Interactions of negatively charged headgroups with alkali cations in water are quantified
by experiment and calculations. We present computational results of sodium and potassium
interactions with methylsulfate and compare them to interactions with acetate. From these
results, former simulations, and from a series of experimental observations we deduce a
Hofmeister-like ordering of headgroups. We further combine this ordering with the law of
matching water affinities in order to obtain general description and predictions of ionheadgroup
interactions. Examples from colloidal chemistry and from biological systems are
provided to illustrate the power of this approach.
*