Liquid-liquid phase transitions of ionic solutions were a challenging subject in Statistical Physics and provoking experimental works.The fundamental aspect of this research is founded in the long-range nature of the Coulomb interactions, which were expected to challenge the universality hypothesis that liquid-gas as well as liquid-liquid phase transitions all belong to the Ising universality class. Phase diagrams and Light-scattering measurements have been reported that led to the conclusion of mean-field criticality in such systems. However, later experiments did not state those results Ising criticality was found in all cases. We review this work and discuss measurements on solutions of room temperature ionic liquids. We demonstrate the various pitfalls that may lead to erroneous conclusions. We name chemical instability, shifting the critical temperature during the measurement, multiple scattering and local heating by the laser beam. Background scattering due to dynamical processes that are slow or fast if compared to the timescale of the scattering experiment, and non-equilibrium inhomogeneities may obscure the results. Such effects become noticeable as inconsistencies, when analyzing the temperature dependency of static and dynamic light-scattering. The amplitudes of the correlation functions that are accessible when using fibre optics turns out to be an important source of information. The 3d-cross-correlation technique enables to separate multiple and single scattering contributions as do simulations of the multiple scattering. The amplitude of the auto-correlation allows distinguishing the scattering intensity caused by the critical fluctuations from the scattering resulting from faster processes.
What will the audience learn ?
- General knowledge about phase transitions
- Knowledge about pitfalls in Light scattering measurements