# Molecular Simulation of the Impact of Defects on Electrolyte Intrusion in Zeolites

Ambroise de Izarra, François-Xavier Coudert, Alain Fuchs, Anne Boutin

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Ambroise de Izarra, François-Xavier Coudert, Alain Fuchs, Anne Boutin. Molecular Simulation of the Impact of Defects on Electrolyte Intrusion in Zeolites. Langmuir, 2023, 39 (51), pp.19056-19063. 10.1021/acs.langmuir.3c03306 . hal-04342488

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Molecular Simulation of the Impact of Defects
on Electrolyte Intrusion in Zeolites

Ambroise de Izarra,†,‡ François-Xavier Coudert,*,‡ Alain H. Fuchs,‡ and Anne Boutin*,†

†PASTEUR, Département de Chimie, École Normale Supérieure, PSL University,
Sorbonne Université, CNRS, 75005 Paris, France

‡Chimie ParisTech, PSL University, CNRS, Institut de Recherche de Chimie Paris, 75005
Paris, France

E-mail: fx.coudert@chimieparistech.psl.eu; anne.boutin@ens.psl.eu
Abstract

We have investigated through molecular simulation the intrusion of electrolytes in two representative pure-silica zeolites, silicalite-1 and chabazite, in which point defects were introduced in varying amounts. We distinguish between two types of defects, considering either “weak” or “strong” silanol nest defects, resulting in different hydration behavior. In presence of weak defects, the hydration process occurs through a homogeneous nucleation process, while with strong defects we observe an initial adsorption followed by a filling of the nanoporous volume at higher pressure. However, we show not only that electrolytes do not penetrate the zeolites, but these defects appears to have only marginal influence on the thermodynamic of electrolyte intrusion. While replacing pure water by the electrolyte solution shifts intrusion pressure towards higher values because of the drop of water saturation vapor pressure, an increase in hydrophilicity of the framework due to point defects has the opposite effect showing that controlling the amount of defects in zeolites is crucial for storage energy applications.
Introduction

In the past years, a new field of application in energy storage was discovered for hydrophobic
nanoporous materials, 1–8 where the non-wetting liquid penetrates the pores under high pres-
sure, resulting in conversion of mechanical energy into fluid–solid interface energy. When the
pressure is released, the mechanical energy can be restored upon release of the fluid. Histor-
ically, intrusion of water in pure-silica zeolites (also called “zeosils”) has been first reported
in 2001 9 and represents the starting point of subsequent intrusion and extrusion experiments
performed on various zeosils. 10–19 Later, Tzanis and co-workers 20 discovered that replacing
pure water by electrolyte solutions is an effective way to increase the mechanical energy
stored in zeosils, due to a shift of intrusion pressure towards higher values. For instance,
it was shown that intrusion of a highly concentrated LiCl solution (\~ 20 M) in silicalite-1
increases intrusion pressure from 96 MPa (for pure water) to 280 MPa, resulting in an in-
crease of stored energy from 10 J/g to 31 J/g. Since this pioneering work, the influence of
the nature of the electrolyte and its concentration on the energetic properties of zeosils has
been widely investigated. 21–37

In a previous study, we have reported a computational investigation of the effect of the
electrolyte solution (nature and concentration) on their intrusion pressure in defect-free pure
silica zeolites.³⁸ We found that due to poorer ion solvation in the pores than in the bulk,
intrusion of electrolyte solutions starts with only intrusion of pure water (and exclusion of
the ions). Moreovre, we showed that the shift of intrusion pressure is more complex than a
simple osmotic effect, because electrolytes are non-ideal solutions. Taking into account the
water activity, which we calculated by the Pitzer model³⁹,⁴⁰ for each electrolyte over a wide
range of concentration, can explain the experimental data available.

