Does cation dehydration drive the binding of metal ions to polyelectrolytes in water? What we can learn from the behaviour of aluminium(iii) and chromium(iii)

Abstract

Much stronger binding is seen in aqueous solutions between the anionic polyelectrolyte potassium poly(vinyl sulfate) and the substitution labile aluminium(III) than with the kinetically inert chromium(III). This strongly supports the idea that entropy driven water loss from the hydration sphere of the metal ion plays a major role in driving binding of the trivalent metal ion to the polyelectrolyte.

Silica Aerogels/Xerogels Modified with Nitrogen-Containing Groups for Heavy Metal Adsorption

“…On the other hand, the materials of formulation B do not interact well with the cations, apart from lead which has considerable removal efficiencies at the tested concentration. In fact, lead is fairly removed by all materials, compared to the other cations, because lead has the lowest charge density of the four, and thus has the less stable hydration sphere, being more labile [ 39 , 40 ].The removal of lead could be due to silanol groups, featured in all silica-based adsorbents or to non-specific processes like electrostatic interactions with the negatively charged [ 41 , 42 ] silica surface. Despite having functional groups containing electron donor atoms, i.e., Lewis bases, the results obtained with aerogels and xerogels of formulations TRIS and U are nearly similar to those of formulation B.…”

“…However, for Cd(II) and Pb(II) the sorption is characterized by an endothermic process and only for Cu(II) the sorption is slightly exothermic, suggesting that the sorption interaction between metal ions and the adsorbent is weaker [ 51 , 52 ]. This also explains the dependence of the Gibbs energy change of copper and nickel (r i = 0.69 Å [ 50 ]) with the temperature, since the stability of hydration shell is higher and, consequently, these metal ions are the less labile [ 39 ]. This is in line with the entropy variation values for the different metal ions; in fact, the water loss from the hydration sphere of more labile metal ions plays a major role in driving the adsorption process [ 39 ].…”

“…This also explains the dependence of the Gibbs energy change of copper and nickel (r i = 0.69 Å [ 50 ]) with the temperature, since the stability of hydration shell is higher and, consequently, these metal ions are the less labile [ 39 ]. This is in line with the entropy variation values for the different metal ions; in fact, the water loss from the hydration sphere of more labile metal ions plays a major role in driving the adsorption process [ 39 ].…”

The effects of charge density distributions of metal ions on single water molecule dissociation of [M(H2O)6]3+ systems

“…Based on the activation energies (figure 6), we have found that Cr(III) cluster is more stable than Al(III) cluster. These results do not only explain the higher number of Al(III) binding to PVS than Cr(III) [3], but they also provide another reasonable explanation for higher membrane degradation level by Fe(III) than that of Al(III) [4] metal ion. Another experimental study [46] on the ion exchange reaction of Al(III) with Fe(III) also supports our results.…”

“…The dehydration has been acknowledged as an essential molecular interaction for water-ion exchange reaction, since some experimental evidences have shown that ion attack at surfactants and polyelectrolyte is followed by its partial interaction of the counter-ions [1,2]. A recent study on counter ion-binding (aqueous Al(III) and Cr(III)) related to the ion attack at electrolyte polyvinyl sulphate (PVS) has brought up that hydration enthalpy from coordinated water release within the primary hydration sphere plays a key role in driving binding to PVS [3]. Similar reactions possibly happen in a humid membrane of fuel cell, when contained water is starting dehydrated at working temperature.…”