Salt effect on the hydrophobic binding of p-toluidino naphthalene sulphonate with cyclodextrins

Abstract

Lithium chloride, lithium perchlorate, cesium bromide, tetra-n-butyl ammonium bromide (TBBr) and guanidine hydrochloride (GnCl) affect the fluorescence properties of 2, 6-p-toluidino naphthalene sulphonate (TNS) bound to α- and β-cyclodextrins (CD) in strikingly different ways. This indicates that the salts have considerable influence on the hydrophobic binding of TNS molecules with CDs. The salting-in agents (LiClO₄, CsBr and GnCl) markedly reduce the fluorescence quantum yield (Φ_f) and lifetime (τ_f) of CD-bound TNS anion. It is suggested that they directly interact with the bound TNS ions. The salting-in agent TBBr increases Φ_f and τ_f. This fluorescence enhancement indicates that the local polarity is reduced by wrapping the four n-butyl groups around the CD-bound TNS ions. The salting-out agent LiCl causes a marginal increase (< 15%) and that at a high concentration (7 M). In the absence of CD in aqueous solution on addition of 0.1 M TBBr; the fluorescence yield of TNS increases sixfold. This is attributed to clustering of TB cations around the TNS anion.