MICELLAR PROPERTIES OF A NEW SERIES OF STEREOCHEMICAL SODIUM CARBOXYLATES BEARING DOUBLE-BONDS NEAR THEIR IONIC HEADS IN AQUEOUS-MEDIA

The critical micelle concentrations (CMCs) of sodium carboxylates bear ing one or two double bonds near the polar head are determined at vari ous temperatures using cryoscopy, conductimetry, and vapor pressure os mometry techniques. CMC increases with temperature in the range 25 to 40 degrees C. The enthalpy for micellization of monoenic and dienic co mpounds is determined above 25 degrees C and found to be negative. The exothermic micellization process is attributed to the rigidity of the polar head group which inhibits molecular interactions in the micella r state. CMC increases with increasing number of double bonds. The fre e energy of micellization per unsaturated CH group (Delta G(m)(CH)) is then determined. The values obtained, which are -1.83, -1.56, -1.27, and -1.18 kJ.mol(-1) at 0, 25, 33, and 40 degrees C, respectively, are higher than those of CH2 group, i.e., -2.53 +/- 0.15 kJ.mol(-1) in th e same temperature range. The unsaturated CH group has thus a less hyd rophobic character. Moreover the Delta G(m)(CH) value of the compounds studied here is found to be lower than the Delta G(m)(CH) value of en d-tail double-bond surfactants. Based on these results, it is conclude d that the closer the double bond to the polar head, the higher the hy drophobic character. (C) 1998 Academic Press.