The effect of NaOH on lowering interfacial tension of oil/alkylbenzene sulfonates solution

Six sodium para-dimethyl alkylbenzene sulfonates (PDABS, abbr. p-S12-5, p-S14-5, p-S16-5, p-S18-5, p-S16-6 and p-S16-8, respectively) have been synthesized. The structures and the purities of the products have also been confirmed by (1)H-NMR and mass spectrometry. Crude oil and its equivalent alkanes were chosen as oil phases. The effect of the NaOH concentration on the interfacial tension (IFT) of PDABS was investigated by using a spinning drop tensiometer. The results showed that, in a certain range of NaOH concentrations, p-S14-5, p-S16-5 and p-S18-5 produced ultra-low IFT (∼10(−3) mN m(−1)). NaOH was roughly similar to NaCl in aqueous solutions in terms of its effect on interfacial concentration and arrangement of PDABS at the interface. With the increase in the concentration of NaOH, the electric double layer of hydrophilic groups in PDABS was compressed to become smaller and the electrostatic interactions between hydrophilic groups weakened, which resulted in an increase in interfacial concentration of PDABS. Continuing to increase the concentration of NaOH, the electrical double layer was further compressed; water molecules could incorporate into the interface through loose hydrophilic groups. Therefore, these two aspects of variations caused IFT to display the trend of decreasing and then increasing with increase of NaOH concentration. However, at higher NaOH concentrations, PDABS molecules were driven into the oil phase by the salting out effect, and this process resulted in a decrease of IFT for water-soluble p-S12-5 and p-S14-5. For oil soluble p-S18-5, this process had little effect on the effective distribution in the oil phase. The effective distribution of PDABS in the oil phase played an important role in stabilizing the interface and reducing IFT. There was no clear evidence that NaOH reduced the IFT between oil and water by showing a synergistic effect between PDABS and active species formed in situ with acidic components in the crude oil.