Anhydrous proton conductor consisting of protamine-monododecyl phosphate composite with self-assembled structure

Abstract

We prepared a protamine-monododecyl phosphate composite by mixing protamine (P) and a monododecyl phosphate (MDP). This P-MDP composite formed an acid-base complex by the electrostatic interaction between cationic protamine and the negatively charged phosphate group. Additionally, according to the X-ray diffraction (XRD) measurements, the composite formed a self-assembled lamellar structure with an interaction between the long alkyl chains of MDP. As a result, the P-MDP composite showed the proton conductivity of 9.5 × 10^-4 S cm^-1 at 120-130 °C under anhydrous conditions. Furthermore, the activation energy of the proton conduction of the P-MDP composite was approximately 0.18 eV. These results suggested that the proton conduction of the P-MDP composite was based on an anhydrous proton conductive mechanism. In contrast, the anhydrous proton conduction of the P-methanediphosphonic acid (MP) composite, which did not form the self-assembled lamellar structure, was ca. 3 × 10^-5 S cm^-1 at 120-130 °C and this value was one order of magnitude lower than that of the P-MDP composite. Therefore, the two-dimensional self-assembled proton conductive pathway of the P-MDP composite plays a role in the anhydrous proton conduction.