Aqueous solution of betaine: Hydration and aggregation

Abstract Despite the large number of applications in medical, pharmaceutical, agricultural and food industries, to date, the hydration and aggregation properties of betaine are object of debate and have not yet been determined by direct structural measurements at the atomistic level. Here we report the results of a neutron diffraction experiment, demonstrating the amphiphilic character of betaine, where only the carboxyl group forms hydrogen bonds with water, although the number of water molecules in the betaine first hydration shell is of the order of 25. Moreover our data evidence considerable direct betaine-betaine aggregation, driven by electrostatic interaction, along with water mediated dimers involving hydrogen bonds. These findings, along with the peculiar shape of the hydration shell of the carboxyl group may be of great interest for pharmaceutical application. The influence of betaine on the structure of water as a solvent is also commented.