Electrochemical and Neutron Reflectivity Characterization of Dodecyl Sulfate Adsorption and Aggregation at the Gold−Water Interface

Chronocoulometry and the thermodynamic analysis of charge density data were employed to describe the energetics of sodium dodecyl sulfate (SDS) adsorption at the Au(111) electrode surface. Thermodynamic data such as the Gibbs excess, Gibbs energy of adsorption, and the film pressure of adsorbed SDS were determined for a broad range of electrode potentials, charge densities, and bulk SDS concentrations. The present results, combined with our previous scanning probe microscopy (SPM) studies, show that adsorption of SDS at the Au(111) electrode surface has a two-state character. At small or moderate absolute charge densities, the adsorbed SDS molecules aggregate into hemicylindrical stripelike micelles. This state is well-ordered. The unit cell of its two-dimensional lattice consists of two vectors that are 44 and 5.0 Å long and are oriented at an angle of 70°. The Gibbs excess data indicate that five SDS molecules are accommodated into the unit cell. At large positive charge densities, the hemimicellar aggregates melt to form a condensed film. The surface concentration of SDS doubles upon transition from the hemimicellar to the condensed state. We have performed neutron reflectivity experiments to determine the thickness of the hemimicellar and condensed films. The neutron reflectivity data indicate that the thickness of the condensed film is equal to 20.5 Å and is only 30% larger than the thickness of the hemimicellar state. The electrochemical and neutron reflectivity data indicate that the properties of the condensed state are best explained by a model of an interdigitated film in which half of the sulfate groups are turned toward the metal and half toward the solution.