Water solubilization capacity and mean emulsion size of phospholipid-based isooctane-alcohol W/O microemulsion

The water solubilization capacity and mean droplet size of water in oil (W/O) microemulsion formed by phospholipids was investigated to understand the role of water content and additive alcohol as a co-solvent. Isooctane (C8*) was employed as the main solvent. The solubilized water content in the W/O microemulsion H2Osoln [mol] was strictly defined in order to compare it with the conventional added water content H2Oadd [mol]. The molar ratio of solubilized water to amphiphile, Wsoln (≡[mol-H2Osoln]/[mol-amphiphile]), was used as an indicator of the water content in the W/O microemulsion. The Sauter mean diameter (d32) was proportional to the 0.33 power of Wsoln. That power term was in good agreement with the assumption that the Wsoln was proportional to the spherical volume of the nanoscale water pool. The empirical equation of d32 PC(nm)=2.3Wsoln0.33 for the phospholipid-added system was obtained regardless of the type of co-solvent alcohol. For the aerosol-OT (AOT) added system, the empirical equation was expressed as d32 AOT(nm)=1.9Wsoln0.33. The mean diameter in both the systems was in good agreement with that in the previous investigations. The additive co-solvent alcohol enlarged the occupied interfacial area of the phospholipids as determined from the change in interfacial tension. The spreading of the molecule–molecule interval of the phospholipids on the microinterface apparently induced the enhancement of the reactivity of phospholipase A2.