Siloxane Rings Research Articles

Preparation of Allyl-Containing PDMS Telehelics by the Anionic Ring-Opening Polymerization

The synthesis of allyl-containing PDMS telechelics from octamethylcyclotetrasiloxane and 1,3-diallyl-1,1,3,3-tetramethyldisiloxane as a stopper by the anionic ring-opening of a siloxane ring is presented. A series of the polymers with the molecular weights from 1000 to 12000 were obtained and characterized by gel permeation chromatography and 1H NMR spectroscopy. The molecular weights of the resulting polymers correspond to the given ones.

Dependence of the formation kinetics of carbon dioxide hydrate on clay aging for solid carbon dioxide storage

Clay-based marine sediments have great potential for safe and effective carbon dioxide (CO2) encapsulation by storing enormous amounts of CO2 in solid gas hydrate form. However, the aging of clay with time changes the surface properties of clay and complicates the CO2 hydrate formation behaviors in sediments. Due to the long clay aging period, it is difficult to identify the role of clay aging in the formation of CO2 hydrate in marine sediments. Here, we used ultrasonication and plasma treatment to simulate the breakage and oxidation of clay nanoflakes in aging and investigated the influence of clay aging on CO2 hydrate formation kinetics. We found that the breakage and oxidation of clay nanoflakes would disrupt the siloxane rings and graft hydroxyl on the clay nanoflakes. This decreased the negative charge density of clay nanoflakes and weakened the interfacial interaction of clay nanoflakes with the surrounding water. Therefore, the small clay nanoflakes enriched in hydroxyl would disrupt the surrounding tetrahedral water structure analogous to the CO2 hydrate, resulting in the prolongation of CO2 hydrate nucleation. These results revealed the influence of the structure-function relationship of clay nanoflakes with CO2 hydrate formation and are favorable for the development of hydrate-based CO2 storage.

Crucial Chemical Revelations in 45S5 Bioactive Glass via Sequential Precursor Integration Order.

Among the wet chemical nanoparticle fabrication techniques, the sol-gel process happens through hydrolysis and subsequent polycondensation reactions. The bioactive glass known as the 45S5 SiO2-Na2O-CaO-P2O5 quaternary system has intricate chemistry, yet its advantages benefit the biomedical field on an enormous scale. The order in which the ethanol and TEOS inclusions are exchanged was investigated in this work because it has a direct impact on the early hydrolysis process. Another strategy involves adding phosphate species to the sol before gelation, modifying the network chemistry, and interpreting the findings. Adding phosphate species before gelation in the biomaterial (E-Si-P) resulted in the formation of hydroxyapatite and other calcium silicate phases at 800 °C. Swapping ethanol and TEOS biomaterials (E-Si and Si-E) resulted in the sodium-calcium silicate phase only. Si-E with strong Si-O-Si siloxane rings demonstrated superior mechanical stability, hemocompatibility, and bioactivity. This compact Si-O-Si decreased the surface area of Si-E. XPS spectra revealed that E-Si-P has the lowest Na 1s binding energy (BE) and the highest BE for Si 2p. More Si-O-/Si-OH groups formed by E-Si make the network weak and decrease the surface area and protein adsorption. These differences significantly influenced the morphology, surface properties, mechanical studies, and compatibility test. This study has further unraveled the protocol to design a biomaterial with mechanical stability and load-bearing ability. In addition, the appropriate protocol to yield the desired property-rich biomaterial with preserved bioactivity, mechanical stability, cytocompatibility, as well and surface porosity has been elaborated in detail.

Ring-opening polymerization of cyclic oligosiloxanes without producing cyclic oligomers.

A long-standing problem associated with silicone synthesis is contamination of the polymer products with 10 to 15% cyclic oligosiloxanes that results from backbiting reactions at the polymer chain ends. This process, in competition with chain propagation through ring-opening polymerization (ROP) of cyclic monomers, was thought to be unavoidable and routinely leads to a thermodynamically controlled reaction mixture (polymer/cyclic oligosiloxanes = 85/15). Here, we report that simple alcohol coordination to the anionic chain ends prevents the backbiting process and that a well-designed phosphonium cation acts as a self-quenching system in response to loss of coordinating alcohols to stop the reaction before the backbiting process begins. The combination of both effects allows a thermodynamically controlled ROP of the eight-membered siloxane ring D4 without producing undesirable cyclic oligosiloxanes.

