Investigations into the phase behavior of nonionic ethoxylated surfactants using H-2 NMR spectroscopy

Deuterium NMR spectroscopy was applied to study the phase behavior of three nonionic surfactants containing the same alkyl chain length (C-13/C-15) and increasing numbers of ethoxylate (EO) units (7, 11, and 20). These surfactants were dissolved in D2O, and the quadrupole interaction of the spin of the deuterium nucleus with the protonated surfactant structural units was investigated. The results showed singlets for the liquid normal micelles, for cubic and inverse micellar phases. In contrast, liquid crystalline structures of the hexagonal and lamellar phases showed a powder pattern with a quadrupole splitting. The hexagonal phase showed a smaller splitting than the lamellar phase. By following the quadrupole splittings as a function of surfactant concentration at constant temperature, the phases produced could be clearly identified. In addition, the results showed a gradual transition from hexagonal to lamellar phases, and in some concentration regions, a mixture of the two phases could be inferred from the results. The influence of temperature (at constant surfactant concentration) was also investigated for some surfactant solutions. The results showed a gradual decrease followed by a sharp reduction in the quadrupole splittings (reaching zero) at the temperature at which melting of the liquid crystalline structure occurred.