Evaluating the concentration of ions in liquid crystal cells: hidden factors and useful techniques

Many of the liquid crystal devices are driven by electric fields. Ions, typically present in molecular liquid crystal materials in minute quantities, can compromise the performance of mesogenic materials (in the simplest case, through a well-known screening effect). Even highly purified liquid crystals can be contaminated with ions during their production and handling. Therefore, measurements of the concentration of ions became an important part of the material characterization of liquid crystals. Interestingly, even a brief analysis of existing publications can reveal a quite broad variability of the values of the concentration of ions measured by different research groups for the same liquid crystals. It reflects the complexity of ion generation mechanisms in liquid crystal materials and their dependence on numerous factors. In this paper, an overview of ion generation mechanisms in liquid crystals and modern ion measurement techniques is followed by the discussion of frequently overlooked factors affecting the measured values of the ion concentration. Ion-generating and ion-capturing properties of the alignment layers (or substrates) of liquid crystal cells are considered and used to evaluate a true concentration of ions in liquid crystals. In addition, practical recommendations aimed at improving the measurements of the ion density in liquid crystals are also discussed.