Gerhard H. Findenegg

Melting and freezing of water in ordered mesoporous silica materials

The melting and freezing of water in a series of mesoporous silica materials with a hexagonal arrangement of unidimensional cylindrical pores and narrow pore-size distribution (MCM-41 with pore diameters from 2.9 to 3.7 nm, and SBA-15 with pore diameters from 4.4 to 11.7 nm) was studied by differential scanning calorimetry (DSC). A lowering of the melting temperature ΔTm = Tmb − Tm(R) up to 50 K was found for water in pores of decreasing radius R. The melting point data can be represented by a modified Gibbs–Thomson equation, ΔTm(R) = K/(R-t), with K = 52 K nm and t = 0.4 nm, in agreement with an earlier study of water in MCM-41 materials of pore width 2 to 4 nm. The value of K agrees with an estimate of the Gibbs–Thomson constant based on thermodynamic data for the normal melting point of ice, and the parameter t can be taken as the thickness of a surface layer of non-frozen water at the pore wall. DSC scans of the freezing of H2O and D2O in partially filled pores of SBA-15 reveal a peak pattern depending on the degree of pore filling ϕ. The different peaks are attributed to different states of the liquid in the pore space, iz. pore water in completely filled regions, and water as an adsorbed film at the pore wall. These two states coexist in a pore filling range ϕmin1) but exhibits substantial supercooling at ϕ<1. On the other hand, the peak attributed to the freezing of film water exhibits no significant supercooling and is found at temperatures near 237 K, almost independent of ϕ and the pore width of the SBA-15 sample. The nature of a further (small) peak near 233 K is not yet understood. Contrary to cooling scans, only one DSC peak is observed in heating scans, independent of the pore filling. This finding indicates that the adsorbed liquid film is metastable relative to the frozen pore liquid.

Pyridine-15N: A mobile NMR sensor for surface acidity and surface defects of mesoporous silica

The hydrogen bond interaction of pyridine with the silanol groups of the inner surfaces of MCM-41 and SBA-15 ordered mesoporous silica has been studied by a combination of solid-state NMR techniques. The pore diameters were varied between 3 and 4 nm for MCM-41 and between 7 and 9 nm for SBA-15. 1 H MAS experiments performed under magic angle spinning (MAS) conditions in the absence and the presence of pyridine-d 5 reveal that the large majority of silanol groups are located in the inner surfaces, isolated from each other but able to form hydrogen bonds with pyridine. On the other hand, low- and room-temperature 1 5 N CPMAS and MAS experiments (CP ≡ cross-polarization) performed on pyridine- 1 5 N show that at low concentrations all pyridine molecules are involved in hydrogen bonds with the surface silanol groups. In the presence of an excess of pyridine, a non-hydrogen-bonded pyridine phase is observed at 120 K in the slow hydrogen bond exchange regime and associates with an inner core phase. From these measurements, the number of pyridine molecules bound to the inner surfaces corresponding to the number of silanol groups could be determined to be n O H 3 nm - 2 for MCM-41 and 3.7 nm - 2 for SBA-15. At room temperature and low concentrations, the pyridine molecules jump rapidly between the hydrogen-bonded sites. In the presence of an excess of pyridine, the hydrogen-bonded binding sites are depleted as compared to low temperatures, leading to smaller apparent numbers n O H . Using a correlation established previously between the 1 5 N and 1 H chemical shifts and the NHO hydrogen bond geometries, as well as with the acidity of the proton donors, the distances in the pyridine-hydroxyl pairs were found to be about r H N = 1.68 A, r O H = 1.01 A, and r O N = 2.69 A. This geometry corresponds in the organic solid state to acids exhibiting in water a pK a of about 4. Roomtemperature 1 5 N experiments on static samples of pyridine- 1 5 N in MCM-41 at low coverage show a residual 1 5 N chemical shift anisotropy, indicating that the jumps of pyridine between different different silanol hydrogen bond sites is accompanied by an anisotropic reorientational diffusion. A quantitative analysis reveals that in this regime the rotation of pyridine around the molecular C 2 axis is suppressed even at room temperature, and that the angle between the Si-O axes and the OH axes of the isolated silanol groups is about 47°. These results are corroborated by 2 H NMR experiments performed on pyridine-4-d 1 . In contrast, in the case of SBA-15 with the larger pore diameters, the hydrogen bond jumps of pyridine are associated with an isotropic rotational diffusion, indicating a high degree of roughness of the inner surfaces. This finding is correlated with the finding by 2 9 Si CPMAS of a substantial amount of Si(OH) 2 groups in SBA-15. in contrast to the MCM-41 materials. The Si(OH) 2 groups are associated with surface defects, exhibiting not only silanol groups pointing into the pore center but also silanol groups pointing into other directions of space including the pore axes, leading to the isotropic surface diffusion. All results are used to develop molecular models for the inner surface structure of mesoporous silica which may be a basis for future simulations of the surfaces of mesoporous silica.

