Dielectric Relaxation of Aqueous Trimethylamineoxide Solutions

Dielectric relaxation was examined for aqueous trimethylamineoxide (TMAO) solutions over a wide concentration (c) range. The dielectric relaxation of TMAO was described by a Debye-type function with a relaxation time of about 3 × 10−11 s, with the strength proportional to c. The number of water molecules tightly hydrated to unprotonated TMAO was estimated to be two. Ab initio calculations predict the magnitudes of the dipoles for individual TMAO and TMAO tightly hydrated by two water molecules, to be 4.9 and 4.2 D, respectively. When the amount of HBr added was increased, dielectric spectra were described by two modes with relaxation times, about 3 × 10−11 and the about 8 × 10−10. The fast relaxation was assigned to the rotational mode of unprotonated TMAO tightly hydrated by two water molecules, and the slow mode to the rotational mode of dimers formed between a protonated and unprotonated TMAO due to hydrogen bonding.