Growth and dissolution on the CaF2 (111) surface observed by scanning force microscopy

Scanning force microscopy (SFM) was used for in situ growth and dissolution experiments on the CaF2 (111) surface in various aqueous solutions in order to investigate chemical processes within the CaF2-water interface. Apart from growth and dissolution, a further process, the formation of protrusions, takes place in the interfacial region. These protrusions are inhomogeneously distributed but uniform in height (2.5-4 nm). The areal density of protrusions depends on the pH value and degree of saturation of the solution. In addition, experiments with in situ exchanged solutions show that the areal density depends further on the sequence of application of the solutions. These and other observations indicate that surface defects which are considered to form surface hydroxyl groups lead to the formation of protrusions. Therefore, we conclude that the protrusions represent surface precipitates which consist of multinuclear calcium (aquo) hydroxo complexes connected to surface hydroxyl groups. The existence of these hydroxo complexes cannot be explained by classical equilibrium thermodynamics of bulk reactions. Their formation is enabled by the different dielectric constant in the electrical double layer. Depending on the composition of solution and the defect density, the complexes reduce the active surface area of CaF2 and therefore affect the growth and dissolution rates.