Effect of nonionic surfactant molecular structure on cloud point extraction of phenol from wastewater

Above a temperature known as the cloud point, aqueous solutions of nonionic ethoxylated surfactants separate into two phases: a coacervate phase concentrated in surfactant and a dilute phase with low surfactant concentration. The coacervate phase contains surfactant aggregates which are micelles or micelle-like and will solubilize any organic solutes originally present in the water, resulting in a liquid–liquid extraction known as a cloud point extraction (CPE). Removal of pollutants from water is an important potential application of CPE and we use phenol as a model organic pollutant in this work. In this study, we investigate the effect of surfactant structure on important equilibrium CPE parameters like cloud point, fractional coacervate volume, and phenol and surfactant partition ratios for alcohol ethoxylates (AEs), a major class of nonionic surfactants. Pure, homogeneous surfactants with linear hydrophobes are studied as well as commercial heterogeneous AEs with both linear and branched hydrophobes. The effects of degree of polymerization in the polyethoxylate group (EO number) as well as hydrophobe size (alkyl carbon number) are systematically investigated as well as the effect of hydrophobe branching. The solubilization equilibrium constant is shown to increase linearly with EO number and is unaffected by alkyl carbon number or hydrophobe branching from which we deduce that the phenol is solubilized with the benzene ring at the surface of the micelle core and the hydroxyl group having attractive interactions with the polyethoxylate chains in the palisade layer of the micelle. The effects of surfactant structural features on net intermicellar attractive forces, micellar excluded volume, and solubilization capacity are discussed. A model is developed which can predict the phenol partition ratio at a given temperature for any AE surfactant dependent on only one simple measured parameter: fractional coacervate volume. This universal model is somewhat unique to phenolics or short chain alcohols because of their solubilization loci in the micelle and mechanism of solubilization. Guidelines for surfactant selection for CPE of phenol or similar solutes are outlined.