Water desorption from microcline (001): insights into the first water layer

Feldspar minerals constitute an abundant group of tectosilicates in the Earth's crust. Consequently, feldspars play a significant role in a plethora of geochemical processes, including weathering, which results in carbon dioxide removal from the atmosphere by the formation of carbonates. Moreover, feldspar dusts are known as highly efficient ice nucleating particles, having a significant impact on the physical properties of mixed-phase clouds. For these processes, the interaction of water with the feldspar surface is decisive. However, little is known about the interaction of water with feldspar surfaces. More specifically, experimental data addressing the binding and in particular the desorption of the first water layer are sparse. Here, we present temperature-programmed desorption (TPD) experiments of water desorbing from the thermodynamically most stable cleavage plane of potassium-rich feldspar, microcline (001). From the interplay of these experimental data with density-functional theory (DFT) results we shed light onto the binding of the first water layer on microcline (001). The coverage-dependent TPD spectra reveal a gradual shift of the peak position from initially 235 K for low coverages towards lower temperatures until a coverage of four water molecules per primitive unit cell is reached. Above this coverage, the peak position remains fixed at about 180 K, even for high coverages. These results are in perfect agreement with DFT simulations, revealing a decrease in the adsorption energy with increasing coverage. When four water molecules per primitive unit cell are reached, the first layer is saturated and further water starts occupying the second layer. Our work confirms previous theory results from the literature and provides molecular-scale insights into the binding of water onto microcline (001).