Sharing of Na+ by Three −COO– Groups at Deprotonated Carboxyl-Terminated Self-Assembled Monolayer-Charged Aqueous Interface

By combining theoretical calculations and experimental observations, we show that Na+ can be shared by three charged −COO– groups of the deprotonated carboxyl-terminated self-assembled monolayers in aqueous solution at lipid–aqueous interface. This is similar to the sharing structure of Na+ usually found in crystals. Notably, this ionic configuration leads to a hydroxyl orientation distribution of interfacial water molecules, showing a maximum toward the bulk solution, in addition to the maximum toward the charged surface expected based on the structure of one-to-one ion-pair formation between Na+ and −COO–. Our results provide new insights for a better understanding of electrochemically and biologically related charged interfaces at the molecular level and should be helpful to design the nanodevices and bioinspired human-made nanomaterials.