Self-Organization of a Wedge-Shaped Surfactant in Monolayers and Multilayers Nicholas Cain, Josh Van Bogaert, Douglas L. Gin, Scott R. Hammond, and Daniel K. Schwartz* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed August 7, 2006. In Final Form: October 12, 2006 The self-organization behavior of a wedge-shaped surfactant, disodium-3,4,5-tris(dodecyloxy)phenylmeth- ylphosphonate, was studied in Langmuir monolayers (at the air-water interface), Langmuir-Blodgett (LB) monolayers and multilayers, and films adsorbed spontaneously from isooctane solution onto a mica substrate (self-assembled films). This compound forms an inverted hexagonal lyotropic liquid crystal phase in the bulk and in thick adsorbed films. Surface pressure isotherm and Brewster angle microscope (BAM) studies of Langmuir monolayers revealed three phases: gas (G), liquid expanded (LE), and liquid condensed (LC). The surface pressure-temperature phase diagram was determined in detail; a triple point was found at ∼10 °C. Atomic force microscope (AFM) images of LB monolayers transferred from various regions of the phase diagram were consistent with the BAM images and indicated that the LE regions are ∼0.5 nm thinner than the LC regions. AFM images were also obtained of self- assembled films after various adsorption times. For short adsorption times, when monolayer self-assembly was incomplete, the film topography indicated the coexistence of two distinct monolayer phases. The height difference between these two phases was again 0.5 nm, suggesting a correspondence with the LE/LC coexistence observed in the Langmuir monolayers. For longer immersion times, adsorbed multilayers assembled into highly organized periodic arrays of inverse cylindrical micelles. Similar periodic structures, with the same repeat distance of 4.5 nm, were also observed in three-layer LB films. However, the regions of organized periodic structure were much smaller and more poorly correlated in the LB multilayers than in the films adsorbed from solution. Collectively, these observations indicate a high degree of similarity between the molecular organization in Langmuir layers/LB films and adsorbed self- assembled films. In both cases, monolayers progress through an LE phase, into LE/LC coexistence, and finally into LC phase as surface density increases. Following the deposition of an additional bilayer, the film reorganizes to form an array of inverted cylindrical micelles. Introduction The control and understanding of the self-assembly behavior of amphiphilic molecules at surfaces have received considerable interest in recent years due to the fact that they spontaneously form highly organized structures with characteristic length scales of 2-5 nm. During the past decade, numerous surfactants have been shown to form organized micelle-like structures at the solid- aqueous solution interface. 1-8 Aggregate formation and the resulting geometries of the self-organized structures, which include spheres, hemispheres, cylinders, and hemicylinders, are dependent upon a variety of system conditions. In particular, surfactant shape and substrate chemistry play fundamental roles in the self-assembly of surface structures. As in bulk surfactant phases, the geometry of aggregates is generally correlated to the molecular shape; that is, roughly cylindrical molecules tend to form bilayers, while wedge-shaped molecules form aggregates with significant spontaneous curvature such as cylinders or spheres. Block copolymers exhibit self-organizing behavior that is fundamentally similar to that observed with surfactants, albeit with somewhat larger length scales. 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