Synthesis and Characterization of a Novel, Colored Lipophilic Additive for Spectral Imaging the Transport in Ionophore Based Ion‐Selective Membranes

Abstract

AbstractThe synthesis and application of a novel colored lipophilic additive (2,4,6‐trinitro‐3‐pentadecylphenol (TNPDP)) as optical tracer for imaging membrane transport is presented. It was found that TNPDP (estimated pKa=5.53) does not dissociate completely in the studied membranes, even in the presence of the highly basic ETH 5294 chromoionophore (pKa=12.0). In voltage polarization experiments the TNPDP compounded, ETH 5294 based membranes behaved similarly to fixed site membranes, i.e., the external field did not induce concentration polarization in TNPDP and the protonated chromoionophore. To interpret this peculiar behavior, original methods have been implemented to i) compare the diffusion properties and determine the diffusion coefficients of multiple species in ion‐selective membranes by hyperspectral imaging, and ii) determine the ion pair formation constant of the ETH 5294 with the novel colored lipophilic anion and a commonly used tetraphenylborate derivative by impedance spectroscopy. Using the “merged membrane” method individual diffusion coefficients of (2.39±0.07)×10−8 cm2/s (N=5), and (2.34±0.02)×10−8 cm2/s (N=2) were determined for the unprotonated form of ETH 5294 and for the undissociated form of TNPDP, respectively. Once in combination with a counter anion, the diffusion coefficient of the protonated ETH 5294 depended on the properties of the lipophilic anion in the membrane. The apparent diffusion coefficient of the protonated ETH 5294 were much smaller when paired with TNPDP ((1.8±0.8)×10−9 cm2/s, N=9) than with tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate (TFPB) ((1.19±0.42)×10−8 cm2/s, N=5). In agreement, the ion pair formation constants of the ETH 5294 with TNPDP (3.14×104 M−1) were almost three orders of magnitude higher than with TFPB (83 M−1).