Counterion-specific clouding in aqueous anionic surfactant: a case of Hofmeister-like series

Two ionic surfactants (sodium dodecylsulphate (SDS) and sodium dodecylbenzenesuphonate (SDBS)) are chosen, and effect of addition of quaternary bromide or chloride (tetra-n-butylammonium bromide, tetra-n-butylphosphonium bromide, tetraphenylphosphonium bromide, tetra-n-pentylammonium bromide and benzyl tributylammonium chloride) has been studied on the cloud point (CP) behaviour in aqueous solution. CP behaviour was observed due to dehydration of headgroup region in presence of quaternary counterion. Order of counterion to decrease CP is as follows: TPeA(+) > BTA(+) > TBP+ > TPhP+ > TBA(+) for SDS and TPeA(+) > TPhP+ > BTA(+) > TBP+ > TBA(+) for SDBS. CP data have been used to select surfactant + salt system. Effect of additives (carbohydrate, amino acid or vitamin), on the CP, has been seen on above such selected systems. Additive may either decrease or increase the CP, depending upon the structure of counterion and/or the additive. Counterion (cations) can be arranged in an order like Hofmeister series.