Ordering Transitions in Micrometer-Thick Films of Nematic Liquid Crystals Driven by Self-Assembly of Ganglioside GM1

摘要

We report an investigation of the self-assembly of the monosialoganglioside (GM1) at interfaces formed between aqueous solutions of 10 µM GM1 (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 GM1 onto the interfaces to be accompanied by continuous ordering transitions within the micrometer-thick films of the LC. At saturation coverage, the GM1 orders the LC in an orientation that is perpendicular to the interface, an orientation that is similar to that caused by phospholipids such as dilaurylphosphatidylcholine (DLPC). This result suggests an interaction between the LC and GM1 that is dominated by the hydrophobic tails of the GM1. Relative to DLPC, however, we observe the dynamics of the LC ordering transition driven by GM1 to be slow (2 hours for DLPC versus 100 hours for GM1). To provide insight into the origins of the slow dynamics of the GM1-induced ordering transition in the LC, we performed two additional measurements. First, we quantified the time-dependent adsorption of GM1 at the LC interface by using fluorescently-labeled GM1. Second, we used the Langmuir-Schaefer method to transfer preorganized monolayers of GM1 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 GM1 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 GM1 (26 °C), suggesting that the phase state of the GM1 micellar aggregates in bulk solution strongly influences the kinetics of the final stages of ordering/adsorption of GM1 at the LC interface. Overall, these results and others presented in this manuscript reveal that it is possible to decorated interfaces of a nematic LC with GM₁, and that the assembly of GM₁ at these interfaces impacts the dynamic and equilibrium ordering of the LC.