Hydration of Ions in Confined Spaces and Ion Recognition Selectivity

Abstract

The hydration of ions in confined spaces, such as the interior of ion-exchange resins, micelles, and surface monolayers, is discussed on the basis of results obtained with X-ray absorption fine structure studies, electrophoresis, and ion-transfer voltammetry. The general trends are that anions are partly dehydrated therein, whereas cations are likely to keep their first hydration shells. For bromide ions, the hydration numbers under various circumstances have been determined. The extents of dehydration depend not only on the structure of the cationic sites electrostatically attracting bromide ions but also on whether the cationic sites are exposed to a solution or are effectively shielded from it. These findings will be useful for designing the systems for ionic recognition and separation.

Nanopore device-based fingerprinting of RNA oligos and microRNAs enhanced with an Osmium tag

“…5e) that purines who are about double the size of pyrimidines yield less ion current obstruction compared to pyrimidines. In addition to proposed rationalizations that attribute the observations with intact homopolymer to special helical structures in a confined environment 1,2 , we wish to propose hydration or solvation/desolvation 67 , as another critical parameter, and envision this phenomenon as follows: There are two different sources of water molecules present within the nanopore at all times. One source are the water molecules solvating the salt ions which are responsible for the observed ion current.…”

“…Solvation inside a nanopore will not resemble solvation in the bulk solution, but desolvation is costly from a thermodynamic point of view. In the confined space the first, perhaps the second too, solvation shell(s) of any functional group within the pore will remain intact 67 , unless it is replaced by a new component in the system. In this context higher solvation requirements can rationalize the typically lower I r of ribooligos compared to deoxyoligos, the typically lower I r of pyrimidines (two functional groups) compared to adenine (one functional group), and the reported here higher I r level of 4-SU compared to U, consistent with the almost double atomic radius of S vs. O and its resulting easier desolvation.…”

Upgrading bio-based acetone to diacetone alcohol by aldol reaction using Amberlyst A26-OH as catalyst

“…Without water addition the strong anion exchange capacity is reduced by 17%, with water addition the reduction is only 6% (Table 1). With water addition a bigger hydrate sphere around the ions of the ion exchange resin is present which protects the quaternary ammonium group against the Hofmann rearrangement [41].…”

Anion exchangers with branched functional ion exchange layers of different hydrophilicity for ion chromatography

“…However, the further elimination of non-ionic interactions between anions and the substrate can be achieved by moving anion exchange groups aside from the hydrophobic polymer surface by using long linkers [15,16]. These effects can also be explained from the view point of hydration of the functional site, as soon as more polarizable anions have higher affinity to more hydrophobic less hydrated anion exchange groups, which leads to their higher retention and sometimes considerable peak asymmetry [17,18]. In this case, moving functional groups aside from the hydrophobic polymer surface leads to their better hydration.…”