Monolayer adsorption of dodecylamine Surfactants at the mica/water interface

Abstract

Monolayer adsorption of aqueous dodecylamine surfactants on the muscovite (001) surface was investigated by means of molecular dynamics simulations. Three systems including different number of surfactant molecules have been designed to examine the effect of surface coverage on the microscopic structure of the mica/water interface. In all surface coverage, hemimicelle structures formed at the muscovite/water interface, with their head groups directed to the surface and the hydrocarbon tails oriented toward the solution. The modified mica surface obtained a hydrophobic character, the hydrophobicity became stronger with the increasing surface coverage. Results from density field showed adsorption sites for water molecules in the first two layers near the muscovite surface. It is found that fraction of gauche conformation decreased with the increasing surface coverage, which is in agreement with IR measurements. Increased chain–chain interactions between alkyl chains and a closer packing of the alkyl chains may account for the relatively lower gauche probability for high surface coverage. Hydrogen bonds between bridging oxygen atoms and surfactant head groups were also investigated, and we observed that they were influenced by interfacial water molecules. Furthermore, surfactants formed hemimicelle structure on the mica surface with a thickness roughly less than that of the molecule chain length, which is in good agreement with experimental AFM data.