Surface acidity of quartz: understanding the crystallographic control

Area of Science:

• * Surface Science
• * Computational Chemistry
• * Geochemistry

Background:

• * Understanding the surface chemistry of quartz is crucial for various applications, including catalysis, environmental remediation, and materials science.
• * The acid-base properties of silanol groups on quartz surfaces dictate their reactivity and interactions with other species.
• * Previous studies have provided insights into quartz surface properties, but a detailed understanding of intrinsic silanol acidity across different facets remains incomplete.

Purpose of the Study:

• * To investigate the surface acid chemistry of quartz growth surfaces, specifically the (101̄0) and (101̄1) facets.
• * To characterize interfacial hydration structures and determine the intrinsic pKa values of surface silanols.
• * To discuss the implications of these acidity constants for metal cation complexation on quartz surfaces.

Main Methods:

• * First Principles Molecular Dynamics (FPMD) simulations were employed to study quartz surface acid chemistry.
• * The vertical energy gap method was utilized to evaluate the intrinsic pKa values of surface silanols.
• * Detailed characterization of interfacial hydration structures was performed.

Main Results:

• * All quartz surface terminations exhibit bimodal acid-base behavior.
• * Doubly-protonated silanol forms (SiOH2) have pKa values below -2.5, indicating minimal protonation under common pH conditions.
• * Surface silanol pKa values were categorized into three groups, with the most acidic at pKa = 4.8 (donor SiOH on (101̄0)-beta) and the least acidic at pKa > 11.0 (inner silanols on (101̄1)).

Conclusions:

• * The calculated pKa values provide a quantitative understanding of quartz surface acidity.
• * The distinct acidity of different silanol groups influences their protonation states across a range of pH conditions.
• * These findings are essential for predicting and understanding the complexation behavior of metal cations on quartz surfaces.