Gt. Dimitrova et al., INVESTIGATIONS INTO THE PHASE-BEHAVIOR OF NONIONIC ETHOXYLATED SURFACTANTS USING H-2 NMR-SPECTROSCOPY, Langmuir, 12(2), 1996, pp. 315-318
Deuterium NMR spectroscopy was applied to study the phase behavior of
three nonionic surfactants containing the same alkyl chain length (C-1
3/C-15) and increasing numbers of ethoxylate (EO) units (7, 11, and 20
). These surfactants were dissolved in D2O, and the quadrupole interac
tion of the spin of the deuterium nucleus with the protonated surfacta
nt 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 hexago
nal phase showed a smaller splitting than the lamellar phase. By follo
wing the quadrupole splittings as a function of surfactant concentrati
on at constant temperature, the phases produced could be clearly ident
ified. In addition, the results showed a gradual transition from hexag
onal 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 investiga
ted for some surfactant solutions. The results showed a gradual decrea
se followed by a sharp reduction in the quadrupole splittings (reachin
g zero) at the temperature at which melting of the liquid crystalline
structure occurred.