According to John A. Pelesko, "self-assembly refers to the spontaneous formation of or-ganized structures through a stochastic process that involves pre-existing components, is reversible, and can be controlled by proper design of the components, the environ-ment, and the driving force" [1]. This is probably one of the most general definitions of the self-assembly. Self-assembly processes are ubiquitous in nature and one can find results of their action on all length and interaction scales, e.g. in the form of seashells or spiral galaxies. Numerous illustrations of self-assembly are also present in abundance in various solutions (micellization), colloids, molecular bilayers, liquid crystals, emul-sions, as well as in all sorts of biological systems [2]. The scope of this work is devoted to the molecular self-assembly at interfaces.
The classical example of self-assembled structures at interfaces are Langmuir layers of amphiphilic molecules at the interface of water. An example of such a mono-layer is depicted in Fig. 1.1 for the case of palmitic acid (15(CH2)COOH) on water [3].
A molecule constituting such a monolayer consists of a polar hydrophilic headgroup and of a hydrophobic, often neutral, tail. The hydrophilic headgroup is anchored on water via electrostatic interactions, whereas the hydrophobic tails stay tilted at∼5◦-29◦[3] to the surface normal on water and interact via the Van der Waals forces among themselves.
This intermolecular interaction between the tails is responsible for the long-range order in the Langmuir films. As a rule the long-range order in a Langmuir self-assembled film increases as the surfactant surface coverage,σ, (or its inverse, the area per surfac-tant) increases (decreases). During this process the monolayer undergoes various phase transitions ranging from a dilute gas phase via low-ordered liquid phase up to
highly-Figure 1.1: A snapshot from a molecular simulation illustrating the Langmuir mono-layer of palmitic acid (15(CH2)COOH) on water for an area per molecule of 22 Å2and a surface pressure of 5.6 mN/m at T =300 K. Color code: alkyl tails (green); polar COOH group, (red); water (blue). Adapted with permission from [W. Lin, A. J. Clark, F. Paesani, Langmuir, 31, 2147 (2015)]. Copyright (2016) American Chemical Society.
ordered condensed amphiphile phases [4]. One can see the signatures of these phase transformations as plateau regions in a surface pressure–area per molecule (or, alterna-tively, in the surface tension–surface coverage) isotherm, which is sketched in Fig. 1.2 [4]. In this context the surface pressure,Π=γ0−γ, means the difference between the surface tension of the bare water,γ0, and of the water surface with a given number of the surfactants on it,γ. Usually the surface tension at a given value of σ is obtained from tensiometry experiments by measuring the force acting on a suspended plate, which should be completely wetted by the material from the trough (see e.g. [4, 5]). The surfactant surface coverage in such experiments is typically controlled by adjusting the area available to the monolayer via appropriately setting the side barriers in a Lang-muir trough (Fig. 1.2 top). LangLang-muir monolayers at the interfaces with water have been known to mankind over centuries and been a subject of ongoing research (e.g. [3]) since the end of the 18-thcentury on. A comprehensive review of these systems is given by Kaganeret al.[4].
Another prominent example of organic self-assembled monolayers (SAMs) are alkyl-thiol (or simply alkyl-thiol) films on gold (Au) [6–9]. In this case a special ordering of the tails, as well as of the headgroups is epitaxially reinforced by the underlying crys-tal structure. The headgroups, typically sulfur (S), of the constituent molecules are
Figure 1.2: A schematic view of a Langmuir trough (top) and a general isotherm of Langmuir monolayer (bottom). Horizontal plateaus in the isotherm correspond to phase coexistence regions at first-order transitions. Reprinted figure with permission from [V.
