Comparison of Linear and Branched Molecular Structures of Two Fluorocarbon Organosilane Surfactants for the Alteration of Sandstone Wettability

The aim of this work is to build a molecular dynamics model to represent the experimental evaluation of the wettability alteration of sandstone surfaces due to the action of partially fluorocarbonated organosilane surfactants with linear (perfluorodecyl) and branched (fluorocarbamate) chains using the contact angles of water and n-decane droplets as the evaluation parameters. The thickness obtained for a monolayer coating with each surfactant was ∼13 Å, in excellent agreement with the experimental measurements of the thickness of a monolayer of fluorocarbonated surfactants in similar systems. The contact angles obtained with molecular dynamics simulations deviate, in most cases, by less than 3° with respect to experimental measurements. The phenomenological model reveals that for the coating with a monolayer, the fluorocarbonated chains of perfluorodecyl were mostly oriented away from the sandstone surface, while the branches of fluorocarbamate are distributed between being adsorbed on the surface and oriented toward the fluid phase. The surface density of the perfluorodecyl monolayer is higher than that obtained for the fluorocarbamate monolayer, since the linear chain in the perfluorodecyl generates weaker lateral repulsive interactions. On the perfluorodecyl coating, the droplets of n-decane and water have an appreciable amount of liquid molecules that are able to penetrate the coating, and in the bulk of these droplets, well-defined adsorption layers appear as a result of the effect of the solid. The configuration of the liquid droplets on the fluorocarbamate does not exhibit any stratification and the thickness of the multilayer coating eliminates the effect of the solid. Thus, the water and n-decane molecules only interact with the outer, less ordered layers of this coating, giving a droplet density profile similar to a fluid–fluid interface. Both surfactants present attractive interactions with the n-decane and water molecules. However, the liquid–liquid attraction forces in the bulk of the droplet are significantly higher than the surfactant-liquid interaction energies, which favors the liquid molecules remaining in the bulk of the droplet. The model developed represents an explanation of the experimental evidence that perfluorodecyl surfactant produces intermediate-wetting coatings, while the fluorocarbamate more efficiently alters the wettability of the surface to water.