Effect of Na+ on solvent extraction of alkaline earth metal cations by proton-ionizable calix[4]arenes

Metal ion recognition by proton-ionizable calixarenes is often affected by interference from the Na+ present in many aqueous solutions. To assess the Na+ -effect on calixarene selectivity towards alkaline earth metal cations (AEMC), competitive solvent extraction of Mg2+, Ca2+, Sr2+ , and Ba2+ from aqueous solutions into chloroform by di-ionizable calix[4]arene N- (trifluoromethylsulfonyl)carboxamides and corresponding carboxylic acids, conformationally mobile and restricted to cone, partial cone and 1,3-alternate conformations, was investigated in the absence and in the presence of aqueous-phase Na+ . The extraction selectivities were found to be altered due to contrasting Na+ -induced changes in the AEMC uptake. Observed increases in loadings of smaller AEMC are rationalized by their co-extraction with Na+.