![]() The solution layer was estimated to be about 17-21 nm thick (details regarding this calculation are given in Supplementary Note 1). Prior to tender-APXPS measurements, we used the “dip and pull” method 21, 22, 23 to form a thin solution layer on the membrane surface. Details of the membrane properties are available in “Methods” and Supplementary Table 1. In this study, aqueous salt solutions with concentrations ranging from 0.001 to 1 M were equilibrated with CR-61, a commercial poly(p-styrene sulfonate-co-divinylbenzene) cation exchange membrane (i.e., a membrane bearing negatively charged sulfonate groups). Probing the Donnan potential with tender APXPS This work opens a new avenue to broadly rationalize ion transport and selectivity in membranes by directly probing the Donnan potential developed at the solution/membrane interface. Our measurements are based on the binding energy shift in membrane related core levels detected by tender ambient pressure X-ray photoelectron spectroscopy (tender-APXPS). We therefore sought to develop an experimental methodology for directly measuring the Donnan potential in a commercial ion-exchange membrane equilibrated with monovalent and divalent aqueous salt solutions (i.e., NaCl and MgCl 2) under equilibrium conditions. Despite the long history and widespread use of IEMs, direct measurement of the Donnan potential and experimental verification of Donnan’s framework has been impossible due to the lack of appropriate interfacial experimental techniques, which has held back the development of novel membranes that are more selective for specific ions (e.g., Li for resource recovery, Ra for hazardous waste management, As for water treatment). To date, Donnan’s theory has been the accepted framework for describing ion sorption phenomena in IEMs, although disagreements exist between experimentally determined ion sorption values and values derived from the original theory 18, 19, 20. The Donnan potential acts to exclude co-ions from entering the membrane 16, where larger absolute Donnan potential values lead to stronger co-ion exclusion from the membrane 17. Due to the presence of fixed ions and the condition of electroneutrality, an unequal distribution of ions exists in a charged polymer, generating an electrical potential, the Donnan potential, at the membrane/solution interface. In 1911, the permselective nature of IEMs was postulated for the first time by Donnan, using thermodynamic equilibrium and electrostatic considerations 15. IEMs contain ionizable groups, bound to the membrane, that allow permeation of oppositely charged ions (counter-ions), while largely rejecting similarly charged ions (co-ions) 10, 11, 12, 13, 14. Similarly, various technologies for water purification and energy storage and conversion rely on ion exchange membranes (IEMs) to selectively control ion transport rates. These channels are responsible for the initiation and continuation of electrical signals in the nervous system 5, the timely delivery of Ca 2+ ions that initiate a contraction in muscle cells 6, the regulation of volume increase/decrease in response to transient changes in the cell environment 7, and pH sensing and survival of bacteria under acid stress 8, 9. ![]() For naturally occurring systems, such as biological cells, well-regulated control of ion transport is achieved through highly specialized ion channels capable of displaying extraordinary degrees of selectivity 3. The electrochemical potential distribution at a membrane/solution interface profoundly influences the transport and selectivity of solutes across semi-permeable membranes and plays an important role in many electrochemical 1, biological 2, 3, and colloidal systems 4. By directly measuring the Donnan potential, we eliminate ambiguities that arise from limitations inherent in current models. Our results highlight the dependence of the Donnan potential on external salt concentration and counter-ion valence, and show a reasonable agreement with current theoretical models of IEMs, which incorporate ion activity coefficients. Here we report the first direct measurement of the Donnan potential of an ion exchange membrane equilibrated with salt solutions. Although there are well-established ways to indirectly estimate the Donnan potential, it has been widely reported that it cannot be measured directly. The Donnan potential results in the partial exclusion of co-ion, providing the basis of permselectivity. When a charged membrane is equilibrated with an electrolyte, an unequal distribution of ions arises between phases, generating the so-called Donnan electrical potential at the solution/membrane interface. Selective transport of solutes across a membrane is critical for many biological, water treatment and energy conversion and storage systems.
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