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 d ifferent ammonium salts, NH4X (X = Cl, Br, I, NO3, Or SCN) at 30 degre es C. The data indicate that an increase in micellar size takes place at all salt concentrations. At 0.1 M NH4X the aggregation number ((n) over tilde) of the micelle depends upon the nature of coion X, the ord er being NO3- similar or equal to SCN- < I- similar or equal to Cl- si milar or equal to Br-, a trend almost opposite to what others have obs erved for spherical micelles. This observation provides information th at the order of preference of added ion is dependent on the size and s hape of the aggregate. It was concluded that the micellar aggregation number (and size) is governed by factors such as intermicellar interac tions which vary at lower and higher salt concentrations and with the micelle/aggregate shapes.