An Unintended Transformation of Surfactant Molecules into Lamellar Structures during Freeze-Drying: Experiments and Simulation

Lyophilization is frequently used for recovering nanomaterials from colloidal dispersions. Such dispersions are often stabilized by surfactant molecules, and the general notion is that, upon lyophilization, surfactant molecules would remain adsorbed on the surface of nanomaterials. In this work, we report a phenomenon that we came across unintentionally while processing a dispersion of 2D materials stabilized by sodium cholate. We found that sodium cholate molecules themselves transform into lamellar structures during lyophilization. Using FTIR spectroscopy and molecular dynamics simulations, we show that, upon the gradual removal of ice, the cholate molecules are expelled, and hydrogen bonds are formed among them. This, in turn, results in their self-assembly into lamellar forms guided by the planar surfaces of the ice. We also show that a similar rearrangement is observed in a structurally distinct cationic surfactant, cetyltrimethylammonium bromide (CTAB), suggesting that such freeze-drying-induced structural rearrangements may represent a general phenomenon across different classes of surfactants. Traditionally, surfactants have been used for stabilizing 2D materials─the fact that they themselves can transform into sheet-like structures during lyophilization is intriguing and offers broad implications in the design of experiments.