Theoretical equations of state and molecular configuration rod-shaped molecules on a metal surface: Alkanes, alcohols, and carboxylic acids on Hg

Abstract

The orientation and configuration of long-chain alkanes, alcohols, and carboxylic acids on a Hg surface have been studied as a function of surface coverage by measuring the surface tension and surface potential. Very detailed information has been obtained concerning the two-dimensional nature of these rod-shaped molecules on a clean smooth metal surface. At low surface density the molecules exhibit a great deal of lateral interaction even though the molecules lie with the long axis adjacent to the surface. As the partial monolayer is compressed, it forms a two-dimensional liquid. When the molecules are in a close-packed horizontal orientation further compression forces the chains to kink so that part of the chains are erect. The erect portions cluster into nuclei which grow to micelles. The number of molecules in a micelle (β) depends on the length of the chain, e.g., of the chain, e.g., β = (0.019 m2 + 2.1) ± 0.74, where m is the number of carbon atoms in the chain. Further compression forces all the molecules into an erect orientation with the long axis perpendicular to the substrate. Fatty acids with m > 15 orient with the carboxyl head group adjacent to the substrate and the tail pointing outward. Fatty acids with m ≦ 15 orient with carboxyl head group in the outer surface with the tail pointed toward the substrate. Equations of state are derived which accurately predict the experimental data for all the alkanes and fatty acids for the two-dimensional liquid and intermediate phase, except for pentane, which behaves ms a two-dimensional gas. The equations of state are based on the proposed model, have no adjustable parameters, and take into consideration interaction with the substrate, interaction between adsorbate molecules, repulsion between similarly oriented induced dipoles, the number of carbon atoms in the chain, the temperature, and the known dimensions of the molecules. The same model is used to calculate the polarizability of the molecules and predict quantitatively the experimental surface potential for the alkanes as a function of surface density. It was then possible to determine the permanent dipole moment of the carboxyl head group of the fatty acids (≈1.8–2.0 debyes, in good agreement with the literature value of 1.7 debyes).