Anomalous dynamics of aqueous solutions of di-propylene glycol methylether confined in MCM-41 by quasielastic neutron scattering

Structure and dynamics of nanoconfined water and aqueous solutions

This review is devoted to discussing recent progress on the structure, thermodynamic, reactivity, and dynamics of water and aqueous systems confined within different types of nanopores, synthetic and biological. Currently, this is a branch of water science that has attracted enormous attention of researchers from different fields interested to extend the understanding of the anomalous properties of bulk water to the nanoscopic domain. From a fundamental perspective, the interactions of water and solutes with a confining surface dramatically modify the liquid's structure and, consequently, both its thermodynamical and dynamical behaviors, breaking the validity of the classical thermodynamic and phenomenological description of the transport properties of aqueous systems. Additionally, man-made nanopores and porous materials have emerged as promising solutions to challenging problems such as water purification, biosensing, nanofluidic logic and gating, and energy storage and conversion, while aquaporin, ion channels, and nuclear pore complex nanopores regulate many biological functions such as the conduction of water, the generation of action potentials, and the storage of genetic material. In this work, the more recent experimental and molecular simulations advances in this exciting and rapidly evolving field will be reported and critically discussed.

Revisiting the glass transition temperature of water-glycerol mixtures in the bulk and confined in mesoporous silica

In this work, we revisited the glass transition temperature (Tg) behavior of bulk and confined water-glycerol solutions as a function of the mixture composition and size of the confinement media, with the aim to shed some light on some controversies found in the literature. In the case of bulk mixtures, some discrepancies are observed due to the differences in the way of calculating Tg from the DSC experiments and differences in the protocols of cooling/reheating. However, unphysical behavior observed below the eutectic composition can be due to the crystallization of water during the cooling of the mixture. We also analyzed the effect of confinement on the glass transition of glycerol aqueous solutions, with glycerol mass fraction, wG, between 0.5 and 1.0, in silica mesoporous samples with pore diameters between 2 and 58 nm. Our results show that the the Tg dependence on pore size changes with the mixture composition. For glycerol-rich samples, Tg decreases with a decreasing pore size. This tendency changes with increasing water concentration below wG ∼ 0.6 for samples with dp between 2 and 8 nm, where two glass transition temperatures appear. We hypothesize that this effect is related to the existence of two liquid phases with different densities. The Tg composition dependence in confined glycerol-water mixtures was analyzed with the Gordon-Taylor equation modified for confined mixtures, which allowed us to calculate the Tg of the pure components as a function of the pore size. This analysis shows that for pores with dp > 20 nm, and for pure water and pure glycerol, Tg decreases with the pore size, attaining an almost constant value for samples with pore sizes between 2 and 8 nm. This Tg pore size dependence is explained considering the competition of two opposite effects: a reduction in Tg with a decreasing pore size given when the length scale of dynamics is comparable to the pore size, and an increment in Tg with a decreasing pore size as a result of increasing interactions of the confined liquid with the pore walls.

Extension and Limits of Cryoscopy for Nanoconfined Solutions

This work investigates the phase behavior of aqueous solutions of glycerol confined in MCM-41 and SBA-15 nanoporous matrixes by calorimetry. Limitations due to overfilling and eutectic freezing are prevented by the absence of an external liquid reservoir and by the glass-forming property of glycerol. Consequently, the stability of nanoconfined ice in equilibrium with aqueous solutions is studied over a wide range of compositions. In confinement, a large temperature depression of the liquidus line is observed. A thermodynamic model accounting simultaneously for the cryoscopic and the Gibbs-Thomson effects gives a consistent view of the phase diagram for large pores (R_p = 4.15 nm). For smaller pores (R_p = 1.8 nm), it reveals that the water activity strongly deviates from the bulk solution with the same composition, indicating the possible role of concentration heterogeneities in determining the onset of ice freezing in strongly nanoconfined solutions.

