Monolayers irreversible adsorption

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

In order to prevent the irrevisible adhesion of MEMS microstructures, several studies have been performed to alter the surface of MEMS, either chemically or physically. Chemical alterations have focused on the use of organosilane self-assembled monolayers (SAMs), which prevent the adsorption of ambient moisture and also reduce the inherent attractive forces between the microstructures. Although SAMs are very effective at reducing irreversible adhesion in MEMs, drawbacks include irreproducibility, excess solvent use, and thermal stability. More recent efforts have shifted towards physical alterations in order to increase the surface roughness of MEMS devices. 

If the activity of the catalyst is slowly modified by chemisorption of materials that are not easily removed, the deactivation process is termed poisoning. It is usually caused by preferential adsorption of small quantities of impurities (poisons) present in the feedstream. Adsorption of extremely small amounts of the poison (a small fraction of a monolayer) is often sufficient to cause very large losses in catalytic activity. The bonds linking the catalyst and poison are often abnormally strong and highly specific. Consequently, the process is often irreversible. If the process is reversible, a change in the temperature or the composition of the gas to which it is exposed may be sufficient to restore catalyst... 

Few studies have been made of benzene chemisorption by the volumetric method. Zettlemoyer et al. (8) have examined the adsorption of benzene vapor at 0°C on powders of nickel and of copper. First, the monolayer coverage of argon (vm) A, was measured. The argon was then removed by pumping and the amount of benzene required to form a monolayer, (vmi) Bz, was measured. Weakly adsorbed benzene was then removed by pumping, after which further benzene adsorption provided the value (vm2) Bz. Some results are reproduced in Table I. On the assumption that the same extent of surface is accessible both for argon and for benzene adsorption, it is clear that complete monolayers of benzene were not achieved, that some (Ni) or all (Cu) of the benzene was adsorbed reversibly. It was considered that only the irreversibly adsorbed benzene was chemisorbed, the remainder being physically adsorbed. Thus chemisorption of benzene on copper appeared not to occur. The heat of adsorption of benzene on nickel at zero... 

Preferential adsorption of a surfactant from a mixture depends on the structures of the amphiphiles and the substrate. Self assembly by physisorption is reversible, while that by chemisorption is irreversible. Thus, surfactants physisorbed in monolayers can be replaced by surfactants which are able to chemisorb. Such behavior was demonstrated by allowing a donor cyanine surfactant, D (capable of physisorption), and OTS (known to chemisorb) to... 

One of the main motivations for the development of chemically modified electrodes is the introduction of (electro-) catalytic species onto the electrode surface. These modified electrodes can be used to improve specificity and product yields in electrochemical synthesis or as the basis for a biosensor. Catalysts can be attached through (irreversible) adsorption onto a suitable substrate (7). These systems mosdy consist of an electrode covered with a monolayer of the electroactive species. In many cases this method does not lead to effective systems, due to instability of the monolayer or low loading with the catalyst. Direct modification of the electrode surface by covalent... 

Bulatnikov et al. (340) studied the effects of promoters on sulfur resistance of iron by measuring the amount of radioactive H2S adsorbed on iron catalysts promoted with A1203 and/or K20. They reported irreversible deactivation of Fe promoted with A1203, A1203 + K20, or KzO after 0.8, 1.5, and 5 monolayers of sulfur had been adsorbed. In other words, the presence of K20 was responsible for increasing sulfur adsorption capacity, although it was not clear upon which portion of the surface sulfur had adsorbed. It was also reported that A1203 was necessary to prevent volatilization of K20. 

Surface redox reactions — or surface -> electrode reactions, are reactions in which both components of the -> redox couple are immobilized on the electrode surface in a form of a -> monolayer. Immobilization can be achieved by means of irreversible -> adsorption, covalent bonding, self-assembling (- self-assembled mono-layers), adhesion, by Langmuir-Blodgett technique (- Langmuir-Blodgett films), etc. [i]. In some cases, the electrode surface is the electroactive reactant as well as the product of the electrode reaction is immobilized on the electrode surface, e.g., oxidation of a gold, platinum, or aluminum electrode to form metal oxide. This type of electrode processes can be also considered as surface electrode reactions. Voltammetric response of a surface redox reaction differs markedly from that of a dissolved... 

NMR adsorption isotherms for Ru/SiOi catalysts have been obtained using explicit calibration (89). Although the pressure over the sample could be adjusted in situ, no volumetric data were taken simultaneously, probably because of the important spillover effects in this catalytic system (see Section III.A). The NMR study was performed at pressures between 10 and 760 Torr and at temperatures between 323 and 473 K (only the 323-K results are reviewed here). The dispersion of the catalyst was determined from the irreversible H NMR signal as 0.29. The metal loading was 8 wt% so that a monolayer coverage on 1 g of catalyst corresponds to 2.8 cm of H2 under standard conditions. It is typical for an NMR sample to contain 0.5 g of material in a 1-cm sample volume, and the pores in the powder make up about half the volume. If such a sample of this catalyst is under 760 Torr of hydrogen, the gas phase corresponds to one-third of a mono-layer, and it can make a detectable contribution to the NMR signal. 

The adsorption of 1,3-cyclohexadiene on Pt(lll) leads to irreversible dehydrogenation to benzene below 400 K, the majority of which is further dehydrogenated at higher temperatnres to form a carbonaceons residue [48], No 1,3-cyclohexadiene desorbed dnring TPD from the Sn/Pt(l 11) alloys, but the monolayer was converted with 100% selectivity to prodnce gas-phase benzene. No carbon was left on the alloy snrfaces after TPD as determined by AES. The ensemble requirement for cyclohexadiene dehydrogenation on these alloys is at most a few (4-5) Pt atoms (see Pig. 2.1). 

Proteins are found either not to desorb or to desorb only with great difficulty from quiescent interfaces. Langmuir and Schaefer (1939) calculated, on the basis of the Gibbs adsorption equation, that compression of a monolayer of protein of molecular weight 35,000 by 15 mN m-1 should increase its solubility by a factor of 1095. This results from the large area occupied by the molecule at the interface and the resultant large pressure increment of solubility. The failure of protein monolayers to desorb readily on compression was thus taken as an indication of irreversibility. 

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