The influence of n-hexanol on the morphology and composition of CTAB micelles

The effect of the addition of n-hexanol as co-surfactant on the structure of cetyltrimethylammonium bromide (CTAB) micelles has been studied using small-angle X-ray and neutron scattering (SAXS, SANS). Contrast variation neutron scattering experiments were performed to determine the structure of both pure CTAB and n-hexanol modified CTAB micelles. The incorporation of n-hexanol leads to an elongation of the ellipsoidal CTAB micelles. The scattering length density of the micellar shell linearly depends on the degree of deuteration of the dispersion medium water and revealed the existence of substantial amounts of water in the micellar shell. The water content in the shell increased from 20 vol-% observed for pure CTAB micelles to 44 vol-% found for n-hexanol modified CTAB micelles. The amount of n-hexanol in the micellar shell was determined by varying the amount of fully deuterated and protonated n-hexanol. These experiments revealed a volume fraction of 26 vol-% of n-hexanol molecules in the micellar core which equals a molar fraction of 50% n-hexanol within the CTAB micelles. The total composition of micellar core and shell was estimated. The packing density of headgroups, water molecules and bromide ions turned out to drastically increase in n-hexanol modified CTAB micelles. These findings contribute to a fundamental understanding of the stabilization mechanism of micelles by alcoholic co-surfactants and the resulting alteration of the morphology and interface composition. These results will facilitate the optimization of processes where CTAB and other comparable surfactants are used as phase transfer catalysts, structure directing agents or stabilizers in colloidal dispersions or emulsions.