Protonation reaction at the air/water interface monitored by a bubble Fabry-Perot interferometer

Abstract

A novel interferometric technique for studies of exothermal chemical reactions at the water surface is presented. The key feature of the technique is the use of an air bubble as the interface which hosts coating molecules and at the same time acts as a subnanometer-sensitive Fabry-Perot interferometer which follows the bubble radius change after a chemical reaction with coating molecules. As an example we report measurements of a protonation reaction with dodecyl-amine (DA). A monolayer of the DA surfactant is formed at the gas-liquid interface of a millimeter-sized bubble immersed in a 10.6 pH solution. After the monolayer formation, the residual DA is removed from the bulk by gentle rinsing. Interference with the DA bulk reaction is avoided. Protonation is induced by HCl injection. The reaction in the thin DA coating layer generates a non-trivial evolution of the bubble radius with amplitude oscillations of the order of 10−3 of the bubble radius with persisting time of a few hundred seconds. Measurements are performed at different DA coverages of the bubble surface, always below the maximum coverage. Control experiments with non-coated bubble show no amplitude radius oscillations. Formation and stability of the DA monolayer are precisely monitored by measurements of the lowest-frequency bubble capillary mode. A possible interpretation of the experiments is given in terms of heat effects, vapor production and surface modification. Molecular Dynamics simulations provide a mechanistic understanding of surface effects triggered by the protonation reaction which promotes desorption of positively charged DA micelles.