Polarization effect and electric potential changes in the stimuli-responsive molecular monolayers under an external electric field
Under the exerted external electric field (Eex), significant polarization effects are demonstrated in the switching process of positively charged oligopeptide chains on the gold surface by using the polarizable force-field-based molecular dynamics (MD) simulations. The changes of electrostatic environment during the conformational switching are described by calling density functional theory (DFT) calculations of atomic charges or fragment-centered dipole moments. The charge-variable polarizable force field (Q-POL) model shows a good agreement with the experimental observations of both the open-circuit state (OC) without Eex and extended/bent conformation (ON/OFF) with upward/downward Eex, while the nonpolarizable force field (Non-POL) with the fixed atomic partial charges fails to reproduce experiments for the ON state. The charged oligopeptide chains (with each residue furnished with a positive charge) challenge the application of the coarse-graining polarizable force field based on the fragment dipoles (Dfrag-POL). Through tracing the dynamics of charge centers of both counterions and oligopeptide chains in the switching process, a qualitative picture is depicted for understanding the polarization phenomena in the dielectric monolayer in OC/ON/OFF states. With the applied electric field, the rearrangement of ions leads to the induced internal E-field, Ein, in the direction opposite to Eex. The values of total electric field, Etotal, and the interfacial potential Φx were evaluated. The relationship between the applied electric potential and the interfacial potential is built for the switching OC/ON/OFF states. The analysis of the induced internal E-field and interfacial potential may shed light on the further modification of theoretical models to better understand the electrical-induced switching mechanism.