Observation of a Two-Dimensional Hydrophobic Collapse at the Surface of Water Using Heterodyne-Detected Surface Sum-Frequency Generation

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