Control of the Structural Charge Distribution and Hydration State upon Intercalation of CO2 into Expansive Clay Interlayers

Numerous experimental investigations indicated that expansive clays such as montmorillonite can intercalate CO2 preferentially into their interlayers and therefore potentially act as a material for CO2 separation, capture, and storage. However, an understanding of the energy-structure relationship during the intercalation of CO2 into clay interlayers remains elusive. Here, we use metadynamics molecular dynamics simulations to elucidate the energy landscape associated with CO2 intercalation. Our free energy calculations indicate that CO2 favorably partitions into nanoconfined water in clay interlayers from a gas phase, leading to an increase in the CO2/H2O ratio in clay interlayers as compared to that in bulk water. CO2 molecules prefer to be located at the centers of charge-neutral hydrophobic siloxane rings, whereas interlayer spaces close to structural charges tend to avoid CO2 intercalation. The structural charge distribution significantly affects the amount of CO2 intercalated in the interlayers. These results provide a mechanistic understanding of CO2 intercalation in clays for CO2 separation, capture, and storage.

Cyclic or Linear? Parameters Determining the Outcome of Glycine Polymerization in Silica Surface Prebiotic Scenarios.

The parameters that determine the formation of linear peptides and cyclic dimers (diketopiperazine, DKP) on silica surfaces of different surface area, silanol and siloxane ring populations, controlled by thermal treatments, are investigated upon glycine deposition from gas and liquid phases. The formed products were characterized by infrared and Raman spectroscopies, X-ray diffraction and thermogravimetric analysis. The results reveal the importance of "nearly-free" silanols to form ester centers as primers for the formation of linear peptides over DKP, on surfaces with medium silanol density (1.4 to 2.7 nm-2 ). Quenched reactivity is seen on isolated silanols (density≤0.7 nm-2 ), while silanols involved in hydrogen bonding (density of 4.5 nm-2 ) weakly interact with Gly resulting in its cyclization to DKP. Deposition of glycine from liquid phase may also form both DKP and linear polymers, depending on its loading and silica surface. These conclusions demonstrate the complexity of glycine surface chemistry in the polymerization reaction and highlight the interest of a surface science approach to evaluate geochemical prebiotic scenarios.

Silsesquioxane Cage-Fused Siloxane Rings as a Novel Class of Inorganic-Based Host Molecules

Abstract Side-opened cage-silsesquioxane, denoted as SO-POSS, was fused with oligo(dimethylsiloxane)s to synthesize inorganic skeleton-based cyclic molecules. The resultant cyclic compounds served as hosts for alkali metal cations (Li+, Na+, K+). Interestingly, ring-transformation occurred in the presence of guest cations depending on the ring size; the SO-POSS-fused cyclic oligo(dimethylsiloxane)s were converted to smaller SO-POSS rings and cyclic oligo(dimethylsiloxane)s. Herein, we report the first study on host-guest chemistry of the cage silsesquioxane-fused siloxane rings.

Unexpected Formation of a Silicon-Centered Spirocyclic Oligosiloxane Bearing Eight Pendant Ferrocene Units

An intriguing but rare silicon-centered spirocyclic compound, spiro[5.5]octaferrocenylpentasiloxane (4) featuring silicon fused six-membered ferrocenyl-functionalized siloxane rings, has been obtained during the thermally induced transformation of triferrocenylsilane Fc3Si–H (1) into triferrocenylsilanol Fc3Si–OH (2), when N,N-dimethylformamide (DMF) was used as a solvent in the presence of the metal carbonyl Mo(CO)6. The unexpected formation of the maximally ferrocenyl substituted silicon centered spirocyclic 4 involves the obtention, and subsequent condensation, of different ferrocenylsilanol intermediates. Spirocyclic silicate 4 has been characterized using a combination of MALDI-TOF mass spectrometry, elemental analysis, and single crystal X-ray diffraction analysis.

Well-defined cyclic silanol derivatives.

Cyclic silanol derivatives (CSDs), possessing siloxane rings consisting of T-unit silicon and oxygen atoms, are considered efficient precursors for the preparation of versatile well-defined building blocks of hybrid materials such as cyclic, cage- or ladder-type SQs. This review provides an outline of the main synthetic routes to numerous stereoregular CSDs with different sizes of siloxane rings since the first example of CSDs reported by Brown et al. in 1965. The typical reaction conditions and chemical shifts of 29Si NMR of all mentioned CSDs in this review are summarized in tables and schemes to recapitulate the state of the art. The synthesis of all-cis-cyclotetrasiloxanes (T4), the most investigated CSDs, and their functionalization by different organic reactions to access various all-cis-T4 with functional groups are methodically presented. Moreover, the potential of CSDs in multiple application fields is discussed to show the possible research directions of this family of compounds in the future.

