Understanding Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids by Insights from Molecular Dynamics Simulations

Abstract

Interest in nanomaterials for subsurface applications has grown markedly due to their successful application in a variety of disciplines, such as biotechnology and medicine. Nevertheless, nanotechnology application in the petroleum industry presents greater challenges to implementation because of the harsh conditions (i.e. high temperature, high pressure, and high salinity) that exist in the subsurface that far exceed those present in biological applications. The most common subsurface nanomaterial failures include colloidal instability (aggregation) and sticking to mineral surfaces (irreversible retention). We previously reported an atomic force microscopy (AFM) study on the calcium-mediated adhesion of nanomaterials in reservoir fluids (S. L. Eichmann and N. A. Burnham, Sci. Rep . 7, 11613, 2017), where we discovered that the functionalized and bare AFM tips showed mitigated adhesion forces in calcium ion rich fluids. Herein, molecular dynamics reveal the molecular-level details in the AFM experiments. Special attention was given to the carboxylate-functionalized AFM tips because of their prominent ion-specific effects. The simulation results unambiguously demonstrated that in calcium ion rich fluids, the strong carboxylate-calcium ion complexes prevented direct carboxylate-calcite interactions, thus lowering the AFM adhesion forces. We performed the force measurement simulations on five representative calcite crystallographic surfaces and observed that the adhesion forces were about two to three fold higher in the calcium ion deficient fluids compared to the calcium ion rich fluids for all calcite surfaces. Moreover, in calcium ion deficient fluids, the adhesion forces were significantly stronger on the calcite surfaces with higher calcium ion exposures. This indicated that the interactions between the functionalized AFM tips and the calcite surfaces were mainly through carboxylate interactions with the calcium ions on calcite surfaces. Finally, when analyzing the order parameters of the tethered functional groups, we observed significantly different behavior of the alkanethiols depending on the absence or presence of calcium ions. These observations agreed well with AFM experiments and provided new insights for the competing carboxylate/calcite/calcium ion interactions.

Ca2+ Ions Decrease Adhesion between Two (104) Calcite Surfaces as Probed by Atomic Force Microscopy

“…Conversely, strongly charged surfaces are more likely to drive the adsorption of more hydrated ions as the high surface charge density is sufficient to favor dehydration and binding of ions as inner-species over their interaction with water molecules in hydration shells. This is also in agreement with the repulsive forces observed for calcite in the presence of hydrated Mg 2+ ions and significantly weaker adhesion reported for different sites on a calcite surface in the presence of Ca 2+ with respect to Ca 2+ -free aqueous solutions in AFM measurements against a modified AFM tip. ,, …”

“…This is also in agreement with the repulsive forces observed for calcite in the presence of hydrated Mg 2+ ions 36 and significantly weaker adhesion reported for different sites on a calcite surface in the presence of Ca 2+ with respect to Ca 2+ -free aqueous solutions in AFM measurements against a modified AFM tip. 37 , 39 , 65 …”

“…AFM force measurements in a dissimilar surface configuration with one calcite surface against a smooth and less-reactive surface (such as colloidal silica, gold, or substrates with modified hydrophilicity) are less experimentally challenging and provide a higher distance resolution. − These works point to the dominant effect of ion-specific hydration on the repulsive forces measured at the smallest surface separations and show a significant decrease of the measured adhesion in the presence of Ca 2+ . The strong ion-specific effects on adhesion between calcite and a functionalized AFM tip have also been demonstrated in a molecular dynamics study for several different crystallographic planes of calcite . However, such measurements cannot be directly related to two interacting calcite surfaces because of the presence of another, chemically different interface.…”

“…The strong ion-specific effects on adhesion between calcite and a functionalized AFM tip have also been demonstrated in a molecular dynamics study for several different crystallographic planes of calcite. 39 However, such measurements cannot be directly related to two interacting calcite surfaces because of the presence of another, chemically different interface. Thus, the complete understanding of the ion-specific forces, which govern the cohesion of two calcite grains in the presence of surface potential-determining or inert inorganic electrolyte ions, is still lacking.…”

Controlling water-mediated interactions by designing self-assembled monolayer coatings

“…The simulation time step was set to 2 fs. The bottom 2 layers of the calcite molecules were restrained (using GROMACS position restraint algorithm) 37 in order to support the surface, while the top 2 layers of the calcite molecules facing the SAM could freely move 45 , 46 . All bonds were constrained using the LINCS algorithm 47 except for water molecules, which were constrained using the SETTLE algorithm 48 .…”