Divalent cations, isotherms

In such systems as (M, Mj (i/2))X (M, monovalent cation Mj, divalent cation X, common anion), the much stronger interaction of M2 with X leads to restricted internal mobility of Mi. This is called the tranquilization effect by M2 on the internal mobility of Mi. This effect is clear when Mj is a divalent or trivalent cation. However, it also occurs in binary alkali systems such as (Na, K)OH. The isotherms belong to type II (Fig. 2) % decreases with increasing concentration of Na. Since the ionic radius of OH-is as small as F", the Coulombic attraction of Na-OH is considerably stronger than that of K-OH. 

In the alkali and alkaline earth nitrate mixtures, the internal mobilities have been systematically investigated, the isotherms being shown in Fig. 15. The internal mobilities of the alkali ions as a function of the molar volume are much smaller than expressed by an equation such as Eq. (12). This means that the internal mobilities of the alkali ions, Mju, are modified by the tranquilization effect caused by the divalent cations. The M ik is assumed to be expressed by... 

Competitive adsorption on sepiolite clay of a monovalent dye (e.g., methyl green or methyl blue) and of the divalent organo-cationic herbicides diquat and paraquat was studied by Rytwo et al. (2002). To evaluate a possible competitive adsorption between the two organic compounds, separate aqueous solutions of each cation were used and adsorption isotherms were obtained. Fig. 8.27 shows the amount of diquat, paraquat, and methyl green adsorbed on sepiolite as a function of total added divalent cation. It may be observed that, when the added amounts were lower than the cation exchange capacity of the sepiolite (O.Mmol kg ), aU cations were completely adsorbed. 

Our current view of the mechanism of membrane fusion in the case of PS vesicles in media containing divalent cations is along the lines of Papahadjopoulos et al. (15), where it was proposed that the key event leading to vesicle membrane fusion is the isothermic phase change... 

The divalent cations studied in more detail were Cu, Zn and Ni. Their isotherms are shown in Figure 9, 10 and 11. All sodium ions were replaced by these divalent ions and their selectivity sequence was ... 

The transformation of the equilibrium (or selectivity) constants and the ion-exchange isotherms can easily be made only for homovalent ion exchange because the ion-exchange isotherms usually do not take into consideration the heterovalent character of the ion exchange. This causes additional serious problems in the evaluation of isotherm parameters. It is shown for the exchange of monovalent and divalent cations that... 

A similar isotherm equation can be derived for the divalent cation, with t, expressing the divalent cation ... 

Kinniburgh D. G., J. A. Barker, and M. Whitfield. 1983. A comparison of some simple adsorption isotherms for describing divalent cation adsorption by ferrihydrite. J. Coll. Interface Sci. 95 370-384. 

Figure 3.6. Calculated isotherms for divalent cation (B ) adsorption on a smectite initially saturated with monovalent cations at three electrolyte concentrations in solution.

Biological membranes, however, also contain lipid components which, in isolation, adopt non-bilayer structures under physiological conditions [114-118]. These non-bilayer structures can be influenced isothermally by a wide variety of physiological factors such as changes in the concentrations of divalent cations, changes in... 

Due to the high water solubility of a divalent cationic head-group, a violo-gen amphiphile 3 (n = 12, m = 5) [56] could not form a stable monolayer on a pure water subphase. A monolayer is stabilized when an anionic polyelectrolyte is added to the water subphase. Pressure-area (ji-A) isotherms of the complex monolayers are shown in Fig. 4. The shape of the pressure-area isotherms is strongly dependent upon the chemical structure of the anionic polyelectrolyte. A condensed monolayer was formed over a highly diluted solution (ca. 0.06 mM)... 

In the discussion of fatty acids given so far it has been assumed that the subphase is sufficiently acid so that these materials are not ionised. However, if the pH is increased so as to be comparable to or larger than the pA"a of the fatty acid, then the situation will obviously be different. If now a divalent or trivalent cation is introduced into the subphase, the structure of the film and the resulting isotherms will be substantially changed. Wostenholme and Schulman [77] were amongst the first workers to make a systematic study of this effect. Binks [78] has given a recent review of the very extensive literature devoted to this phenomenon. The matter will be returned to in Section 3.3. 

Cation Exchange in Mordenite. Until 1974 there had been few systematic studies of ion exchange in mordenite and only Rees had determined isotherms. Peculiarities were found which had not been observed in A, X, and Y zeolites or in chabazite, in that only limited exchange of divalent ions could be achieved. Using a natural mordenite from Harbourville, Nova Scotia, Rees found for... 

The uptake(pH) curves were generated for two hypothetical materials, namely, alumina and silica whose surface area is 100 m /g. and whose pristine surface charging behavior corresponds to the model curves presented in Figs. 5.72 and 5.85, respectively. The model curves representing specific adsorption of Pb on these materials at low initial concentration ([surface sites] > > [total Pb]) were calculated to produce pHso S for 10 g solid/dm and in the presence of 10 mol dm inert electrolyte solution. For sufficiently low total Pb concentration the adsorption isotherms at constant pH are linear and the course of the calculated uptake(pH) curves is independent of the Pb initial concentration. The above solid to liquid ratio is typical for studies of specific adsorption, and pHjo = 5 is a realistic value for Pb adsorption on silica and alumina at this solid to liquid ratio, in view of the results of actual adsorption experiments compiled in Table 4.1. Lead has higher affinity to solid surfaces than most other divalent metal cations. The choice of the model curves from Fig. 5.72 (alumina) and 5.85 (silica), rather than model curves derived from any other set of experimental data analyzed in Section III, or calculated using any other model than TLM, or any other set of TLM parameters was arbitrary. This choice does not imply that TLM is favored over other models or that the experimental data used to derive these model curves are more reliable than other results used as examples in Section III,... 

Criscenti and Sverjensky (1999, 2002) continued to build the internally consistent set of triple layer model equilibrium constants developed by Sverjensky and Saliai (1996) and Sahai and Sverjensky (1997a,b) by reexamining sets of adsorption edge and isotherm data for divalent metal cation adsorption onto oxide surfaces. In contrast to previous investigations, they found tliat the adsorption of transition and heavy metals on solids such as goethite, y-ALOs, corundum, and anatase, which have dielectric constants between 10 and 22, was best described by surface complexes of the metal with the electrolyte anion. Metal (M +j adsorption from NaNOs solutions is described by... 

If we assume that divalent and monovalent cations form only 1 1 complexes with the membrane. surface charges, algebraic manipulation of the Langmuir adsorption isotherm and equation / / gives ... 

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