Synthesis and characterization of ladder structured ormocer resin of siloxane backbone and methacrylate side chain

Available resin-based hard tissue restorations are highly influenced by the problems associated with post-polymerization shrinkage and toxicity of uncured monomers. To address this scenario, we have synthesized an ormocer resin (LSM) of inorganic siloxane backbone using the monomer 3-(trimethoxy silyl)propyl methacrylate (3-TMSPM) by modified sol gel method. The synthesized polycondensed LSM resin is fashioned with three-dimensional network of ladder structured Si-O-Si backbone which grow continuously to form higher molecular weight (Mn-18956 KDa) prepolymer. Four membered siloxane rings along with intact methacrylates constituted the monomeric unit of the prepolymer LSM. High thermal stability indicated the presence of intense siloxane network whereas low glass transition temperature and melting peak demonstrated the flexibility and partial crystallinity of the LSM resin. Low linear polymerization shrinkage (LPS) of cured LSM resin (0.422 ± 0.04%) witnessed the presence of inorganic siloxane backbone and performed better than cured bisphenol glycidyl methacrylate (BisGMA)composite (0.916 ± 0.09%). Non-cytotoxicity and cytocompatibility of the cured LSM resin proved the eligibility of the resin for hard tissue restorative applications.

Macromonomers as a Novel Way to Investigate and Tailor Silicon-Oxycarbide-Based Materials Obtained from Polymeric Preceramic Precursors.

It has been shown that bifunctional monomers (D units), which are used to increase the carbon content in silicon oxycarbide precursors, can form volatile oligomers, thus affecting the amount of carbon available during the transition into the final material in the annealing process. Additionally, an uneven distribution of carbon-rich mers may lead to the formation of a free-carbon phase, instead of the incorporation of carbon atoms into the silicon matrix. In this study, a novel two-step approach was utilized. Firstly, a macromonomer containing a number of structural units with precise structure was synthesized, which was later polycondensed into a ceramic precursor. Chlorodimethylsilane modified 2,4,6,8-tetramethylcyclotetrasiloxane was used as a silicon oxycarbide precursor monomer containing both T and D structural units (i.e., silicon atoms bonded to three and two oxygen atoms, respectively), with well-defined interconnections between structural units. Such a macromonomer prevents the formation of small siloxane rings, and has a very limited number of possible combinations of structural units neighboring each silicon atom. This, after investigation using IR, XRD, TG and elemental analysis, gave insight into the effect of “anchoring” silicon atoms bonded to two methyl groups, as well as the impact of their distribution in comparison to the materials obtained using simple monomers containing a single silicon atom (structural unit).

Acid–base sensor based on sol–gel encapsulation of bromothymol blue in silica: application for milk spoilage detection

In this work, silica capsules containing bromothymol blue (BTB) were synthesized by a modified Stober sol–gel method. These silica capsules were dispersed into a high-density polyethylene (HDPE)/ethylene-vinyl acetate (EVA) polymeric matrix and then applied in milk spoilage detection tests. The effect of the KOH/BTB molar ratio used in the sol–gel synthesis on the structural, textural, and morphological characteristics of the resulting silica capsules was also investigated. Field emission gun scanning electron microscopy images of the silica capsules showed particles with spherical morphology and size ranging from 0.1 to 1 µm. All silica capsules presented a very low porosity and an encapsulated BTB content in the range of 12–37 µmol g−1. The increase of the KOH/BTB molar ratio resulted in an increase in the relative amount of six-fold siloxane rings and a decrease in the BET surface area of the silica capsules. Spoilage kinetics of milk at room and fridge temperature were much different, as evidenced by pH milk measurements. The milk storage temperature and the KOH content in the silica capsules affected the intensity and rate of blends color change. Blends (HDPE/EVA/silica capsules) with lower KOH content in the capsules showed faster kinetics and higher intensities of color change during the milk spoilage at both tested temperatures. These results showed the potential of the HDPE/EVA/silica capsules as a base material to a sensor packaging, which allows the verification of the milk quality by the end-users.

