Abstract
Clathrates are open crystals in which molecules are arranged in a hierarchy of polyhedral cages that encapsulate guest molecules and ions. As well as holding fundamental interest, molecular clathrates serve practical purposes, such as for gas storage, and their colloidal counterparts also appear promising for host–guest applications. Here using Monte Carlo simulations, we report the entropy-driven self-assembly of hard truncated triangular bipyramids into seven different host–guest colloidal clathrate crystals with unit cells ranging from 84 to 364 particles. The structures consist of cages that are either empty or occupied by guest particles, which can be different from or identical to the host particles. The simulations point to crystallization occurring through the compartmentalization of entropy between low- and high-entropy subsystems for the host and the guest particles, respectively. We use entropic bonding theory to design host–guest colloidal clathrates with explicit interparticle attraction, providing a route to realize such systems in the laboratory.

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Data availability
All data that support the conclusions in this manuscript are present in the main text or the supplementary materials. Source data are provided with this paper.
Code availability
Source code for HOOMD-blue is available at https://github.com/glotzerlab/hoomd-blue. Sample codes are included in the Supplementary Information.
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Acknowledgements
This work was supported as part of the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under award no. DE-SC0000989. T.V. was supported by the Office of the Undersecretary of Defense for Research and Engineering, Newton Award for Transformative Ideas during the COVID-19 Pandemic, Award HQ00342010030. With support from XSEDE award DMR 140129, simulations were conducted on resources provided by the Extreme Science and Engineering Discovery Environment (XSEDE)42, which is supported by National Science Foundation grant no. ACI-1053575. Computational resources and services were also supported by Advanced Research Computing at the University of Michigan, Ann Arbor. The funders had no role in the conceptualization, design, data collection, analysis, decision to publish, or preparation of the manuscript.
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S.L. designed the research and performed HPMC simulations and related data analysis. T.V. performed EBT calculations and related data analysis. S.L. and T.V. performed MD simulations. S.C.G. directed the research. All authors contributed to data analysis and manuscript preparation.
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Extended data
Extended Data Fig. 1 TBP truncation.
The truncation parameter S is defined as the length ratio of the maximum truncation length (ltr_max). The ltr_max is defined as the maximum distance of the vertex truncation without overlapping the two truncation directions. The regular TBP (S = 0.0) vertices are: (1,1,1), (−1,−1,−1), (2,2,−4), (2,−4,2) and (−4,2,2). All the TBPs were rescaled to have unit volume.
Extended Data Fig. 2 Cavity size measurement.
a, The truncated TBP tips (left) that point toward a cage center form a cavity (right). The radius of the cavity is determined by the average distance between the center of the cavity and the truncated TBP tips. b, The distribution of the radius of the cavity B obtained from a bulk crystal phase of S = 0.52 system (Clath I – A0B1) at three different pressures.
Extended Data Fig. 3 Free energy calculation.
A schematic diagram of the free energy calculation of the guest and host sublattices in a single component system. The free energies of the guest truncated TBP sublattice and the host truncated TBP sublattice were calculated along two paths (A and B). For each path, we selected multiple states (1, 2, 3 …) between the reference state and the equilibrium state that are connected to subsequent states by a continuous and reversible harmonic potential. See Methods for more details.
Extended Data Fig. 4 Free energy matching between states.
Free-energy matching between two states (B1, A2) represented in Extended Data Fig. 3. a, Potential energy U(γ) change of between the A1 and the A2 states of S = 0.56 and ϕ = 0.60. As we gradually decrease the spring constant γ of the harmonic potential holding the host particles in place, we continue tracking the translational and the rotational displacements of the host particles. b,c, When the translational and the rotational displacements of the host particles of the A2 matches with that of B1, the spring constant is prevented from further decreasing. Here, the γ of A2 is 1.0 (ln γ = 0).
Extended Data Fig. 5 Melting pressure measurement.
a, Gibbs free energy of the fluid and the solid phases of S = 0.42 system. The free energy of the fluid was calculated by thermodynamic integration and that of the solid was calculated by the Frenkel-Ladd method (Methods). The red arrow indicates the pressure where the free energy of the fluid and the solid crosses. b, The equation of state of S = 0.42 system, obtained by melting Clath II – A0D1. The crystal melts at P* ~ 9.7, which is a similar pressure obtained in a.
Extended Data Fig. 6 Reorganization of clathrate structure by the guest removal.
