Performance of a rigid rod statistical mechanical treatment to predict monolayer ordering: a study of chain interactions and comparison with molecular dynamics simulation
A statistical mechanical model that treats hydrocarbon self-assembled monolayer (SAM) chains as rigid rods is examined to interrogate the mechanisms involved in monolayer ordering. The statistical mechanical predictions are compared to fully atomistic molecular dynamics simulations of SAMs with different packing densities. The monolayer chain order is examined as a function of surface coverage, chain-surface interactions, and chain–chain interactions. Reasonable interaction potentials are deduced from ab initio electronic structure calculations of small model systems. It is found that the chain-surface interaction is the most important parameter in formation of flat-lying monolayer phases, while formation of standing phase monolayers is driven most importantly by increased density of molecules at the surface. A brief discussion of the utility and validity of the rigid rod treatment is given in light of the molecular dynamics results.
KeywordsSelf-assembled monolayers Molecular dynamics Rigid rod statistical mechanics Alkylthiols Surface ordering
I am grateful for support through The University of Memphis College of Arts and Science Faculty Research Grant Program and through the Computational Research on Materials at The University of Memphis (CROMIUM). Computational resources were also provided on the Lonestar system at the Texas Advanced Computing Center (TACC) under the National Science Foundation XSEDE grant number TG-CHE110109. Additionally, I thank Prof. Louis A. Madsen for helpful discussions of an early version of this work.
- 8.J.I. Siepmann, I.R. McDonald, Simulations of self-assembled monolayers of thiols on gold. Thin Films 24, 205–226 (1998)Google Scholar
- 9.A. Ulman (ed.), Self-Assembled Monolayers of Thiols, vol. 24, Thin Films (Academic Press, San Diego, 1998)Google Scholar
- 38.Gaussian 03, Revision C.02, M.J. Frisch, G.W. Trucks, H.B.Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian, Inc., Wallingford CT, 2004Google Scholar
- 40.C.D. Sherrill, T. Takatani, E.G. Hohenstein, An assessment of theoretical methods for nonbonded interactions: comparison to complete basis set limit coupled-cluster potential energy curves for the benzene dimer, the methane dimer, benzene–methane, and benzene-H\(_2\)S. J. Phys. Chem. A 113, 10146 (2009)CrossRefGoogle Scholar
- 47.We note that the choice of the reference atoms can influence greatly the obtained chain-tilt angle, so making a consistent and universal choice for the definition of the chain-tilt angle is important for these types of studies. Google Scholar