Positive adsorption

The adsorption equation shows that a solute may very strongly lower the surface tension of a solvent, but cannot strongly raise it, since although T may reach high values by positive adsorption (in some cases, as with solutions of some aniline dyes, the pure solute appears as a thin skin on the surface), it can never sink below that of the pure solvent by negative adsorption. 

When the two phases in contact are condensed phases and the entire volume is taken up by incompressible substances, positive adsorption of one component must be attended by negative adsorption (desorption) of other components. This phenomenon is called adsorptive displacement. 

The study of the interfacial liquid-liquid phase however is complicated by several factors, of which the chief is the mutual solubility of the liquids. No two liquids are completely immiscible even in such extreme cases as water and mercury or water and petroleum the interfacial energy between two pure liquids will thus be affected by such inter-solution of the two homogeneous phases. In cases of complete intersolubility there is evidently no boundary interface and consequently no interfacial energy. On addition of a solute to one of the liquids a partition of the solute between all three phases, the two liquids and the interfacial phase, takes place. Thus we obtain an apparent interfacial concentration of the added solute. The most varied possibilities, such as positive or negative adsorption from both liquids or positive adsorption from one and negative adsorption from the other, are evidently open to us. In spite of the complexity of such systems it is necessary that information on such points should be available, since one of the most important colloidal systems, the emulsions, consisting of liquids dispersed in liquids, owe their properties and peculiarities to an extended interfacial phase of this character. 

If an aqueous solution of some colouring matter be agitated with powdered charcoal and a determination of the concentration of the colouring matter in the solution be made both before and after the operation, the solution will be found to be much less concentrated due to a selective removal of the colouring matter by the charcoal. Such selective removal is frequently termed positive adsorption. 

Or, in general, positive adsorption rising to a maximum will be followed by zero and eventually negative adsorption, due to the fact that solvent and solute are both adsorbed. 

Osaka Mem. Coll. Sci. Kyoto Univ. vi. 257, 1915) has obtained positive adsorption in the case of sodium and potassium nitrates and for potassium bromide and iodide, and negative adsorption in the case of sodium and potassium sulphate as well as potassium iodide. 

Surface tensions for the interface between air and aqueous solutions generally display one of the three forms indicated schematically in Figure 7.14. The type of behavior indicated by curves 1 and 3 indicates positive adsorption of the solute. Since dy/dc and therefore dy/d In c are negative, E must be positive. On the other hand, the positive slope for curve 2 indicates a negative surface excess, or a surface depletion of the solute. Note that the magnitude of negative adsorption is also less than that of positive adsorption. 

Interfacial pressure th, the change in interfacial tension as a result of sorption (usually positive adsorption) of the surface-active material. It may be regarded as a measure of the tendency of adsorbed species at the interface (or biface) to enlarge the area occupied by the BLM. 

Acid dissociation (or negative adsorption of H+) produces negative surface sites. Basic dissociation (as in Reaction 2, equivalent to positive adsorption of H+) produces positive surface sites, which because the probability of existence of a bare M+ is small, probably occur through a combination of Reactions 2 and 3 as... 

Both Rum(EDTA) and Rum(HEDTA) (HEDTA = 7V-2-hydroxyethylenediaminetriacetato-) are adsorbed at mercury electrodes, but if a ligand is present which can bind the labile sixth coordination position, adsorption is reduced. Thiocyanate is effective in reducing the quantity of complex adsorbed, possibly due to the formation of a thiocyanato complex.93... 

Table 5. Properties of adsorption complexes of methane with studied systems favored position, adsorption energy, methane-to-cluster charge transfer, activation barrier and stretching frequency of C-H bond.

Adsorption position Adsorption energy (kcal/mol) AQ (methane —> cluster) Activation energy (kcal/mol) Vc-Hdis (cm 1)... 

The Freundlich equation, empirical in origin, relates positive adsorption to a power function of c, as follows ... 

Polysaccharides interfaced with water act as adsorbents on which surface accumulations of solute lower the interfacial tension. The polysaccharide-water interface is a dynamic site of competing forces. Water retains heat longer than most other solvents. The rate of accumulation of micromolecules and microions on the solid surface is directly proportional to their solution concentration and inversely proportional to temperature. As adsorbates, micromolecules and microions ordinarily adsorb to an equilibrium concentration in a monolayer (positive adsorption) process they desorb into the outer volume in a negative adsorption process. The adsorption-desorption response to temperature of macromolecules—including polysaccharides —is opposite that of micromolecules and microions. As adsorbate, polysaccharides undergo a nonequilibrium, multilayer accumulation of like macromolecules. 

For polysaccharide dispersions, SV is exceedingly small relative to Vi. Equations (3.11) and (3.12) are mathematical propositions that the exchangeable energy stored in a dispersed polysaccharide solute is equal to the energy absorbed from an external source and any increase in surface area of the solute is consequently a repository of +A . Conversely, aggregation and desorption correspond to a loss of energy, felt as heat in the latter occurrence ( —A ) when a dry polyaccharide powder is wetted (positive adsorption). 

The conceptual meaning of Eq. 4.1 is that nP is the excess moles of substance i in the reacted mixture, relative to the content of a reference substance j in the mixture and to the composition of the separated aqueous solution, indicated by the molalities nij and mj. In the example of the slurry and supernatant solution, np is the excess moles of i in the slurry, as compared to the content of the reference substance j and to an aqueous solution that has the mixed composition indicated by mj and mj. The right side of Eq. 4.1 can be positive (adsorption), zero (no surface excess), or negative (desorption), depending entirely on the relative behavior of the substances i and j when two phases containing them react. In applications of Eq. 4.1 to the reactions of soils with aqueous solutions, the reference substance j is invariably chosen to be liquid water (j = w) ... 

The increase of the surface tension of water by the addition of an electrolyte was traditionally related (via Gibbs adsorption equation) to the negative adsorption of ions on the interface. However, some electrolytes decrease the interfacial tension [29], hence should be positively adsorbed. Therefore, if the van der Waals interactions would repel all the ions from the interface, some additional interactions have to be included to explain the positive adsorption. 

Fig. 5. Graphical solution of Eq. (38). If Wj >0, there is a positive adsorption of anions in the potential well if the electrolyte concentration is sufficiently low. The increase of the electrolyte concentration displaces Idi I toward larger values, [f the electrolyte concentration is sufficiently high, the anion adsorption in the well becomes negative exp((Wj + eijj)/W)< l).

Karraker and Radke [18] proposed another explanation of this minimum. At low electrolyte concentrations, the positive adsorption of OH leads to a decrease of surface tension, while at sufficiently high ionic strengths, the depletion of Cl", repelled by the negative surface charge as well as by the van der Waals and image forces, becomes dominant and the surface tension increases. 

Fig. 31 shows the positive adsorption of alcohol and the negative adsorption of water, on the above conventions, from pure water to pure alcohol. In dilute solution it does not matter what convention is used for defining T, the T are all very nearly equal. 

Diagram (6) shows similar, but opposite, phenomena with the capillary active cations, tetramethyl and tetraethyl ammonium. The phenomena are reversed the rising part of the curve is not affected the falling part, and the maximum, are depressed and the maximum is shifted to more positive potentials. This is because the adsorption tends to form a double layer in the water, with the positive ions nearer the surface than the negative, and this positive adsorption potential aids in attracting electrons to the mercury surface and neutralizing it, so that the maximum is reached with the application of smaller negative potentials E. 

Since the activity coefficient f2 is always positive, a fall in surface tension with increasing concentration indicates positive adsorption a rise, negative adsorption, of the solute 2. 

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