The effect of cosurfactants on sodium dodecyl sulfate micellar structures at a graphite surface

Sodium dodecyl sulfate (SDS) self assembles at hydrophobic graphite surfaces forming linear parallel hemicylinders with approximately 5 nm spacing between adjacent hemicylinders. In this paper, we report the effect of cosurfactants on the SDS surface micelle structures. It is known from the literature that cosurfactants influence the relaxation times of bulk phase micelles. Specifically, the relaxation times of SDS micelles increase dramatically when cosurfactants, such as non-ionic 1-dodecanol (C12OH) or cationic dodecyl trimethyl ammonium bromide (C12TAB) are added, resulting in the formation of stable micelles. Soft contact AFM imaging was used to determine if these increased relaxation times in bulk solution influence the formation of surface micelle structures at the graphite surface. Surface micelles formed by adsorption from SDS only solutions were significantly different from those surface micelles formed from mixed surfactant solutions. The addition of cosurfactants produced kinks and disrupted the otherwise highly ordered linear hemicylindrical SDS structures observed at graphite surfaces. In one case (SDS/C12OH) swollen surface micelles with a spacing of more than 8 nm were observed. Finally, whereas surface micelles form instantaneously from SDS only solutions, the time taken for the equilibrium surface state to be established in the presence of the C12OH cosurfactant was found to be more than 4 h.