Specific Ion Effects in Hydrogels

Abstract

Specific ion effects on the structure and function of many biological macromolecules, their associations, colloidal systems, interfacial phenomena, and even "simple" electrolytes solutions are ubiquitous. The molecular origin of such phenomena is discussed either as a salt-induced change of the water structure (the hydrogen bond network) or some specific (solvent mediated) interactions of one or both of the ions of the electrolyte with the investigated co-solute (macromolecules or colloidal particles). The case of hydrogels is of high interest but is only marginally explored with respect to other physico-chemical systems because they are formed through the interactions of gelling agents in the presence of water and the added electrolyte. In addition, hydrogels in a physiological environment, in which they are used for biomedical applications, may be subjected to fluctuations in their ionic environment. In this review, specific ion effects on the properties of hydrogels (made from macromolecules or small-molecular-weight gelators) are reviewed and discussed. In particular, the importance of specific ion binding to the molecules constituting the gel network versus the effect of the same ions on the structure of water is discussed.

Keywords: chaotropes; hydrogels; kosmotropes; specific binding; specific salt effects.

Scheme 1

The main anions and cations and their classification in the Hofmeister series as well as some characteristic attributes of kosmotropic and chaotropic ions. Adapted from [14] with authorization.

Figure 1

Correlation between the entropy of hydration and the viscosity B coefficient (A) and between the polarizability of the ion pair solubilized in water with the viscosity B coefficient (B). In panel B, the full and dashed lines correspond to a linear correlation and to the limits of the 95% confidence interval, respectively. The values of the viscosity B coefficients and of the entropy of hydration are taken from Table 2 of [18], whereas the polarizability values are taken from Table 1 of [19].

Figure 2

(I) Depression of the LCST of PNIPAM versus the hydration entropy of the used anion. A good correlation is found for kosmotropes (full and dashed lines, the nature of the anions in the form of a sodium salt) but not for chaotropes, which are clustered in the upper-left part of the figure. (IIa) Change in the LCST of PNIPAM for sodium salts of chaotropic anions. The lines correspond to the fits of Langmuir isotherms to the experimental data. (IIb) Depression of the LCST of PNIPAM versus the surface tension increment of the aqueous electrolyte solutions. This time, the correlation is good for chaotropes (full line, the nature of the anions in the form of a sodium salt) and bad for kosmotropes, which are clustered in the lower-right part of the figure. Reproduced from [28] with authorization.

Figure 3

(I) Typical gelation kinetics of 10% (w/v) bovine gelatin solutions in the presence of sodium salts at an ionic strength of 100 mM: (a): NaCl, (b) NaI, and (c) NaSCN. (IIa) variation of the constant k (defined in Equation (2) as the proportionality constant between the logarithm of the ionic strength and the logarithm of the gelation time) as a function of the Jones and Dole viscosity B coefficient of the different used sodium salts (indicated on each experimental point) in the gelatin solutions. (IIb) The same data as in panel (IIa) but restricted to halide anions. The full and dashed lines correspond to linear regressions and to the limits of the 95% confidence interval, respectively. Modified from [19] with authorization from the American Chemical Society.

Figure 4

Correlation between the slope of the peak temperature measured during DSC scans on hydroxypropylmethylcellulose hydrogels as a function of the salt concentration versus the entropy of hydration (panel I) and the viscosity B coefficient (panel II). For all those anions (indicated in the figures), sodium was the common cation. Data taken from Table II of [45].

Figure 5

Correlations between the expansion ratio of poly(methyl methacrylate)₈₈-block-poly(2-diethylamino)ethyl methacrylate)₂₂₃-poly(methyl methacrylate)₈₈ as measured by SAXS. (I) Influence of the viscosity B coefficient, (II) influence of the surface charge density, and (III) influence of the entropy of hydration. The full lines correspond to linear regressions and the dashed lines to the 95% confidence intervals. The data for the thiocyanate anion (open circles in part II and III) have not been taken into account for linear regression. Reproduced from [18] with authorization.