Rational Design of a Hydrophobic Ion-Pair Sensor for Potentiometric Determination of Cationic Surfactants in Disinfectants: Combined Experimental and DFT Study

Cationic surfactants are widely used in disinfectants, creating a need for rapid and reliable analytical methods for their determination in complex formulations. In this study, a new hydrophobic ion-pair, 1,3-didecyl-2-methylimidazolium tetrakis(perfluorophenyl)borate (DDMIm–TPFPhB), was developed and applied as an ionophore in a potentiometric sensor. The ion-pair was incorporated into a PVC membrane and evaluated by direct potentiometric measurements and titrations. The sensor exhibited near-Nernstian responses toward selected cationic surfactants (56.8–59.1 mV per decade), low detection limits (1.4–2.2 × 10−6 M), and stable signal behavior, along with good selectivity and stability over a pH range of 3–9. Application on commercial disinfectant samples showed good agreement with a commercial ion-selective electrode. According to the charge decomposition analysis performed using density functional theory calculations, the number of electrons donated from perfluorotetraphenyl borate to 1,3-didecyl-2-methylimidazolium is 0.25 e. In contrast, the back-donation from the cation to the anion is only 0.05 e, indicating a relatively substantial overall charge transfer of 0.20 e. This pronounced charge transfer, together with dominant dispersion interactions, contributes to enhanced ion-pair stability within the membrane phase, which is reflected in reduced signal drift and improved analytical performance. These findings establish a direct link between molecular-level interactions and sensor behavior, providing a rational basis for the design of potentiometric sensors for real-sample analysis.