Ion specific hydration in nano-confined electrical double layers

We probe counter-ion specific effects in nano-confined electrical-double-layers (EDLs) by means of direct force measurements across a model slit pore in the extended surface forces apparatus. By variation of solution composition, we compare four different counter-ions with dissimilar hydration properties, namely Na⁺, K⁺, Cs⁺ as well as H₃O⁺, all confined between (0 0 1) mica surfaces. We discuss the results in terms of a recently proposed π–transition model, evoking hydrated ion layering as the recurrent structural element in highly confined EDLs. We demonstrate that the π–transition essentially proceeds in multiple steps in coherence with the event of single- or multiple- hydrated-ion layering.