The short-chain n-alcohol-induced changes in phase behaviors of aqueous mixed 1,3-propanediyl bis(dodecyl dimethylammonium bromide) (12-3-12) and sodium dodecyl sulfonate (AS) system have been investigated. For the 12-3-12/AS/HO mixed system, there are two kinds of aqueous two-phase systems with excess cationic surfactant (ATPS-C). The molar ratio of 12-3-12 to AS (MR) and the total surfactant concentration ( m) in the top phase are smaller than those in the bottom phase of ATPS-C. It is worth noting that the addition of ethanol or n-propanol leads to different influences on the ATPS-C. Molecular dynamics (MD) simulation results illustrate that the different influences ascribe to the difference in the cosurfactant effect of ethanol and n-propanol. When ethanol is used as additive, the difference in m leads to the difference in interactions between surfactants and ethanol for the two coexisting phases of ATPS-C, determining the difference in their combination ability with the mixed solvent. It is the main reason for the ethanol-induced phase inversion of the first kind of ATPS-C. When n-propanol is added, in addition to m, MR is also a key factor influencing the interactions between 12-3-12 and AS and between surfactants and n-propanol due to the stronger cosurfactant effect of n-propanol. MD simulations indicate that vesicles with smaller MR are easier and faster to form. These vesicles spontaneously accumulate at the top phase accompanied by certain amount of mixed solvent transferred from the bottom phase of ATPS-C. Meanwhile, the competition for the mixed solvent arising from the surfactant-rich bottom phase prevents the transferring. The two factors work together to cause the increase of m in the top phase of ATPS-C with the addition of n-propanol, leading to n-propanol-induced phase concentration inversion rather than phase inversion of ATPS-C. On the basis of the experimental results and MD simulations, ethanol-induced phase inversion mechanism or n-propanol-induced phase concentration inversion mechanism of ATPS-C has been proposed.
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