Foamstability: The interplay between salt-, surfactant- and critical micelle concentration

Abstract

It is central to understand the underlying factors that affect the generation and stabilization of foam. Nevertheless, reports on how foam stability is impacted by surfactant concentration, electrolyte concentration, and electrolyte nature are ambiguous. This paper summarizes the foaming properties below and above the critical micelle concentration (CMC) at various electrolyte concentrations. The commonly used surfactant, alpha olefin sulfonate (AOS), was used along with NaCl in a range of concentrations. The foam height, foam volume stability, bubble count, and foam structure were studied to determine the foam behavior at given conditions. The optimum surfactant concentration for the formation of a stable micellar film was found to be just above the CMC value and the foam stability on the addition of salts is related to the surfactant concentration. There exists another critical concentration (not CMC) that is important to determine the impact of salts on the stability of the foam. The addition of NaCl does not have the same impact below and above this critical concentration. If the concentration of surfactant is below the critical concentration, addition of NaCl reduces the half-life and foam stability. However, at higher surfactant concentrations, the presence of NaCl improves the half-life and foam stability. This study clarifies the ambiguities that are present in the literature pertaining to the effect of salt on foam stability and foaming properties.