The rheology of highly concentrated emulsions stabilized with different surfactants

An investigation was performed into the effect of surfactants on the rheology of water-in-oil highly concentrated emulsions (HCE). The surfactants were oligomers of the PIBSA-type with different headgroups and low-molecular-weight sorbitan monooleate (SMO). The rheological properties of HCE are presented by flow curves with clearly expressed yield stress and dynamic modulus which does not depend on frequency but does on the amplitude of deformations. The changes in modulus and the yield stress depend on the nature of the headgroups as well as the addition of low molecular weight surfactant. It was shown that an increase in the surfactant concentration results in the decrease in the theological parameters. This shows the significance of micellar structure on the theological behaviour of HCE. The dependencies of elastic modulus as well as the yield stress on droplet size are deviated from scaling by Laplace pressure. This means that some additional arguments to explain the elasticity of compressed HCE emulsions should be included for comparison with the classical models based on the conception of increase in the surface area of droplets. Finally, it was found that the scaling of shear modulus with reciprocal squared droplet size fulfil the zero intercept condition for this variation. (C) 2010 Elsevier B.V. All rights reserved.