Stem layer adsorption

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study 

Material Potential-Determining Ion Point of Zero Charge 

Electrolyte adsorption on metals is important in electrochemistry [167,168]. One study reports the adsorption of various anions an Ag, Au, Rh, and Ni electrodes using ellipsometry. Adsorbed film thicknesses now also depend on applied potential. 

A detailed study by Grieser and co-workers [169] of the forces between a gold-coated colloidal silica sphere and a gold surface reveals the preferential adsorption of citrate ions over chloride to alter the electrostatic interaction. 

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Figure V-8 illustrates that there can be a pH of zero potential interpreted as the point of zero charge at the shear plane this is called the isoelectric point (iep). Because of specific ion and Stem layer adsorption, the iep is not necessarily the point of zero surface charge (pzc) at the particle surface. An example of this occurs in a recent study of zircon (ZrSi04), where the pzc measured by titration of natural zircon is 5.9 0.1...

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

The adsorption appears to be into the Stem layer, as was illustrated in Fig. V-3. That is, the adsorption itself reduces the f potential of such minerals in fact, at higher surface coverages of surfactant, the potential can be reversed, indicating that chemical forces are at least comparable to electrostatic ones. The rather sudden drop in potential beyond a certain concentration suggested to... 

Certain counterions may be held in the compact region of the donble layer by forces additional to those of purely electrostatic origin, resulting in their adsorption in the Stem layer. Specifically... 

Further evidence for three distinct modes of adsorption can be seen in the electrophoretic behavior of alumina in the presence of sodium dodecyl sulfonate. Below 6 X 10 5M, the electrophoretic mobility is nearly independent of concentration, but at this concentration the slope of the mobility-us.-concentration curve abruptly changes. At 3 X 10"4M dodecyl sulfonate concentration, the electrophoretic mobility reverses its sign, indicating that the charge in the Stem layer now exceeds the surface charge in absolute magnitude. 

Inorganic Ions. Because of electrostatic attraction, positive ions are attracted to negatively charged surfaces and have a higher concentration near the surface than in the bulk. Negative ions are repelled from the negative surface and have a lower concentration near that surface. Ions which are very strongly bound (/zADS > kT) are in the Stem layer, whereas those that can move into and out of the ionic atmosphere (nADS < kT) are in the Helmholtz layer. The effect of ionic attraction or repulsion from the surface is to enhance or reduce the nonionic adsorption coefficient ... 

Finally, the Schulze-Hardy provides a very useful rule of thumb, but it is limited in that it only addresses first-order effects. Secondary effects that will have a bearing on aggregation and CCCs include specific adsorption of ions in the Stem layer,... 

Radio frequency heating, 500 Steam stripping, 500 Vacuum extraction, 500 Aeration, 501 Bioremediation, 501 Soil flushing/washing, 502 Surfactant enhancements, 502 Cosolvents, 502 Electrokinetics, 503 Hydraulic and pneumatic fracturing, 503 Treatment walls, 505 Supercritical Water Oxidation, 507 Solid Solution Theory, 202 Solubility products, 48-53 Metal carbonates, 433-434 Metal hydroxides, 429-433 Metal sulfides, 437 Sorption, 167 See Adsorption Specific adsorption, 167 See Chemisorption Stem Layer, 152-154 Sulfate, 261... 

Fig. 3.25 presents the aqueous solutions in the absence of a surfactant at constant ionic strength (HC1 + KC1) [186,197], It can be seen that at pH > 5.5, op-potential becomes constant and equal to about 30 mV. At pH < 5.5 the potential sharply decreases and becomes zero at pH 4.5, i.e. an isoelectric state at the solution surface is reached. As it is known, the isoelectric point corresponds to a pH value at which the electrokinetic phenomena are not observed. Since in the absence of the potential of the diffuse electric layer, the electrokinetic potential (zeta-potential) should also be equal to zero, the isoelectric point can be used to determine pH value at which isoelectric state is controlled by the change in pH. This is very interesting, for it means that the charge at the surface of the aqueous solutions is mainly due to the adsorption of H+ and OH" ions. Estimation of the adsorption potential of these ions in the Stem layer (under the assumption that the amounts of both ions absorbed are equal) showed that the adsorption potential of OH" ions is higher. It follows that ( -potential at the solution/air interface appears as a result of adsorption of OH" ions. 

The slope Idiff/dx in fig. 3.20b is higher than the potential gradient across the Stem layer in fig. 3.20a because at fixed potential v °. is higher than in the situation without specific adsorption. At the same time, is lower. This Is the basis for an otherwise unaccountable observation at given surface potential (l.e. at fixed pAg, pH, etc.) specific adsorption leads to a higher surface charge but at the same time renders the sol less stable in the colloid sense. 

Figure 3.23. Surface charge for a Gouy-Stem layer with specific adsorption of the cation at the outer Helmholtz plane for four combinations of the inner layer capacitance C = k ...

Figure 3.24. Surface charge for a Gouy-Stem layer with speciflc adsorption at an Inner Helmholtz plane of varying positions. The numbers at the curves equal P/lfi + y] In fig. 3.20b. 10" M (1-1) electrolyte, K = 10 dm mol". T= 298.16 K. ff°(max) = =...

More pertinent for our purposes is the problem to what extent ionic double layers and the interfacial polarization are additive. In the absence of specific adsorption the problem is relatively minor the countercharge resides in the diffuse part, where solvent polarization is negligible (or neglected by definition) and the Interfacial polarization more or less coincides with the Stem layer, ignoring any contribution from the solid. To a first rough approximation the two potential drops are spatially separated and hence additive, so that... 

Adsorption isotherms were obtained for four amino acids in an investigation of their interaction with calcium montmorillonite and sodium and calcium illite. Linear isotherms were obtained in the study of their adsorption by the calcium clay. These isotherms were described in terms of a constant partition of solute between the solution and the adsorbent Stem layer. Free-energy values were calculated using the van t Hoff relation [16,27]. 

An intermediate situation between cases (b) and (c) is that of a Stem layer with specific adsorption, for which the general charging formalism is available from sec. II.3.6f. The result is, see [11.3.6.65],... 

If a hydrolyzed metal ion is adsoibed, its OH will be included in the proton balance similariy, in case of adsorption of protonated anions, their will be included in the proton balance. Some colloid chemists often place these specifically bound cations and anions in the Stem layer. From a coordination chemistry point of view, it does not appear very meaningful to assign a surface-coordinating ion to a layer different from H or OH in a =MeOH group. 

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