Hydrocarbon chain packing in the micellar core of surfactants studied by ¹H NMR relaxation

Abstract

 ¹H NMR spin–lattice and spin–spin relaxation of different types (cationic cetyltrimethyl ammonium bromide, anionic sodium dodecyl sulfonate and nonionic Triton X-100) of surfactants in water solution were studied. Simulation of the decay curves of proton relaxation shows that the spin lattice relaxation of all the samples exhibits exponentially, while the spin–spin relaxation for several protons on the hydrophobic chains forming the micellar core is bi-exponential. The fast relaxing component is attributed to the part of the segments of the hydrophobic chain, situated near or on the surface of the micellar core, while the slower relaxing component is attributed to the rest part staying in the interior. The latter exchanges with the former in equilibrium. Thus, a part of each certain segment of the hydrophobic chain has an opportunity to stay in the surface layer of the micellar core and spend some time on the interface experiencing hydrophilic environment. Generally, the protons on the methylene carbon of the hydrophobic chain nearest to the polar head have more chance to spend time in the hydrophilic environment. However, it seems to be dependent on the chemical structure of the surfactant molecule. Large size of the polar group of CTAB shows steric hindrance on the packing of the hydrophobic chain. Quantitative results are given. The fact, that the fraction of slow relaxing protons on the hydrophilic ethylene oxide long chain of Triton X-100 dominates over that of fast relaxing protons, and that their T ₂ values are larger than those of the protons on the hydrocarbon chain in the interior of the micellar core, suggests that the ethylene oxide chain does not participate in the formation of the micellar core.