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

H-1 NMR spin-lattice and spin-spin relaxation of different types (cationic cetyltrimethyl ammonium bromide, anionic sodium dodecyl sulfonate and nonio nic Triton X-100) of surfactants in water solution were studied. Simulation of the decay curves of proton relaxation shows that the spin lattice relax ation of all the samples exhibits exponentially, while the spin-spin relaxa tion for several protons on the hydrophobic chains forming the micellar cor e 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 o f the micellar core, while the slower relaxing component is attributed to t he rest part staying in the interior. The latter exchanges with the former in equilibrium. Thus, a part of each certain segment of the hydrophobic cha in has an opportunity to stay in the surface layer of the micellar core and spend some time on the interface experiencing hydrophilic environment. Gen erally, the protons on the methylene carbon of the hydrophobic chain neares t to the polar head have more chance to spend time in the hydrophilic envir onment. However, it seems to be dependent on the chemical structure of the surfactant molecule. Large size of the polar group of CTAB shows steric hin drance on the packing of the hydrophobic chain. Quantitative results are gi ven. The fact, that the fraction of slow relaxing protons on the hydrophili c ethylene oxide long chain of Triton X-100 dominates over that of fast rel axing protons, and that their T-2 values are larger than those of the proto ns 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.