Taku Iiyama

Study of the structure of a water molecular assembly in a hydrophobic nanospace at low temperature with in situ X-ray diffraction

Abstract The X-ray diffraction patterns of water molecules adsorbed in two kinds of slit-shaped graphitic micropores (micropore width: 0.75 and 1.13 nm) were measured from 143 to 303 K, and analyzed by an electron radial distribution function analysis. The formation of an ordered assembly structure of water molecules in the graphitic micropores at 303 K was evidenced. The structure of the water molecules in the 0.75 nm pore was more ordered compared with that of the 1.13 nm pore, although the water molecules became less mobile with lowering temperature. The water molecular assembly in the graphitic micropores had no clear phase transition point.

Melting temperature elevation of benzene confined in graphitic micropores

Abstract Melting and freezing behaviors of benzene confined in graphitic micropores of different pore widths were examined by the use of differential scanning calorimetry (DSC) over the temperature range of 180–320 K. A single diffuse DSC peak due to melting was observed for confined benzene above the melting temperature of bulk benzene. The melting temperature of confined benzene was in the range of 294–298 K, which is higher than that of bulk benzene by 16–20 K. The observed enthalpy of fusion of confined benzene was in the range of 4–5 J mol −1 , which is much smaller than that of bulk benzene (10.7 kJ mol −1 ).

Cluster-Mediated Water Adsorption on Carbon Nanopores

The adsorption isotherms of water at 303 K and N2 at 77 K on various kinds of porous carbons were compared with each other. The saturated amounts of water adsorbed on carbons almost coincided with amounts of N2 adsorption in micropores. Although carbon aerogel samples have mesopores of the great pore volume, the saturated amount of adsorbed water was close to the micropore volume which is much small than the mesopore volume. These adsorption data on carbon aerogels indicated that the water molecules are not adsorbed in mesopores, but in micropores only. The adsorption isotherms of water on activated carbon having micropores of smaller than 0.7 nm in width had no clear adsorption hysteresis, while the water adsorption isotherms on micropores of greater than 0.7 nm had a remarkable adsorption hysteresis above P/P0 = 0.5. The disappearance of the clear hysteresis for smaller micropores suggested that the cluster of water molecules of about 0.7 nm in size gives rise to the water adsorption on the hydrophobic micropores; the formation and the structure of clusters of water molecules were associated with the adsorption mechanism. The cluster-mediated pore filling mechanism was proposed with a special relevance to the evidence on the formation of the ordered water molecular assembly in the carbon micropores by in situ X-ray diffraction.

Structural mechanism of water adsorption in hydrophobic micropores from in situ small angle X-ray scattering

The in situ small-angle X-ray scattering (SAXS) of water adsorbed on pitch-based activated carbon fibers (ACF) of pore width wa 0:8 and 1.1 nm were measured at 303 K and diAerent relative pressures. The SAXS spectrum of ACF of wa 1:1 nm in the direction of desorption did not overlap that in the adsorption direction, showing hysteresis. The density fluctuation data from Ornstein‐Zernike (OZ) analysis for the SAXS profiles indicated that water molecules form great clusters in the course of adsorption, but desorption proceeds through uniform molecular evaporation. ” 2000 Elsevier Science B.V. All rights reserved.

Confinement in Carbon Nanospace-Induced Production of KI Nanocrystals of High-Pressure Phase

An outstanding compression function for materials preparation exhibited by nanospaces of single-walled carbon nanohorns (SWCNHs) was studied using the B1-to-B2 solid phase transition of KI crystals at 1.9 GPa. High-resolution transmission electron microscopy and synchrotron X-ray diffraction examinations provided evidence that KI nanocrystals doped in the nanotube spaces of SWCNHs at pressures below 0.1 MPa had the super-high-pressure B2 phase structure, which is induced at pressures above 1.9 GPa in bulk KI crystals. This finding of the supercompression function of the carbon nanotubular spaces can lead to the development of a new compression-free route to precious materials whose syntheses require the application of high pressure.

Effect of a quaternary ammonium salt on propylene carbonate structure in slit-shape carbon nanopores.

The effect of addition of tetraethylammonium tetrafluoroborate (Et(4)NBF(4)) on the structure of propylene carbonate (PC) confined in slit-shaped carbon nanopores of activated carbon fiber (pore width = 1.0 nm) was studied by synchrotron X-ray diffraction and reverse Monte Carlo simulation. PC molecules are randomly packed in the slit carbon nanopores of 1 nm in the absence of Et(4)NBF(4). Addition of Et(4)N(+) and BF(4)(-) ions promotes formation of considerably ordered double layers of PC molecules even in the highly restricted slit pore space. PC molecules can accept these ions efficiently. This structural modulation function of PC molecular assemblies should contribute to the evolution of supercapacitance in carbon nanopores.

