Titration microcalorimetry of micelle formation in aqueous solutions containing alkylpolyoxyethylene glycol ethers, C12H25(OCH2CH2)mOH where m=3, 4 and 5, and octadecyltrimethylammonium bromide

Abstract

Titration microcalorimetric results are reported for alkylpolyoxyethylene glycol ethers, C₁₂H₂₅(OCH₂CH₂)_mOH where m=3, 4 or 5 at ambient pressure and 298.2 K. The enthalpograms yield estimates of critical micelle concentrations (c.m.c.) which for these surfactants cover the range from 3.25(±0.22)×10⁻⁵ to 5.26(±0.49)×10⁻⁵ mol dm⁻³ with increasing integer m. The enthalpograms are complicated. Although the recorded enthalpies of injection are exothermic, the dominant process accompanying injection of an aliquot into the sample cell is not simple micelle deaggregation. Rather these surfactant solutions above the c.m.c. contain both micelles and higher aggregates. In a similar fashion, enthalpograms for aqueous solutions containing octadecyltrimethylammonium bromide, C₁₈H₃₇N⁺ Me₃Br⁻ (OCTAB) do not follow the previously reported pattern for n-hexadecyltrimethylammonium bromide (CTAB), pointing to the dramatic impact of an additional methylene group in the alkyl chain of the cationic surfactant on the aggregation of this surfactant in aqueous solution.