Bridging oxygen as a site for proton adsorption on the vitreous silica surface

Abstract

Molecular dynamics computer simulations were used to study the protonation of bridging oxygen (Si-O-Si) sites present on the vitreous silica surface in contact with water using a dissociative water potential. In contrast to first-principles calculations based on unconstrained molecular analogs, such as H(7)Si(2)O(7)(+) molecules, the very limited flexibility of neighboring SiO(4) tetrahedra when embedded in a solid surface means that there is a relatively minor geometric response to proton adsorption, requiring sites predisposed to adsorption. Simulation results indicate that protonation of bridging oxygen occurs at predisposed sites with bridging angles in the 125 degrees-135 degrees range, well below the bulk silica mean of approximately 150 degrees, consistent with various ab initio calculations, and that a small fraction of such sites are present in all ring sizes. The energy differences between dry and protonated bridges at various angles observed in the simulations coincide completely with quantum calculations over the entire range of bridging angles encountered in the vitreous silica surface. Those sites with bridging angles near 130 degrees support adsorbed protons more stably, resulting in the proton remaining adsorbed for longer periods of time. Vitreous silica has the necessary distribution of angular strain over all ring sizes to allow protons to adsorb onto bridging oxygen at the surface, forming acidic surface groups that serve as ideal intermediate steps in proton transfer near the surface. In addition to hydronium formation and water-assisted proton transfer in the liquid, protons can rapidly move across the water-silica interface via strained bridges that are predisposed to transient proton adsorption. Thus, an excess proton at any given location on a silica surface can move by either water-assisted or strained bridge-assisted diffusion depending on the local environment. The result of this would be net migration that is faster than it would be if only one mechanism is possible. These simulation results indicate the importance of performing large size and time scale simulations of the structurally heterogeneous vitreous silica exposed to water to describe proton transport at the interface between water and the silica surface.

Atomistic computer simulations of water interactions and dissolution of inorganic glasses

“…63 With advances in potential development the role of the original defective surface in forming silanol concentrations were identified. 54,56,64 Classical MD simulations have also been able to reproduce both of the silica-water reaction mechanisms found in ab initio simulations 65,66 due to the inclusion of DFT reaction pathways and energies. 59 Classical MD simulations tend to favor siloxane bond breakage by absorption of a proton onto a bridging oxygen site, noted by Lockwood and Garofalini 65 and Rimsza, van Duin, and Du.…”

Prediction of Specific Biomolecule Adsorption on Silica Surfaces as a Function of pH and Particle Size

“…Prior force fields often required fixed atoms to avoid collapse of the models in the simulation, neglected the pH dependence of the surface chemistry, and involved other drastic approximations so that even approximate predictions of specific binding of biomolecules were essentially impossible. [43][44][45][46][47][48][49][50][51][52][53][54][55][56] The new, thermodynamically consistent silica parameters are compatible with comprehensive harmonic force fields for biopolymers, organic molecules, and inorganic compounds such as 7 CHARMM, AMBER, PCFF, COMPASS, CVFF, and INTERFACE. 26,37 The compatibility enables insight into a limitless number of silica hybrid materials by the possible combination of thousands of distinct silica surface structures with billions of distinct biopolymers, surfactants, and receptor molecules across a wide range of concentrations and solution conditions.…”

Reactions between water and vitreous silica during irradiation

“…This model benefits from being able to reproduce the behavior of bulk water, bulk silica, and the water-silica interface in terms of structure, chemistry, transport processes, energetics, and other properties [21,23,26,27]. It also has been used previously to simulate the effects of irradiation in silica [16] and to reproduce the characteristic rapid formation of damage followed by picosecond-scale healing produced by other MD models [13,28].…”

“…From this temperature, the system was quenched to a temperature of 300 K via intermediate steps in a manner identical to previous work [21].…”

“…For example, abnormally high proton conduction has been observed in mesoporous silica [17][18][19][20], and evidence suggests that defects play a significant role in H + mobility in water-silica systems [21][22][23][24][25]. Given the fact that ballistic radiation creates large concentrations of defects in silica, it is not unreasonable to consider how radiation damage affects the mobility of H + at the silica surface and into the bulk glass.…”