Encapsulation efficiencies of vesicles formed by the nonionic surfactant 1,2-dioctadecyl-rac-glycerol-3-omega-methoxydodecylethylene glycol (abbreviated as 2C(18)E(12)) and its phospholipid counterpart, distearoylphosphatidylcholine (DSPC) at 298 K, were determined by the entrapment of the water-soluble dye, carboxyfluorescein (CF) to be 0.045 +/- 0.001 and 0.03 +/- 0.04 Lmol(-1) for 2C(18)E(12) vesicles prepared using low osmolarity (270 m Osm) Krebs-Henseleit (K-H) buffer and a modified 'high salt' (1600 m Osm) variant of K-H buffer, respectively, and 0.64 +/- 0.01 and 0.31 +/- 0.04 L mol(-1) for DSPC vesicles prepared under the same conditions and in the same buffers. Freeze fracture electron microscopy studies confirmed the presence of vesicles when 2C(18)E(12) and DSPC were dispersed in water and both buffer solutions. Small angle neutron scattering (SANS) studies, using D2O in place of H2O, showed that when 2C(18)E(12) vesicles were prepared in the 'high salt' variant of K-H buffer as opposed to K-H buffer or water, a higher proportion of multilamellar vesicles (MLV) were formed. Furthermore when prepared in the 'high salt' variant of K-H buffer, the 2C(18)E(12) bilayers were thinner, and when present in the form of MLV exhibited a smaller layer of water separating the bilayers. However, even in the absence of electrolyte, 2C18E12 formed surprisingly thin bilayers due to the penetration of the polyoxyethylene chains into the hydrophobic chain region of the bilayer. Due to the dehydrating effect of the high concentration of electrolyte present in the 'high salt' variant of K-H, the polyoxyethylene head groups penetrated further into the hydrophobic region of the bilayer making the bilayer even thinner. In the case of the DSPC vesicles, although the SANS study showed an increase in the relative proportion of multilamellar to unilamellar vesicles when samples were prepared in the 'high salt' variant of K-H buffer, no differences were observed in the thickness and the d-spacing of the vesicle bilayers. Variable temperature turbidity measurements of 2C(18)E(12), and DSPC vesicles prepared in water indicated phase changes at 320 +/- 0.5 and 327 +/- 0.5 K, respectively, and were unchanged when the 'high salt' variant of K-H buffer was used as hydrating medium. Taken together, these results suggest that a low phase transition temperature was not the reason for the poor entrapment efficiency of 2C(18)E(12) vesicles but rather the very 'thin' hydrophobic barrier formed by the penetration of the polyoxyethylene chains into the hydrophobic region of the bilayer.