Stabilisation of emulsions by using a combination of an oil soluble ionic surfactant and water soluble polyelectrolytes. I: Emulsion stabilisation and Interfacial tension measurements

Traditionally emulsions are formed using a surfactant to stabilise the oil water interface and prevent coalescence of the droplets, although other materials such as particles or proteins can be used to stabilise emulsions. Bancroft's rule is a good guide as to whether a water in oil or oil in water emulsion is formed, namely if the surfactant is water soluble an oil in water emulsion is formed, whilst if the surfactant is oil soluble a water in oil emulsion is formed. In this paper we demonstrate a novel way to stabilise oil in water emulsions by using a combination of an oil soluble surfactant (hexadecyl amine) and a high molecular weight polyelectrolyte, polyacrylic acid. It was found that neither component on its own produced a stable emulsion but in combination stable emulsions were formed. To explore this effect further the interfacial tensions of various mixtures of hexadecylamine and polyacrylic acid were determined, and whilst the interfacial tension of pure hexadecyl amine and polyacrylic acid both at 10(-4) mol/dm(3) is 23 mN/m and 35 mN/m, respectively, when used in combination the interfacial tension drops to 6 mN/m and under the correct conditions of pH to less than 1 mN/m. It is proposed that the hexadecyl amine adsorbs at the oil water interface, leaving the droplets positively charged, but that this on its own is insufficient to stabilise the emulsion as the concentration of surfactant at the interface is likely to be low due to the solubility of the hexadecylamine in the oil droplet. However the negatively charged polymer, polyacrylic acid then adsorbs on the positively charged droplet, anchoring the hexadecylamine to the interface and forms a steric barrier preventing coalescence of the droplets. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.