity of the environment may result in different bonding geometries than in bulk. In addition, it is possible to distinguish two types of hydrogen bonds in the studied systems: those established between water molecules and those established between the water molecules and the acceptor oxygen atoms of the clay surfaces. Therefore, we have tested the geometric criteria45 in our systems. The distance and angle combined distribution function (Figure 7) shows s more likely distance of 2.7 Å and null angle for all systems and types of hydrogen bonding. The angle and length distribution are comparable in the L16 and L10-PY20, yet a noticeable difference is found in the L5-LDS45. The distributions are narrower in hydrogen bonds between water molecules and wider for hydrogen bonds between water and the clay surface. This fact suggests that the hydrogen bonds in L5-LDS45 between water molecules, respect to the other two systems, are stronger, while the hydrogen bonds establish with the clay surface are weaker. From these results, we consider the maximum bonding conditions under distances of 3.25 Å and angles under 30°. These conditions encompass almost all potential hydrogen bonds and have been employed for the following analysis. We have calculated the average number of hydrogen bonds per water molecule (Figure 8). Compared to bulk water, the confinement reduces the average number of hydrogen bonds per water molecule from 3.5 to 2.8 in laponite,46,47 and decreases even more when dye molecules are incorporated into the clay. This is because the confinement and the dye molecules

Figure 8. (a) Average number of hydrogen bonds per water molecule in the three studied systems. The two types of possible hydrogen bonds, water−water and water−surface, are depicted in light and dark blue, respectively. The corresponding value for bulk water is given for comparison. (b) Hydrogen-bond lifetimes of dye-free laponite (in black) and loaded with pyronin Y and LDS-722 (in red and blue, respectively). Continuous lines represent lifetime of hydrogen bonds between water molecules, and dashed lines represent the lifetime of hydrogen bonds between water molecules and the clay surface.

reduce the possibilities of forming hydrogen bonds. Hence, the insertion of dyes in the interlayer space entails a decrease in the number of water molecules translates into a sparser hydrogenbond network. We have also analyzed the strength of the hydrogen bonds in the systems. In general, the smaller the distance between the hydrogen of the donor molecule and the acceptor oxygen, the higher the bond strength.48 By looking at the length of the hydrogen bond and the bond angle distribution in Figure 7, we can suggest that the strength of the hydrogen bonding is similar in the L16 and L10-PY20, while the water−water bonds are stronger and the water−clay bonds weaker in the L5-LDS45. However, for a more quantitative analysis another two parameters have been measured: the dipolar moment distribution of water molecules and the hydrogen bond lifetime. The presence of charges in clay sheets and cations in the interlaminar space intensifies the donor and acceptor character on water molecules, thus enhancing the strength of the hydrogen bonds. Such effect should translate in an increase of 4342



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the dipolar moment of the water molecules that we can evaluate, since the CLAYFF uses a polarizable SPC model to describe water molecules.27 However, we found that the dipolar moment distributions are almost equal for the three studied systems. The most probable value varies by just 0.1 D, which suggests that the hydrogen bond strength is similar in the studied systems. Several studies use the hydrogen bond lifetime as an indicator of the bond strength.49,50 For a system with a constant number of water molecules that relationship holds, and the lifetime of a hydrogen bond can be related to its strength. However, recent studies have proved that the hydrogen bonding lifetime is mainly influenced by the amount of surrounding molecules with which can establish new hydrogen bonds. Then, the hydrogen bond breakage is more dependent on the local water structure rather than the instantaneous hydrogen bond strength.51 We clearly observe that correlation in our materials (Figure 8) with decreasing lifetimes as the number of water molecules in the system increases. Thus, we cannot extract any conclusion comparing different systems. However, it must be noted that, for a given system, the lifetime of the hydrogen bonds established between water molecules and the clay sheets is always lower than lifetime of the hydrogen bonds between water molecules. This fact evidence that the hydrogen bonds between water molecules and the clay surface are weaker than the hydrogen bonds between water molecules. Molecular Forces That Governs the Mechanical Properties. The cohesion between clay sheets can be described by attending to two types of interactions: first, the electrostatic interactions between the charged clay sheets and the interlaminar cations; second, the hydrogen-bond network among water molecules and the clay surface. We have monitored the evolution of both forces, plotting the energy variation as a function of the strain (Figure 9). Positive values represent a decrease in the interacting energy. Under tensile deformation, the interaction energy decrease depends mostly on the Coulombic forces between the Na+ cation and the laponite sheets, while the influence of water molecules is minor. In fact, up to the elastic limit, the total energy decrease matches with the contribution of the cations, and only when water starts to rearrange both curves can be distinguished. This is consistent with the type of deformation: when the layers are pulled apart, the distance between charges increases and the drop in Coulombic energy is critical. However, when a shear strain is applied, we observe a more important contribution of the hydrogen bond network to the energy decrease. The result is consistent with recently reported data on friction between quartz surfaces,52 where a correlation between the hydrogen-bond network density and the friction was found. In this case the distance between charged layers and the cations does not change, and the Coulombic energy decrease may be associated mainly to a rearrangement of coordination shells around the cation due to the lateral movement. Therefore, the role of the hydrogen bond breaking and water rearrangement becomes equally significant to the Coulombic energy, or even more important. The decrease of the mechanical properties in the hybrid systems, with respect to the dye-free clay, can be attributed to changes in the interlaminar space structure that modify these two forces. Yet, the energy change trends previously described are general; the different relative importance of Coulombic and hydrogen bond forces in the hybrid systems is noteworthy.

