Concentration and temperature dependence of the helical pitch in cholesteric xanthan gum solutions

Solutions of partially depolymerized xanthan gum which form cholesteric lyotropic liquid crystal (LLC) phases, can be used to produce structurally colored, non-brittle photonic films. A key factor which determines the photonic properties of the dried xanthan films is the helical pitch p in the solid cholesteric structure retained from the liquid crystal phase, which enables the films to selectively reflect visible light. However, detailed experiments and a sophisticated theoretical model for unraveling the evolution of helical pitch over a broad concentration range in LLC systems containing semiflexible building blocks have remained elusive. In this work, we systematically study the temperature and concentration dependence of the helical pitch in aqueous solutions of ultrasonically treated xanthan gum over an extended concentration range up to 30 wt%. This investigation reveals that while the inverse pitch, i.e. the helical twist, decreases linearly with temperature, it grows much more dramatically with concentration. Current theories cannot explain such strong non-linear concentration dependence of the inverse pitch at high concentration. Therefore, we propose a new molecular statistical model based on the so-called generalized van der Waals theory (GVWT) of the nematic phase, taking into account the concentration dependent steric intermolecular correlations as well as the effective attraction interaction. This model successfully interprets the divergence of the inverse pitch at high concentrations and estimates reasonable minimum achievable pitch values in the dried films as observed for the xanthan samples. These findings reinforce our understanding of chiral interactions in semiflexible LLCs, more deeply bridging the cholesteric ordering and structural coloration in solid films.