Density measurement of 1-d confined water by small angle neutron scattering method: pore size and hydration level dependences

Small angle neutron scattering (SANS) is used to measure the absolute density of water contained in 1-D cylindrical pores of a silica material MCM-41-S with pore diameters of 19 and 15 A. By being able to suppress the homogeneous nucleation process inside the narrow pore, one can keep water in the liquid state down to at least 160 K. From a combined analysis of SANS data from both H(2)O and D(2)O hydrated samples, we determined the absolute value of the density of 1-D confined water. We found that the average density of water inside the fully hydrated 19 A pore is 8% higher than that of the bulk water at room temperature. The temperature derivative of the density shows a pronounced peak at T(L) = 235 K signaling the crossing of the Widom line at ambient pressure and confirming the existence of a liquid-liquid phase transition at an elevated pressure. Pore size and hydration level dependences of the density are also studied.     

Investigation of confined ionic liquid in nanostructured materials by a combination of SANS, contrast-matching SANS, and nitrogen adsorption

Small-angle neutron scattering (SANS), contrast-matching SANS, and nitrogen adsorption have been utilized to investigate the confined ionic liquid (IL) [bmim][PF(6)] phase in ordered mesoporous silica MCM-41 and SBA-15. The results suggest that the pores of SBA-15 are completely filled with IL whereas a small fraction of the pore volume, the pore "core", of MCM-41 is empty. The contrast-matching SANS measurements confirm the enhanced solubility of water in IL. In addition, they provide strong evidence that water does not enter the empty pore core of MCM-41, possibly because of the preferred orientation of the IL molecules in the adsorbed layer.     

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.     

Characterisation of porous alumina membrane by adsorption in conjunction with SANS

An alumina membrane was studied by water adsorption in conjunction with small-angle neutron scattering (SANS). The SANS data were fitted according to a polydisperse homogeneous permeable sphere model. Pore size distributions and radial distribution functions were calculated for pores as small as 19 Å too. The lower limit of the pore size distribution is in agreement with the predictions of Kelvin equation.     

Acrylamide-based magnetic nanosponges: a new smart nanocomposite material

Nanocomposite materials consisting of CoFe2O4 magnetic nanoparticles and a polyethylene glycol-acrylamide gel matrix have been synthesized. The structure of such materials was studied by means of small-angle scattering of X-rays and polarized neutrons, showing that the CoFe2O4 nanoparticles were successfully and homogeneously embedded in the gel structure. Magnetic, viscoelastic, and water retention properties of the nanocomposite gel confirm that the properties of both nanoparticles and gel are combined in the resulting nanomagnetic gel. Scanning electron microscopy highlights the nanocomposite nature of the material, showing the presence of a gel structure with different pore size distributions (pores with micron and nano-size distributions) that can be used as active sponge-like nanomagnetic container for water-based formulations as oil-in-water microemulsions.     

Thermal properties and vibrational density of states of a nanoconfined discotic liquid crystal

Neutron scattering is employed to investigate the vibrational density of states (VDOS) of the discotic liquid crystal 2,3,6,7,10,11-hexakis[hexyloxy] triphenylene (HAT6) confined to the pores of alumina oxide membranes with different pore sizes. Additionally, the phase transitions were studied by differential scanning calorimetry. The transitions were observed down to the smallest pore size. The decrease of the transition enthalpies versus inverse pore size for both transitions implies an increase of the amount of disordered amorphous material. By extrapolation of its pore size dependence, a critical pore diameter for structure formation of 17 nm is estimated. Similar to the bulk, excess contributions to the VDOS (Boson peak) are also observed for confined HAT6. The Boson peak gains in intensity and shifts to lower frequencies with decreasing pore diameter. This is discussed in the framework of a softening of HAT6 induced by the confinement due to a less-developed plastic crystalline state inside the pores compared to the bulk.     

Effect of heat treatment on pore structure in nano-crystalline NiO: A small angle neutron scattering study

Nano-crystalline nickel oxide powder was synthesized by a precipitation route. Powder samples were heat treated at 300, 600 and 900 °C, and pore structure evolution was followed by small angle neutron scattering (SANS) technique. SANS measurements were carried out also on pelleted samples in order to study the modifications of pore morphology due to heat treatment. SANS data reveal scattering from pores at two different length scales. The pore structure at various heat treatment temperatures does not follow any scaling behavior.     

