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The structure of self-assembled monolayers (SAMs) on the gold (111) surface is still a matter of debate despite a considerable experimental and theoretical effort. We address the problem from a new perspective, studying the influence of electrostatic interactions on the degree of disorder in COOH-terminated SAMs. We show that the HS(CH2)n-1COOH molecules carry two dipole moments associated with their head- and tail-groups respectively. Depending on the coupling of these dipole moments along the molecules, the structure of the COOH-SAMs either resembles the structure of the corresponding alkanethiol monolayers (short molecules, strong dipole coupling), or shows a more complex behavior (long molecules, weak dipole coupling). In the latter case the monolayer exhibits a crystalline-like order with respect to the hydrocarbon chains and a high degree of disorder with respect to the carboxylic head-groups. These results resolve the controversy of experimental data on the degree of order in COOH-monolayers with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy (Himmel et.al. Langmuir 1997, 13, 4943), probing the tail-groups, showing that the monolayer is largely disordered, and the infrared (IR) data (Nuzzo et.al. J. Am. Chem. Soc. 1990, 112, 558) on the C-H stretching modes suggesting a crystalline-like order of the hydrocarbon chains. |