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 relevant 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 by molecular dynamics simulations. To develop the simulations, the atomistic model of illite was constructed following the Löwenstein’s rule. The oily phase was modeled by 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; whereas, 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 show that in general, in the case of the inverse emulsions, water has a high affinity for both illite surfaces and its contact angle is below to 45º regardless to pH. However, the heptane/heptanoic acid inverse emulsions on the edge surface were the exception to that behavior, i.e., the contact angles calculated for the water droplets reveled mixed wettability because of the hydrogen bonds between the carboxylic functional groups (pH << 4.4) and the surface silanols and alumminols. On the other hand, oil droplets suspended in water did not adhere to the illite surfaces and contact angles were not measurable; nevertheless, the heptane/heptanoic acid droplets (pH << 4.4) showed heights around 2 and 4 Å above the basal and edge surfaces, respectively. This behavior was due 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 over 8 Å from the surface, presumably due to a charge repulsion effect between the mineral surface and the surface of the droplets.