The effects of electrolyte anions on lowering the interfacial tension of oil/ alkylbenzene sulfonates solution

Abstract

The effects of different electrolyte anions on the interfacial tension (IFT) of surfactant solution of Para-dimethyl alkylbenzene sulfonate (PDABS) were discussed in detail. The results showed that the electrolyte can reduce the IFT by changing the ionic strength of the aqueous solution, compressing the electric double layer of hydrophilic group of the surfactant and increasing the adsorption concentration of the active substance at the interface. For the different electrolyte anions, the effect of this factor showed the same trend of changing IFT. With the increase of electrolyte anions concentration, IFT first decreased rapidly and then decreased slowly, meanwhile, the surfactant was continuously distributed into the oil phase. When the oil-water partition coefficient was 1, the IFT had a minimum value, IFTmin. Because the anions of electrolytes were different, the polarity and structure of water molecules near the interface were changed, resulting in the different decrease of IFT. The hydrolysis of CO32− produced OH−, which made the water molecules polarized highly, and the CO32− with relatively large ionic radius had a weak binding force on water molecules, resulting in the largest difference between the two sides of the interface and the decrease of IFT was the slowest. NaCl had no ability to produce OH− by hydrolysis, and its ionic radius was also small, so the IFT decreased the most rapidly. For the same reason, the decrease degree of IFT in NaOH and NaHCO3 aqueous was between Na2CO3 and NaCl. With the increase of the lipophilic chain length of PDABS, the solubility of PDABS in water was greatly affected by the electrolyte concentration, and the IFTmin moved to the low electrolyte concentration. The movement of p-S18-4 was most obvious, which had reached IFTmin at the concentration of 180.0 mol·L−1, and there was a second low value of IFT under the interaction of the change of single-layer adsorption micro-structures and spontaneous emulsification.