Structural characterization of surfactant aggregates adsorbed in cylindrical silica nanopores

The self-assembly of nonionic surfactants in the cylindrical pores of SBA-15 silica with a pore diameter of 8 nm was studied by small-angle neutron scattering (SANS) at different solvent contrasts. The alkyl ethoxylate surfactants C(10)E(5) and C(12)E(5) exhibit strong aggregative adsorption in the pores as indicated by the sigmoidal shape of the adsorption isotherms. The SANS intensity profiles can be represented by a sum of two terms, one accounting for diffuse scattering from surfactant aggregates in the pores and the other for Bragg scattering from the pore lattice of the silica matrix. The Bragg reflections are analyzed with a form factor model in which the radial density profile of the surfactant in the pore is approximated by a two-step function. Diffuse scattering is represented by a Teubner-Strey-type scattering function which indicates a preferred distance between adsorbed surface aggregates in the pores. Our results suggest that adsorption starts with formation of discrete surface aggregates which increase in number and eventually merge to interconnected patches as the plateau value of the adsorption isotherm is approached. A grossly different behavior, viz. formation of micelles as in solution, is found for the maltoside surfactant C(10)G(2), in agreement with the observed weak adsorption of this surfactant in SBA-15.     

Detailed structure of hairy mixed micelles formed by phosphatidylcholine and PEGylated phospholipids in aqueous media

Aqueous dispersions of mixed egg yolk phosphatidylcholine (PC) and poly(ethylene glycol) (PEG) modified distearoyl phosphatidylethanolamine (DSPE) were investigated with the purpose of determining shape, size, and conformation of the formed mixed micelles. The samples were prepared at a range of DSPEPEG to PC molar ratios ([DSPEPEG/PC] from 100:0 to 30:70) and with, respectively, DSPEPEG2000 and DSPEPEG5000, where 2000 and 5000 refer to the molar masses of the PEG chains. Particle shape and internal structure were studied using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The contrast of the micelles is different for X-rays and neutrons, and by combining SANS and SAXS, complementary information about the micelle structure was obtained. The detailed structure of the micelles was determined in a self-consistent way by fitting a model for the micelles to SANS and SAXS data simultaneously. In general, a model for the micelles with a hydrophobic core, surrounded by a dense hydrophilic layer that is again surrounded by a corona of PEG chains in the form of Gaussian random coils attached to the outer surface, is in good agreement with the scattering data. At high DSPEPEG contents, nearly spherical micelles are formed. As the PC content increases the micelles elongate, and at a DSPEPEG/PC ratio of 30:70, rodlike micelles longer than 1000 angstroms are formed. We demonstrate that by mixing DSPEPEG and PC a considerable latitude in controlling the particle shape is obtained. Our results indicate that the PEG chains in the corona are in a relatively unperturbed Gaussian random coil conformation even though the chains are far above the coil-coil overlap concentration and, therefore, interpenetrating. This observation in combination with the observed growth behavior questions that the "mushroom-brush"transition is the single dominating factor for determining the particle shape as assumed in previous theoretical work (Hristova, K.; Needham, D. Macromolecules 1995, 28, 991-1002).     

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.     

Structural investigation of the confinement of finite amounts of trehalose in water-containing sodium Bis(2-ethylhexyl)sulfosuccinate reversed micelles

The structural effect of trehalose confined in water-containing sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles at water to AOT molar ratio W = 5 and 10 as a function of the trehalose to AOT molar ratio T (0 < T < 0.1) has been investigated by small-angle neutron scattering (SANS). SANS data analysis is consistent with the hypothesis that trehalose is encapsulated within the quite spherical hydrophilic micellar cores of water-containing reversed micelles, causing an increase of the aggregate size and a decrease of the polydispersion. Moreover, SANS results suggest that the trehalose confinement in water-containing reversed micelles involves marked changes on the molecular packing of the water-containing micellar cores. In particular, according to the obtained findings, we can hypothesize the intercalation of the trehalose molecules between the polar surfactant headgroups. The preferential solubilization in this specific nanodomain could explain the trehalose capability to prevent, upon dehydration, the transition to a gel phase, hindering serious damage to biostructures.     

Neutron scattering study on dynamics of water molecules in MCM-41. 2. Determination of translational diffusion coefficient

Quasielastic neutron scattering (QENS) spectra of water-filled MCM-41 samples (pore diameters: 21.4 and 28.4 Angstrom) were measured over the temperature range 238-298 K and the momentum transfer range 0.31-0.99 A(-1) to investigate the dynamics of confined water molecules. The spectra, which consist mainly of contributions from the translational diffusion of water molecules, were analyzed by using the Lorentzian and the stretched exponential functions. Comparison of the fits indicated that the latter analysis is more reliable than the former one. The fraction of immobile water molecules located in the vicinity of the pore walls, which give an elastic component, was found to be 0.044-0.061 in both pores. The stretch exponent beta was determined as 0.66-0.80. It was shown that the translational diffusion of water molecules in the pores is decelerated by confinement and that the deceleration becomes marked with a decrease in pore size. The ratios of the translational diffusion coefficient D(T) of confined water to that of bulk water at room temperature were within a range of 0.47-0.63.     

