We investigate the ordering behavior of molecules in salt solutions confined within ultra-narrow pores. The results of these studies are relevant for, e.g., the understanding of weathering and corrosion of geological rock formations and construction materials.

Lay summary

Water is all around us in Nature. Water is never pure but often is salty. We find it in the narrow pores present in geological rock, soil, and in building and road constructions. It is well-known that the weathering and corrosion of rock and construction materials are strongly influenced by the quantity and distribution of the salt ions within the pores and by their immediate chemical environment. The following questions arise. Is the salt solution within the narrow pore still liquid or crystallized? Can the salt ions move in and out of the pore? Does swelling of the pore occur? The answers to these questions have a direct bearing on the integrity of the materials in question and thus on our daily life.

We confine the salty solution between two flat mica surfaces at nanometer* distance and try to determine the positions of the salt ions within the extremely narrow slit thus formed. This will tell us whether the molecules (salt ions and water) within the mica slit will line up like in a regiment of soldiers (‘solid-like’) or whether they move more freely like in bulk solution. In addition, we can tell whether the salt ions are surrounded by a shell of water molecules or not. Mica is a mineral commonly found in geological formations, which makes our studies important for understanding the properties of salted water directly below our feet.

* 1 nanometer = 0.000000001 meter