Effect of temperature and concentration on the self-assembly process of L‑α‑phosphatidylcholine emulsions in water

L‑α‑phosphatidylcholine (L‑α‑PC) is a zwitterionic phospholipid that spontaneously self‑assembles into bilayer structures in aqueous media. The aim of this work was to establish a unified description of phosphatidylcholine self‑assembly as a function of concentration and temperature using complementary structural and thermodynamic techniques. L‑α‑PC dispersions were prepared in deionized water by probe ultrasonication and characterized over the concentration range of 0–2.5 mM and temperatures between 25 and 40 °C. Dynamic light scattering (DLS), ζ‑potential measurements, scanning electron microscopy (SEM), isothermal titration calorimetry (ITC), density, and sound velocity measurements were employed to investigate aggregation, colloidal stability, morphology, energetics, and volumetric properties.

DLS measurements revealed a concentration‑dependent self‑assembly process characterized by three aggregation regimes and a critical transition region centered near 1.0 mM. Around this concentration, the average hydrodynamic diameter increased by approximately 30 nm, indicating cooperative bilayer formation. ζ‑potential measurements showed a marked decrease toward highly negative values, confirming the development of stable vesicular dispersions. SEM observations supported this transition, revealing the evolution from loosely associated aggregates to well‑defined vesicle‑like structures. Volumetric measurements exhibited significant changes in apparent molar volume and adiabatic compressibility near 1.0 mM and 32.5 °C, indicating enhanced molecular packing, reduced volume fluctuations, and partial dehydration of the lipid interface. ITC measurements identified two thermodynamic transitions located near 1.0 and 2.0 mM. The enthalpy change associated with the first transition was close to zero below 32.5 °C, while positive enthalpic contributions were observed above this temperature. In both transition regions, the self‑assembly process was found to be predominantly entropy‑driven.

These results demonstrate that L‑α‑phosphatidylcholine self‑assembly is governed by cooperative concentration‑ and temperature‑dependent transitions, providing a comprehensive thermodynamic description of lipid organization and vesicle formation in aqueous media.