Raman Study of Vibrational and Rotational Relaxation of Liquid Benzene-d6 Confined to Nanoporous Silica Glasses

Fluids and Electrolytes under Confinement in Single-Digit Nanopores

Confined fluids and electrolyte solutions in nanopores exhibit rich and surprising physics and chemistry that impact the mass transport and energy efficiency in many important natural systems and industrial applications. Existing theories often fail to predict the exotic effects observed in the narrowest of such pores, called single-digit nanopores (SDNs), which have diameters or conduit widths of less than 10 nm, and have only recently become accessible for experimental measurements. What SDNs reveal has been surprising, including a rapidly increasing number of examples such as extraordinarily fast water transport, distorted fluid-phase boundaries, strong ion-correlation and quantum effects, and dielectric anomalies that are not observed in larger pores. Exploiting these effects presents myriad opportunities in both basic and applied research that stand to impact a host of new technologies at the water-energy nexus, from new membranes for precise separations and water purification to new gas permeable materials for water electrolyzers and energy-storage devices. SDNs also present unique opportunities to achieve ultrasensitive and selective chemical sensing at the single-ion and single-molecule limit. In this review article, we summarize the progress on nanofluidics of SDNs, with a focus on the confinement effects that arise in these extremely narrow nanopores. The recent development of precision model systems, transformative experimental tools, and multiscale theories that have played enabling roles in advancing this frontier are reviewed. We also identify new knowledge gaps in our understanding of nanofluidic transport and provide an outlook for the future challenges and opportunities at this rapidly advancing frontier.

Probing electrolyte–silica interactions through simulations of the infrared spectroscopy of nanoscale pores

The structural and dynamical properties of nanoconfined solutions can differ dramatically from those of the corresponding bulk systems. Understanding the changes induced by confinement is central to controlling the behavior of synthetic nanostructured materials and predicting the characteristics of biological and geochemical systems. A key outstanding issue is how the molecular-level behavior of nanoconfined electrolyte solutions is reflected in different experimental, particularly spectroscopic, measurements. This is addressed here through molecular dynamics simulations of the OH stretching infrared (IR) spectroscopy of NaCl, NaBr, and NaI solutions in isotopically dilute HOD/D2O confined in hydroxylated amorphous silica slit pores of width 1–6 nm and pH [Formula: see text]. In addition, the water reorientation dynamics and spectral diffusion, accessible by pump–probe anisotropy and two-dimensional IR measurements, are investigated. The aim is to elucidate the effect of salt identity, confinement, and salt concentration on the vibrational spectra. It is found that the IR spectra of the electrolyte solutions are only modestly blue-shifted upon confinement in amorphous silica slit pores, with both the size of the shift and linewidth increasing with the halide size, but these effects are suppressed as the salt concentration is increased. This indicates the limitations of linear IR spectroscopy as a probe of confined water. However, the OH reorientational and spectral diffusion dynamics are significantly slowed by confinement even at the lowest concentrations. The retardation of the dynamics eases with increasing salt concentration and pore width, but it exhibits a more complex behavior as a function of halide.

Molecular-level mechanisms of vibrational frequency shifts in a polar liquid

A molecular-level analysis of the origins of the vibrational frequency shifts of the CN stretching mode in neat liquid acetonitrile is presented. The frequency shifts and infrared spectrum are calculated using a perturbation theory approach within a molecular dynamics simulation and are in good agreement with measured values reported in the literature. The resulting instantaneous frequency of each nitrile group is decomposed into the contributions from each molecule in the liquid and by interaction type. This provides a detailed picture of the mechanisms of frequency shifts, including the number of surrounding molecules that contribute to the shift, the relationship between their position and relative contribution, and the roles of electrostatic and van der Waals interactions. These results provide insight into what information is contained in infrared (IR) and Raman spectra about the environment of the probed vibrational mode.

Ultrafast dynamics in complex fluids observed through the ultrafast optically-heterodyne-detected optical-Kerr-effect (OHD-OKE)

The ultrafast molecular dynamics of complex fluids have been recorded using the optically-heterodyne-detected optical-Kerr-effect (OHD-OKE). The OHD-OKE method is reviewed and some recent refinements to the method are described. Applications to a range of complex fluids, including microemulsions, polymer melts and solutions, liquid crystal and ionic liquids are surveyed. The level of detail attainable with the OHD-OKE method in these complex fluids is discussed. The prospects for future experiments are discussed.

