Formation of transparent solid lipid nanoparticles by microfluidization: influence of lipid physical state on appearance

Hypothesis: This study investigated the influence of liquid-solid transition and particle size on the optical properties of nanoemulsions. The hypothesis was that the crystallization of lipid droplets influences the nanoemulsion appearance.

Experiments: Liquid and solid nanoemulsions (10 wt% octadecane, 1-5 wt% sodium dodecylsulfate) were formed by high-pressure microfluidization (5000-28,500 psi) at 45 °C. Solid lipid nanoparticles were formed by cooling the nanoemulsions to 5 °C and then heating to ambient temperature, whereas liquid nanoemulsions were formed by maintaining them at 25 °C.

Findings: Results indicated that lipid nanoparticles ranging from 136 nm down to 36 nm were generated, and were stable to particle aggregation. The melting and onset temperatures of the nanoparticles decreased with decreasing particle diameter. Upon crystallization of the lipid, the absorbance increased by about 140% for nanoemulsions with 136 nm particle diameter, but only 5% for nanoemulsions with 36 nm particle diameter. These results were explained in terms of changes in refractive index upon droplet solidification that alter their scattering behavior. These results show that solidification of nanoemulsions results in a shift of the transparent-to-turbid transition regime. The practical consequences for emulsion manufacturers are that solid nanoemulsions must be smaller than liquid nanoemulsions to remain transparent.