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

Abstract

Abstract:2 H 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.

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

“…In Figure 3 b, we see that the I(a/b) processes are characterized by small width parameters in the range 0.3–0.6, indicating that the related rotational motions occurred on broadly distributed time scales. Consistently, previous studies on hydrogen-bonded liquids in various types of confinements reported broad distributions of correlation times [ 14 , 15 , 28 ]. More precisely, the relaxation processes of confined liquids showed a broadening on the low-frequency flank, which did not occur for the processes of bulk liquids.…”

“…We restricted this analysis to temperatures in the vicinity of the minimum, where the resulting correlation times hardly depended on the exact shape of the spectral density. To obtain synonymous time constants from the H STE study, we used the and parameters obtained from the Kohlrausch fit to calculate peak correlation times according to [ 28 ] …”

“…While they were interpreted in terms of core-shell models for confined alcohols [ 33 , 97 ], they were attributed to coexisting liquid and solid phases or high-density and low-density liquid phases for confined water [ 78 , 86 , 98 ]. In our case of binary mixtures, another possible explanation resulted from the finding that preferential interactions with the pore walls could lead to microphase separation and, thus, to bimodal dynamics [ 28 , 29 , 40 , 43 , 44 , 99 , 100 ].…”

“…It is well established that nano-sized confinements strongly influence the structural and dynamical properties of liquids, including hydrogen-bonded compounds [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. Prominent observations are the suppression of crystallization and changes in the glass transition temperatures .…”

Small Molecules, Non-Covalent Interactions, and Confinement

“…To tackle this problem, 2 H NMR was used to investigate reorientation dynamics of unfreezable water (D 2 O) in MCM-41 and SBA-15 pores over wide temperature ranges towards the glass transition [ 45 , 176 , 181 , 183 , 213 , 214 , 215 , 216 , 217 , 218 , 219 ]. In particular, spin-lattice relaxation, line-shape analysis, and stimulated-echo experiments were combined to ensure broad dynamic ranges and the pore-size was systematically varied to study possible finite-size effects.…”