Electrical properties of soap + water + amphiphile systems. Electrical conductance and sodium ion activity of aqueous 2% sodium dodecyl sulphate with added homologous n-aliphatic C2–7alcohols

Abstract

The electrical conductance of aqueous 2 % sodium dodecyl sulphate at 25°C has been studied as a function of increasing concentration of n-aliphatic homologous alcohols with a chain length of 2-7 carbon atoms. Observed increases in conductance have been directly correlated with increases in counter-ion activity of the detergent which were observed using a sodium-responsive glass electrode. An attempt has been made to estimate the distribution of the added alcohol between the micellar phase and aqueous phase (containing free soap ions). As the chain length of the alcohol increases the greater becomes the tendency to penetrate into the micelle. This causes separation of the charged soap ions, repulsion decreases and counter-ions are released. The contribution of the released sodium ions and the dodecyl sulphate ions to the total conductance has been estimated. Whereas the contribution due to sodium ions increases for each alcohol the contribution due to dodecyl sulphate ions decreases; this decrease, however, becomes progressively less marked as the alcohol series is ascended. For hexanol, the rise in sodium ion activity can entirely account for the rise in electrical conductance. The physical significance and limitations of the theoretical approach used in the paper are discussed and a brief review of previous work is presented.