Probing the Organization of Charged Self-Assembled Monolayers by Using the Effects of pH, Time, Electrolyte Anion, and Temperature, on the Charge Transfer of Electroactive Probes

The effect of the solution pH and the electrolyte anion on the capacitive behavior observed for a self-assembled monolayer (SAM) of the ionizable 11-amino-1-undecanethiol (AUT) deposited on polycrystalline Au was checked by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using pure electrolyte solutions. The double layer capacitance (C-dl) decreased with increasing the pH and was shown to be dependent on the electrolyte anion, suggesting that the equilibria controlling the degree of charge at the monolayer may affect the order of the thiol chains at the outermost part of the film. The effect of these parameters, plus the temperature and the time of electrode-solution contact, were also checked by monitoring the electron transfer (ET) process of electroactive Ru(NH3)(6)(+3) and Fe(CN)(6)(-3) redox probes in solution. The results obtained at short contact times showed that while the structure of the monolayer was not shown to affect significantly the charge transfer process for attractive probe-end group interactions, Fe(CN)(6)(-3) probes, the process seemed to be controlled by the probe access into the layer when they were repulsive, Ru(NH3)(6)(+3). The results indicated that the anion, the pH, and the temperature can affect the initial structure of the monolayer. The evolution observed in the redox response when the contact time was increased was associated with the establishment of a disordering process affecting the thiol organic chains the kinetics of which was determined from the impedance data. An apparent rate constant (k) of the order of 10(-4) s(-1) was obtained, agreeing reasonably well with the previously reported value. Additional results obtained for the unionizable 1-DT film suggest that the access of both probes into the monolayer was the limiting step, which was already expected from the absence of electrostatic interactions. The rate constant fell within the same order of magnitude of that found for AUT.