Clouding in fatty acid dispersions for charge-dependent dye extraction.

The clouding phenomenon in non-ionic surfactant systems is a common feature that remains rare for ionic detergents. Here, we show that fatty acid (negatively charged) systems cloud upon cooling hot dispersions depending on the concentration or when adding excess guanidine hydrochloride. The clouding of these solutions yields the formation of enriched fatty acid droplets in which they exhibit a polymorphism that depends on the temperature: upon cooling, elongated wormlike micelles transit to rigid stacked bilayers inside droplets. Above this transition temperature, droplets coalesce yielding a phase separation between a fatty acid-rich phase and water, allowing extraction of dyes depending on their charge and lipophilicity. Positively charged and zwitterionic dyes were sequestered within the droplets (and then in the fatty acid-rich upper phase) whereas the negatively charged ones were found in both phases. Our results show an additional case of negatively charged surfactant which exhibit clouding phenomenon and suggest that these systems could be used for extracting solutes depending on their charge and lipophilicity.