Cation Hydration Energy and Surface Hydration Have Crucial Role On Thermodynamics of Clay Swelling

The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behavior after cation exchange with the surrounding medium is important for their efficient utilization. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na+, Li+, and H+ ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths d = 4−40 Å). We found that water adsorption in Na-, Li- and H-mica pores is qualitatively similar; however significant quantitative differences are observed, especially at smaller d. Higher water density in H-mica pores (ρH) was expected due to the smaller size of H+ ions having higher hydration energy. However, a counter-intuitive trend of ρLi > ρNa > ρb (bulk density) > ρH was observed due to adsorption energy where the contribution of mica framework atoms was also found to be significant. The disjoining pressure (Π), swelling free energy (∆Ωex), and several structural properties of confined water and ions were calculated to perform thermodynamic analysis of the system. Our detailed calculations have captured the structural evolution of ions and water, especially the transitions from mono- to bi- and multilayer as a function of d. Oscillatory behavior in the Π and ∆Ωex profiles with diminishing to zero for d ≥ 11 Å is observed in all three mica systems. A shift in the location of global minima of ∆Ωex towards the higher d values and ∆Ωex becoming more repulsive is observed in the increasing order of hydration energy of Na+, Li+, and H + ions. The ∆Ωex for Na-mica is characterized by global minima at d = 6 Å corresponding to crystalline swelling, a significant barrier for crystalline swelling from d = 6 to 9 Å and lower for crystalline (d = 9 Å) to osmotic swelling (d > 12 Å). For Li-mica, the energy barrier for crystalline to osmotic swelling is lesser compared to the Na-mica system, whereas for H-mica the ∆Ωex > 0 for all d thus favoring osmotic swelling. We found that the hydration of cations by surface atoms plays a key role in the thermodynamics of clay swelling. The Na+ ions hydrate more number of surface oxygens, act as anchors, and hold the mica pore at d = 6 Å by sharing hydrating water with ions of opposite sides forming an electrostatically connected bridge of mica Na-water-Na-mica. The Li+ ions do hydrate surface oxygen atoms, albeit lesser number and sharing of hydration shell with nearby Li+ ions is also minimum. Both, surface hydration and water sharing, is minimum in the H+ ion case, as they are mostly present in the center of the pore as diffusive ions; thus exerting a consistent osmotic pressure on the mica frameworks, favoring swelling.

Free Energies of Hydrated Halide Anions: High Through-Put Computations on Clusters to Treat Rough Energy-Landscapes

With a longer-term goal of addressing the comparative behavior of the aqueous halides F−, Cl−, Br−, and I− on the basis of quasi-chemical theory (QCT), here we study structures and free energies of hydration clusters for those anions. We confirm that energetically optimal (H2O)nX clusters, with X = Cl−, Br−, and I−, exhibit surface hydration structures. Computed free energies, based on optimized surface hydration structures utilizing a harmonic approximation, typically (but not always) disagree with experimental free energies. To remedy the harmonic approximation, we utilize single-point electronic structure calculations on cluster geometries sampled from an AIMD (ab initio molecular dynamics) simulation stream. This rough-landscape procedure is broadly satisfactory and suggests unfavorable ligand crowding as the physical effect addressed. Nevertheless, this procedure can break down when n≳4, with the characteristic discrepancy resulting from a relaxed definition of clustering in the identification of (H2O)nX clusters, including ramified structures natural in physical cluster theories. With ramified structures, the central equation for the present rough-landscape approach can acquire some inconsistency. Extension of these physical cluster theories in the direction of QCT should remedy that issue, and should be the next step in this research direction.

Free Energies of Hydrated Halide Anions: High throughput Computations on Clusters to Treat Rough Energy-Landscapes

With a longer-term goal of addressing the comparative behavior of the aqueous halides F$^-$, Cl$^-$, Br$^-$, and I$^-$ on the basis of quasi-chemical theory (QCT), here we study structures and free energies of hydration clusters for those anions. We confirm that energetically optimal $(\mathrm{H_2O})_n\mathrm{X}$ clusters, with X = Cl$^-$, Br$^-$, and I$^-$, exhibit \emph{surface} hydration structures. Computed free energies based on optimized surface hydration structures utilizing a harmonic approximation, typically (but not always) disagree with experimental free energies. To remedy the harmonic approximation, we utilize single-point electronic structure calculations on cluster geometries sampled from an AIMD (\emph{ab initio} molecular dynamics) simulation stream. This \emph{rough-landscape} procedure is broadly satisfactory and suggests unfavorable ligand crowding as the physical effect corrected. Nevertheless, this procedure can break down when $n \gtrsim 4$, with the characteristic discrepancy resulting from a relaxed definition of clustering in the identification of $(\mathrm{H_2O})_n\mathrm{X}$ clusters, including ramified structures natural in \emph{physical cluster theories.} With ramified structures, the central equation for the present rough-landscape approach can acquire some inconsistency. Extension of these physical cluster theories in the direction of QCT should remedy that issue, and should be the next step in this research direction.

Adenine nucleotide translocase regulates airway epithelial metabolism, surface hydration and ciliary function

ABSTRACTAirway hydration and ciliary function are critical to airway homeostasis and dysregulated in chronic obstructive pulmonary disease (COPD), which is impacted by cigarette smoking and has no therapeutic options. We utilized a high-copy cDNA library genetic selection approach in the amoeba Dictyostelium discoideum to identify genetic protectors to cigarette smoke. Members of the mitochondrial ADP/ATP transporter family adenine nucleotide translocase (ANT) are protective against cigarette smoke in Dictyostelium and human bronchial epithelial cells. Gene expression of ANT2 is reduced in lung tissue from COPD patients and in a mouse smoking model, and overexpression of ANT1 and ANT2 resulted in enhanced oxidative respiration and ATP flux. In addition to the presence of ANT proteins in the mitochondria, they reside at the plasma membrane in airway epithelial cells and regulate airway homeostasis. ANT2 overexpression stimulates airway surface hydration by ATP and maintains ciliary beating after exposure to cigarette smoke, both of which are key functions of the airway. Our study highlights a potential for upregulation of ANT proteins and/or of their agonists in the protection from dysfunctional mitochondrial metabolism, airway hydration and ciliary motility in COPD.This article has an associated First Person interview with the first author of the paper.