Characterization of organic adsorbates on model glass surfaces

Model silicate glasses in planar thin film structures are prepared using sol/gel techniques. A typical structure consists of 3.0–12.0 nm of glass on vapor deposited Ag. In this study, a pure silica and a 50/50 binary alumina-silica films are examined. Poly(methyl methacrylate) (PMMA) is adsorbed from chlorobenzene solution onto the surfaces. Infrared reflection (IR) spectroscopy, ellipsometry and quartz oscillator microgravimetry (QOM) are applied to characterize the PMMA/silicate glass interfaces formed. The QOM and IR data show that PMMA adsorption on the pure silica surface is irreversible with respect to removal by pure solvent rinse while on the binary glass surface and on a pure alumina surface, solvent rinses remove the initially adsorbed polymer. These results are interpreted in terms of Bronsted acid-base interactions involving the basic properties of the PMMA C=O group, the acidic nature of the silica and the more basic nature of the alumina containing surfaces. Further evidence for this interpretation is given by the IR spectral data which show broadening of the C=O stretching mode to lower frequencies for the irreversible adsorbed polymer on silica as compared with simulated spectra of non-surface bonding PMMA thin films. This spectral perturbation is interpreted as evidence for a hydrogen bonding interaction between OH groups on the silica surface and the C=O groups. The overall conclusion is that the surface of a 50/50 binary alumina-silica composition is dominated by the basic nature of alumina.     

Structural origin of apparent surface basicities of p‐aminosulanamide polymorphs

Perichromism of two polymorphic forms of p‐aminosulfonamide (SNM) affecting spectroscopic properties of adsorbed thymol blue (TB) was studied. The consistent protocol for assessment of indicator interactions with solids was proposed and validated. This relies on the deconvolution of spectra into individual contributions both in solution and on the surface. The obtained relationships between pH and degree of protonation of indicator allows for surface basicity quantification. It has been found that γ‐SNM is significantly more basic than β‐form with difference exciding 3 pH units. Besides, the signal profiles measured in solution and adsorbed on either of SNM form were used for examination of TB perichromism. Although, only small red‐shift has been notices, this effect is not identical for both polymorphs. The differences in apparent surface pH of polymorph are explained in terms of structural properties. The morphology analysis allows for identification of habits responsible for structural origin in observed differences of polymorphs basicities. The noticed coincidental distributions of active centres of TB conformers and amine basic/acidic sites on γ‐polymorph might be considered as the source of apparent pH differences. Thus, basicities of studied system are to be related to morphology differences of polymorphs and recognition by iorn‐pairs formation rather than changes in characters of proton donating or accepting sites.     

An interpretation of glass chemistry in terms of the optical basicity concept

The basicity of an oxide glass can be measured experimentally from the frequency shifts in the ultra-violet (UV) (s-p) spectra of probe ions such as Pb²⁺ and can be expressed on the numerical scale of optical basicity Λ (ideally Λ lies between zero and unity). It is possible to relate Λ with (i) the constitution, and (ii) the electronegativity of the cations (e.g. Na⁺, Si⁴⁺, etc.) of the glass, and the relationship allows microscopic optical basicities λ to be assigned to individual oxides and oxy-groups in the glass. These microscopic optical basicities are used for interpreting various aspects of the physics and chemistry of glass including refractivity, network coordination number changes, chemical durability, the glass electrode, UV transparency and the host behaviour of glass towards metal ions generally. Changes in glass basicity in going from one alkali metal oxide to another are also discussed. Finally, the concept of optical basicity, both as an experimentally obtained quantity and as a number calculated from glass constitution and electronegativity, is discussed in relation to the traditional approach to acid-base behaviour in glass.     

