Dissociation Mechanisms of Dissolved Alkali Silicates in Sodium Hydroxide The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Dupuis, Romain et al. "Dissociation Mechanisms of Dissolved Alkali Silicates in Sodium Hydroxide." Journal of Physical Chemistry C 124, 15 (March 2020): 8288–8294 © 2020 American Chemical Society As Published http://dx.doi.org/10.1021/acs.jpcc.0c01495 Publisher American Chemical Society (ACS) Version Final published version Citable link https://hdl.handle.net/1721.1/129405 Terms of Use Creative Commons Attribution 4.0 International license Detailed Terms https://creativecommons.org/licenses/by/4.0/ This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. pubs.acs.org/JPCC Article Dissociation Mechanisms of Dissolved Alkali Silicates in Sodium Hydroxide Romain Dupuis,* Roland Pellenq, Jean-Baptiste Champenois, and Arnaud Poulesquen Cite This: J. Phys. Chem. C 2020, 124, 8288−8294 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Recent accelerated simulations of the decondensa- tion of silicates by sodium hydroxide open a window on understanding complex mechanisms of the depolymerization of silicate chains. Herein, complex mechanisms of decondensation that involve two water molecules (or OH− groups) are unveiled. The study of two different solutions, having the same chemical composition but in different concentration, help one to draw more general conclusions on the dissociation mechanism in silicate solutions. We find that the dissociation is not always assisted by single water molecules but that in about 20% of the cases two water molecules (or OH−) are present in the near environment. The results underline the importance to consider explicit water solvent in which water molecules are reactive. I. INTRODUCTION the final products we wanted to synthesize. The first one For a long time, efforts have been done to investigate and (named Zsol) is usually used to synthesize some aluminosi- − · control the silicate chemistry.1 9 It is the base for the licate zeolites. The initial concentrations are [Si] = 2.25 mol −1 + · −1 formation or phase transformation of liquids, gels, and L and [Na ] = 1.31 mol L , which correspond to molar minerals, yet it is unclear how water and silicate chains ratios of SiO2/Na2O = 2.44 and H2O/Na2O = 85. A sodium − interplay at the atomic scale, depending on the chemistry of hydroxide solution ([NaOH] = 1.31 mol·L 1) is added to this the solution and on the shape of the interface.10,11 It is mainly solution in order to study the silicate decondensation process, due to the complexity with experimentally characterizing the as explained by Dupuis et al. in 2019.17 The final 12 structure of silicates in solution. Besides, molecular dynamics concentrations are [Si] = 1.125 mol·L−1 and [Na+] = 1.31 · −1 fi have helped to rationalize the outcomes of experiments. Until mol L ( nal molar ratios are SiO2/Na2O = 1.72 and H2O/ recently, it was also challenging to reproduce the bond Na2O = 85). The second one (named Gsol) is commonly used formation or dissociation by atomistic simulations due to the to synthesize alkali-activated materials as geopolymers. Two high activation energy barrier (a few tenths of an electronvolt) − solutions were characterized by varying the concentration of to make or break Si−O bonds,13 15 but new methods have 14,16,17 sodium hydroxide at fixed silicate and water content ([Si] = See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. been used to produce reliable silicate structures. This −1 + −1 Downloaded via MASSACHUSETTS INST OF TECHNOLOGY on June 10, 2020 at 18:43:54 (UTC). 5.8 mol·L and [Na ] = 9.66 mol·L , which correspond to opens the paths to study in detail the statistics of the molar ratios of SiO2/Na2O = 1.2 and H2O/Na2O = 11.5, and mechanisms of decondensation for silicate chains. Indeed, − − ff fi [Si] = 5.8 mol·L 1 and [Na+] = 13 mol·L 1, which correspond long-time trajectories o er a large set of con gurations of 9 interest to observe multiple bond-breaking incidents. to molar ratios of SiO2/Na2O = 0.89 and H2O/Na2O = 11.5). In this work, we employ statistical analysis to describe the The NMR spectra corresponding to the two solutions are dissociation mechanisms, triggered by the addition of sodium presented in Figure 1 in the initial state and after hydroxide, that has been reproduced by accelerated molecular decondensation. As usually observed, the mean connectivity dynamics. The loss of connectivity is compared to experiments of the silicate species decreases when the ratio SiO2/Na2O for two different solutions. Both systems contain sodium decreases.9,18 silicates in solution. In particular, we detail how several water − molecules or OH groups could