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Updated: May 13, 2026
Interfacial Molecular-level Structures of Polymers and Biomacromolecules Revealed via Sum Frequency Generation Vibrational Spectroscopy
Published on: August 13, 2019
Electrodewetting of Surfactant-Laden Drops on Silicon Oxide: Molecular Insights from Sum-Frequency Generation
Billura Shakhayeva1, Bas Ter Beest2, Frieder Mugele2
1Institute of Physical Chemistry, Center for Soft Nanoscience, University of Münster, Corrensstraße 28/30, 48149 Münster, Germany.
Abstract:
Electrodewetting (EDeW) is an emerging approach to reversibly control surface wettability via ionic surfactants such as dodecyl trimethylammonium bromide (DTAB). In this study, we investigate EDeW on hydrophilic silicon oxide layers supported on conductive Si substrates, where positive electrode potentials induce a reversible increase in water contact angle from ∼10° to ∼30°. The effect is highly sensitive to solution pH, surfactant concentration, and applied potential; however, the molecular mechanisms at the three-phase contact line remain poorly understood. To address this, we employed ellipsometry and vibrational sum-frequency generation (SFG) spectroscopy to probe molecular structure changes during EDeW using aqueous DTAB solutions at pH 2, a condition necessary to suppress autophobing of the SiO2 surface. Contact angle measurements revealed that significant dewetting occurs only at a DTAB concentration of 0.15 mM (∼0.1× CMC), with both much lower and higher concentrations showing negligible EDeW. SFG spectra recorded at various distances from the contact line showed that even without applied potential, DTAB spontaneously spreads over millimeter-scale distances on the substrate, forming a heterogeneous DTAB-modified nanoscopic water layer. This prewetting layer contains both strongly hydrogen-bonded and weakly bonded H2O molecules, along with silanol groups. Upon EDeW, no significant changes in SFG spectra were observed far from the contact line, but locally, the exposed surface exhibits a thicker and more disordered DTAB-rich layer, consistent with a hemimicellar structure. Our data suggest that EDeW involves electrophoretic transport of DTAB along the liquid-vapor interface toward the contact line, where surfactant deposition modifies the solid-vapor interface to become more hydrophobic. This mechanism explains the observed contact angle increase and highlights the critical role of interfacial surfactant organization in the EDeW.
