(510c) Dynamic Response of Low-Density Self-Assembled Monolayers on Gold

We report the dynamic range of properties exhibited by low-density self-assembled monolayers (LD-SAMs) in response to an applied electrical potential. Using two different methods to synthesize uniform, loosely-packed SAMs from thiols on gold substrates, we can systematically control the surface coverage of the LD-SAMs (from a 5% coverage to a fully packed SAM). SAMs of different packing densities allow us to characterize the importance of intermolecular interactions in dictating the conformational orientation of the thiol chains as well as in stabilizing the film against reductive desorption. The packing density, uniformity, and electrochemical response of these sub-monolayer films ? relative to traditional well-ordered, fully-packed SAMs ? have been studied using x-ray photoelectron spectroscopy, contact angle measurements, and electrochemical techniques. Contact angle measurements and electrochemical impedance spectroscopy show that the films made from carboxylic acid-terminated thiols undergo a reversible change in surface wetting and insulating properties in response to charging of the underlying gold electrode. We also show that the films can exist in three distinct states, depending upon whether the gold substrate is negatively-charged, positively-charged, or neutral. We have found that the energy required to induce these changes in the film depends upon both the intermolecular spacing between the surface-bound thiols as well as the electrochemical environment. We show the direct relationship between the insulating properties (interfacial capacitance and ionic permeability) on surface coverage and compare the measured optimum surface coverage with theoretical predictions.