Molecular Association of Water-Soluble Calixarenes with Several Stilbene Dyes and Its Application to the Facile Determination of Cationic Surfactant Concentrations

The molecular association of water-soluble p-sulfonatocalix[n]arenes (1_n; n = 4, 6, and 8) with several stilbene dyes (D) has been studied by spectrophotometric and ¹HNMR methods. It was found that 1_n reacts in 1: 1 stoichiometry with, D such as 4-[(4-dimethylamino)styryl]-1-methylpyridinium (St-4Me), 2-[(4-dimethylamino)styryl]-1-methylpyridinium (St-2Me) and 2-[(4-dimethylamino)styryl]-1-ethylpyridinium (St-2Et) iodides to form their complexes (1_nD). The apparent association constants for 1₈-complexes are larger by about ten times than those of 1₄^- and 1₆-complexes. Examinations of the CPK molecular models and the spectrophotometric studies suggested that 1₄ complexes D mainly with its 1-methyl- or 1-ethylpyridinium group, whereas 1₆ complexes D either with its pyridinium ring or with its protonated dimethylamino group in a similar probability, and in 1₈-complexes the molecule is wholly incorporated into the cavity of 1₈. The absorbance of D, which was decreased by association with 1_n, was regenerated by the addition of other colorless cationic surfactants (A) such as cetylethyldimethylammonium ion. Studies on the substitution reaction of 1_nD with A indicated that a 1 : 1 complex (1_nA) is formed by releasing a free dye molecule (D). This was applied to a convenient determination of the cationic surfactant concentration in water.