Hofmeister series, anions-cations ordering

COAGULATION (Hofmeister Series). A definite order of arrangement of anions and cations according lo their powers of coagulation when their salts are added in quantity lo lyophilic sols. Thus, the order of cations... 

Note Some of Grahame s values for and included in this table. For a common cation, the sequence of anions in order of increasing adsorption is similar to that of the Hofmeister series in coagulation studies, and it is evident that specific adsorption properties are involved. 

S ts can be used to precipitate proteins by salting out effects. The effectiveness of various salts is determined by the Hofmeister series, with anions being effective in the order citrate > PO4" > SO4" > CH3COO > Cl > NO3 , and cations according to NH4 > > Na ... 

The order of cations and anions regarding these structure breaking and making properties is related to their position in the lyotropic or Hofmeister series, such as... 

The activity and stability of mushroom tyrosinase in [BMIM] containing ILs were analyzed in detail with respect to the influence of Hofmeister effect of the anions on the catalytic performance of the enzyme and active site structure as well as reaction mechanism [120]. Enzyme activity was in the opposite order to that of the Hofmeister series [BMIM][PFg]>[BMIM][BF ]>[BMIM][MeSO ] which was attributed to the more influential kosmotropic effect of the cation [121]. 

The conformational stability of biomolecules is greatly dependent on the solvent species. It is also affected by coexisting solutes such as salts (e.g., NaCl). The salt effects [47, 48, 49] on the solubility and the conformational stability of proteins in aqueous solutions are experimentally known to follow the order called the Hofmeister series. The series for anions is [S04 > CHsCOO" > Cl > Br > NOa" > CIOJ > 1 > CNS ], and that for cations is [(CH3)4N+ > NH > Rb+,K+,Na+, Cs+ > Li+ > Mg + > Ca + > Ba +j. In each of these series, the species to the left decrease the solubility of proteins and stabilize their native structures. The species to the right, on the contrary, increase the solubility and cause destabilization of the native structures. Though the Hofmeister series is not valid for acidic and basic proteins [50, 51], it is generally applicable to neutral proteins. The series, except for divalent cations, is also applicable to the other neutral substances such as benzene [52]. That is, the effects of monovalent ions on the solubility of various neutral substances follow the Hofmeister series. The microscopic mechanisms of these experimentally known properties, however, have not been elucidated yet. 

It is furthermore concluded that whereas the anion Hofmeister series is generally well established, with only few reversals that can be explained ad-hoc, this is not the case regarding the cation Hofmeister series. There are in general no head cations established in a series such as (5.8), erroneously termed chaotropic, nor general tail cations , erroneously termed cosmotropic, but for each surface the hydrophilic and hydrophobic character or balance between these properties is responsible for the order of cations interacting directly with sites at the surface, in addition to the ion properties mentioned above, see Table 5.4 for the latter. 

Preferential adsorption of one type of ions onto the particle surface coupled with the formation of a diffuse layer of the counterions (ions of opposite charge) leads to electrostatic stabilization due to repulsion between the double layers (Chapter 4). The valence and radius of the counterions can modify the repulsion between the particles and so can influence the stability of the suspension. Counterions with higher valence are more effective for causing flocculation Schulze-Hardy rule), while for ions of the same valence, the smaller ions are more effective. For monovalent cations, the effectiveness of flocculation is in the order Li > Na > K+ > NH4, while for divalent cations, Mg " > Ca " > Sr " > Ba. This sequence is known as the Hofmeister series. For common anions, the effectiveness of flocculation is in the order 804 > Cl > NOJ. 

Ion-specific effects on enzyme activity could be explained through their kosmo-tropicity (Hofmeister series) [80]. The Hofmeister series describes the strength of interaction of ions with water. Kosmotropic ions interact strongly with water and tend to produce ordered water structures, while chaotropic ions interact less with water than water itself The ability of ions to modify the water structure influences the protein hydration environment, and combined with direct interactions between ions and proteins, the protein structure can be affected. The suggested hypothesis is that kosmotropic anions and chaotropic cations stabilize enzymes, while chaotropic anions and kosmotropic cations destabilize them [80]. In a number of reports. 

Cations and anions in the presence of the same counter ion can be arranged in the following orders (Hofmeister series) based on their salting out effects ... 

FIGURE 7.6 Typical ordering of cations and anions in a Hofmeister series. Source Reprinted from Ref. [36] with permission. 

Hofmeister series chem An arrangement of anions or cations in order of decreasing... 

Salts are known to influence several properties of aqueous solutions in a systematic way (122,123). The effect of different aiuons and cations seems to be ordered in a sequence this theory was already proposed by Hofmeister in 1888 (124) from a series of experiments on the salts ability to precipitate hen-egg white protein. Numerous other properties of aqueous salt solutions are also found to be systematically salt dependent, such as the surface tension or the surface potential (122). However, the exact reason for the observed specific cation and anion sequences is still not fully understood (125). Model calculations (126), as well as nuclear magnetic relaxation experiments (127), propose a delicate balance between ion adsorption and exclusion at the solute interface. This balance is tuned by the solvent (water) stmcture modification according to the ion hydration (128, 129) and hence is possibly subject to molecular details. 

The increase in retention at a given concentration depends on both the anion and the cation of the salt. Speciflcally, the retention increment depends on the molal surface tension increment of the salt. Table 13.1 contains this value for various salts. The higher the value, the greater is the retention at a pven concentration. It should be noted that the order in the table parallels the Hofmeister salting-out series (12). 

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