Langmuir adsorption isotherm solvents

A solvent can be adsorbed from a solvent mixture on the surface of silica gel according to the Langmuir adsorption isotherm as previously discussed. 

J. Haberman T.C. Castorina, The Surface Chemistry of HMX , PATR 3089 (1963) [A study was made of the solution-adsorption properties of HMX using a 14C labeled quarternary ammonium salt, stearyl tri-methyl ammonium bromide (STAB). The solvent system used consisted of 10% ethanol and 90% w. The solution-adsorption system isotherms for 10-micron HMX were measured. The authors conclude that an H-type Langmuir adsorption isotherm is obtd, the adsorption of STAB on HMX is reversible and therefore physi-... 

The thermodynamics and dynamics of interfacial layers have gained large interest in interfacial research. An accurate description of the thermodynamics of adsorption layers at liquid interfaces is the vital prerequisite for a quantitative understandings of the equilibrium or any non-equilibrium processes going on at the surface of liquids or at the interface between two liquids. The thermodynamic analysis of adsorption layers at liquid/fluid interfaces can provide the equation of state which expresses the surface pressure as the function of surface layer composition, and the adsorption isotherm, which determines the dependence of the adsorption of each dissolved component on their bulk concentrations. From these equations, the surface tension (pressure) isotherm can also be calculated and compared with experimental data. The description of experimental data by the Langmuir adsorption isotherm or the corresponding von Szyszkowski surface tension equation often shows significant deviations. These equations can be derived for a surface layer model where the molecules of the surfactant and the solvent from which the molecules adsorb obey two conditions ... 

Fig. 6.2. A plot of the Langmuir adsorption isotherm for three non-polar solvents (chloroform, butyl chloride and benzene) in n-heptane. It can be seen that the data fit very closely to the monolayer function y = x/(A + Bx). Reproduced from Scott and...

In LC, at very low concentrations of moderator in the mobile phase, the solvent distributes itself between the two phases in much the same way as the solute. However, as the dilution is not infinite, the adsorption isotherm is not linear and takes the form of the Langmuir isotherm. 

This equation is the mono-layer adsorption isotherm of solvent (B) and is exactly the same as the previously derived Langmuir adsorption equation but somewhat differently expressed. 

The more dispersive solvent from an aqueous solvent mixture is adsorbed onto the surface of a reverse phase according to Langmuir equation and an example of the adsorption isotherms of the lower series of aliphatic alcohols onto the surface of a reverse phase (9) is shown in figure 9. It is seen that the alcohol with the longest chain, and thus the most dispersive in character, is avidly adsorbed onto the highly dispersive stationary phase, much like the polar ethyl acetate is adsorbed onto the highly polar surface of silica gel. It is also seen that... 

Sinee replaeement of an adsorbed solvent moleeule by an adsorbate molecule generally results in a ehange of the eomposition of the first layer of the eleetrolyte solution being in contact with the electrode, the different values of the dieleetrie properties of the solvent and adsorbate moleeules as well as their different size result in a change of the value of Cdl- Evaluation of the changes of Cdl as a function of the concentration of the adsorbate in the solution phase results in adsorption isotherms. Various isotherms (Frumkin, Langmuir etc.) have been used in the further evaluation [75Damj. Beeause of interferenees of the kineties of the adsorption proeess in particular by slow transport and slow adsorption tensammetry... 

For instance, the time course of SPE demonstrates that the solvent phase surfactant concentration steadily decreases (Fig. 3) [58]. The w/o-ME solution s water content decreases at the same rate as the surfactant [58]. The protein concentration at first increases, presumably due to the occurrence of Steps 2 and 3 above, but then decreases due to the adsorption of filled w/o-MEs by the solid phase (Fig. 3) [58]. Additional evidence supporting the mechanism given above is the occurrence of a single Langmuir-type isotherm describing surfactant adsorption in the solid phase for several SPE experiments employing a given protein type (Fig. 4) [58]. Here, solid-phase protein molecules can be considered as surfactant adsorption sites. Similar adsorption isotherms occurred also for water adsorption [58]. 

Adsorption isotherms are used to quantitatively describe adsorption at the solid/ liquid interface (Hinz, 2001). They represent the distribution of the solute species between the liquid solvent phase and solid sorbent phase at a constant temperature under equilibrium conditions. While adsorbed amounts as a function of equilibrium solute concentration quantify the process, the shape of the isotherm can provide qualitative information on the nature of solute-surface interactions. Giles et al. (1974) distinguished four types of isotherms high affinity (H), Langmuir (L), constant partition (C), and sigmoidal-shaped (S) they are represented schematically in Figure 3.3. 

When one of the solvents has a limited and low solubility, C, we find the classic Langmuir absorption isotherm is obtained with a sli t modification to the activity axis as shown in Figure 9.13. Such a solubility limit can be obtained by precipitation cn- micellization in the case of surfactants. Micellization is the association of ionized surfactant molecules into structures where the hydrophobic parts of the surfactant molecules expel water. Micelles have different structures (i.e., spheres, cylinders, and lamellar structures) depending on the surfactant molecule and its concentration of surfactant in solution. Each structure has a different maximum concentration, C , which limits adsorption. In Figure 9.13, the activity is replaced by concentration in the dilute solution case and the ooncentraticm axis C2 becomes dimensionless by division by the solubility limit when the constant b is replaced by 6C in equation 9.40. 

Determinations of the adsorption isotherms for a number of organic solvent-water systems in contact with hydrocarbonaceous stationary phases have shown that a layer of solvent molecules forms on the bonded-phase surface and that the extent of the layer increases with the concentration of the solvent in the mobile phase. For example, methanol shows a Langmuir-type isotherm when distributed between water and Partisil ODS (56). This effect can be exploited to enhance the resolution and the recoveries of hydrophobic peptides by the use of low concentrations, i.e., <5% v/v, of medium-chain alkyl alcohols such as tm-butanol or tert-pentanol or other polar, but nonionic solvents added to aquo-methanol or acetonitrile eluents. It also highlights the cautionary requirement that adequate equilibration of a reversed-phase system is mandatory if reproducible chromatography is to be obtained with surface-active components in the mobile phase. 

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