Silica-zirconia mixed oxide samples by an hybrid materials based innovative preparation procedure: Influence of preparation procedure and composition on active sites

In this work, the interaction of amorphous silica-zirconia mixed oxide samples obtained from inorganic-organic silica-based hybrid materials with pyridine and CO₂ was studied to investigate their acid/base character. Several silica-zirconia mixed oxide powders characterized by different [Zr/Si] atomic ratios were prepared and treated at increasing temperatures both in a conventional muffle and with microwave technology. The powder samples were characterized with Diffuse Reflectance Infrared Fourier Transform (DRIFT) and X-ray Photoelectron (XP) spectroscopies. The surface acidic and basic active sites were investigated (with DRIFT spectroscopy) by chemisorbing probe molecules (pyridine, carbon dioxide). The obtained results revealed the presence of both Lewis and Brønsted acidic sites on the amorphous silica-zirconia mixed oxide powder surfaces. Several acidic sites characterized by different strength were observed; the acidic sites distribution is markedly influenced by the sample composition and by the heat treatment: more numerous acidic sites form on the surface of the samples treated with microwaves with respect to the muffle treated ones; the increment of the temperature and the decrease of the zirconium content cause a significant decrement of the acidic sites. No basic sites were revealed.     

Effect of optical basicity on Er3+ up-conversion luminescence in GeO2–R2O glasses

The effect of optical basicity on Er³⁺ up-conversion luminescence in germanate glasses is investigated under 980 nm excitation. The intensity of green and red up-conversion luminescence decreased with the increase in radius of alkali ion or Li₂O content, implying that up-conversion luminescence strongly relates to the optical basicity of glass host. On the other hand, as increasing the optical basicity, the red emission intensity decreased significantly, while the green emission intensity decreased slightly. It has been proposed that the up-conversion luminescence intensity was dominated by the optical basicity, which theoretically estimated from glass composition. The interaction mechanism between up-conversion process and optical basicity was proposed.     

Investigation of alkali borosilicate glass durability using tritium tracing, β-autoradiography, scanning electron microscopy and ion beam analysis

A Li-Na-containing borosilicate glass was submitted to aqueous corrosion in static mode in tritiated solutions of various pH at room temperature and 90 °C. Tritium counting, β-autoradiography, scanning electron microscopy (SEM) and ion beam analysis (IBA) techniques have been used to investigate the composition variations of the leached glass surfaces. In an acidic medium, the glass surface is covered by a thick hydrated silica layer; mobile elements like Li, Na and B and transition elements like Fe and Mo are strongly depleted. Near pH 7, relative enrichments of aluminium and iron are shown together with strong Li, Na and B depletions. In a basic medium, the glass surface exhibits iron and molybdenum enrichments whereas mobile elements seem to be kept at their nominal concentration level at the glass surface. The tritium activity and distribution on the leached glass surface is shown to depend strongly on the pH of the tritiated solutions. These observations are correlated with the observed changes of the surface morphology and chemistry characterized by IBA and SEM analysis. Elemental solubility versus pH of various chemical species have been calculated for comparison purpose with the experimental observations.     

Prospects of sol-gel processes

The ultrastructural control of materials through sol-gel processes offers significant promise for the achievement of reliable performance in glass, glass-ceramics, ceramics and composites. This will be attainable only if the fundamental chemistry of the sol-gel process is understood. Several examples based on this approach will be presented for optical glass, structural ceramics and electromagnetic materials. New concepts, such as ceramic or glass molecular composites and optically active gels, will be discussed. It will be shown how these new concepts are derived from an understanding of polymer chemistry and chemical reactivity. Advanced sol-gel basic research directions and their prospects will be discussed.     

Electronic structure and properties of oxide glasses (II) Basicity measured by copper(II) ion probe in Na2OP2O5, K2OB2O3 and K2SO4ZnSO4 glasses

The correlation between the basicity of oxygens measured by the Cu(II) ion probe and the non-bonding electron density on oxygens in alkali borate glasses was considered. The basicity was measured for K₂OB₂O₃, Na₂OP₂O₅ and K₂SO₄ZnSO₄ glasses and categorized into two types, δ and π, according to the symmetry property of the bonding between a Cu(II) ion and oxygen. The π basicity for borate and phosphate glasses showed an abrupt increase in the vicinity of 17 and 50 mol% alkali oxide, respectively. The values of π-type basicity varied with the composition of glass, being larger in the order: sulfate < phosphate ? borate, whereas δ basicity was constant irrespective of the glass composition. Such a change of the basicity with the composition of glass was interpreted in terms of behavior of non-bonding levels of the ligand oxygens in a glass network.     

