Beyond liquid water: Understanding the structure of water confined in hydrophobic ZIF cavities to design shock absorbers
Beyond liquid water: Understanding the structure of water confined in hydrophobic ZIF cavities to design shock absorbersPromotor(en): V. Van Speybroeck, S.M.J. Rogge /28115 / Model and software development, Nanoporous materials
Background and problem
Water is ubiquitously used on Earth in all its three elementary phases: solid ice, liquid water, and water vapor. However, the behavior of water can be engineered even beyond these three well-known phases. For instance, when confined in small hydrophobic cavities, water molecules tend to cluster together differently than in liquid water or in ice, forming a “phase” that strongly depends on the properties of the cavity. We recently exploited this behavior in collaboration with experimental partners at the University of Oxford to design efficient shock absorption materials, in which liquid water is forced inside the hydrophobic cavities of a solid-state material.1 As this process requires energy, it succeeds in absorbing and dissipating part of the original shock’s energy.2
Molecular modeling sheds unique light on what happens in these hydrophobic cavities. Using various modeling tools, the interactions among the different water molecules as well as the interactions between the water molecules and the hydrophobic cavity in which they are confined can be predicted starting from the Schrödinger equation. By correlating these interactions with the macroscopic behavior, novel design principles can be discovered to engineer state-of-the-art materials, for instance for shock absorption. However, to this end, it is crucial to clearly understand which design parameters dominate this macroscopic behavior. For instance, Figure 1 demonstrates a set of zeolitic imidazolate frameworks (ZIFs), which all consists of hydrophobic cavities.3 However, the fashion in which these cavities are stacked (the topology of the material), the atoms these cavities are built from, as well as the size of the cavities and their connecting apertures differ.
Figure 1: Five zeolitic imidazolate framework (ZIF) materials built from hydrophobic cavities. These solid-state materials differ by the way these cavities are stacked (the topology of the material), the atoms these cavities are built from, as well as the size of the cavities (LCD) and their connecting apertures (PLD).
In this thesis, it is our intention to exploit the power of molecular modeling to understand how these various design parameters affect the behavior of water confined in these materials and to extract new design principles for efficient shock absorption materials. This goal consists of two subtasks. First, we aim to completely characterize the structure of water confined in the ZIFs’ cavities. Second, we aspire to accurately determine the ZIFs’ energy absorption capacity and hence adequacy to be used as a shock absorber by quantifying the free energy barriers for water intrusion. As a proof of concept, Figure 2 shows a cluster analysis of water confined inside the nanocavities of ZIF-8.1 From this analysis, one can deduce the structure of the water clusters, which in turn determines the energy absorption efficiency of the material. In ZIF-8, water is known to fill the cavities sequentially by means of the formation of water clusters. The filling of a next cavity is substantially facilitated by the interaction with water clusters that are already present. Similarly, Figure 3 directly shows the energy barriers associated with the water intrusion phenomenon for ZIF-8.
Figure 2: A cluster analysis of water confined in the nanocavities of ZIF-8 as a function of the simulation time, showing when interactions between different clusters and cavities take place. Adapted from reference 1 with permission from Springer Nature.
To identify the structure of confined water at experimental operating conditions, we will follow their trajectory inside the hydrophobic cavities using so-called molecular dynamics (MD) and enhanced MD simulations. These simulations will be performed at a force field level, describing the interatomic interactions by means of an effective potential that is fitted to data obtained by solving the Schrödinger equation. The student will be actively coached to make him/her acquainted with the advanced simulations techniques early in the thesis year, and to transfer necessary programming skills needed to perform the research. Following this computational approach, it will be possible to elucidate the underlying principles determining the structural organization of water in ZIFs and to provide design principles on how ZIF materials can be tuned to optimize their performance as shock absorbers.
Figure 3: Free energy needed for water molecules to hop between adjacent hydrophobic cavities. The maximum in free energy decreases once sufficient water molecules start to cluster together, explaining why clustering plays a dominant role in explaining the material’s shock absorption capacity. Adapted from reference 1 with permission from Springer Nature.
- Study programmeMaster of Science in Engineering Physics [EMPHYS], Master of Science in Physics and Astronomy [CMFYST]Keywordsconfined water, design principles, hydrophobic cavities, molecular simulations, cluster formation, hydrogen bondingReferences
1Y. Sun, S.M.J. Rogge, A. Lamaire, S. Vandenbrande, J. Wieme, C.R. Siviour, V. Van Speybroeck, J.-C. Tan, Nat. Mater. 20: 1051, 2021.
2G. Fraux, F.-X. Coudert, A. Boutin, A. Fuchs, Chem. Soc. Rev. 46: 7421, 2017.
3A. Phan, C.J. Doonan, F.J. Uribe-Romo, C.B. Knobler, M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 43: 58, 2010.