Investigation of Hofmeister Effects in the Attraction of Ions in Solution to Organic Moieties

Hofmeister effects, also known as specific ion effects, originally described the ranked ability of ions to influence protein solubility in bulk solutions. It should be recognized that these interactions largely occur at the protein surfaces, as proteins are typically large, folded structures with only their surface sites exposed to activities in aqueous solutions. This makes Langmuir monolayers a useful model for investigating ion-specific interactions relevant to proteins by looking at individual hydrophilic groups separately.

We have studied ion adsorption to various Langmuir monolayers using the element specific and surface sensitive measurement X-Ray Fluorescence Near Total Reflection (XFNTR). Our results directly confirmed ion specificity between the anions Cl-, Br-, I- and ClO4- and amine, acid, and guanidine groups by observing differences in ion populations in the interfacial region. Notably, the rank ordering of the degree of attraction of these ions varied with head groups. Additionally, we discovered that this ranking can be modifying by varying the pH. This may potentially explain the reported ‘alternation’ or sometimes even ‘reversal’ of Hofmeister’s series in different proteins or under various conditions.