Adsorbates segregated

The system of adsorption isotherms (2.93) allows for the discussion of the critical phenomena in the multi-component adsorption layer. When there is an interaction between die adsorbed particles and the adsorbate segregation occurs. For the sake of clarity, as an example we will present below the derivation for a two-component layer, however the conclusions are qualitatively the same for this case and for the cases where more than two components form the adsorbed layer. 

In the case of mixtures of gases of different molecular size, an adsorbent of D > 2 will effect some segregation by size. This segregation will also affect the probability of bimolecular reactions between molecules of different sizes [168]. 

Scheme 5.1. Alloy formation and segregation in bimetallic systems with one of the metals present as a minority. The scheme qualitatively predicts whether two elements form a surface alloy or a solid solution. The results are valid in vacuum. As soon as an adsorbing gas is...

The surface after the reaction was found, using AES, to be clean with the exception of the presence of strongly adsorbed nitrogen and some small amounts of sulfur that segregated from the crystal bulk during the course of the reaction. Exposure of this surface to reaction conditions resulted in a reaction rate similar to that over the clean surface. A layer of PeN was prepared by exposing the surface to 5x10 Torr of ammonia at 673 K for 10 hrs. ( ). ... 

In the last two sections the formal theory of surface thermodynamics is used to describe material characteristics. The effect of interfaces on some important heterogeneous phase equilibria is summarized in Section 6.2. Here the focus is on the effect of the curvature of the interface. In Section 6.3 adsorption is covered. Physical and chemical adsorption and the effect of interface or surface energies on the segregation of chemical species in the interfacial region are covered. Of special importance again are solid-gas or liquid-gas interfaces and adsorption isotherms, and the thermodynamics of physically adsorbed species is here the main focus. 

Fig. 23 a, b. a An amplitude micrograph of PMMA brushes adsorbed on mica from a dilute solution in chloroform measured by tapping mode SFM. b Three-dimensional SFM micrograph of single brush-molecules indicated by the arrow in (a). The axial modulations have been explained by phase segregation of the side chains into nodules (insert) [82]... 

How surface adsorbed or segregated impurities may have influenced the results mentioned above is an important question. At the temperatures of the Ni experiments it is likely that desorption or dissolution into the crystal are rapid processes. In fact it was to address this question that we carried out an extensive series of experiments on the surface concentration of C as a function of temperature for various Ni surfaces[18,19]. The data indicates that neither C nor O is likely to remain on the surface but small concentrations of S cannot be raled... 

In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

In other papers by the same group, the effects of sulfur adsorbed or segregated on the Ni surface on corrosion or passivation were described, including the sulfur-induced enhancement of dissolution and the blocking of passivation. It was shown how the conditions of stability of adsorbed sulfur monolayers could be predicted on thermodynamical grounds and this was illustrated by a potential-pH diagram for adsorbed sulfur on nickel in water at 25 °C. (See Refs. 22, 25-29 and papers cited therein.)... 

Island Formation and Segregation. Island formation affects the kinetics because often only the adsorbates at the edges of islands can react with other molecules. (Exceptions are adsorbates that form islands and that can react with each other, and if there is an Eley-Rideal mechanism.) To form islands it seems necessary to have an attractive interaction that keeps adsorbates together. Indeed, if there is just one type of adsorbate an attractive interaction leads to island formation at low temperatures, whereas a repulsive one does not. If there is more than one type of adsorbate, then one can even have island formation if all lateral interactions are repulsive. The term segregation may be more appropriate for such a situation. 

Table 5.1. Adsorption properties of metal monolayers on metal substrates. The clean substrate properties are also given for comparison. Substrates are ordered by lattice type (fee, bcc, hep, cubic, diamond and rhombic). The structures, nearest neighbor distances and heats of vaporization refer to the bulk material of the substrate or the adsorbate. VD, ID and S stand for vapor deposition, ion beam deposition and surface segregation, respectively. TD, WF and TED stand for thermal desorption, work function measurements and transmission electron diffraction, respectively...

Calculations from SCF theory of the mixed layer structure, and of the interaction potential for a pair of mixed layers as a function of interlayer separation, suggest that the mixed layer has a heterogeneous morphology perpendicular to die interface (Parkinson et al., 2005). This localized segregation arises from the excluded volume interaction between spaced-out casein chains and the dense brush-like layer that was invoked in the simple SCF model to represent the p-lactoglobulin adsorbed monolayer. 

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