Structure of the monolayer formed at an air-water interface by a novel nonionic (vesicle-forming) surfactant

Neutron specular reflection measurements together with isotopic substitution have been used to determine the molecular architecture of the monolayer formed at the air-water interface by a synthetic nonionic vesicle-forming surfactant, 1,2-di-O-octadecyl-rac-glycerol-3-(omega-methoxydodecakis(ethylene glycol)). The structure of the monolayer was determined with the surface pressure maintained at 28 and 34 mN/m, where the individual surfactant molecules are calculated to occupy areas of 125 +/- 1 and 116 +/- 2 Angstrom(2), respectively. At 28 mN/m, the widths of the Gaussian distributions for the surfactant hydrophobe and head group layers are determined to be 16.5 +/- 1 and 20 +/- 1 Angstrom, respectively, with the associated solvent modeled with a tanh distribution of thickness 9 +/- 1 Angstrom. An extensive overlap is observed between the hydrophobe and head group layers, with the mixed chain region having a thickness of around 12 Angstrom. As the result of the modest increase in surface pressure to 34 mN/m, the surfactant monolayer is made slightly thicker and more ordered as expected, with the widths of the hydrophobe, head group, and solvent layers increased to 17 +/- 1, 22 +/- 1, and 9.5 +/- 1 Angstrom, respectively. The higher pressure, however, also leads to an increased thickness for the mixed chain region (13 Angstrom), and causes a greater proportion of the alkyl chains to be immersed in the solvent (similar to 58% versus 38%).