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)

Abstract

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 ¹H-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 the ¹H-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 the complex depending on the polymer concentration in solution.