In order to study the influence of molecular structure on the formation of a monolayer,two molecules have been considered,namely N-stearoyldopamine(DOPA)and 4-stearyl-catechol(ST).The difference between these two molecules is the amide group in DOPA.We conclude that for both kinds of molecules,the π-interaction between the catechol group and the Au(111)surface is important.Compared to experimental results,the catechol groups are found either parallel or perpendicular to the Au(111)surface in MD simulation.The difference between DOPA and ST systems is that when there are fewer molecules on the Au(111)surface,in the DOPA system,the amount of catechol groups perpendicular with their hydroxyls orienting towards the surface is less than that of the ST system.Further analysis of catechol groups and amide groups indicates that various kinds of hydrogen bonds formed in the DOPA system have a profound influence on the structure and regularity of the monolayer.The surface pressure-area isotherms for N-stearoyldopamine(DOPA)and 4-stearylcatechol(ST)monolayers are obtained by means of molecular dynamics simulations and compared to experimental isotherms.Upon using TIP4P/2005 for water and OPLS force fields for the organic material and a relatively large system size,the simulated results are found to be consistent with experiments.When the surface pressure is high,a regular molecular orientation is observed for ST molecules,whereas regular orientations are only observed in local domains for DOPA molecules.The differences between DOPA and ST monolayers are attributed to the amide groups in DOPA molecules,which are useful for both steric effects and the formation of hydrogen bonds in the DOPA monolayers.This study clearly demonstrates that hydrogen bonds,due to the presence of the amide group in DOPA,are the cause of the disorder in its Langmuir monolayers.Thus,the conclusion may be helpful in making ordered organic monolayers in the future.