Characterization of adsorbate-induced ordering transitions of liquid crystals within monodisperse droplets

The ordering of liquid crystals (LCs) within micrometer-sized droplets is known to depend strongly on the presence of interfacial adsorbates, although the exact sequence of ordered equilibrium states that accompany a change in interfacial anchoring from tangential to perpendicular has not been established. In this paper, we report use of a methodology that permits the preparation of monodisperse LC droplets in aqueous phases to investigate ordering transitions in the LC droplets that accompany the adsorption of amphiphiles at the aqueous-LC droplet interface. By using an amphiphile that undergoes reversible adsorption at the aqueous-LC interface (sodium dodecylsulfate, SDS), we identified six distinct topologically ordered states of the LC droplets as a function of increasing concentration of SDS. We exploited the reversible adsorption of the SDS to LC droplets with diameters of 8.0+/-0.2 microm to confirm that these topological states are equilibrium ones. We also exposed LC droplets to a continuous gradient in concentration of SDS to document the continuous transitions between topological states and to confirm the absence of additional, intermediate topological states. The formation of the LC droplets as aqueous dispersions also enabled an investigation of ordering transitions in LC droplets driven by biomolecular interactions. Surprisingly, enzymatic hydrolysis of the phospholipid L-dipalmitoyl phosphatidylcholine (L-DLPC) by phospholipase A2 at the interfaces of the LC droplets was observed to trigger the same progression of topologically ordered states of the LC as was observed with SDS. Overall, the results presented in this paper resolve prior conflicting data in the literature by providing an unambiguous set of observations regarding topologically ordered states encountered in LC droplets. This paper provides a data set against which future theories and simulations of LCs can be compared to develop a fundamental understanding of the competition between volumetric and interfacial effects in droplets. The results also suggest that topological ordering transitions in LC droplets can be exploited to report interfacial enzymatic reactions.