Electric double layer in concentrated solutions of ionic surfactants

TL;DR

This paper develops a non-local theoretical model for concentrated ionic surfactant solutions, revealing temperature-dependent phase separation behaviors and electric double layer structures using coupled thermodynamic and electrostatic analysis.

Contribution

It introduces a novel non-local model combining Cahn-Hilliard theory with electrostatics to analyze ionic surfactant solutions at various temperatures.

Findings

Unstable solutions have two critical temperatures with discontinuous characteristic lengths.

Below the lower critical temperature, solutions form layered structures with periodic electric potential.

Between the critical temperatures, solutions exhibit oscillatory-decaying electric double layers.

Abstract

A simple non-local theoretical model is developed considering concentrated ionic surfactant solutions as regular ones. Their thermodynamics is described by the Cahn-Hilliard theory coupled with electrostatics. It is discovered that unstable solutions possess two critical temperatures, where the temperature coefficients of all characteristic lengths are discontinuous. At temperatures below the lower critical temperature ionic surfactant solutions separate into thin layers of oppositely charged liquids spread across the whole system and the electric potential is strictly periodic. At temperatures between the two critical temperatures separation can occur only near the solution surface thus leading to an oscillatory-decaying electric double layer. At temperatures above the higher critical temperature as well as in stable solutions there is no separation and the electric potential decays exponentially.