Observation of a Two-Dimensional Hydrophobic Collapse at the Surface of Water Using Heterodyne-Detected Surface Sum-Frequency Generation
J. Phys. Chem. Lett. , Volume 14 - Issue 41 p. 9285- 9290
We study the effect of sodium chloride (NaCl) on the properties of the interface of water and the surfactant dodecyl sulfate (DS-) using heterodyne-detected vibrational sum-frequency generation spectroscopy. We find that the signal of the O-H stretch vibrations of oriented water molecules at the interface is highly nonlinearly dependent on the NaCl concentration. This nonlinear dependence is explained by a combination of screening of the electric field of surface-bound DS- ions pointing into the bulk and screening of the Coulomb repulsion between the headgroups of the DS- ions in the surface plane. The latter effect strongly increases the oriented water signal within a limited NaCl concentration range of 10-100 mM, indicating a two-dimensional hydrophobic collapse of the surfactant layer. The occurrence of collapse is supported by model calculations of the surface potential and surface surfactant density.
|European Union Horizon 2020|
|J. Phys. Chem. Lett.|
Sengupta, S, Versluis, J, & Bakker, H.J. (2023). Observation of a Two-Dimensional Hydrophobic Collapse at the Surface of Water Using Heterodyne-Detected Surface Sum-Frequency Generation. J. Phys. Chem. Lett., 14(41), 9285–9290. doi:10.1021/acs.jpclett.3c01530