Structural Changes in Silica Glass under the Action of Electron Beam Irradiation: The Effect of Irradiation Dose

Structural changes in silica glass, produced by electron irradiation, with electrons energies of 50 keV and doses of 5–80 mC/cm2 were studied by Raman spectroscopy in frequency ranges of 300–550 and of 700–1500 cm–1. It is shown that for irradiation doses less than 5 mC/cm2 the decrease of siloxane rings concentration in glass takes place. The further increase of dose results in the increase of siloxane rings concentration in glass network after irrsdiation. And for doses of more than 40 mC/cm2 the whole destruction of glass structure and its structural units takes place. The obtained results for silica glass were compared with similar results for crystalline quartz.

SiOCH Hits the Gas

There is an increasing need for on-site and real-time analyses of complex gas mixtures in many application fields such as industry, air quality, security and medicine. One challenge consists in developing complete and portable multi-gas analysis systems allowing in situ and real-time quantitative analysis of complex gas mixtures. A promising approach is based on the integration of resonating devices on silicon chips by using “standard” microelectronic technologies. The fabrication of these gravimetric gas sensors based on Nano Electro Mechanical System (NEMS) requires the use of a chemical sensitive layer to ensure a better collection and concentration of the target molecules. This enhances the sensitivity in conjunction with the limit of detection. The integration of a chemical sensitive layer on to NEMS fabricated on Si wafers leads to specific material requirements. The use of solvent is prohibited in order to avoid the degradation of the Si nanocantilevers due to capillarity forces,Very thin film thickness (less than few hundred of nanometers) should be deposited on the Si beam in order to avoid filling the gap between the resonant cantilever and the electrodes,The deposition technique should provide good uniformity over a large surface (typically on 200 mm Si wafers) for a collective sensor fabrication.And the material should be optimized to absorb the highest level of targeted gas for a given thickness, reversibly and quickly. In this work, the objective was to develop materials deposited using microelectronic compatible techniques in order to detect light alkanes and aromatic volatile organic compounds such as BTEX (for Benzene, Toluene Ethylbenzene and Xylene). Organosilicate materials are potentially good candidates for this application especially because they are nonpolar or weakly polar materials. In order to better understand the impact of the material chemistry on the detection of hydrocarbon gas and optimize the sensitivity of the chemical layer, a large panel of organosilicate thin films were deposited by different chemical vapor deposition (CVD) techniques. First, SiOCH were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), this technique being the most used in the microelectronic industry for dielectric thin-film deposition. Several precursors (diethoxymethylsilane, trimethylsilane or octamethyltrisiloxane) and deposition conditions were used in order to vary their chemical composition and their physical properties. In these SiOCH, the carbon content is mainly under the form of methyl groups bonded to silicon. At the same time, to allow a better flexibility in term of chemical functions, filament assisted chemical vapor deposition (FACVD) was used for the deposition of organosilicate thin films. Indeed, FACVD allow a better control over the precursor fragmentation pathway in comparison to plasma-based techniques. Then, it is possible to add polar groups (such as ethoxy to in case of FACVD of methyltriethoxysilane) or to introduce siloxane rings in the materials (in case of initiated CVD of cyclosiloxane). Finally, the introduction of additional porosity was investigated as another way to potentially improve the sensitivity of the chemical sensitive layers. By using a porogen approach, it is possible to introduce up to 40 % porosity in a PECVD SiOCH. Using a foaming approach, we can go even further (> 50 %). Film properties after deposition and after potential annealing were investigated using multiple characterization techniques (ellipsometry, FTIR and Tof-SIMS). Porosity was characterized by ellipsometric porosimetry and grazing incidence small angle X-ray Scattering. In these material, pores (if any) are nanometrics and porosity is mainly open and interconnected. Thin film response under toluene or pentane exposures was studied using Quartz Crystal Microbalance functionalized with the different SiOCH.Through the synthesis and characterization of these various SiOCH thin films, the role of hydrophobicity, carbon content and specific chemical bonding can be highlighted. It is shown that the hydrophobic nature of SiOCH materials composed of a Si-O-Si backbone with methyl groups bonded to silicon, combined with the presence nanoporosity, lead to very high sensibility. The presence of isolated polar bonds such as ethoxy groups seems beneficial especially for BTEX detection. High partition coefficients toward toluene and pentane can be obtained, at least ten time higher than those measured on more classical polymers such as PDMS. Both deposition techniques (PECVD or FACVD) are able to produce good chemical sensitive layers for the detection of light alkanes and aromatics VOCs and these organosilicates constitute promising solution for the functionalization of NEMS-based gas sensors.Acknowledgment: Developments on FACVD were performed in the frame of a collaboration with TEL, US-Technology Development Center, America.

