Influence of added electrolytes on the lyotropic phase behavior of triethylammoniodecyloxycyanobiphenyl bromide (OCB-C10NEt3Br)

The influence of added electrolytes (NaBr, NaCl, Na2SO4, Na3PO4 Na2HPO4, Na SCN) on the lyotropic mesophase behavior of triethylammoniodecyloxycyanobip enyl 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 intera ggregate dimensions, and NMR was employed to monitor water ordering. Only l amellar and micellar phases have been observed. Tn 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 s urfactant solution from a more concentrated one. It does not occur within t he extensive lamellar phase region. X-ray diffraction and NMR results demon strate that the major influence of the electrolytes is to reduce the area p er headgroup within the aggregates. The effects of the electrolytes follow the Hofmeister series and are attributed to adsorption or desorption of ani ons at the aggregate surface rather than to changes in "water structure". S odium 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 sugges ted to account for this, which, if valid, offers a potential route to obtai ning true amphitropic mesophases (i.e., materials with both thermotropic an d lyotropic liquid crystalline order).