Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors

The leaching process of lipophilic anionic additives (e.g., tetraphenylborates) from neutral carrier based membranes into an aqueous sample is quantified. While the necessary lipophilicity (i.e., equilibrium partition coefficient) of the additive for a certain lifetime of the sensor is identical to the one of neutral compounds described earlier, its quantification needs to consider the co-extraction equilibria with the counterions between both phases. It is shown theoretically and with liquid-liquid extraction experiments that the lifetime of the sensor depends for a given tetraphenylborate both on the nature of the counterion and its concentration in the aqueous and organic phase. Moreover, the complexation of the cationic counterion by the ionophore will shift the partition equilibrium of the anionic site in direction of the membrane phase, thus rendering the additive more lipophilic. By using previously measured ion-exchange constants determined on thin membrane phases (optical sensors) together with the extraction data presented here, quantitative lipophilicity data is indirectly accessible. It is shown that ionophores forming highly stable complexes with the sample cation (e.g., valinomycin for K+ or ETH 1001 for Ca2+) induce a sufficiently high lipophilicity of the tetraphenylborate derivative according to the criteria presented here. However, only the most lipophilic tetraphenylborates (i.e., tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) show acceptably slow leaching behavior in membranes containing no ionophore.