Comparisons of the effects of pH on the interfacial tension-lowering activity of surfactants Triton X-100 and Triton SP-190

A comparative study was conducted to investigate the effects of pH on the equilibrium and dynamic interfacial tension-lowering activity of a commercial splittable surfactant Triton SP-190 and a common surfactant Triton X-100 at the n-pentadecane/water interface. Equilibrium and dynamic interfacial tensions at the interface of n-pentadecane/aqueous phase containing surfactants were measured by using a Wilhelmy plate tensiometer and a drop volume tensiometer, respectively. It was found that the addition of inorganic acids did not have an effect on the equilibrium and dynamic interfacial tension-lowering activity of Triton X-100 at the interface. However, though the equilibrium interfacial tension-lowering activity of Triton SP-190 was not significantly affected by acids, the dynamic interfacial tension-lowering activity of Triton SP-190 was strongly inhibited at lower pH, resulting in a slower decreasing rate of dynamic interfacial tension. This phenomenon might be explained by the presence of minor surface-active components in the commercial Triton SP-190 surfactant, which dominate the equilibrium interfacial tension behavior at the interface and are not affected by acids. Based on the pH-dependent dynamic interfacial tension data, the percentages of the Triton SP-190 surfactant split under acidic conditions were then estimated. Moreover, the influence of pH was considerably enhanced at small adsorption times and low Triton SP-190 concentrations.