Structure and Orientation of Tetracarboxylic Acids at Oil-Water Interfaces

Fouling caused by tetracarboxylic acids in transport and separation process chains involving petroemulsions occurs when the interfacial concentration of tetraacids becomes large enough for calcium ions in the water phase to "crosslink" the adsorbed tetraacid molecules and form a precipitate. At present, the structure and orientation of tetraacid molecules at oil-water interfaces, which influences the precipitation behavior, has not been studied in detail. In this work, molecular dynamics simulations of indigenous and synthetic tetracarboxylic acid compounds are presented to describe the structure and spatial orientation of tetraacid molecules at oil-water interfaces. Molecular distributions relative to the oil-water dividing surface along with the length and orientation angle distributions of the acidic arm groups are presented. The probability distributions determined here that describe the tetraacids at an oil-water interface can be employed to reconstruct the density of carboxylic acid groups at the oil-water interface. The interfacial carboxylic acid density can be employed to determine the fraction of adsorbed tetraacid molecules that are "crosslinked" with calcium ions based on the distances between carboxylic acid groups. The simulations presented also form a basis to calculate interfacial molecular areas and virial coefficients to employ in molecular mixed monolayer adsorption isotherms.