Computer simulation and analysis of self-assembled alkylthiol monolayers on the surface of liquid mercury

Abstract

In this dissertation I investigate the structure and thermodynamics of alkylthiol (thiol) surfactants on the surface of liquid mercury (Hg) by means of the large-scale Molecular Dynamics (MD) techniques. Simulations in the canonical as well as in the constant temperature-constant stress ensembles are employed in order to comprehensively study these systems at various conditions. Mercury is treated atomistically, whereas the alkylthiol surfactants are modeled within the united-atom formalism, in which alkyl and methyl groups are represented as separate superatoms. The most of results on the self-assembly are presented for octadecanethiol and dodecanethiol systems on liquid mercury. Special care is devoted to the choice of an appropriate model for the modeling of the surface of liquid mercury, because of its unique properties, such as strong surface layering and very high surface tension as for a liquid at room temperatures I start by analyzing the latest most successful models of liquid mercury in their ability to reproduce the above properties of liquid mercury surface. For this purpose I use classical MD simulations in the canonical and isothermal-isobaric ensembles as well as Monte Carlo techniques as an auxiliary tool. Specifically, atomistic models of liquid mercury such as (i) the density-independent Raabe model, (ii) the density-independent double-exponent model, (iii) the ad-hoc density-dependent double-exponent model, and (iv) two modifications of the embedded-atom models, were compared with each other. Additionally, the liquid-state theory is used to rationalize the phase behavior of these models. Based on the detailed evaluation of the above models I propose an optimized density-independent force field for liquid mercury, which yields the description of the surface of liquid mercury, which is comparable to the generally more accurate embedded-atom models. At the same time my optimized force field for liquid Hg allows to gain substantial savings in computational time in comparison to the these models. In the following, I investigate the surface coverage-driven self-assembly of alkylthiols on liquid mercury by the MD simulations in the canonical ensemble. Two coexistence regions are found, namely, (i) the coexistence of the agglomerated alkylthiols laying flat (laying-down) with their tails on mercury with the patches of the bare Hg surface, and (ii) the coexistence of the laying-down thiols with the highly-ordered crystalline islands of the standing-up alkylthiols. It is shown that the crystalline phase is not formed immediately as the full coverage of the laying-down molecules is achieved (as one would expect from the microscopic picture), but rather the monolayer of the oversaturated laying-down molecules continues to exist up to a specific threshold value of the surface coverage. This finding perfectly agrees with the general predictions of the condensation/evaporation theory for finite systems. The deviations of the surface tension from a plateau level in the coexistence region between the laying-down and crystalline alkylthiol phases can be well explained by the corrections due to the line tension of the boundary between the two phases derived in the simplest approximation to this theory. The structure of the crystalline thiolate islands on liquid mercury is characterized in detail and the oblique packing of the alkylthiol headgroups is found. Finally, the influence of such factors as temperature, surfactant morphology, alkylthiol tail length, lateral pressure and the alkylthiol packings in the crystalline phase higher than that found at coexistence of the crystalline islands with the laying-down phase is investigated. The MD simulations of the temperature and morphology effects are performed in the canonical ensemble for the crystalline alkylthiol islands at coexistence with the respective laying-down phases. The temperature, at which the crystalline island of alkylthiols on liquid mercury "melts", is estimated. By comparing different types of alkylthiol binding to Hg atoms it is found that the crystalline phase of alkylthiols, in which a single surfactant is attached to a single mercury atom, features a higher level of disorder compared to the respective phases of alkylthiol, when two surfactants can bind to the same mercury atom. To study the effects of different tail lengths, lateral pressure and alkyl thiol packing on the crystalline phases MD simulations in the constant temperature-constant stress ensemble are utilized. This approach allows to avoid the effect of the finite boundaries of the crystalline thiolate islands and to obtain "purified" estimates of their structure. The simulation of the alkylthiols of various lengths reveal no substantial differences. The simulations under lateral compression indicate that the crystalline alkylthiol monolayers on liquid Hg are more stable upon increase of the lateral pressure compared to Langmuir monolayers on water. My simulations also point out that thiol monolayers undergo a tilting transition upon increase of the lateral packing in the crystalline phases of the standing-up thiols.