(592c) Effects of Surfactant Structural Parameters on the Relative Position of Surfactant Head Groups in Mixed Cationic-Carbohydrate Surfactants Micelles

Mixtures of cationic and non-ionic surfactants are of sustained interest for industrial applications and ongoing research due to the synergistic effects that arise due to favorable interactions among headgroups in cationic-non-ionic mixed micelles.  This synergy has been studied primarily through its effects on micellization thermodynamics, where favorable effects are observed that are not present in mixed micelles composed of ionic surfactants with the same charge.¹ Although several investigations have reported global thermodynamics of mixed surfactants with the help of a regular solution theory-based interaction parameter (β) this parameter is only an indicator of net enthalpy changes associated with mixing in micelles.² The relative positions of the surfactants in the mixed micelle and the interaction sites responsible for the observed enthalpy changes in cationic-non-ionic surfactants are not as thoroughly understood, but are critical in applications where one or both surfactants are intended to interact with their environment, such as in sensing and materials synthesis applications. 

Here, we use nuclear magnetic resonance (NMR) and fluorescence spectroscopy to study carbohydrate headgroup positioning and interactions in mixed micelles of alkyltrimethylammonium-based cationic and carbohydrate-based non-ionic surfactants.  These surfactants are of interest to our group for synthesis of mixed oxides for adsorption and catalysis, but they have a wide range of potential applications.  Our main objective is to determine combinations of surfactants that “push” the carbohydrate to the exterior of the micelle so that the carbohydrate is available for interactions during materials synthesis.  Chain lengths of both surfactants and the type of linker in the carbohydrate surfactant are varied.  NOESY spectroscopy, relaxation analysis, and binding of selective fluorescent probes are used to understand the relative positioning of headgroups.  Results indicate that the sugar head group is always located inside the corona region of mixed micelle prepared using carbohydrate surfactants with ether linkages, even when the tail lengths are tuned to favor carbohydrate exposure.  The situation changes when new surfactants are used that contain a polar triazole linker, where clear evidence is found that the carbohydrate headgroup is “pushed out” to the exterior of the micelle.  The use of surfactants with triazole linkers thus represents a potential pathway to developing micelle-based systems with sugar headgroups that are able to interact with their environment for templating, sensing, and selective binding applications.

References

1. Vora et al. Journal of Surfactants and Detergents, 1999, 2, 213-221.
2. Ruiz et al. Surfactant Science Series, CRC Press, 2009,143, 413.