Influence of Added Electrolytes on the Lyotropic Phase Behavior of Triethylammoniodecyloxycyanobiphenyl Bromide (OCB-C10NEt3Br)

Abstract

The influence of added electrolytes (NaBr, NaCl, Na2SO4, Na3PO4, Na2HPO4, NaSCN) on the lyotropic mesophase behavior of triethylammoniodecyloxycyanobipenyl bromide (OCB-C10NEt3) has been studied using optical microscopy, X-ray diffraction, and NMR. Optical microscopy was employed to map the existence regions of the lamellar mesophase, while X-ray diffraction gave its interaggregate dimensions, and NMR was employed to monitor water ordering. Only lamellar and micellar phases have been observed. In all of the electrolytes except NaSCN, partial miscibility of surfactant and water is induced above a certain concentration. This usually involves the separation of a dilute surfactant solution from a more concentrated one. It does not occur within the extensive lamellar phase region. X-ray diffraction and NMR results demonstrate that the major influence of the electrolytes is to reduce the area per headgroup within the aggregates. The effects of the electrolytes follow the Hofmeister series and are attributed to adsorption or desorption of anions at the aggregate surface rather than to changes in “water structure”. Sodium thiocyanate causes the occurrence of a “re-entrant” isotropic phase, in part because the lamellar phase is stabilized to low concentrations. The displacement of OCB groups from the micellar surface by SCN ions is suggested to account for this, which, if valid, offers a potential route to obtaining true amphitropic mesophases (i.e., materials with both thermotropic and lyotropic liquid crystalline order).