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Systematic dependence of kinetic and thermodynamic barriers to homogeneous silica nucleation on NaCl and amino acids

P. Dove,N. Han,A. Wallace

Published 2019 in Journal of Materials Research

ABSTRACT

The kinetics of silica polymerization was measured in silicic acid solutions containing a suite of 0.1 M amino acids, 0.1 M citric acid, 0.7 M NaCl, and 0.10 M NaCl (Control). Fitting a modified classical rate model to measurements of induction time (τ) at 20 °C for a series of supersaturated solutions, we estimate the thermodynamic barrier (Δ G _c), interfacial free energy (γ), and kinetic barrier (Δ_a g ^k) for silica nucleation. For 0.10 M NaCl solutions, γ_Control = 54.9 ± 1.6 mJ/m^2 and Δ_a g ^k_Control = 2.29 × 10^−19 J/mol. These values are consistent with previous reports for amorphous and fused silica materials. To facilitate comparisons with the treatments, Δ_a g ^k_Control is converted to a molar basis and used as a reference datum, such that Δ_a g ^k_Control = 0.0 J/mol. The effects of salt and organic acids on nucleation rate have thermodynamic and kinetic origins, respectively. Faster nucleation rates measured in 0.7 M NaCl solutions arise from a lower interfacial free energy, such that γ_0.7 M NaCl = 51.4 ± 1.7 mJ/m^2. Organic acids increase rate through biomolecule-specific reductions in Δ_a g ^k. Catalytic effects are greatest for lysine (Δ_a g ^k_lysine = −1685 ± 315) and citric acid (Δ_a g ^k_citric = −1690 ± 96 J/mol). Reductions in the kinetic barrier correlate with net positive charge of the amino acids and dissociation of the amine $\left( {{K_{\alpha - {\rm{N}}{{\rm{H}}_3}}} + } \right)$ ( K α − NH 3 + ) group and thus the abundance of the conjugate base. Citric acid, lacking amine groups, promotes the greatest rate enhancement, thus demonstrating the role(s) of additional kinetic factors in promoting nucleation rate. Catalytic activity correlates with multiple physical and chemical properties of the organic acids.

PUBLICATION RECORD

  • Publication year

    2019

  • Venue

    Journal of Materials Research

  • Publication date

    2019-02-01

  • Fields of study

    Materials Science, Chemistry

  • Identifiers
    DOI 10.1557/JMR.2018.474
  • External record

    Open on Semantic Scholar

  • Source metadata

    Semantic Scholar

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