Langmuir irreversible adsorption

C.M. Cuppett, L.J. Doneski and M J. Wirth, Irreversible adsorption of lysozyme to polishing marks on silica. Langmuir 16 (2000) 7279-7284. 

Soderquist and Walton s72) model of the protein adsorption process (discussed earlier, Fig. 13) reduces to the classical Langmuir (small-molecule) adsorption as t -> 0. As t - oo, their model predicts that adsorption is irreversible. A model which reduces to Langmuir adsorption at short contact time and to irreversible adsorption at very... 

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... 

Monolayers can be formed on electrode surfaces by irreversible adsorption or covalent attachment, or, in the form of organized assemblies, by Langmuir-Blodgett transfer and self-assembly techniques. 

Although, this model describes enzyme adsorption, other studies have shown that the lipase distribution on the support surface can be presented by a Freundlich model (Knezevic et al., 1998 Mojovic et al., 1998) that describes nonidentical adsorption on the support surface, or by a Redlich-Peterson model (Al-Duri and Yong, 2000) that combines the Langmuir and Freundlich isotherms. The Langmuir model reflects irreversible adsorption and is based on the assumption that every adsorption site is identical and energetically equivalent. However, this is a condition that is rarely... 

Fig. 2. Upper diagram Illustration of the basic features of the dynamic model at low and high protein concentrations, respectively. Some different desorption time constants were used. C is the normalized concentration C C, /s,. Lang-muir-like adsorption of a molecule in two different orientations is denoted by the dashed lines. The parameters were chosen to obtain a good fit between the Langmuir case and the dynamic model at low concentrations. It is also indicated that an irreversible adsorption (dashed-dotted line) would lead to a constant adsorption at low concentrations equal to 0, + 02 = 1/ for the dynamic model. Note that (0j -I- 62) corresponds to the number of adsorbed protein molecules/per unit area. (The number is actually Afo(0, + 02)O Lower diagram 0, -t- 02 for 42 = 4 41 comparing the dynamic model with reversibility (solid line) with an irreversible adsorption (dashed-dotted line) and a Langmuir isotherm (dashed line) at intermediate concentrations. The irreversible adsorption is only approximate, given by equations (9) and (10). We have also shown schematically the packing of the protein molecules in different concentration regions...

A plot of r versus C2 gives the adsorption isotherm. Two types of isotherms can be distinguished a Langmuir type for reversible adsorption of surfactants (Fig. 4.10) and a high affinity isotherm (Fig. 4.11) for irreversible adsorption of polymeric surfactants. 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

In the steady-state approach (equations (35) and (36)), no attempt is made to isolate the adsorption step from the internalisation of solutes. In this case, a Langmuir adsorption via membrane carriers is coupled to an irreversible and rate-limiting internalisation of the solute carrier complex [186], The process can be described by the Michaelis-Menten equation ... 

Results were modelled on the basis of CO adsorption/desorption equilibrium with oxygen adsorption as the rate determining step and an irreversible Langmuir-Hinshelwood surface reaction. It was assumed that adsorption and... 

When the equilibrium constant b is large (highly favorable adsorption) the Langmuir isotherm approaches irreversible or rectangular form,... 

A Ithough the adsorption of polymers onto solid surfaces has been thor-oughly studied (I), relatively few studies can be found in the literature on the adsorption of proteins onto polymer surfaces. In 1905, Landsteiner and Uhliz (2) discussed the interaction of serum proteins with synthetic surfaces. Blitz and Steiner (3) showed that albumin adsorption onto solid surfaces increased with increasing albumin concentration and that adsorption was nearly irreversible. Hitchcock reported (4) that adsorption of egg albumin onto collodion membranes followed a Langmuir isotherm with maximum adsorption occurring near the isoelectric point. Later, Kemp and Rideal (5) reported that protein adsorption onto solids conforms with Langmuir adsorption. 

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. 

In an electrolyte containing a dilute SPS concentration, disulfide adsorption occurs coincident with displacement of the less strongly bound PEG-based inhibitor. The adsorption or exchange process can be described by irreversible Langmuir kinetics [112,135, 281], The effective rate constant, fcads, is potential dependent, the rate increasing exponentially with overpotential T) according to... 

Note that the Langmuir model is an equilibrium relationship, and its application assumes adsorption is instantaneous and reversible in terms of polymer concentration. When polymer adsorption is considered to be irreversible, the Langmuir model cannot be used directly when the polymer concentration is declining. An additional parameter, Cp p x, must be used to track the adsorption history so that Eq. 5.33 applies. 

These reactions are based on the assumptions only irreversible dissociative adsorption of oxygen (2), and instantaneous desorption of the carbon dioxide produced (3)(4)(5). In the temperature range of this study (300-650 K) these assumptions were found to be valid. Reaction (3) represents a reaction according to the Langmuir-Hinshelwood model, whereas reactions (4) and (5) represent reactions according to an Eley-Rideal model. 

During the adsorption step, the binding of solute by the affinity ligand is often very tight, essentially irreversible. For Langmuir, or constant separation factor adsorption, the equilibrium relation is... 

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