Mechanism of Ion Transfer in Supported Liquid Membrane Systems: Electrochemical Control over Membrane Distribution

A polarization study carried out on a thin supported liquid membrane separating two aqueous compartments is presented. Transfer of both the ionized and uncharged form of an organic tracer dye, rhodamine B ([9-(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene]-diethylammonium chloride), across supported liquid membranes composed of one of 1-octanol (octan-1-ol), 1,9-decadiene (deca-1,9-diene), 1,2-dichlorobenzene, or nitrophenyl octyl ether (1-(2-nitrophenoxy)octane) was studied using cyclic voltammetry and UV–vis absorption spectrophotometry. Concentration analysis indicates that the high membrane concentration of rhodamine B determines the ionic transfer observed via voltammetry, which is consistent with the low aqueous ionic concentration and large membrane/aqueous distribution of the molecule. The observed double-transfer voltammogram, although it has been largely neglected in previous literature, is a logical consequence of the presence of two liquid–liquid interfaces and is rationalized in terms of ion transfer across the two interfaces on either side of the membrane and supported by voltammograms obtained for a series of ions of varied lipophilicity. The bipolar nature of the voltammetric response offers an effective way of mass transport control via changing polarity of the applied voltage and finds immediate use in extraction, purification, and separation applications.


S-2 Rhodamine B stability under UV-vis light
The stability of the molecule under UV light was confirmed by collecting the UV-vis spectra of a 2 µM solution rhodamine B solution for 100 h as shown in Fig. S-2. Over the course of UV-vis irradiation and absorbance detection, the shape of the spectra was not changed and the overall absorbance decreased by a mere 1.5%, indicating the slight adsorption of RB on the walls of the lower PTFE cell, which was also visible as a slight colouring on the PTFE.

S-3 Spectrophotovoltammetry in an analyte-free SLM system
A blank supported liquid membrane system, containing only the organic solvent, aqueous buffer, and both aqueous and organic electrolytes, was analysed using cyclic voltammetry. The comparison of the voltammetric data obtained for 1-octanol, 1,9-decadiene, 1,2-dichlorobenzene and nitrophenyl octyl ether is shown in Fig. S-3a. With exception of 1-octanol, all solvents exhibit a large baseline potential window of about 1.2 V width (for |I| < 2 µA), centred around 0 V, which is suitable for transfer of ions within this potential range. The potential window is limited by the transfer of either tetradodecylammonium (TDDA + ) or tetrakis(4-chlorophenyl)borate (TPBCl 4 -) organic electrolyte ions, which both exit the membrane at extreme potentials depending on their standard transfer potential value.
The phase to which the respective ions transfer depends on the polarity applied to the SLM as indicated in Fig. S-3a. Detailed voltammetric study using TDDACl and KTPBCl 4 as the membrane electrolytes has proven that TPBCl 4 anion (rather than TDDA + cation) limits the potential window. 1 NPOE produces a very low and featureless current-potential response, which makes this solvent an excellent choice for electrochemical measurements in this SLM system. Both 1,9-decadiene and ODCB result in a large potential window comparable to NPOE but the current-potential response is inferior. 1-octanol, on the other hand, has a poor response with a potential window of a mere 0.4 V (for |I| < 10 µA) due to its high resistivity, which makes it an unsuitable system for electrochemical experiments in this configuration.
Rotation of the SLM about its vertical axis results in stable hydrodynamic conditions (including mixing of the aqueous phases) and enables reversible transfer of the electrolyte in and out of the membrane to be measured via UV-vis spectrophotometry simultaneously with the electrochemical measurement. only slightly more lipophilic than TPB − it is reasonable to expect that TPBCl 4 − transfers at lower overpotentials than Na + . Same reasoning can be applied to the case of into-membrane direction of tetraalkylammonium transport (Fig.S-4b). The standard Gibbs energy of tetradodecylammonium cation transfer across NPOE/water interface, which has not been reported in literature, was estimated to be about −110 kJ mol −1 based on extrapolation of standard the Gibbs energies of tetra(-methyl,-ethyl,propyl, -butyl and pentyl)ammonium cations. [5][6] Therefore, the analyte transport into the membrane is coupled to chloride anion co-transfer (standard Gibbs energy of chloride transfer is 43.1 kJ mol −1 5 ).

S-5 Effects of partial ionisation of rhodamine B on membrane transfer
The case of fully ionized species B + is considered: as Fig. S-5 demonstrates, there is only one ionized tranport 'channel' for B + to transfer between phases A, M and B. Therefore, changes in concentration on aqueous and membrane side of each interface are only affected by ionic flux induced by voltammetry (assuming steady-state permeation or concentration equilibrium). If this were the case for RB, one would expect the reverse peak current (aqueous-to-membrane) to be higher because the concentration change in each phase is of opposite sign and equal absolute value, whereas the diffusion in the aqueous phase is much faster thus resulting in larger peak current according to Eq. (4) (main text).
However, the observed reverse current, corresponding to transfer from the aqueous phases back to membrane (peak 2 and 4 in Fig.4, main text) has a peak value 2 -4 times smaller, depending on the solvent. This is consistent with the complex distribution/dissociation equilibrium occurring at liquidliquid interface for partially ionized molecules, 7 such as rhodamine B. As shown in Fig  The coloured arrows represent the transfer routes of RB + cation during voltammetry.

S-6 Double-transfer voltammogram for a series of cations and anions
Fig. S-6 shows the cyclic voltammetric transfer data obtained for series of cations (S-6a) and anions (S-6b) to complement the data already presented in the main text. Table S-6 lists the half-wave potentials and peak separarations for all studied cations and anions. curve is the analyte-free 'blank' voltammogram. Scan rate was 40 mV s −1 and the system was not stirred.