Correlation of the thermal stability of phospholipid-based emulsions and the microviscosity measurements using fluorescence polarization.

The fluorescence polarization technique was used to measure the microviscosity of a series of phospholipid-based emulsions. Fourteen different oil-in-water emulsions containing 20% medium chain length triglycerides, various concentrations and types of phospholipids, and 2.2% glycerin were prepared by microfluidization and pH-adjusted to 4.0 or 7.4. Microviscosity was measured by determining the anisotropy of a fluorophore probe (1,6-phenyl 1,3,5-hexatriene) which was found to obey Perrin's equation as has been previously reported for liposomes and membrane bilayers. Moreover the method was validated by comparing viscosities of phospholipid-oil mixtures measured by rheometry and fluorescence polarization. The viscosities determined by fluorescence polarization were within 6% of the values obtained by classic rheometry. Emulsions were also subjected to thermal stress at 121 degrees C. The droplet growth rate was estimated by measuring the time-dependent mean droplet diameter using photon correlation spectroscopy. The logarithm of the droplet growth rate was found to be directly proportional to the interfacial rigidity (i.e., the inverse microviscosity) which suggested that coalescence rather than molecular diffusion is the primary mechanism of droplet growth under these conditions of thermal stress.