Surface Properties and Micelle Formation of a Series of Alkyl Octakis (oxyethylene) Ethers in the Aqueous Solutions and Their Temperature Dependences

Abstract

The values of cmc, area per molecule and equilibrium surface tension at cmc of a homologous series of alkyl octakis (oxyethylene) ethers (CnE8 : n=10 to 15) were determined from the concentration dependence of surface tension in the temperature range 15.0°C to 40.0 °C. The areas per molecule and equilibrium surface tension values at the cmc decreased with an increasing carbon number and they showed zigzag curves by the difference in even and odd carbon numbers. These findings may be attributed to the difference in the molecular orientation between the molecules with even carbon number and ones with odd carbon number on the air-water interface. On the other hand, the plots of log cmc vs. the number of carbon atoms (N) in the alkyl chain exhibited a linear relationship, and the difference between even and odd carbon numbers was not observed on the bulk properties. The free energy change relating to micelle formation, ΔGm, was calculated from the temperature dependence of the cmc. ΔHm and ΔSm values were estimated from the slopes and the intercepts of the log cmc versus the reciprocal of temperature plots, respectively. This result indicates that ΔSm dominates over the micellization. Furthermore, the values of ΔGm (-CH2-), ΔHm (-CH2-), ΔSm (-CH2-) and TΔSm (-CH2-) per methylene group calculated from the slope of each thermodynamic parameter against the carbon number were -0.68 kcal/mol, -0.33 kcal/mol, 1.16 cal/ (mol·deg), and 0.35 kcal/mol, respectively. It, was concluded that the summation of ΔHm (-CH2-) and TΔSm (-CH2-) contributed to the micellization.