Silanol group effect in C18 bonded phases in HPLC

Summary If any residual (free) silanol groups remain at the surface of silica gel after bonding treatment, they may affect the retention of solutes since the dissociated groups (SiO^–) will attract cations. The silanol group effect on the retention of cationic solutes will increase with increasing pH of the mobile phase but the effect will decrease with increasing hydrophobic-ion concentration at the C₁₈ surface because such ions can mask the residual silanol groups. A method for the separation of metal complexes with 2-(5-bromo-2-pyridylazo)-diethylaminephenol (5-Br-PADAP) has been developed. The hydrophobic ion in the MeOH/H₂O mobile phase was tetrabutylammonium (TBA).     

Electronic structure of the surface compounds ≡SiOElCl3 (El, = C,Si, Ge,Sn) and their thermal stability

The SCF MO LCA0 method in the MNDO approximation has been used to consider the electronic structure of the surface compounds SlODlCl₃ (El = C, Si, Ge, Sn), modelled by extended clusters. The thermal stabilities of these groups are compared and conclusions ore drawn about the probable mechanism of the interaction of the surface silanol groups of SiO₂ with ElCl₄ molecules.Institute for Surface Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 1, pp. 19–22, January–February, 1991. Original article submitted March 11, 1990.     

Adsorption of Cationic Polyelectrolyte on Quartz from Aqueous Background Electrolyte Solution at pH 3

Adsorption isotherm of cationic polyelectrolyte, poly(styrene-co-dimethylaminopropylmaleimide), (molecular mass is 2 × 10⁴) on the surface of fused quartz in aqueous 10^–4 M KCl solution at pH 3 was measured by the method of capillary electrokinetics. The limiting coverage of adsorption layer corresponds to surface charge ₀ = 0.82 C/mol that exceeds the value obtained earlier at pH 6.5. However, if one takes into account the higher charge of a macromolecule at pH 3, the values of packing density of copolymer molecules in completely filled adlayers appeared to be close: 7.88 × 10¹⁰ at pH 3 and 7.27 × 10¹⁰ cm^–2 at pH 6.5. The average binding energy of the molecules and the quartz surface calculated by the Langmuir equation is equal approximately to 21kT and lies between the values of the energy of electrostatic (25.4kT) and hydrophobic (17.7kT) adsorption at pH 6.5 calculated earlier. It can be assumed that, at pH 3, charged units of a macromolecule form ion–dipole bonds with silanol groups, while uncharged groups form hydrophobic bonds with siloxane surface sites.     

Infrarot-spektralphotometrische Bestimmung der ?freien? Silanolgruppen in pyrogenen Kiesels?uren

Zusammenfassung Die technischen Eigenschaften pyrogener Kieselsäuren werden entscheidend vom Gehalt an »freien« Silanolgruppen mitbestimmt. Die IR-Absorption dieser Gruppen in CCl₄ gehört zu einer Bande bei 3700 cm^–1. Es wurde festgestellt, daß die maximale Extinktion in CCl₄ unter bestimmten Bedingungen unabhängig von der Anzahl ebenfalls vorhandener »gebundener« Silanolgruppen (max. Extinktion bei etwa 3350–3450 cm^–1) sowie auch weitgehend unabhängig vom Wassergehalt (Feuchtigkeit) ist. Diese Tatsache wurde zur Ausarbeitung eines Routineverfahrens benutzt.

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Infrared-spectrophotometric determination of free silanol groups in pyrogenic silicic acids

Summary The technical properties of pyrogenic silicic acids are decisively co-determined by their content of so-called feee silanol groups. Their infrared absorption in a CCl₄ suspension belongs to a band at about 3,700cm^–1. It has been found that under certain conditions their maximum absorbance in CCl₄ is independent of the number of so-called bonded silanol groups (maximum absorption at about 3,350–3,450cm^–1), additionally existing in all commercial pyrogenic silicic acids, and also in wide ranges of the water content (humidity). This fact has been used as a basis for a quantitative routine determination of free silanol groups.

Zum Gedenken an Heinz Dannenberg     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: I. Methane Chemisorption on a Reactive Silica Surface

Experimental and quantum chemical study of reactive silica surface methylation is carried out. The main product of the reaction is the (Si–O)₂Si(H)(CH₃) groups, which are formed via a radical-chain process with the participation of methane molecules and paramagnetic and diamagnetic defects on the oxide surface. Spectral (optical and IR) characteristics of the groups participating in the process (Si–O)₂Si^·–CH₃, (Si–O)₂Si(H)(CH₃), and (Si–O)₂Si(CH₃)(CH₃) are determined. Information on the kinetics of separate steps of the process is obtained including rate constants and the activation energies of steps.     

