Acid-base dissociation mechanisms at the silica-water interface: A DFT study

Lowe, Benjamin, Skylaris, Chris-Kriton and Green, Nicolas (2016) Acid-base dissociation mechanisms at the silica-water interface: A DFT study. 251st American Chemical Society National Meeting & Exposition - Computers in Chemistry, San Diego, United States. 13 - 17 Mar 2016.

Abstract

One of the fundamental effects of the hydration of silica surfaces is the production of surface charges. This phenomenon is believed to be primarily a consequence of proton uptake and release at the silica/water interface [1]. Hydrated silica surfaces have been the focus of much experimental and simulation work due to their importance in nanodevices such as Ion-Sensitive Field Effect Transistors (IS-FETs) [2], however the electrostatics and dynamics of these surfaces are not well understood. For example, there is still no consensus on whether the partial charge or unit charge model is more representative of the underlying charging mechanism [3].

In this work, Density Functional Theory (DFT) calculations are used to investigate the fundamental protonation/deprotonation reactions at the silica/water interface. The silica surface was modelled as a periodic slab, whereas the solvent was modeled as a hydrated cluster containing hydronium or hydroxide ions. These reactions were found to be highly exothermic and activationless, and showed a rate of reaction limited by reorientation of the nearby water molecules. A new mechanism of surface protonation was demonstrated where hydronium ions protonate the surface without transfer of their protons to the surface, this occurred via stabilising the dissociation of intermediate water molecules. Understanding these kinetics is vital to interpreting signal-to-noise characteristics of chemically sensitive nanodevices such as nano-IS-FETs. [4]

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Contributors

Author: Chris-Kriton Skylaris

Author: Nicolas Green

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