Interfacial properties of sodium para-dimethyl alkylbenzene sulfonate in the presence of monovalent metal counterions at the oil-water interface

The interfacial properties of sodium para-dimethyl alkylbenzene sulfonate surfactant p-S14-41 (formula is C22H37SO3Na) at the oil-water interface in the presence of monovalent metal counterions (Li+, Na+, K+ and Cs+) are investigated by the spinning drop tensiometry. In the presence of monovalent metal counterions (Na+, K+ and Cs+), the interfacial tension (IFT) of oil-water first decreases and then increases with the increase of counterion concentration, while for Li+ concentration in the studied concentration range, the IFT decreases continuously. The reason why the latter (Li+) does not appear the optimum salinity is that the salt concentration is not high enough. The most noteworthy feature is that not only the different counterions affect differently the optimum salinity but also the concentration of p-S14-4 in the studied concentration range for the same counterion has no effect on the optimum salinity. Besides, the optimum salinity is affected by the electrostatic interaction, water-oil ratio (WOR), salting-out effect, and the hydrated radius of counterion, under the synergistic effect of which, the optimum salinity value order under various monovalent metal counterions is as follows: Li+ > Na+ > Cs+ > K+. The influence mechanism of counterions on the adsorption of p-S14-4 at the oil-water interface is also discussed in detail.