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

Area of Science:

• Materials Science
• Colloid and Surface Chemistry
• Nanotechnology

Background:

• Lyophilization is a common technique for recovering nanomaterials from colloidal dispersions.
• Surfactants are typically used to stabilize these dispersions, with the assumption they remain adsorbed on nanomaterials post-lyophilization.

Purpose of the Study:

• To investigate the behavior of surfactant molecules during the lyophilization of 2D material dispersions.
• To characterize the structural transformations of surfactants under freeze-drying conditions.

Main Methods:

• Fourier-transform infrared (FTIR) spectroscopy to analyze molecular changes.
• Molecular dynamics (MD) simulations to model surfactant behavior during ice crystal formation and sublimation.
• Processing of 2D material dispersions stabilized by sodium cholate and cetyltrimethylammonium bromide (CTAB).

Main Results:

• Sodium cholate molecules were observed to self-assemble into lamellar structures during lyophilization.
• Expulsion of cholate molecules and formation of hydrogen bonds, guided by ice surfaces, led to self-assembly.
• Similar lamellar rearrangement was observed with a different surfactant, CTAB, indicating a general phenomenon.

Conclusions:

• Surfactants can undergo significant structural rearrangements, forming lamellar structures, during lyophilization.
• This freeze-drying-induced self-assembly challenges the conventional understanding of surfactant behavior in nanomaterial recovery.
• The findings offer new perspectives for designing experiments involving lyophilization and surfactant-stabilized materials.