Cold-burst method for nanoparticle formation with natural triglyceride oils

Abstract: Preparation of nanoemulsions of triglyceride oils in water usually requires high mechanical energy and sophisticated equipment. Recently, we showed that alpha-to-beta (viz. gel-to-crystal) phase transition, observed with most lipid substances (triglycerides, diglycerides, phospholipids, alkanes, etc.), may cause spontaneous disintegration of micro-particles of these lipids, dispersed in aqueous solutions of appropriate surfactants, into nanometer particles/drops using a simple cooling/heating cycle of the lipid dispersion (Cholakova et al. ACS Nano 14 (2020) 8594). In the current study we show that this "cold-burst process" is observed also with natural oils of high practical interest, incl. coconut oil, palm kernel oil and cocoa butter. Mean drop diameters of ca. 50 to 100 nm were achieved with some of the studied oils. From the results of dedicated model experiments we conclude that intensive nano-fragmentation is observed when the following requirements are met: (1) The three phase contact angle at the air-water-solid lipid interface is below ca. 30 degrees; (2) The equilibrium surface tension of the surfactant solution is below ca. 30 mN/m and the dynamic surface tension decreases rapidly. (3) The surfactant solution contains non-spherical surfactant micelles. e.g. ellipsoidal micelles or bigger supramolecular aggregates; (4) The three phase contact angle measured at the contact line (frozen oil-melted oil-surfactant solution) is also relatively low. The mechanism(s) of the particle bursting process is revealed and, on this basis, the role of all these factors is clarified and discussed. We explain all main effects observed experimentally and define guiding principles for optimization of the cold-burst process in various, practically relevant lipid-surfactant systems.