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1. Interfacial CO 2 -mediated nanoscale oil transport: from impediment to enhancement

   https://doi.org/10.1039/D0CP03930F  

   Moh, Do Yoon ; Fang, Chao ; Yin, Xiaolong ; Qiao, Rui ( October 2020 , Physical Chemistry Chemical Physics)

   null (Ed.)

   CO 2 -based enhanced oil recovery is widely practiced. The current understanding of its mechanisms largely focuses on bulk phenomena such as achieving miscibility or reducing oil density and viscosity. Using molecular dynamics simulations, we show that CO 2 adsorption on calcite surfaces impedes decane transport at moderate adsorption density but enhances decane transport when CO 2 adsorption approaches surface saturation. These effects change the decane permeability through 8 nm-wide pores by up to 30% and become negligible only in pores wider than several tens of nanometers. The strongly nonlinear, non-monotonic dependence of decane permeability on CO 2 adsorption is traced to CO 2 's modulation of interfacial structure of long-chain hydrocarbons, and thus the slippage between interfacial hydrocarbon layers and between interfacial CO 2 and hydrocarbon layers. These results highlight a new and critical role of CO 2 -induced interfacial effects in influencing oil recovery from unconventional reservoirs, whose porosity is dominated by nanopores. 

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2. Interaction Energy Analysis of Monovalent Inorganic Anions in Bulk Water Versus Air/Water Interface

   https://doi.org/10.3390/molecules26216719  

   Herbert, John M. ; Paul, Suranjan K. ( November 2021 , Molecules)

   Soft anions exhibit surface activity at the air/water interface that can be probed using surface-sensitive vibrational spectroscopy, but the structural implications of this surface activity remain a matter of debate. Here, we examine the nature of anion–water interactions at the air/water interface using a combination of molecular dynamics simulations and quantum-mechanical energy decomposition analysis based on symmetry-adapted perturbation theory. Results are presented for a set of monovalent anions, including Cl−, Br−, I−, CN−, OCN−, SCN−, NO2−, NO3−, and ClOn− (n=1,2,3,4), several of which are archetypal examples of surface-active species. In all cases, we find that average anion–water interaction energies are systematically larger in bulk water although the difference (with respect to the same quantity computed in the interfacial environment) is well within the magnitude of the instantaneous fluctuations. Specifically for the surface-active species Br−(aq), I−(aq), ClO4−(aq), and SCN−(aq), and also for ClO−(aq), the charge-transfer (CT) energy is found to be larger at the interface than it is in bulk water, by an amount that is greater than the standard deviation of the fluctuations. The Cl−(aq) ion has a slightly larger CT energy at the interface, but NO3−(aq) does not; these two species are borderline cases where consensus is lacking regarding their surface activity. However, CT stabilization amounts to <20% of the total induction energy for each of the ions considered here, and CT-free polarization energies are systematically larger in bulk water in all cases. As such, the role of these effects in the surface activity of soft anions remains unclear. This analysis complements our recent work suggesting that the short-range solvation structure around these ions is scarcely different at the air/water interface from what it is in bulk water. Together, these observations suggest that changes in first-shell hydration structure around soft anions cannot explain observed surface activities. 

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3. Pore‐Scale Modeling of PFAS Transport in Water‐Unsaturated Porous Media: Air–Water Interfacial Adsorption and Mass‐Transfer Processes in Thin Water Films

   https://doi.org/10.1029/2023WR034664  

   Chen, Sidian ; Guo, Bo ( August 2023 , Water Resources Research)

   Air–water interfacial adsorption complicates per‐ and polyfluoroalkyl substance (PFAS) transport in vadose zones. Air–water interfaces can arise from pendular rings between soil grains and thin water films on grain surfaces, the latter of which account for over 90% of the total air–water interfaces for most field‐relevant conditions. However, whether all thin‐water‐film air–water interfaces are accessible by PFAS and how mass‐transfer limitations in thin water films control PFAS transport in soils remain unknown. We develop a pore‐scale model that represents both PFAS adsorption at bulk capillary and thin‐water‐film air–water interfaces and the mass‐transfer processes between bulk capillary water and thin water films (including advection, aqueous diffusion, and surface diffusion along air–water interfaces). We apply the pore‐scale model to a series of numerical experiments—constrained by experimentally determined hydraulic parameters and air–water interfacial area data sets—to examine the impact of thin‐water‐film mass‐transfer limitations in a sand medium. Our analyses suggest: (a) The mass‐transfer limitations between bulk capillary water and thin water films inside a pore are negligible due to surface diffusion. (b) However, strong mass‐transfer limitations arise in thin water films of pore clusters where pendular rings disconnect. The mass‐transfer limitations lead to early arrival and long tailing behaviors even if surface diffusion is present. (c) Despite the mass‐transfer limitations, all air–water interfaces in the thin water films were accessed by PFAS under the simulated conditions. These findings highlight the importance of incorporating the thin‐water‐film mass‐transfer limitations and surface diffusion for modeling PFAS transport in vadose zones.

    

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4. Investigations of water/oxide interfaces by molecular dynamics simulations

   https://doi.org/10.1002/wcms.1537  

   Wang, Ruiyu ; Klein, Michael L. ; Carnevale, Vincenzo ; Borguet, Eric ( June 2021 , WIREs Computational Molecular Science)

   Water/oxide interfaces are ubiquitous on earth and show significant influence on many chemical processes. For example, understanding water and solute adsorption as well as catalytic water splitting can help build better fuel cells and solar cells to overcome our looming energy crisis; the interaction between biomolecules and water/oxide interfaces is one hypothesis to explain the origin of life. However, knowledge in this area is still limited due to the difficulty of studying water/solid interfaces. As a result, research using increasingly sophisticated experimental techniques and computational simulations has been carried out in recent years. Although it is difficult for experimental techniques to provide detailed microscopic structural information, molecular dynamics (MD) simulations have satisfactory performance. In this review, we discuss classical and ab initio MD simulations of water/oxide interfaces. Generally, we are interested in the following questions: How do solid surfaces perturb interfacial water structure? How do interfacial water molecules and adsorbed solutes affect solid surfaces and how do interfacial environments affect solvent and solute behavior? Finally, we discuss progress in the application of neural network potential based MD simulations, which offer a promising future because this approach has already enabled ab initio level accuracy for very large systems and long trajectories.

   This article is categorized under:

   Theoretical and Physical Chemistry > Spectroscopy

   Molecular and Statistical Mechanics > Molecular Interactions

   Structure and Mechanism > Molecular Structures

    

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5. Effects of Ionic Strength on Arsenate Adsorption at Aluminum Hydroxide–Water Interfaces

   https://doi.org/10.3390/soils2010001  

   Xu, Tingying ; Catalano, Jeffrey G ( March 2018 , Soil Systems)

   Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by zeta-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions. 

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