What are steric effects? A purely continuum model

Alain Boldini, Maurizio Porfiri

Research output: Chapter in Book/Report/Conference proceedingConference contribution


In electrolyte solutions, the application of an external voltage elicits a series of complex microscopic phenomena. When there is no charge transfer between solution and electrodes (non-Faradaic processes), an extremely thin electric double layer is formed on each of the electrodes, screening the bulk of the solution from the external electric field. In the simplest double layer models, the volume of ions in the solution is neglected. For relatively small voltage values, one can easily reach values of concentrations in the double layers over the physical packing limit. To address this issue, steric effects associated with the finite volume of ions are introduced, toward limiting the maximum concentration in the double layers. These effects are often introduced at a microscopic scale, through modifications of the entropy of mixing. However, the macroscopic interpretation of these models remains elusive. Here, we propose a purely continuum model of steric effects in electrolyte solutions. We show that including steric effects at a microscopic scale is equivalent to requiring that each constituent of the solution is incompressible at a macroscopic level. Incidentally, the macroscopic model easily extends steric effects to multiple ions of different sizes, a challenging task for microscopic models. We highlight the consequences of our model on electrolyte solutions and ionic membranes. In particular, we show how our model constitutes a simple mathematical formulation for actuators with ionic liquid solvents. Our effort supports the creation of physics-based models of ionic actuators, facilitating their mathematical description.

Original languageEnglish (US)
Title of host publicationElectroactive Polymer Actuators and Devices (EAPAD) XXV
EditorsHerbert R. Shea
ISBN (Electronic)9781510660717
StatePublished - 2023
EventElectroactive Polymer Actuators and Devices (EAPAD) XXV 2023 - Long Beach, United States
Duration: Mar 13 2023Mar 15 2023

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


ConferenceElectroactive Polymer Actuators and Devices (EAPAD) XXV 2023
Country/TerritoryUnited States
CityLong Beach


  • Electrolyte solutions
  • incompressibility
  • ions
  • steric effects

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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