Electrochemical chromatic change of deionized latex suspensions

Well-deionized suspensions of polystyrene sulfonate latex are known to exhibit iridescence, where color changes with the voltage applied to transparent electrodes. The reflection spectra were blue-shifted at voltages more positive than 1.0 V, whereas the spectra were red-shifted at voltages more negative than -1.2 V. They were invariant to voltages from -1.2 to 1.0 V. The wavelength versus voltage curve was similar to the current versus voltage curve, and hence electrode reactions may participate in the chromatic change. The spectra in the quiescent suspension varied gradually for 1 min, whereas those in the stirred suspension were stable after the voltage application. The thickness of the layer exhibiting the chromatic change grew with time and was proportional to the square-root of time, suggesting a diffusion-controlled process. The diffusion coefficient was on the order of 10(-4) cm(2) s(-1) for the 2.0 V application. This implies that the diffusion of H+ was generated electrochemically from water. The generation of H+ was monitored with pH chemical indicators and was found to be the diffusion-controlled process. The H+ should be compensated with anions to keep electric neutrality. There is only one kind of anion: the sulfonic latex particles. The latex particles are then accumulated near the electrode and, hence, exhibit the blue-shifted chromatic change. This elucidation contrasts with the conventional understanding that the external electric field causes the dilution or the accumulation of the latex particles.