Morphologies near cloud point in aqueous ionic surfactant: scattering and NMR studies

Clouding phenomenon in ionic surfactant solution is fairly a new addition to the conventional phenomenon observed with nonionic counterpart. Various scattering and spectroscopic techniques, dynamic light scattering (DLS), small angle neutron scattering (SANS), and nuclear magnetic resonance (NMR), have been used to draw information regarding the aggregate morphologies (formed by an ionic surfactant, tetra-n-butylammonium dodecylsulphate, TBADS) when the surfactant solution passes through the cloud point (CP). DLS measurements have shown that two morphologies are present when the system approaches the CP. The data revealed that individual micelles (~5 nm) convert to giant aggregates (~500 nm) over the range of temperature including the CP. SANS experiments have been performed to draw the information regarding individual micellar fraction below and above the CP. NMR spectra at different temperatures have been collected for TBADS solution. The broadening and the downfield shift of -N-CH(2) and -O-CH(2) proton peaks support the micellar growth as the sample was heated. The above peaks show splitting (into two each) confirming the presence of two morphologies around the CP. It is noted that conversion of only a small fraction of individual micelles is responsible for the clouding.