Self-Diffusion and Collective Diffusion of Charged colloids Studied by Dynamic Light Scattering

TL;DR

This study investigates the dynamics of charged microemulsion droplets using dynamic light scattering, revealing a previously unobserved slow diffusive mode linked to local charge fluctuation relaxation.

Contribution

It introduces the observation of a new slow diffusive relaxation mode in charged microemulsions, distinct from known collective diffusion processes.

Findings

Charged microemulsions exhibit two relaxation modes, fast and slow.

The slow mode is attributed to local charge fluctuation relaxation.

The slow mode's diffusion coefficient matches the self diffusion of droplets.

Abstract

A microemulsion of decane droplets stabilized by a non-ionic surfactant film is progressively charged by substitution of a non-ionic surfactant molecule by a cationic surfactant. We check that the microemulsion droplets remain identical within the explored range of volume fraction (0.02 to 0.18) and of the number of charge per droplets (0 to 40) . We probe the dynamics of these microemulsions by dynamic light scattering. Despite the similar structure of the uncharged and charged microemulsions the dynamics are very different . In the neutral microemulsion the fluctuations of polarization relax, as is well known, via the collective diffusion of the droplets. In the charged microemulsions, two modes of relaxation are observed. The fast one is ascribed classically to the collective diffusion of the charged droplets coupled to the diffusion of the counterions. The slow one has, to our knowledge, not been observed previously neither in similar microemulsions nor in charged spherical colloids. We show that the slow mode is also diffusive and suggest that its possible origine is the relaxation of local charge fluctuations via local exchange of droplets bearing different number of charges . The diffusion coefficient associated with this mode is then the self diffusion coefficient of the droplets.