Liposomes and Lipid Droplets Display a Reversal of Charge-Induced Hydration Asymmetry

The unique properties of water are critical for life. Water molecules have been reported to hydrate cations and anions asymmetrically. We show here that this behavior extends to charged nanoscale interfaces. We investigated charge hydration asymmetry using nonlinear light scattering methods on lipid nanodroplets and liposomes in water made of negative, zwitterionic, and positive lipids. Nanodroplets covered with negatively charged lipids induce the strongest water ordering, while droplets covered with positively charged lipids induce the weakest water ordering. Surprisingly, this charge-induced hydration asymmetry is reversed around liposomes. This opposite behavior in charge hydration asymmetry is caused by a delicate balance of electrostatic and hydrogen bonding interactions: In nanodroplets, the electrostatic effects from negatively (positively) charged lipids and oil phase interfere constructively (destructively). Electrostatic contributions drive the asymmetric hydration of positively charged liposomes, whereas a destructive interference between electrostatic and hydrogen bonding contributions drives the asymmetric hydration of negatively charged liposomes. These findings highlight the importance of not only the charge state but also the specific distribution of neutral and charged lipids in cellular membranes.