Hydrotrope-Induced Inversion of Salt Effects on the Cloud Point of an Extended Surfactant

Abstract

The authors report on the effects of electrolytes spanning a range of (NaOc, NaSCN, NaNO3, NaBr, NaCl, NaBu, NaOAc, Na2SO4, Na2HPO4, and Na2CO3) and (LiCl, NaCl, KCl, CsCl, and choline chloride) on the aq. soly. of an extended surfactant. The surfactant is anionic with a long hydrophobic tail as well as a significant fraction of propylene oxide groups and ethylene oxide groups (C12-14-PO16-EO2-SO4Na, X-AES). In the absence of electrolytes, X-AES exhibits a cloud-point temp. that decreases with increasing surfactant concn. After the addn. of salts to the surfactant solns., various shifts in the soly. curves are obsd. These shifts follow precisely the same Hofmeister series that is found for salting-in and salting-out effects in protein solns. In the presence of different concns. of sodium xylene sulfonate (SXS), the soly. of the surfactant increases. In this context, SXS can be considered to be a salting-in salt. However, when the electrolytes are added to an aq. soln. of X-AES and SXS the Hofmeister series reverses for divalent anions such as Na2SO4, Na2HPO4, and Na2CO3. Studies on the phase behavior and micelle structures using polarization microscopy, freeze-etch TEM, and NMR measurements indicate a dramatic change in the coexisting phases on the addn. of SXS.