publications

2022

1. J Phys Chem B

   Dielectric Properties of Water in Charged Nanopores

   Thomas R Underwood, and Ian C Bourg

   The Journal of Physical Chemistry B 2022

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   In this study, we examine the spectral dielectric properties of liquid water in charged nanopores over a wide range of frequencies (0.3 GHz to 30 THz) and pore widths (0.3 to 5 nm). This has been achieved using classical molecular dynamics simulations of hydrated Na-smectite, the prototypical swelling clay mineral. We observe a drastic (20-fold) and anisotropic decrease in the static relative permittivity of the system as the pore width decreases. This large decrement in static permittivity reflects a strong attenuation of the main Debye relaxation mode of liquid water. Remarkably, this strong attenuation entails very little change in the time scale of the collective relaxation. Our results indicate that water confined in charged nanopores is a distinct solvent with a much weaker collective nature than bulk liquid water, in agreement with recent observations of water in uncharged nanopores. Finally, we observe remarkable agreement between the dielectric properties of the simulated clay system against a compiled set of soil samples at various volumetric water contents. This implies that saturation may not be the sole property dictating the dielectric properties of soil samples, rather that the pore-size distribution of fully saturated nanopores may also play a critically important role.

   @article{underwood2022dielectric,
     title = {Dielectric Properties of Water in Charged Nanopores},
     author = {Underwood, Thomas R and Bourg, Ian C},
     journal = {The Journal of Physical Chemistry B},
     volume = {126},
     number = {14},
     pages = {2688--2698},
     year = {2022},
     publisher = {American Chemical Society},
     doi = {10.1021/acs.jpcb.1c09688},
   }

2020

1. J Phys Chem C

   Large-scale molecular dynamics simulation of the dehydration of a suspension of smectite clay nanoparticles

   Thomas R Underwood, and Ian C Bourg

   The Journal of Physical Chemistry C 2020

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   Fine-grained sediments and sedimentary rocks play important roles in a variety of modern energy technologies from petroleum geology to geological carbon sequestration and radioactive waste management. However, despite their utility and ubiquity, many of their properties remain poorly understood. In particular, the ability to predict the permeability and mechanics of these media remains a persistent fundamental challenge in the geosciences. In the present work, we show how large-scale classical molecular dynamics (MD) simulations can help interpret the properties of fine-grained sedimentary material. All-atom simulations containing 30 discrete clay particles are utilized to understand the evolution of a clay nanoparticle suspension during its progressive dehydration. Microstructural (pore size distribution, tortuosity, anisotropy), thermodynamic (enthalpy and free energy of hydration, anion exclusion), mechanical (total suction), and transport properties (diffusion coefficient tensors of water and sodium) are calculated and compared to the experiment. Overall, our results provide new insight into the coupled chemistry, mechanics, and transport properties of disordered nanoparticle assemblages and shed light upon the important role of water films in controlling these properties.

   @article{underwood2020large,
     title = {Large-scale molecular dynamics simulation of the dehydration of a suspension of smectite clay nanoparticles},
     author = {Underwood, Thomas R and Bourg, Ian C},
     doi = {10.1021/acs.jpcc.9b11197},
     journal = {The Journal of Physical Chemistry C},
     volume = {124},
     number = {6},
     pages = {3702--3714},
     year = {2020},
     publisher = {American Chemical Society},
   }

2018

1. Energy Fuels

   Understanding model crude oil component interactions on kaolinite silicate and aluminol surfaces: toward improved understanding of shale oil recovery

   Shansi Tian, Valentina Erastova, Shuangfang Lu, and 7 more authors

   Energy & Fuels 2018

   Bib

   @article{tian2018understanding,
     title = {Understanding model crude oil component interactions on kaolinite silicate and aluminol surfaces: toward improved understanding of shale oil recovery},
     author = {Tian, Shansi and Erastova, Valentina and Lu, Shuangfang and Greenwell, H Chris and Underwood, Thomas R and Xue, Haitao and Zeng, Fang and Chen, Guohui and Wu, Chunzheng and Zhao, Rixin},
     journal = {Energy \& Fuels},
     volume = {32},
     number = {2},
     pages = {1155--1165},
     year = {2018},
     publisher = {American Chemical Society}
   }

2. Sci Rep

   The water-alkane interface at various Nacl salt concentrations: a molecular dynamics study of the readily available force fields

   Thomas R Underwood, and H Chris Greenwell

   Scientific reports 2018

   Bib

   @article{underwood2018water,
     title = {The water-alkane interface at various Nacl salt concentrations: a molecular dynamics study of the readily available force fields},
     author = {Underwood, Thomas R and Greenwell, H Chris},
     journal = {Scientific reports},
     volume = {8},
     number = {1},
     pages = {1--11},
     year = {2018},
     publisher = {Nature Publishing Group}
   }

