The molecular association of water-soluble p-sulfonatocalix[n]arenes (1n; n = 4, 6, and 8) with several stilbene dyes (D) has been studied by spectrophotometric and 1HNMR methods. It was found that 1n 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 (1nD). 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 1n, was regenerated by the addition of other colorless cationic surfactants (A) such as cetylethyldimethylammonium ion. Studies on the substitution reaction of 1nD with A indicated that a 1 : 1 complex (1nA) is formed by releasing a free dye molecule (D). This was applied to a convenient determination of the cationic surfactant concentration in water.
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