Bending elastic properties of a block copolymer-rich lamellar phase doped by a surfactant: a neutron spin-echo study.

The influence of the short alkyl-chain ionic surfactant OTAB on the dynamic behavior of an inverse block copolymer-rich lamellar phase was investigated by neutron spin-echo spectroscopy (NSE). The observed intermediate scattering function can be described by a sum of two contributions. For high scattering vectors the model of Zilman-Granek plus a slow diffusional mode can be used to describe the experimental data and the bending elastic modulus κ for a polymer-rich membrane is calculated. At low scattering vectors the relaxation curves are strongly influenced by de Gennes narrowing arising from the structure factor of the Lα phase. Hence, the computed relaxation rates in this q-range are inversely proportional to the static structure factor. The present study demonstrates the necessity to directly investigate the dynamic behavior of lamellar phases and that an analysis of the width of the Bragg peaks can be insufficient to derive information about the single membrane elasticity, especially when both κ and B[combining macron] depend on the composition of the membrane.

Repeated sorption of water in SBA-15 investigated by means of in situ small-angle x-ray scattering.

The effect of repeated cycles of water adsorption/desorption on the structural stability of ordered mesoporous silica SBA-15 is studied by small-angle x-ray scattering (SAXS). In situ sorption measurements are conducted using a custom-built sorption apparatus in connection with a laboratory SAXS setup. Two striking irreversible changes are observed in the sorption isotherms as derived from the integrated SAXS intensity. First, the capillary condensation pressure shifts progressively to lower relative pressure values with increasing number of sorption cycles. This effect is attributed to chemisorption of water at the silica walls, resulting in a change of the fluid-wall interaction. Second, the sorption cycles do not close completely at vanishing vapour pressure, suggesting that progressively more water remains trapped within the porous material after each cycle. This effect is interpreted to be the result of an irreversible collapse of parts of mesopores, originating from pore wall deformation due to the large Laplace pressure of water acting on the pore walls at capillary condensation and capillary evaporation.

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.

Carboxylic acid-doped SBA-15 silica as a host for metallo-supramolecular coordination polymers.

The adsorption of a metallo-supramolecular coordination polymer (Fe-MEPE) in the cylindrical pores of SBA-15 silica with pure and carboxylic acid (CA) carrying pore walls has been studied. Fe-MEPE is an intrinsically stiff polycation formed by complexation of Fe(II)-acetate with an uncharged ditopic bis-terpyridine ligand. The adsorption affinity and kinetics of the Fe-MEPE chains is strongly enhanced when the pore walls are doped with CA, and when the pH of the aqueous medium or temperature is increased. The initial fast uptake is connected with a decrease of pH of the aqueous solution, indicating an ion-exchange mechanism. It is followed by a slower (presumably diffusion-controlled) further uptake. The maximum adsorbed amount of Fe-MEPE in the CA-doped material corresponds to a monolayer of Fe-MEPE chains disposed side-by-side along the pore walls. The stoichiometry of Fe-MEPE in the pores (determined by XPS) was found to be independent of the loading and similar to that of the starting material. The mean chain length of Fe-MEPE before and after embedding in the CA-doped matrix was studied by solid-state 15N NMR using partially 15N-labeled Fe-MEPE. It is shown that the average chain length of Fe-MEPE is reduced when the complex is incorporated in the pores.

Structural characterization of water and ice in mesoporous SBA-15 silicas: II. The 'almost-filled' case for 86 Å pore diameter.

Neutron diffraction measurements for D(2)O in SBA-15 silica of pore diameter 86 Å have been made in a temperature range from 300 to 100 K. The pore-filling factor for the liquid phase is 0.95, resulting in an 'almost-filled' sample. The nucleation and transformation of the ice phase were determined for cooling and warming cycles at two different rates. The primary nucleation event at 258 K leads to a defective form of ice-I with predominantly cubic ice features. For temperatures below the main nucleation event, the data indicate the formation of an interfacial layer of disordered water/ice that varies with temperature and is reversible. The main diffraction peak for the water phase shows similar features to those observed in earlier studies, indicating enhanced hydrogen bonding and network correlations for the confined phase as the temperature is decreased. A detailed profile analysis of the triplet peak is presented in the accompanying paper (Seyed-Yazdi et al 2008 J. Phys.: Condens. Matter 20 205108).

Structural characterization of water/ice formation in SBA-15 silicas: III. The triplet profile for 86 Å pore diameter.

The diffraction results for the formation of ice in 86 Å diameter pores of a SBA-15 silica sample are analysed to provide information on the characteristics of the ice created in the pores. The asymmetric triplet at ∼1.7 Å(-1), which involves several overlapping peaks, is particularly relevant to the different ice phases and contains a number of components that can be individually identified. The use of a set of three peaks with an asymmetric profile to represent the possibility of facetted growth in the pores was found to give an unsatisfactory fit to the data. The alternative method involving the introduction of additional peaks with a normal symmetric profile was found to give excellent fits with five components and was the preferred analytic procedure. Three peaks could be directly linked to the positions for the triplet of hexagonal ice, I(h), and one of the other two broad peaks could be associated with a form of amorphous ice. The variation of the peak intensity (and position) was systematic with temperature for both cooling and heating runs. The results indicate that a disordered state of ice is formed as a component with the defective crystalline ices. The position of a broad diffraction peak is intermediate between that of high-density and low-density amorphous ice. The remaining component peak is less broad but does not relate directly to any of the known ice phases and cannot be assigned to any specific structural feature at the present time.