2016

1. J Phys Chem C

   Wetting effects and molecular adsorption at hydrated kaolinite clay mineral surfaces

   Thomas Underwood, Valentina Erastova, and H Chris Greenwell

   The Journal of Physical Chemistry C 2016

   Abs Bib

   In this study, classical molecular dynamics simulations have been used to understand the key interactions and surface structure of a set of organic molecules at the hydrated surfaces of the 1:1 clay mineral kaolinite. Decane, decanoic acid, and decanamine have been modeled at both the hydroxylated and silicate surfaces of kaolinite. Additionally, the effect of pH is observed via looking at the protonated decanamine and decanoate anion forms. The key results show that relative affinity of the organic molecules to the kaolinite surface may be readily switched between the hydroxylated and the silicate surfaces according to the pH and the nature of the organic head functional group. Decane molecules readily form droplets atop the silicate surface and do not adsorb to the hydroxyl surface, as do protonated decanoic acids. In stark contrast, decanoate anions do not adsorb to the silicate surface, yet adsorb to the hydroxyl surface through an anion exchange mechanism. Decanamine readily adsorbs to both silicate and hydroxyl surfaces, though the hydroxyl–amine interactions are mediated through water bridges. Once charged, the decanamine remains adsorbed to both surfaces, however, both interactions are ionically mediated, rather than through van der Waals and hydrogen bonds. Furthermore, protonated decanamine is observed to adsorb to the hydroxyl surface via anion bridges, a phenomenon that is typically associated with positively charged layered double hydroxides rather than negatively charged clay minerals.

   @article{underwood2016wetting,
     title = {Wetting effects and molecular adsorption at hydrated kaolinite clay mineral surfaces},
     author = {Underwood, Thomas and Erastova, Valentina and Greenwell, H Chris},
     journal = {The Journal of Physical Chemistry C},
     volume = {120},
     number = {21},
     pages = {11433--11449},
     year = {2016},
     publisher = {American Chemical Society}
   }

2. Clays Clay Miner

   Ion adsorption at clay-mineral surfaces: the Hofmeister series for hydrated smectite minerals

   Thomas Underwood, Valentina Erastova, and H Chris Greenwell

   Clays and Clay Minerals 2016

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   Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form: K+ > Na+ > Ca2+ > Cs+ > Ba2+. The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide-soil interactions, and enhanced oil recovery.

   @article{underwood2016ion,
     title = {Ion adsorption at clay-mineral surfaces: the Hofmeister series for hydrated smectite minerals},
     author = {Underwood, Thomas and Erastova, Valentina and Greenwell, H Chris},
     journal = {Clays and Clay Minerals},
     volume = {64},
     number = {4},
     pages = {472--487},
     year = {2016},
     publisher = {Springer International Publishing},
   }

2015

1. J Phys Chem C

   Molecular dynamic simulations of montmorillonite–organic interactions under varying salinity: an insight into enhanced oil recovery

   Thomas Underwood, Valentina Erastova, Pablo Cubillas, and 1 more author

   The Journal of Physical Chemistry C 2015

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   Enhanced oil recovery is becoming commonplace in order to maximize recovery from oil fields. One of these methods, low-salinity enhanced oil recovery (EOR), has shown promise; however, the fundamental underlying chemistry requires elucidating. Here, three mechanisms proposed to account for low-salinity enhanced oil recovery in sandstone reservoirs are investigated using molecular dynamic simulations. The mechanisms probed are electric double layer expansion, multicomponent ionic exchange, and pH effects arising at clay mineral surfaces. Simulations of smectite basal planes interacting with uncharged nonpolar decane, uncharged polar decanoic acid, and charged Na decanoate model compounds are used to this end. Various salt concentrations of NaCl are modeled: 0‰, 1‰, 5‰, and 35‰ to determine the role of salinity upon the three separate mechanisms. Furthermore, the initial oil/water-wetness of the clay surface is modeled. Results show that electric double layer expansion is not able to fully explain the effects of low-salinity enhanced oil recovery. The pH surrounding a clay’s basal plane, and hence the protonation and charge of acid molecules, is determined to be one of the dominant effects driving low-salinity EOR. Further, results indicate that the presence of calcium cations can drastically alter the oil wettability of a clay mineral surface. Replacing all divalent cations with monovalent cations through multicomponent cation exchange dramatically increases the water wettability of a clay surface and will increase EOR.

   @article{underwood2015molecular,
     title = {Molecular dynamic simulations of montmorillonite--organic interactions under varying salinity: an insight into enhanced oil recovery},
     author = {Underwood, Thomas and Erastova, Valentina and Cubillas, Pablo and Greenwell, H Chris},
     journal = {The Journal of Physical Chemistry C},
     volume = {119},
     number = {13},
     pages = {7282--7294},
     year = {2015},
     publisher = {American Chemical Society},
   }