Growth of Sodium Dodecyl Sulfate Micelles in Aqueous Ammonium Salts

Small-angle neutron-scattering (SANS) measurements have been performed on 0.3 M sodium dodecyl sulfate (SDS) micellar solutions containing different ammonium salts, NH 4 X (X = Cl, Br, I, NO 3, or SCN) at 30 °C. The data indicate that an increase in micellar size takes place at all salt concentrations. At 0.1 M NH 4 X, the aggregation number ( ñ ) of the micelle depends upon the nature of coion X, the order being NO 3 - ≃ SCN - < I - ≃ Cl - ≃ Br -, a trend almost opposite to what others have observed for spherical micelles. This observation provides information that the order of preference of added ion is dependent on the size and shape of the aggregate. It was concluded that the micellar aggregation number (and size) is governed by factors such as intermicellar interactions which vary at lower and higher salt concentrations and with the micelle/aggregate shapes.