Electrostatic binding among equilibrating 2-D and 3-D self-assemblies.

Six organic additives, each bearing a different number of anionic charges, were added to a large excess of cationic surfactant (dodecyltrimethylammonium bromide, DTAB). The surface-tension vs log [DTAB] plot for solutions containing DTAB/trianion = 15:1 showed an abrupt break (routinely taken as the critical micelle concentration, CMC) at 2.9 mM. This constitutes a 5-fold decrease compared with a CMC of 15 mM for pure aqueous DTAB. There is a 10-fold decrease in the break-point concentration caused by a mere 3 mol % of hexanion. Corresponding CMC values from DTAB/trianion mixtures, measured by both conductivity and diffusion NMR, gave normal values of 14 mM. The unusual discrepancy between the CMC based on surface tension and on the two "bulk" methods was attributed to saturation of the air/water interface by a DTAB/trianion complex far below the concentration at which the micelles form. Thus, the sharp break seen in surface-tension "CMC plots" need not in fact attest to actual micelle formation as is almost universally assumed in colloid chemistry.