Freezing and Melting of Water Confined in Silica Nanopores

In nanosized pores, liquid water can be thermodynamically stable down to temperatures well below the limit of homogeneous nucleation of bulk water ( approximately 235 K). Studies of water in such pores therefore offer an opportunity to reveal the anomalous behavior of deeply supercooled water. Herein we focus on recent studies of the limits of freezing and melting of water in the cylindrical pores of ordered mesoporous silicas with pore diameters in the range of 2-10 nm, based on vapor sorption measurements, calorimetric studies, NMR spectroscopy and cryoporometry, and neutron diffraction studies.

Aggregation of Silica Nanoparticles Directed by Adsorption of Lysozyme

The interaction of the globular protein lysozyme with silica nanoparticles of diameter 20 nm was studied in a pH range between the isoelectric points (IEPs) of silica and the protein (pH 3-11). The adsorption affinity and capacity of lysozyme on the silica particles is increasing progressively with pH, and the adsorbed protein induces bridging aggregation of the silica particles. Structural properties of the aggregates were studied as a function of pH at a fixed protein-to-silica concentration ratio which corresponds to a surface concentration of protein well below a complete monolayer in the complete-binding regime at pH > 6. Sedimentation studies indicate the presence of compact aggregates at pH 4-6 and a loose flocculated network at pH 7-9, followed by a sharp decrease of aggregate size near the IEP of lysozyme. The structure of the bridged silica aggregates was studied by cryo-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering. The structure factor S(q) derived from the scattering profiles displays characteristic features of particles interacting by a short-range attractive potential and can be represented by the square-well Percus-Yevick potential model, with a potential depth not exceeding 3k(B)T.

Fluids in pores: experimental and computer simulation studies of multilayer adsorption, pore condensation and critical-point shifts

The phase behaviour of a fluid in mesopores just below the critical temperature (T,) has been studied experimentally and by molecular dynamics simulations. Experimental results were obtained for the adsorption of sulfur hexafluoride (SFg) on Controlled-Pore Glass (CPG-10) for reduced temperatures Tr = TTT, of 0.857, 0.920 and 0.985. Pore condensation occurs at the two lowest temperatures, whereas at the highest temperature (TI = 0.985) a phase transition cannot be detected, indicating that the fluid inside the pore is in a supercritical state. It follows that the pore critical temperature (Tcp) is lower than the bulk critical temperature, An analysis of unstable and metastable limits for the underlying system is given. Molecular dynamics computer simulations have been performed for a system of Lennard-Jones molecules, where the parameters values chosen simulated the SF6-CPG system. Phase transitions are only present for the lowest temperatures, but the isotherm at Tr = 0.985 increases monotonically in agreement with experiment. If the fluid- fluid interaction E~~ is decreased, pore condensation occurs at higher relative fugacities f/fsat. Pore condensation occurs closer to the saturation pressure as the radius R is increased, in agreement with the experimental findings.

Neutron diffraction and NMR relaxation studies of structural variation and phase transformations for water/ice in SBA-15 silica: I. the over-filled case

Neutron diffraction and NMR relaxation measurements have been made of water/ice in SBA-15, a mesoporous silica constituting an ordered array of cylindrical mesopores of pore diameter ~86 A, over the temperature range 180–300 K in a cooling and heating cycle. The over-filled sample shows the initial formation of hexagonal ice on the outside of the silica grains, followed by the nucleation of cubic ice inside the pores at a lower temperature. Neutron scattering profiles for the cubic ice peaks are significantly broadened and indicate a defective structure, as observed in previous experiments on ice formation in sol–gel and MCM-type silicas. Below the pore freezing temperature the intensity of the cubic ice peaks exhibit a significant increase, down to the lowest experimental temperature, indicating a reversible conversion of defective ice to ordered ice crystals. The peak profile analysis for the two ice patterns indicates a systematic variation in the position as a function of temperature, giving values of the expansion coefficients that are slightly lower than other measurements for the bulk phase. NMR results on proton relaxation as a function of temperature indicate the presence of a mobile phase for temperatures below pore freezing that supports the view that there is interconversion between brittle and plastic phases of ice.

