Hydrogen Bonded Structures of Confined Water Molecules and Electric Field Induced Shift of Their Equilibrium Revealed by IR Electroabsorption Spectroscopy

Suppressed terahertz dynamics of water confined in nanometer gaps

Nanoconfined waters exhibit low static permittivity mainly due to interfacial effects that span about one nanometer. The characteristic length scale may be much longer in the terahertz (THz) regime where long-range collective dynamics occur; however, the THz dynamics have been largely unexplored because of the lack of a robust platform. Here, we use metallic loop nanogaps to sharply enhance light-matter interactions and precisely measure real and imaginary THz refractive indices of nanoconfined water at gap widths ranging from 2 to 20 nanometers, spanning mostly interfacial waters all the way to quasi-bulk waters. We find that, in addition to the well-known interfacial effect, the confinement effect also contributes substantially to the decrease in the complex refractive indices of the nanoconfined water by cutting off low-energy vibrational modes, even at gap widths as large as 10 nanometers. Our findings provide valuable insights into the collective dynamics of water molecules which is crucial to understanding water-mediated processes.

Critical In-Plane Density of Polyelectrolyte Brush for the Ordered Hydrogen-Bonded Structure of Incorporated Water

A polymer electrolyte brush is a reasonable platform to confine water molecules within a nanoscopic area to study their role in the function of interacting media because of their adjustable nanospace and charge by changing the in-plane density and side chains of the brush. Here, we demonstrate how the in-plane spacing of cationic polymer brush chains, poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMTAC), affects the hydrogen bond configuration of incorporated water using soft X-ray emission spectroscopy. At the critical in-plane density σ = 0.30 chains/nm² of PMTAC, tetrahedrally coordinated water molecules started to melt into distorted or broken hydrogen-bonded configurations. Considering the charge on the quaternary ammonium cations, the electric field required to form a tetrahedrally coordinated hydrogen-bonded configuration was estimated as ∼500 kV cm^-1 and is effective up to ∼1 nm from the surface of the polymer chain. These findings are useful for designing specific interface properties and the resultant surface function of polyelectrolyte-based materials.

Observation of the Pockels Effect in Ionic Liquids and Insights into the Length Scale of Potential-Induced Ordering

Ionic liquids (ILs) under electric fields play essential roles in the electrochemical utilization of ILs. Recently, long-range organization of ILs in the vicinity of charged (and even neutral) surfaces has been revealed, but experimental evidence for such an ordering is still limited and its spatial length scale remains controversial. Here, we use confocal Raman microspectroscopy to investigate the effect of an applied electric potential on the IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and its analogues in a space-resolved manner. Much to our surprise, the observed Raman difference spectra of the ILs obtained with and without an applied potential exhibit uniform intensity changes independent of vibrational modes of cations and anions, a finding in sharp contrast with the electric field effects on molecular liquids that we have previously observed. We interpret this unexpected finding in terms of the Pockels effect that occurs as a result of a potential-induced ordering of the IL near an IL-electrode interface. The refractive index changes due to the applied potential are estimated using the experimental Raman intensity changes. The results allow us to deduce that the length scale of the ordering in the ILs is tens to hundreds of nanometers, extending more than would be expected for the electrical double layer but not as far as a micrometer scale.

Interactions of water confined in a metal-organic framework as studied by a combined approach of Raman, FTIR, and IR electroabsorption spectroscopies and multivariate curve resolution analysis

Water in nanoconfinement shows distinct properties that are markedly different from those of bulk water. These unique properties stem not only from the water-water interaction but also from the interactions between water and the surrounding confining environment. Here we used a combined approach of vibrational spectroscopies (Raman, FTIR, and IR electroabsorption) and a multivariate curve resolution technique to study the interactions of water in a heterogeneous confining environment within a prototype of pillared layer-type metal-organic frameworks (MOFs), CPL-1 ([Cu2(pzdc)2(pyz)]n, where pzdc = 2,3-pyrazinedicarboxylate, pyz = pyrazine). The OH stretching Raman spectrum of hydrated CPL-1 microcrystals revealed that the adsorbed water molecules resemble the subpopulation of bulk water whose hydrogen bond is weak. Multivariate curve resolution analysis of FTIR spectra monitoring water desorption from CPL-1 allowed for accurate assignments of the framework's carboxylate vibrational modes associated with water-filled and empty nanopores of the MOF, and for quantitative determination of the number fraction of these pores. Furthermore, building on the assignments so made, IR electroabsorption measurements showed that the hydrogen-bonding interaction with water adsorbed in CPL-1 has little impact on the response to electric fields of the framework vibrational modes. The present findings altogether provide a solid basis of elucidating water confined in CPL-1 and demonstrate the potential of the combined vibrational spectroscopic method for interrogating the interactions within MOFs.

Tautomer Structures in Ketose-Aldose Transformation of 1,3-Dihydroxyacetone Studied by Infrared Electroabsorption Spectroscopy

The acyclic form of monosaccharides exists in a structural equilibrium, with aldose having the aldehyde group and ketose the ketone group (ketose-aldose equilibrium). A basic catalyst facilitates their transformation, which affects the chemical properties of the monosaccharide. In this study, we investigated the ketose-aldose transformation of 1,3-dihydroxyacetone (1,3-DHA), one of the simplest systems of the ketose-aldose equilibrium. We examined the effects of piperidine as the basic catalyst and used IR electroabsorption spectroscopy to study the responses to an external electric field. We analyzed the changes in IR absorption by considering the changes in the molecular orientation and number of molecules in response to the external electric field. The results of the analysis revealed the permanent dipole moment μ_P, an angle η between μ_P and μ_T (the transition moment of the molecular vibration), and the equilibrium constants. The ketose-aldose transformation of 1,3-DHA can be explained in terms of the equilibrium of three states. In the presence of piperidine, a five-state equilibrium was concluded. On the basis of the experimental data, we propose plausible models of dihydroxyacetone, E-enediols, Z-enediol, or glyceraldehyde for each state. The results of our structural analysis of these tautomers provide a detailed understanding of the ketose-aldose transformation of acyclic saccharides and the effects of the basic catalyst.

Nano-/Micro-confined Water in Graphene Hydrogel as Superadsorbents for Water Purification

Confined water has been proven to be of great importance due to its pervasiveness and contribution to life and many fields of scientific research. However, the control and characterization of confined water are a challenge. Herein, a confined space is constructed by flexibly changing the pH of a graphene oxide dispersion under the self-assembly process of a graphene hydrogel (GH), and the confined space is adjusted with variation from 10.04 to 3.52 nm. Confined water content in GH increases when the pore diameter of the confined space decreases; the corresponding adsorption capacity increases from 243.04 to 442.91 mg g^-1. Moreover, attenuated total reflectance Fourier transform infrared spectroscopy and Raman spectroscopy are utilized to analyze the hydrogen bonding structure qualitatively and quantitatively, and correlation analysis reveals that the improvement in the adsorption capacity is caused by incomplete hydrogen bonding in the confined water. Further, confined water is assembled into four typical porous commercial adsorbents, and a remarkable enhancement of the adsorption capacity is achieved. This research demonstrates the application potential for the extraordinary properties of confined water and has implications for the development of highly effective confined water-modified adsorbents.

Understanding the effect of nanoconfinement on the structure of water hydrogen bond networks

Dynamic hydrogen bond trails in water confined between two phospholipid membranes traced by the information flow model.