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We present an overview of the studies of ion pairing in aqueous media of the past decade. In these studies, interactions between ions, and between ions and water, are investigated with relatively novel approaches, including dielectric relaxation spectroscopy, far-infrared (terahertz) absorption spectroscopy, femtosecond mid-infrared spectroscopy, and X-ray spectroscopy and scattering, as well as molecular dynamics simulation methods. With these methods, it is found that ion pairing is not a rare phenomenon only occurring for very particular, strongly interacting cations and anions. Instead, for many salt solutions and their interfaces, the measured and calculated structure and dynamics reveal the presence of a distinct concentration of contact ion pairs (CIPs), solvent shared ion pairs (SIPs), and solvent-separated ion pairs (2SIPs). We discuss the importance of specific ion-pairing interactions between cations like Li+ and Na+ and anionic carboxylate and phosphate groups for the structure and functioning of large (bio)molecular systems. [ABSTRACT FROM AUTHOR]

The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in geoenvironmental engineering. This study investigated the crystalline swelling behavior of montmorillonite with different interlayer cations including monovalent Na, K, and Cs and divalent Ca and Sr by molecular dynamics simulations and experimental measurements including X-ray diffraction and water vapor adsorption. The stepwise swelling behavior for Na- and Ca/Sr-montmorillonite demonstrated good agreement between molecular dynamics (MD) simulated and experimental results. The swelling curves of K- and Cs-montmorillonite remaining in the 1-hydrated state even at high relative humidity showed systematic discrepancies between MD simulations and experimental results. The comparative discussion between the experimental and MD derived swelling curves offer insights that may help to understand the swelling mechanism of montmorillonite and the dependence on interlayer cations. The water content in the interlayer and external surface in five homoionic montmorillonite was strongly correlated to the hydration number of each ion. On the basis of these comparisons, the hydration free energy of interlayer cations seems to contribute greatly to the total driving force for the crystalline swelling of montmorillonite. An additional key factor is the preference of an outer- or inner-sphere complex of interlayer cations and the distribution of cations in the interlayer space. This effect has a considerable impact in the case of monovalent cations and results in different swelling behavior between the outer-sphere Na and inner-sphere K and Cs ions. • Effects of interlayer cations on swelling behaviors of montmorillonite were investigated. • Molecular dynamics simulations and experimental results are systematically compared. • Differences in measured stepwise swelling behaviors were interpreted by MD simulations. • Main factor affecting crystalline swelling behavior is the hydration free energy of interlayer cations. • Preference of outer- or inner-sphere complex of interlayer cations is an additional key factor. [ABSTRACT FROM AUTHOR]