As the concentration of Eu³⁺ increases, the sign of the water band inverts from positive to negative. The sign of the water band corresponds to the direction of the water molecule present at the interface, and the interfacial water molecules showing negative sign point their hydrogen atoms downward. The hydrogen-down orientation of the interfacial water molecules is due to the bonding to Eu³⁺, which adsorbs to the extractant (HDEHP) at the interface (Fig.4-8(c)).