Tuning the position of head groups by surfactant design in mixed micelles of cationic and carbohydrate surfactants.

HYPOTHESIS

Emerging applications of carbohydrate/cationic surfactant mixtures require not only synergistic mixing, but also accessible sugar headgroups at the exterior of micelles. A previous study showed that the glucoside headgroups of octyl-β-d-glucopyranoside aggregate at the interior of mixed micelles with equimolar cetyltrimethylammonium bromide rather than mixing with trimethylammonium groups at the corona. The current study tests the hypothesis that structural characteristics of the surfactants (the relative lengths of the alkyl tails and the type of linker) can be tuned to shift the carbohydrate groups to micelle surfaces.

EXPERIMENTS

The structural arrangement of 30 mM equimolar mixed micelle solutions in DO is investigated using NMR. The dynamics in different regions are probed using H spin-lattice (T) and spin-spin (T) relaxation measurements, and relative positioning by nuclear Overhauser effect spectroscopy (NOESY). Additional micellar properties are determined using solvatochromic fluorescent probes.

FINDINGS

Matching surfactant alkyl tail lengths is found ineffective at "pushing out" the carbohydrate headgroups due to a large mismatch in interactions between the headgroups and DO. However, inserting a novel polar triazole group between the carbohydrate head group and the hydrophobic tail (e.g. in n-octyl-β-d-xylopyranoside) using click chemistry is able to "pull out" the carbohydrate, thus giving accessible sugar moieties at the surface of mixed micelles.