Investigations on NaDEHP and AOT: computer simulation and surface tension measurements

This paper studies two structurally related surfactants NaDEHP and AOT with the surface tension measurements and the computer simulation. The surface activities of AOT and NaDEHP with different temperatures and different NaCl concentrations are investigated. The interaction parameters (βM) of NaDEHP–CTAB and AOT–CTAB molecules in their respective mixed micelles are obtained according to the equation proposed by Rubingh et al. (Solution Chemistry of Surfactants 1 (1979) 337) The mixing energy and Huggins parameter of AOT–CTAB mixed system and NaDEHP–CTAB mixed system are also obtained by computer simulation and the results are consistent with our surface tension measurements. The following results are obtained: (i) the surface tension of AOT is reduced with the increase of temperature when the temperature is below 40 °C, but it has the opposite trend above 40 °C; this phenomenon is not observed in the NaDEHP case; (ii) the lower βM value of AOT–CTAB suggests the stronger interaction between AOT and CTAB molecules in their mixed system. It is novel to find that the βM of NaDEHP–AOT mixed systems is also a negative value, which suggests there is a weak attractive interaction between NaDEHP and AOT molecules. Comparing the experimental values with those obtained from theoretical calculation we find the CMCs of NaDEHP–AOT mixed systems do not agree well with the ideal mixed model, whereas the effect of NaCl on AOT agrees well with the empirical formula.