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Nanoemulsion stability above the critical micelle concentration: A contest between solubilization, flocculation and Krafft precipitation

The relative importance of micelle solubilization and Krafft temperature on the appraisal of the flocculation rate is studied using a dodecane-in-water nanoemulsion as a model system. In 0.5 mM solutions of sodium dodecylsulfate (SDS), neither the critical micelle concentration (CMC) nor the Krafft point of the surfactant are attained between 300 and 700 mM NaCl and 20 < T < 25 °C. Hence, the addition of salt to a SDS-stabilized nanoemulsion only induces aggregation. Conversely, a surfactant concentration of 7.5 mM SDS promotes micelle solubilization or crystal precipitation depending on the physicochemical conditions. Solubilization decreases the absorbance of the system while flocculation and Krafft precipitation increase it. In this paper, the actual variation of the absorbance above the CMC was followed during five minutes for 300, 500 and 700 mM NaCl. The initial 60-second changes were used to determine apparent aggregation rates. The solubilization capacity of the systems was appraised using plots of absorbance vs. volume fraction of oil for different salt concentrations and observation times. The variation of the Krafft point as a function of the salt concentration was also studied using surfactant solutions and emulsions. Finally, small-scale simulations accounting for micelle solubilization were implemented in order to advance in the comprehension of the problem.