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The influence of sodium dodecyl sulfate (SDS) and magnetite nanoparticles (NPs), individually and simultaneously, on the interfacial tension of n-hexane−water was studied over the temperature range from (288.2 to 308.2) K. Results show that SDS in the range from (0 to 2.00•10 −4 ) mol•dm −3 is adsorbed positively; while, NPs in the mass fraction range from (0 to 5.00•10 −4 ) are adsorbed negatively and cause interfacial tension to rise. Interestingly, with the presence of both SDS and NPs, interfacial tension is reduced more than with SDS only, because the repulsive interactions between SDS molecules are weakened in the presence of positively charged NPs. An almost linear decrease in interfacial tension with temperature is also relevant in all cases. The experimental data, below the critical micelle concentration of SDS, were nicely reproduced by the Szyszkowski adsorption isotherm. Accordingly, obtained SDS saturated interface excess is reduced with NPs mass fraction, and on the other hand, adsorption tendency increases with temperature and NPs mass fraction.

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The influence of individual and binary adsorption of octylphenol decaethylene glycol ether (Triton X-100) and dodecyl trimethyl ammonium chloride (DTMAC) surfactants with bulk compositions within (1.50 • 10 -6 to 4.80 • 10 -4 ) mol • L -1 on the interfacial tension of a recommended system for liquid-liquid extraction investigations, toluene + water, at 25.0 °C is studied. The drop-weight method has been used to measure the interfacial tension, and a maximum relative standard deviation of ( 0.01 has been appropriate for the concentration of surfactant solutions. From the data and in agreement with the commonly used Szyszkowski equation, a much higher adsorption effectiveness and adsorption tendency for Triton in comparison with DTMAC is obtained. The influence of DTMAC in lowering interfacial tension can be enhanced significantly by adding Triton with bulk mole fraction less than 0.01. For bulk mole fractions up to about 0.2, the estimated interfacial composition and attractive interaction from the theory of nonideal interactions in binary mixtures show different trends. Accordingly, the attractive interaction parameter finds a maximum absolute value at the bulk mole fraction of about 0.01 and lower values at bulk mole fractions between 0.05 and 0.1. Moreover, investigations on the experimental and ideal area per molecule at the interface show variations in agreement with the interaction parameter.

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