(left) Guest particles are removed from Clath I – A0B1 (S = 0.52), and the system is equilibrated at P* = 11.0. (middle and right) Part of host particles (red particles) spontaneously fill the empty cavity. In these simulation snapshots, all blue and red spheres represent the centers of truncated TBPs, and grey bonds connect nearest neighbors.
Extended Data Fig. 7 Clathrate Phase Diagram from EBT.
Phase diagram of various clathrate morphologies as computed from EBT. Colors correspond to same phases as in Fig. 2 of the main text. Scatter points are from simulations.
Extended Data Fig. 8 Histogram of Orientation Free Energy per Particle from EBT.
(left) D-cage of Clath II – A0D1. (right) B-cage of Clath I – A0B1. Distribution of D-cage is continuous, but this is a distinct set of lower free energy configurations for the B-cage.
Extended Data Fig. 9 Cage Particle Bonding Free Energy.
Computed bonding free energy between particles making up the clathrate cages. σ is the in-sphere diameter of the truncated TBP. Free energy well sits at ~ 3 kT.
Extended Data Fig. 10 Rotational motion of guest in binary clathrate systems.
a-b, Accumulated points of a direction vector (from particle center to a vertex) of (a) a tetrahedron guest in a A-cage and (b) a truncated TBP guest in a B-cage of a Clath I – A1B1 crystal phase (S = 0.52) at a constant volume fraction (ϕ = 0.62) for 3 × 107 MC sweeps, represented on a unit sphere (left) and spherical coordinates (r = 1.0σ) (right). c, Rotational displacement of the host truncated TBPs (blue), the guest truncated TBPs (green) and the guest tetrahedra (yellow) of the Clath I – A1B1 (S = 0.52) at ϕ = 0.62 for 107 MC sweeps. d, Accumulated points of direction vectors of (left) a dodecahedron guest in a A-cage and (right) a truncated TBP guest in a B-cage of a Clath I – A1B1 crystal phase (S = 0.52) at a constant volume fraction (ϕ = 0.62) for 106 MC sweeps, represented on a unit sphere. (left) For the dodecahedron guest, the direction vectors are from particle center to six randomly selected vertices of the particle, so there are six different colors. (right) For the truncated TBP guest, the direction vector is the same vector with Fig. 4a.
Supplementary information
Supplementary Information
Supplementary Figs. 1–4, Tables 1–3 and Videos.
Supplementary Video 1
HPMC simulation of the self-assembly of a one-component system of hard truncated TBPs from the fluid to the host–guest clathrate crystal phase, with S = 0.52 at ϕ = 0.55. The duration of the movie spans 9 × 107 MC sweeps. The resulting crystal is Clath I–A0B1. Red truncated TBPs are guest particles in B-cages. Host TBPs are shown as a network representation (grey bonds; Methods).
Supplementary Video 2
Single guest particles (red) rotating continuously around an axis in the B-cages of the Clath I–A0B1 colloidal crystal of Supplementary Video 1 at P* = 13.
Supplementary Video 3
Freely rotating truncated TBP single guest particles (red) in the Clath II–A0D1 colloidal crystal with S = 0.42 at P* = 11.
Supplementary Video 4
Truncated TBP tetramer guest particles (blue) discretely rotating among four orientations in the Clath II–A1D4 colloidal crystal with S = 0.64 at P* = 15. The blue spheres represent the centres of the guest truncated TBPs.
Supplementary Video 5
Stabilization of B-cage made of truncated TBPs. Simulations are performed using a recently developed anisotropic Lennard–Jones potential implemented in HOOMD-blue27. Host particles comprising the cage are mutually attractive with a well depth ε of 5 kT. The guest particle is modelled using a WCA, repulsive interaction with all host particles. Simulations were performed in the NVT ensemble at T = 1.0 kT/ε for 107 timesteps with dt = 0.0005σ(m/ε)0.5 in simulation units, where m is mass.
Supplementary Data
Source data for Supplementary Figs. 2 and 4b.
Supplementary Code
Sample codes; source code for HOOMD-blue is available at https://github.com/glotzerlab/hoomd-bluemp4.
Source data
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Lee, S., Vo, T. & Glotzer, S.C. Entropy compartmentalization stabilizes open host–guest colloidal clathrates. Nat. Chem. 15, 905–912 (2023). https://doi.org/10.1038/s41557-023-01200-6
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DOI: https://doi.org/10.1038/s41557-023-01200-6