Theoretical study of metal/silica interfaces: Ti, Fe, Cr and Ni on β-cristobalite.

The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between β-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.

Hydroxyl radical oxidation of cyclic methylsiloxanes D4 ∼ D6 in aqueous phase.

Cyclic methylsiloxanes (CMS) were listed as candidates of substances of very high concerns in 2018 by the REACH. These compounds can enter environmental waters, and potentially cause harmful effects to aquatic organisms and human beings. Until now, reaction mechanisms of these pollutants with hydroxyl radicals (HO) in aqueous phase were unknown. In this study, reaction mechanisms of three typical CMS (D4 ∼ D6) with HO in aqueous phase were investigated by employing both UV/H2O2 experiments and density functional theoretical calculations. Bimolecular reaction rate constants (kHO·) of D4 ∼ D6 with HO were determined as kHO·(D4) = 8kHO·(D5) = 12kHO·(D6) = 6.6 × 108 L mol-1 s-1. Half-lives of HO oxiding D4 ∼ D6 ranged from 12 to 140 days at [HO] = 10-15 mol L-1 in sunlit surface water, and were comparable to (D4, D5) or much shorter (D6) than hydrolytic half-lives. The reactivity to HO decreased with the increasing size of siloxane ring in aqueous phase, in an order totally opposite to that in gaseous phase. Calculation results indicated that HO oxidation of the three CMS proceeded spontaneously through an exothermic H-abstraction process at the first step. Water molecules participated into H-abstraction of CMS and caused energy barrier of D5 higher than that of D4. Thus, H-bonds formed by water molecules were responsible for the reverse reactivity of CMS in aqueous phase. This work provided basic evidences suggesting environmental persistence of CMS in aqueous phase completely different from that in gaseous phase.

The effect of UV irradiation and thermal treatments on structural properties of silver-containing photo-thermo-refractive glasses: Studies by Raman spectroscopy

Structural changes in silver-containing photo-thermo-refractive (PTR) silicate glasses during photo-thermo-crystallization process were studied by methods of Raman spectroscopy. Raman spectra were measured in frequency ranges 300-700 cm−1 and 900-1300 cm−1 after UV irradiation and subsequent thermal treatments at T = 360 OC and 510 OC. Raman spectra of silver-containing PTR glass were compared with Raman spectra of glasses without Ag, and glasses without Ag, Ce and Sb. It was shown that mentioned external treatments result in the sequence of structural changes, which may be interpreted as breaks and restoration of siloxane rings, appearance of new rings, appearance and disappearance of non-bridging oxygen centers. These processes can influence on the features of photo-thermo-crystallization processes in PTR glasses.

