Study of the pH effects on water–oil–illite interfaces by molecular dynamics†

Abstract

Illite mineral is present in shale rocks, and its wettability behavior is significant for the oil and gas industry. In this work, the pH effects on the affinity between the (001) and (010) crystallographic planes of illite K₂(Si₇Al)(Al₃Mg)O₂₀(OH)₄ and direct and inverse emulsions were studied using molecular dynamics simulations. To develop the simulations, an atomistic model of illite was constructed following Löwenstein's rule. The oily phase was modeled using heptane, toluene, and mixtures of heptane/heptanoic acid, heptane/heptanoate, heptane/hexylamine and heptane/hexylammonium. For the heptane/heptanoate and heptane/hexylammonium mixtures, Na⁺ and Cl⁻ ions were used to neutralize the excess electrical charge of the droplets, respectively. The affinity of the mineral surface to the oil models was estimated by the contact angle for systems where it was possible. However, for systems where the droplets did not adhere to the mineral, a methodology based on the height of the droplet on the surface was proposed. The results showed that, in general, for the inverse emulsions, water exhibited a high affinity for both illite surfaces, with its contact angle remaining below 45° regardless of pH. However, the heptane/heptanoic acid inverse emulsions on the edge surface were an exception to this behavior. Specifically, the contact angles calculated for the water droplets revealed mixed wettability due to hydrogen bonds between the carboxylic functional groups (pH ≪ 4.4) and the surface silanols and aluminols. Oil droplets suspended in water, on the other hand, did not adhere to the illite surfaces, and contact angles were not measurable. Nevertheless, the heptane/heptanoic acid droplets (pH ≪ 4.4) showed heights of approximately 2 Å and 4 Å above the basal and edge surfaces, respectively. This behavior was attributed to the hydrogen bonds formed between the carboxylic functional groups and the water molecules located on the mineral surfaces. Finally, the heptane/heptanoate (pH ≫ 4.4) and heptane/hexylammonium (pH ≪ 10.64) droplets were localized at distances greater than 8 Å from the surface, presumably due to a charge repulsion between the mineral surface and the surface of the droplets.