Figure 9. Cohesion energy change under tensile strain and shear strain. Positive values indicate a decrease of the cohesion. The total decrease of cohesion energy is represented in black, and the contribution of the hydrogen bond network and the cations are shown in blue and green, respectively. The SI includes a detailed description of the approach followed to compute these contributions independently.

When pyronin is incorporated at a 20% CEC, the water molecules arrange in a monolayer in the center of the interlaminar space. Although the number of hydrogen bonds is lower than in the dye-free clay, monolayer distributions of water are energetically favorable hydration states in clays.53 Furthermore, water monolayers may present long-range polarization effects and ordering that increase their stability.54 Therefore, the relative weight of the hydrogen bond network interactions to the cohesion is more important for the pyroninloaded clay than the dye-free clay, as can be seen in Figure 9. This is especially noticeable on the mechanical response under shear strain, where both systems present similar shear strength and yield stress. In contrast, when LDS-722 is incorporated at a 45% CEC, water distributes as a distorted bilayer. Although the water content is not even enough to reach the monolayer hydration state, the swelling induced by the high amount of absorbed dye forces such arrangement. This results in a sparse and probably discontinuous hydrogen bond network. Moreover, the amount of water is so low that they cannot even form a full hydration shell around Na+. Hence, the Coulombic interaction between the cations and the layers is less screened, and a higher relative weight of the Coulombic interaction is observed (Figure 9). The low contribution of the hydrogen-bond network to the cohesion is again more noticeable under shear strain, and the hybrid system presents a drastic decrease of the shear strength. Finally, the specific role of the dye on the mechanical properties was checked. For an appropriate comparison, we replaced the pyronin by LDS-722 in the L10-PY20 hybrid material. Under these conditions, the contribution of water molecules and cations should be the same, and any difference attributable only to the dye. We evaluated the system under a 4343



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shear strain since the differences are more noticeable than in tension. The stress buildup is remarkably similar in both hybrid materials, independent of the dye, especially during the elastic region (see the SI). Therefore, we can conclude that that the dye itself does not have a great impact on the mechanical properties, aside from the modification of the interlayer molecular arrangement.

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ASSOCIATED CONTENT

S Supporting Information *

Computed ChelpG charges of the dyes, local strain spatial representations of the L10-PY20 and L5-LDS45 under shear and tensile deformations, shear strain−stress curve of the L10LDS20, and the approach used for the contribution of cations and hydrogen bonds to the cohesion energy. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS Hybrid materials consisting on organic dyes incorporated into clay thin films are promising for the development of photoactive devices, since a homogeneous microscopic order of the dye can be achieved. Nevertheless, other factors must be tested to ensure a proper operation of the devices, such as their mechanical performance. With that objective, we have employed atomistic simulation methods to study the molecular forces governing the strength of clay-dye hybrid materials in realistic conditions, i.e., experimental dye loads and interlaminar water contents. It has been proven that the mechanical performance of the hybrid dye-clay materials is dependent only on the cohesion between layers, while the layers themselves do not participate in the failure mechanism and can be considered “solid blocks”. The two main forces contributing to the cohesion between layers, namely, the Coulombic forces and the hydrogen-bond network, have been characterized. The Coulombic force is more significant when a tensile strain is applied, while the hydrogen bonding becomes relevant under shear. In general, the inclusion of dyes decreases the cohesion between layers. However, their impact is more dependent on how they alter the interlaminar space structure, because of the cationic exchange (CEC), than the dye itself. On one hand, the dye can induce changes in the water distribution, forming stable monolayers and bilayers with good mechanical performance, or intermediate unfavorable states with a distorted water distribution and worse mechanical properties. On the other hand, the number of hydrogen bonds per water molecule decreases when dyes are present, forming a sparser hydrogenbond network. For the specific hybrid systems studied in this work, we found varied impact of the dye in the mechanical properties. Under tensile deformations, both hybrid systems behave similarly, since the contribution of water is less important. However, under shear strain, the differences become more significant: (i) The incorporation of pyronin molecules with a relatively low CEC allows water molecules to arrange in a monolayer. This distribution compensates the decrease in the number of hydrogen bonds and explains the slight worsening of the mechanical properties. (ii) The large CEC of the LDS-722 reduces the water content dramatically, forcing an unfavorable water distribution and a sparser hydrogen bond that turns into poor mechanical properties. In summary, we have predicted the mechanical properties of hybrid organic-clay materials and related them to the atomic arrangement in the interlaminar space. Therefore, we believe that our findings could be relevant not only for the studied systems, but, generally, for any small/medium-sized cationic molecule incorporated between clay sheets.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish Government and the Basque Country Government through the MAT2010-20646204-04 and the Etortek 2014 projects, respectively. H.M. and V.M. acknowledge, respectively, the Juan de la Cierva and Ramón y Cajal (RYC-2011-09505) postdoctoral contracts from the Spanish Ministerio de Industria y Competitividad. E.D-R. acknowledges the IT339-10 contract from the Basque Country Department of Education, Research, and Universities, and the FPI grant from the UPV/EHU. N.E. acknowledges the UPPAUPV/EHU coadvised Ph.D. grant. The computing resources from the SGIker (UPV/EHU) and the i2Basque project are gratefully acknowledged.

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