Competitive adsorption/displacement mechanism

Surface dilatational rheology is a very sensitive technique to analyze the competitive adsorption/displacement of protein and LMWE emulsifier at the air-water interface (Patino et al., 2003). A common trend is that the surface dilatational modulus increases as the monolayer is compressed and is a maximum at the highest surface pressures, at the collapse point of the mixed film, and as the content of LMWE in the mixture increases. At higher TT, the collapsed protein residues displaced from the interface by LMWE molecules have important influence on the dilatational characteristics of the mixed films. The mechanical properties of the mixed films also demonstrate that, even at the highest tt, the LMWE is unable to displace completely protein molecules from the air-water interface. 

In Figure 2, kinetics of T2 removal using QAC treated beads is presented. It is obvious that the competitive adsorption between viruses and BSA molecules also reduced the adsorption rate. In both cases, viruses were inactivated rapidly at the initial 2 hour mark and titer reduction slowed down after that. This inconsistency with the first-order inactivation model may be due to various interfering mechanisms such as displacement, molecular orientation, multilayer effects, surface heterogeneity, and virion clumping. 

Differential eleetroehemieal mass speetrometry was used by Vidal-Iglesias et al. to study the HCOOH oxidation on Pd submonolayers deposited on Pt(lOO) and Pt(lll) [170], It was found that the adsorption of S04 inhibits HCOOH oxidation. Competitive adsorption between hydrogen and sulfate was responsible for the voltammetric peak at 0.26 V vs. RHE on Pd, while at 0.3 V vs. RHE, Had was eompletely displaced by the adsorbed sulfate. At E > 0.3 V the S04 adsorption was stronger, further diminishing the HCOOH oxidation current on the Pd modified electrodes [170]. The deposition mechanism and surface analysis of Pd on Pt(l 11) was published by Ball et al. [171]. 

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

It is interesting also to compare the results of the present experiment, which shows directly that a competitive mechanism occurs in the co-adsorption of NO and CO, with previous studies on several surfaces of the platinum group metals. On Pt(lll) and Pt(110), Lambert and Comrie (65) have inferred from thermal desorption data that gaseous CO displaces molecular NO from the surface and causes also a conversion between two thermal desorption states of molecular NO. Similarly, Campbell and White (55) report that adsorbed CO inhibits the oxidation of CO by NO at low temperature on polycrystalline Rh. They attribute this to the occupation of sites by CO which are required for NO adsorption and dissociation. Conrad et al. (66) have used UV-photoelectron spectroscopy to observe directly the displacement of molecular NO by gaseous CO from Pd(110) and polycrystalline Pd surfaces. Thus, it appears that adsorption of molecular CO and NO is competitive on these... 

The retention mechanism in the normal phase is often referred to as adsorption chromatography. It is described as the competition between analyte molecules and mobile-phase molecules on the surface of the stationary phase. It is assumed that the adsorbing analyte displaces an approximate equivalent amount of the adsorbed solvent molecules from the monolayer on the surface of the packing throughout the retention process [18]. 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. This retention model, proposed by Snyder, was originally used to describe retention with silica and alumnina adsorbents, but several other studies have shown that this model may also be used for polar bonded phases, such as diol, cyano, and amino bonded silica [10,19]. 

The semiempirical model of adsorption chromatography, analogous to that of Martin and Synge, was established only in the late 1960s by Snyder [3] and Soczewinski [4] independently, and it is often referred to as the displacement model of solute retention. The crucial assumption of this model is that the mechanism of retention consists in competition among the solute and solvent molecules for the active sites of the adsorbent and, hence, in a virtually... 

Finally, we have shown that the stress response to compression of a mixed system (protein -I- protein or protein - - surfactant) is not significantly affected by the nature of the bonding mechanism of the bond-forming species. In contrast, the "competitive desorption" induced by compression favors the displacement of one or other of the species depending on the extent of breakability of the bonds. This phenomenon seems to arise as a consequence of the delicate balance between a "bond-enhanced adsorption effect" and a "drag-desorption mechanism."... 

Water certainly competes with NH3 for the formation of both species I and II, but allows the formation of new species like species III. The competition of water with ammonia causes a decrease in the FT-IR spectra of the band characteristic of species I (5symNH3 mode near 1220 cm ) while it results in an increase of a band at lower frequency (near 1160 cm-1) assigned to species III. In the TPD experiments, treatment with water vapour at r.t. after NH3 adsorption, can lead to the displacement of ammonia by water, thus transforming species II into species III. When the sample is heated, since water desorbs at a temperature lower than ammonia, species III transforms into species I, while species II cannot be formed. This mechanism can explain why TPD experiments following water treatment show ammonia desorbing only at high temperature, while the low temperature signal is completely absent. 

Retention mechanisms of adsorption chromatography have been extensively studied. There are two popular models for this process. The displacement model, originally proposed by Snyder, treats the distribution of solute between a surface phase, usually assumed to be a monolayer, and a mobile phase as a result of a competitive solute and solvent adsorption. A treatment of this model, including the significance of predictions of solvent strength and selectivity in terms of mobile-phase optimization strategies, has been published by Snyder (81). 

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