Drew Parsons, Assoc. Prof., University of CagliariCambridge Fluids Network - fluids-related seminars25 April 2024 11:30amOpen Plan Area, Institute for Energy and Environmental Flows, Madingley Rise CB3 0EZThe 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