Rheology of the Electric Double Layer in Electrolyte Solutions
- 18 May 2020
- journal article
- research article
- Published by American Chemical Society (ACS) in Analytical Chemistry
- Vol. 92 (12), 8244-8253
- https://doi.org/10.1021/acs.analchem.0c00475
Abstract
Electric double layers (EDLs) are ionic structures formed on charged surfaces and play an important role in various biological and industrial processes. An extensive study in the past decade has revealed the structure of the EDL in concentrated electrolyte solutions of both ordinary salts and ionic liquids. However, how the EDL structure affects their material properties remains a challenging topic due to technical difficulties of these measurements at nanoscale. In this work, we report the first detailed characterization of the viscoelasticity of the EDL formed over a wide range of ion concentrations, including concentrated electrolyte solutions. Specifically, we investigate the complex shear modulus of the EDL by measuring the resonant frequency and the energy dissipation of a quartz crystal microbalance (QCM), a surface-sensitive device, immersed in aqueous solutions containing three types of solutes: an ionic liquid, 1-butyl-3-methyl\-imidazolium chloride (BmimCl); an ordinary salt, sodium chloride (NaCl); and a non-electrolyte, ethylene glycol (EG). For the two electrolyte solutions, we observe a monotonic decrease in the resonant frequency and a monotonic increase in the energy dissipation with increasing ion concentrations due to the presence of the EDL. The complex shear modulus of the EDL is estimated through a wave propagation model in which the density and shear modulus of the EDL decay exponentially towards those of the bulk solution. Our results show that both the storage and the loss modulus of the EDL increase rapidly with increasing ion concentrations in the low ion concentration regime (<1 M), but reach saturation values with similar magnitude at a sufficiently high ion concentration. The shear viscosity near the charged QCM surface is approximately 50 times for NaCl solutions and 500 times for BmimCl solutions of the bulk solution value at the saturation concentration. We also demonstrate that QCM can be utilized for analyzing the rheological properties of the EDL, thus providing a complementary, low-cost, and portable alternative to conventional laboratory instruments such as the surface force apparatus. Our results elucidate new perspectives on the viscoelastic properties of the EDL and can potentially guide device optimization for applications such as biosensing and fast charging of batteries.Funding Information
- Schweizerischer Nationalfonds zur F?rderung der Wissenschaftlichen Forschung
- Japan Society for the Promotion of Science (17K06173, 18H01135, 19K15641, 20K20237)
- Okinawa Institute of Science and Technology Graduate University
- Natural Sciences and Engineering Research Council of Canada
This publication has 62 references indexed in Scilit:
- Self-assembly in the electrical double layer of ionic liquidsChemical Communications, 2011
- Double Layer Structure of Ionic Liquids at the Au(111) Electrode Interface: An Atomic Force Microscopy InvestigationThe Journal of Physical Chemistry C, 2011
- Resonance shear measurement of nanoconfined ionic liquidsPhysical Chemistry Chemical Physics, 2010
- Layering and shear properties of an ionic liquid, 1-ethyl-3-methylimidazolium ethylsulfate, confined to nano-films between mica surfacesPhysical Chemistry Chemical Physics, 2009
- Ionic-liquid materials for the electrochemical challenges of the futureNature Materials, 2009
- Ionic liquid lubricants: designed chemistry for engineering applicationsChemical Society Reviews, 2009
- Characteristics of the series resonant-frequency shift of a quartz crystal microbalance in electrolyte solutionsThe Analyst, 2006
- Viscoelastic Acoustic Response of Layered Polymer Films at Fluid-Solid Interfaces: Continuum Mechanics ApproachPhysica Scripta, 1999
- Importance of electrical double layers in structural and diffusional properties of deionized colloidal suspensionsColloids and Surfaces A: Physicochemical and Engineering Aspects, 1996
- Comparison of double layer potentials in lipid monolayers and lipid bilayer membranesThe Journal of Membrane Biology, 1972