Effects of confinement on supercooled tetrahedral liquids

Quantitative Understanding of Liquid Dynamics at Interfaces from a Free-Energy Landscape Perspective

On the basis of molecular dynamics simulations of water and ethanol in nanopores, we devise a methodology to determine the free-energy landscape (FEL) imposed by an interface on an adjoining liquid directly from the particle trajectories. The methodology merely uses the statistical mechanical relation between occupancy and energy and, hence, is particularly suitable in complex situations, e.g., for disordered or rough atomistic interfaces and molecular liquids, as encountered in many biological, geological, and technological situations. Moreover, we show that the thus-obtained FEL enables a quantitative understanding of interface effects on liquid dynamics. Specifically, by determining the local minima and barriers of the FEL and using an Arrhenius-like relation, we reproduce the very strong spatial variation of the structural relaxation time of water and ethanol across nanopores over a broad temperature range. We anticipate that the proposed FEL approach is transferable to various other liquids and interfaces.

Structural and Dynamical Properties of Liquids in Confinements: A Review of Molecular Dynamics Simulation Studies

Molecular dynamics (MD) simulations are a powerful tool for detailed studies of altered properties of liquids in confinement, in particular, of changed structures and dynamics. They allow, on one hand, for perfect control and systematic variation of the geometries and interactions inherent in confinement situations and, on the other hand, for type-selective and position-resolved analyses of a huge variety of structural and dynamical parameters. Here, we review MD simulation studies on various types of liquids and confinements. The main focus is confined aqueous systems, but also ionic liquids and polymer and silica melts are discussed. Results for confinements featuring different interactions, sizes, shapes, and rigidity will be presented. Special attention will be given to situations in which the confined liquid and the confining matrix consist of the same type of particles and, hence, disparate liquid-matrix interactions are absent. Findings for the magnitude and the range of wall effects on molecular positions and orientations and on molecular dynamics, including vibrational motion and structural relaxation, are reviewed. Moreover, their dependence on the parameters of the confinement and their relevance to theoretical approaches to the glass transition are addressed.

NMR Relaxometry Accessing the Relaxation Spectrum in Molecular Glass Formers

It is a longstanding question whether universality or specificity characterize the molecular dynamics underlying the glass transition of liquids. In particular, there is an ongoing debate to what degree the shape of dynamical susceptibilities is common to various molecular glass formers. Traditionally, results from dielectric spectroscopy and light scattering have dominated the discussion. Here, we show that nuclear magnetic resonance (NMR), primarily field-cycling relaxometry, has evolved into a valuable method, which provides access to both translational and rotational motions, depending on the probe nucleus. A comparison of ¹H NMR results indicates that translation is more retarded with respect to rotation for liquids with fully established hydrogen-bond networks; however, the effect is not related to the slow Debye process of, for example, monohydroxy alcohols. As for the reorientation dynamics, the NMR susceptibilities of the structural (α) relaxation usually resemble those of light scattering, while the dielectric spectra of especially polar liquids have a different broadening, likely due to contributions from cross correlations between different molecules. Moreover, NMR relaxometry confirms that the excess wing on the high-frequency flank of the α-process is a generic relaxation feature of liquids approaching the glass transition. However, the relevance of this feature generally differs between various methods, possibly because of their different sensitivities to small-amplitude motions. As a major advantage, NMR is isotope specific; hence, it enables selective studies on a particular molecular entity or a particular component of a liquid mixture. Exploiting these possibilities, we show that the characteristic Cole-Davidson shape of the α-relaxation is retained in various ionic liquids and salt solutions, but the width parameter may differ for the components. In contrast, the low-frequency flank of the α-relaxation can be notably broadened for liquids in nanoscopic confinements. This effect also occurs in liquid mixtures with a prominent dynamical disparity in their components.

Confinement effects on glass-forming mixtures: Insights from a combined experimental approach to aqueous ethylene glycol solutions in silica pores

We perform nuclear magnetic resonance, broadband dielectric spectroscopy, and differential scanning calorimetry studies to ascertain the dynamical behaviors of aqueous ethylene glycol (EG) solutions in silica pores over broad temperature ranges. Both translational and rotational motions are analyzed, and the pore diameter (2.4–9.2 nm) and the EG concentration (12–57 mol. %) are varied, leading to fully liquid or partially crystalline systems. It is found that the translational diffusion coefficient strongly decreases when the diameter is reduced, resulting in a slowdown of nearly three orders of magnitude in the narrowest pores, while the confinement effects on the rotational correlation times are moderate. For the fully liquid solutions, we attribute bulk-like and slowed down reorientation processes to the central and interfacial pore regions, respectively. This coexistence is found in all the studied pores, and, hence, the range of the wall effects on the solution dynamics does not exceed ∼1 nm. Compared to the situation in the bulk, the concentration dependence is reduced in confinements, implying that the specific interactions of the molecular species with the silica walls lead to preferential adsorption. On the other hand, bulk-like structural relaxation is not observed in the partially frozen samples, where the liquid is sandwiched between the silica walls and the ice crystallites. Under such circumstances, there is another relaxation process with a weaker temperature dependence, which is observed in various kinds of partially frozen aqueous systems and denoted as the x process.

Prolonged Association between Water Molecules under Hydrophobic Nanoconfinement

We present an investigation of the dynamics of water confined among rigid carbon rods and between parallel graphene sheets with molecular dynamics simulations. Diffusion coefficients, activation energy of diffusion, and residence-time correlation functions as a function of confinement geometry reveal a retardation of water dynamics under hydrophobic confinement compared to bulk water. In fact, water under various confinements possesses longer associations with its neighbors and exhibits diffusion dynamics characteristic of a lower temperature. Analysis of the residence-time correlation functions reveals long and short residence times, which we relate to the diffusion coefficient and activation energy of diffusion, respectively. Additional investigations reveal how the level of confining surface hydrophobicity affects water dynamics, further broadening our understanding of water diffusion inside diverse media. Overall, this study sheds light on the physical origin of retarded water dynamics under hydrophobic confinement and the close relationship between residence times and diffusion behavior.

MD simulations of charged binary mixtures reveal a generic relation between high- and low-temperature behavior

Experimental studies of the glassy slowdown in molecular liquids indicate that the high-temperature activation energy E_∞ of glass-forming liquids is directly related to their glass transition temperature T_g. To further investigate such a possible relation between high- and low-temperature dynamics in glass-forming liquids, we analyze the glassy dynamics of binary mixtures using molecular dynamics simulations. We consider a binary mixture of charged Lennard-Jones particles and vary the partial charges of the particles and, thus, the high-temperature activation energy and the glass transition temperature of the system. Based on previous results, we introduce a phenomenological model describing relaxation times over the whole temperature regime from high temperatures to temperatures well inside the supercooled regime. By investigating the dynamics of both particle species on molecular and diffusive length scales along isochoric and isobaric pathways, we find a quadratic charge dependence of both E_∞ and T_g, resulting in an approximately constant ratio of both quantities independent of the underlying observable, the thermodynamic ensemble, and the particle species, and this result is robust against the actual definition of T_g. This generic relation between the activation energy and the glass transition temperature indicates that high-temperature dynamics and the glassy slowdown are related phenomena, and the knowledge of E_∞ may allow us to approximately predict T_g.