Uncontaminated Hydrophobic/Water Interfaces Are Uncharged: A Reply

In our communication Why are hydrophobic/water interfaces negatively charged? we offer a simple explanation for the negative charge observed at hydrophobic interfaces with water.[1] We demonstrate that traces of carboxylic acid surfactant at these interfaces account qualitatively and quantitatively for the properties of surfactant-free emulsions. This is a classic example of colloidal behavior where traces of surface-active substances can control the surface behavior and hence the macroscopic properties of the colloidal dispersion.[2] Our demonstration rests on three simple but solid experimental observations: 1) Through straightforward acid-base titrations we detect trace amounts of acids at a concentration of 7 mmolL-1 in 99% pure hexadecane oil and 1 mmolL-1 in 99.8% pure hexadecane oil. For 100 nm diameter droplets in an aqueous phase, with 99% pure oil, the concentration that we detect yields a surface charge of 0.065 enm-2 at pH 7. 2) We observe that these carboxylic acids accumulate at the interface of oil droplets in surfactant-free emulsions. Indeed, through electrophoretic mobility measurements we find a strong variation of the negative charge with pH, as reported by a large number of authors.[5, 6] The freeenergy associated with this variation is 20 times the thermal energy kT, which matches exactly the reaction free-energy between hydroxide ions and carboxylic acids. 3) The key observation was obtained by comparing different emulsions made with the same method, at constant pH, but with two oils of different purity (99%hexadecane and 99.8% hexadecane). All other parameters being kept constant, the concentration of impurities changed dramatically the surface properties of the droplets and particularly their colloidal stability. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.