THERMODYNAMICS OF MICELLAR SYSTEMS - VOLUMES, OSMOTIC COEFFICIENTS, AND AGGREGATION NUMBERS OF A HOMOLOGOUS SERIES OF 1,2,3-ALKANE TRIOLS IN WATER

Aqueous solutions of a homologous series of alkanetriols, from 1,2,3-hexanetriol to 1,2,3-nonanetriol, have been examined with different techniques, including density determination, freezing point depression, and vapor pressure osmometry. Their hydrophobic or micellar parameters were determined at different temperatures (0, 20, 30, and 40°C): volumes of monomers or micelles, volume changes during micelle formation, osmotic coefficient, activity of the solute and solvent, nonideal free energy of the solute, and relative chemical potential. 1,2,3-Hexanetriol does not form micelles and tends to form hydrophobic aggregates at 2.09 (20°C) and 1.80 (30°C) mol · dm-3. 1,2,3-Heptanetriol is a key compound since it is difficult to speak about clusters or real micellization at 1.03 (30°C) and 0.98 (40°C) mol · dm-3. 1,2,3-Octanetriol and 1,2,3-nonanetriol self associate in a manner similar to that of conventional surfactants. The CMCs respectively obtained are 0.370 to 0.320 mol · dm-3 (temperature range of 0 to 40°C) and 0.098 mol · dm-3 (40°C). Calculations using the phase separation model led to the following values for the aggregation numbers: 2.3 ± 0.2 for 1,2,3-hexanetriol, 7.6 ± 0.1 for 1,2,3-heptanetriol and 31.9 ± 0.1 for 1,2,3-octanetriol. The values directly obtained by freezing point depression (31.5 ± 1.8) or vapor pressure osmometry measurements (27.1 ± 0.9) at 40°C for 1,2,3-octanetriol show that the phase separation model is thus satisfactorily applied for this compound. The calculated free energy of the micelle formation agrees with published data for similar nonionic surfactants. © 1993 by Academic Press, Inc.