Critical composition in organized surfactant solutions

Abstract

Micellar organized solutions are systems in which the size of the solvent molecule and the organized aggregate are widely different. The solubility of the small water molecules in the surfactant phase is large while the solubility of the large surfactant micellar aggregates in water is very small. There is in aqueous non-ionic surfactant systems a good correlation between the experimental critical compositions and the values estimated from the aggregation number of the micelles close to the respective critical temperatures. The critical compositions of some non-ionic surfactants in the aqueous solutions in weight fractions are: R 6(OCH 2CH 2) 2OH, 0.117; R 8(OCH 2CH 2) 4OH, 0.071; R 12(OCH 2CH 2) 5OH, 0.012; R 12(OCH 2CH 2) 6OH, 0.020; R 14(OCH 2CH 2) 6OH, 0.0087; R 16(OCH 2CH 2) 6OH, 0.0049. Thus, the surfactant phase has the capability to incorporate large amounts of water. Furthermore, there is a linear decrease in the plot of the logarithm of the critical compositions vs the hydrocarbon chain length of the surfactants. This characteristic property of an organized solution, i.e. a marked asymmetry in the critical composition, is explained by the large aggregation number for the micelles close to the lower critical consolute temperature, and decreases geometrically with the increase of hydrocarbon chain length of the surfactants. At the lower critical consolute temperature the aggregation number of the micelles increases geometrically with the hydrocarbon chain length. The upper critical solution temperature in R 10(CH 2CH 2) 4OHH 2O is 300°C, and the critical composition is about 0.37 wt%. The system is now no longer a typical organized solution and the aggregation number is about 3.