Hofmeister effects in membrane biology: The role of ionic dispersion potentials

Membrane biology is notorious for its remarkable, and often strong dependence on the supposedly irrelevant choice of ion pair of background salt solution. While experimentally well known, there has been no progress towards any real theoretical understanding until very recently. We have demonstrated that an important source behind these Hofmeister effects is the ionic excess polarizabilities of ions in solution. Near an interface an ion experiences not only an electrostatic potential, but also a highly specific ionic dispersion potential. At biological concentrations (around 0.1 M and higher) when the electrostatic contribution is highly screened this ionic dispersion potential has a dominating influence. We present the result of model calculations for the interfacial tension and surface potential that demonstrates that inclusion of ionic dispersion potentials is an essential step towards predictive theories. Our results are compared with experimental surface and zeta potential measurements on phospholipid bilayers, zirconia, and cationic micelles.