Bonding and ion-ion interactions of Mn ions in fluoride-phosphate and boro-silicate glasses probed by EPR and fluorescence spectroscopy

Electron paramagnetic resonance (EPR) and fluorescence spectroscopy are sensitive and selective methods for probing coordination and bonding of Mn²⁺ ions in glasses. Both methods provide additional information on Mn-Mn ion interactions and cluster formation. Mn²⁺ was found to be tetrahedrally coordinated in boro-silicate glasses of high optical basicity, and octahedrally coordinated in low alkaline boro-silicate glasses (duran-type) as in fluoride-phosphate glasses. Broad emission bands and multicomponent fluorescence decay curves in duran glasses indicate very strong Mn-Mn ion interactions and the presence of multiple Mn²⁺ sites. Site distribution is more homogenous in metaphosphate glasses, though concentration quenching is apparent at high Mn-levels. As the Mn-content increases the EPR spectra show exchange narrowing due to a decrease in the Mn-Mn distances in the duran series, but show extreme linewidth broadening due to increased cluster sizes at constant Mn-Mn distances for metaphosphate glasses. For the fluoride-phosphate and boro-silicate systems investigated, fluorescence lifetimes are found to decrease as the wavelength of the emission maximum increases and with increasing g-values of the sextet at g = 2. For octahedral coordination of Mn²⁺ ions the EPR hyperfine splitting constant decreases linearly with increasing optical basicity, as a result of an increasing covalent character of the Mn²⁺-ligand bond.     

Effect of leaching concentration and time on the morphology of pores in porous glasses

Herein, porous glass was prepared by the acid leaching of phase-separated sodium borosilicate glass. The effects of hydrochloric acid (HCl) leaching concentrations varying from 0.1 to 3 M and of leaching times varying from 2 to 48 h on pore size and shape are reported here. The porous structure evolution was investigated through small angle X-rays scattering measurements and were compared to the classical nitrogen adsorption technique. We argue that the composition of these samples is not significantly modified with the acidic concentrations of the solution or with the leaching times tested. However, as expected a priori, we clearly show strong differences in porous structure. The investigation of different HCl leaching concentrations (from 0.1 to 3 M), with a small variation between each concentration, has enabled the observation of an interesting result. That is, we clearly show a non-linear effect of acidic concentration on the porous structure of these materials. At low acid concentrations, the specific area of these materials is relatively low. Increasing the HCl concentration to approximately 0.7 M leads to a strong increase in a specific BET area and total pore volume. However, further increasing of the acid concentration of the leaching solution leads to a decrease of both the specific surface area and the total pore volume. All of these results due to the presence of colloidal silica inside the pore structure can be explained by the well-known Zhdanov's model. These nanoparticles formed at an intermediate acid concentration seem to cluster when the acidity increases. The effect of the leaching time is less significant than the HCl concentration; however, this same assumption can be explained by the experimental results.     

On the effect of glass composition in the dissolution of glasses by water

The dissolution rate in aqueous solution has been measured for three families of glasses of technological interest (borosilicates, aluminosilicates and lead-silicates). Borosilicates have been altered at a slightly basic pH (8–9), whereas the aluminosilicates and the lead-silicate glasses have been corroded in acidic solutions. In these conditions, silica is always less soluble than all other glass components. In each glass family, the effect of the silica content on the glass dissolution rate has been studied. The three series display a similar behavior as a function of the glass composition, namely a sharp increase of the dissolution rate with the content in soluble oxides. These experimental results have been explained in the framework of a Monte-Carlo simulation of glass dissolution. The simulations show that the fast extraction of the soluble species as compared to that of silica generates a porous surface layer, which becomes thicker and thicker when the percentage of soluble oxide increases in the glass. This increases the interface area between the glass and the solution, and consequently the dissolution rate in the same proportion. The numerical simulations reproduce quite well the experimental variations of the dissolution rates with glass composition.     

