Abstract: The formation and structural variation of lyotropic liquid crystal in mixed cationic surfactants (alkyl (C8,C12,C14)trimethylamonium bromide,alkyl (C12,C14)pyridinium bromide)and anionic surfactants (sodium alkyl(C8,C12) sulfate ) systems were studied.When the molar ratio of cationic to anionic surfactant was close to 1,the liquid crystal changed from hexangonal structure to lamellar or cubic structure in concentrated aqueous solution.Variation of nonpolar chain lengthes of surfactants has obvious effects on the phase behavior.When both of cationic and anionic surfactants have short carbon chain length at equimolar mixed ratio,a mixture of large amount of lamellar or cubic liquid crystal and small amount of precipitate was obtained.With the carbon chain length increasing amount of precipitate increased.The amount of coexisted liquid crystal decreased,and even disappeared as well.The alkyltrimethylamonium interacted with sodium alkylsulfate more intensively than with alkyl pyridinium.Therefore,the insoluble mixture produced at wider molar ratio of the two surfactants.

Key words: Mixed surfactant, Sodium alkyl sulfate, Alkyl trimethylamonium bromide