A SANS study for structural transition of micelles of cationic octadecyl trimethyl ammonium chloride and anionic ammonium dodecyl sulfate surfactants in aqueous solutions

The sizes and structures of micelles formed in aqueous solutions of cationic octadecyl trimethyl ammonium chloride (OTAC) and anionic ammonium dodecyl sulfate (ADS) surfactants were investigated using small angle neutron scattering (SANS). For aqueous OTAC solutions, SANS data indicate that the structure of micelles is of sphere with core and hydrated shell up to OTAC 50 mM. As the total OTAC concentration increases, SANS data show that the micelles become larger and the transition in micellar shape from sphere to oblate ellipsoid takes place.For aqueous ADS solutions, SANS data point to the micelle structure of sphere with core and shell but this time up to ADS 100 mM. Above this concentration the spherical micelles changes to oblate ellipsoid with core and hydrated shell.The surfactant concentration at which micelle shape changes is termed as the shape transition concentration which is 50 mM for OTAC and 100 mM for ADS. Such transitions may be explained qualitatively by the distortion of the hydrocarbon chains at high surfactant concentrations. When micelles are large, a number of hydrocarbon chains of the surfactant molecules should be accommodated in the core of the micelle and therefore the chains are likely to be distorted. That is why the micelle shape changes from sphere to ellipsoid and the ellipsoidal axial ratio (Ra/Rb or Ra,core/Rb,core) must increase. This is reflected in the linear relationship between the ellipsoidal axial ratio and the aggregation number.

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► The size and shape of micelles of OTAC and ADS have been determined by the SANS data.
► The micelle shape changes from sphere to ellipsoid.
► 50 mM may be the shape transition concentration for OTAC micelles. For ADS micelles the shape transition concentration is 100 mM.
► The micelle shape is likely to be ellipsoids with large values of the axial ratio.