Intricate coupling between ion-ion and ion-surface correlations in double layers as illustrated by charge inversion-combined effects of strong Coulomb correlations and excluded volume.

Many-body correlations in electrolyte systems are important when the electrostatic coupling and/or the volume fraction of ions are not low. Such correlations are ignored in the traditional theories of electrolytes based on the Poisson-Boltzmann approximation. In the general case, the ion density profiles (ion-surface correlation functions) and the ion-ion correlation functions in diffuse electric double layers are strongly interdependent. Both have to be included in the treatment of the system to capture many essential properties. In this work the coupling between the ion-ion and ion-surface correlations and effects of this coupling are illustrated explicitly and graphically (visually). The average forces that act on the ions in the double layer are analysed. This leads to an understanding of mechanisms in action in the inhomogeneous electrolyte near a surface. Charge separation in an electrolyte outside an uncharged surface and charge inversion of highly charged surfaces are thereby used as examples of what insights can be gained by this kind of approach. Some links to mechanisms behind like-charge attraction are also discussed.