Light scattering measurements on microemulsions: estimation of droplet sizes

Different scattering methods were used as tools to assess the size of droplets in highly diluted microemulsions. These were obtained after dilution of a self-emulsifying system made up of an oil, a surfactant and ethanol. Typical methods, often used in size and shape determination of particles, such as SAXS and USAXS suffer in the present case from a lack of electrondensity contrast. It becomes clear from our extensive use of dynamic light scattering that one should be careful in interpreting the latter data as well. Sample preparation and the subsequent handling of the samples during the experiments strongly affect reproducibility of the results. There is a need for well-defined protocols at the level of sample preparation and data handling. In the present research one uses extensively dynamic light scattering (DLS) in the back scattering mode and strengths and pitfalls, inherent to the backscattering technique, are discussed. It is crucial to be aware of droplet size distributions (monomodal/bimodal/multimodal) while reporting mean radii (Rh) as this radius is only relevant in the case of well-defined monomodal distributions. Moreover, one should asses the shape of the droplets prior to data interpretation, as usual in scattering methods, by an independent method. Anyway the shape of the time correlation functions of the scattered intensity should be reported or at least inspected as they provide information on the reproducibility of the experiments hence safegarding the value of the physical meaning of the final value of droplet size (Rh). Preferentially static light scattering (SLS) measurements should always support DLS experiments as the angular dependence is very sensitive to the presence of large particles.