ORGANIZED STRUCTURE OF LITHIUM PERFLUOROOCTANESULFONATE AT THE GRAPHITE-SOLUTION INTERFACE

The structure of aggregates of lithium perfluor ooctanesulfonate (LiFO S) adsorbed to the interface between graphite and aqueous solution hav e been measured. This fluorocarbon surfactant produces aggregates whic h are long (similar to 100 nm) and thin (similar to 5 nm), and about o ne molecule (similar to 1.3 nm) deep. The aggregates lie in straight, parallel arrays on the surface with a characteristic repeat di stance, or period, perpendicular to the long axis, As the bulk concentration of LiCl is increased, the period decreases, but as the bulk concentrat ion of LiFOS is increased, the period increases. The decrease in perio d on addition of salt is similar to that observed for sodium dodecyl s ulfate (SDS) and is explicable in terms of electrostatic forces. The i ncrease in period on addition of surfactant is in sharp contrast to th e behavior of SDS and may be due to a higher surfactant packing-parame ter for LiFOS. (C) 1997 Academic Press.