Mechanism of freezing of water in contact with mesoporous silicas MCM-41, SBA-15 and SBA-16: role of boundary water of pore outlets in freezing

The freezing mechanism of water contacted with mesoporous silicas with uniform pore shapes, both cylindrical and cagelike, was studied by thermodynamic and structural analyses with differential scanning calorimetry (DSC) and X-ray diffraction (XRD) together with adsorption measurements. In the DSC data extra exothermic peaks were found at around 230 K for water confined in SBA-15, in addition to that due to the freezing of pore water. These peaks are most likely to be ascribed to the freezing of water present over the micropore and/or mesopore outlets of coronas in SBA-15. Freezing of water confined in SBA-16 was systematically analysed by DSC with changing the pore size. The freezing temperature was found to be around 232 K, close to the homogeneous nucleation temperature of bulk water, independent of the pore size when the pore diameter (d) < 7.0 nm. Water confined in the cagelike pores of SBA-16 is probably surrounded by a water layer (boundary water) at the outlets of channels to interconnect the pores and of fine corona-like pores, which is similar to that present at the outlet of cylindrical pores in MCM-41 and of cylindrical channels in SBA-15. The presence of the boundary water would be a key for water in SBA-16 to freeze at the homogeneous nucleation temperature. This phenomenon is similar to those well known for water droplets in oil and water droplets of clouds in the sky. The XRD data showed that the cubic ice I(c) was formed in SBA-16 as previously found in SBA-15 when d < 8.0 nm.

Organized structures of methanol in carbon nanospaces at 303 K studies with in situ X-ray diffraction

Abstract The X-ray diffraction of methanol molecules confined in carbon micropores having different pore widths (micropore widths, 0.7 and 1.1 nm) was measured at 303 K and the structural results were compared with those of ethanol. The methanol molecules confined in micropores of 0.7 nm have a more solid-like ordered structure than in 1.1 nm. The nearest-neighbor peak of the electron radial distribution function of confined methanol did not shift from that of bulk methanol, but the amplitude was much greater than that of bulk methanol. These results were different from those of confined ethanol. Methanol molecules confined in carbon micropores have a rigid organized structure even at 303 K. However, no remarkable orientation effect, as in confined ethanol, was observed.

Partial breaking of the Coulombic ordering of ionic liquids confined in carbon nanopores

Ionic liquids are composed of equal quantities of positive and negative ions. In the bulk, electrical neutrality occurs in these liquids due to Coulombic ordering, in which ion shells of alternating charge form around a central ion. Their structure under confinement is far less well understood. This hinders the widespread application of ionic liquids in technological applications. Here we use scattering experiments to resolve the structure of the widely used ionic liquid (EMI-TFSI) when it is confined inside nanoporous carbons. We show that Coulombic ordering reduces when the pores can only accommodate a single layer of ions. Instead, equally-charged ion pairs are formed due to the induction of an electric potential of opposite sign in the carbon pore walls. This non-Coulombic ordering is further enhanced in the presence of an applied external electric potential. This finding opens the door for the design of better materials for electrochemical applications.

Organoclays in Water Cause Expansion That Facilitates Caffeine Adsorption

This study investigates the adsorption of caffeine in water on organically modified clays (a natural montmorillonite and synthetic saponite, which are smectite group of layered clay minerals). The organoclays were prepared by cation-exchange reactions of benzylammonium and neostigmine with interlayer exchangeable cations in the clay minerals. Although less caffeine was uptaken on neostigmine-modified clays than on raw clay minerals, uptake was increased by adding benzylammonium to the clays. The adsorption equilibrium constant was considerably higher on benzylammonium-modified saponite (containing small quantities of intercalated benzylammonium) than on its montmorillonite counterpart. These observations suggest that decreasing the size and number of intercalated cations enlarges the siloxane surface area available for caffeine adsorption. When the benzylammonium-smectite powders were immersed in water, the intercalated water molecules expanded the interlayer space. Addition of caffeine to the aqueous dispersion further expanded the benzylammonium-montmorillonite system but showed no effect on benzylammonium-saponite. We assume that intercalated water molecules were exchanged with caffeine molecules. By intercalating benzylammonium into smectites, we have potentially created an adaptable two-dimensional nanospace that sequesters caffeine from aqueous media.