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

Abstract

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 s...