Micellization of Sodium Dodecyl Benzene Sulphonate in Aqueous Quaternary Bromides

The effect of addition of symmetrical quaternary bromides (tetramethylammonium bromide, Me₄NBr; tetraethylammonium bromide, Et₄NBr; tetra-n-propylammonium bromide, Pr₄NBr; tetra-n-butylammonium bromide, Bu₄NBr; tetra-n-butylphosphonium bromide, Bu₄PBr, and tetraphenylphosphonium bromide, Φ₄PBr) on the micellization of sodium dodecylbenzenesulphonate (SDBS) has been studied at 25 °C using conductivity measurements. The critical micelle concentration (CMC) and dissociation degree (β) are determined from the conductivity vs. [surfactant] plots at various fixed [salt]. As usual, the presence of salts bring about a reduction in CMC but salts containing bulky counterions show progressively increased effect both on CMC and β. A bulky counterion seems to weaken hydration forces with concomitant decrease in the CMC. The behaviour is further explained in the light of hydrophobic interactions (among alkyl/phenyl chains of the concerned salt and surfactant) in addition to the neutralization of surface charge due to salt addition. Connecting atom (N or P) of the counterion seems to play an important role in such interactions as salts with P were more effective in reducing the CMC and thus seem to be the ideal candidate to vary hydration forces in the anionic surfactant systems.