Dielectric studies of segmental dynamics in poly(dimethylsiloxane)/titania nanocomposites

Differential scanning calorimetry, thermally stimulated depolarization currents and dielectric relaxation spectroscopy techniques, covering together a broad frequency range of 10⁻⁴ to 10⁶ Hz, were employed to investigate the effects of in situ synthesized titania nanoparticles on thermal transitions, segmental dynamics and interfacial interactions in poly(dimethylsiloxane)/titania nanocomposites. Titania particles (TiO₂, 20-40 nm in diameter) were prepared and well dispersed into the polymer network through sol-gel technique, aiming at stable and mechanically reinforced systems. The interactions between polymer and fillers were found to be strong, supressing crystallinity and affecting the temperature development of the glass transition. The segmental relaxation associated with the glass transition consists of three contributions, arising, in the order of decreasing mobility, from the bulk (unaffected) amorphous polymer fraction (α relaxation), from polymer chains restricted between condensed crystal regions (α_c relaxation), and from the semi-bound polymer in an interfacial layer with strongly reduced mobility due to interactions with hydroxyls on the nanoparticle surface (α? relaxation). The thickness of the interfacial layer was estimated to be in the range of 3-5 nm. Measurements using different thermal protocols proved very effective in analyzing the origin of each relaxation and the respective effects of filler addition.     

The vanadyl ion as a spectroscopic probe:: Measurement of optical basicity in the visible / near-infrared region

Previously optical basicity was always measured using s-p absorption spectra of Tl⁺, Pb²⁺ and Bi³⁺, but since these spectra are in the UV region, media containing oxides of transition metal aons have been excluded from such measurements. The present work shows that for VO²⁺ the d-d transition ²B₂ → ²E red-shifts with increasing optical basicity, and a study in the Na₂O---P₂O₅ glass system allows comparison with results obtained previously with Tl⁺ and other probe ions. It is shown that under controlled conditions determination of optical basicity with VO²⁺ is viable, and since the ²B₂ → ²E spectral band occurs in the visible / near-IR region, UV transparency of the medium is no longer necessary. The VO²⁺ probe therefore offers a means of obtaining optical basicities of glasses containing e.g. PbO, or of metallurgica slags containing e.g. Fe₂O₃ or MnO.     

MNDO studies of basicity in borate glasses

The cluster approximation was adopted for the borate glasses. The geometries and basicity of various isomers of H₈B₉O₁₈⁻, H₈B₁₁O₂₁⁻ and H₈B₁₂O₂₃²⁻ were studied by the MNDO semi-empirical molecular orbital method. The optimized geometries of pentaborate, triborate and diborate structure in the clusters showed good agreement with the experimentally determined ones. The changes of bond length and charge rearrangements accompanying the formation of a four-fold coordinated boron atom (⁴B) accorded with the prediction of Gutmann's bond length variation rule. The basicity in the borate glass was defined as the electron donability of oxygens in the glass network. MO interaction between the occupied orbitals of the clusters and the LUMO of the acidic site in (H₂B₉O₁₄)₂O and the proton affinity of the clusters were estimated. The basicities of the clusters which contained ⁴B and that of the clusters which contained non-bridging oxygen atoms (NBO) were compared. The appearance of NBO in high alkali borate glass was interpreted.     

ESR and optical absorption of Cu2+ in Na2OSiO2 glasses

ESR and optical absorption spectra of Cu²⁺ in xNa₂O(100?x)SiO₂ glasses were measured, where x ranges from 12 to 70 mol% Na₂O. This glass system was divided into three composition regions, 12 ? x ? 37, 37 ? x ? 55 and 55 ? x, from the composition dependence of the ligand field transition energy and spin hamiltonian parameters of Cu²⁺. Two boundary compositions (37 and 55 mol%) between the two adjacent regions agreed with the eutectics in the equilibrium phase diagram. Two types of Cu²⁺-complexes, with less basic ligands (HFS-1) and much more basic ones (HFS-2), were detected in ESR for ultra-high soda glasses (x ? 55). The distribution of the ESR parameters due to the fluctuation of ligand fields was negligible for HFS-2 compared with that for other glasses. The Cu²⁺ ion responsible for HFS-2 was considered to distribute in the microphase of orthosilicate. Imagawa's basicity, the covalency of the bondings between Cu²⁺ and ligands, was calculated by using Maki and McGarvey's analysis. The basicity of σ-type symmetry remained constant, irrespective of the glass composition, and the value was identical with those for other oxyanionic glasses. The π-type basicity was also constant for the glasses of x ? 55. Two different basicities, each corresponding to HFS-1 or 2, were obtained for the glasses of x ? 55. The value derived from HFS-1 was identical with those for x < 55 glasses, whereas that derived from HFS-2 suggested the formation of much more basic ligands.     

