Direct Visualization of the Potential-Controlled Transformation of Hemimicellar Aggregates of Dodecyl Sulfate into a Condensed Monolayer at the Au(111) Electrode Surface

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Abstract

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to present the first direct images of the potential-controlled phase transition between the hemimicellar and condensed states of a dodecyl sulfate (SDS) film at the Au(111) electrode surface. The adsorbed SDS forms stripe-shaped hemimicellar aggregates at small or moderate charge densities at the electrode. High-resolution STM images of these aggregates revealed that adsorbed SDS molecules are ordered and form a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. We propose that each unit cell contains two flat-laying SDS molecules stretched out along the longer axis of the cell with the hydrocarbon tails directed toward the interior of the cell. The remaining SDS molecules in the hemimicelle assume a tilted orientation. This long-range structure is stabilized by the interactions of sulfate groups belonging to the adjacent cells. The sulfate groups of the flat-laying SDS molecules are arranged into a characteristic (√3 × √7) structure in which the sulfate groups along the √7 direction are bridged by hydrogen-bonded water molecules. When the positive charge on the metal either becomes equal to or exceeds the charge of adsorbed surfactant, the surface aggregates melt to form a condensed film. The transition between the hemimicellar and condensed states of the film is reversible. The hemimicellar aggregates may be re-formed by decreasing the charge density at the electrode surface. The charging and discharging of the gold electrode can be easily controlled by a proper variation of the electrode potential.

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