Influence of Aqueous Phase Salt and Oil Phase Surfactants\nand Viscosity on the Dynamic Interfacial Tension and Coalescence Timescales

Liquid–liquid\nseparation is a critically important process\nin the treatment of emulsions that can occur in our environment, such\nas oily stormwater, shipboard bilgewater, or off-shore oil spill treatment.\nEffective filtration systems, including coalescing filters, are essential\nfor mitigating these environmental pollutants. Achieving this requires\na comprehensive understanding of liquid–liquid interface dynamics\ninfluenced by additives and surfactants. Furthermore, understanding\nthe impact of surfactants on emulsion stability in saline environments\nis vital for optimizing filtration processes and ensuring the protection\nof marine and freshwater ecosystems. In this work, these effects are\nhighlighted using measurements performed across a range of droplet\nsize, surfactant concentration, viscosity ratios, and saline presence.\nDynamic IFT measurements are conducted using the pendant drop method\nfor water in light mineral oil, with and without salt in the water\nphase. The effect of salt addition is also highlighted by using microfluidic\ncoalescence experiments, in which it was found that the addition of\nsalt increases the dimensionless drainage time below the critical\nmicelle concentration. The second focus of this work is to study the\neffect of bulk phase viscosity on the stability. Dynamic IFT measurements\nare performed at both millimeter and micrometer scales using pendant\ndrop experiments and microfluidic tensiometry, respectively, involving\nlight and heavy mineral oils with varying SPAN80 surfactant concentrations.\nThe surfactant diffusivity and interfacial adsorption and desorption\nrates are then extracted by fitting a surfactant diffusion and equation\nof state equations to the dynamic IFT measurements. The results of\nthe IFT decay, surfactant diffusivity, and adsorption rates are compared\nat two different viscosity ratios. This study also compares the times\nrequired for IFT relaxation with the film drainage times in water-in-oil\nsystems. The comparison aids in comprehending the impact of competing\ntimescales during film drainage. The findings presented in this paper\noffer valuable insights into the design and optimization of liquid–liquid\nfiltration systems, especially when operating under challenging environmental\nconditions, such as in saline environments. The principles explored\nhere can be applied to improving industrial water treatment and in\nthe design of advanced filtration technologies for chemical and petrochemical\nindustries, particularly those involving flow, contributing to more\nsustainable and efficient practices in handling emulsified waste streams.