Displacement adsorption mechanism

This plot represents the variation of an excessively adsorbed amount of acetonitrile with the variation of the equilibrium concentration of acetonitrile in the bulk solution. In the adsorption system the influence of adsorption forces exerted by the adsorbent surface are limited in their distance consequently, we should have limited volume where adsorbed analyte accumulates. It is also assumed that liquid is uncompressible and that molar volumes of both components do not change under the influence of adsorption forces. This leads to the displacement adsorption mechanism. 

Alcohols are hydroxylated alkyl-compounds (R-OH) which are neutral in reaction due to their unionizable (OH) group (e.g., methanol, ethanol, isopropanol, and w-butanol). The hydroxyl of alcohols can displace water molecules in the primary hydration shell of cations adsorbed onto soil-solid and sediment-solid clay particles. The water molecule displacement depends mainly on the polarizing power of the cation. The other adsorption mechanisms of alcohol hydroxyl groups are through hydrogen bonding and cation-dipole interactions [19,65],... 

Figure 5-1. Hypothetical representation of the adsorption mechanism of retention in normal-phase chromatography. S denotes sample molecule, E denotes molecule of strong polar solvent, and X and Y are polar functional groups of the stationary phase. Prior to retention, the surface of stationary phase is covered with a monolayer of solvent molecules E. Retention in normal-phase chromatography is driven by the adsorption of S molecules upon the displacement of E molecules. The solvent molecules that cover the surface of the adsorbent may or may not interact with the adsorption sites, depending on the properties of the solvent. (Reprinted from reference 1, with permission.)...

In particular, according to this adsorption mechanism, each adsorbate molecule A cuts from Sc a smaller cluster Sa of solvent molecules with equivalent to A dimensions and displaces it towards the bulk solution, where it is disintegrated into g monomeric solvent molecules. Thus, irrespective of the size of the original solvent clusters Sc the adsorbate molecules do not see either these clusters or the monomeric solvent molecules but only the clusters Sa, which have always dimensions equal to those of the adsorbate molecules. In this respect the adsorbed layer behaves as if it were composed of adsorbate A molecules and solvent clusters Sa with equal dimensions. For this reason, this adsorption mechanism is compatible with a value of n close to unity. Moreover, if it is analysed within the frames of classical thermodynamics, we obtain that the adsorption equilibrium may be described by the following equation [11] ... 

If the solvent does not associate on the adsorbing surface, or its association does not affect the adsorption mechanism which takes place via the displacement of single solvent molecules, then the equilibrium is described from the well known system of equations ... 

In the present work the adsorption mechanism is even more complicated because in addition to molecular orientation the crude oil/water interface is already saturated by naturally present surface active material which may have to be displaced from the interface before adsorption can take place. 

In this problem, the mass transport of a surfactant during miscible displacement in a saturated medium is evaluated. Besides convection and dispersion, the additional mechanism which must be considered is adsorption. Most studies have assumed that the adsorption mechanism occurs rapidly compared with the mechanisms of convection and dispersion. Thus, an equilibrium isotherm usually of the Langmuir type is assumed. 

In the mesoporous range for MCM-41, each steps of the water adsorption mechanism are well identified by neutron diffraction the physisorption of the first molecules on the rough MCM-41 wall, then the whole filling of the MCM-41 mesopore by water molecules. The usual molecular water organisation is measured (confined liquid phase, confined nanociystallites of cubic ice). The confinement effect, the displacement of the whole water phase diagram towards the low temperature side is quantifi decreasing the MCM-41 pore size increases the temperature displacement. 

This quantity provides information about the excess of component-adsorbent interactions averaged over all surface domains from which the solvent has been displaced by the adsorbing solute species. In consequence, it is not easy to monitor subtle changes in the adsorption mechanism based on usually small variations of the Adpih values with increasing quantity of adsorption. Compared to the differential molar enthalpy of displacement, the enthalpy Adpih is less sensitive to the energetic heterogeneity of the solid surface. 

Regardless of the specifics of the adsorption mechanism due to dispersion forces, the result is important scientifically and technologically because it is always present, whether as an independent effect or acting in conjunction with the other mechanisms discussed below. The importance of the effect in conjunction with other forces is illustrated by the propensity for surfactants with longer hydrophobic tails to displace similarly charged lower-molecular-weight materials and inorganic ions from solid surfaces. 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

Studies on mechanisms are described by Balzer [192]. In the case of anionics the residual oil in the injection zone is removed via displacement into the adjacent reservoirs ether carboxylates show their good adaptation to differences in temperature and salinity. Further it was found from interfacial tension measurements, adsorption and retention studies, and flooding tests that use of surfactant blends based on ether carboxylates and alkylbenzensulfonates resulted... 

Consequences of the Snyder and Soczewinski model are manifold, and their praetieal importance is very signifieant. The most speetaeular conclusions of this model are (1) a possibility to quantify adsorbents ehromatographic activity and (2) a possibility to dehne and quantify chromatographic polarity of solvents (known as the solvents elution strength). These two conclusions could only be drawn on the assumption as to the displacement mechanism of solute retention. An obvious necessity was to quantify the effect of displacement, which resulted in the following relationship for the thermodynamic equilibrium constant of adsorption, K,, in the case of an active chromatographic adsorbent and of the monocomponent eluent ... 

No carrier is completely specific for a given trace metal metals of similar ionic radii and coordination geometry are also susceptible to being adsorbed at the same site. The binding of a competing metal to an uptake site will inhibit adsorption as a function of the respective concentrations and equilibrium constants (or kinetic rate constants, see below) of the metals. Indeed, this is one of the possible mechanisms by which toxic trace metals may enter cells using transport systems meant for nutrient metals. The reduced flux of a nutrient metal or the displacement of a nutrient metal from a metabolic site can often explain biological effects [92]. 

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