Dynamics of Silica Aerogel’s Hydrophobic Groups: A Quasielastic Neutron Scattering Study

Silica aerogels can be hydrophobized with a variety of functional groups, resulting in different material properties. Here we compare the mobility of hydrophobic groups using neutron spectroscopy. Data were collected between 1.5 and 300 K on the MARS backscattering spectrometer at SINQ (PSI) for three different silica aerogels and octakis(trimethylsiloxy)silsesquioxane (Q8M8) as a reference. The mobility persists to below 10 K for silica aerogels whose surfaces are decorated with trimethylsilyl (TMS) and ethoxy groups. The high low-temperature mobility of the hydrogen in these surface functional groups originates from the large rotational freedom in the aerogel’s open mesopores. In contrast, the mobility freezes in below 30 K for Q8M8, consistent with a reduced rotational freedom within the dense Q8M8 crystal structure. At room temperature, both the rotations of hydrogen within the methyl groups and the rotations of the entire TMS groups contribute to the mean square displacements for both the silylated silica aerogels and the Q8M8 reference. For the methyltrimethoxysilane (MTMS) based aerogel, the room temperature mean square displacements are limited to methyl rotations only. In addition, the mobility is completely frozen in below 30 K, presumably because ∼80% of the methyl groups are not on the surface but located inside the predominantly amorphous SiO1.5CH3structure. No distinct methyl tunneling peaks were detected at 1.5 K for any of the samples, although a weak but statistically significant shoulder is present in the energy spectrum for the MTMS based aerogel.