Computational Study of Isotropic-to-Nematic Transition in Salt-Free Coacervates

Complex coacervation between oppositely charged polyelectrolytes (PEs) has attracted considerable interest because it serves as a model of intracellular compartmentalization and early stage prebiotic evolution. Recent experimental and theoretical studies have found this phase separation can be accompanied by the liquid crystal ordering (LCO) within the coacervate phase over limited ranges of flexibility of the PE chains (e.g. double-stranded DNA). Our work uses molecular dynamics simulations to demonstrate the existence of the isotropic-to-nematic transition in salt-free coacervates as the stiffness of the semiflexible chains increases. Two particular cases of coacervates are considered: (i) oppositely charged semiflexible PEs and (ii) semiflexible polyanions and flexible polycations. By comparing coacervates with the corresponding solutions of neutral polymers, we show that electrostatic interactions in coacervates facilitate LCO, in accordance with our earlier theoretical predictions. Our simulations also reveal that, near the interface between the nematic coacervate and supernatant phases, PEs prefer to be aligned parallel to the interface. This work provides molecular-level insight into the physics of intra-coacervate LCO and the role of Coulomb interactions in these systems.