Effect of organic alkali on interfacial tensions of surfactant solutions against crude oils

Abstract

Abstract The dynamic interfacial tensions (IFTs) of petroleum sulfonate and organic alkali combined solutions against crude oils with different acid numbers have been investigated by a spinning-drop interfacial tensiometer. The influences of surfactant concentration, organic structure and concentration on the IFTs were expounded. The experimental results show that the petroleum acids can react with the organic alkalis and produce the surface-active soaps at the interface in situ. Both the ultralow transient and equilibrium IFTs can be reached at optimum conditions. The structures and contents of petroleum acids play important roles in controlling IFTs between the crude oils and the organic alkaline solutions. It is believable that fatty acids contribute to the lower transient IFT in short time and aromatic acids with condensed ring determine the equilibrium IFTs. The crude oil with higher acid number usually shows lower IFT values, however the structures of petroleum acids are more crucial factors and the crude oils with similar acid numbers may behave quite different dynamic IFT behaviors. The concentrations and structures of organic alkalis also strongly affect the IFTs of crude oil–alkaline solution systems. An obvious IFT minimum appears as a function of organic alkali concentration and the increase of molecular size of the organic counterion results in a loosely packed film and an increment of IFT. By adding petroleum sulfonate to organic alkaline solutions, the IFTs may be easily reduced to an ultralow value through the formation of close-packed mixed adsorption film containing surfactant, petroleum acid and soap molecules. For organic alkalis with larger molecular size, the loose mixed adsorption film is beneficial for the synergistic effect among petroleum acids, soaps and surfactant molecules, which can enlarge the applying reservoirs of organic alkali.