Lower consolute boundaries of a poly(oxyethylene) surfactant in aqueous solutions of monovalent salts

Abstract

Lower consolute boundaries up to 50 vol% surfactant have been determined for n-octylpenta(oxyethylene glycol)(C8E5) in the presence of the monovalent salts NaF, LiCl, NaCl, KCl, CsCl, NaBr and NaI. We find that the lower consolute boundaries are shifted, with only small changes in shape, to lower or higher temperatures (salting-out or salting-in, depending on the salt used). The amount and sign of the miscibility shift is determined almost solely by the anion, and the shift is related to the surface charge density of the ion. The shifts in the lower consolute boundary cannot be explained by structural changes in the bulk water structure due to the addition of salts. We have applied the Flory–Huggins lattice theory to relate the shifts of the lower consolute boundaries to changes in micelle–solvent interactions. This analysis shows that small changes in the free energy of interaction between oxyethylene and water can explain the shifts in temperature of the lower consolute boundary. We discuss the use and limitations of the Flory–Huggins theory, as applied to micellar phase separation. The salt effects are explained in terms of salt-deficient or salt-rich regions around the oligo-oxyethylene chains.