Phase Diagram and Glass Transition of Confined Benzene

We used differential scanning calorimetry, neutron scattering, and proton NMR to investigate the phase behavior, the structure, and the dynamics of benzene confined in a series of cylindrical mesoporous materials MCM-41 and SBA-15 with pore diameters, d, between 2.4 and 14 nm. With this multitechnique approach, it was possible to determine the structure and, for the first time to our knowledge, the density of confined benzene as a function of temperature and pore size. Under standard cooling rates, benzene partially crystallizes in SBA-15 matrixes (4.7 <or= d <or= 14 nm) but not in MCM-41 (2.4 <or= d <or= 3.5 nm). Structure factors of the confined phases were recorded at different temperatures and compare to those of the bulk. The confined liquid has the same structure as the bulk above the bulk melting point. In SBA-15, the confined crystals are defective and have the same structure as the bulk. In MCM-41, the liquid undergoes a glass transition at low temperature regardless of the cooling rate or the thermal history of the sample. The density as a function of temperature was measured by neutron scattering contrast matching, and the glass transition temperatures were determined from the density versus temperature curves. The pore size dependence of T(g) does not show any evidence of finite size effects. A temperature versus pore diameter phase diagram of confined benzene is proposed combining liquid, supercooled liquid, crystal states, and glassy states. NMR relaxation time measurements showed that the dynamics of the confined liquids are slower than those of the bulk above its melting point. In the partially crystallized samples, the liquid and the crystal have the same relaxation times. The activation energies of reorientation motions in the confined phases, determined from spin lattice relaxation times, are smaller than the bulk ones.

Theoretical Studies of Solute Vibrational Energy Relaxation at Liquid Interfaces

Recent advances in the theoretical understanding of solute vibrational energy relaxation at liquid interfaces and surfaces are described. Non-equilibrium molecular dynamics simulations of the relaxation of an initially excited solute molecule are combined with equilibrium force autocorrelation calculations to gain insight into the factors that influence the vibrational relaxation rate. Diatomic and triatomic nonpolar, polar, and ionic solute molecules adsorbed at the liquid/vapor interface of several liquids as well as at the water/CCl(4) liquid/liquid interface are considered. In general, the vibrational relaxation rate is significantly slower (a factor of 3 to 4) at the liquid/vapor and liquid/liquid interface than in the bulk due to the reduced density, which gives rise to a reduced contribution of the repulsive solvent-solute forces on the vibrational mode. The surface effects on the ionic solutes are much smaller (50% or less slower relaxation relative to the bulk). This is due to the fact that ionic solutes at the interface are able to keep part of their solvation shell to a degree that depends on their size. Thus, a significant portion of the repulsive forces is maintained. A high degree of correlation is found between the peak height of the solvent-solute radial distribution function and the vibrational relaxation rate. The relaxation rate at the liquid/liquid interface strongly depends on the location of the solute across the interface and correlates with the change in the density and polarity profile of the interface.

Molecular Dynamics Study of Anisotropic Translational and Rotational Diffusion in Liquid Benzene

Equilibrium NPT and NVT molecular dynamics simulations were performed on liquid benzene over an extended range of temperature (from 260 to 360 K) using the COMPASS force field. Densities and enthalpies of vaporization (from cohesive energy densities) were within 1% of experiment at all temperatures. tumbling and spinning rotational diffusion coefficients, D(perpendicular) and D(parallel), computed as a function of temperature, agreed qualitatively with the results of earlier reported experimental and computational investigations. Generally, it was found that D(parallel)/D(perpendicular) approximately 1.4-2.5 and the activation energy for tumbling was significantly greater than for spinning about the C6 axis [Ea(D(perpendicular)) = 8.1 kJ mol(-1) and Ea(D(parallel)) = 4.5 kJ mol(-1)]. Calculated translational diffusion coefficients were found to be in quantitative agreement with experimental values at all temperatures [deviations were less than the scatter between different reported measurements]. In addition, translational diffusion coefficients were computed in the molecule-fixed frame to yield values for Dxy (diffusion in the plane of the molecule) and Dz (diffusion perpendicular to the plane). It was found that the ratio Dxy/Dz approximately 2.0, and that the two coefficients have roughly equal activation energies. This represents the first atomistic molecular dynamics study of translational diffusion in the molecular frame.

