Abstract
The charging and dissolution of mineral surfaces in contact with flowing liquids are ubiquitous in nature, as most minerals in water spontaneously acquire charge and dissolve. Mineral dissolution has been studied extensively under equilibrium conditions, even though non-equilibrium phenomena are pervasive and substantially affect the mineral-water interface. Here we demonstrate using interface-specific spectroscopy that liquid flow along a calcium fluoride surface creates a reversible spatial charge gradient, with decreasing surface charge downstream of the flow. The surface charge gradient can be quantitatively accounted for by a reaction-diffusion-advection model, which reveals that the charge gradient results from a delicate interplay between diffusion, advection, dissolution, and desorption/adsorption. The underlying mechanism is expected to be valid for a wide variety of systems, including groundwater flows in nature and microfluidic systems.
Highlights
The charging and dissolution of mineral surfaces in contact with flowing liquids are ubiquitous in nature, as most minerals in water spontaneously acquire charge and dissolve
The local changes in the surface potential upon flow are tracked through the intensity of the OH-stretch band in vibrational sum frequency generation (v-SFG) spectra, which reports on the alignment and polarization of interfacial water molecules and, the surface charge
Visible (Vis) and infrared (IR) laser pulses overlap in space and time at the CaF2water interface, such that a nonlinear optical v-SFG process generates a response at a frequency equal to the sum of the two incident frequencies
Summary
The charging and dissolution of mineral surfaces in contact with flowing liquids are ubiquitous in nature, as most minerals in water spontaneously acquire charge and dissolve. A few years ago, Lis et al.[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 This observation implies that fluid flow can directly affect a chemical equilibrium. The local changes in the surface potential upon flow are tracked through the intensity of the OH-stretch band in v-SFG spectra, which reports on the alignment and polarization of interfacial water molecules and, the surface charge Using this approach, we show that the surface charge depends on the flow rate and on the position in the channel, along the flow direction. Our approach of combining quantitative theory with position-resolved v-SFG experiments allows determining whether the surface is charged by desorption or adsorption when the dissolution mechanism is known
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