Comparative Studies of Structurally Similar Surfactants: Sodium Bis(2-ethylhexyl)phosphate and Sodium Bis(2-ethylhexyl)sulfosuccinate

Abstract

Sodium bis(2-ethylhexyl)phosphate (NaDEHP) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) are studied by atomic-level molecular modeling, using UFF force field and surface tension measurements. Five representative geometries of NaDEHP are shown, and the corresponding structural parameters such as the energy, area, and volume are analyzed. The effects of additives such as NaCl, water-soluble polymer, and alcohol on the surface activity of NaDEHP and AOT solutions are investigated by surface tension measurements. The results suggest the following: (i) The bond angle of the hydrophobic chains of NaDEHP is smaller than that of AOT. (ii) AOT is more sensitive to NaCl than NaDEHP, that is, the surface tension of AOT is obviously decreased as a small amount of NaCl is added, but it has only a slight influence on the NaDEHP system. (iii) Two transition points appear in the surface tension curves of AOT with addition of 0.5 wt % PVP (poly(vinylpyrrolidone)), but in the NaDEHP surface tension curves two transition points appear only when the PVP concentration is high enough, for example, 2.0 wt % in this experiment, which suggests that the interaction between AOT and PVP is stronger than that between NaDEHP and PVP. (iv) The surface tension and critical micelle concentration are decreased with the increase of the carbon number and the concentration of the alcohol. (v) The mean squared end-to-end distance (〈r2〉) of the polymer chain obtained from the simulation is reduced by adding NaDEHP and AOT.