Systematic dependence of kinetic and thermodynamic barriers to homogeneous silica nucleation on NaCl and amino acids
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Systematic dependence of kinetic and thermodynamic barriers to homogeneous silica nucleation on NaCl and amino acids Patricia M. Dove1,a), Nizhou Han1, Adam F. Wallace2 1
Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA Department of Geological Sciences, University of Delaware, Newark, Delaware 19716, USA a) Address all correspondence to this author. e-mail: [email protected] 2
Received: 27 July 2018; accepted: 18 November 2018
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 (s) at 20 °C for a series of supersaturated solutions, we estimate the thermodynamic barrier (DGc), interfacial free energy (c), and kinetic barrier (Dagk) for silica nucleation. For 0.10 M NaCl solutions, cControl = 54.9 ± 1.6 mJ/m2 and DagkControl = 2.29 × 1019 J/mol. These values are consistent with previous reports for amorphous and fused silica materials. To facilitate comparisons with the treatments, DagkControl is converted to a molar basis and used as a reference datum, such that DagkControl = 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 c0.7 M NaCl = 51.4 ± 1.7 mJ/m2. Organic acids increase rate through biomolecule-specific reductions in Dagk. Catalytic effects are greatest for lysine (Dagklysine = −1685 ± 315) and citric acid (Dagkcitric = −1690 ± 96 J/ mol). Reductions in the kinetic barrier correlate with net positive charge of the amino acids and dissociation of the amine ðKa‐NH3 +Þ 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.
Introduction The polymerization of silicic acid from aqueous solution has been investigated for more than a century [1], with the majority of studies focused on inorganic systems and the formation of amorphous silica as small particles, sols, and gels. Thorough reviews of these systems are presented in the chemistry, materials, and geological literature [2, 3, 4, 5, 6, 7, 8]. More recently, the geochemical community has recognized the significance of biological silica mineralization in the global biogeochemical cycling of silicon. In modern seas, diatoms, sponges, and radiolaria sequester H4SiO4 and polymerize amorphous silica to produce functional skeletal structures— sometimes within a matter of hours [9]. Although low Si concentrations prevent a significant amount of silicic acid
ª Materials Research Society 2019
polymerization in seawater, bio
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