Pure-silica zeolites consistently display a strong hydrophobic character, but its exact ex-
tent can depend on the synthesis conditions of the material. Zeosils are typically obtained
through hydrothermal synthesis⁴¹ using fluoride (F⁻) or hydroxide (HO⁻) anions. Patarin
and co-workers⁹ highlighted that the intrusion pressure of water was higher for silicalite-1
(F⁻) than for silicalite-1 (OH⁻), differing by 20 MPa. This shift of intrusion pressure may be attributed to the presence of internal defect in the framework, as silicalite-1 synthesized through the hydroxide route is known to contain more defects than using the fluoride route. ⁴²⁻⁴⁴ Recently, it has been shown that intrusion of NaCl electrolyte solution in chabazite occurs in two steps:²¹ first only water molecules penetrate into the nanocages at 44 MPa, followed by intrusion of the ions at ~260 MPa. The Rietveld analysis of the high-pressure phase revealed that the Na⁺ cation interacts with the oxygen atom of the framework, while the Cl⁻ anion is very close to the silanol defects. This motivated the present work where we investigate several models of silicalite and chabazite containing point defects, in order to probe their influence on electrolyte intrusion in the zeosils.

In the same manner as in the previous works of Cailliez and Trzpit,¹¹,⁴⁵ we tune the hydrophobicity of the framework by introducing various amounts of hydrophilic defects, modeled as silanol nests, to qualitatively reproduce the change in adsorption thermodynamics in zeolites as a function of the number and strength of defects. These defects cause the zeolites to become less hydrophobic and more hydrophilic, because the water-zeolite interaction energy increases and becomes higher than the bulk cohesion energy of water (which is of -44 kJ.mol⁻¹ in the TIP4P model). Starting from equilibrated zeolite-water systems, we attempt to introduce electrolyte pairs (or triplets in case of divalent-cation electrolytes) inside the frameworks. Our simulations highlight that silanol defects have very little influence on electrolyte solvation compared to defect-free zeolite frameworks: the thermodynamic work performed to introduce a first electrolyte unit in the system is similar to that in defect-free frameworks. We conclude that point defects, such as silanol nests, do not provide a satisfactory explanation *per se* for the electrolyte intrusion in zeolites observed in some experimental cases.
# Materials and Methods

## Zeolite Models

We used rigid zeolite frameworks and the atomic coordinates of the orthorhombic (Pnma) structure of silicalite-1 determined by Olson et al.⁴⁶ (unit cell parameters: *a* = 20.07 Å; *b* = 19.92 Å; *c* = 13.42 Å) and the trigonal (R$\bar{3}$m) structure of chabazite determined by Calligaris et al.⁴⁷ (unit cell parameters: *a* = *b* = *c* = 9.459 Å; α = β = γ = 94.07°). The simulation boxes contain, respectively, four and twelve unit cells for silicalite-1 and chabazite, in order to have large enough systems to avoid side effects due to use of periodic boundary conditions. Due to the strong nature of the Si–O bonds, changes in zeolite structure upon water and electrolyte adsorption are generally small²¹ and the rigid framework approach is a realistic approximation.⁴⁸

## Defect Model

It is known from NMR experiments that silanol defects are among the most common points defects encountered in zeolites.⁴⁹ These defects are modeled through a silicon atom vacancy, resulting in a “silanol nest” initially proposed by Barrer⁵⁰ and integrated in silicalite-1 by Cailliez and Trzpit¹¹,⁴⁵ to study its influence on water intrusion. We have chosen here to introduce 1 or 4 silanol nests in each unit cell of the silicalite-1 model, and 0.25 or 0.5 nests in each unit cell of the chabazite model (see Figure 1). We decided to focus our attention on these two zeolites to disentangle the role of silanol defects on the thermodynamic of electrolyte intrusion, from that of pore size. In the silicalite-1 model, we selected and removed from the framework a silicon atom at the intersection between a straight and a zigzag channel (T1 site). For chabazite, we chose to remove a silicon atom that was common to both large and small cages. The resulting four dangling oxygen atoms were then saturated by a hydrogen atom and the O–H bond length (0.9572 Å) was set to the same value as in the Ti4P water molecule model, as well as the Si–O–H angle set to 109.5°. In real zeolites, the silanol defects
are most often assumed to be randomly distributed — although some recent studies have shown the potential for multiple connected defects to exist in specific cases.⁵¹ We discard here frameworks having irregular defect distribution because the use of periodic boundary conditions would lead to spurious effect, but instead use a regular spacing of the defects.¹¹,⁴⁵ It must be mentioned here that we are not exploring high defect content models because it has been proven that silicalite-1 model containing only one defect per cell provides a reasonable description of the thermodynamics of water intrusion.¹¹