Surface aggregate structure of nonionic surfactants on silica nanoparticles

The self-assembly of two nonionic surfactants, pentaethylene glycol monododecyl ether (C12E5) and n-dodecyl-β-maltoside (β-C12G2), in the presence of a purpose-synthesized silica sol of uniform particle size (diameter 16 nm) has been studied by adsorption measurements, dynamic light scattering and small-angle neutron scattering (SANS) using a H2O/D2O mixture matching the silica, in order to highlight the structure of the surfactant aggregates. For C12E5, strong aggregative adsorption onto the silica beads, with a high plateau value of the adsorption isotherm above the CMC was found. SANS measurements were made at a series of loadings, from zero surfactant up to maximum surface coverage. It is found that the spherical core-shell model nicely reproduces the SANS data up to and including the local maximum at q = 0.42 nm−1 but not in the Porod region of high q, indicating that the surface area of the adsorbed surfactant is underestimated by the model of a uniform adsorbed layer. A satisfactory representation of the entire scattering profiles is obtained with the model of micelle-decorated silica beads, indicating that C12E5 is adsorbed as spherical micellar aggregates. This behaviour is attributed to the high surface curvature of the silica, which prevents an effective packing of the hydrophobic chains of the amphiphile in a bilayer configuration. For the maltoside surfactant β-C12G2 very weak adsorption on the silica beads was found. The SANS profile indicates that this surfactant forms oblate ellipsoidal micelles in the silica dispersion, as in the absence of the silica beads.

Interfacial tension of alkylglucosides in different APG/oil/water systems

The interfacial performance of pure alkylglucosides (C8G1, C10G1 and C12G1) and of technical grade alkylpolyglucoside (APG) surfactants was investigated in three different water/oil systems (decane, isopropylmyristate and 2-octyldodecanol). From the dependence of the interfacial tension on the surfactant concentration below the CMC the cross-sectional area of the molecules at the decane/water interface was estimated. The plateau values of the interfacial tension at the CMCσc are independent of temperature and almost independent of added electrolyte in the decane/water system. The ability of the surfactants to lower the oil/water interfacial tension is most pronounced for the nonpolar oil. The partition coefficient of the surfactant between oil and water phase (kc) was estimated from the CMC and the observed break point of the interfacial tension after equilibration of the two phases. In decane/water,kc is nearly zero for all surfactants studied. For the polar oils,kc increases with the chain length of the surfactant up tokc≈10 for C12G1 in octyldodecanol/water. The values ofσc in the different oil/water systems appear to be correlated withkc and exhibit a minimum nearkc=1.

Fluid adsorption in ordered mesoporous solids determined by in situ small-angle X-ray scattering

The adsorption of two organic fluids (n-pentane and perfluoropentane) in a periodic mesoporous silica material (SBA-15) is investigated by in situ small-angle X-ray scattering (SAXS) using synchrotron radiation. Structural changes are monitored as the ordered and disordered pores in the silica matrix are gradually filled with the fluids. The experiments yield integrated peak intensities from up to ten Bragg reflections from the 2D hexagonal pore lattice, and additionally diffuse scattering contributions arising from disordered (mostly intrawall) porosity. The analysis of the scattering data is based on a separation of these two contributions. Bragg scattering is described by adopting a form factor model for ordered pores of cylindrical symmetry which accounts for the filling of the microporous corona, the formation of a fluid film at the pore walls, and condensation of the fluid in the core. The filling fraction of the disordered intrawall pores is extracted from the diffuse scattering intensity and its dependence on the fluid pressure is analyzed on the basis of a three-phase model. The data analysis introduced here provides an important generalisation of a formalism presented recently (J. Phys. Chem. C, 2009, 13, 15201), which was applicable to contrast-matching fluids only. In this way, the adsorption behaviour of fluids into ordered and disordered pores in periodic mesoporous materials can be analyzed quantitatively irrespective of the fluid density.

Hydrogen Bonding of Water Confined in Controlled-Pore Glass 10-75 Studied by 1 H-Solid State NMR

The adsorption of water in the mesoporous silica material with cylindrical pores of uniform diameter, Controlled Pore Glass 10-75 (CPG), was studied by 1H-MAS solid state NMR spectroscopy. From the NMR spectra it is evident that inside the mesopores of the silica different water environments exist, which are characterized by their individual chemical shift. All observed hydrogen atoms are either surface –SiOH groups or hydrogen bonded water molecules. It is found that there exist some stronger bound water molecules on the surface which are not removable even by heating at a vacuum pump. As a tentative assignment these water molecules are attributed to surface defects or inaccessible cavities in the CPG 10-75. At intermediate water filling levels, the principal signal is a single NMR line with continuously varying chemical shift. This finding is interpreted as the result of a radial water filling mechanism. That is, the filling of the pore grows from the pore surface towards the pore axis. Finally it is shown that water is a sensor for surface and structural inhomogeneity and that a coexistence of inner pore and outer bulk water exists in the system.