Cosmotropic anions

Recently, Liu et al. have broadened the field of ion-specific effects on polyelectrolyte multilayers by investigation of the deposition of PSS/ PDADMAC multilayers in mixed electrolyte solutions (PDADMAC poly(diallyldimethy-lammonium chloride)) [22]. It appeared that the effects of anions of a mixed electrolyte solution on the deposition of PEMs are nonadditive. Moreover, in a mixed electrolyte solution containing both chaotropic and cosmotropic anions, the multilayer buildup is dominated by chaotropic anions. 

The PSS/PDADMAC multilayer growth in a single electrolyte solution containing one type of chaotropic anions (e.g., Br, ClOs ) is nonlinear (increasing increment with increasing number of deposited layers). In contrast to this, in the presence of cosmotropic anions like E, CHaCOO, H2PO, and a linear growth... 

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. 

The observed ion-specific effects were associated with the hydrophobicity of the ions and the ions affinity to the polyelectrolyte. Later, Khtzing et al. explained this effect by the difference between cosmotropic and chaotropic ions [37]. The influence of specific anions on certain properties of PEMs has been investigated extensively by Salomaki et al. [28-30]. The anions tested within this work followed the famous Hofmeister series and gave an increasing storage shear modulus with increasing ion size. The latter result is in contradiction to the increase in chain mobihty found later by Nazaran et al. [24]. 

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