Isotropic-isotropic phase separation and spinodal decomposition in liquid crystal-solvent mixtures

• Published in: Soft matter 2019, Vol.15 (3), p.644-654
• Main Authors: Reyes, Catherine G, Baller, Jörg, Araki, Takeaki, Lagerwall, Jan P. F
• Format: Article
• Language: English
• Subjects: Binary mixtures; Calorimetry; Coexistence; Complexity; Concentration gradient; Contamination; Cooling; Critical temperature; Decomposition; Differential scanning calorimetry; Ethanol; Liquid crystals; Liquid phases; Microscopy; Miscibility; Nucleation; Phase diagrams; Phase separation; Spinodal decomposition; Water pollution
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c9sm00921c

Phase separation in mixtures forming liquid crystal (LC) phases is an important yet under-appreciated phenomenon that can drastically influence the behaviour of a multi-component LC. Here we demonstrate, using polarising microscopy with active cooling as well as differential scanning calorimetry, that the phase diagram for mixtures of the LC-forming compound 4′- n -pentylbiphenyl-4-carbonitrile (5CB) with ethanol is surprisingly complex. Binary mixtures reveal a broad miscibility gap that leads to phase separation between two distinct isotropic phases via spinodal decomposition or nucleation and growth. On further cooling the nematic phase enters on the 5CB-rich side, adding to the complexity. Significantly, water contamination dramatically raises the temperature range of the miscibility gap, bringing up the critical temperature for spinodal decomposition from ∼ 2 °C for the anhydrous case to >50 °C if just 3 vol% water is added to the ethanol. We support the experiments with a theoretical treatment that qualitatively reproduces the phase diagrams as well as the transition dynamics, with and without water. Our study highlights the impact of phase separation in LC-forming mixtures, spanning from equilibrium coexistence of multiple liquid phases to non-equilibrium effects due to persistent spatial concentration gradients. Phase separation in mixtures forming liquid crystal (LC) phases is an important yet under-appreciated phenomenon that can drastically influence the behaviour of a multi-component LC.