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Specific Ion Effects in Colloidal Surface Forces

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The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of the interactions of colloid particles has provided a useful framework for understanding general trends determining adsorption and aggregation of micro- and nanoparticles. The point-charge (Poisson-Boltzmann or Debye-Hückel) theory of electrolytes characterises the nature of the electrolyte solely by its pH and Debye length or ionic strength. So conventional theory is incapable of predicting the ion-specific distinction between, for instance, NaCl and KCl solutions, or between phosphate and citrate pH buffer solutions. But ion-specific phenomena (Hofmeister effects) are ubiquitous, and observed in protein aggregration, enzyme adsorption on nanoparticles, particle diffusion coefficients, charge reversal effects, bubble coalescence, lipid self-assembly, electrode capacitance.

Ion specificity essentially arises from the distinct electron structure of different ions. We identify two competing consequences. On the one hand, electronic polarisability drives ion dispersion forces [1], leading to adsorption of coions, or excess adsorption of counterions resulting in charge reversal [2]. On the other hand, the size of the electron cloud drives ionic steric forces, resulting in a limit to the concentration of adsorbed ions that results, for instance, in a diminution of electrode capacitance [3].

We account for these effects as additional nonelectrostatic contributions to the total chemical potential of ions, applied in a modified Poisson-Boltzmann model. For basic development of the ideas we use symmetry to simplify the geometry to 1D calculations. But implementing the solution using finite element methods, we obtain a framework that will be used to model the complex 3D geometries of porous electrodes and self-assembled lipid crystal phases. One long term aim is to predict the phase transitions between hexagonal, cubic and micellar phases relevant to, for instance, the physiology of RNA (COVID) vaccines.


[1] Importance of Accurate Dynamic Polarizabilities for the Ionic Dispersion Interactions of Alkali Halides. D.F. Parsons, B.W. Ninham. Langmuir 2010, 26(3), 1816–1823.

[2] Buffer-specific effects arise from ionic dispersion forces. D.F. Parsons, C. Carucci, A. Salis. Phys. Chem. Chem. Phys., 2022, 24, 6544.

[3] Thermodynamics beyond dilute solution theory: Steric effects and electrowetting. D. Tadesse, D.F. Parsons. In: Encyclopedia of Solid-Liquid Interfaces (2024).

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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