Structure and stability of colloidal liquid aphrons

Abstract

The structure and stability of colloidal liquid aphrons (CLAs) have been investigated using a variety of experimental techniques, i.e. cryo-TEM, DSC, and light scattering. The findings support the structural model proposed by Sebba who suggested that polyaphron phases resemble a biliquid foam while the individual CLAs, when dispersed in a continuous aqueous phase, consist of spherical, micron-sized, oil droplets surrounded by a thin, aqueous “soapy-shell”. First-order half-lives of CLAs dispersed in a stirred vessel over a range of continuous phase ionic strengths, pH, and temperatures were also determined for the first time. These allowed quantitative comparison of CLA stability when dispersed under various conditions and of the influence of including various concentrations of lipase or erythromycin-A in the aphron formulation. Based on these results, a mechanism for the breakdown of dispersed CLA structure is proposed which involves destabilisation and loss of the “soapy-shell” followed by coalescence of the oil cores of the aphrons. However, direct evidence for the structure of the surfactant-laden interfaces responsible for the stabilisation of aphrons is still required if the structural model proposed by Sebba is to be fully confirmed. The similarities and differences between CLAs and high-internal-phase-ratio emulsions (HIPREs) are also discussed.