1H and 2D-DOSY study of the micellar properties of a viscoelastic surfactant (cetyltrimethylammonium p-toluenesulfonate) and its physicochemical interactions with a carbohydrate based polymer (sodium alginate)

Labelled 2D-DOSY Spectrum for a 2.0 mM CTAT solution in D 2 O • The self-assembly and morphology of a long chain cationic surfactant is affected when an aromatic counterion is present. • The chemical shifts and the diffusion coefficients of both the amphiphile ion and the counterion were consistent with two self-assembly processes occurring in solution. • Counterion binding is enhanced when the transition to rod-like aggregates occurs. • The interactions of the surfactant with the anionic carbohydrate polymer are complex processes involving multiple equilibria. The self-assembly of a long chain cationic surfactant (cetyltrimethylammonium p-toluene sulfonate or CTAT) and its interaction with a carbohydrate-based polymer (sodium alginate) was investigated by measuring the 1 H-chemical shifts and the diffusion coefficients of the surfactant by itself and mixed with the polymer in solution at 298.2 K. Firstly, the self-assembly of the surfactant was investigated by measuring both 1D 1 H-chemical shifts and diffusion coefficients from 2D-DOSY experiments as a function of the concentration of the amphiphile in deuterium oxide. An analysis of the chemical shifts and the diffusion coefficients of both the amphiphile ion and the counterion were consistent with two self-assembly processes occurring in solution: an initial self-assembly of the surfactant into spherical micelles with a transition to rodlike micelles occurring at a higher amphiphile concentration. The counterion diffusion coefficients from the 2D-DOSY experiments were used to calculate the fraction of counterions associated with the aggregates as the self-assembly evolved between the two types of aggregates. In the case of the polymer/surfactant system, the condensation of the surfactants onto the anionic carbohydrate polymer is a complex process involving a strong-cooperative binding of the amphiphile ions to the polymer and an equilibrium between spherical micelles, rod-like micelles, free surfactant, and complex depending on the polymer concentration in solution.