Small‐angle neutron scattering studies on sodium dodecylbenzenesulfonate‐tetra‐n‐butylammonium bromide systems

Abstract

AbstractA phase study was completed on aqueous sodium dodecylbenzenesulfonate (SDBS) and tetra‐n‐butyl‐ammonium bromide (Bu4NBr) systems, and consolute boundaries were drawn through cloud points. Samples were selected from both miscibility regions [under the lower consolute boundary (LCB) and above the upper consolute boundary (UCB)] for small‐angle neutron scattering (SANS) studies. In the first set of experiments, the effect of varying Bu4NBr concentration on micellular parameters of 100 mM SDBS was studied at 30°C. The pure SDBS micelle has an aggregation number (ns) of 51, and the effective charge on the monomer (α) is 0.17. With the addition of Bu4NBr, the ns of SDBS micelles increases while α decreases. The system with [Bu4NBr]=39.5 mM (an above‐UCB sample) showed clouding near room temperature (≈29°C) and had a high ns value (300) and a low α (=0.09). The data indicated that the micelles lose ionic character in the presence of Bu4NBr. The temperature effect on this sample shows that α remains almost constant, while ns decreases on heating. A similar effect was observed with samples of lower Bu4NBr concentration (32 or 25 mM) in the presence of 100 mM SDBS. The same type of temperature effect was seen on a sample of under‐LCB region (50 mM SDBS+32 mM Bu4NBr); the ns values increased significantly as the LCB was approached. The overall SANS observations suggest that the micelles have low ionic character together with high ns values (a case of micellar growth) near LCB/UCB.