Ultraslow Dynamics at a Charged Silicon-Ionic Liquid Interface Revealed by X-ray Reflectivity

The interfacial structure of the room temperature ionic liquid methyltrioctylammonium bis-(trifluoromethylsulfonyl)imide ([MTOA]+[NTF2](-)) near a silicon electrode was investigated using specular X-ray reflectivity. Using this technique, we have previously observed "crowding", i.e., formation of a thick anion layer on a positively charged electrode. We now report that this layer develops over time scales in the range similar to 400-1100 s. This is different from the time scales reported in other experiments, and is inconsistent with most theoretical predictions. A tentative explanation is proposed which assumes that the formation and dispersion of the crowding layer requires collective reordering of anions/ cations through the electrochemical cell. We suggest that because of the presence of multiple time scales in these systems, the observed time scales will vary depending on the time scale of the measurement.