Reactivity of bis(trimethylsilyl) sulfate with respect to the surface of silica

It was established by IR spectroscopy and chemical analysis that bis(trimethylsilyl) sulfate reacts chemically with isolated silanol groups of silica even at room temperature, and is quantitatively chemisorbed at 373 K. Sulfidation of the silica does not occur. Chemisorption of [(CH₃)₃Si]₂SO₄ on the surface of silicon(IV) oxide is not complicated by side processes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 377–380, May–June, 1989.     

Dissociation Constants of Silanol Groups of Silic Acids: Quantum Chemical Estimations

Dissociation constants of silanol groups on the silica surface are calculated with the DFT quantum chemical method using B3LYP and M06 functionals. Structural features of silanol fragments and the presence of hydrogen-bonded water clusters are shown to have significant effects on pK_a values of silanol groups. In particular, pK_a values are shown to vary widely depending on the features of the system of hydrogen bonds.     

Synthesis of Bioactive Organic-Inorganic Hybrids with γ-Methacryloxypropyltrimethoxysilane

Bioactive organic-inorganic hybrids were synthesized through sol-gel processing starting from -methacryloxypropyltrimethoxysilane. NMR-spectroscopic studies showed the presence of silanol groups (Si–OH) and Si–O–Si bonds. In vitro tests of the hybrids for bioactivity with a simulated body fluid (Kokubo solution) indicated that only calcium-containing hybrids could form apatite on their surfaces. Thus the presence of calcium ions was no less important to deposit apatite than the formation of silanol groups or Si–O–Si bonds.     

Electrosurface and Ion-Selective Properties of Track-Etched Nanofilters: The Effect of Polyvalent Metal Adsorption

The effect of polyvalent metal adsorption on the performance and ion selectivity of poly(ethylene terephthalate) track-etched membranes with pores of 10 nm in diameter was studied. Membrane samples were prepared from the track-etched membranes with pores of 20 nm in diameter by thermal shrinkage. It was shown that an effective pore diameter decreases and selectivity of track-etched membranes increases upon filtration of Al(NO₃)₃ and Cr(NO₃)₃ solutions. The results obtained are explained by ion adsorption leading to the formation of complexes between polyvalent metals and carboxyl groups on the pore surface that is confirmed by IR spectroscopy data. The study of electrosurface properties of modified membranes and the dependence of ion selectivity of track-etched membranes on the concentration of Al³⁺ ions in 10^–2 M KCl solution indicates the decrease of membrane negative surface charge resulted from Al³⁺ adsorption and membrane charge reversal at Al³⁺ concentration in a solution higher than 10^–6 M. The dependences of the ion selectivity on pH and Al³⁺ concentration C _Al in a solution are similar. At pH < 3 and C _Al > 10^–6 M, the _1–2 > _1–1 > _2–1 ion selectivity series characteristic of the initial negatively charged membranes for the 1–1, 1–2, and 2–1 electrolyte solutions is reversed into the _2–1 > _1–1 > _1–2 series characteristic of positively charged membranes.     

Reactivity of trimethylcyanosilane toward the surface of silica

The reaction of the surface of silica with (CH₃)₃SiCN vapor was studied by IR spectroscopy. It was established that the chemisorption of trimethylcyanosilane belongs to the processes of electrophilic substitution of a proton in free silanol groups. The activation energy, calculated from the kinetic curves, amounts to 22±1 kJ/mole of grafted trimethylsilyl groups.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 1, pp. 117–120, January–February, 1987.     

Electrophysical Properties of a Poly(ethylene terephthalate) Film Modified in a Cyclohexane Plasma

The structure of a thin layer applied on a poly(ethylene terephthalate) (PET) film using ion-assisted chemical vapor deposition of cyclohexane was studied by electron spectroscopy for chemical analysis and IR spectroscopy. It was found that the film was composed of linear (–₂–)_n chains bearing six-membered cyclohexane rings, including those containing carbonyl groups as substituents, and carbon chains free of H atoms. The plasma-synthesized cyclohexane film was found to be semiconducting. The deposition of a film 10–120 nm in thickness on the surface of PET 30 m in thickness resulted in an increase in the bulk conductivity over the temperature range 20–200°C and in a considerable increase in the electric strength.     

