Water in confinement of epoxy layer and hydroxylated (001) γ-alumina: An atomistic simulation view
Introduction
Developing protective organic coatings for metallic substrates is a multi-disciplinary topic. There are many factors influencing the efficiency of such coatings. More specifically, organic coatings of aluminum alloy substrates, which hold a special place in industrial productions and scientific purposes [1,2]. Inevitable diffusion of water molecules in the matrix of an organic coating is the starting point of many other complex phenomena, such as corrosion and anticorrosion reactions. Naturally, the subject of protection is capable of reacting with water. From a technical point of view, an organic coating is modified to have better compatibility with a substrate that has already reacted with atmospheric water (hydroxylation reactions) [3], which means water has a role in dictating the development of organic coatings [4]. By continuation of transport of water molecules in the matrix of a coating, water reaches the substrate. A coating will fail when there is an excessive amount of water present at the interface of coating/substrate. Additionally, confined water at the interface can grow to a macroscopic size (blister) and become a source of bulk water for their neighboring confinements [5]. Water at the interface is the medium in which many of the important properties of a protective coating such as disbonding, corrosion and/or inhibition reactions, and formation of passive films are determined [6,7]. Changes in behavior of water in confinement affect these reactions. Water in confinement is not the same as the bulk water. The well-known hydrogen bond network of bulk water is responsible for many of characteristic behaviors of water. This pattern is disrupted in confined geometries [8]. The confining sides also have a significant role in determining the behavior of water molecules. The structure of coating/substrate confinement has its own characteristic properties. The substrate is hydroxylated alumina, which is an inorganic compound, rigid, and crystalline with periodic features [9]. The other side of confinement is an epoxy layer, an organic material, without periodicity. These are unique features of this type of confinement that justifies more in-depth studies specific to this type of confinement.
Water molecules in confinement have been extensively studied using neutron scattering and X-Ray methods [[10], [11], [12]]. In addition, water in different hydrophobic or hydrophilic confinement has been studied before [[13], [14], [15]]. One of the reliable methods in studies of confined water is the computational approach. The ab initio or molecular dynamics simulations are frequently used for studies of different confined systems [16,17]. Molecular dynamics is a powerful tool in the study of such systems, given utilizing an appropriate force field. Simulations of such confinements require accurate descriptions of different phases such as the epoxy coating, inorganic substrate, and water molecules. Confined systems are mainly governed by long range non-bond interactions, which is important to be accurately reproduced by the force fields. In our previous work, we used ReaxFF method in simulations of epoxy/water/aluminum systems [18]. In this work, we studied that ReaxFF description to obtain a comprehensive view on behavior of water molecules confined at the substrate of a hydroxylated gamma-alumina interface and epoxy coating.
Section snippets
Simulation methods
The initial confined water configurations were obtained from Yoshizawa et al., which are equilibrated configurations using density functional calculations [9]. These configurations are adequate starting points for studying the behavior of confined water using atomistic simulations. Please refer to the original article for more details on the calculations. Each simulation cell has six Diglycidyl Ether of Bisphenol-A (DGEBA, n = 2) molecules over a 27.9 Å × 24.9 Å hydroxylated (001) γ-alumina
Results and discussion
Fig. 1 shows the graphical representation of confined systems. Fig. 1a, b, c shows the configurations of conf-70w, conf-140w, conf-210w, respectively. The mesh volume around water molecules between the substrate and epoxy layer is visible. The epoxy layer and the hydroxylated γ-alumina substrate are shown. Some of the unique features of this type of confinement can be seen as well. The organic coating layer has less compact and orderly features while the periodic structure of alumina substrate
Conclusions
The behavior of confined water changes rapidly as the size of confinement decreases. Water molecules position themselves near the hydroxylated substrate from the oxygen side, where there can be only a limited number of water molecules can be placed. With more water molecules in confinement, the volume available for them increases and approaches to that of bulk value. The majority of water molecules are placed at 3–5 Å distance from the substrate. There are fewer water molecules with higher
CRediT authorship contribution statement
Mohammad Soleymanibrojeni:Conceptualization, Data curation, Formal analysis, Writing - original draft, Writing - review & editing.Hongwei Shi:Conceptualization, Funding acquisition, Writing - review & editing, Supervision.Fuchun Liu:Writing - review & editing.En-Hou Han:Conceptualization, Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51571202).
References (50)
- et al.
Microcontainers with 3-amino-1,2,4-triazole-5-thiol for enhancing anticorrosion waterborne coatings for AA2024-T3
Prog. Org. Coat.
(2019) - et al.
Microcontainer-based waterborne epoxy coatings for AA2024-T3: effect of nature and number of polyelectrolyte multilayers on active protection performance
Mater. Chem. Phys.
(2020) - et al.
Sub-micrometer mesoporous silica containers for active protective coatings on AA 2024-T3
Corros. Sci.
(2017) - et al.
Structure and kinetics of water in highly confined conditions: a molecular dynamics simulation study
J. Mol. Liq.
(2018) - et al.
Density-dependent phase transition in nano-confinement water using molecular dynamics simulation
J. Mol. Liq.
(2018) Fast parallel algorithms for short-range molecular dynamics
J. Comput. Phys.
(1995)- et al.
VMD: visual molecular dynamics
J. Mol. Graph.
(1996) - et al.
The random walk’s guide to anomalous diffusion: a fractional dynamics approach
Phys. Rep.
(2000) - et al.
Hydrogen bond dynamics at vapour–water and metal–water interfaces
Chem. Phys. Lett.
(2004) - et al.
Charging behavior at the alumina–water interface and implications for ceramic processing
J. Am. Ceram. Soc.
(2007)