Experimental Characterization of the Solubilization of Weakly Polar Compounds in Aqueous Nonionic Surfactant Solutions

In this work, the influence of the molecular structure of the surfactant on the self-assembly as well as the solubilization of otherwise insoluble solutes in surfactant solutions was investigated. By systematically varying the hydrophilic and hydrophobic chain length of nonionic CiEj surfactants, the molecular structure of the surfactant could be successfully linked to relevant quantities such as the critical micelle concentration (CMC), the aggregation number (Nagg) and the hydrodynamic radius (Rh) of the surfactant aggregates as well as the composition of solute/surfactant aggregates upon solubilization of a solute. It was shown that different solubilization mechanisms exist. Nonpolar components are incorporated in the hydrophobic core of the surfactant aggregates, while weakly polar substances are incorporated both in the palisade layer and in the hydrophilic corona layer. Weakly polar long chain aldehydes and n-amines are incorporated into a surfactant aggregate such that their alkyl chain points to the hydrophobic core and the aldehyde/amine group points to the hydrophilic corona. The structure of the solute significantly influences the size (Nagg, Rh) of solute/surfactant aggregates. Thus, depending on the solute, no change in size, a steady increase in size, or a decrease followed by an increase in the size of the solute/surfactant aggregates was observed.