Structural features of molecular-colloidal solutions of C60 fullerenes in water by small-angle neutron scattering

Highly stable and reproducible molecular-colloidal water solutions of C60 fullerenes (FWS) obtained by transferring fullerenes from an organic solution into an aqueous phase with the help of ultrasonic treatment are investigated by means of small-angle neutron scattering (SANS). A polydispersity in the size of detected particles up to 84 nm is revealed. These particles are slightly anisotropic and have a characteristic size of approximately 70 nm. Along with it, there are some indications that a significant part of fullerenes composes particles with the size of the order of 1 nm. The contrast variation based on mixtures of light and heavy water shows that the mean scattering length density of the particles is close to that of the packed fullerene associates as well as that the characteristic size of possible fluctuations of the scattering length density within the particles does not exceed 2 nm. A smooth surface resulting in the Porod law for the scattering is detected. A number of models discussed in the literature are considered with respect to the SANS data.     

Three-dimensional confinement-related size changes to mixed-surfactant vesicles

The effect of three-dimensional confinement on the size and morphology of a vesicular surfactant mesophase obtained by mixing micellar solutions of cetyltrimethylammonium bromide and dodecylbenzenesulfonic acid has been studied using small-angle neutron scattering (SANS). The confined spaces were generated by the random close packing of polystyrene beads of radius Rb=1.5, 0.25, and 0.1 microm, creating voids of characteristic dimensions R approximately 0.22 Rb=3300, 550, and 220 A, respectively. These void length scales were comparable to or less than the radii of vesicles formed in the system under conditions of no confinement. Vesicles, made by mixing 0.8 wt % micellar solutions of surfactant in a water/D2O mixture that is contrast-matched with the polystyrene beads, were added in a SANS scattering cell without beads, as well as three cells with the different sized beads. The SANS data from the sample without confinement was best fitted by a core-shell model and not by spheres or disks, confirming the presence of vesicles. The data from samples in the confined domains also showed vesicles as the dominant structure. The most important result is that the mean size of these vesicles decreases as the confinement length scale is reduced. A simple thermodynamic model accounting for the balance between increased enthalpy when vesicles with curvature higher than the preferred one are formed, and increased free volume entropy for smaller vesicles supports the experimental data. While these results are focused on a specific vesicle system, the broad principles behind changes in microstructure produced by confinement are applicable to other surfactant aggregates. The results of this study are potentially important for understanding the flow of drug delivery vehicles through microcapillaries, in the recovery of oil from fine pores in rocks using surfactant containing fluids, micellar enhanced ultrafiltration, or in other situations where the size of surfactant aggregate structures approach the length scales between confining walls.     

Hydrogen confinement in carbon nanopores: extreme densification at ambient temperature

In-situ small-angle neutron scattering studies of H(2) confined in small pores of polyfurfuryl alcohol-derived activated carbon at room temperature have provided for the first time its phase behavior in equilibrium with external H(2) at pressures up to 200 bar. The data were used to evaluate the density of the adsorbed fluid, which appears to be a function of both pore size and pressure and is comparable to the density of liquid H(2) in narrow nanopores at ～200 bar. The surface-molecule interactions responsible for densification of H(2) within the pores create internal pressures that exceed the external gas pressure by a factor of up to ～50, confirming the benefits of adsorptive storage over compressive storage. These results can be used to guide the development of new carbon adsorbents tailored for maximum H(2) storage capacities at near-ambient temperatures.     

Melting and freezing of water in cylindrical silica nanopores

Freezing and melting of H(2)O and D(2)O in the cylindrical pores of well-characterized MCM-41 silica materials (pore diameters from 2.5 to 4.4 nm) was studied by differential scanning calorimetry (DSC) and (1)H NMR cryoporometry. Well-resolved DSC melting and freezing peaks were obtained for pore diameters down to 3.0 nm, but not in 2.5 nm pores. The pore size dependence of the melting point depression DeltaT(m) can be represented by the Gibbs-Thomson equation when the existence of a layer of nonfreezing water at the pore walls is taken into account. The DSC measurements also show that the hysteresis connected with the phase transition, and the melting enthalpy of water in the pores, both vanish near a pore diameter D* approximately equal to 2.8 nm. It is concluded that D* represents a lower limit for first-order melting/freezing in the pores. The NMR spin echo measurements show that a transition from low to high mobility of water molecules takes place in all MCM-41 materials, including the one with 2.5 nm pores, but the transition revealed by NMR occurs at a higher temperature than indicated by the DSC melting peaks. The disagreement between the NMR and DSC transition temperatures becomes more pronounced as the pore size decreases. This is attributed to the fact that with decreasing pore size an increasing fraction of the water molecules is situated in the first and second molecular layers next to the pore wall, and these molecules have slower dynamics than the molecules in the core of the pore.     

