Chromonic Liquid Crystal Mixtures

 
The natural formation of complex structures of small molecules in water is of fundamental biological interest. We study liquid crystal phases formed by self-assembly of nanoDNA and chromonic molecules in water. NanoDNAs shorter than a few dozen bases are synthesized and mixed with chromonic materials, which are typically flat molecules with nearly planar aromatic cores and ionic peripheries, in water (Figure 1). Both nanoDNAs and chromonic molecules stack to form rod-like aggregates. At sufficiently high concentrations, these aggregates become long enough on average to meet the Onsager criterion for forming liquid crystal phases. Isotropic, nematic, and columnar phases may be formed, depending on the ternary ratios of the DNA, chromonics, and water. Analysis of the phases of such mixtures using micro-spectroscopy and polarizing optical microscopy reveals a tendency for the component materials to mix well in phases with low order but to demix in higher-order phases. Depolarized transmission light images of thin cells of nanoDNA and sunset yellow, and sunset yellow and disodium cromoglycate (DSCG), both show demixing in the columnar phase (Figure 2).

chromonics

Figure 1 : Structures of chromonic molecules sunset yellow (SSY) and DSCG, and of Dickerson Dodecamer nanoDNA1. All of these materials stack to form rod-like structures.

phase separation

Figure 2. Phase separation of nanoDNA/chromonic mixtures in the columnar ('M') phase observed using polarized optical microscopy. Top: Mixture of nanoDNA and sunset yellow. The red domains, mostly in the form of thin lines, have a higher concentration of sunset yellow. Bottom: Phase separation of sunset yellow and disodium cromoglycate. The DSCG-rich regions (the circular, indigo domains) are phase-separated from the sunset yellow-rich regions surrounding them.

References

[1] The nanoDNA structure is from H. R. Drew et al., Proc. Natl. Acad. Sci. USA 78, 2179 (1981).

Text and images contributed by Youngwoo Yi, Greg Smith, and Akihiro Yamaguchi .