![]() ![]() The interface-specificity of v-SFG results from the selection rule that inversion symmetry must be broken for a v-SFG signal to be generated. At solid-water interfaces, vibrational sum frequency generation (v-SFG) is a well-established method that is interface-specific and provides molecular information 2, 4, 13, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27. One needs an interface-specific method to obtain a fundamental understanding of the events occurring at interfaces. However, the mechanism by which the flow alters the surface charge is poorly understood as of yet 15. ![]() A flow-dependent surface charge not only paves the way for novel electrokinetic effects 14, but it also has important consequences for the interpretation of zeta potential measurements. 13 concluded that a change in surface potential could be explained by a flow-induced change of the concentration of ions that screen the surface charge. 11 that considers net dissolution as the driving force. 12, which would lead to a surface charge gradient and a 1D toy model by Lian et al. In fact, several different hypotheses have been put forward for the change of the v-SFG signal upon flow, for instance, a surface conduction model by Werkhoven et al. Therefore, a fully self-consistent quantitative model is missing. The surface charging reaction itself cannot supply the excess of ions in the channel over the bulk concentration, as the number of reactive ions on the surface is too small to support a steady flux of ions, especially over many flow cycles. For instance, the question remains how a concentration difference between the reservoir and channel is generated. Even though the flow-induced disturbance of the charging equilibrium qualitatively explains several experimental features, there are also inconsistencies. A similar explanation has also recently been given by Xi et al. The dilution changes the charging equilibrium and increases the surface charge, which manifests itself by an increase of the v-SFG response. This dilution is caused by the concentration difference between the fresh solution from the reservoir and that in the flow channel. For the mineral fluorite (CaF 2) under acidic conditions, the change of the surface charge upon flow was argued to be due to the dilution of reactive ions partaking in the surface charging reaction 4. This observation implies that fluid flow can directly affect a chemical equilibrium. 4 presented the first experimental evidence that the surface potential of mineral surfaces (silica and calcium fluoride) in contact with water changes substantially when liquid flow is applied. They are also central in technological applications in areas as diverse as froth flotation 6, electrophoresis 7, water desalination 8, soil remediation 9, and even dentistry 10. Reactions at the interface between a charged solid surface and a flowing fluid play a key role on macroscopic scales in geochemical cycles 1, 2, 3, 4, as well as on microscopic scales in micro- and nanofluidic systems 5. ![]()
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