Ionic strength adsorption

Fig. 8. Residual adsorbed amount of dT35 onto bare latex particles as a function of ionic strength. Adsorption was performed onto bare latex particles with Ns=1.0 mg nr2 at pH 5 and 10 2 mol/1 ionic strength. The desorption was carried out after two washes using buffer (at various ionic strengths, pH 10 and 1% Triton x-405) [22]...

Arafat et al. [4] studied the effect of the concentration of inorganic electrolyte on adsorption of benzene, toluene and phenol from aqueous solution at pH 11.6 on one commercial and two modified activated carbons and obtained very different results for these three adsorbates. The uptake of benzene was rather insensitive to the ionic strength. The uptake of toluene systematically decreased when the ionic strength increased. Finally the uptake of phenol was enhanced on addition of 0.5 mol dm KCl, but further addition of salt depressed the uptake and with 0.8 mol dm"" KCl the uptake dropped below that observed at low ionic strength. Adsorption of phenol on activated carbons was recently studied by other research groups [12,13], but without emphasis on the possible effects of pH dependent surface charging. 

Determination of the extent of adsorption at various pH values and constant ionic strength (adsorption edge)... 

Radovic et al. (2000) review a selection of the considerable available literature which attempts to explain the processes of adsorption of gold and silver complexes on carbon surfaces. No unequivocal message emerges from this review. Radovic et al. (2000) report that, in the presence of oxygen, the species adsorbed was M" [Au(CN)2 ] . At low ionic strengths adsorption was considered to involve ion-exchange sites. 

Later, some publications postulated the reduced viscosity to decrease again at very high dilution [144-147], thus questioning the validity of the Fuoss-Strauss extrapolation procedure. However, due to the enormous experimental difficulties (influence of dust particles, atmospheric carbon dioxide altering the ionic strength, adsorption problems) these results were mostly considered as unreliable. In recent years measurements with highly sensitive and sophisticated viscometers [148-158] provided overwhelming evidence for the presence of a maximum and proved the Fuoss-Strauss extrapolation to be one of the big errors in polyelectrolyte history. 

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. 

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

Typical adsorption isotherms are shown in Figs. 16 and 17. Despite the large experimental scatter, a steep increase in adsorption can be seen at low concentrations, followed by a plateau at concentrations exceeding the CMC. Similar behavior has been observed before with model surfactants [49-54] and has also been predicted by modem theories of adsorption [54]. According to Fig. 16, adsorption increases modestly with salinity provided that the calcium ion concentration remains low. The calcium influence, shown in Fig. 17, cannot be explained by ionic strength effects alone but may be due to calcium-kaolinite interactions. 

Pullerits et al.m studied specific adsorption of I from an aqueous solution at constant ionic strength. 

Domenek S., Petit E., Ducept F., Mezdoura S., Brambati N., Ridoux C., Guedj S., Michon C. 2008. Influence of concentration and ionic strength on the adsorption kinetics of gelatin at the air/water interface. Colloids and Surfaces A Physicochem. Eng. Aspects 331, 48-55. 

Mahoney JJ, Langmuir D (1991) Adsorption of Sr on kaolinite, illite, and montmorillonite at high ionic strengths. Radiochim Acta 54 139-144... 

Separations in hydrophobic interaction chromatography have been modeled as a function of the ionic strength of the buffer and of the hydrophobicity of the column, and tested using the elution of lysozyme and ovalbumin from octyl-, butyl- and phenyl-Sepharose phases.2 The theoretical framework used preferential interaction analysis, a theory competitive to solvophobic theory. Solvophobic theory views protein-surface interaction as a two-step process. In this model, the protein appears in a cavity in the water formed above the adsorption site and then adsorbs to the phase, with the free energy change... 

---