2H-solid state NMR and DSC study of isobutyric acid in mesoporous silica materials.

Solid state deuterium NMR has been used to study the molecular motion of d(6)-isobutyric acid (d(6)-iBA) in the pure (unconfined) state and confined in the cylindrical pores of two periodic mesoporous silica materials (MCM-41, pore size 3.3 nm and SBA-15, pore size 8 nm), and in a controlled pore glass (CPG-10-75, pore size ca. 10 nm). The line shape analysis of the spectra at different temperatures revealed three rotational states of the iBA molecules: liquid (fast anisotropic reorientation of the molecule), solid I (rotation of the methyl group) and solid II (no rotational motion on the time scale of the experiment). Transition temperatures between these states were determined from the temperature dependence of the fraction of molecules in these states. Whereas the solid I-solid II transition temperature is not affected by confinement, a significant lowering of the liquid-solid I transition temperature in the pores relative to the bulk acid was found for the three matrix materials, exhibiting an unusual dependence on pore size and pore morphology. Complementary DSC measurements on the same systems show that the rotational melting (solid I-liquid) of d(6)-iBA in the pores occurs at a temperature 20-45 K below the thermodynamic melting point. This finding indicated that the decoupling of rotational and translational degrees of freedom in phase transitions in confined systems previously found for benzene is not restricted to molecules with non-specific interactions, but represents a more general phenomenon.

Adsorption isotherms of nonionic surfactants in SBA-15 measured by micro-column chromatography.

In this work, micro-column liquid chromatography has been employed for the study of the adsorption of ethylene glucol monoether (C8E4) and pentaethylene glucol monoether (C10E5) nonionic surfactants into the nanopores of SBA-15 silica at two temperatures. The adsorption process for both the surfactants has been investigated in the range of concentration from very dilute solution to "just above" critical micelle concentration (CMC). The adsorption data for both the surfactants are characterized as typical LC-shaped isotherms with plateau near the CMC. A simple two-step adsorption model has been applied to represent an experimental data. An attempt to estimate the thickness of the surfactant layer in the pores, from the experimental data, has been made.

Evidence of microphase separation in controlled pore glasses.

The phase separation of a mixture of water and isobutyric acid (iBA) confined in the pore space of Controlled Pore Glass (CPG) 10-75 has been studied by 1H NMR relaxometry and 1H-pulsed field gradient (PFG) diffusion measurements. For an acid-rich mixture (mass fraction 54 wt% iBA), evidence of a phase separation process in the pores was obtained, which occurs in a temperature window between 32 and 39 degrees C, as indicated in the PFG data by an anomalous temperature dependence of the diffusion coefficient and in the relaxation data by a bi-exponential magnetization decay. The phase separation temperature of the mixture in the pore is slightly lower than in the bulk mixture of the same composition (41 degrees C) and extends over a finite temperature range. A qualitative model of the phase separation process in the pores is developed, which assumes a temperature-dependent domain-like structure of the liquid below the phase transition temperature and a breakdown of these domains upon reaching the transition temperature.

De-mixing dynamics of a binary liquid system in a controlled-pore glass. A neutron spin-echo spectroscopy and small angle neutron scattering study.

The temperature-induced microphase separation of the binary liquid system iso-butyric acid+heavy water (iBA + D(2)O) in a mesoporous silica glass (CPG-10-75) of nominal pore width 7.5 nm was investigated by neutron spin-echo spectroscopy (NSE) and neutron small-angle scattering (SANS). Two mixtures of different composition were studied at different scattering angles at temperatures above and below the bulk phase transition temperature. The phase separation in the pore space is found to occur at a lower temperature than the bulk transition and extends over a significant temperature range. The effective diffusion coefficient derived from NSE at low scattering angles is found to decrease by one order of magnitude from 70 degrees C to 20 degrees C. This observation is attributed to the growing size of concentration fluctuations having a cut-off at ca. 8 nm, which corresponds to the mean pore size. The dynamics of the concentration fluctuations appears to be strongly influenced by the confinement in the pores, as it differs strongly from the bulk behaviour. These results are consistent with the preliminary results of the SANS study.

X-ray reflectivity study of smectic wetting and prewetting at the free surface of isotropic liquid crystals.

We study the structures of free-surface-wetting layers above the isotropic to smectic-A transition of three liquid-crystal compounds that show different kinds of growth of the wetting film as the bulk transition is approached: layer-by-layer, continuous, and continuous with prewetting. The smectic-A surface phase of the layer-by-layer compound consists of well-defined layers and possesses a sharp boundary to the isotropic bulk phase, whereas in the two continuous compounds sinusoidal density oscillations with a continuously decaying amplitude are found. In the continuous case with prewetting, the wetting film below the prewetting transition does not show an essential difference to the continuous case without prewetting.