Liquid-liquid criticality in the dielectric constant and refractive index: A perspective

The critical region in the phase diagram of condensed matter systems such as fluids or fluid mixtures is characterized by the anomalous behaviour of specific thermodynamic properties. Here, we discuss the progress of understanding the behaviour of two intimately related properties, the dielectric constant $\varepsilon$ and the refractive index n, when approaching the liquid-liquid critical point in binary liquid mixtures. Phenomenological scaling approaches, in particular, complete scaling formulation, are highlighted. In addition, experimental evidence provided by dielectric spectroscopy and optical measurements supporting the asymmetry-related critical features in $\varepsilon$ and n in the one- and two-phase regions is assessed. We revisit recent experimental data and point out peculiar patterns of behaviour of polar-polar liquid mixtures as compared to (much more frequently studied) polar-non-polar mixtures. We point the necessity of additional research efforts towards the study of polar-polar mixtures, which would shed light into the influence of (microscopic) system-dependent parameters on asymmetry-related features of liquid-liquid criticality