Force Fields for Carbohydrate-Divalent Cation Interactions

Critical Packing Density of Water-Mediated Nonstick Self-Assembled Monolayer Coatings

Nanoparticle-mineral surface interactions are relevant in many biological and geological applications. We have previously studied nanoparticle coatings based on closely packed bicomponent polyol-fluoroalkane self-assembled monolayers (SAMs) that can have tunable stickiness on calcite surfaces by changing the compositions of fluoroalkanes in SAMs, where the coatings show nonstick properties if fluoroalkanes can effectively perturb hydration layers on calcite surfaces. However, when applying coatings on nanoparticles, it can be challenging to predict the maximum achievable coating density. Here, we study how would water-mediated SAM-calcite interactions change with different SAM coating densities. Molecular dynamics simulations show that compositionally repulsive, closely packed polyol-fluoroalkane SAMs become adhesive to calcite surfaces with decreasing coating densities. Our modeling shows that this results from the collapsing of fluoroalkanes into the voids of SAMs, where fluoroalkanes can no longer perturb hydration layers on calcite surfaces. Interestingly, we find that the nonstick-stick transition occurs when the volume fractions of the voids on SAMs are greater than the volume fractions of hydrophilic coating molecules.

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

Engineered nanoparticles have been broadly used in biological and geological systems. Hydrophilic molecules such as polyols have been used as coatings on nanoparticle surfaces due to their good biocompatibility and solubility in saline water. However, polyol coatings can cause huge retention of nanoparticles when encountering mineral surfaces. Here, molecular dynamics simulations enlightened that the strong adhesion of hydrophilic coatings to mineral surfaces stemming from the partitioning of the hydroxy groups on the hydrophilic molecules to the well-defined bound hydration layers on the mineral surfaces. To mitigate the nanoparticle adhesion, we investigated introducing small percentages of omniphobic fluoroalkanes to form a bicomponent system of hydrophilic and fluoroalkanes, which greatly perturbed the hydration layers on mineral surfaces and resulted in nonstick surface coatings. Our results provide important insight for the design of tunable “stickiness” nanoparticle coatings in different mineralogies, such as applications in subsurface environments or targeted delivery in mineralized tissues.

Molecular Assembly of Surfactant Mixtures in Oil-Swollen Micelles: Implications for High Salinity Colloidal Stability

Alkylbenzene sulfonates are one of the most important synthetic surfactant families, considering their wide applicability, cost-effectiveness, and overall consumption levels. Nevertheless, their low salt tolerance (especially divalent ion contents) prevented their wider applications such as enhanced oil recovery in high salinity reservoirs. Here, using experiments and atomistic molecular dynamics simulations, we demonstrated that the high salinity colloidal stability of alkylbenzene sulfonates can be dramatically increased by mixing with zwitterionic cosurfactants in oil-swollen micelles. By mixing with cosurfactants we had two important observations. (1) The polydispersity of surfactant mixture oil-swollen micelles were largely decreased due to the less rigid oil/water interfaces with mixed surfactants, which formed fewer but larger uniform micelles. (2) Strong dehydration of sulfonates due to the shielding from protruding more extended zwitterionic cosurfactants at the oil/water interfaces. Both observed molecular assembly characteristics triggered by the cosurfactants effectively reduced the total exposures of sulfonates to water phase that may form divalent ion bridging and large aggregates, and thus increased their high salinity colloidal stability. Lastly, it was observed that the dehydration of sulfonates was the highest at flat oil/water interfaces (very large oil-swollen micelles), which justified that adding trace amount of mineral oils may boost the high salinity colloidal stability even further.

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

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.

Silver-mediated oxidative 1,2-alkylesterification of styrenes with nitriles and acids via C(sp3)–H functionalization

A new silver-mediated 1,2-alkylesterification of alkenes with nitriles and acids promoted by a catalytic amount of nickel catalyst for producing acyloxylated nitriles has been developed via a C(sp³)–H functionalization process.

Calcium-Mediated Adhesion of Nanomaterials in Reservoir Fluids

Globally, a small percentage of oil is recovered from reservoirs using primary and secondary recovery mechanisms, and thus a major focus of the oil industry is toward developing new technologies to increase recovery. Many new technologies utilize surfactants, macromolecules, and even nanoparticles, which are difficult to deploy in harsh reservoir conditions and where failures cause material aggregation and sticking to rock surfaces. To combat these issues, typically material properties are adjusted, but recent studies show that adjusting the dispersing fluid chemistry could have significant impact on material survivability. Herein, the effect of injection fluid salinity and composition on nanomaterial fate is explored using atomic force microscopy (AFM). The results show that the calcium content in reservoir fluids affects the interactions of an AFM tip with a calcite surface, as surrogates for nanomaterials interacting with carbonate reservoir rock. The extreme force sensitivity of AFM provides the ability to elucidate small differences in adhesion at the pico-Newton (pN) level and provides direct information about material survivability. Increasing the calcium content mitigates adhesion at the pN-scale, a possible means to increase nanomaterial survivability in oil reservoirs or to control nanomaterial fate in other aqueous environments.

Hydration Repulsion between Carbohydrate Surfaces Mediated by Temperature and Specific Ions

Stabilizing colloids or nanoparticles in solution involves a fine balance between surface charges, steric repulsion of coating molecules, and hydration forces against van der Waals attractions. At high temperature and electrolyte concentrations, the colloidal stability of suspensions usually decreases rapidly. Here, we report a new experimental and simulation discovery that the polysaccharide (dextran) coated nanoparticles show ion-specific colloidal stability at high temperature, where we observed enhanced colloidal stability of nanoparticles in CaCl₂ solution but rapid nanoparticle-nanoparticle aggregation in MgCl₂ solution. The microscopic mechanism was unveiled in atomistic simulations. The presence of surface bound Ca²⁺ ions increases the carbohydrate hydration and induces strongly polarized repulsive water structures beyond at least three hydration shells which is farther-reaching than previously assumed. We believe leveraging the binding of strongly hydrated ions to macromolecular surfaces represents a new paradigm in achieving absolute hydration and colloidal stability for a variety of materials, particularly under extreme conditions.