Nanoemulsion formation by the phase inversion temperature method using polyoxypropylene surfactants.

Surfactants with temperature-sensitive polyoxyethylene (POE) chains are widely used to prepare nanoemulsions by the phase inversion temperature (PIT) method. It is therefore anticipated that surfactants with temperature-sensitive polyoxypropylene (POP) chains could also be used to prepare nanoemulsions by the PIT method. POP surfactants were synthesized through electrostatic interactions between hydrophilic POP diamines and hydrophobic long-chain fatty acids. The synthesized POP surfactants were used as emulsifiers to prepare n-tetradecane-in-water nanoemulsions by the PIT method. Electrical conductivity measurements were used to determine the PITs of the water/POP surfactant/n-tetradecane systems. The effects of surfactant concentration, NaCl concentration, number of POP units, and degree of unsaturation of hydrocarbon chains on the PIT and the nanoemulsion droplet size were investigated. The droplet size and morphology of the nanoemulsions were characterized by dynamic light scattering and cryogenic transmission electron microscopy, respectively. Nanoemulsions are formed by the PIT method using POP surfactants when the POP chains are short (∼2.5-6.1 POP units). The formation of nanoemulsions with droplet radii of 20-300 nm and spherical morphology occurs because of the temperature-dependent hydration of the short POP chains.