Elucidating the Bimodal Acid-base Behavior of the Water-silica Interface from First Principles

TL;DR

This study uses first-principles simulations to uncover the structural origins of the bimodal acid-base behavior of silica surfaces, revealing that strained or defective regions host highly acidic silanol groups.

Contribution

It identifies the structural motifs responsible for the high acidity of certain silanol groups on silica surfaces, challenging previous hypotheses.

Findings

Highly acidic silanol groups are likely in strained or defected regions.

Chemical connectivity and hydrogen bonding do not account for high acidity.

Ring-opening reactions of silica rings occur in contact with water.

Abstract

Understanding the acid-base behavior of silica surfaces is critical for many nanoscience and bio-nano interface applications. Silanol groups (SiOH) on silica surfaces exhibit two acidity constants--one as acidic as vinegar--but their structural basis remains controversial. The atomic details of the more acidic silanol site govern not just the overall surface charge density at near neutral solution pH, but also how ions and bio-molecules interacts with and bind to silica immersed in water. Using initio molecular dynamics simulations and multiple representative crystalline silica surfaces, we determine the deprotonation free energies of silanol groups with different structural motifs. We show that previously proposed motifs related to chemical connectivity or inter-silanol hydrogen bonds do not yield high acidity. Instead, a plausible candiate for pKa=4.5 silanol groups may be found in locally strained or defected regions with sparse silanol coverage. In the process, irreversible ring-opening reactions of strained silica trimer rings in contact with liquid water are observed.