Local structure of a phase-separating binary mixture in a mesoporous glass matrix studied by small-angle neutron scattering

The mesoscopic structure of the binary system isobutyric acid + heavy water (D(2)O) confined in a porous glass (controlled-pore silica glass, mean pore width ca. 10 nm) was studied by small-angle neutron scattering at off-critical compositions in a temperature range above and below the upper critical solution point. The scattering data were analyzed in terms of a structure factor model similar to that proposed by Formisano and Teixeira [Eur. Phys. J. E 1, 1 (2000)], but allowing for both Ornstein-Zernike-type composition fluctuations and domainlike structures in the microphase-separated state of the pore liquid. The results indicate that the phase separation in the pores is shifted by ca. 10 K and spread out in temperature. Microphase separation is pictured as a transition from partial segregation at high temperature, due to the strong preferential adsorption of water at the pore wall, to a tube or capsule configuration of the two phases at low temperatures, depending on the overall composition of the pore liquid. Results for samples in which the composition of the pore liquid can vary with temperature due to equilibration with extra-pore liquid are consistent with this picture.     

Morphology study of a series of azobenzene-containing side-on liquid crystalline triblock copolymers

A series of azobenzene containing side-on liquid crystalline ABA triblock copolymers were investigated.This triblock series possesses the same central liquid crystal block B and various lengths of the amorphous block A.Transmission electron microscopy(TEM),small angle X-rays and neutron scattering(SAXS and SANS) were used to study their morphologies.After annealing the samples over weeks at a temperature within the nematic temperature range of block B, different morphologies(disordered,lamellar,perforated layer and hexagonal cylinder) were observed by TEM.The alignment behavior of these azo triblock copolymers in the magnetic field for artificial muscle application,as well as the phase period and the order-disorder transition(ODT) were studied in situ by SANS.     

Properties of Water Confined in Periodic Mesoporous Organosilicas: Nanoimprinting the Local Structure

The properties of materials confined in porous media are important in scientific and technological aspects. Topology, size, and surface polarity of the pores play a critical role in the confinement effects, however, knowledge regarding the guest–pore interface structure is still lacking. Herein, we show that the molecular mobility of water confined in periodic mesoporous organosilicas (PMOs) is influenced by the polarity of the organic moiety. Multidimensional solid‐state NMR spectroscopy directly probes the spatial arrangement of water inside the pores, showing that water interacts either with only the silicate layer or with both silicate and organic layers depending on the alternating surface polarity. A modulated and a uniform pore filling mode are proposed for different types of PMOs.     

Structure of methanol confined in MCM-41 investigated by large-angle X-ray scattering technique

Large-angle X-ray scattering (LAXS) measurements over a temperature range from 223 to 298 K have been made on methanol confined in mesoporous silica MCM-41 with two different pore diameters, 28 A (C14) and 21 A (C10), under both monolayer and capillary-condensed adsorption conditions. To compare the structure of methanol in the MCM-41 pores with that of bulk methanol, X-ray scattering intensities for bulk methanol in the same temperature range have also been measured. The radial distribution functions (RDFs) for the monolayer methanol samples showed that methanol molecules are strongly hydrogen bonded to the silanol groups on the MCM-41 surface, resulting in no significant change in the structure of adsorbed methanol with respect to the pore size and temperature. On the other hand, the RDFs for the capillary-condensed methanol samples showed that hydrogen-bonded chains of methanol molecules are formed in both pores. However, the distance and number of hydrogen bonds estimated from the RDFs suggested that hydrogen bonds between methanol molecules in the pores are significantly distorted or partly disrupted. It has been found that the hydrogen bonds are more distorted in the smaller pores of MCM-41. With decreasing temperature, however, the hydrogen-bonded chains of methanol in the pores were gradually ordered. A comparison of the present results on methanol in MCM-41 pores with those on water in the same pores revealed that the structural change with temperature is less significant for confined methanol than for confined water.