X-Ray Fluorescence Directly Reveals Ion Specificity near Ionizable Interfaces.

The Hofmeister effect refers to the drastically different effects that apparently similar ions (e.g., Cl^- and I^-) have on protein solubility and stability. This phenomenon has been intensively studied, but not only the underlying mechanisms remain obscure, but even the order of ions within the ‘Hofmeister series’ is still not well-established.

We have studied the interactions of monovalent ions with floating octadecylamine monolayers—amines are one of the major hydrophilic functional groups in proteins. We used X-Ray Fluorescence Near Total Reflection (XFNTR), which has the advantage of being both element-specific and surface-sensitive. It detects ions in the interfacial region directly, in contrast to indirect methods looking at the effects of ions on the surface films or on water molecules (SFG, FTIR, etc.). Our XFNTR results confirm ion-specific adsorption to amine groups at neutral solution pH, with the adsorbed ion population following the familiar Hofmeister series. Further, when the bulk solution pH is lowered, the observed ion specificity is lost, with all ions studied exhibit similar affinity to amines. This fading of the Hofmeister effect in response to pH changes may potentially explain the reported ‘alternation’ or sometimes even ‘reversal’ of the Hofmeister series under various specific conditions. A possible explanation for these observations will be discussed.