Properties of Hydrogen-Bonded Liquids at Interfaces

Effects of interfaces on hydrogen-bonded liquids play major roles in nature and technology. Despite their importance, a fundamental understanding of these effects is still lacking. In large parts, this shortcoming is due to the high complexity of these systems, leading to an interference of various interactions and effects. Therefore, it is advisable to take gradual approaches, which start from well designed and defined model systems and systematically increase the level of intricacy towards more complex mimetics. Moreover, it is necessary to combine insights from a multitude of methods, in particular, to link novel preparation strategies and comprehensive experimental characterization with inventive computational and theoretical modeling. Such concerted approach was taken by a group of preparative, experimentally, and theoretically working scientists in the framework of Research Unit FOR 1583 funded by the Deutsche Forschungsgemeinschaft (German Research Foundation). This special issue summarizes the outcome of this collaborative research. In this introductory article, we give an overview of the covered topics and the main results of the whole consortium. The following contributions are review articles or original works of individual research projects.

2H NMR Studies on the Dynamics of Pure and Mixed Hydrogen-Bonded Liquids in Confinement

2H NMR is used to ascertain dynamical behaviors of pure and mixed hydrogen-bonded liquids in bulk and in confinement. Detailed comparisons of previous and new results in broad dynamic and temperature ranges reveal that confinement effects differ for various liquids and confinements. For water, molecular reorientation strongly depends on the confinement size, with much slower and less fragile structural relaxation under more severe geometrical restriction. Moreover, a dynamical crossover occurs when a fraction of solid water forms so that the dynamics of the fraction of liquid water becomes even more restricted and, as a consequence, changes from bulk-like to interface-dominated. For glycerol, by contrast, confinement has weak effects on the reorientation dynamics. Mixed hydrogen-bonded liquids show even more complex dynamical behaviors. For aqueous solutions, the temperature dependence of the structural relaxation becomes discontinuous when the concentration changes due to a freezing of water fractions. This tendency for partial crystallization is enhanced rather than reduced by confinement, because different liquid-matrix interactions of the molecular species induce micro-phase segregation, which facilitates ice formation in water-rich regions. In addition, dynamical couplings at solvent-protein interfaces are discussed. It is shown that, on the one hand, solvent dynamics are substantially slowed down at protein surfaces and, on the other hand, protein dynamics significantly depend on the composition and, thus, the viscosity of the solvent. Furthermore, a protein dynamical transition occurs when the amplitude of water-coupled restricted backbone dynamics vanishes upon cooling.

Dynamics of aqueous binary glass-formers confined in MCM-41

Dielectric permittivity measurements were performed on water solutions of propylene glycol (PG) and propylene glycol monomethyl ether (PGME) confined in 21 Å pores of the silica matrix MCM-41 C10 in wide frequency (10(-2)-10(6) Hz) and temperature (130-250 K) ranges. The aim was to elucidate how the formation of large hydrogen bonded structural entities, found in bulk solutions of PGME, was affected by the confined geometry, and to make comparisons with the dynamic behavior of the PG-water system. For all solutions the measurements revealed four almost concentration independent relaxation processes. The intensity of the fastest process is low compared to the other relaxation processes and might be caused by both hydroxyl groups of the pore surfaces and by local motions of water and solute molecules. The second fastest process contains contributions from both the main water relaxation as well as the intrinsic β-relaxation of the solute molecules. The third fastest process is the viscosity related α-relaxation. Its concentration independency is very different compared to the findings for the corresponding bulk systems, particularly for the PGME-water system. The experimental data suggests that the surface interactions induce a micro-phase separation of the two liquids, resulting in a full molecular layer of water molecules coordinating to the hydrophilic hydroxyl groups on the surfaces of the silica pores. This, in turn, increases the geometrical confinement effect for the remaining solution even more and prevents the building up of the same type of larger structural entities in the PGME-water system as in the corresponding bulk solutions. The slowest process is mainly hidden in the high conductivity contribution at low frequencies, but its temperature dependence can be extracted for the PGME-water system. However, its origin is not fully clear, as will be discussed.