<figure><img src="image_2.png" /><figcaption>Figure 1: Schematic view of four unit cells of the silicalite-1 structure and twelve unit cells of the chabazite structure along the *b* axis with defects shown as yellow spheres. (a) Model of silicalite-1 with 1 silanol nest defect per unit cell. (b) Model of silicalite-1 with 4 defects per unit cell. (c) Model of chabazite with 0.25 silanol nest defect per unit cell. (d) Model of chabazite with 0.5 silanol nest defect per unit cell.</figcaption></figure>

## Local defect optimization procedure

After introduction of silanol defects as described above, we performed a local optimization of the defect structure using the force field described in the next section, by allowing each H
atom to freely rotate around the Si–O bond while the rest of the framework remains frozen. We used the RASPA simulation package⁵² in which we implemented the version of the pivot algorithm proposed by Steinhauser.⁵³ For each oxygen atom in a triplet Si–O–H, we define the unit vector $\vec{n}$ pointing from Si to O atom:

$$ \vec{n} = \frac{\overrightarrow{SiO}}{\|\overrightarrow{SiO}\|} \qquad (1) $$

Then, the vector $\overrightarrow{OH}$ is rotated of an angle $\alpha$ around the direction $\mathbf{n}$ and the new coordinate of the H atom $\overrightarrow{OH'}$ is calculated using the following relationship:

$$ \overrightarrow{OH'} = \overrightarrow{OH} \cos \alpha + (1 - \cos \alpha)(\overrightarrow{OH} \cdot \vec{n})\vec{n} + \sin \alpha(\vec{n} \times \overrightarrow{OH}) \qquad (2) $$

The new hydrogen atom coordinates are accepted according to the Metropolis criterion. For each structure, we run 200,000 MC pivot moves for which an hydrogen H of the framework is chosen randomly. An example of some of resulting silanol nests after optimization in silicalite-1 and chabazite are presented in Figure 2.

<figure><img src="image_3.png" /><figcaption>Figure 2: Silanol nest defects obtained from the local optimization procedure in silicalite-1 (a) and chabazite (b). The H atoms are in gray and the hydrogen bond lengths are labeled in Å.</figcaption></figure>

We notice that three silanol groups are involved in a trimer of hydrogen bonds character-
ized by an imperfect O-H-O alignment, resulting from the applied steric constraints. These
silanol nest geometric structures are in very good agreement with classical studies of Trzpit
and Cailliez,¹¹,⁴⁵ the DFT calculations of Sokol et al.⁵⁴ as well as those obtained from ab
initio cluster calculations.⁵⁵

Forcefield

Molecular simulations were performed in the classical limit, with no bond breaking or for-
mation, meaning that the materials undergo no structural change upon intrusion of water
or electrolyte. We used the forcefield proposed by Desbiens and co-workers,⁵⁶ tailored to
reproduce intrusion of TIP4P water around ~ 100 MPa in defect-free silicalite-1, within the
rigid framework approach and the Kiselev approximations.⁴⁸ Hence, the partial charge of
both framework oxygen and silicon atoms were fixed to -0.6e and +1.2e respectively (see
Table S1). Regarding the silanol nest defect, we distribute the charge carried by the vacant
silicon atom on the four hydrogen atoms in order to keep the zeolite neutral. We distinguish
between two types of defects that differ only by their charge distribution:¹¹ the so-called
“weak defect” results from a partial charge of +0.3e carried by each hydrogen atom while
the “strong” defect has the partial charge of hydrogen atom increased to +0.6e and its oxy-
gen charge decreased accordingly (Table S2). Water molecules are modeled with the rigid
TIP4P model⁵⁷ and the electrolytes are represented as point charges and Lennard-Jones
centers. We considered the following monovalent electrolytes: LiCl, NaCl, KCl, RbCl, CsCl,
KF, KBr and KI. We also explored the following divalent electrolytes to probe influence
of charge ion: MgCl₂, CaCl₂, SrCl₂ and BaCl₂. The details of the force field are given in
Tables S3 and S4.⁵⁸,⁵⁹

