ORIENTATIONAL BINDING OF SUBSTITUTED NAPHTHOATE COUNTERIONS TO THE TETRADECYLTRIMETHYLAMMONIUM BROMIDE MICELLAR INTERFACE

The orientational binding of four naphthoate anions to the tetradecyltrimethylammonium bromide (TTAB) micellar interface has been studied by recording surface tension and H-1 NMR data. The critical micelle concentration (cmc) values for 1:1 stoichiometric mixtures of TTAB and sodium 1-naphthoate and sodium 2-naphthoate are 0.62 and 0.33 mM, respectively. The fluorescence measurements for TTAB/1-naphthoate 1:1 mixtures also gave a cmc value of 0.65 mM. The TTAB/1-OH-2-naphthoate and TTAB/3-OH-naphthoate mixtures have cmc values of 0.10 and 0.17 mM for a 2:1 stoichiometry. The organic counterions all stabilize the formation of the micellar aggregates. The orientational assignments of all the aromatic anions were determined from NMR chemical shift data. The changes in chemical shift with increasing mole fraction of anion indicate a change in micelle structure from the globular to prolate or rodlike, which is also clearly observable for samples of 1:1 stoichiometry since the exhibit viscoelasticity at dilute concentrations. The aromatic induced chemical shifts of the alkyl chain methylene protons demonstrate the deep penetration into the palisade layer by these anions. The nuclear Overhauser spectroscopic data (NOESY) provide further support for the conclusion of deeper penetration since an intermolecular NOE is observed between the counterion and the alkyl chain. The critical assignment of the anions' orientation with respect to the micellar interface elucidates the subtle control of the aggregate packing constraints to define the aggregate shape.