Spontaneous Organization of Carboxylic Acid Monolayer Films in Ultrahigh Vacuum. Kinetic Constraints to Assembly via Gas-Phase Adsorption

The organization and bonding of vapor-phase-deposited, monomolecular C₁-C₄ and C₆ n-carboxylic acid films supported on a Cu(100) surface are described. It is shown that, contrary to adsorption from solution, closest-packed monolayers of the n-carboxylic acids are not readily obtained due to the existence of a severe kinetic barrier (presumably largely steric in origin) to their formation. For all the acids studied, at least two discrete bonding states have been identified: (1) a symmetrically bonded, low-converage state and (2) a high-coverage state showing significant tilting of the carboxyl head group with respect to the plane of the surface. The implications of these results as regards molecular self-assembly are discussed.