Rheology of the Electric Double Layer in Electrolyte Solutions

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-methylimidazolium chloride (BmimCl); an ordinary salt, sodium chloride (NaCl); and a nonelectrolyte, 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 toward 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 of the EDL 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.

Rheological Scaling of Ionic Liquid-Based Polyelectrolytes in the Semidilute Unentangled Regime from Low to High Salt Concentrations

“…The characteristic decay length (much larger than the value predicted by the DH theory) was considered as the screening length in the IL. Similar long-range exponential decay of separation forces, therefore larger screening lengths, have been confirmed by several experimental − and theoretical − studies for both ILs and ordinary salts. Moreover, the ionic correlation effect on the charge screening is found to become more important for salts having large ion diameters, such as ILs …”

“…When the salt concentration becomes sufficiently high, the scaling predictions of η sp and λ shown in eqs – are expected to become inapplicable due to the breakdown of the DH theory . Indeed, several research groups recently reported an unexpectedly large screening length in concentrated salt solutions, including both ionic liquids and ordinary salts. ,,− , In particular, Smith et al showed that the screening length in the low-salt concentration regime decreased with increasing salt concentrations, consistent with the DH prediction. With a further increase in the salt concentration, the screening length reached a minimum, followed by an upturn due to strong ionic correlations at high salt concentrations.…”

Studying Soft Interfaces with Shear Waves: Principles and Applications of the Quartz Crystal Microbalance (QCM)

“…Unfortunately, there are numerous sources of artifacts at the low end of the QCM’s sensitivity range. Among these are the viscoelasticity of the double layer [ 48 , 217 ], nanobubbles [ 218 ], slip [ 219 ], roughness [ 216 ], piezoelectric stiffening [ 176 , 220 ], and static stress, which bends the plate [ 53 ].…”

Bifunctional Metal Meshes Acting as a Semipermeable Membrane and Electrode for Sensitive Electrochemical Determination of Volatile Compounds

“…Meshes with a 40 μm hole diameter resulted in a higher amount of vapor collected in the electrolyte compared with the usual PTFE membrane. In this groundbreaking fashion, the diffusion of this analyte vapor dissolved in an electrolyte receptor occurs from an electric double layer (EDL) to bulk that is opposite to that one noted in electrochemical cells (such phenomenon was abbreviated as reverse diffusion throughout this article). , While this kind of diffusion is supposed to gradually reduce the target amount at the EDL as shown in Scheme C, diminishing the currents, analyses using the own membrane as an electrode were revealed to enable highly sensitive determinations as discussed next.…”