Phase separations in aerosol OT reverse micellar solutions in dodecane: A curious case of polar domain composition driven transition from lower consolute temperature (LCT) to upper consolute temperature (UCT) regime

• Phase behaviors of AOT microemulsions with different polar domains were investigated.
• Polar domain composition determines the temperature dependence of the phase behavior.
• Water and FA show opposite variation of microemulsions properties with temperature.
• This leads to a change in phase separation characteristics from LCT to UCT regime.

AOT-based microemulsions have been studied quite extensively on account of their numerous industrial applications and their existence as simple ternary phases. Properties of these microemulsions with higher alkanes as the oil continuous phase are governed by the presence of a strong inter micellar attractive interaction. In water in oil (w/o) microemulsions the inter micellar attraction strengthens with increase in temperature and leads to the observation of critical behavior on approaching the lower consolute temperature (LCT). In this manuscript we show that the phase behavior of AOT-based reverse micellar solutions in dodecane transforms dramatically when the polar domain is changed from water to formamide (FA) and even to aqueous solution of urea and NaCl. In all these cases the temperatures dependence of the microemulsion properties undergo complete reversal, which brings about a transition in their phase separation characteristics from lower consolute temperature (LCT) to upper consolute temperature (UCT) regime. Such systematic transformation in the phase behavior as a function of polar domain composition, which has been explained based on the influence of the polar domain on the properties of the interfacial AOT layer, is first of its kind in any reverse micellar system.

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