Aggregation behaviour of sodium dodecyl sulphate micelles in aqueous solutions studied by membrane osmometry

Measurements of the micellar molecular weight of sodium dodecyl sulphate (NaDDS) by the membrane osmometry method have been carried out over a wide range of concentration of added electrolyte, NaCl, at 40°C. The micellar molecular weight was found to increase sharply at NaCl concentrations above 0.5 mol dm–3, which agrees with literature data in which the aggregation number was estimated from diffusion constants measured using the light-scattering method under the same experimental conditions. The variation of the second virial coefficient with osmotic pressure has been analysed, in order to estimate the various interactions which give rise to the sign and magnitude of the virial coefficient. The second virial coefficient is related to the change in shape of the NaDDS micelle with NaCl concentration.