Electroreflectance study of potential dependent phase changes of dodecyl sulfate adlayer on a Au(1 1 1) Electrode

• Two-step phase changes of dodecyl sulfate on a Au(1 1 1) electrode showed electroreflectance (ER) signals.
• ER spectral feature was of the electroreflectance of Au.
• Phase change from interdigitated bilayer to hemi-micellar phase was found to be slower than reverse process.
• Second harmonic ER response was recorded.
• Kinetics of the transitions was discussed semi-quantitatively based on a simplified model.

Electroreflectance (ER) methods (potential-modulated UV-visible reflectance spectroscopy) were used to track the potential dependent phase changes of an n-dodecyl sulfate (DS−) adlayer on a Au(1 1 1) electrode surface. In the aqueous solution containing Na-DS (SDS) at a concentration lower than cmc, two step non-faradaic phase changes, which have been well known so far, were clearly observed as ER signals, although the surfactant is colorless. On the ER spectral structure basis, the origin of the signal was the electroreflectance from Au, which relies on the change of surface free electron density on the Au surface. The second harmonic frequency ER signal helped us gain perspective on the extent of non-linearity and kinetics of the reflectance change in response to the potential modulation. Results of the ER measurements enabled us to find that adsorptive hemi-micelle formation-desorption at -0.18 V (Ag/AgCl/sat-KC1) is faster process than the phase change process between interdigitated bilayer and the hemi-micellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-growth type nature in response to a potential step, and this phase change appeared slower than the reverse change.