ORGANIZATION OF SODIUM DODECYL-SULFATE AT THE GRAPHITE-SOLUTION INTERFACE

The aggregated structure of sodium dodecyl sulfate (SDS) adsorbed to t he grapite-solution interface has been determined. Atomic force micros copy reveals that SDS adsorbs in periodic structures when the solution concentration is in the range 2.8-81 mM. Using previously obtained ad sorption isotherms, we deduce that these structures are hemicylindrica l, but we are not able to determine their length. The long axes of the hemicylinders lie parallel in grains which typically extend over (500 nm)(2) to (1000 nm)(2), but the grain size can be reduced by adsorpti on of other species from solution. Two basic types of grain boundaries have been identified: broad boundaries, where the periodicity of both grains continue into the boundary for several periods, and narrow bou ndaries, where one or both of the hemicylindrical arrays terminate wit hin a short distance. The period within each grain decreases when the concentration of SDS or the concentration of added NaCl is increased a nd approaches the diameter of bulk micelles at high concentration. In NaCl solutions, the period is proportional to the solution Debye lengt h. We propose that this is a result of a decrease in interaggregate sp acing rather than a decrease in aggregate size. Using a simple geometr ic argument, we suggest that the curvature of surfactant aggregates on hydrophobic surfaces will usually be lower than that of aggregates in bulk solution with which they are in equilibrium.