Adsorbent secondary effects

Connors and Jozwiakowski have used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids [12]. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. As an example of this behavior, selected reflectance spectra obtained for the adsorption of indolinonaphthospiropyran onto silica gel are shown in Fig. 1. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased the 550 nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect is most likely due to the presence of pyran-pyran interactions, which become more important as the concentration of sorbant is increased. 

Gillis-D Hamers explains this shift by the interaction of the free silanols with the surrounding hydrophilic centres (adsorbed water, bridged and free silanols). The free silanol peak shift is considered as a secondary effect although the free silanols are in no direct interaction with the other silanols, still their presence causes slight changes in the electronic environment of the free O-H bond, resulting in a small peak shift. 

The adsorbent has only a secondary effect on the adsorbate-adsorbate interaction. For this reason, we will focus our attention on the second term, adsorbate-adsorbent potential, and refer to this term as [Pg.82]

Equation (6-6) [and (6-8)] can be corrected to include secondary adsorbent activity effects of the type under discussion ... 

The first stage in chemisorption probably consists in the transfer of electrons either to or from the cations of the lattice, leading to the formation of a charge transfer bond. This bond will be more stable at low than at high temperatures. Secondary effects occur at higher temperatures, such as reaction with the oxygen ions of the lattice or the creation or destruction of lattice vacancies. When the adsorbed gas is an ion, there will be a space charge or barrier layer which will affect the character of the adsorption isotherm, modify the rates of adsorption, and create lattice vacancies. 

When enzymes are adsorbed, the rate at which they act may be depressed because of their removal in part at least by the adsorbent (McLaren, 1954a,b McLaren and Estermann, 1956). This effect is especially marked if they are trapped in the interlamellar spaces the effect is usually not great when 2 1 clay minerals are not present. It is sometimes difficult to interpret experimental data because secondary effects, such as adsorption of substrate or of toxic products, may be more important than the main transformation under study. 

The most important type of inhibitor to the oil industry is the filming inhibitors. One end of the inhibitor molecule is adsorbed onto the metal surface. The non-polar tail of the inhibitor molecule is oriented in a direction generally vertical to the metal surface. It is believed that the hydrocarbon (non-polar) tails mesh with each other in a sort of zipper effect to form a tight film that repels aqueous fluids, estabhshing a barrier to the chemical and electrochemical attack of fluids on the base metal. A secondary effect is the physical adsorption of hydrocarbon molecules from the process fluids by the hydrocarbon tails of the adsorbed inhibitor molecules. This increases both the thickness and effectiveness of the hydrophobic barrier to corrosion. 

With external lubrication, concern is focused on the interface between the compound and the process equipment. The surface of the latter is generally an oxidized polar layer with a pattern of microscopic cracks and pits. The polar sections of external lubricants are adsorbed on this layer and in these sites. Compound flow at the lubricated surface is then hindered only by the interaction of nonpolar ends of lubricant molecules adsorbed on the surface with others borne by the polymer. The primary effect is less resistance to flow secondary effects may include higher output, lower heat buildup and energy consumption, improved surface appearance, and easier release. The effectiveness of an additive as an external lubricant should be determined with laboratory-scale equivalents of the process equipment. Small extmders, calenders, and spiral flow molds are... 

This section presents a general overview on the mode of action of dispersants in refractory castables, focusing especially on the ability of these molecules to adsorb and modify the surface chemistry of particles (Parts A—C), the secondary effects that may arise from the addition of dispersants to the castable matrix (Part D), the possible interactions between dispersants and cement particles (Part E), and novel routes that have been applied to design the dispersant molecule in order to optimize the rheological behavior of castables and concretes (Part F). 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

Reaction of adsorbed inhibitors In some cases, the adsorbed corrosion inhibitor may react, usually by electro-chemical reduction, to form a product which may also be inhibitive. Inhibition due to the added substance has been termed primary inhibition and that due to the reaction product secondary inhibition " . In such cases, the inhibitive efficiency may increase or decrease with time according to whether the secondary inhibition is more or less effective than the primary inhibition. Some examples of inhibitors which react to give secondary inhibition are the following. Sulphoxides can be reduced to sulphides, which are more efficient inhibitorsQuaternary phosphonium and arsonium compounds can be reduced to the corresponding phosphine or arsine compounds, with little change in inhibitive efficiency . Acetylene compounds can undergo reduction followed by polymerisation to form a multimolecular protective film . Thioureas can be reduced to produce HS ions, which may act as stimulators of... 

On the contrary, on oxygen-modified metal surfaces where secondary reactions between the adsorbed oxygen and ethylene decomposition products can easily occur, the effect of oxygen on the adsorptive capacity of the... 

Passivation phenomena on the whole are highly multifarious and complex. One must distinguish between the primal onset of the passive state and the secondary phenomena that arise when passivation has already occurred (i.e., as a result of passivation). It has been demonstrated for many systems by now that passivation is caused by adsorbed layers, and that the phase layers are formed when passivation has already been initiated. In other cases, passivation may be produced by the formation of thin phase layers on the electrode surface. Relatively thick porous layers can form both before and after the start of passivation. Their effects, as a rule, amount to an increase in true current density and to higher concentration gradients in the solution layer next to the electrode. Therefore, they do not themselves passivate the electrode but are conducive to the onset of a passive state having different origins. 

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

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