Salt-induced changes of colloidal interactions in critical mixtures

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.Comment: 5 pages, 4 Figure

The effect of antagonistic salt on a confined near-critical mixture

“…the surfaces of particles [17][18][19][20]. Temperature can be changed in a reversible manner, and reversible structural changes can be induced [20,21,25].…”

“…Theoretical investigations based on the theory of Onuki and Kitamura [6] focused very briefly on colloid particles immersed in the critical mixture with salt [16]. In contrast, the effective potentials between a flat substrate and a colloid particle immersed in the critical mixture with hydrophilic salt, and between two particles in such mixture were measured in several impressive experiments [17][18][19][20][21]. Similarity between the structure factors in the investigated system and sponge phases suggests that an oscillatory force between confining surfaces, observed experimentally for surfactant mixtures [14], could occur in a presence of antagonistic salt too.…”

Casimir amplitudes and capillary condensation of near-critical fluids between parallel plates: Renormalized local functional theory

“…On approaching the solvent criticality, this interaction becomes longranged and universal in the strong adsorption limit 16,32 . Recently, the near-critical colloid-wall interaction has been measured directly at the critical composition 40,41 . However, the interaction should be much larger at offcritical compositions.…”

Temperature-sensitive colloidal phase behavior induced by critical Casimir forces

“…c) Electronic mail: ducnguyen@uva.nl d) Electronic mail: p.g.bolhuis@uva.nl e) Electronic mail: ps@peterschall.de ilar temperature control of phase behavior for many different colloids. 10 The question is then how to predict the phase behavior: because of the complexity of the critical Casimir interactions, the possible importance of many-body effects and the potential coupling between ions and solvent concentration fluctuations, 11,12 there has been much discussion recently about the nature of the interactions and the relevant terms to be included. [13][14][15][16] Effective pair interactions have been discussed to provide a good description of colloidal phase behavior in reasonably dilute suspensions, 13 and such effective interactions have been computed recently based on scaling functions calculated using the Ising model and combined with the Derjaguin approximation, 13 and using simple solvent models with short-range interactions.…”