Water

Water is one of the most abundant chemical compounds on Earth. The biosphere consists of more than 90% of water; up to 5% of the gas in the atmosphere is water vapor; the lithosphere (the Earth’s crust) contains, on average, about 15% water and living organisms on our planet typically contain 50-90% water. Because of its ubiquity, water is considered as a very normal and common substance, yet, from a physical-chemical point of view, it is actually very unique and extraordinary. By virtue of its unique properties, water is the medium, par excellence, in which life has evolved and is sustained. To be more precise, in living organisms water fulfills the following functions: it is essential for the formation and maintenance of the structures of cell walls, membranes, proteins, and the like; it is the environment for biochemical pathways; often it participates as a reagent in biochemical reactions; it is the medium in which components are transported; and it regulates the temperature of warm-blooded organisms. Also, among non-living (natural) systems, for instance in the biomedical, pharmaceutical, and the agri-food sectors, most (nano-structured) objects owe not only their existence to their aqueous environment, but also their shape and spatial structure, which are to a large extent determined by the interaction with water. Two questions arise immediately: (1) which properties make water so special, and (2) how can these properties be explained and understood on a molecular level?