Anti-freezing surfactants form an adsorption layer at
the solid–water
interface to inhibit the nucleation and growth of ice. However, this
mechanism has not been elucidated at the molecular scale because of
the difficulties in visualizing such adsorption structures. In this
study, we overcome this limitation by directly visualizing the three-dimensional
(3D) adsorption structures of anti-freezing surfactants, hexadecyltrimethylammonium
bromide (C16TABs), on sapphire (0001) surfaces through
3D scanning force microscopy. We present molecularly resolved two-dimensional/3D
images of the adsorption structures in solutions of 1, 10, and 100
ppm. At 1 ppm, the molecules form a monolayer with a flat-lying configuration.
At 10 ppm, the molecular orientation is closer to the upright configuration,
with a relatively large tilt angle. At 100 ppm, the molecules form
a bilayer with almost upright configurations, providing excellent
screening of the sapphire surface from water. Owing to the steric
and electrostatic repulsion between adjacent molecular head groups,
the surface of the bilayer exhibits relatively large fluctuations,
inhibiting the formation of stable ice-like structures. The understanding
of molecular-level mechanisms provides important guidelines for improving
the design of anti-freezing surfactants for practical applications
such as car coolants.