Surface Deactivation Research Articles

One-component polyurethane systems by deactivation of dispersed solid isocyanates

One-component isocyanate curing systems employed to date may be cured either by the action of atmospheric moisture or by splitting off a low molecular weight protective group by thermal treatment. Both processes are limited in their application, however. One-component polyurethane (PU) curing systems with a long shelf-life can be produced by surface deactivation of insoluble solid particulate isocyanates followed by their dispersion in polyols. During this process only a small proportion of the isocyanate groups available at the surface react with long-chain aliphatic polyamines and this leads to superficial stabilization via the formation of a protective polymer shell. At a higher degree of conversion or if amines with a lower molecular weight are employed, this leads to the formation of a rigid urea wall similar to a microcapsule which behaves in a completely different manner and can only be activated thermally with great difficulty. On the other hand, such surface-stabilized dispersions can be cured at 100–120 °C to yield homogeneous films. These one-component two-phase systems combine a long shelf-life with a relatively low stoving temperature, with cure being achieved without the splitting off and release of low molecular weight compounds. Systems with a wide range of properties capable of producing layer thicknesses from thin films up to moldings can thus be produced.

A stochastic analysis of catalyst deactivation in presence of external transfer limitations

The paper analyzes the simplest case of concentration independent deactivation of a catalyst surface in the presence of external transport limitations and accounts for the variations around the mean value of the activity of the catalysts leading to a stochastic formulation. The results reveal interesting features which would not be evident from the conventional deterministic analysis.

Fouling-resistant, polymer-modified graphite electrodes

Oxidation of several biochemically important phenols (phenol and tyrosine) and methoxyphenols (homovanillic acid, eugenol, vanillin and p-methoxyphenol) at spectroscopic graphite electrodes results in deactivation or fouling of the electrode surface. As has been previously well described, fouling is often due to the build-up of the oxidation products of these compounds at the electrode surface. Electrodes coated with gamma-irradiated immobilized films of poly( N-vinylpyrrolidone) do not exhibit the decrease in response with time observed at bare electrodes during the oxidation of these compounds. In some instances, this may be due to the structure of the polymer which may prevent large amounts of oxidation products from forming. In other instances, however, the nature of the products formed may differ at the coated electrodes. In this situation, responses as high as those initially obtained at bare electrodes may be obtained without the decrease in response as a function of time associated with bare electrodes.

Mediation of oxidation reactions at noble metal anodes by low levels of in situ generated hydroxy species

The study of organic oxidation reactions at noble metal electrodes in aqueous media is frequently complicated by surface deactivation effects. Hydrazine is a very useful model compound for this type of work as it is both highly reactive and free of complications due to poisoning. It was demonstrated that for several metals (Au, Pt, Pd and possibly Rh) the potential for the onset of oxidation of dissolved species at low pH coincided approximately with that for hydrous oxide reduction in the absence of oxidisable species. Compact monolayer oxides, and high levels of adsorbed hydrogen, have an inhibiting effect—the latter is particularly marked with Ir and Ru. With the other metals (and also, according to earlier data, with both Ag and Cu in base) the effective oxidising agents were apparently low levels of hydrous oxide species generated at low coverage, adatom sites at the electrode—solution interface. There may be also some activation of the dissolved species due to adsorption or coordination at active centres on the electrode surface.

The preparation of immobilized stationary phase fused silica capillary columns with OV‐1701‐vinyl deactivation

AbstractA new simplified method was developed for the preparation of immobilized stationary phase fused silica capillary columns coated with OV‐1‐vinyl, SE‐54, and OV‐1701‐vinyl. This methodology includes the application of the moderately polar stationary phase OV‐1701‐vinyl as a surface deactivation agent that may also participate in the dicumyl peroxide initiated free radical immobilization process. Practical details of the procedure are presented and applications of the laboratory‐produced columns are illustrated with capillary gas chromatographic separations of standard mixtures. Possible contributions of the OV‐1701‐vinyl deactivation layer to both crosslinking and surface bonding in the immobilization process are discussed.

