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-(ω-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 Å2, 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 Å, respectively, with the associated solvent modeled with a tanh distribution of thickness 9 ± 1 Å. An extensive overlap is observed between the hydrophobe and head group layers, with the mixed chain region having a thickness of around 12 Å. 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 Å, respectively. The higher pressure, however, also leads to an increased thickness for the mixed chain region (13 Å), and causes a greater proportion of the alkyl chains to be immersed in the solvent (∼58% versus 38%).