Salt Effect on Critical Micelle Concentrations of Nonionic Surfactants, N-Acyl-N-methylglucamides (MEGA-n)

Salting-out effects on critical micelle concentrations (cmc's) of N-acyl-N-methylglucamides (MEGA-8, -9, and -10) were examined for 16 kinds of salts. The cmc's were determined by using a surface tension or a fluorescent probe method. Salt constants (k(s)'s) were estimated from logarithmic plots of the cmc against the salt concentration and the magnitude of k(s) varied with the different types of salt as follows: for cation, Ca>Na>K>Cs>Li, and for anion, SO(4)>CO(3)>SO(3)>HPO(4)>F>Cl>Br>NO(3)>I>SCN. These orders were similar to the Hofmeister lyotropic series. The salting effect was divided into two components, the contributions from the hydrocarbon tail of a surfactant and its hydrophilic head group. The order of the magnitude of k(s)'s depended mainly on the former of the two components, although the latter contribution was comparable in magnitude to the former. The former contributed to a decreasing cmc (salting out), while the latter contributed to an increasing cmc (salting in). The hydrophilic group of MEGA-n was similar to a hexaoxyethlene chain of POE nonionic surfactants in the magnitude of the contribution to k(s). Copyright 2001 Academic Press.