Effects of alcohol on partition equilibrium of phenol red in micellar solutions and o/w microemulsions of anionic surfactants

The effects of alcohol, viz., propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, pentan-1-ol, pentan-2-ol and pentan-3-ol on partition equilibrium of phenol red (PR) in anionic micelles and in o/w microemulsions of sodium dodecyl sulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS) with buffered aqueous pseudophase. The results have been correlated with incorporation of the alcohol to different regions in the micelles and oil-water interface and enhanced incorporation of the dye to the oil-water interface with decreasing droplet curvature. Preferred incorporation of alcohol in the interfacial headgroup region lowers the partition equilibrium constant, Kp initially up to an alcohol to surfactant w/w ratio of 1:1 in both micellar solutions and o/w microemulsions. The Kp increases as the ratio exceeds 1:1 due to increased solubilization of the alcohol towards the hydrocarbon chain region of the surfactants rather than in the headgroup region. In the microemulsion systems, the Kp increases with increase in oil volume fraction due to enhanced incorporation of the dye in the palisade layer of the oil-water interface with decreased droplet interface curvature. At a given volume fraction of oil, the partition equilibrium constant increases on changing the alcohol in the order: propan-2-ol < propan-1-ol < butan-2-ol < butan-1-ol < pentan-3-ol < pentan-2-ol < pentan-1-ol. The trend has been attributed to synergistic surface activity between the surfactant and alcohol. The partition of the dye into the micelles or the oil-water interface was higher in the case of SDBS than in SDS.