SEARCH · Search NASA
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Hydration water dynamics, structure, and thermodynamics are crucially important to understand and predict water-mediated properties at molecular interfaces.
The nature of the interface between the solute and the solvent in a colloidal solution has attracted attention for a long time. For example, the surface of colloidal nanocrystals (NCs) is specially created to impart colloidal stability in a variety of polar and nonpolar solvents. This work focuses on a special type of colloids where the solvent is a molten inorganic salt or organic ionic liquid. The stability of such colloids is difficult to rationalize because solvents with high density of mobile charges efficiently screen the electrostatic double-layer repulsion, and purely ionic molten salts represent an extreme case where the Debye length is only similar to 1 angstrom. We introduce a detailed investigation of NC dispersions in molten salts and ionic liquids using small-angle X-ray scattering (SAXS), atomic pair distribution function (PDF) analysis and molecular dynamics (MD) simulations. Our SAXS analysis confirms that a wide variety of NCs (Pt, CdSe/CdS, InP, InAs, ZrO 2 ) can be uniformly dispersed in molten salts like AlCl 3 /NaCl/KCl (AlCl 3 /AlCl 4 - ) and NaSCN/KSCN and in ionic liquids like 1-butyl-3-methylimidazolium halides (BMIM + X - , where X = Cl, Br, I). By using a combination of PDF analysis and molecular modeling, we demonstrate that the NC surface induces a solvent restructuring with electrostatic correlations extending an order of magnitude beyond the Debye screening length. These strong oscillatory ion-ion correlations, which are not accounted by the traditional mechanisms of steric and electrostatic stabilization of colloids, offer additional insight into solvent-solute interactions and enable apparently "impossible" colloidal stabilization in highly ionized media.
Nanoparticles are attractive in a wide range of research genres due to their size-dependent properties, which can be in contrast to those of micrometre-sized colloids or bulk materials. This may be attributed, in part, to their large surface-to-volume ratio and quantum confinement effects. There is a growing awareness that stress and strain at the particle surface contribute to their behaviour and this has been included in the structural models of nanoparticles for some time. One significant oversight in this field, however, has been the fact that the particle surface affects its surroundings in an equally important manner. It should be emphasized here that the surface areas involved are huge and, therefore, a significant proportion of solvent molecules are affected. Experimental evidence of this is emerging, where suitable techniques to probe the structural correlations of liquids at nanoparticle surfaces have only recently been developed. The recent validation of solvation shells around nanoparticles has been a significant milestone in advancing this concept. Restructured ordering of solvent molecules at the surfaces of nanoparticles has an influence on the entire panoply of solvent–particle interactions during, for example, particle formation and growth, adhesion forces in industrial filtration, and activities of nanoparticle–enzyme complexes. This article gives an overview of the advances made in solvent–nanoparticle interface research in recent years: from description of the structure of bulk solids and liquids via macroscopic planar surfaces, to the detection of nanoscopic restructuring effects. Water–nanoparticle interfaces are given specific attention to illustrate and highlight their similarity to biological systems.
Amphiphilic monolayers composed of end groups with distinct polar and nonpolar functional groups offer rapid and reversible interfacial adaptation in response to environmental stimuli such as a change in interfacial medium polarity. We have synthesized and characterized a suite of monolayers with functional groups of competing polarity designed to reconfigure their interfacial chemical composition in response to solvent polarity. In these films, the end group is designed to be able to reorient and expose the functional groups that minimize the interfacial free energy between the film and the environment. Using a combination of spectroscopic, computational, and wettability studies, we have investigated the responsive interfacial behavior of different end groups upon exposure to environments with varying polarities. Contact angle measurements across a series of polar and dispersive probe liquids reveal trends that reflect the underlying molecular flexibility and composition. Vibrational sum frequency generation (SFG) spectroscopy and atomistic molecular dynamics (MD) simulations confirm solvent-driven reorientation of the end groups, with restructuring observed at the interface. To quantify these effects, we have developed a surface energy calculation model that incorporates solvent-induced surface rearrangements into the estimations. Our findings reveal a strong dependence of surface energy and switching behavior on the length and flexibility of the functionalities in the end group, which affects the exposure of certain interfacial compositions under different solvents. These results offer new insights into the design of adaptive monolayers and provide a framework for evaluating solvent-responsive surfaces.
