  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.
