(107f) Stability of DPPC Vesicles in Aqueous Electrolyte Solutions and Comparisons to Predictions From the DLVO Theory

Dipalmitoylphosphatidylcholine (DPPC) lipid is the major component of lung surfactant, which stabilizes the lung by reducing the surface tension at the air/liquid interface of the lung alveoli. DPPC is zwitterionic and insoluble in aqueous solutions, forming dispersions of DPPC vesicles only after extensive sonication above the chain melting temperature, 41º C. The vesicle dispersions are thermodynamically unstable, and they can remain colloidally (kinetically) stable for days, depending on the concentration, temperature, and ionic strength. Their colloidal stability is important for the shelf life and for producing low dynamic surface tension minima (DSTM) because the coagulated dispersions have higher DSTM.

The stability of the DPPC vesicle dispersions in aqueous solutions, characterized by the stability ratio W (the inverse of which is a measure of the probability that a collision will lead to coagulation), was determined with a series of measurements of dispersed particle sizes with dynamic light scattering (DLS) at 25º C. Several vesicular dispersions in water, 1 mM, 10 mM, 150 mM NaCl solution, or phosphate buffer saline (PBS) solution were tested to find out how electrostatic forces may affect the stability ratio W. The zeta potentials were -30, -14, 1±6, 2±3, and -20 mV, respectively. These values are not consistent, probably due to their relatively low values. Nonetheless, the results indicate that electrostatic effects may play a role and some ions may adsorb on the surface of the zwitterionic vesicles. Moreover, the values of zeta potential varied with time as some coagulation proceeded.

The stability ratio W was also predicted from the interaction potential Φmax, and the Fuchs-Smoluchowski theory for dilute dispersions. This quantity was determined from a new dimensionless group formulation of the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, with measured values of vesicle radius, zeta potential, Debye length (from the ionic strength), and published measured values of the Hamaker constant of the lipid and water. The predicted W values were generally smaller than the measured values, indicating that some other forces (hydration or other) play a role, or that the DLVO predictions were inaccurate. The effective values of Φmax were estimated from the data and the DLVO formalism.