Shear-DSC study on the crystal-isotropic phase transition of semidilute aqueous cetyltrimethylammonium bromide (CTAB) solutions

Abstract

A differential scanning calorimeter equipped with a steady shear flow system (Shear-DSC) was developed, and the effects of shearing on the crystal-isotropic phase transition were investigated for aqueous cetyltrimethylammonium bromide (CTAB) solutions at 4, 18, and 26 wt%. At 18 wt%, where rod-like micelles are formed in the isotropic phase, the transition temperature Ttrs changes with the shear rate γ˙. At low shear rate (γ˙<1s−1), Ttrs decreases as γ˙ increases due to an increase in the surface energy of the crystal grains. At high shear rate (γ˙>30s−1), Ttrs increases as γ˙ increases because of the orientational ordering of rod-like micelles induced by shearing in the isotropic phase, leading to the formation of a nematic structure. The latter change is suppressed for the solution at 26 wt% because the nematic phase is formed without shearing, whereas it disappears at 4 wt% indicating that the spherical micelles are not deformed by shearing.