Viscoelastic behaviour of cetyl trimethyl ammonium bromide/sodium salicylate/water system:: Effect of solubilisation of different polarity oils

Rheological behaviour of cetyl trimethyl ammonium bromide (CTAB)/sodium salicylate/oil/water system (being termed as oil-in-water microemulsions) was studied. Three monomers with different polarities, viz. acrylic acid, methylmethacrylate and styrene, were used to formulate isotropic oil-in-water microemulsions. Compositions of all the systems studied were on the boundary of the semidilute and concentrated regime (<5 wt.%). CTAB/NaSal mole ratio was kept constant ([CTAB]/[NaSal]=1) and effect of the solubilisation of oils on the viscoelasticity was studied. All measurements were made in linear viscoelastic region. Microemulsions formulated were found to obey Maxwellʹs model. Various parameters such as G′, G′′, η*, η0 and τm obtained by rheological studies were correlated with polarity and partitioning behaviour of monomers in between continuous phase and micelles. Characteristic relaxation time τm for all the systems studied was in the range 0.02–0.56 s. The mechanism of the relaxation behaviour of the systems was discussed on the basis of the ‘existing models’ proposed for the thread-like micellar systems. It is seen that the nature of oil is quite important in deciding the viscoelastic properties of above microemulsions. The behaviour of the oils was compared with that of hydrocarbons and alcohols.