ABOUT THE DETERMINATION OF THE CRITICAL M ICELLIZATION CONCENTRATION

The influence of small amounts of highly surface active impurities (do decanol, ethoxylated dodecanol) on the c.m.c. of sodium dodecyl sulfat e was investigated using a surfactant-sensitive electrode. The results were compared with corresponding determinations of the surface tensio n. The c.m.c. values determined after the addition of small amounts (< 0.5 mol %) did not show measurable changes within the investigated co ncentration range. The corresponding sigma-log c-isotherms show a mini mum in the c.m.c. region down to 11.6 mN/m. For this reason the determ ination of the c.m.c. by means of surface tension measurements is impo ssible. Comparative determinations of the c.m.c. values of other syste ms gave identical findings. In solutions of sodium dodecyl sulfate the electrode shows near Nernstian response with a sharp beak at the c.m. c., suggesting that no dimers or trimers are formed below the c.m.c. C orresponding results obtained with cetyl trimethyl ammonium bromide an d tetradecyl pyridinium bromide having a larger hydrophobic molecule s egment deviate already far below the c.m.c. from the linear Nerstian s lope. This deviation from the Nernstian behavior could be an indicatio n of premicellar aggregations.