Mixtures: adsorption from solvents

Figure 3. Plateau adsorption from solvent/non-solvent mixtures, curve 1, PMM I 2, RC 08 3, RC 51 A, RC 86 5, PS III. 8 marks the theta-point. Vertical lines are (Vns) for phase separation at c = 10 2 g cm-3.

The discussion so far has been confined to systems in which the solute species are dilute, so that adsorption was not accompanied by any significant change in the activity of the solvent. In the case of adsorption from binary liquid mixtures, where the complete range of concentration, from pure liquid A to pure liquid B, is available, a more elaborate analysis is needed. The terms solute and solvent are no longer meaningful, but it is nonetheless convenient to cast the equations around one of the components, arbitrarily designated here as component 2. 

Such displacement effects, although often very pronounced, have not yet been studied systematically. They will be the subject of the present paper. We will discuss the adsorption of polymer from a mixture of two solvents and we will see that in some cases drastic effects occur as a function of the mixture composition. Also, we explore some consequences and practical applications of displacement. It turns out that displacement studies not only increase our insight on the role of the solvent in polymer adsorption but can also be used to determine the segmental adsorption energy. So far, experimental data for this quantity were very scarce. Some illustrative experiments will be discussed briefly. 

A division Into "adsorption from dilute solution" and "adsorption from binary (and multicomponent) mixtures covering the entire mole fraction scale" appears to be useful. For simplicity, we shall designate mixtures covering the entire mole fraction scale as binary mixtures, as opposed to dilute solutions. This distinction is a consequence of issues (1) - (3) above, and reflected in thermodynamic and statistical interpretations. For instance, in dilute solutions locating the Gibbs dividing plane is not a problem, but for a mixture in which one of the components cannot confidently be identified as the solvent, it is. 

We recall sec. 1.2.22, and in particular fig. 1.2.13 where the consequences of basing the Gibbs plane on a major or a minor component are illustrated. Statistically, adsorption from dilute solution is easy when the solvent may be interpreted primitively, i.e. as a structureless continuum. Then, much of chapter 1 may be applied after minor modification. For binary mixtures this becomes more problematic. In practice, adsorption from (dilute) solution is more frequently met than that from binarj mixtures. 

Derivation Selective crystallization or solvent extraction of m-, p-mixture separation from mixed-xylene feedstocks by adsorption. 

The first examples of the P-K reaction required high temperatures and pressures to succeed. Several improvements in yield and reaction rate have resulted from continued research efforts over the past 30 years. These include the addition of silica gel200 to the reaction mixture (adsorption of the alkyne-Co complex onto silica may restrict molecular motion, allowing the ene-yne system to interact more readily also, lack of solvent would allow bimolecular reactions to proceed faster) the use of tertiary amine A-oxidcs 201-202 the application of photochemical conditions to ease departure of CO in the rate-determining step and the inclusion of various Lewis bases, which help stabilize intermediate Co complexes.203... 

Studies have been made of adsorption from mixed solvents but the conclusions to be drawn have limited scope.2 In the simplest case—that of a mixture of similar solvents such as benzene and toluene—the adsorption is often proportional to the composition of the mixture and it can be predicted when the adsorption from each separate solvent is known. From a mixture of dissimilar solvents, the adsorption is seldom predictable.2 One of the solvents may have a predominating influence even when present in minor proportions and not infrequently adsorption is less from a mixture of two solvents than from either solvent used alone. Minimum or maximum adsorption may be found at an intermediate mixture of the components, and such behavior recalls other abnormal behavior (e.g., vapor pressure and solubility) observed in mixed solvents. 

Generally, wastewaters are complex mixtures of solutes, which require theoretical approaches to predict multicomponent adsorption equilibria flxtm pure component adsorption data. The Ideal Adsorbed Solution model (IAS) was first established for a mixed gas adsorption by Myers and Prausnitz [9], and then extended to a multi-solute adsorption from dilute liquid solution by Radke and Prausnitz [10]. The model is based on the fundamental hypothesis that the multicomponent solution has the same spreading pressure s as that of the ideal single solution of the i component, the spreading pressure being the difference between the interfacial tension of the pure solvent and that of the solution containing the solute. This hypothesis is described by the Gibbs equation ... 

Ash et al. (1973) extended the preceding theory to the adsorption from liquid mixtures. It follows very similar lines to the gas/solid situation and so, in the interests of concision, it will not be reproduced here. For a binary mixture of solute 2 in solvent 1, the analogue of the gas/solid equation (17.17) is... 

This selectivity is subject to a strong solvent effect, and is probably determined by the polarity and solubihty of the substrates. The least soluble thiol usually adsorbs preferentially. Thus, in a mixture of an alkanethiol and an from ethanol solution. Adsorption from acetonitrile, on the other hand, does not show any preference, and adsorption from isooctane results in the preferential adsorption of a hydroxythiol . The preference of long-chain thiols over the short-chain ones and compounds with bulky substituents also depends on the solvent, being much less pronounced in non-polar solvents. Adsorption of octadecanethiol is preferred over adsorption of r-butylthiol in isooctane solution by a factor of only 40-100 (the corresponding ratio in ethanol is 290-710, vide supra), and by just a factor of 3-4 over adsorption of straight-chain butanethiol (compared with a 20-30 ratio for C22 vs C12 preference in ethanol, vide suprd). ... 

No data on solutions are included either. Although there is considerable information in the literature on certain polymers, it is dependent on the particular choice of solvent and not amenable to systematic tabulation. The same is true of the wealth of adsorption from solution onto solids and spread film Langmuir trough data. Equations are available for calculating the surface tension of simple liquid mixtures that could be applied to polymers [14] and for calculating polymer solvent solution [15] surface tensions. 

Method of ellipsometry allows to observe in situ adsorption of proteins or other surface active substances at interfaces, it was applied for studies cleaning processes. The main observation is the increase of protein adsorption from a mixture with surfactant with dilution of solvent due to exchange of surfactant for protein when the concentration of surfactant decreses below a critical concentration in the range of its CCM. 

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. 

Let us consider the adsorption and the normal-phase partition chromatography systems employing binary mobile phases composed of the solvents A and B (with the nonpolar solvent A, for which P 0). If we want to change the separation selectivity of this system, the simplest way is to employ the isoeluotropic mixture in which solvent B is replaced by solvent C. The volume fraction of solvent C can be estimated from the relationship ... 

The sample solution band (test dye mixture), applied from the edge of the layer, formed a partly separated starting zone (frontal chromatography stage). After adsorption of the sample by the adsorbent layer, the eluent was introduced under the solvent distributor, and the marker (azobenzene) was spotted. The movements of the marker and the dye zones were recorded on a transparent foil (84). By connecting the points representing the upper and lower boundaries of the zones, a dynamic picture of the movement and separation of the zones could be obtained. 

This cmde product is purified by chromatography using adsorption alumina as described above initially using a 2 1 toluene/acetonitrile solvent mixture and changing solvents as described below. The first visible band to elute from this column is a small amount of the red-purple monometallic complex. After this band is eluted the solvent is changed to 1 2 toluene/acetonitrile to elute the... 

---