Surface Impurities Are Not Responsible For the Charge on the Oil/Water Interface: A Comment

A recent communication by Roger and Cabane[1] on the origin of negative charge on the interface between oil and water states that: "The basic assumption of all previous experimental and theoretical studies has been that these systems have "pristine" oil/water interfaces, that is, oil molecules in contact with water molecules, although the possibility of contamination by anionic, surface-active impurities has been mentioned." It is asserted that long-chain carboxylic acids could originate as impurities from the oil phase. These impurities might be responsible for the unusually large surface charge on the oil/water interface. Covering the interface with carboxylic ions would lead to a reduced interaction of oil with water. This claim is supported by a selfconsistent data set[1] comprised of 1) droplet mobility data as a function of pH, 2) droplet size distribution measured for different grades of impurity and with different concentrations of carboxylic acid, and 3) a titration experiment to quantify the amount of acid in the hexadecane oil phase. While this data is of interest its weakness appears to be that none of the employed methods are in situ background-free measurements of interfacial chemical structure. The surface sensitivity of the employed methods relies on the application of crucial assumptions about the surface structure, mean field theories, and non-equilibrium methods. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.