Properties of surfactant - stabilized liquid - liquid interfaces and the relation to emulsion stability
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The stability of emulsions is of major importance to numerous industrial processes in which they are involved. The production of crude oil includes challenges due to the presence of indigenous molecules including asphaltenes and naphthenic acids. This yields formation of kinetically stable emulsions which can cause corrosion, deposits, increase of production costs. The emulsions properties are also relevant to food manufacture, agrochemicals, paint and road constructions processes. Gaining a better insight of the interface behaviour in presence of stabilizing agents is therefore of great interest. This work was developed within the aims of the project “Multiphase Flow Assurance Centre (FACE)” and has been started in an attempt to investigate the properties of oil-water interfaces in presence of surfactants with an emphasis on the rheology features granted by adsorption and the resulting stability of such interfaces. The process of adsorption of Span 80 at liquid-liquid interfaces has been identified as diffusion controlled using the Ward and Tordai methodology. The influence of the oil properties on the diffusion and interfacial characteristics of the Span 80 molecules has been demonstrated by means of ring and profile analysis tensiometry. The rheological response to oscillatory variation of the interface area has been demonstrated as mostly elastic and in good agreement with the Lucassen van den Tempel model. The stability of binary droplets to coalescence, studied by means of a Drop Bubble Micro manipulator, has been shown to depend on the surfactant concentration and the oil phase properties. The formation of a precipitate at the interface has been put forward to explain the stabilisation mechanism. The applicability of a newly developed shear rheology technique to several surfactants exhibiting a wide array of interfacial behaviours has been questioned. It has been shown that the ISR 400 is able to determine that the air-water interface behaves as a Newtonian like fluid in presence of a fatty alcohol, in agreement with other authors work. The instrument was deemed not adequate for determination of the viscoelastic properties of asphaltenes, Span 80 and naphthenic acid model compound. The effect of varying physical parameters on the emulsion droplet size has been showed and a phenomenological model for the evaluation of the average droplet diameter was put forward. The model was capable of predict the tendency in a qualitative manner and it was concluded that more work is required for precise quantitative results.