The phase behaviors and properties of alkyl trimethyl ammonium bromide with different carbon-chain length/sodium dodecyl sulfate (SDS)/NaBr/H 2 O systems at the total surfactant concentration of 0.1 mol Á L À1 at 333 K were investigated. The experimental results indicate that the phase behaviors of different systems have similar distributing rules. The carbon-chain length of cationic surfactant can influence the position of aqueous two-phase system region, especially for aqueous two-phase system with cationic surfactant in excess, but only have slight effect on aqueous two-phase system with anionic surfactant in excess. The ATPSc region narrows as the length of carbon-chain increases, while the trend for ATPSa is opposite. Increasing the carbon-chain length to some extent will be favorable to shortening the formation time of ATPS greatly. The investigations illustrate that the above rules result from a hydrophobic interaction relationship between surfactant molecules.

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