Brazilian Journal of Chemical Engineering MOLECULAR VALIDATED MODEL FOR ADSORPTION OF PROTONATED DYE ON LDH

Hydrotalcite-like compounds are anionic clays of scientific and technological interest for their use as ion exchange materials, catalysts and modified electrodes. Surface phenomenon are important for all these applications. Although conventional analytical methods have enabled progress in understanding the behavior of anionic clays in solution, an evaluation at the atomic scale of the dynamics of their ionic interactions has never been performed. Molecular simulation has become an extremely useful tool to provide this perspective. Our purpose is to validate a simplified model for the adsorption of 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonic acid (MBSA), a prototype molecule of anionic dyes, onto a hydrotalcite surface. Monte Carlo simulations were performed in the canonical ensemble with MBSA ions and a pore model of hydrotalcite using UFF and ClayFF force fields. The proposed molecular model has allowed us to reproduce experimental data of atomic force microscopy. Influences of protonation during the adsorption process are also presented.


INTRODUCTION
Layered double hydroxides (LDH) are an important class of natural compounds and also easily obtained by synthesis.They have a permanent positive charge on their surface.In addition, LDHs can exchange anions that lie between the layers.For these characteristics, these compounds are of great scientific and technological interest and are used as ion exchange materials (Dutta et al., 1991), catalysts (Reichle et al., 1986) and modified electrodes (Itaya et al., 1987).LDHs with magnesium and aluminum atoms on their layers are known as hydrotalcite and the framework of the layers are similar to brucite (Mg(OH) 2 ).We can imagine a starting structure, electrically neutral, being composed by one brucite layer (Figure 1a).
As magnesium is isomorphically replaced by Al 3+ , a permanent positive charge is created that is balanced by intercalation species (Figure 1b).In hydrotalcite, CO 3 2-and water molecules are the intercalation species in the interlayer space.
The permanent positive charge is responsible for the extraordinary LDH performance in the adsorption of anionic dyes (Zhu et al., 2005;Abdelkader et al. 2011;Boudiaf et al. 2012;Aguiar et al., 2013).To be able to understand and predict the behavior of LDH's for anionic dyes adsorption, it is important to analyze on an atomic-scale, the surface phenomena that result in the macroscopic properties.Experimental techniques are limited for clarifying these phenomena.
Molecular simulation has been used successfully in the study of LDH.Wang et al. (2001)   rges of the M rence).

Forcefield
The system LDH/dye was described using the LJ potential for repulsion-dispersion forces plus Coulombic contributions between point charges: where ε ij (kcal/mol) represents the depth of the potential well, σ ij (Å) is the finite distance at which the energy of interaction is zero and r ij (Å) is the distance between the molecular centers i and j.In the second term, q i and q j are point charges separated by the distance r ij .The cross LJ terms were obtained using the usual arithmetic and geometric combination rules (Lorentz-Berthelot).
For the interactions between MBSA molecules and the LDH framework, we used the ClayFF force field (Cygan et al., 2004).This force field has been developed especially for clay minerals.
The interaction between MBSA molecules are computed based in the UFF force field (Rappe et al., 1992).Table 2 presents the parameter's values for each atomic species (r 0 = σ / 1.1224).

Computational Details
The electrostatic potential was calculated by Ewald method.The Ewald method accuracy was set for an energy tolerance of 0.001 kcal/mol a cut-off of 15.2 Å.This means that larger cut-offs will give energy variation in the third decimal place as done in a similar study (Lima et al, 2015).All LJ potential had also a 15.2 Å truncation.We also did tests with increasing LJ cut-offs and obtained similar results.After 1.5 x 10 6 equilibration steps, more 1.0 x 10 6 steps were used to obtain the average values of the thermodynamic properties and the most stable configuration.
The simulations in the canonical ensemble (NVT) were performed with a variable number of MBSA anions until the surface was saturated.The number of molecules at saturation will be compared with that obtained experimentally by Cai et al. (1994) for MBSA 1-.The molecular simulation algorithm of Cerius2 (Accelrys) was used to obtain the NVT equilibrium configurations.During the Monte Carlo simulations, the MBSA molecules and LDH framework were considered rigid.While this approach is acceptable for LDH, it is not suitable for dye molecules.However, MBSA is a special case because its structure consists basically of two aromatic rings with reduced mobility.Furthermore, the molecule has previously been minimized and we did not expect significant conformation changes.

Model Validation
In order to compare simulated and experimental data (Cai et al., 1994), an increased number of MBSA molecules were introduced in the simulation cell until saturation.During simulation, MBSA 1-molecules are translated and rotated inside the simulation cell until equilibrium is reached (Figure 5).

Figure 5:
The evolution of the energy in the system LDH/MBSA -1 at 298 K.The most stable configuration occurred after about 30,000 steps.
At equilibrium, the molecules of MBSA 1-assumed a perpendicular position with the LDH surface.This same orientation of the anions with respect to the hydrotalcite surface was verified experimentally by Cai et al. (1994) for MBSA 1-adsorption onto In the sim acking of 1 mately 12.2 m ace was full ion capacity 994) that res o 10.5 mole Thus, in the e molecules.C urface in the xperimental he models fo owards the p

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