Evaluation of an immobilized cyanosilicone, 60-CN, as a stationary phase for capillary gas chromatography

The influence of several factors, such as the method used for the synthesis of the stationary phase, the type of fused-silica capillary tubing, surface deactivation, stationary phase immobilizatioin, column rinsing and ageing, on the properties of capillary columns coated with silicones having 60% cyano substitution has been investigated. The column properties studied were efficiency, deactivation, acid base status, polarity and thermal stability. The column properties were found to be generally improved by stationary phase immobilization. However, an increase in the HETP of n-alkanes is a disadvantage that may be encountered. Well deactivated, relatively efficient columns were prepared with good reproducibility. The properties were largely retained after column ageing at 220°C.

A Study on the Thickness of Rhodium Oxide Film Produced by the Surface Deactivation Treatment of Rhodium-Plated Contact Reed Switches

Surface deactivation treatment, i.e., oxygen treatment at high temperatures, was invented to prevent adsorption and catalysis of rhodium-plated contact surfaces. This oxygen treatment was also found to be effective to inhibit soft sticking, a kind of locking phenomena that is one of the most serious problems in the field of electrical contacts. So far, oxygen on rhodium-plated contact surfaces treated with oxygen at 450°C or above is found to be present in the state of rhodium oxide (Rh 2 0 3 ). However, research on the thickness of rhodium oxide film remained to be carried out. This time the rhodium-plated contact surfaces are examined precisely to obtain information about the thickness of rhodium oxide film produced by oxygen treatment at 450°C or above, by utilizing ellipsometry and Auger electron spectroscopy. As a result of these investigations, it is found that the thickness of rhodium oxide film by oxygen treatment at 450°C is about 60 A and that the thickness of rhodium oxide film increases as the oxygen treatment temperature rises.

Studies on self-sustained reaction-rate oscillations: II. The role of carbon and oxides in the oscillatory oxidation of carbon monoxide on platinum

CO oxidation on a platinum foil was studied in a high pressure flow cell (10 2−10 2 Pa) and an UHV chamber (10 −8 −5 × 10 4 Pa) both interfaced to a surface infrared spectrometer. Real-time surface infrared and calorimetry experiments performed in the cell during oscillatory oxidation indicated a slow periodic variation (∼ 40%) in the number of active sites, the period of which was commensurate with that of the reaction-rate oscillations. Auger spectroscopy performed in the UHV chamber showed that surface carbon quantitatively accounted for the surface deactivation, as evidenced by the inverse correlation of the number of surface sites active towards CO adsorption with the surface carbon concentration and by the demonstration that, at the oscillation temperatures, carbon can diffuse from the bulk to the surface, oxygen can remove surface carbon and adsorbed CO can block carbon diffusion. Although silicon oxide was always detected on the surface with infrared spectroscopy, no periodic variation in it could be observed during the reaction-rate oscillations. Auger studies confirmed that the maximum and the variations in surface concentration of silicon oxide could not account for the variations in the number of active sites. A mechanism is therefore proposed in which carbon is driving the long-period self-sustained oscillations in the rate of CO oxidation on Pt.

Synthesis of three alkyldihydrochlorosilanes and their application in studies of steric factors in the surface deactivation of porous silica

The synthesis and bonding to silica of ethyl-, octyl-, and octadecyldihydrochlorosilane are reported. These silanes are show to react to a greater extent with surface hydroxyls than analogous alkyldimethylchlorosilanes, and this results in chromatographic stationary phases with reduced residual silanol activity.

Catalytic deactivation dynamics of zeolites in multicycle processes

Catalytic deactivation dynamics of zeolites in multicycle processes has been calculated. It has been revealed that in naphthalene sorption on NaX zeolites “surface deactivation” is predominant with its similarity criterion close to unity and a high fraction of deactivation. Calculated data for several multicycle processes are given.