The Study of Sol-Gel Protective Coatings Using Combined Analytical Tools

Trialkoxysilane organic-inorganic hybrids give versatile options to design various functional materials on a molecular level [1]. Among possible effective products there are also protective coatings for different metals/alloys. Versatility of approaches reflects in this work through application of: (i) short bis end-capped trialkoxysilanes, which enable formation of compact coatings; (ii) bis end-capped trialkoxysilanes with long poly(dimethylsiloxane) chain in-between the terminal trialkoxysilyl groups and (iii) cyclic tetrasiloxane composed of siloxane ring that is functionalized with trimethoxysilyl groups. All described precursors enabled formation of crosslinked structures through sol-gel reactions of hydrolysis and condensation. In the case of bis end-capped trialkoxysilane-based coatings, the effect of addition of polyhedral oligomeric silsesquioxanes (POSS) on the protective efficiency is also regarded. POSS have ordered structures composed of hard silica core rounded by organic shell [2]. The functional groups can be reactive (alkoxysilyl, vinyl, amine, …) or just offer a desired property, for example, introduce hydrophobicity (alkyl). Through reactive functional groups POSS can firmly bond into the crosslinked structure of the coating. It is already confirmed that POSS can influence chemical, scratching, mechanical, thermal and electrochemical performance of coatings. Despite quite some reports on behavior of POSS in sol-gel and other polymeric materials exist, the application of cyclic siloxane-based precursors have been highly overlooked [3]. The protective coatings prepared from the above described precursors are dip-coated on aluminum alloy AA 2024. The coatings are first characterized using standard techniques like infrared (IR) and Raman spectroscopy, contact angle measurements, surface energy value, scanning electron (SEM) and atomic force (AFM) microscopy. Potentiodynamic polarization, impedance spectroscopy or exposure in salt spray chamber is used to test the protective efficiency of coatings. Moreover, there are applied also advanced analytical techniques that combine electrochemical technique with either vibrational spectroscopic (IR, Raman) [4] or AFM technique [5]. It is evident that solely electrochemical measurement gives a sum of all processes that occur in the electrochemical cell. The coupling of another technique gives additional data. For instance, on identification of reaction intermediates or products, interfaces, type of adsorbed species and their orientation, intercalation mechanisms in the intercalation compounds, etc. In case of protective coatings, in this way one may obtain information on processes that occur in coatings during forced polarization to high anodic potentials. IR or Raman spectroscopy serves to identify the bonds in coatings that are the most prone to cleavage, detect hydration of coatings during their exposure to electrolyte, eventually even follow the breaking of Al-O-Si bonds that serves to adhesion of protective coating to the alloy substrate. AFM, on the other hand, records the changes in morphology after application of potential pulses. Accordingly, there are applied three different combined techniques for the study of corrosion processes on alkoxysilane-based coatings described above. These techniques are ex situ IR reflection-absorption (IR RA) and in situ Raman spectroelectrochemistries, and in situ electrochemical AFM. Listed techniques have a potential to upgrade the value of obtained results compared to sole techniques. Each of them, however, is subjected to special characteristics of the coupled spectroscopic or microscopic technique. Consequently, the geometries of the in situ cells are different with regard to usual electrochemical cells. Namely, the in situ cells should enable the approach of IR radiation, laser beam or cantilever and also to obtain the maximized signals. The sealing of in situ cells may require a certain level of skill and materials’ knowledge. The in situ cells were herein designed as three-electrode cells. Anyhow, the inevitable developments in instrumentation and building materials will further enlarge the enormous potential of coupled techniques. In order to characterize the functioning of the in situ cells, the electrochemical impedance spectroscopy can be applied. The impedance measurement of equal alkoxysilane-based protective coatings in various in situ cells should give answer to query if various geometry designs influence their electrochemical response.

Structural, electronic and adhesion characteristics of zinc/silica interfaces: ab initio study on zinc/β-cristobalite.

The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. With the goal of identifying its microscopic origin, we report an extensive ab initio study on the structural, electronic, and adhesion characteristics of a variety of model zinc/β-cristobalite interfaces, representative for different oxidation conditions. We show that the weakness of the zinc-silica interaction at non polar interfaces is driven by the presence of surface siloxane rings. These latter are drastically detrimental to interface adhesion when intact and their breaking is impeded by a large energy barrier. Conversely, the characteristics of polar interfaces are principally driven by the capacity of zinc to screen the surface compensating charges and to form O-Zn bonds. This screening is especially efficient in an oxygen-rich environment where the substrate-induced partial oxidation of the zinc deposit produces a considerable enhancement of interface adhesion. The identified microscopic mechanisms of interface interactions furnish precious guidelines towards practical attempts to improve adhesion. In particular, processes which enable breaking the surface siloxane rings are expected to noticeably reinforce the interaction at non-polar interfaces.

Octadecyl-modified silicas obtained by non-hydrolytic condensation of a C18-hybrid silica sol on a silica surface

Octadecyl-modified silicas bearing different amounts of octadecyl groups were synthesized by non-hydrolytic condensation of a C18-hybrid silica sol on spheroidal silica. The modified materials were characterized by a set of complementary techniques, including diffuse reflectance infrared spectroscopy (DRIFTS), attenuated total reflectance infrared spectroscopy (ATR), solid-state magic angle spin 1H, 13C and 29Si nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analyses (TGA), N2 adsorption/desorption porosimetry and scanning electron microscopy (SEM). The octadecyl-modified silicas presented weight loss percentages of octadecyl ranging from 0.5 to 8.5wt%. An increase in the octadecyl content on the silica surface resulted in an increase in the alkyl chain order and the six-fold siloxane rings. The use of a high C18-hybrid silica sol amount in the synthesis led to the formation of a porous hybrid silica gel layer on the surface of the spheroidal commercial silica.