Central Role of Bicarbonate Anions in Charging Water/Hydrophobic Interfaces

Aqueous interfaces are ubiquitous in Nature and play a fundamental role in environmental or biological processes or modern nanotechnologies. These interfaces are negatively charged, and despite several decades of research, the rationale behind this phenomenon is still under debate. Two main controversial schools of thought argue on this issue; the first relies on the adsorption of hydroxide anions on hydrophobic surfaces, whereas the second one supports a self-rearrangement of water molecules at the interface bearing hydronium ions. Here, we report on two series of independent experimental studies (nanoprecipitation and interfacial tension measurements) that demonstrate that in the pH 5-10 range the negative interfacial charge of the colloids mostly stems from bicarbonate ions, whereas at lower and higher pH, protons and hydroxide ions contribute, with bicarbonate ions, to the interfacial charging. This new interpretation complies with previous studies and opens new perspectives to this striking physical chemical issue.