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The effect of additives on the phase behavior of aqueous solutions of sodium dodecyl sulfate (SDS) and cetyl trimethyl ammonium bromide (CTAB) has been studied in detail for extended applications. The results showed that both the widths of an anionic two-phase aqueous system (ATPSa) region with SDS in excess and a aqueous cationic two-phase system (ATPSc) region with CTAB in excess are very narrow, less than 1% in absence of any additives. However, phase regions of ATPS move toward the corresponding isotropic single phase regions and become wider in the presence of inorganic additives such as NaBr in the aqueous surfactant solutions. This is because the addition of inorganic salt increases the concentration of inorganic counter-ions and therefore the electrostatic interactions between counter-ions and surfactant aggregates. Meanwhile, the organic salts, sodium salicylate and tetrabutylammonium bromide, have been found to have a greater capacity to expand the range of ATPS because the electrostatic interactions between organic ions and aggregates were adjustable through the deformation of the electron cloud of organic ions. Organic additives with different properties can effectively extend the range of either ATPSc or ATPSa. Wherein, sodium salicylate can expand ATPSc efficiently and tetrabutylammonium bromide can expand ATPSa significantly. [ABSTRACT FROM AUTHOR]