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Sodium Dodecyl Sulfate Organization at the Graphite?Solution Interface

Time:2015/11/28 5:45:47

Growth of Sodium Dodecyl Sulfate Micelles with Detergent Concentration

Sodium alkyl sulfate micelles are predicted to grow as a power law of the total counterion salt concentration, Y-aq, in the aqueous phase as follows: N-A = kappa(2)(Y-aq)(gamma), where N-A is the micelle aggregation number and kappa(2) and gamma are constants which are estimated from the critical micelle concentration and the apparent degree of counterion attachment. For sodium dodecyl sulfate, N-A = 162(Y-aq)(1/4) is predicted and is shown to be in good agreement with aggregation numbers measured by a number of techniques over a wide range of Y-aq whether this quantity is varied by changing the concentrations of the detergent, the added counterion salt, or both. Values of kappa(2) and gamma are estimated for six members of the sodium alkyl sulfate famil

Organization of Sodium Dodecyl Sulfate at the Graphite?Solution Interface

The aggregated structure of sodium dodecyl sulfate (SDS) adsorbed to the graphite61solution interface has been determined. Atomic force microscopy reveals that SDS adsorbs in periodic structures when the solution concentration is in the range 2.86181 mM. Using previously obtained adsorption isotherms, we deduce that these structures are hemicylindrical, 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 adsorption of other species from solution. Two basic types of grain boundaries have been identified:65 broad boundaries, where the periodicity of both grains continue into the boundary for several periods, and narrow boundaries, where one or both of the hemicylindrical arrays terminate within a short distance. The period within each grain decreases when the concentration of SDS or the concentration of added NaCl is increased and approaches the diameter of bulk micelles at high concentration. In NaCl solutions, the period is proportional to the solution Debye length. We propose that this is a result of a decrease in interaggregate spacing rather than a decrease in aggregate size. Using a simple geometric 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