Experimental Evidence for Algebraic Double-Layer Forces

Uncharged Monolithic Carbon Fibers Are More Sensitive to Cross-Junction Compression than Charged

Textile-based wearable robotics increasingly integrates sensing and energy materials to enhance functionality, particularly in physiological monitoring, demanding higher-performing and abundant robotic textiles. Among the alternatives, activated carbon cloth stands out due to its monolithic nature and high specific surface area, enabling uninterrupted electron transfer and energy storage capability in the electrical double layer, respectively. Yet, the potential of monolithic activated carbon cloth electrodes (MACCEs) in wearables still needs to be explored, particularly in sensing and energy storage. MACCE conductance increased by 29% when saturated with Na2SO4 aqueous electrolyte and charged from 0 to 0.375 V. MACCE was validated for measuring pressure up to 28 kPa at all assessed charge levels. Electrode sensitivity to compression decreased by 30% at the highest potential due to repulsive forces between like charges in electrical double layers at the MACCE surface, counteracting compression. MACCE's controllable sensitivity decrease can be beneficial for garments in avoiding irrelevant signals and focusing on essential health changes. A MACCE charge-dependent sensitivity provides a method for assessing local electrode charge. Our study highlights controlled charging and electrolyte interactions in MACCE for multifunctional roles, including energy transmission and pressure detection, in smart wearables.

Charge regulation indicates water expulsion from silica surface by cesium cations

HYPOTHESIS

Since the discovery of the Hofmeister effect in 1888, the varied propensity of ions to proteins, DNA and other surfaces has motivated research aimed at deciphering the underlying ion specific adsorption mechanism. Experimental and numerical studies have shown that in agreement with Collins' heuristic law of matching water affinity, weakly hydrated (chaotropic) ions adsorb preferentially to hydrophobic surfaces. Here, we show that this preference is driven by expulsion of bound water molecules from the surface by the adsorbing ions.

EXPERIMENTS

Using AFM spectroscopy of the force acting between two silica surfaces, we characterize surface charge regulation by adsorbed Na⁺ and Cs⁺ ions at different salt concentrations, pH values and temperatures. These data are analyzed in the framework of a recent theory of charge regulation, relating it to change in surface entropy.

FINDINGS

Upon binding to the silica, cesium cations expel water molecules from the surface to create additional adsorption sites for more ions. Cs⁺ adsorption is thus driven by the release of hydrating water molecules and the resulting increased surface entropy. The model indicates that on average, the binding of three cesium cations releases enough water molecules to make room for two additional bound cations. Na⁺ does not exhibit such behavior.

Relation of EDL Forces between Clay Particles Calculated by Different Methods

Calculation of the electrostatic double layer force (EDL force) between clay particles is relevant as it is closely related to important macroscopic mechanical behaviors of clays. The popular method to calculate the EDL force is to integrate the electric potential and Maxwell stress along the boundary enclosing a simply connected domain within which a clay particle resides. The EDL force has also been calculated by the integration of the electrostatic force density over the preceding domain. However, the subtle relation of the EDL forces calculated by the different existing methods has not yet been investigated. By means of theoretical analysis and finite element simulation, it was shown that the force calculated by the integration of Maxwell stress along the complete boundary enclosing a multiply connected domain in which the clay particle is excluded, and that along the partial boundary enclosing the preceding simply connected domain represents the electrical attractive force and osmotic repulsive force, respectively, while the integration of the potential along both the same complete and partial boundary denotes the osmotic force. Numerical results showed that the calculated EDL force deviates from its actual value significantly with the decrease in distance between the chosen integral boundary and particle surface, and the deviation varies with surface potential and angle between particles. Moreover, the recommended minimum distance was proposed to be 10 times the thickness of the particle based on the present simulation results.

Rigorous treatment of pairwise and many-body electrostatic interactions among dielectric spheres at the Debye-Hückel level

Electrostatic interactions among colloidal particles are often described using the venerable (two-particle) Derjaguin-Landau-Verwey-Overbeek (DLVO) approximation and its various modifications. However, until the recent development of a many-body theory exact at the Debye-Hückel level (Yu in Phys Rev E 102:052404, 2020), it was difficult to assess the errors of such approximations and impossible to assess the role of many-body effects. By applying the exact Debye-Hückel level theory, we quantify the errors inherent to DLVO and the additional errors associated with replacing many-particle interactions by the sum of pairwise interactions (even when the latter are calculated exactly). In particular, we show that: (1) the DLVO approximation does not provide sufficient accuracy at shorter distances, especially when there is an asymmetry in charges and/or sizes of interacting dielectric spheres; (2) the pairwise approximation leads to significant errors at shorter distances and at large and moderate Debye lengths and also gets worse with increasing asymmetry in the size of the spheres or magnitude or placement of the charges. We also demonstrate that asymmetric dielectric screening, i.e., the enhanced repulsion between charged dielectric bodies immersed in media with high dielectric constant, is preserved in the presence of free ions in the medium.

Repulsive and attractive depletion forces mediated by nonadsorbing polyelectrolytes in the Donnan limit

In mixtures of colloids and nonadsorbing polyelectrolytes, a Donnan potential arises across the region between surfaces that are depleted of the polyelectrolyte and the rest of the system. This Donnan potential tends to shift the polyelectrolyte density profile toward the colloidal surface and leads to the local accumulation of polyelectrolytes. We derive a zero-field theory for the disjoining pressure between two parallel flat plates. The polyelectrolyte is allowed to enter the confined interplate region at the cost of a conformational free energy penalty. The resulting disjoining pressure shows a crossover to a repulsive regime when the interplate separation gets smaller than the size of the polyelectrolyte chain, followed by an attractive part. We find a quantitative match between the model and self-consistent field computations that take into account the full Poisson-Boltzmann electrostatics.

Effect of Nonzero Solid Permittivity on the Electrical Repulsion between Charged Surfaces

The electrical repulsion between two charged solid surfaces separated by an electrolyte is studied as a function of the permittivity ϵ_s of the solid in the limit in which potentials are small, and the gap between the plane solid surfaces is small compared to the Debye length κ^-1 within the electrolyte. The solid surfaces are uniformly charged in a central region |x|< L outside which they are uncharged. When ϵ_s = 0, ions from the charge cloud between the charged surfaces spill out into regions of length O(κ^-1) beyond x = ± L, thereby reducing the pressure between the surfaces from that predicted by Derjaguin-Landau-Verwey-Overbeek theory for infinite, uniformly charged surfaces. When ϵ_s>0, ions spill out over much larger O(L) regions, thereby reducing still further both the electrical potential between the solid surfaces and the repulsive force between them. However, this reduction becomes smaller as κL becomes large.