Structure of vesicles formed from non-ionic dialkylglycerol poly(oxyethylene) ether surfactants: effect of electrolyte and cholesterol

Five non-ionic dialkylglycerol poly(oxyethylene) ether surfactants, designated 2CmEn (where m, the number of carbons in each alkyl chain = 16 or 18, and n, the number of oxyethylene units = 12, 16 or 17) have been examined for their ability to form vesicles when dispersed in water or in an aqueous solution of 154 mM NaCl, alone or in the presence of 50 mol% cholesterol. Freeze fracture electron microscopy and light scattering showed that regardless of the hydrating fluid, all the non-ionic surfactants, with the exception of 2C16E17 and 2C18E17, formed vesicles in the absence of cholesterol – 2C16E17 and 2C18E17 instead formed micellar aggregates. All surfactants, however, formed vesicles in the presence of 50 mol% cholesterol. Small angle neutron scattering studies of the surfactant vesicles enabled the bilayer thickness and repeat distance (d-spacing) to be determined. The bilayers formed by all the non-ionic surfactants in the absence of cholesterol were surprisingly thin (∼50 Å for the E12 containing surfactants and ∼64 Å for 2C18E16) most likely due to the intrusion of oxyethylene groups into the hydrophobic core of the bilayers. In contrast, however, the non-ionic surfactants exhibited a relatively large d-spacing of around ∼130–150 Å. The addition of 50 mol% cholesterol had a dramatic effect on the thickness of the vesicle bilayer, increasing its size by 10–20 Å, most probably because of an extrusion of oxyethylene from the hydrophobic region of the bilayer and/or a reduction in the tilt on the surfactant alkyl chains. Additionally the presence of cholesterol in a vesicle tended to reduce slightly both the d-spacing and the thickness of the water layer separating the bilayers. The presence of NaCl, even at the low concentrations used in the study, did affect the properties of the bilayer such that it reduced the d-spacing and, in the case of cholesterol-containing systems, also reduced bilayer thickness.