Effect of surfactant structure on the orientation of liquid crystals at aqueous-liquid crystal interfaces

It is known that the orientations assumed by thermotropic liquid crystals (LCs) in contact with water are sensitive to the types and concentrations of surfactants and/or polymers present in the aqueous phase. This work expands upon these past observations by developing criteria for surfactants that give rise to a particular orientation of a contacting nematic LC formed formed from 4'-pentyl-4-eyanobiphenyl (5CB). We observe surfactants that have a bolaform structure ((11-hydroxyundecyl)trimethylammonium bromide (HTAB), dodecyl-1,12-bis(trimethylammonium bromide) (DBTAB), 11-(ferrocenylundecyl)trimethylammonium bromide (FTMA)) and which adopt looped configurations at air-water/oil-water interfaces cause planar anchoring of 5CB. In contrast, classical surfactants (alkyltrimethylammonium halides (C(n)TABs, n > 8), sodium dodecyl sulfate (SDS), and N,N-dimethylferrocenylalkylammonium bromides (FC(n)ABs, n n > 12)) that assume tilted orientations at air-water/oil-water interfaces can give rise to a homeotropic orientation of 5CB. By comparing SDS, dodecyl trimethylammonium halide (DTAB), and tetra(ethylene glycol) monododecyl ether (C12E4), we conclude that the nature of these headgroups does not measurably influence the orientation of the LC. However, the orientation of the LC is found to depend on the aliphatic chain length and the areal density of the adsorbed surfactant. When using surfactants with short alkyl chain lengths (n = 8 for C(n)TAB and n = 7 and 12 for FC(n)AB), we observe the orientation of 5CB to remain parallel to the interface up to concentrations at which the 5CB begins to be solubilized by the surfactant. These results, when combined, lead us to conclude that interactions between the aliphatic chains of the surfactant and 5CB, which are influenced by the conformation of the surfactant, largely dictate the orientation of the 5CB.