Phase transformations in lipid A-diphosphate initiated by sodium hydroxide

Area of Science:

• Colloid and surface science
• Materials science
• Biophysics

Background:

• Charged lipid A-diphosphate clusters form self-assembled structures in aqueous dispersions.
• Understanding their phase behavior is crucial for applications in drug delivery and biomaterials.
• Previous studies focused on NaCl removal, not NaOH addition, for crystallization.

Purpose of the Study:

• To explore the fluid phase transitions of charged-stabilized lipid A-diphosphate clusters.
• To investigate the effects of sodium hydroxide (NaOH) concentration and particle number density on phase behavior.
• To characterize the crystalline phases and their transitions to fluid phases.

Main Methods:

• Small-angle X-ray scattering (SAXS) for structural analysis.
• Electron microscopy for visualizing particle morphology.
• Titration with NaOH (c(S)) to induce and study phase transitions.
• Varying particle number density (n).

Main Results:

• A new fluid disordered phase and a crystalline body-centered cubic (bcc) phase were observed with increasing NaOH concentration.
• The bcc phase had a lattice constant of 35.6 nm.
• Transitions between fluid and bcc phases were observed, influenced by NaOH concentration and particle density.
• Effective charge (z(eff)) and screening parameter (k) were determined, correlating with counterion condensation and many-body effects.
• 70 nm lipid A-diphosphate clusters formed at the melting line for all particle number densities.

Conclusions:

• NaOH addition effectively induces crystallization and melting in lipid A-diphosphate dispersions.
• The study identified novel fluid and crystalline phases, including a bcc structure.
• The findings provide insights into the electrostatic interactions governing self-assembled lipid systems and their phase behavior.
• Results align with universal melting-line simulations when effective charge is considered.