The dynamics of substituted benzoate anions undergoing counterion condensation in lamellar phase dispersions

We have studied the interaction of three ortho-substituted benzoate anions (2-ethylbenzoate, 2-chlorobenzoate, and 2-hydroxybenzoate) and three para-substituted benzoate anions (4-ethylbenzoate, 4-chlorobenzoate, and 4-hydroxybenzoate) with lamellar phase dispersions of the di-chain cationic surfactants 2,3-diheptadecyl ester ethoxypropyl-1,1,1-trimethyl-ammonium chloride (DHTAC) and dioctadecyl-dimethyl-ammonium chloride (DODMAC) using avoided level crossing muon spin resonance (ALC-μSR) spectroscopy. Highly polarized spin probes were produced in situ by the addition of muonium to the aromatic anions, and the muon and methylene proton hyperfine coupling constants (hfccs) of these radical anions were determined from the Δ1 and Δ0 resonance fields in the ALC-μSR spectra. The motionally averaged dipolar muon hfccs, |Dμ‖|, were determined from the width and amplitude of the Δ1 and Δ0 resonances and used to estimate the extent of reorientational motion. The results are consistent with the counterions being electrostatically trapped near the oil/water interface of the surfactant bilayer and undergoing large amplitude anisotropic motion. This motion is generally more restricted in DODMAC bilayers compared with DHTAC, which is likely related to the relative flexibility of the headgroups to which the counterions are loosely bound. The motion of the ortho isomers is generally more restricted than the para isomers, while there is no obvious trend regarding the ethyl, chloro, and hydroxy substituents.