Salt-induced changes of colloidal interactions in critical mixtures

TL;DR

This study investigates how salt influences colloidal interactions in critical binary liquid mixtures, revealing a crossover from attraction to repulsion linked to ionic distributions, distinct from salt-free behaviors.

Contribution

It uncovers salt-dependent crossover behaviors in colloidal interactions and suggests a coupling between ionic distributions and concentration profiles in critical mixtures.

Findings

Salt induces a temperature-dependent crossover from attractive to repulsive forces.

Bulk critical fluctuations remain unaffected by salt addition.

Observed effects are likely due to ionic and concentration profile coupling, not critical Casimir forces alone.

Abstract

We report on salt-dependent interaction potentials of a single charged particle suspended in a binary liquid mixture above a charged wall. For symmetric boundary conditions (BC) we observe attractive particle-wall interaction forces which are similar to critical Casimir forces previously observed in salt-free mixtures. However, in case of antisymmetric BC we find a temperature-dependent crossover from attractive to repulsive forces which is in strong contrast to salt-free conditions. Additionally performed small-angle x-ray scattering experiments demonstrate that the bulk critical fluctuations are not affected by the addition of salt. This suggests that the observed crossover can not be attributed alone to critical Casimir forces. Instead our experiments point towards a possible coupling between the ionic distributions and the concentration profiles in the binary mixture which then affects the interaction potentials in such systems.