Complexation of counter-ions in ionic micellar solutions: A small-angle neutron scattering study

The properties of LDS and SDS micellar solutions upon the addition of [2.2.2]-cryptand (C222) are studied by small-angle neutron scattering (SANS). The interfacial thickness of the micelle for three ligand to surfactant molar ratios (L/S=0.5, 1.0 and 1.5) increases from 5.5 Angstrom (interfacial thickness of the micelles without ligand) to 10 Angstrom, indicating that the ligand is located at the interface. The micellar net surface charge and the average aggregation number decrease as L/S increases, producing a slight decrease of the micellar size and a more spherical shape. The ionization degree of the LDS micelles is almost constant as L/S increases, whereas it decreases slightly for SDS micelles. This supports the higher binding ability of C222 for sodium ions. The ligand/surfactant molar ratio at the interface is at most 1/2 when the ligand is added to the surfactant solution (LDS and SDS) in equimolar amounts. This implies that only half of the total amount is partitioned at the interface, while the remaining ligand molecules are in the bulk solution. Further addition of the ligand increases its amount in the bulk solution.