H. Fruhner et G. Czichocki, ABOUT THE DETERMINATION OF THE CRITICAL M ICELLIZATION CONCENTRATION, Tenside, surfactants, detergents, 33(4), 1996, pp. 310-314
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.