Equilibrium and dynamic behavior of a system containing a mixture of anionic and nonionic surfactants

Abstract

Mixtures of the pure nonionic surfactant C₁₂E₃ and an anionic surfactant of the alkyl ethoxy sulfate type were studied. NaCl brine was added to these mixtures and the phase behavior was determined at 30°C as a function of surfactant composition and salinity. A larger range of compositions was found where the lamellar liquid crystalline phase coexisted with a micellar solution in these oil-free systems. The Phase Inversion Temperature (PIT) was determined as a function of surfactant composition and concentration, salinity, and hardness with n-hexadecane as the hydrocarbon. A new contacting technique was developed to observe with videomicroscopy the dynamic behavior which occurred when aqueous solutions of these surfactants were brought into contact with small drops of n-hexadecane of about 100 μm in diameter. This method more closely simulates the situation during actual detergency processes than that employed previously, and enables the entire solubilization process to be observed, including the final stages. Solubilization of hydrocarbon into a middle-phase microemulsion and excellent hydrocarbon soil removal were observed near the PIT, as in our previous work with nonionic surfactants. Thus, our previous conclusions about the solubilization-emulsification mechanism of hydrocarbon detergency are confirmed for surfactant mixtures similar to those employed in many current products.