Effect of amino-acid-based polar oils on the Krafft temperature and solubilization in ionic and nonionic surfactant solutions

The Krafft temperature and solubilization power of ionic and nonionic surfactants in aqueous solutions are strongly affected by added polar oils such as amino-acid-based oils (e.g., N-acylamino acid esters, AAE), because they tend to be solubilized in the surfactant palisade layer. The Krafft temperatures of 5 wt.% sodium dodecyl sulfate (SDS)-water and octaoxyethylene octadecyl ether (C18EO8)-water systems largely decreases upon addition of AAE and 1-hexanol, whereas it decreases very slightly in isopropyl myristate (IPM) and n-dodecane. The lowering of the Krafft temperature can be explained by the same mechanism as the melting-temperature reduction of mixing two ordinary substances. Namely, the polar oils are solubilized in the surfactant palisade layer of micelles and reduce the melting temperature of hydrated solid-surfactant (Krafft temperature). On the other hand, non-polar oil such as dodecane is solubilized deep inside micelles and makes an oil pool. The solubilization of non-polar oil is enhanced by mixing surfactant with AAE due to an increase in micellar size.