On the role of tetramethylammonium cation and effects of solvent dynamics on the stability of the cage-like silicates Si₆O₁₅ ^6- and Si₈O₂₀^8- in aqueous solution. A molecular dynamics study

We have undertaken explicit solvent molecular dynamics simulations to investigate the preferential stabilization of the silicate octamer Si ₈O₁₅⁸ over the hexamer Si₆O ₁₅^6- in relation with the ability of tetramethylammonium (TMA) to form an adsorption layer around these cage-like polyions. We have found that the hexamer cannot support such a layer and as a result is vulnerable to hydrolysis. The dynamics of TMA desorption off the surface of the hexamer is investigated in connection with the solvent dynamics. We have studied the energetics of this preferential stabilization by calculating the relative change in the free energies of formation between the complexes Si₈O ₂₀^8-·8TMA and Si₆O₁₅ ^6-·6TMA and found the former to be more stable by 70 kcal/mol. We also find that the energetics are consistent with experimental data, suggesting that the hexamer is a long-lived metastable species. Furthermore, we have studied the solvent structure and dynamics in the vicinity of both the bare polyions and their complexes with TMA. We have found that, as anticipated, both the octamer and the hexamer participate in hydrogen bonds with the water molecules, regardless of whether a TMA adsorption layer exists or not. In fact, we find that the presence of a TMA adsorption layer has a rather profound effect on the stability of these hydrogen bonds-it increases their lifetime by at least a factor of 2 relative to that of the hydrogen bonds between water and the bare polyions.