Ion-transport and diffusion coefficients of non-plasticised methacrylic–acrylic ion-selective membranes
Introduction
Transport processes through ion-selective membranes have preoccupied researchers in their understanding of the ion-selective mechanism. Bulk diffusion through the membrane is identified as an intrinsic process in the response mechanism leading to the potentiometric response and influencing the limits of the response. Many methods have been devised to study the transport through the membrane, including the use of stacks of membranes, each 40–50 μm thick, which are separated and analysed after an experiment [1]. More recently, the advent of chromoionophores has allowed spectrophotometric methods to be employed and experimentally observed diffusion potentials and ionic site concentrations to be rationalised against theoretical prediction [2], [3]. Schneider et al. reported a method for in situ imaging of the concentration profiles through membranes containing an H+-sensitive chromoionophore, with a diffusion pathway >600 μm. From these images, diffusion coefficients could be obtained for data taken up to 24 h. The spatially resolved data relates the degree of deprotonation, α, of the chomoionophore across the membrane to the transport process, thus allowing diffusion coefficients to be obtained by fitting diffusion equations to the concentration profiles. However, diffusion coefficients could also be obtained without the spatial resolution of this method, by following the bulk change in protonation of the membrane.
The present interest in polymer matrices other than PVC and the attempts to remove plasticiser have renewed interest in the mechanisms of ion-transport involving ionophores in different supporting polymers. Concomitant with this general aim is a focus on the lower detection limit of many conventional ion-selective electrodes (ISEs) in the field of ion sensing [4], [5], [6]. It has been shown that the lowering of the detection limit of an ISE not only depends on its membrane selectivity but also the analyte ions that may be released from the membrane itself due to ionic fluxes [7], [8]. Diffusion coefficients through the membrane therefore become important in the design of the membrane. There are various processes that lead to the release of analyte ions from the membrane [7]: (1) coextraction of analyte cations and counterions from the inner electrolyte solution. These are transported across the membrane as ionophore complex and counteranion towards the diffusion layer at the sample side; (2) simple partitioning of analyte cations and lipophilic anionic additives from the membrane into the sample solution; (3) a counter-diffusion process from partial exchange of analyte cations with interfering cations that cause transport of the analyte cations across the membrane towards the direction of the sample. In this case, for primary and interferent ions of the same charge, the detection limit may be expressed as:where cI and cJ are the surface concentration of the analyte cation and interference cation, respectively, Kex the ion-exchange constant, DIm and DIaq the diffusion coefficients of the ion in the membrane and in aqueous phase, δaq and δm the thicknesses of the diffusion layers involved and RT− the concentration of the lipophilic anionic sites in the membrane.
The release of analyte cations from the membrane to the diffusion layer on the sample side [7] eventually leads to a poorer detection limit. Lower concentration of analyte cation in the inner solution, thicker membrane, lower concentration of ion exchanger, lower diffusion coefficient (DIm) and stirring (δaq) have all been proposed as ways that will assist in achieving a lower detection limit.
Transport behaviour, characterised by the diffusion coefficients of various migrating species in the membrane, plays an important role in establishing the analyte cation concentration in the membrane and diffusion layer [9]. Measured apparent diffusion coefficients will reflect the dominant limiting mass transfer process and may be influenced by both ion and ionophore concentration, depending on the details of the transport mechanism involving the ionophore. For a given polymer, reduction of the diffusion coefficient of the ion in the polymeric membrane has been proposed by covalent immobilisation of membrane materials [10] such as ionophores. Clearly, ion diffusion requires movement from one immobile ionophore site to the next, conceivably influenced by inter-site distance, but is the diffusion coefficient just influenced by the carrier or does the polymer matrix contribute a significant or minor role?
The non-plasticised methacrylic–acrylic-based polymer membranes have been chosen for further investigation because they have been proven to yield ISEs for many ions that give comparable analytical response to that of the highly plasticised PVC membranes [10], [11]. Diffusion coefficients can therefore be compared in different polymers with and without plasticiser. Covalent immobilisation of ionophore is also possible with these polymers [10]. In this work, we have investigated the ion-transport behaviour using a spectrophotometric method, based on the basic principle of the ion-selective optode and the use of chromogenic reagents [12], [13], [14]. The aim of the study was to obtain estimates of diffusion coefficients and compare results using ionophores which form 1:1 and 1:2 complexes with analyte ions.
Section snippets
Reagents
Copolymers synthesised from various compositions of methyl methacrylate and n-butyl acrylate (MB16, MB1:10, MB32; see Table 1) were used. Similar polymers with acryloylamido-18-crown-6 (AAB18C6) or acryloylamido-15-crown-5 (AAB15C5) immobilised were also used. The syntheses of these copolymers were described elsewhere and their chemical composition are shown in Table 1. High molecular weight PVC was purchased from Aldrich. Ionophores, chromoionophores and other membrane components:
Bulk spectrophotometric measurement to give estimation of diffusion coefficient
The process occurring in an optode film containing just chromoionophore (Cm) that binds H+ and lipophilic anions (R−) has been derived many times from the ion-exchange reaction between an aqueous (aq) and the organic (org) membrane phase [17]:Assuming that concentrations in the membrane are proportional to activities, the response function of the membrane is [18], [19]:where Kexch is the exchange constant for the equilibrium [2]; the degree of deprotonation, α
Conclusions
The values of the apparent diffusion coefficients observed for the acrylate containing optode films were of the order of 10−11 cm2 s−1. The diffusion coefficients obtained from this study are the same order of magnitude as those obtained from measurement with polyetherpolyamide membranes with covalently bound 18-crown-6 determined by conductivity measurements. Other reported diffusion coefficients for sodium and rubidium ions in a polydibenzo-18-crown-6-ether polymer, determined by radio
Acknowledgements
We would like to thank the Faculty of Science and Technology, National University of Malaysia (UKM) for a research leave granted to Lee Yook Heng to complete this study at the Institute of Biotechnology, Cambridge University.
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Present address: Faculty of Science and Technology (FST), National University of Malaysia (UKM), Bangi, 43600 Selangor, Malaysia.