Characterization of Alkylsilane Self-Assembled Monolayers by Molecular Simulation

Abstract

Self-assembled monolayers (SAM) of dodecyltrichlorosilane (DTS) and octadecyltrichlorosilane (OTS) on silica are studied by molecular dynamics simulations at 298 K and 1 bar. The coverage (number of alkylsilane molecules per surface area) is systematically varied. The results yield insight into the properties of the alkylsilane SAMs, which complement experimental studies from the literature. Relationships are reported between thickness, tilt angle, and coverage of alkylsilane SAMs, which also hold for alkylsilanes other than DTS and OTS. They are interpreted based on the information on molecular ordering in the SAMs taken form the simulation data. System size and simulation time are much larger than in most former simulation works on the topic. This reduces the influence of the initial configuration as well as the periodic boundary conditions and hence minimizes the risk of artificial ordering. At the same time, more reliable statistics for the calculated properties can be provided. The evaluation of experimental data in the field is often based on strongly simplified models. The present simulation results suggest that some of these lead to errors, concerning the interpretation of experimental results, which could be avoided by introducing more realistic models.

Structural Characterization of Alkylsilane and Fluoroalkylsilane Self-Assembled Monolayers on SiO2 by Molecular Dynamics Simulations

“…Castillo et al 35 obtained values unsatisfactorily lower than the experimental ones (less than 10 • at full coverage), probably because they simulated SAMs on a perfectly crystalline SiO 2 substrate rather than on an amorphous one. Our average values for OTS reported in Table 1 are found to be in better agreement with experiments.…”

“…the angle formed by the long molecular axis with respect to the normal to the surface, where the long axis is defined as the end-to-end distance vector. It is well known that alkylated SAMs form tilted monolayers on most substrates,65,66 and that their tilt angle decreases with coverage 35. Specifically for OTS on silica, reported tilt angles start from 27-30 • at low coverage 51,67 and reach 16-21 • at full coverage 51,59.…”

Atomistic simulations of charge transport in photoswitchable organic-graphene hybrids

“…The analysis of the 2D RDFs allows estimating the average first neighbor distance, which amounts to about 5 Å and corresponds to the distance between the origin and the first peak of the 2D RDF. However, differently from more standard SAMs formed by long alkyl chains [30][31][32], no longer-range positional order is present in any of the samples: all SAMs have a liquid-like structure from this point of view.…”

“…3.1. Structure of the SAMs SAMs molecular organization was characterized in terms of thickness, tilt angle, order parameter, rms surface roughness (table 1) and 2D radial distribution function (figure S6) [30][31][32].…”

“…The optimization of the interfacial characteristics requires an atomic scale knowledge of the molecular organization in both states of the photoswitch, investigated here using atomistic models. Molecular dynamics (MD) simulations have been widely employed to unravel the structural and dynamical properties of SAMs on SiO 2 [30][31][32][33]. On the other hand, while the detailed description of the photoisomerisation mechanism of azobenzene derivatives calls for high-level quantum-chemical methods [34][35][36][37], such techniques are too expensive for the large surface-SAM systems of interest.…”