The carbon dioxide (CO 2 ) retention capacity and adsorption/desorption energetics of layered nano-porous oxide materials depend critically on the hydration level and the nature of molecular interactions among H 2 O, CO 2 , charge-balancing cations and the oxide/hydroxide layers. Molecular-scale understanding of the structure, dynamics and interfacial energetics of H 2 O/CO 2 binary mixtures confined in the interlayer nano-pores is paramount to geological CO 2 storage efforts in clay-rich materials. This Article investigates the effects of supercritical CO 2 (scCO 2 ) in the hydrated interlayer galleries of the hydrophilic smectite mineral (Na-montmorillonite) under geochemically relevant conditions using classical molecular dynamics simulations and enhanced sampling free energy methods. For the compositions investigated, the interactions among the cations, intercalated fluid species, and the basal surfaces result in structures with H 2 O and CO 2 coexisting in a single layer at the center of the interlayer. The water molecules in this central H 2 O/CO 2 layer cluster around and hydrate Na + ions desorbed from the basal surfaces, whereas CO 2 -CO 2 hydrophobic interactions favor mutual clustering of CO 2 molecules. This arrangement results in dynamic percolation paths that facilitate single file-like anisotropic lateral diffusion of ---2 CO 2 . The water clusters around the Na + ions act as two-dimensional nano-pores for the diffusion of Na + between the basal surfaces and across the central H 2 O/CO 2 layer, whereas the CO 2 -rich regions are not permeable to Na + . The near-surface Na + ions occur in two distinct types of coordination environments with distinct NMR spectral fingerprints. Type-I near-surface Na + ions are coordinated by two basal oxygen atoms and four water molecules, whereas for type-II one of the coordinating water molecules is replaced by a CO 2 molecule. The activation energies for a H 2 O and a CO 2 molecule to move out of the first coordination shell of a near-surface Na + are ~2.75 kcal/mol and ~0.5 kcal/mol, respectively. The activation barriers for site-hopping of a H 2 O molecule within the first coordination shell of near-surface and displaced Na + ions are ~1.6 kcal/mol whereas those for site-hopping of CO 2 around the near-surface and displaced Na + ions are ~1.8 kcal/mol and ~3.5 kcal/mol, respectively. The results provide a detailed picture of the interlayer structure and energetics of diffusional motion of cations and intercalates.---3 I. INTRODUCTIONThe interaction of water with hydrophilic surfaces is driven by water-water and watersurface hydrogen bonding (H-bonding) and the ion-water-substrate interactions involving the charge-balancing, exchangeable surface ions. 1-6 On external hydrophilic surfaces and in twodimensional nano-confinement between surfaces, the interplay of these molecular interactions minimizes the water-surface interfacial energy and maximizes the contact area leading to welldefined layers of water molecules with densities greater than in bulk water. 1-8 In ...
The microscopic understanding of uptake and retention of supercritical carbon dioxide by expandable layered aluminosilicate minerals is of central importance for large-scale geological sequestration of CO2. At the molecular scale, the interlayer charge-balancing cations (CBCs) play a crucial role in CO2 adsorption by these cost-effective and environmentally friendly materials. This article investigates the influence of CBCs on the structure, dynamics, and energetics of CO2–H2O mixtures nanoconfined in four different montmorillonites with different CBCs (Cs+, K+, Na+, and Ca2+) using molecular dynamics and enhanced sampling simulations. The results reveal that both CO2 and H2O coexist in a single layer at the center of the interlayer, that CO2 molecules orient parallel to the basal surface and form two-dimensional percolation paths that facilitate their lateral diffusion, and that the near-neighbor CO2 molecules preferentially adopt a distorted slipped-parallel geometry in all of these systems. The calculated activation barriers for CO2 and H2O to diffuse away from the first coordination shells of cations show that Cs+–CO2 and Ca2+–H2O affinities are relatively higher than those of the other cations. The residence times in the cation coordination shells, relative orientations, and diffusion constants of intercalates are also quantified. The presented results can be useful for rational design of engineered layered minerals with improved CO2 retention capacity.
Insulin has been the cornerstone of diabetes treatment since its discovery over a century ago. Despite this, several aspects of insulin engagement with its target and off-target receptors remain unknown, thus limiting the use of structure-guided design of novel analogs. A recent spurt in structural information for the insulin and insulin-like growth factor 1 receptors (IR and IGF-1R), were leveraged in this study to gain a deeper, molecular-level understanding of ligand recognition at these targets. Molecular dynamics (MD) and free energy perturbation (FEP) were utilized to map the drivers of potency and specificity of several insulin variants including disease-linked mutations. The remarkable accuracy of MD-FEP in capturing functional trends (>80% of cases) across various insulin analogs underscored the method’s potential. The ability to deconvolute observations into receptor contributions, conformational perturbations and (or) solvent-network changes lays the foundations for its use in predictive design of future insulins and other therapeutic peptides.
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