Laboratoire de Physique Théorique
Toulouse - UMR 5152

Three members of the Statistical Physics group of the LPT have proposed in an article appearing in Physical Review Letters that a novel ionic “liquid-vapor” phase transition occurring for electrolytes in weakly charged nanopores could explain the time-dependent electrical conductivity fluctuations observed across certain biological and artificial ion channels.

Sketch of a cylindrical nanopore (radius a) filled with counterions and coions, showing the surface charge density and the ionic screening in the pore and in the bulk.

Transport through biological ion channels can exhibit such seemingly unexpected behavior in line with their natural functions that it has long been thought that at least some of the underlying mechanisms must have specific biochemical underpinnings. One outstanding example is the so-called gating phenomena controlling conductivity fluctuations between high and low conducting states. It therefore came as a great surprise when similar fluctuations were measured using patch-clamp methods for single well-characterized artificial nanopores.

Using a variational field theory, the authors have shown this novel ionic “liquid-vapor” phase transition could provide a common physical mechanism, underlying the observed conductivity fluctuations (without the need to invoke any specific biochemical ones). The conductivity fluctuations would then be a manifestation of the fluctuation induced switching between the high (liquid) and low (vapor) density phases of the nano-confined electrolyte. This phase transition is the ionic analog of the confinement modified liquid-vapor water “capillary evaporation” transition occurring in hydrophobic nanopores.

A better understanding of the functioning of ion channels could lead to novel and highly efficient artificially fabricated nanofilters, for example for seawater desalination and high performance particle and macromolecule (DNA,...) sensors.

Details can be found in the article Ionic Capillary Evaporation in Weakly Charged Nanopores, Sahin Buyukdagli, Manoel Manghi, and John Palmeri, Physical Review Letters 105, 158103 (2010)

Phase diagram for various membrane dielectric constants. Critical lines correspond to phase boundaries between the ionic-penetration phase (liquid phase L, above) and the ionic exclusion one (vapor phase V, below).