Ion-specific thermodynamic properties of colloids and proteins

Franz Hofmeister established in 1888 that different salt solutions with the same ionic charges have different efficiencies in precipitating proteins from whole egg white. We will discuss how this can be understood from the modified Poisson–Boltzmann equation that accounts for ion specificity via the ion-surface non-electrostatic potential of mean force (NE-PMF) from molecular dynamics simulations. Using this approach, it is at least in principle possible to capture the important physics of the system due to the inclusion of ion-surface van der Waals forces, short range hydration, image potential and different solvent-mediated forces. The method has been proved to be efficient and suitable for describing phenomena where the water structure close to the interface plays an essential role. As an illustrative example, we demonstrate why the double layer force between two gold electrodes coated with hydrophobic self-assembled monolayers in different electrolytes can be highly ion specific. Important thermodynamic properties related to protein aggregation, essential in biotechnology and pharmaceutical industries, can be obtained from the method shown here.