Aqueous Solutions of Zwitterionic Surfactants with Varying Carbon Number of the Intercharge Group. 3. Intermicellar Interactions

The effect of the presence of salt on the structure of, and interactions between, zwitterionic micelles has been studied using radioactive tracer self-diffusion and elastic and quasi-elastic light scattering. Intermicellar interactions were found to become significantly more repulsive in the presence of salt. Electrostatic interactions calculated from the known micellar charges do not account for the experimental results. The origin of those salt-induced repulsive interactions is discussed with the help of experimental data about a series of zwitterionic surfactants having polymethylene intercharge groups of variable length. An increase of micelle excluded volume by reorientation of the zwitterionic headgroups was found to be consistent with both elastic and quasi-elastic light-scattering data and with their variation with surfactant molecular structure. It is likely that electrostatic screening by salt adsorption at the micellar surface allows more conformational freedom, switching from a zwitterion orientation parallel to the interface to a more radial average orientation in the presence of an increasing NaCl concentration.