The effect of mixed micelle formation on the dynamics of phase partitioning for a polydisperse ethoxylated nonionic surfactant

Mixed surfactants are common in practical applications, and their behaviors are often observed to deviate from the behaviors of pure surfactants with similar average properties in striking, and sometimes unexpected ways. The focus of this work was on exploring the role of mixed micelle formation in a mixed ethoxylated nonionic surfactant on the dynamics of partitioning between immiscible phases. The work examined partitioning of an ethoxylated nonylphenol surfactant between water and tetrachloroethylene (PCE) in a stagnant system at two different phase volume ratios. A model describing partitioning rate was developed and calibrated to experimental data, and then used to predict the long-term dynamic approach to equilibrium in mixed and pure systems. The work suggests that micelle formation slows partitioning time by limiting the concentration of monomers in water; in pure systems, time to approach equilibrium increases with increasing concentration, but then reaches a maximum. However, in mixed systems, time to approach equilibrium continues to increase with increasing concentration over the physically-possible concentration range. For the mixed system studied here, partitioning could be expected to take two orders of magnitude longer at very high concentrations than a similar pure system. Because of the similarity of the mathematics, this result might be expected for any monomer-driven interaction, such as adsorption or interfacial tension reduction.