Dynamics and local environment of an aromatic counterion bound to di-chain cationic surfactant bilayers studied by avoided level crossing muon spin resonance: evidence for counterion condensation

Abstract

Avoided level crossing muon spin resonance (ALC-μSR) has been used to study the reorientational dynamics of muon-spin-labelled 2,4,6-trimethylbenzoate (246TMB⁻) counterions and their interaction with DODMAC (dioctadecyldimethylammonium chloride) bilayers in the L_α and L_β liquid crystalline states. The muoniated radical anion formed by the addition of muonium to the secondary carbons of the aromatic ring of 246TMB⁻ is used as a local spin probe. The muon and methylene proton hyperfine parameters and the electron spin relaxation rate (λ_e) of the muoniated spin probe were determined as a function of temperature by modelling the ALC-μSR spectra with Monte Carlo numerical simulations. The observation of a Δ₁ resonance indicates that 246TMB⁻ is undergoing anisotropic motion and doesn't reside in the aqueous layer in either the L_α and L_β phases. The lack of an abrupt change in the hyperfine parameters or λ_e when the system goes from the L_β to the L_α lamellar liquid crystalline phases suggests that 246TMB⁻ is located at the oil–water interface rather than within the bilayer. The hyperfine parameters indicate that 246TMB⁻ is undergoing large amplitude reorientational motion about a preferred orientation resulting from the bilayer's electric field. The interaction between 246TMB⁻ and the bilayer decreases and the amplitude of the wobbling-in-a-cone motion increases with increasing temperature. The temperature dependence of the electron spin relaxation rate indicates the barrier to reorientation is 41.7 kJ mol⁻¹.