Temperature programmed desorption of hydrogen from 5 % Rh/ZrO2 catalyst

A considerable effect of the conditions of hydrogen adsorption (temperature, time) and the number of measurement cycles HT−OT on results of hydrogen TPD for Rh/ZrO2 catalyst has been observed. Deactivation of rhodium surface at room temperature, conditioned by high temperature reduction seems to be due to a strongly temperature-activated hydrogen adsorption.

Carbon on Pt(111): Characterization and influence on the chemisorptive properties

Fully dehydrogenated carbon deposits in the monolayer range have been characterized on the Pt(111) face by LEED, Auger and Δφ observations. These techniques pointed to the graphitic character of such deposits at least beyond about 0.5 monolayer of carbon. The gradual deactivation of the Pt surface towards CO or C 6H 6 adsorption with increasing carbon coverage has been explained by the nucleation and the growth of numerous inactive graphitic islands. The loss of chemisorptive properties only occurs for a complete carbon monolayer which may be viewed as a passive film masking the metal surface. Some weakening in the binding energy of CO, C 6H 6 and H 2 has been evidenced on a surface partially covered with carbon. This additional effect of carbon has been discussed mainly in relation with Δφ measurements.

A study on the Surface Deactivation Treatment of Rhodium-Plated Contact Reed switches

As a member of the platinum group, rhodium is al adsorptive and catalytic element. Therefore, rhodium-plated contac surfaces adsorb organic impurities which become polymers durin operations having a low-level load. Oxygen treatment at high tempera tures is an effective way of preventing adsorption and catalysis, thu enabling the production of high-reliability reed switches with stabl contact resistance. In earlier work the optimum treatment temperatur was experimentally determined to be 450°C. However, research on th interaction between oxygen and the rhodium-plated contact surfac remained to be carried out. Therefore, rhodium-plated contact surface are examined precisely to determine how oxygen reacts with surface treated at around 450°°C by utilizing Auger electron spectroscopy reflection high-energy electron diffraction, molecular optical laser exam iner, and X-ray photoelectron spectroscopy. As a result, oxygen ol rhodium-plated contact surfaces treated with oxygen at 450°C or above is found to be present in the state of rhodium oxide (Rh <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> 0 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> )

Deactivation of fused-silica capillary columns with polymethylhydrosiloxanes : Optimization of reaction conditions

An evaluation of reaction conditions for surface deactivation of fused-silica capillary columns with polymethylhydrosiloxanes (PMHS) is given. Deactivation was accomplished by a dehydrocondensation reaction between the silylhydride bonds of PMHS and the surface silanols of fused silica. This provided a bonded, low surface tension film wettable by polydimethylsiloxane stationary phases. The percentage of silylhydride groups in the polymer influenced the required reaction time at 300°C. Hydrolysis of SiH bonds by physically adsorbed water played a role in providing complete surface coverage of the deactivation layer. The exclusion of oxygen was imperative during the dehydrocondensation reaction. Reaction temperatures for thermally initiated dehydrocondensation of silanol and silylhydride, silylhydride oxidation, and silylhydride hydrolysis were determined.

Deactivation of fused silica capillary columns with cyanopropylhydrosiloxanes

AbstractA method is described for surface deactivation and modification of fused silica capillary columns with a cyanopropyl‐containing reagent. The deactivation procedure involved a dehydrocondensation reaction between a bis(cyanopropyl)methylhydropolysiloxane reagent and surface silanol groups at an optimum temperature of only 250°C. Actual critical surface tension measurements were made using the capillary rise method. Excellent deactivation for acidic and basic compounds at the low ng level, and wettability for nonpolar and polar polysiloxane stationary phases were obtained. A procedure was developed to remove acidic impurities that are present in polar stationary phases.

