Spectroscopic study of the melting and reconstruction of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles from their frozen states

Abstract

The confinement of water in organic self-assemblies with nanometer-sized pores is ubiquitous in nature. Water pools in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles have been intensively studied as a representative model of such confined water. However, the freezing and melting behavior of such water pools is poorly understood owing to their poor structural stability under phase transition. In the present work, the melting of iced water pools accompanied by a reconstruction of AOT reverse micelles was studied with infrared spectroscopy. For all AOT reverse micelles tested (Rw: 1.2–4.4nm), a characteristic ice–water coexistence phase was observed during melting. The results provide experimental evidence of the previously proposed core–shell structure of the water pool. For larger frozen reverse micelles (Rw>2.3nm), shifts of the υ(OH) bands were observed. The spectra of the shifted υ(OH) bands were similar to those for the melted water pools in the smaller micelles. The mechanism of the reconstruction of AOT reverse micelles from their frozen states and the corresponding transient changes in the local environments were also discussed.