Understanding the surface tension reduction of the water/air interface due to the synergistic effect of the SDS-CTAB mixture: A molecular dynamics simulation study

Using molecular dynamics simulation, the surface tension is calculated and compared with experimental data to provide a microscopic interpretation of the synergistic effect on water surface tension reduction caused by a binary mixture composed of anionic and cationic surfactants. For the anionic and cationic surfactants, the sodium dodecyl sulphate (SDS) and the cetyl trimethylammonium bromide (CTAB) were used, respectively. Besides the electrostatic attraction between the anionic and cationic surfactants, resulting in a synergistic effect, other properties were analysed to study the reduction of the surface tension from a molecular level. For different SDS:CTAB composition of distinct properties of the pseudobinary mixture, such as the hydrogen bonds, order of the tails, and the arrangement of the surfactants at the interface were calculated. Those properties present different behaviours at the maximum and minimum values of the surface tension. The CTAB tails present a higher order than the SDS tails at all SDS:CTAB compositions however, the first ones show a maximum order at the minimum in the surface tension. The excess configurational energy was also calculated and shows a minimum value in the same composition as the minimum value of the surface tension. [GRAPHICS] .