Chemical imaging in a surface forces apparatus: confocal raman spectroscopy of confined poly(dimethylsiloxane)

Confocal Raman spectroscopy has been implemented within the molecularly thin films of a surface forces apparatus. Applying this technique to an initial system, we investigate the confinement and shear-induced changes in the Raman spectra of poly(dimethylsiloxane) (PDMS) liquids confined between atomically smooth mica surfaces at thicknesses less than the unperturbed radius of gyration of the polymer. We focus on stretch vibrations of the PDMS methyl group, whose net orientation is perpendicular to the chain backbone. When PDMS was confined to a thickness of approximately the unperturbed radius of gyration (3.5 nm) but no shear, the Raman intensity of the methyl group was anisotropic in the x-y plane, signifying that chains oriented preferentially parallel to the confining surfaces. Relative to the bulk fluid, the relative intensity of the asymmetric to symmetric carbon-hydrogen stretch (2965 and 2907 cm-1, respectively) was enhanced, indicating that asymmetric vibration was enhanced by confinement. Measurements using polarized radiation showed coherent planar anisotropy in the x-y plane whose direction varied stochastically from experiment to experiment. It seems that although coherent in-plane alignment was favored, no preferential alignment direction was favored in the absence of shear. Application of shear caused the time-averaged polymer conformations to become more nearly isotropic in the plane of shear. These measurements are considered to represent the first chemical imaging of chemical species within the contact area of a surface forces apparatus.

Ultrafast Orientational Dynamics of Nanoconfined Benzene

Ultrafast optical Kerr effect spectroscopy has been used to study the orientational dynamics of benzene and benzene-d(6) confined in nanoporous sol-gel glass monoliths with a range of average pore sizes. All of the observed orientational diffusion of confined benzene is found to occur on a slower time scale than in the bulk, even in pores with diameters that are significantly larger than a benzene molecule. The orientational dynamics of benzene-d(6) are found to be inhibited to a lesser extent than those of benzene, which is attributed to the differences in wetting properties of the two liquids on silica. The decays are fit well by a sum of two exponentials, the faster of which depends on pore size. Similar results are found in pores that have been modified with trimethylsilyl groups, although the relaxation is faster than in unmodified pores. Comparison to Raman line width data for confined benzene-d(6) suggests that the liquid exhibits significant structuring at the pore walls, with the benzene molecules lying flat on the surfaces of unmodified pores.

Water motion in reverse micelles studied by quasielastic neutron scattering and molecular dynamics simulations

Motion of water molecules in Aerosol OT [sodium bis(2-ethylhexyl) sulfosuccinate, AOT] reverse micelles with water content w(0) ranging from 1 to 5 has been explored both experimentally through quasielastic neutron scattering (QENS) and with molecular dynamics (MD) simulations. The experiments were performed at the energy resolution of 85 microeV over the momentum transfer (Q) range of 0.36-2.53 A(-1) on samples in which the nonpolar phase (isooctane) and the AOT alkyl chains were deuterated, thereby suppressing their contribution to the QENS signal. QENS results were analyzed via a jump-diffusion/isotropic rotation model, which fits the results reasonably well despite the fact that confinement effects are not explicitly taken into account. This analysis indicates that in reverse micelles with low-water content (w(0)=1 and 2.5) translational diffusion rate is too slow to be detected, while for w(0)=5 the diffusion coefficient is much smaller than for bulk water. Rotational diffusion coefficients obtained from this analysis increase with w(0) and are smaller than for bulk water, but rotational mobility is less drastically reduced than translational mobility. Using the Faeder/Ladanyi model [J. Phys. Chem. B 104, 1033 (2000)] of reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function F(S)(Q,t) for water hydrogens. Comparison of the time Fourier transform of this F(S)(Q,t) with the QENS dynamic structure factor S(Q,omega), shows good agreement between the model and experiment. Separate intermediate scattering functions F(S) (R)(Q,t) and F(S) (CM)(Q,t) were determined for rotational and translational motion. Consistent with the decoupling approximation used in the analysis of QENS data, the product of F(S) (R)(Q,t) and F(S) (CM)(Q,t) is a good approximation to the total F(S)(Q,t). We find that the decay of F(S) (CM)(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior is due to lower water mobility close to the interface and to confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay. However, rotational mobility of O-H bond vectors in the interfacial region remains fairly high due to the lower density of water-water hydrogen bonds in the vicinity of the interface.