Hydrogenation of the silanone groups (≡Si-O)2Si=0. Experimental and quantum-chemical studies

The reactivity of bis(siloxy)silanone groups (Si-0)₂Si=O stabilized on a silica surface with respect to H₂ molecules was studied. The reaction was found to give the (Si-O)₂SiH(OH) groups. The rate constant for this process was determined. Its activation energy in the 300–580 K temperature range is 13.4±0.3 kcal mol^–1, and the enthalpy is 54±5 kcal mol^–1. The activation energy for the reverse reaction,viz., elimination of a hydrogen molecule, is equal to 65 kcal mol^–1. Quantum-chemical calculations of hydrogenation of F₂Si=O and (HO)₂Si=O, which are the simplest molecular models of the silanone groups, were performed. Data on the geometrical and electronic structures of transition states and on the effects of substituents at the silicon atom on the reactivity of the silanone groups in this process were obtained. The optical absorption band of the surface silanone groups was quantitatively characterized. Its maximum is located at 5.65±0.1 eV; the extinction coefficient at the maximum (220 nm) is (3±0.5) · 10^–18 cm² molec.^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1951–1958, August, 1996.     

X-ray Photoelectron Spectroscopy of the Surface of Solid Electrolyte La0.88Sr0.12Ga0.82Mg0.18O3 – α

The composition and chemical state of elements on the surface of solid oxygen-conducting electrolyte La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 –} is studied by x-ray photoelectron spectroscopy before and after annealing it at 973 K in a CO₂ atmosphere. Products of interaction between doped lanthanum gallate and CO₂ are localized in a surface layer 8–10 nm thick. The annealing does not lead to the formation of chemical carbon compounds with metals on the electrolyte surface. Surface layers of the electrolyte both in the initial state and after the annealing are rich in strontium oxide and contain some lanthanum hydroxide.     

Characterization of Hollow Chemical Garden Fibers from Metal Salts and Water Glass

Hollow fibers formed from water glass and metal salts of IIA(Ca), VIIB(Fe, Co, Ni) and IB(Cu) groups were characterised in this study. Fragile fibres obtained herein broke down into small pieces during isolation and drying. Quantitative information about morphology, chemical composition and surface structure of the fibres were obtained. The diameter and wall thickness of the fibers were around 50 and 3 . respectively. They had particulate inner and smooth outer surfaces. Fibers had variable composition with metal (II) oxide/SiO₂ ratio in the range 0.31 to 1.02. While group VIIB metal (II) fibres were amorphous, group IIA and IB metal (II) fibres were partially crystalline All the fibres had pores both in micro pore and meso pore region. The B.E.T surface area from N₂ adsorption data was in the range of 10–249 m³ g^–1 and 8–176 m² g^–1 from Langmuir and B.E.T models respectively.     

Effect of Metal Alkoxide Complexes on Condensation Reactions of Hydrolyzed Phenyltriethoxysilane

Metal alkoxide complexes, in which alkoxy groups were substituted with ethyl acetoacetate (EAcAc), were found to accelerate condensation reactions of hydrolyzed phenyltriethoxysilane (PTES) and affect the structure of phenylsiloxane networks. The ²⁹Si NMR study revealed that the acceleration effect increased in the order Al(O-sec-C₄H₉)_3–x (EAcAc) _x < Ta(OC₂H₅)_5–x (EAcAc)_x < Ti(OC₂H₅)_4–x (EAcAc) _x . However, the GPC study revealed that the solution contained the high molecular weight species in the order Al(O-sec-C₄H₉)_3–x (EAcAc) _x < Ti(OC₂H₅)_4–x (EAcAc) _x < Ta(OC₂H₅)_5–x (EAcAc) _x . In the case of Ti(OC₂H₅)_4–x (EAcAc) _x , the smaller size of phenylsiloxane networks with a low content of silanol groups was formed in the solution. In the case of Ta(OC₂H₅)_5–x (EAcAc) _x , the larger size of phenylsiloxane networks with a high content of silanol groups was formed in the solution. In the case of Al(O-sec-C₄H₉)_3–x (EAcAc) _x , the small size of phenylsiloxane networks with a high content of silanol groups was formed in the solution. During gelation, the catalytic effect of the metal alkoxide complexes on the development of phenylsiloxane networks was also found. However, the average molecular weight of the gels increased in the order Ta(OC₂H₅)_5–x (EAcAc) _x < Ti(OC₂H₅)_4–x (EAcAc) _x Al(O-sec-C₄H₉)_3–x (EAcAc) _x . The metal alkoxide complexes were hydrolyzed and finally incorporated into the phenylsiloxane networks of gels to form M—O—Si bonds, which were indicated by the results of FT-IR analysis.     

