Coadsorption of two different carboxylic acids, benzenetribenzoic acid and trimesic acid, was studied at the liquid-solid interface in two different solvents (heptanoic and nonanoic acid). Independent alteration of both concentrations in binary solutions resulted in six nondensely packed monolayer phases with different structures and stoichiometries, as revealed by means of scanning tunneling microscopy (STM). All of these structures are stabilized by intermolecular hydrogen bonding between the carboxylic acid functional groups. Moreover, phase transitions of the monolayer structures, accompanied by an alteration of the size and shape of cavity voids in the 2D molecular assembly, could be achieved by in situ dilution. The emergence of the various phases could be described by a simple thermodynamic model.

A simple model system for the 2D self-assembly of functionalized organic molecules on surfaces was examined in a concerted experimental and theoretical effort. Monolayers of 1-halohexanes were formed through vapor deposition onto graphite surfaces in ultrahigh vacuum. Low-temperature scanning tunneling microscopy allowed the molecular conformation, orientation, and monolayer crystallographic parameters to be determined. Essentially identical noncommensurate monolayer structures were found for all 1-halohexanes, with differences in image contrast ascribed mainly to electronic factors. Energy minimizations and molecular dynamics simulations reproduced structural parameters of 1-bromohexane monolayers quantitatively. An analysis of interactions driving the self-assembly process revealed the crucial role played by small but anisotropic electrostatic forces associated with the halogen substituent. While alkyl chain dispersion interactions drive the formation of a close-packed adsorbate monolayer, electrostatic headgroup forces are found to compete successfully in the control of both the angle between lamella and backbone axes and the angle between surface and backbone planes. This competition is consistent with energetic tradeoffs apparent in adsorption energies measured in earlier temperature-programmed desorption studies. In accordance with the higher degree of disorder observed in scanning tunneling microscopy images of 1-fluorohexane, theoretical simulations show that electrostatic forces associated with the fluorine substituent are sufficiently strong to upset the delicate balance of interactions required for the formation of an ordered monolayer. The detailed dissection of the driving forces for selfassembly of these simple model systems is expected to aid in the understanding of the more complex self-assembly processes taking place in the presence of solvent.haloalkanes ͉ conformation ͉ simulations F uture advances in nanoscale science and engineering are expected to rely increasingly on the controlled bottom-up assembly of molecular arrays. The successful creation of targeted molecular device structures demands a fundamental understanding of the interactions governing 2D self-organization. Simple functionalized hydrocarbon molecules are known to form self-ordered structures at a variety of surfaces and interfaces (1-32) and can serve as ideal model systems to study the underlying forces driving the selfassembly process.Numerous experimental (1-10, 13, 14, 16-19, 21, 22, 24, 27, 28, 30-41) and theoretical (39, 42, 43) studies have investigated the self-assembled monolayers formed when a melt or solution containing alkanes or their derivatives is brought in contact with the basal plane of a graphite substrate. Scanning tunneling microscopy (STM) (2-6, 8-10, 13, 14, 16-19, 24, 27, 28, 31-38, 40, 41) and diffraction-based probes (7,39,44,45) have been used to characterize the crystallographic monolayer parameters as well as the orientation and conformation of the constituent molecular species. For many alkane...

The structural properties of self-assembled monolayers of short 1-bromoalkanes and n-alkanes on graphite were investigated by a combination of ultrahigh vacuum scanning tunneling microscopy (UHV-STM) at 80 K and theoretical methods. STM images of 1-bromohexane reveal a lamellar packing structure in which the molecules form a 57° ± 3° lamella-molecular backbone angle and a head-to-head assembly of the bromine atoms (Müller, et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5315). STM images of 1-bromoheptane also show a head-to-head 60° ± 3° lamella-molecular backbone pattern; however, the molecules pack in a herringbone structure. The odd/even chain-length alternation in the monolayer morphologies of 1-bromoalkanes is similar to that observed for the self-assembly of short n-alkanes on graphite, suggesting that the bromine atom acts effectively as an extension of the carbon backbone. The analogy, however, is incomplete. Odd and even short n-alkanes (hexane, heptane, octane) display 60° herringbone and rectangular (not 60°) lamella-molecular backbone configurations, respectively. The balance of intermolecular forces and packing considerations responsible for this odd/even alternation in monolayer morphology for short 1-bromoalkanes on graphite is examined here according to classical molecular dynamics simulations and in light of the structural properties of analogous n-alkane assemblies.