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.

Nanoprecipitation of polymethylmethacrylate by solvent shifting: 1. Boundaries.

Nanoparticles of polymethylmethacrylate (PMMA) have been produced through the solvent shifting process (also called the "Ouzo process") in which water (nonsolvent) was added to a solution of PMMA in acetone or tetrahydrofuran (solvent). At low concentrations of PMMA in the initial solution, and for large additions of water, the process yielded PMMA nanoparticles with a narrow distribution of particle sizes. The mean particle diameter varied as a power law of the initial PMMA concentration in the solvent, in agreement with the predictions from the Smoluchowski equation for an aggregation process that has definite "start" and "stop" times. At higher PMMA concentrations, the mixing process yielded microparticles coexisting with PMMA nanoparticles. The boundary that separates the Ouzo region of compositions (PMMA nanoparticles only), from the "non-Ouzo" region (nano- and microparticles) has been determined. This boundary does not appear to have any relation to the spinodal decomposition line of the ternary solutions: the transition from Ouzo to non-Ouzo behavior must have another unknown origin.