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

Titration microcalorimetric results are reported for alkylpolyoxyethylene g lycol ethers, C12H25(OCH2CH2)(m)OH where m=3, 4 or 5 at ambient pressure an d 298.2 K. The enthalpograms yield estimates of critical micelle concentrat ions (c.m.c.) which for these surfactants cover the range from 3.25(+/- 0.2 2)x10(-5) to 5.26(+/- 0.49)x10(-5) mol dm(-3) with increasing integer m. Th e enthalpograms are complicated. Although the recorded enthalpies of inject ion are exothermic, the dominant process accompanying injection of an aliqu ot into the sample cell is not simple micelle deaggregation. Rather these s urfactant solutions above the c.m.c. contain both micelles and higher aggre gates. 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 (C TAB), pointing to the dramatic impact of an additional methylene group in t he alkyl chain of the cationic surfactant on the aggregation of this surfac tant in aqueous solution.