Bulk Nanostructure of Mixtures of Choline Arginate, Choline Lysinate, and Water

Neutron diffraction with empirical potential structure refinement was used to investigate the bulk liquid nanostructure of mixtures of choline arginate (Ch[Arg]), choline lysinate (Ch[Lys]), and water at mole ratios of 1Ch[Arg]:1Ch[Lys]:6H₂O (balanced), 1Ch[Arg]:1Ch[Lys]:20H₂O (balanced dilute), 3Ch[Arg]:1Ch[Lys]:12H₂O (Arg^- rich), and 1Ch[Arg]:3Ch[Lys]:12H₂O (Lys^- rich). The Arg^- and Lys^- anions tend not to associate due to electrostatic repulsion between charge groups and weak anion-anion attractions. This means that the local ion structures around the anions in these mixtures resemble the parent single-component systems. The bulk liquid nanostructure varies with the Arg^-:Lys^- ratio. In the Lys^--rich mixture (1Ch[Arg]:3Ch[Lys]:12H₂O), Lys^- side chains cluster into a continuous apolar domain separated from a charged domain of polar groups. In the balanced mixture (1Ch[Arg]:1Ch[Lys]:6H₂O), Lys^- side chains form discrete apolar aggregates within a continuous polar domain of Arg^-, Ch⁺, and water, and in the Arg^--rich mixture (3Ch[Arg]:1Ch[Lys]:12H₂O), the distribution of Lys^- and Arg^- is nearly homogeneous. Finally, in the balance dilute system (1Ch[Arg]:1Ch[Lys]:20H₂O), a percolating water domain forms.