Two-step droplet formation in monodisperse nanodroplet generation in quenched hydrothermal solution as revealed by spontaneous transformation of nanodroplets to swollen micelles in octane‑in‑water nanoemulsions

Mechanistic Insights into Micelle-Enhanced Nanofiltration for Heavy Metal Removal: Transformation of Ion Transport and Fouling Phenomena

Toxic heavy metals are widely present in typical scenarios, such as mines and electroplating wastewater, presenting significant risks to biological and environmental safety. Membrane processes encounter a challenge in effectively intercepting heavy metals due to their small hydration radius. This research showcases the high efficiency of micelle-enhanced nanofiltration (MENF) in removing heavy metals. At the critical micelle concentration, sodium dodecyl sulfate demonstrated a high removal of Cu2+, Ni2+, Zn2+, and Cd2+ while maintaining substantial potential for complexation of heavy metals. The formation of micelles and the bonding of heavy metals with surfactants bolstered the resistance of heavy metal ions to transmembrane transport. The presence of heavy metals in ionic form in wastewater facilitated their complexation with surfactants or micelles. Notably, the valence state and concentration of interfering ions in the environment could slightly influence the removal of heavy metals by MENF. Additionally, MENF displayed remarkable antifouling properties. The loose gel layer created by surfactant molecules and the micelle enhanced the membrane permeability and reduced the scaling tendency of heavy metals. This study contributes to an improved understanding of the mechanisms involved in heavy metal rejection by using MENF.

Nanoemulsions Stable against Ostwald Ripening

Ostwald ripening, the dominant mechanism of droplet size growth for an O/W nanoemulsion at high surfactant concentrations, depends on micelles in the water phase and high aqueous solubility of oil, especially for spontaneously formed nanoemulsions. In our study, O/W nanoemulsions were formed spontaneously by mixing a water phase with an oil phase containing fatty alcohol polyoxypropylene polyoxyethylene ether (APE). By monitoring periodically the droplet size of the nanoemulsions via dynamic light scattering, we demonstrated that the formed O/W nanoemulsions are stable against Ostwald ripening, i.e., droplet growth. In contrast, the nanoemulsion droplets grew with the addition of micelles, demonstrating the pivotal role of the presence of micelles in the water phase in the occurrence of Ostwald ripening. The influence of the initial phase of APE, the oil or water phase in which APE is present, on the micelle formation is discussed by the partition coefficient and interfacial adsorption of APE between the oil and water phase using a surface and interfacial tensiometer. In addition, the spontaneously formed O/W nanoemulsion, which is stable against Ostwald ripening, can be used as a nanocarrier for the delivery of water-insoluble pesticides. These results provide a novel approach for the preparation of stable nanoemulsions and contribute to elucidating the mechanism of instability of nanoemulsions.

Solid lipid nanoparticles and nanoemulsions with solid shell: Physical and thermal stability

HYPOTHESIS

Nanoemulsions (NE) and solid lipid nanoparticles (SLN) used for drug delivery should have a solid shell to be stable during long shelf life and become liquid at human body temperature. The core components of lipid nanoparticles can be partially incorporated into the shell and affect the physical and thermal stability.

EXPERIMENTS

We prepared NE and SLN by the phase inversion temperature (PIT) method. Solidification of the surfactants Tween60 and Span 60 on the surface of NE droplets with paraffin oil resulted in the formation of the solid shell. SLN contained stearic acid in the core and the same surfactants in the solid shell. The size, structure and stability of the NE and SLN were studied by DLS and cryo-TEM. Their crystallization and melting were analyzed using DSC.

FINDINGS

The lipid nanoparticles were resistant to aggregation and sedimentation and hold up to at least two cycles of heating to 50-60 °C and subsequent cooling to 5 °C, even though the upper temperatures were higher than the melting point of the surfactant shell. The expected liquid core/solid shell morphology of NE was confirmed. SLN were composed of a semi-liquid core of supercooled stearic acid melt and coated with a solid surfactant shell, so they can be treated as NE. Stearic acid molecules penetrated the shell, leading to an increase in its melting point.