Study of the uptake of inorganic ions and organic acids at theα-alumina-electrolyte interface in a colloidal system by radiochemical techniques and microelectrophoresis

The interactions of the alumina-electrolyte in the presence of inorganic ions (copper, selenium) and/or organic acids (benzoic acid, glutamic acid, phenol) have been studied. The acid-based properties of-alumina have been determined in NaCl media (pzc=8.5±0.3). Three types of hydroxy groups with different acidity constants have been found. The adsorption of copper and selenium have been described by the formation of the surface complexes: (=Al¹–OH, CuCl) and (=Al¹, SeO₃)^–. The interaction of copper with the surface oxygen atoms is strong and so is the interaction of selenium with the surface aluminium atoms, which involves a shift of the iep of alumina. For the three organic acids studied (HX), the adsorption equilibrium is:=Alⁱ–OH+HX(=Alⁱ, X)+H₂O. The interaction of the X^– group and the aluminium atoms is strong following the order: glutamic acid > benzoic acid > phenol. The uptake of selenium is uninfluenced by the presence of organic acids with carboxylic, amino-acidic and phenolic groups. The uptake of copper is highly increased by the presence of organic acids but it is not influenced by the presence of the phenolic groups. Both of these behaviours can be extrapolated to inorganic ions of the same chemical structure.     

Effect of alkaline modification of products of thermal vacuum and hydrothermal treatment of NH4Na-Y zeolite according to the IR spectral data

The effect of alkaline modification on the structure of the products of heat treatment of NH₄Na-Y zeolite (53% NH ₄ ⁺ , Si/Al - 2.37) in a vacuum at 573 K and in water vapors at 873 K was investigated with the IR spectrum of vibrations of the zeolite framework in the 400–1200 cm^–1 region. It was shown that the high-frequency shift of the bands in the spectra of the products obtained, the stretching vibration of v_as(TO₄) tetrahedrons in particular (T=Si, Al) at 1023 cm^–1 by 6 and 19 cm^–1, is determined by a decrease in the excess negative charge of the framework due to weakening and hydrolytic splitting of Al-O bonds of the deammoniated units with the formation of bridging Si-O(H)...Al and terminal Si-OHHO-Al hydroxyl groups. Treatment of these samples with an aqueous solution of KOH (pH 13.4) at 293 and 353 K restores the normal framework Si-O-Al bonds at the sites of formation of bridging and terminal hydroxyl groups. In the second case, restoration is hindered by substitution of H⁺ by K⁺ with some silanol groups.Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow. I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199164 St. Petersburg. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1733–1739, August, 1992.     

Calculations of the chemical shifts of E2s and E2p of Si in model clusters of an SiO2 surface

The electronic configuration of the HOSiO₃ ^3– cluster, which simulates an isolated OH group on an SiO₂ surface, has been calculated by the SCF-X-scattered-wave method with overlapping spheres. The calculation of the binding energies of the Si core electrons confirmed the previously drawn conclusion [see Teor. Éksp. Khim.,16, No. 4, 458 (1980)] that the formation of pairs of ions (Si⁺, SiO^–) and radicals (Si·, SiO·) upon the dehydroxylation of SiO₂ should be accompanied by negative chemical shifts of E_2s and E_2p of Si, while the formation of di- and trisiloxane bridges should be accompanied by positive chemical shifts. A comparison of the values of E_2s and E_2p of Si for various clusters was conducted with the use of corrections which take into account the differences in the radii of the Watson spheres. Extrapolation of the dependence of qSi on E_2s and E_2p of Si revealed that the charge of Si in crystalline silicon is equal to 0.6–0.8. The height of the barrier to the hindered rotation of a surface OH group on SiO₂ calculated in the framework of the model adopted is equal to 0.052 eV, which is in good agreement with the experimental value (0.038 eV).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 513–522, September–October, 1985.     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: III. Products of >Si

Permenov  D. G.  Radzig  V. A. 《Kinetics and Catalysis》2004,45(2):273-278