Preliminary investigation of the effects of gamma-radiation on the surface of raw fused-silica tubing

This paper is a preliminary report concerning the effects of gamma-radiation on the surface of raw fused-silica capillary tubing. Included in this report is a discussion of how these effects pertain to the preparation of columns coated with polar liquid phases. Improved deactivation and wettability by polar phases are observed when raw fused-silica tubing is subjected to various dosages of gamma-radiation. It was also found that capillary columns exhibiting optimum chromatographic performance require a specific pretreatment dosage which increases with the polarity of the liquid phase. It is proposed that this correlation may be related to surface dehydration (surface deactivation) combined with an increase in surface irregularities at the molecular level (improved wettability). The role of gamma-radiation in altering the surface of fused-silica columns is discussed in terms of the surface phenomena described and the subsequent chromatographic performance of coated columns.

Energy characteristics of a waveguide CO2laser

The composition of the laser active medium is assumed to be constant and the rate equations for the average numbers of vibrational quanta are used to simulate numerically the operation of a waveguide CO2 laser as a linear amplifier and as an oscillator. These equations allow for the quantum flux (to levels higher than the minimum of the Treanor distribution function of the antisymmetric mode of the CO2 molecule) which is due to the high degree of vibrational excitation of the laser medium and they also allow for surface deactivation of CO2 molecules at the inside walls of the waveguide. The calculations are made for two typical diameters of the discharge channel, 1.5 and 2 mm. The results are given in the form of graphs in which the discharge current is an independent variable.

Synthesis of LC Reversed Phases of Higher Efficiency by Initial Partial Deactivation of the Silica Surface

Pretreating silica with a small amount of end-capping reagent, followed by exhaustive octadecylation, yields reversed phases of higher efficiency than phases made in the usual manner. Increases in plate numbers by a factor of two to three for various test solutes have been achieved, along with reductions in peak tailing. This effect is most likely due to the production of a chemically modified surface with a more homogeneous distribution of octadecyl groups.

Role of surface groups in affecting the chromatographic performance of certain types of fused-silica glass capillary column : III. The deactivation and coating of certain specially prepared high-molecular-weight, high-viscosity non-polar and polar stationary phases on untreated fused-silica surfaces in a single step

High-molecular-weight, high-viscosity, non-polar, silanol-terminated polydimethylsiloxane polymers with and without phenyl substitution when applied in a single step to untreated fused-silica glass capillary tubing efficiently deactivated and wetted these surfaces. Upon heating, there is evidence that hydrogen bonding occurred between the silanol moieties of the polymer and those of the fused-silica surface. This formed a covalent linkage between the polymer and the glass. Concomitant condensation of the polymer occurs, effecting crosslinking of the SiOSi type. Successful single-step deactivation and uniform wetting was also noted with a wide variety of high-molecular-weight, high-viscosity, Polar cyanopropyl silicone polymers with vinyl or tolyl groups. In many instances, however, difficulties were encountered in the cross-linking of these polymers in the presence of free radical generators. Under certain experimental conditions, the mechanisms that give rise to the surface deactivation of untreated fused-silica glass in the presence of certain other non-polar and polar polymers with various chemical compositions are unknown at this time.

Immobilization of stationary liquids in reversed- and normal-phase liquid chromatography : Production and testing of materials for bonded-phase chromatography

New types of silanization reagents for small-particle silica packing materials and a solventless vacuum technique for carrying out the reaction were tested with the aim of conveniently achieving a more complete reaction of the silanol groups. Materials for bonded-phase chromatography with better quality and reproducibility were obtained. In a second series of experiments we attempted to fix monomeric and oligomeric siloxanes and also certain unsaturated hydrocarbons on either unsilanized or presilanized materials by cross-linking reactions with γ-radiation or peroxides. With the presilanized materials of the Si- O- Si(CH 3) 3 type we obtained better surface deactivation and fixation of the siloxane oligomers on the surface alkylsilyl groups by the cross-linking reaction. All silicas should be examined for particle size and structure before and after chemical treatment and analysed for carbon content in bulk and in the surface layer, for permeability and for tailing behaviour after packing of a column. Surface area determinations are also advisable. Chromatographic measurements for efficiency and polarity (selectivity) gave information about the practical value of these materials.