Adsorption-induced phenomena

Surfactant adsorption on the thin layer support cannot be avoided. It is the cause of a micellar gradient concentration between the solvent front and the mobile phase reservoir. Adsorption up to saturation of the sorbent depletes the surfactant concentration in the mobile phase. A double solvent front was observed, the upper one was a dilute non micellar surfactant solution, the lower second front corresponded to the micellar front [22-25, 27]. The problem is that this phenomenon introduces potential error in the identification of the exact Rf parameters of the solutes, inducing accuracy concerns of the K coefficients [28]. This adsorption-induced micellar concentration gradient was used to separate the polar solutes in the nonmicellar region (between the two solvent fronts) from the hydrophobic solutes separated by the slow moving micellar phase [22,27]. 

Parallel events in the field of in situ IR spectroscopy (for a review of sulfate IR studies, see Ref. 26) resulted in a coupled shift to molecular level bi-sulfate anion was recognized in sulfate adlayer even in solutions with predominating sulfate. It was com-pletey new situation, when chemical equilibrium is affected by adsoibate-surface interactioa In usual terms of solution equilibria, the effect corresponds to increase of pKa from its bulk value (ca. 2) to 3.3-4.7 (pKa is potential-dependent). To agree this situation with bulk thermodynamics, one should simply use electrochemical potential instead of chemical. The phenomena of adsorption-induced protonation is relative to UPD, when adsoibate-surface interaction shifts redox equihbria. In more molecular terms, the species determined as bi-sidfate ions are probably interfacial ion pairs, i.e., the phenomenon can be considered as coadsorptioa This situation is screened in purely thermodynamic analysis, as excess surface protonation is hidden in Gibbs adsorptions of sulfate and H. However it becomes important for any further model consideration, as it can affect lateral interactions and the order in the adlayer. The excess adsorption-induced protonation of various anions is a very attractive field. In particular it is the only chance to explain why multicharged oxoanions can form complete mono-layers on platinum. 

It is not exceptional that adsorption of the first molecules promotes adsorption of the subsequent one and so on. This phenomenon is called cooperative adsorption. It may be the result of lateral attraction between adsorbed molecules or by adsorption-induced changes in the interface leading to exposure of sites having a higher affinity for the adsorbate. In such a cooperative process, adsorption of the next molecule is coupled to that of the previous one. Let us discuss this phenomenon on the basis of lateral attraction between adsorbed molecules. 

ADP Adenosine di-phosphate, a product which can be used to induce platelet aggregation Adsorption Stuface phenomenon by which molecules of a gas or a liquid are fixed on the stuface of a solid, with various types of bonding... 

In the discussion of the reversibility of the Rehbinder effect, it was implied that there is a thermodynamically stable interface present between the mutually saturated solid phase and liquid medium and that the effect vanishes when the liquid medium is ranoved, for example, by evaporation. These two peculiarities make the Rehbinder effect principally different from the corrosion caused by the action of aggressive media. At the same time, one must realize that complete segregation is not possible various processes can cover a fairly broad spectrum from idealized cases involving purely mechanical failure to purely corrosive processes (or dissolution). The Rehbinder effect, which involves the adsorption-induced lowering of strength, stress-facilitated corrosion, and corrosive fatigue, often occupies intermediate positions in these series. In this type of phenomenon, the action of external forces and the action of chemically active media both contribute to the net result in certain proportions. 

If the adsorbate builds a superstructure or induces a reconstruction with a larger unit cell than the one of the bare substrate, the size of the new Brillouin zone will be reduced. Previously independent reciprocal space regions may therefore coincide after adsorption, a phenomenon that is usually referred to as folding of the zone. The folding process is shown in Fig. 3 for the case of a (100) face covered with c(2x2) and p(2x2) overlayers. As one can see, for the former case the size of the zone is halved so that the r and the... 

In Section 1.2.1, we discussed the phenomenon of adsorbate-induced chirality whereby the adsorption of achiral species (e.g., glycine) results in the formation of two mirror equivalent domains on the surface. It has recently been shown that the presence of relatively small mole fractions of chiral dopants can result in the exclusive... 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

The concept of a pore potential is generally accepted in gas adsorption theory to account for capillary condensation at pressures well below the expected values. Gregg and Sing ° described the intensification of the attractive forces acting on adsorbate molecules by overlapping fields from the pore wall. Adamson has pointed out that evidence exists for changes induced in liquids by capillary walls over distances in the order of a micron. The Polanyi potential theory postulates that molecules can fall into the potential field at the surface of a solid, a phenomenon which would be greatly enhanced in a narrow pore. 

Laser-induced desorption via the DIET process is a structure-sensitive phenomenon. Firstly, we describe the recent results for adsorbed NO on Pt(l 1 1), since the adsorption structure of this system has been misunderstood for a long time. Adsorbed species giving rise to the 1490 cm-1 NO stretching vibrational mode had been believed to be adsorbed at bridge sites [34, 35]. Recently it has been shown that this species is adsorbed at the threefold fee hollow site. This problem was pointed at first using LEED analysis by Materer et al. [36, 37]. A similar problem is the occupation of the fee and hep threefold hollow sites in a ratio of 50/50 described by Lindsay et al. [38] on the basis of a photoelectron diffraction investigation of NO on Ni(l 1 1) at a coverage of 0.25 monolayer. 

Relatively Important is the phenomenon of dijfusiophoresis and its counterpart plug or capillary osmosis. For both the driving force is a concentration gradient, either of an electrolyte or of a non-electrolyte. Consider for instance fig. 4.40. The presence of the gradient leads to at least an (osmotic) pressure gradient p(0) around the particle in the double layer. Moreover, by specific adsorption it can also lead to concentration polarization and x" may depend on 9. In this way driving forces are established to induce the particle to move. 

Another factor responsible for the observed variability in positive results may be adsorption of ammonium and other nitrogen compounds onto humic substances (the main photoreactive fraction of DOM). Hence, detection of photoammonification may to some extent reflect photochemicaUy induced desorption of ammonium from large organic molecules, rather than the mineralization of organic nitrogen. This phenomenon, however, would be limited to fresh and low salinity waters, since at higher salinities ammonium ions are replaced by cationic macroconstituents of seawater. 

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