participate in bond Received: February 20, 2020 dissociation. Revised: March 16, 2020 Published: March 25, 2020 II. METHODS II.A. Experimental Samples. The chemical composition of the two sodium silicate solutions were adjusted according to © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.jpcc.0c01495 8288 J. Phys. Chem. C 2020, 124, 8288−8294 The Journal of Physical Chemistry C pubs.acs.org/JPCC Article Figure 1. NMR spectra as a function of the chemical composition for (a) Zsol and (b) Gsol. II.B. Simulated Solutions. In order to study the decondensation of Gsol and Zsol upon the addition of sodium hydroxide, we have first built two structures for the simulation. The stoichiometry and the connectivity of the initial structure have been chosen to be in agreement with the experimental data obtained for Zsol (see Table 1). The two structures that has been simulated are given in Figure 2. The colloidal liquid is composed of 50 SiO4 groups, initially organized in 4 silicate species, 2344 water molecules, and 28 Na+ and OH− groups. The simulation cell size has been set to 42 × 42 × 42 Å3 in order to correspond to the experimental density of 1.135 kg/m3. The system Gsol is composed of 40 SiO groups, initially organized in 14 silicate 4 fi species ranging from monomers to quadrimers, 294 water Figure 2. Snapshot of the original con gurations. Silicon, oxygen, and molecules, and 94 Na+ and OH− groups. The simulation cell sodium atoms are represented in yellow, red, and blue, respectively. The water molecules have been omitted for clarity. size has been set to 30 × 20 × 20 Å3, in agreement with the experimental density that has been measured experimentally (about 1.35 kg/m3). described. The temperature has been controlled using the II.C. Simulation Details. In order to simulate the effect of Nose−́Hoover thermostat.26 Brief-PT runs were simulated adding sodium hydroxide to these two systems, we have used a using the NVT ensemble. If the average distribution of the Qn method based on the parallel tempering (PT) technique, does not evolve for 1000 steps, the convergence is achieved. which consists of replicating the initial state and running For both systems, the convergence has been reached after dynamics at different temperatures in order to enhance energy about 200 ps of PT; note that this does not correspond to real barrier crossing. After 10 steps, if it is energetically favorable, time. The Qn distributions have been compared to nuclear replicas are exchanged.19,20 As detailed by Dupuis et al.,16 in magnetic resonance measurements made after decondensation. brief-PT, the simulation is interrupted regularly (herein every The agreement with the experiments is good for both solutions 1000 steps) and all replica are repopulated with the (see Table 1). In the view of previous works, the brief-PT configuration that has been obtained at the lowest temperature. technique enables one to study decondensation in quantitative A set of 16 temperatures ranging from 300 to 800 K has been agreement with experimental data.16,17 used. II.D. Statistical Analysis. In order to study the In order to account for chemical reactions, we have used the decondensation mechanism, we have proceeded to a statistical reactive force field ReaxFF21 using the parameters given by analysis along the trajectories that have been simulated. In Hahn et al.,22 in which parameters for Na are refitted on the particular, we have developed an algorithm to study the basis of the parameters of Fogarty et al.23 The parameters for mechanism of decondensation (see Figure 3). In this Si/O/H have been previously tested and used to study various algorithm, we are seeking for water molecules or hydroxides silicate systems.16,22,24,25 In particular, it is possible to that enter the near environment during the dissociation, which reproduce the polymerization and depolymerization of silicates defines one event. The environment of the dissociation, as using this set of parameters. Moreover, for silica-base gel shown in Figure 3, is the space defined by the sum of two ffi formation, this set of parameters has shown good e ciency to spheres of radius Rc centered on the position of the two silicon simulate the polymerization of pure silica gels.14 atoms that are being dissociated. After each dissociation, we All simulations have been carried out with LAMMPS code rewound the dynamics and counted the number of events that using the python interface. The time step was set to 0.2 fs to occurred during the last 250 000 steps of the simulation. Since ensure that the dynamics of water molecules is correctly water molecules can be dissociated in the near environment 8289 https://dx.doi.org/10.1021/acs.jpcc.0c01495 J.