POTENTIOMETRIC DISCRIMINATION OF ORGANIC AMINES BY A LIQUID MEMBRANE-ELECTRODE BASED ON A LIPOPHILIC HEXAESTER OF CALIX[6]ARENE

A novel type of poly(vinyl chloride) matrix liquid membrane electrode that is capable of discriminating the steric shapes of nonpolar moieties of organic amines at neutral pH has been developed. A new lipophilic hexaester of calix[6]arene having a well-defined inclusion cavity for organic guests [37,38,39,40,41,42-hexakis(decyloxycarbonylmethyloxy)calix[6]arene] was synthesized and exploited as a sensory element. This electrode displayed the strongest potentiometric responses to the primary amines having no substituent adjacent to the amino group such as 1-octylamine, 2-phenylethylamine, and dopamine. An examination of potentiometric selectivities for different series of organic amines, as well as a H-1-NMR study on the geometry of the host-guest complexes, strongly supported a mode of potentiometric discrimination that depends on the availability of a stable inclusion complex involving the formation of tripodal hydrogen bonds between the NH3+ group of the guest and the C=O groups of the host. Such a mode of potentiometric discrimination based on the recognition of the steric shapes of nonpolar moieties was not attained by the electrodes based on dibenzo-18-crown-6 or potassium tetrakis-(p-chlorophenyl)borate having no inclusion cavity for organic guests; both of these electrodes showed a selectivity that reflects simple lipophilicity of the guests.