We used Lorentz-Berthelot mixing rules for cross-terms in Lennard-Jones potential, and
Ewald summation was used to account efficiently for electrostatic interactions. We used a
cutoff of 8.0 Å for both the Lennard-Jones potential and the separation between real space
and reciprocal space in the Ewald summation.
Adsorption of water and electrolytes

Adsorption isotherms of pure water in the zeolites were computed with GCMC simula-
tions using the RASPA simulation package.⁵² We screened several values of the chemical
potential of the (fictitious) bulk fluid reservoir to compute the number of adsorbed water
molecules, by using the relationship between chemical potential of water and hydrostatic
pressure $\mu_{\text{water}}(P)$.⁵⁶ In order to explore the phase space, translation, rotation and swap
moves were applied to water molecules, in a ratio of 1:1:2 respectively. Each simulation con-
sists of 4 × 10⁵ equilibration cycles followed by 2 × 10⁵ production cycles where the average
quantities were calculated. In addition, we characterized the thermodynamics of water in-
trusion by showing the evolution of potential energy per water molecule as a function of
intrusion pressure.

In order to probe electrolyte insertion in zeolites, we used our recent implementation of
the Nonequilibrium Candidate Monte Carlo (NCMC) move⁶⁰ in the osmotic ensemble with a
relative probability set to 10⁻³. The starting configurations were the silicalite-1 and chabazite
materials in equilibrium with a water reservoir at 400 and 200 MPa respectively, in which we
will attempt to introduce ions pairs. Briefly, the osmotic ensemble is sampled using a hybrid
Monte Carlo move using nonequilibrium molecular dynamics, or NCMC, initially proposed
by Nilmeier⁶¹ and later applied by Ross and co-workers⁶² to sample the distribution of
NaCl electrolyte around biomolecules. We previously calculated the chemical potential of ion
exchange $\Delta\mu_{C_nA_m}(P)$ in the semi-grand canonical ensemble for “alchemically” transforming
($n+m$) water molecules chosen randomly into an electrolyte unit $C_nA_m$ (where $C^{m+}$ and
$A^{n-}$ are the ionic species) in bulk electrolyte solution at fixed concentration. This chemical
potential, used in combination with NCMC moves to insert or delete ions at constant number
of particles in the system, defines the osmotic ensemble, where the fictitious reservoir is an
electrolyte solution at a concentration imposed by the chemical potential. We tabulated this
quantity in our previous work, and use here the values corresponding to molalities of 4.77
and 4.62 mol/kg for monovalent and divalent bulk electrolytes, respectively.³⁸ The NCMC
move consists of $T = 200$ steps, each one involving a perturbation of the system through interpolation of non-bonded parameters of the chosen molecules to be transformed, followed by relaxation with 200 MD steps with a timestep of 1 fs in the NVE ensemble. All simulations used a Nosé-Hoover thermostat, setting the temperature at 300 K. In addition, translations were performed both on ions and water while rotations were applied only to water molecules, in a ratio of 1:1:1. Simulation are performed until 1000 moves in the osmotic ensemble have been attempted.

