Sorption of linear alcohol ethoxylate surfactant homologs to soils

Sorption onto five saturated soils of the homologs within the commercial surfactant mixture Brij 35 (registered trademark of ICI Americas) was investigated. Brij 35 is a mixture of linear ethoxylated alcohols, having an average of 23 ethoxy (EO) groups per molecule and alcohol chain of primarily 12 carbons in length (C₁₂H₂₅(OCH₂CH₂)₂₃OH). In experiments, saturated soils were exposed to various concentrations of the surfactant mixture for specified times, the slurries were centrifuged to separate the phases, the aqueous phases were extracted with 1,2-dichloroethane, and the residual homologs were derivatized with 3,5-dinitrobenzoyl chloride and analyzed by normal phase HPLC. Homologs containing 4–43 EO groups were chromatographically separated at near baseline. At aqueous Brij 35 concentrations below the critical micelle concentration (cmc), the proportion of each homolog sorbed to each of the soils increased with increasing EO chain length through the homologous series. As a result, in experiments where a significant proportion of the surfactant adsorbed, significant shifts in the aqueous phase compositions occurred to mixtures with lower mean EO numbers. A sharp break in the adsorption isotherms occurs at the cmc.