Recent total scattering experiments have opened up the possibility to study nanoparticle formation in situ and to observe the structural transformation from precursor clusters to adult particles. Organic ligand molecules interact with precursors of metal oxide nanoparticles, yet their influence onto the evolution of crystallinity during particle formation has not been addressed in detail; nor have in situ total scattering experiments ventured into the field of low-concentration, room-temperature syntheses in organic solvents to date. In this report, we follow the crystallization of ZnO nanoparticles in ethanol in the presence of different organic ligands. Low coordinated zinc precursor clusters rapidly polymerize upon base addition to particles of ca. 1 nm in diameter. In situ SAXS experiments reveal that the overall particle size increases to 2 to 4 nm with advancing reaction time. Complementary in situ PDF experiments show smaller crystalline domain sizes, which are only one third to half as large as the particle diameter. The ZnO particles thus feature a crystalline core surrounded by a disordered shell. Both, the core and the shell diameter are influenced by the different surface-bound organic ligands, which prevent an immediate relaxation to fully crystalline particles. A slow crystallization takes place in solution. We assume a dynamic equilibrium of the ligand and solvent molecules at the particle surface, which enables gradual bond restructuring. With suitably adjusted synthesis conditions, in our case by a continuous base addition, we show how to bypass the disordered intermediates, allowing the spontaneous nucleation of fully crystalline nanoparticles.
Ceria-based materials possessing mesoporous structures afford higher activity than the corresponding bulk materials in CO oxidation and other catalytic applications, because of the wide pore channel and high surface area. The development of a direct, template-free, and scalable technology for directing porosity inside ceriabased materials is highly welcome. Here in this paper, a family of mesoporous transition-metaldoped ceria catalysts with specific surface areas up to 122 m 2 g -1 is constructed by mechanochemical grinding. No templates, additives, or solvents are needed in this process, while the mechanochemistry-mediated restructuring and the decomposing of the organic group led to plentiful mesopores. Interestingly, the copper species are evenly dispersed in the ceria matrix at the atomic scale, as observed in high resolution scanning transmission electron microscopy in high angle annular dark field. The copper-doped ceria materials show good activity in the CO oxidation.
The differential capacitance profile of electrochemical interfaces reflects the physical properties of the double layer. For carbon electrodes and ionic liquid based electrolytes, these capacitance profiles are not fully understood. In this work, we utilize constant voltage molecular dynamics simulations to compute differential capacitance profiles of ionic liquids [BMIm + ][BF 4 - ] and [BMIm + ][TFSI - ] mixed with acetonitrile and 1,2-dichloroethane, at model graphene electrodes. We find that both pure and 10% mole fraction ionic liquid electrolytes exhibit camel shaped capacitance profiles with two peaks on either side of a minimum centered at the potential of zero charge. This profile shape results from electric field induced rearrangement of ion structure within the inner layer closest to the electrode interface. At low potential, the ionic liquid inner layer is concentrated with non-polar tri-fluoromethyl and butyl functional groups of the anions and cations, corresponding to the minimum of the capacitance profiles. With increasing voltage, electrostatic interactions of polar/charged functional groups with the electrode surface compete with these non-polar interactions, leading to ion rearrangement that increases the inner layer charge density and results in higher capacitance. After the ion restructuring is complete, the response saturates and capacitance diminishes. The presence of organic solvent significantly changes the composition of the inner layer. For example, strong non-polar interactions between dichloroethane molecules and the graphene surface substantially block ion/electrode contact at moderate potentials. Overall, our simulations highlight the dynamic nature of the inner region of organic electrolyte double layers, and the sensitive dependence on electrolyte composition and applied voltage.
Abstract Efficient direct air capture (DAC) of CO 2 will require strategies to deal with the relatively low concentration in the atmosphere. One such strategy is to employ the combination of a CO 2 ‐selective membrane coupled with a CO 2 capture solvent acting as a draw solution. Here, the interactions between a leading water‐lean carbon‐capture solvent, a polyether ether ketone (PEEK)‐ionene membrane, CO 2 , and combinations were probed using advanced NMR techniques coupled with advanced simulations. We identify the speciation and dynamics of the solvent, membrane, and CO 2 , presenting spectroscopic evidence of CO 2 diffusion through benzylic regions within the PEEK‐ionene membrane, not spaces in the ionic lattice as expected. Our results demonstrate that water‐lean capture solvents provide a thermodynamic and kinetic funnel to draw CO 2 from the air through the membrane and into the bulk solvent, thus enhancing the performance of the membrane. The reaction between the carbon‐capture solvent and CO 2 produces carbamic acid, disrupting interactions between the imidazolium (Im + ) cations and the bistriflimide anions within the PEEK‐ionene membrane, thereby creating structural changes through which CO 2 can diffuse more readily. Consequently, this restructuring results in CO 2 diffusion at the interface that is faster than CO 2 diffusion in the bulk carbon‐capture solvent.
Liquid/liquid (L/L) interfaces play a key, yet poorly understood, role in a range of complex chemical phenomena where time-evolving interfacial structures and transient supramolecular assemblies act as gatekeepers to function. Here, for this study, we employ surface-specific vibrational sum frequency generation combined with neutron and X-ray scattering methods to track the transport of dioctyl phosphoric acid (DOP) and di-(2-ethylhexyl) phosphoric acid (DEHPA) ligands used in solvent extraction at buried oil/aqueous interfaces away from equilibrium. Our results show evidence for a dynamic interfacial restructuring at low ligand concentrations in contrast to expectation. These time-varying interfaces arise from the transport of sparingly soluble interfacial ligands into the neighboring aqueous phase. These results support a proposed “antagonistic” role of ligand complexation in the aqueous phase that could serve as a holdback mechanism in kinetic liquid extractions. These findings provide new insights into interfacially controlled chemical transport at L/L interfaces and how these interfaces vary chemically, structurally, and temporally in a concentration-dependent manner and present potential avenues to design selective kinetic separations.
