Ordering transitions in micrometer-thick films of nematic liquid crystals driven by self-assembly of ganglioside GM_1

摘要

We report an investigation of the self-assembly of the monosialoganglioside (GM_1) at interfaces formed between aqueous solutions of 10 μM GM_1 (at 25 °C) and micrometer-thick films of the nematic liquid crystal (LC) 4′-pentyl-4-cyanobiphenyl (5CB). We observe the process of spontaneous transfer of GM_1 onto the interfaces to be accompanied by continuous ordering transitions within the micrometer-thick films of the LC. At saturation coverage, the GM_1 orders the LC in an orientation that is perpendicular to the interface, an orientation that is similar to that caused by phospholipids such as dilauroylphosphatidylcholine (DLPC). This result suggests an interaction between the LC and GM_1 that is dominated by the hydrophobic tails of the GM_1. Relative to DLPC, however, we observe the dynamics of the LC ordering transition driven by GM_1 to be slow (2 h for DLPC versus 100 h for GM_1). To provide insight into the origins of the slow dynamics of the GM_1-induced ordering transition in the LC, we performed two additional measurements. First, we quantified the time-dependent adsorption of GM_1 at the LC interface by using fluorescently-labeled GM_1. Second, we used the Langmuir-Schaefer method to transfer preorganized monolayers of GM_1 from an air-water interface to the aqueous-LC interface. Results obtained from these two experiments are consistent with a physical picture in which the final stages of spontaneous adsorption/ordering of GM_1 at the aqueous-LC interface dictate the dynamics of the LC ordering transition. This rate limiting process underlying the ordering transition was substantially accelerated by heating the system above the phase transition temperature of GM_1(26 °C), suggesting that the phase state of the GM_1 micellar aggregates in bulk solution strongly influences the kinetics of the final stages of ordering/adsorption of GM_1 at the LC interface. Overall, these results and others presented in this manuscript reveal that it is possible to decorate interfaces of a nematic LC with GM_1, and that the assembly of GM_1 at these interfaces impacts the dynamic and equilibrium ordering of the LC.