Effect of n-Octanol on the Structure at the Supramolecular Scale of Concentrated Dimethyldioctylhexylethoxymalonamide Extractant Solutions

Using small-angle X-ray scattering (SAXS) and interfacial tension measurements, we show that concentrated solutions of dimethyldioctylhexylethoxymalonamide diluted in dodecane are organized in reverse aggregates that have many features in common with reverse micelles. The critical micellar concentration is determined from interfacial tension measurements using the pendant-drop technique, and the aggregation number is given by fitting SAXS data on the absolute scale. The effect of the addition of n -octanol, a modifier widely used in liquid/liquid extraction, is studied by SAXS. It is shown that, for small amounts of alcohol, the modifier acts as a cosurfactant; it swells the reverse micelles by adsorbing onto the aggregates and increases the surface per extractant polar head. When n -octanol is added in larger quantities, a new microstructure appears, and the amount of octanol required for this transition depends on the amount of diamide in solution (for example, octanol concentration ≥ 1 M for 0.7 M diamide). Therefore, a hydrogen-bond network is present in solution, and the presence of areas of high electronic density separated by a distance varying from 15 to 20 Å is found to explain the scattering patterns.