We report adsorption behavior of imidazolinium-type surfactant molecules in different aggregation states on metal-water interfaces studied using all-atom molecular dynamics simulations. Surfactant molecules with two different alkyl tail lengths, a 10-carbon and a 17-carbon tail (henceforth referred to as imid-10 and imid-17, respectively), have been considered. Six layers of face-centered cubic lattice of gold atoms submerged in water represent the metal-water interface. Our simulations reveal that, in infinite dilution, both types of surfactant molecules strongly adsorb onto the metal-water interface in a configuration with their alkyl tail lying parallel to the surface. This adsorption occurs through a barrierless transition with an adsorption free energy of ∼30 k T and is found to be enthalpically driven and entropically unfavorable. Surfactant micelles, on the other hand, experience a long-range repulsion from the metal surface at distances as large as 50-60 Å due to the presence of a large "corona" around the micelles that comprises counterions and their solvation layer. Surfactant micelles have an adsorption free energy barrier of ∼13-16 k T, which is associated with the removal of adsorbed water from the metal surface. Micelles are thermodynamically stable in the bulk aqueous phase, and the adsorbed micellar state is only metastable.
Some recent efforts toward studying adsorption, aggregation, and self-assembly of corrosion inhibitor molecules near metal/water interfaces via classical molecular simulations are reported. Two different approaches have been used. In the first approach, a coarse-grained model of corrosion inhibitor molecules is studied, and the following key findings are found: (a) hydrophobic interactions between the alkyl tails of corrosion inhibitor molecules are important for the formation of adsorbed self-assembled layers on the metal surface, (b) the morphology of the adsorbed layers are strongly influenced by molecular geometry, and (c) the relative strength of interactions between polar head and metal and between alkyl tail and metal are important determinants of adsorbed conformations. In the second approach, fully atomistic simulations are performed for a bulk aqueous phase and near metal/water interfaces of two kinds of model inhibitor moleculesimidazolinium-type and quaternary ammonium-type surfactants. From these simulations, the following are concluded: (a) these inhibitor molecules aggregate in the bulk phase as spherical micelles, (b) the unaggregated inhibitor molecules have a strong tendency to adsorb onto metal surfaces while inhibitor micelles show only a weak tendency to adsorb, and experience a long-range repulsion from the surface. Finally, it is discussed how the coarse-grained and fully atomistic simulations present a unified molecular picture of adsorption and self-assembly of corrosion inhibitor molecules on metal surface.
Abstract:The computational modeling of corrosion inhibitors at the level of molecular interactions has been pursued for decades, and recent developments are allowing increasingly realistic models to be developed for inhibitor-inhibitor, inhibitor-solvent and inhibitor-metal interactions. At the same time, there remains a need for simplistic models to be used for the purpose of screening molecules for proposed inhibitor performance. Herein, we apply a reductionist model for metal surfaces consisting of a metal cation with hydroxide ligands and use quantum chemical modeling to approximate the free energy of adsorption for several imidazoline class candidate corrosion inhibitors. The approximation is made using the binding energy and the partition coefficient. As in some previous work, we consider different methods for incorporating solvent and reference systems for the partition coefficient. We compare the findings from this short study with some previous theoretical work on similar systems. The binding energies for the inhibitors to the metal hydroxide clusters are found to be intermediate to the binding energies calculated in other work for bare metal vs. metal oxide surfaces. The method is applied to copper, iron, aluminum and nickel metal systems.
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