We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(O i Pr) 3 ) to thermally pretreated silica under solvent-free conditions. VO(O i Pr) 3 is found to react with both site-isolated silanol (Si–OH) groups and strained siloxane (≡Si–O–Si≡) bridges at the silica surface. Solid-state 51 V and 13 C MAS NMR confirms the formation of two slightly different vanadium species associated with the two anchoring mechanisms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ Raman spectroscopy, and thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) were used to study the subsequent calcination, revealing the formation of a transient V—OH intermediate upon the release of propene, followed by the formation of isolated VO 4 surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and 51 V MAS NMR of the calcined material confirm the conversion of the two original vanadium sites to a species with a single isotropic shift, confirming the formation of isolated, tetrahedral VO 4 sites.
Tetrathiafulvalene (TTF) crystals grown from the melt are organized as spherulites in which helicoidal fibrils growing radially from the nucleation center twist in concert with one another. Alternating bright and dark concentric bands are apparent when films are viewed between crossed polarizers, indicating an alternating pattern of crystallographic faces exposed at the film surface. Band-dependent reorganization of the TTF crystals was observed during exposure to methanol vapor. Crystalline growth appears on bright bands at the expense of the dark bands. After a 24 h period of exposure to methanol vapor, the original spherulites were completely restructured, and the films comprise isolated, concentric circles of crystallites whose orientations are determined by the initial TTF crystal fibril orientation. While the surface of these outgrowths appears faceted and smooth, cross-sectional SEM images revealed a semiporous inner structure, suggesting solvent-vapor-induced recrystallization. Collectively, these results show that crystal twisting can be used to rhythmically redistribute material. Crystal twisting is a common and often controllable phenomenon independent of molecular or crystal structure and therefore offers a generalizable path to spontaneous pattern formation in a wide range of materials.
Abstract not provided
Green organic solvents such as ionic liquids (ILs) have versatile use yet are inhibitory to microbes even at low concentrations of 0.5–1.0% (v/v) ILs. We discovered the oleaginous yeast Yarrowia lipolytica can grow in 10% (v/v) of 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), which makes it more tolerant than most engineered microorganisms and naturally screened isolates. However, the underlying mechanism of IL tolerance in Y. lipolytica is not understood. Through adaptive laboratory evolution, in combination with physiological characterization and omics analysis, we shed light on the underlying mechanism of how Y. lipolytica restructures its membrane to tolerate different types of ILs at high levels up to 18% ILs. Particularly, we discovered that sterols play a key role for exceptional IL tolerance in Y. lipolytica .
In this work, solutions of room temperature ionic liquids (RTILs) and water were studied by observing the reorientational dynamics of the fluorescent probe perylene. Perylene is solvated in the alkyl regions of the RTILs. Its D 2h symmetry made it possible to extract dynamical information on both in-plane and out-of-plane reorientation from time resolved fluorescence anisotropy measurements. Perylene reorientation reports on its interactions with the alkyl chains. The RTILs were a series of 1-alkyl-3-methylimidazolium tetrafluoroborates (C n mimBF 4 , where n is the number of carbons in the alkyl chain), and the effects on perylene’s dynamics were observed when varying the alkyl chain length of the cation (n = 4, 6, 8, and 10; butyl, hexyl, octyl, decyl) and varying the water content from pure RTIL to roughly three water molecules per RTIL ion pair. Time correlated single photon counting was used to measure the fluorescence anisotropy decays to determine the orientational dynamics. The friction coefficients for both the in-plane and out-of-plane reorientation were determined to eliminate the influence of changes in viscosity caused by both the addition of water and the different alkyl chain lengths. The friction coefficients provided information on the interactions of the perylene with it alkyl environment and how these interactions changed with chain length and water content. As chain length increased, the addition of water had less of an effect on the local alkyl environment surrounding the perylene. The friction coefficients generally increased with higher water contents; the inplane orientational motion was hindered significantly more than the out-of-plane motion. The restructuring of the alkyl regions is likely a consequence of a rearrangement of the ionic imidazolium head groups to accommodate partial solvation by water, which results in a change in the arrangement of the alkyl chains. At very high water content, BmimBF4 broke this general trend, with both in-plane and out-of-plane rotational friction decreasing above a water content of one water per ion pair. This decrease indicates a major reorganization of the overall liquid structure in high water content mixtures. In contrast to BmimBF 4 , the longer chain length RTILs are not infinitely miscible with water, and do not show evidence of a major reorganization before reaching saturation and phase-separating. The results suggest that phase separation in longer chain length BF 4 RTILs is a consequence of their inability to undergo the reorganization of the alkyl regions necessary to accommodate high water concentrations.