Heat Capacity Changes Accompanying Micelle Formation upon Burial of Hydrophobic Tail of Nonionic Surfactants

The heat of micellization Δ H m for five nonionic surfactants (C 10 E 5, C 10 E 6, C 10 E 7, C 10 E 8, C 8 E 5 ) was measured as a function of temperature by using an isothermal titration calorimeter. The abbreviation C i E j stands for the nonionic surfactant CH 3 (CH 2 ) i - 1 (OCH 2 CH 2 ) j OH. The heat capacity change upon micellization Δ C p was determined from Δ H m 's. It is found that Δ C p is constant for four C 10 E j 's. That is Δ C p is independent of the chain length of hydrophilic headgroups. Furthermore, Δ C p is directly proportional to the removal of water accessible nonpolar surface area Δ A np . The ratio Δ C p /Δ A np is equal to −1.28 ± 0.06 J mol -1 K -1 Å -2 for C 10 E j 's and −1.34 ± 0.06 J mol -1 K -1 Å -2 for C 8 E 5, consistent with that observed upon protein folding (−1.34 ± 0.33 J mol -1 K -1 Å -2 ) of Spolar et al. ( Biochemistry 1992, 31, 3947.).