Solvent adsorption isotherm prediction

The model has been further tested in terms of the behavior of solvent molecules isotherms for binary-solvent mixtures A/B adsorption energies (ch values) of the polar solvent B in such mixtures as a function of surface coverage 6b, etc. Again, good agreement of experimental data with calculated values is observed. An important requirement of the model and related experimental correlations is that the solvent molecule must be treated (thermodynamically) in the same manner as solute molecules. Thus, if. solute adsorption energies are measured for a molecule X, the behavior of X as a component (solvent) of the mobile phase should then be predictable. 

In adsorption chromatography the relevant functions are the adsorption isotherm parameters. Since there are no theoretical tools available to predict isotherms from physico-chemical data of the solute, solvent and adsorbent, these adsorption isotherm parameters have to be determined experimentally. When measuring the data, it is important to use a broad concentration range, i.e., including both the linear part of the isotherm as well as concentration close to saturation of the stationary phase. Despite the fact that there are several methods available to obtain adsorption isotherm parameters, the experimental determination of the isotherms is still far from being routine work. 

The preferential uptake of polar solvents by the column can be largely eliminated when starting the gradients at an initial concentration of the polar solvent higher than 3%. If this is not possible, the effects of the preferential adsorption of the polar solvent on the retention can be predicted by numerical calculation from the parameters of the adsorption isotherm of the polar solvent on the column packing material (usually the Langmuir type). 

The adsorption of binary organic mixtures by a porous carbon was studied by Takeuchi and Furaya, the components being chosen such that some were accepted by and some excluded from 0.5 nm micropores. The experimental results could be represented by a combination of Freundlich adsorption isotherms, the parameters of which could be related to the physical properties of the adsorptives, and it was found possible to predict satisfactorily the adsorption isotherms of new systems. The adsorption by and desorption from active carbon has been reported by Andreikova, Kondratov, and Kogan, who found that adsorption from (unspecified) organic solvents decreased in the series phenol > quinoline > phenanthrene > acenaphthene > naphthalene. In desorption, acidic compounds are best desorbed with a mixture of methanol and dichloroethane but for basic compounds benzene is most effective. 

The performance of demulsifiers can be predicted by the relationship between the film pressure of the demulsifier and the normalized area and the solvent properties of the demulsifier [1632]. The surfactant activity of the demulsifier is dependent on the bulk phase behavior of the chemical when dispersed in the crude oil emulsions. This behavior can be monitored by determining the demulsifier pressure-area isotherms for adsorption at the crude oil-water interface. 

The influence of the molecular weight distribution of diblock copolymers on their segregating properties is considered in Sect. 4.2.4. It describes first experimental study [255] on a bimodal mixture of short symmetric and long asymmetric copolymers added to a polymeric matrix. Shorter copolymers were found to adsorb preferentially at the homopolymer interfaces in accord with brush formation observed from a solvent host matrix [274-276]. The mean field model is able [255] to predict the segregation isotherm of the bimodal mixture of copolymers, basing on single component adsorption data [254]. 

It was found that the obtained isotherms are markedly different from those which take into account only short-range interactions, like the Flory-Guggenheim isotherm for local adsorption. The dipole-dipole interactions play an important role in the properties of adsorbed layers, since they smear the adsorbed solute molecules among the solvent molecules, increasing the adsorbate surface solubility. Another interesting implication caused by the electric field of the adsorbed dipoles is the increase in the adsorption of a neutral adsorbate in the presence of specifically adsorbed anions, a property which is not predicted by previous models although experimental evidence of this phenomenon does exist. 

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