Ergodic-to-nonergodic phase inversion and reentrant ergodicity transition in DNA-nanoclay dispersions

We have observed DNA concentration and hydration dependent inversion from ergodic to non-ergodic phase followed by reentry into the ergodic phase in DNA-nanoclay (laponite) dispersions at room temperature (25 degrees C), using results obtained from dynamic light scattering (DLS) and rheology data. The interaction between the DNA strand and the anisotropically charged discotic platelets of laponite (L) was found to be strongly hierarchical in DNA concentration. For a fixed laponite concentration (C-L = 1% (w/v)) and varying DNA concentration (C-DNA) from 0.3-2.3% (w/v), we observed three distinct phase regions characterized by the following: region (i): C-DNA < 1.0% (w/v), ergodic region with weak DNA-L attractive interaction, region (ii): 1.0% < C-DNA < 1.6% (w/v), non-ergodic regime having strong DNA-L associative interaction and region (iii): C-DNA > 1.6% (w/v), showing phase reentry into the ergodic regime due to repulsion between DNA strands. Hydration study in these three regions revealed that a loss in the abundance of amorphous water, signified by Raman frequency 3460 cm(-1), caused the ergodic to nonergodic phase transition. In summary, it is shown that maximum stability and interaction between DNA and nanoclay platelets occurred at an intermediate concentration of DNA where the hydration was at its minimum. The present system is qualitatively different from the hard-sphere/polymer systems for which reentrant phase transition has been reported in the literature. However, some similarity between the two classes of systems is not ruled out.