Ions at interfaces: the central role of hydration and hydrophobicity

It is increasingly being accepted that solvation properties of ions and interfaces (hydration of ions, hydrophobic or hydrophilic character of interfaces) play a fundamental role in ion-surface interaction in water.. However, a fundamental understanding of the precise role of solvation in ionic specificity in colloidal systems is still missing, although important progress has been made over the last years. We present in this contribution experimental evidences (including also ions not usually included in specific ion studies) together with Molecular Dynamics (MD) simulations that highlight the importance of the hydration of ions and surfaces in order to understand the origin of ionic specificity. We first show that both surface polarity and ion hydration determine the sorting of ions according to their ability to induce specific effects (the so-called Hofmeister series). We extend these classical series by considering the addition of the inorganic anions IO3-, BrW and CIO3-, which present unusual properties as compared with the ions considered in classical Hofmeister series. We also consider big hydrophobic organic ions such as tetraphenylborate anion (Ph4B-) and tetraphenylarsonium cation (Ph4As+) that in the context of the Hofmeister series behave as super-chaotropes ions. (C) 2016 Elsevier Ltd. All rights reserved.