title: Self-assembly of linactants: micelles and lyotropic liquid crystals in two dimensions.
abstract: We report observations of phenomena within two-component Langmuir monolayers that are analogous to surfactant self-assembly in 3D solutions. A partially fluorinated fatty phosphonic acid played the role of 2D surfactant (linactant), and a perfluorinated fatty acid acted as the 2D solvent. Langmuir-Blodgett monolayers were prepared from mixtures of these compounds and examined using atomic force microscopy. Above a critical linactant mole fraction of approximately 0.013, distinctive monodisperse structural features were observed with a characteristic diameter of approximately 30 nm and a relative height of 1.4 nm. As the linactant concentration was further increased, the number density of these features increased linearly with concentration, whereas the size remained approximately the same. A quantitative analysis of these observations suggested that the features corresponded to self-limiting clusters composed of approximately 2000 linactant molecules and that the dispersed clusters represented a 2D micellar phase. Above a linactant mole fraction of 0.63, the clusters organized into a local hexagonal structure with short-range positional order indicative of a 2D "lyotropic" liquid crystalline phase; the correlation length increased systematically with increasing linactant concentration.
authors.affiliation: Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, USA.
authors.forename: Stephanie M
authors.initials: SM
authors.lastname: Malone
authors.forename: Siwar
authors.initials: S
authors.lastname: Trabelsi
authors.forename: Shishan
authors.initials: S
authors.lastname: Zhang
authors.forename: T Randall
authors.initials: TR
authors.lastname: Lee
authors.forename: Daniel K
authors.initials: DK
authors.lastname: Schwartz