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Molecular Dynamics Investigation of Surface Potential and Electrokinetic Phenomena at the Amorphous Silica/Water Interface

Chen, Si-Han
http://orcid.org/0000-0002-4506-3318
http://rave.ohiolink.edu/etdc/view?acc_num=osu1534510054324125

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Chemistry.
The static and dynamic properties of water and ions at an amorphous silica/water electrolyte interface deviate significantly from those in the bulk liquid. The complicated interfacial properties have been extensively studied with various experimental methods, including nonlinear spectroscopy for water polarization and surface potential, and electrokinetic measurements for zeta potential and electrical conductance. We have conducted classical molecular dynamics (MD) simulations for salt ions near the amorphous silica/water interface. The di fference of the ion distributions between MD simulations and those obtained from the Gouy-Chapman (GC) model and the constant capacitance (CC) model is significant. By computing the intensity of the second harmonic generation (SHG), we revealed that the interpretation of the experimental results using the GC and CC models is misleading. To explain why electrokinetic properties of an aqueous electrolyte near a charged silica surface diff er from the one predicted by the Gouy-Chapman-Stern (GCS) model, we performed non-equilibrium MD simulations by applying an electric field to model electroosmotic flow, and with a pressure gradient to model Poiseuille flow. Instead of observing mobile ions in a layer where water is immobile, as postulated in the GCS model, we discovered mobile fluid with increased eff ective fluid viscosity and reduced ion mobility near the silica surface. Our studies show that at low salt concentrations, the GCS model underestimates the electrical conductance by omitting the convection contribution of ion current inside the hypothetical Stern layer. On the contrary, at high salt concentrations, the GCS model overestimates the electrical conductance due to the assumption of homogeneous ion mobilities throughout the mobile layer, whose values equal the bulk mobilities. Extracting static and dynamic properties from the simulation, we proposed a continuum model that provides a better description of ion currents near the amorphous silica/water interface. In the last part of this thesis, we directly analyze experimental electrokinetic data. Combining the continuum model adapted by the simulations and the charge regulation model for the silica surface, we are able to emulate ion distributions and velocity profiles for a slit-type nano-channel with an arbitrary channel height, and a wide range of salt concentration and pH values. The calculated electrical conductance and streaming conductance are found to be equal to the experimental values when appropriate eff ective concentrations of salt and protons are provided.
Sherwin Singer (Advisor)
Barbara Wyslouzil (Committee Member)
Heather Allen (Committee Member)
154 p.
Analytical Chemistry; Energy; Engineering; Nanoscience; Nanotechnology; Physical Chemistry
amorphous silica; electrical double layer; second harmonic generation; Langmuir adsorption; Gouy-Chapman; Poisson-Boltzmann; Stern conductance; electrokinetics; electroosmotic flow; streaming current

Recommended Citations

Citations

  • Chen, S.-H. (2018). Molecular Dynamics Investigation of Surface Potential and Electrokinetic Phenomena at the Amorphous Silica/Water Interface [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534510054324125

    APA Style (7th edition)

  • Chen, Si-Han. Molecular Dynamics Investigation of Surface Potential and Electrokinetic Phenomena at the Amorphous Silica/Water Interface. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1534510054324125.

    MLA Style (8th edition)

  • Chen, Si-Han. "Molecular Dynamics Investigation of Surface Potential and Electrokinetic Phenomena at the Amorphous Silica/Water Interface." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534510054324125

    Chicago Manual of Style (17th edition)

osu1534510054324125
298
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This open access ETD is published by The Ohio State University and OhioLINK.