M. Kaganer, H. Mohwald, P. Dutta, Rev. Mod. Phys., 71, 779 (2015).] Copyright (2016) by the American Physical Society.
bound to the Au substrate via covalent bonds. Less frequently selen is used as a head-group [7, 10]. Alkythiol SAMs on other crystalline substrates such as graphene, gallium arsenide, silver, zinc selenide, copper, germanium etc. have been also experimentally studied [6, 11–16]. Alkylthiol SAMs on gold gained enormous attention from experi-mental physicists (see e.g. Refs. [6–9, 17–25]). The alkylthiol SAMs allow to modify surface properties of solid materials in a controlled way, since the thiol endgroups can be easily exchanged [6]. This finds numerous applications in nano- and biotechnology, e.g. for creation of biocompatible surfaces, (bio)sensors, corrosion and wear protec-tion, nanopatterning, controlling of such macroscopic properties as wetting, adhesion, frictionetc. [6–8, 26]. Seminal experimental observations were described by Nuzzoet al.[23] and Poirier et al. [25]. The former proposed the √
3×√ 3
R30◦ packing of the sulfur headgroups that is commensurate with the underlying Au (111) surface [23], which is schematically depicted in Fig. 1.3. The notation √
3×√ 3
R30◦ denotes the hexagonal arrangement of the sulfur headgroups, which is rotated by 30◦ relatively to the underlying Au (111) surface and has the nearest-neighbor distance of dau√
3 with
Figure 1.3: Schematic view of the typical √ 3×√
3
R30◦ packing of sulfur head-groups (black) on Au (111) surface (yellow). The sulfur headhead-groups are arranged in a hexagonal manner with the nearest-neighbor distance ofdau√
3, wheredau=2.88 Å is the nearest-neighbor distance of the hexagonal structure of the gold atoms in the sub-strate. The unit cell of the sulfur groups is rotated by 30◦with respect to the underlying Au (111) surface. Adapted with permission from [L. Ramin, A. Jabbarzadeh, Langmuir, 28, 4102 (2012)]. Copyright (2016) American Chemical Society.
dau being the lattice spacing of Au. Poirier et al. gave the first description of the self-assembly mechanism on Au (111) [25]. In the case of other crystalline substrates the order of the thiol SAMs has been also found to be commensurate with the one of the underlying crystal lattice [6].
SAMs are usually created by a molecular deposition of thiols from a gas or a liquid phase onto a substrate [6–9, 27]. This process usually involves multiple stages, which are sketched in Fig. 1.4. Here, unlike the Langmuir monolayers, the alkyl tails do not experience strong repulsive interaction with the substrate and adsorb initially onto the metal surface with their tails laying flat on it (Fig. 1.4a). This stage typically lasts 2–15 min. In the following molecules in such conformations will be called laying-downmolecules. The layer of the laying-down thiols is subsequently completed as the surfactant surface coverage, σ, grows (Fig. 1.4b). When σ exceeds the value of the
Figure 1.4: Schematic view of the alkylthiol self-assembly on the surface of liquid mer-cury. This chart illustrates general stages of alkylthiol self-assembly on metal surfaces.
full coverage of the laying-down conformations, the backbones of thiols reorient and the nucleation of thestanding-upphase is observed (Fig. 1.4c), where thiols stand tilted at a sharp angle to the surface normal. This is followed by several protracted stages of the self-assembly, which are often associated with the increase of ordering within the monolayer of the tilted molecules. The final stages of the self-assembly can last from a couple of hours to several days and result in thiol SAMs that feature long-range crystalline order (Fig. 1.4d). The tilt angle of the SAMs on crystalline substrates is typically 30◦[6].
Computer simulation studies of the self-assembled alkylthiol monolayers on metals [28–46] focus on the SAMs on crystalline surfaces (mostly Au) as well. Moreover, in the vast majority of the simulations only the structure of the densely packed monolayers of thiols standing tilted to the surface normal (i.e. at the final stage of the self-assembly) was probed. Apart from several simplified model systems [47–49], no investigation of molecular self-assembly as a function of the number of alkylthiol surfactants on metallic (liquid) surfaces has been attempted.
Figure 1.5: Potential structure of a dense self-assembled monolayer of alkylthiol sur-factants on the surface of liquid mercury at room temperature. Color code: alkyl tails, green; sulfur headgroups, yellow; bulk mercury, red; mercury atoms with thiols ad-sorbed onto them (bound mercury), purple.