Mechanistic approach on heat induced growth of anionic surfactants: a clouding phenomenon

Abstract

Different anionic surfactants: tetra-n-butylammonium dodecylsulphate (TBADS), tetra-n-butylammonium-α-sulfonato myristic acid methyl ester (TBAMES) and tetra-n-butylphosphonium dodecylsulphate (TBPDS) were synthesized. Though all the surfactants are ionic in nature, they show the clouding phenomenon on heating. The effect of temperature on the solution behaviour (micellization and clouding) of the synthesized surfactants was studied by conductometry, dynamic light scattering (DLS), nuclear magnetic resonance (NMR), small angle neutron scattering (SANS) and polarising optical microscopy (POM) studies. The experimental cmc data of TBADS exhibits a U-shaped curve when plotted against temperature. Conductivity measurements follow different trends with temperature for various fixed concentrations of TBADS. Conductivity increases with temperature at lower TBADS concentrations, while an unusual decrease was observed with temperature for higher TBADS concentrations. NMR data with temperature also show peculiarity similar to that observed conductometrically. The broadening and splitting of the NMR peaks with increasing temperature is in agreement with the growth of micelles and formation of two morphologies. The grown aggregates tend to exhibit a stronger attractive potential relative to individual spherical micelles. The increase in δ (ppm) values along with the broadening of the peaks in the NMR spectra on increasing temperature corroborates the dehydration of the counterion and the increase in attractive forces among the grown aggregates. This proposition has been supported by the DLS and SANS results. It is experimentally confirmed that the formation of bigger aggregates through the fusion of grown micelles, which is facilitated by the dehydrated counterion on increasing the temperature and the onset of attractive interactions, is the key process involved in the clouding phenomenon of ionic surfactant solutions. The Langer-Schwartz's nucleation theory and the growth of micelles beyond the critical droplet size, resulting in a phase separation, are confirmed through the POM results. Thus, the solution response complements with both conventional ionic and nonionic surfactants, depending upon the concentration/temperature. This added feature can be due to the substitution of Na⁺ by a quaternary counterion in a typical anionic surfactant.