# Results & Discussion

## Influence of defects on water intrusion

We first investigated the effect of local defects on pure water intrusion inside silicalite-1 and chabazite. In Figure 3, we plot the intrusion isotherm of TIP4P water using GCMC simulations for defect-free model of silicalite-1 and chabazite, and compare it to the isotherms with either weak or strong silanol defects. The “weak” silanol defect model provides a representation of defects observed in zeolites synthesized in hydroxide ($HO^−$) media. In order to keep a zeolite model as simple as possible, the “strong” defect model attempts to mimic more hydrophilic defects, such as $Si–O^−$, $Na^+$ without modifying the defect geometry, but only its charge distribution.¹¹,⁴⁵

Let us first discuss the intrusion of water in presence of weak defects. In all cases, the isotherms are of type V according to the IUPAC classification,⁶³ and condensation takes place above bulk water vapor pressure ($P_0 \sim 5300$ Pa for the TIP4P model⁵⁶), as in the case of the defect-free models. This indicates that the framework system remains hydrophobic, regardless of the defect content.¹¹ By increasing the number of defects, the forced intrusion (capillary evaporation) is shifted towards lower pressure. The similarity of qualitative hydration behavior regardless of defect content (i.e., the same shape of the isotherms) is worth being examined in more detail by examining the thermodynamics of water intrusion. In Fig-
<figure><img src="image_4.png" /><figcaption>Figure 3: Water intrusion isotherms computed through GCMC simulations at 300 K, for silicalite-1 (top) and chabazite (bottom). Black curves correspond to defect-free zeolites. Blue and red full curves: models with weak silanol nest defects. Blue and red dashed curves: models with strong silanol nest defects.</figcaption></figure>

ure 4, we show the evolution of potential energy per water molecule adsorbed in silicalite-1 and chabazite as a function of water pressure.

In presence of weak defects (top and bottom left panels on Figure 4), the evolution of potential energy is consistent with the hydrophobic nature of the framework because at low water content, the potential energy per water molecule is below the bulk cohesion energy of water — indicating that water would rather stay in the bulk rather than penetrate the framework. The presence of defects helps lowering the water-water energy to −27 kJ.mol⁻¹.molec⁻¹ for silicalite-1 and −32 kJ.mol⁻¹.molec⁻¹ for chabazite upon condensation, occurring at lower intrusion pressure in presence of weak defects (e.g. around ~70 and
<figure><img src="image_5.png" /><figcaption>Figure 4: Potential energy per water molecule as a function of water pressure, for various models of silicalite-1 (top panels) and chabazite (bottom panels). Left: weak defects; right: strong defects. The total potential energy (represented in solid lines) is split into two contributions: the water-zeolite energy (dashed lines) and the water-water energy (dotted lines). Black curves: defect-free zeolites; blue curves: low defect concentration (1 defect per cell in silicalite-1; 0.25 defect per cell in chabazite); red curves: large defect concentration (4 defect per cell in silicalite-1; 0.50 defect per cell in chabazite). The green horizontal line indicates bulk cohesion energy of water at 300 K.</figcaption></figure>

40 MPa in silicalite-1 for 1 and 4 defects per cell, and ~80 and 50 MPa in chabazite for 0.25 and 0.50 defects per cell) than for defect-free zeolites (e.g. 95 MPa for silicalite-1 and 90 MPa for chabazite). Hence, the water-water interaction energy is dominant in the condensation mechanism and results in total energy –40 kJ.mol⁻¹.molec⁻¹ which is close to the bulk liquid value. The snapshots of water intrusion in silicalite-1 containing one weak defect per cell (see Figure S1) confirm that condensation of water occur through rather homogeneous nucleation of water molecules, followed by a collapse of the water cluster into the bulk phase, as for defect-free silicalite-1.⁴⁵

In presence of strong defects, however, the picture is drastically changed and the isotherms
<figure><img src="image_6.png" /><figcaption>Figure 5: Framework of silicalite-1 with 1 strong defect per unit cell, and density of adsorbed water as a cloud of blue spheres, for various values of intrusion pressure.</figcaption></figure>

