Among the many mysteries of water is the origin of the Hofmeister series. This is a 120-year-old list of ions that describes their effects on the structure of water – most notably, how they alter the strength of hydrophobic interactions with proteins. The science behind the series has remained a puzzle since the list was created by Franz Hofmeister when he was studying proteins in Prague in the late 1880s.
Now Yan Levin of the Federal University of Rio Grande do Sul in Brazil and colleagues may have found the key to understanding the series. Their detailed calculations of the polarization of water around various ions and its effect on surface tension provides a list that not only matches experimental measurements of surface tension, but also reproduces the Hofmeister series, making a link between polarization, water–ion–air surface effects and water–ion–protein interactions.
The work has wider implications because it also explains the tendency of highly reactive halogen ions to move to the surfaces of water droplets – an important and until now mysterious phenomenon in atmospheric chemistry.