Acoustic Evaluation of Ion-exchange

The acoustic properties of ion-exchange resins strongly depend on the hydration natures of counterions as well as the cross-linking percentage and base materials. In this paper, ionexchange resin particles are discriminated on the basis of the acoustic nature of counterions using a coupled acoustic-gravity field. Ion-exchange resin particles are aggregated at the equilibrium positions, which are determined by the densities and compressibility of the particles, in a half-wavelength matching cell filled with water. The aggregation positions of tetrabutylammonuim (TBA), hydrogen, and caesium ion-form resins become lower in this order. The high compressibility of the TBA-form resin is an important factor for the highest aggregation position, whereas the high density of the Cs-form resin is responsible for the lowest aggregation position. When a mixture of resins with different counterions is introduced into the observation cell, the separation of two aggregation zones is observed. This approach thus allows the visual identification of resins and the evaluation of the extent of an ion-exchange reaction. INTRODUCTION Ion-exchange experiments usually involve the processes for the determination of ions in the solution phase by an appropriate method such as spectrometry and titration. A single determination of a selectivity coefficient based on this traditional routine takes a relatively long time period. Thus, it has been difficult to study the kinetic or dynamic aspects involved in ionexchange reactions particularly when the reaction is progressed within a short time. The development of in situ or real-time measurements has thus been important for the design of new materials and the further understanding of ion-exchange phenomena. The utilization of a physical force field is a possible choice. However, physical force is usually a function of the sizes of the particles studied, and, if applied to complex samples such as ion-exchange resins, it leads to very complicated results of less use. A size-independent physical field is thus suitable for ion-exchange studies. The swelling properties of ion-exchange resins strongly depend on the hydration nature of counterions. Ion-exchange selectivity has been discussed from this perspective, and the thermodynamic and structural features involved in ion-exchange reactions have been revealed to some extent. For a cation-exchange resin, water contents (or extents of swelling) decrease in the order that is predictable from the hydration energies of countercations. If a difference in the swelling between resins is probed by an appropriate method, the extent of an ion-exchange reaction should also be evaluated. The swelling of resins results in different densities and compressibility, which are acoustically discriminated. Thus, resin particles with different counterions are expected to behave differently in an acoustic field. We proposed a coupled acoustic-gravity field, and showed that the aggregation coordinates of a particle therein is not affected by its size.[1-4] This physical field should thus be advantageous over other physical fields particularly if we handle particles with various chemical compositions as well as with a wide size distribution; an ion-exchange resin is the case.. In this paper, the behavior of ionexchange resin particles with different countercations in the coupled acoustic-gravity field is discussed. 19 INTERNATIONAL CONGRESS ON ACOUSTICS – ICA2007MADRID