Simultaneous study of molecular and micelle diffusion in a technical microemulsion system by dynamic light scattering

The application of dynamic light scattering (DLS) is well-established for measuring diffusion coefficients related to either molecular or translational micelle diffusion. The simultaneous determination of both transport properties should be feasible, but has not been reported in the literature yet.

Different diffusion modes present in a microemulsion and selected subsystems consisting of a polyol mixture, a binary surfactant mixture, and carbon dioxide (CO₂) were investigated systematically by DLS at temperatures of (314, 333, and 353) K and corresponding pressures of (10, 13, and 16) MPa.

Diffusion coefficients related to molecular and translational micelle diffusion could be measured simultaneously and increase with increasing temperature. From the translational diffusion coefficients, an increase in the hydrodynamic diameter of the micelles from their non-swollen to the CO₂-swollen state being in agreement with literature data for the same and similar microemulsions was found. The effective diffusion coefficients related to the faster molecular diffusion process only observable in the presence of CO₂ are not affected significantly by the surfactant. The time-dependent parts of the recorded intensity correlation functions related to molecular diffusion processes are heterodyne because the scattered light modulated by molecular concentration fluctuations is superimposed with light scattered by the micelles.