Elucidating the Bimodal Acid−Base Behavior of the Water−Silica Interface from First Principles

Abstract

Understanding the acid-base behavior of silica surfaces is critical for many nanoscience and bionano 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 biomolecules interact with and bind to silica immersed in water. Using ab 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 intersilanol hydrogen bonds do not yield high acidity. Instead, a plausible candiate for pK(a) = 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.

In situ Observation of Desorption Reaction of Cytochrome c from Solid/Liquid Interfaces with Slab Optical Waveguide Spectroscopy

“…10 The acid-base behavior of surface silanols was later studied in detail from first principles. 11,12 Their results were almost consistent with that proposed by Ong et al, except for some details. Under the present experimental condition with pH 7.4, silanol site with pKa 4.5 is completely deprotonated and that with pKa 8.5 will be deprotonated by about 10%.…”

Molecular Dynamics Simulations of Water Structure and Diffusion in Silica Nanopores

“…We note that the acidity of silanol groups may depend not only on hydrogen bonding to surface O atoms, but also on the electrostatic screening of charged >SiO -sites by ions in the electrical double layer 11 and on the dielectric screening of those sites by interfacial water 77,81,84 , both of which may vary with confinement or interfacial curvature.…”

“…Therefore, if confinement influences the structure of surface water in silica nanopores, it also could influence the acidity of silanol groups. Indeed, several studies suggest that silanol groups located in silica nanopores have a different acidity than those located on flat silica surface 5,80,81 . :…”

Arsenic Adsorption onto Minerals: Connecting Experimental Observations with Density Functional Theory Calculations

“…POTIM is a time-step variable and its value depends on the type of calculation being performed. Note that because some DFT methods may over structure and freeze water at 300 K, some authors have used higher temperatures to overcome this problem [170]. Another method is to use D instead of H [171], so that a 1 fs time step can be used instead of a 0.5 fs time step.…”