Microheterogenities and Crossover from Ising to Mean Field Criticality in Solutions of Ionic Liquids

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Structure and dynamics of solutions of Ionic Liquids will be investigated on a mesoscopic scale by static and dynamic light scattering. Near a critical solution point the critical fluctuations dominate the scattering, but other equilibrium and non-equilibrium inhomogeneities contribute and will be analyzed applying new techniques. Corresponding state analysis indicates that in non-polar solvents the liquid-liquid phase transition is determined by the Coulomb interactions, while with increasing polarity solvophobic interactions become dominant. It was shown that the liquid-liquid phase transitions of ionic solutions always belong to the Ising universality class. While the nature of the critical point is unchanged, when Coulomb forces or solvophobic interactions drive the phase transition, it is not clear if this is true also for the crossover from Ising to regular behaviour. Some theories predict a large, others a small non-classical region with crossover to a tricritical point, when Coulomb forces drive the phase transition. However, non-critical mesoscopic structures may erroneously be interpreted as crossover. To clarify the situation, we will investigate the light-scattering of solutions of Ionic liquids in solvents of different dielectric constant including hydrocarbons, alcohols and water. Solutions of Ionic Liquids that are able to form liquid crystals will be investigated as in such systems crossover to a tricritical point is quite likely.