Preparation and structure of organic-inorganic hybrid precursors for new type low-melting glasses

Organic-inorganic hybrid precursors for new type low-melting glasses without pollution elements such as Pb and F have been synthesized through a non-aqueous acid-base reaction process. The hybrid compounds consist of -Si-O-P- framework, in which some of the bridging oxygens of a Si tetrahedron are substituted by organic functional groups. Terminating the oxide framework by the organic functional groups lowers the network dimension. Orthophosphoric acid (H₃PO₄) and dialkyldichlorosilane (Me₂SiCl₂ and Et₂SiCl₂) were employed as starting materials. The formation of P-O-Si linkage was confirmed by IR spectra. The larger viscosity increase at higher Si concentrations is correlated to the formation of the linkage. It is proposed that an acid-base polycondensation reaction of P-OH+Si-Cl→P-O-Si+HCl↑ dominates the network formation. When dimethylsilane SiMe₂ is substituted by divalent Sn, an organic-inorganic hybrid low-melting glass in the system of SnO-Me₂SiO-P₂O₅ can be derived, the glass transition temperature of which is about 29 °C. So the present acid-base reactions provide a new process by which precursors for new type low-melting glass are synthesized.     

Solubility of tin dioxide in soda-lime silicate melts

In the present work, the solubility of tin dioxide is assessed as a function of time, temperature and basicity in simple ternary glasses: NC3S, NC4S, NC5S and NC6S (N: Na₂O, C: CaO, S: SiO₂). An increase of silica contents in the glass composition leads indeed to a decrease of the glass basicity. First, a kinetic study of the dissolution has been performed. Consequently, the solubility limits of tin dioxide have been determined after 2 h of heat treatment: this duration is long enough to reach the dissolution equilibrium, and short enough to limit the sodium oxide losses in the melt at high temperatures. Nevertheless the specific case of the most acid glass has been underlined, as its higher viscosity implies longer heating times. At equilibrium state, SnO₂ solubility depends on the temperature (Arrhenius law) and on the glass basicity. In the 1200 °C–1400 °C temperature range, in these soda–lime glasses, the solubility of tin dioxide is between 1.3 and 2.1 at.% Sn and the temperature dependence of solubility exhibits a single mechanism of dissolution. Furthermore, the basicity dependence of the solubilization process is also discussed, and the presence of another oxidation state of tin (Sn^II) is thus proposed.     

Silicate nanoparticles by bioleaching of glass and modification of the glass surface

Bioleaching is examined as a low temperature (50 °C) soft chemical approach to nanosynthesis and surface processing. We demonstrate that fungus based bioleaching of borosilicate glass enables synthesis of nearly monodispersed ultrafine (∼5 ± 0.5 nm) silicate nanoparticles. Using various techniques such as X-ray diffraction, X-ray photoelectron spectroscopy and FTIR we compare the constitution and composition of the nanoparticles with that of the parent glass, and establish the basic similarities between the two. The bioleaching process is shown to enhance the non-bridging oxygen component and correspondingly influence the Si-O-Si network. The root mean square roughness of glass surface is seen to increase from 1.27 nm for bare glass to 2.52 nm for 15 h fungal processed case, this increase being equivalent to that for glass annealed at 500 °C.     

Understanding and Controlling the Morphology of ZnO Crystallites under Hydrothermal Conditions

The morphological features of ZnO crystallites influenced by solution basicity under hydrothermal conditions have been studied from the standpoint of the incorporation of the growth units. A crystal chemistry approach is developed to understand and thus to control a desirable morphology of the crystallites. The effects of the additive OH⁻ on the crystal morphology of ZnO crystallites are qualitatively but satisfactorily explained by a mechanism considering two aspects: (1) solution structures and the structural forms of growth units under a certain growth condition, i.e. the interactions between the solvent molecules and the growth units; and (2) the influence of the solvent molecules or additives on growth interfaces, particularly on two polar faces of ZnO crystallites in terms of the inhibition or promotion of the incorporation of various growth units depending on the solution basicity. Since the incorporation rates of the growth units are different on positive and negative polar faces, the relative growth rates of these faces are different and thus lead to the habit modifications. The approach clearly demonstrates that the hindrance of crystal growth is a consequence of surface adsorption processes.