Atomic Force Microscopy of Adsorbed Surfactant Micelles

Atomic Force Microscopy of Adsorbed Surfactant Micelles 

WILLIAM A. DUCKER Virginia Tech, Blacksburg, Virginia, U.S.A. 

The concept of surface aggregation for surfactants was first introduced to explain abrupt changes in interfacial properties as a function of smfactant concentration. Gaudin and Fuerstenau [1] inferred the existence of smface aggregates from zeta potential measmements, which showed an increase in the gradient of smfactant adsorption at a particular concentration. This concentration was approximately a constant fraction of the critical micelle concentration (cmc), which suggested that the surface process was similar to bulk micellization [2]. Unlike simple monovalent ions, surfactant ions reversed the charge of solids even when they were not lattice ions. This ad- 

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Zhu and Gu [7] used a simple mass action model to fit adsorption isotherms for nonionic surfactants. They noted that adsorption often increased with concentration in two steps, so they modeled adsorption with two binding constants. The first binding constant accounted for binding of the surfactant to the surface, and the second also accounted for attractive interactions between the adsorbed surfactant molecules. The latter are expected to be important at a high surface density of surfactant. 

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Atomic force microscopy (AFM) [19-21] provides a more detailed molecular picture of adsorbates at concentrations above the CMC. Adsorbed cationic surfactant C14TAB (tetradecyltrimethylammo-nium bromide) was arranged in surface micelles on silica and in long cylinders on mica. Double-chained DDAB was organized in bdayers on mica. These structures result from the balance of adsorbate-surface interactions and aggregate curvature controlled by intermolecular interactions. 

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