Specific Ion Effects in Colloidal Surface Forces

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• Drew Parsons, Assoc. Prof., University of Cagliari
• Thursday 25 April 2024, 11:30-12:30
• Open Plan Area, Institute for Energy and Environmental Flows, Madingley Rise CB3 0EZ.

If you have a question about this talk, please contact Chris Richardson.

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.

References

[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. https://dx.doi.org/10.1021/la902533x

[2] Buffer-specific effects arise from ionic dispersion forces. D.F. Parsons, C. Carucci, A. Salis. Phys. Chem. Chem. Phys., 2022, 24, 6544. https://dx.doi.org/10.1039/d2cp00223j

[3] Thermodynamics beyond dilute solution theory: Steric effects and electrowetting. D. Tadesse, D.F. Parsons. In: Encyclopedia of Solid-Liquid Interfaces (2024). https://dx.doi.org/10.1016/B978-0-323-85669-0.00137-9

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

This talk is included in these lists:

• All Talks (aka the CURE list)
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• Institute for Energy and Environmental Flows (IEEF)
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• Open Plan Area, Institute for Energy and Environmental Flows, Madingley Rise CB3 0EZ
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