| dc.description.abstract | We model and simulate the Self?Assembled Monolayer (SAM) of thiols (with a general chemical formula CH?�(CH?)?朣朅u) on a gold surface with the (111) index exposed. This system has been studied extensively by both experiments and simulations. Experimental studies have focused on analysing spectroscopic properties-such as the spectra of symmetric and asymmetric stretches of the methylene unit next to the methyl end-which have led to interesting, though sometimes subjective, interpretations of order?to?disorder transitions with temperature, especially for long thiols. Almost all simulations to date have been molecular?dynamics simulations, which also predict transitions, typically interpreted as sudden changes in untilting rates and/or orientational order parameters. However, the conclusions reached by different experiments and simulations based on different observables are not consistent.
We adopt the special Monte Carlo method developed by Siepmann and Frenkel, known as Configurational Bias Monte Carlo, to simulate the above system for a range of thiol chain lengths and temperatures. We finally arrive at a physical understanding that unifies the effects of seemingly different properties of SAMs on their order.
The most tangible indicator of disorder is the formation of gauche defects, and other properties such as tilt angle, tilt distribution, twist distribution, bond?order parameter, etc., are linked-though not always in obvious ways-to the presence of gauche defects and to each other. Twist distribution and the bond?order parameter are two new properties that we introduce for studying defects in SAMs. We also compare our results with interpretations of disorder drawn from electrochemical experiments performed simultaneously by a separate research group. These experiments tentatively suggest a transition to disorder beyond 80癈 for almost all thiol chain lengths above C12 (thiols with 12 carbon atoms). Closely related to the above properties are the internal energy and its various components; for example, torsional energy is directly linked to gauche defects. Based on variations in these energies with changes in the molecular properties (and scientifically meaningful combinations of these properties) as molecule size or temperature changes, our simulations also point to similar transition values.
Having identified the transition boundaries, we propose a hypothesis for a general theory based on relationships between two directly linked internal?energy components. In addition to two scaling relations predicted by this theory, the hypothesis introduces the concept of a term we call the 揹ivergence,� which incorporates all deterministic and non?deterministic relations between energy components. It is the limiting value of this divergence term that determines the transition boundaries in the phase plot.
We also conducted preliminary comparative studies between fully packed SAMs and SAMs with 90% surface coverage for short thiols (C8). The transition temperature for the low?coverage SAM is found to be lower than that for the fully packed monolayer. However, beyond the transition, the low?coverage SAM appears to slow down considerably in forming gauche defects-contrary to what is suggested by the electrochemical experiments mentioned above. We attempt to explain this anomaly by hypothesising the presence of molecular migration in low?coverage SAMs that persists at higher temperatures. | |
