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

Abstract

The molecular dynamics of solutions of di-propylene glycol methylether (2PGME) and H 2 O (or D 2 O) confined in 28 Å pores of MCM-41 have been studied by quasielastic neutron scattering and differential scanning calorimetry over the concentration range 0-90 wt.% water. This system is of particular interest due to its pronounced non-monotonic concentration dependent dynamics of 2PGME in the corresponding bulk system, showing the important role of hydrogen bonding for the dynamics. In this study we have elucidated how this non-monotonic concentration dependence is affected by the confined geometry. The results show that this behaviour is maintained in the confinement, but the slowest diffusive dynamics of 2PGME is now observed at a considerably higher water concentration; at 75 wt.% water in MCM-41 compared to 30 wt.% water in the corresponding bulk system. This difference can be explained by an improper mixing of the two confined liquids. The results suggest that water up to a concentration of about 20 wt.% is used to hydrate the hydrophilic hydroxyl surface groups of the silica pores, and that it is only at higher water contents the water becomes partly mixed with 2PGME. Hence, due to this partial micro-phase separation of the two liquids larger, and thereby slower relaxing, structural entities of hydrogen bonded water and 2PGME molecules can only be formed at higher water contents than in the bulk system. However, the Q-dependence is unchanged with confinement, showing that the nature of the molecular motions is preserved. Thus, there is no indication of localization of the dynamics at length scales of less than 20 Å. The dynamics of both water and 2PGME is strongly dominated by translational diffusion at a temperature of 280 K.

More room for microphase separation: An extended study on binary liquids confined in SBA-15 cylindrical pores

“…There is currently a growing number of observations reporting on microphase separation induced under nanoporous confinement in liquid mixtures, which are under normal bulk conditions fully miscible. 14,15,16,47,48 However direct evidence for such a phenomenon form structural investigations are scarce. 19,20 In this article, we exploit both the unique properties of neutron scattering regarding isotopic labeling and the additional concentration parameter offered for These conclusions were obtained from the analysis of the diffraction pattern produced by filled and unfilled SBA-15 pore lattices, which can be directly related to the radial concentration profile of the confined mixture across the channel.…”

“…6,7,[9][10][11][12][13] The influence of the confinement has been extended to the dynamics of binary liquids, providing indirect evidence for a microphase separation of fully miscible mixtures of alcohol aqueous mixtures confined in MCM-41 mesostructured porous silicates. [14][15][16][17][18] In a recent study, we conducted a thorough structural analysis to characterize the nature of microphase-separated systems. 19,20 We determined the concentration profile across the nanochannels by implementing an original application of neutron diffraction properties with isotopic substitution and derived a quantitative model to characterize the binary liquid structure.…”

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

“…to altered phase diagrams [15]. For alcohol-water mixtures, it was argued that surfaceinduced demixing manifests itself in changed dynamical behaviors [16][17][18][19]. In this contribution, we discuss the dynamical behaviors of several alcohol-water mixtures in various types of confinements to gain detailed insights into confinement effects for HB mixtures.…”

Extension and Limits of Cryoscopy for Nanoconfined Solutions

“…It is worth mentioning that microphase separation phenomena induced by nanoconfinement and interfacial interaction have been reported for different binary liquid mixtures based on dynamical information. 39,[46][47][48] Moreover, direct structural evidences were also obtained from neutron scattering methods and molecular simulation. [49][50][51] The microphase segregation facilitating ice formation in water-rich domains was also invoked to rationalize the enhanced tendency for partial crystallization for glycol-water mixtures.…”