progressively transition from type V to type IV (Figure 3). This evolution observed for silicalite-1 is in agreement with previous studies:¹¹,⁴⁵ There is some water uptake at low pressure and the transition is less sharp when the defect content is increased. When strong defects are present, the water-zeolite interaction becomes the dominant factor in the condensation process as it becomes larger than the bulk cohesion energy of water at very low pressure. In that case, nucleation of water happens more heterogeneously at low pressure, around the defect sites, and each of these small water clusters acts as a seed in the condensation at higher pressure. This is shown by MC snapshots illustrating the heterogeneous nature of water condensation for silicalite-1 around the strong defects (Figure 5). The type IV nature of the observed isotherms is thus explained by favorable adsorption of a moderate amount of water around the strong defects, followed by a sudden filling of the nanoporous volume. The effect of various type of silanol defects on water intrusion in silicalite-1 and chabazite are in qualitative agreement with previous studies focused on silicalite-1.¹¹,⁴⁵
Influence of defects on electrolyte intrusion

We now turn our attention to the impact of these weak and strong defects on the adsorption
of electrolytes inside the nanopores of the zeolites. Despite the large body of literature on
the intrusion of electrolyte solutions in zeolites, there have been very few studies attempting
to characterize the composition of the intruded fluid, especially as a function of the intrusion
pressure.<sup>21,22,30</sup> The behavior most commonly observed is that water molecules penetrate
the nanoporous volume at intrusion pressure, potentially followed by electrolyte penetration
at a much higher pressure (e.g., around one order of magnitude of increase in case of 2
mol/L NaCl solution in chabazite<sup>21</sup>). Because previous molecular simulations of electrolyte
intrusion in defect-free zeolites showed that ions intrusion is thermodynamically unfavorable
in the sub-GPa range of pressure,<sup>60</sup> our goal was to probe electrolyte intrusion in those same
zeolites in the presence of point defects.

Therefore, we performed MC simulations in the osmotic ensemble in order to insert a
first electrolyte unit in water, zeolite systems originally equilibrated with a water reservoir
at values of pressure that represent saturation uptake: 400 MPa for silicalite-1 and 200 MPa
for chabazite. From these starting configurations, Figure 6 reports the histograms of the
alchemical work associated with the insertion of a single NaCl pair in various zeolite model
containing defects. In this analysis, we discarded both the Silicalite-1 and chabazite mod-
els containing respectively 4 and 0.5 strong silanol defects per unit cell, because of their
hydrophilic character which does not mimick experimental behavior. For all other zeolite
models, we can see that the profile of the alchemical work histogram is similar in both
shape, average and extent to the parent defect-free zeolite. This shows that the presence of
defects, although it impacts strongly the adsorption of water, has marginal influence on the
thermodynamics of electrolyte intrusion.

In order to provide an estimation of the acceptance probability of the insertion of an
NaCl pair, we plot a vertical line in each histogram of Figure 6 indicating the value of
the alchemical work associated to an intrusion probability of 10<sup>-2</sup> for a NaCl electrolyte at
<figure><img src="image_7.png" /><figcaption>Figure 6: Histogram of the alchemical work for inserting a NaCl electrolyte unit $W_{NaCl}$ in various defect models of silicalite-1 (left panel) and chabazite (right panel). The vertical lines indicate the value of $W_{NaCl}$ corresponding to an insertion probability of $P_{acc} = 10^{-2}$ for a NaCl electrolyte solution at molality of 4.77 mol/kg. For comparison, the histogram of alchemical work for defect-free zeolites is displayed in black for each case.</figcaption></figure>
molality of 4.77 mol/kg using the following acceptance probability:³⁸

$$ P_{\text{acc}} = \min \left( 1, \frac{\prod_{i=0}^{m+n-1} (N_{\text{H}_2\text{O}} - i)}{\prod_{j=1}^{n} (N_{C^{m+} + j}) \prod_{k=1}^{m} (N_{A^{n-} + k})} e^{-\beta(W_{C_n A_m} - \Delta\mu_{C_n A_m})} \right) \\ = \min (1, N_{\text{H}_2\text{O}} (N_{\text{H}_2\text{O}} - 1) e^{-\beta(W_{C_n A_m} - \Delta\mu_{C_n A_m})}) \quad (3) $$

All MC moves attempted to insert a NaCl unit in the zeolites filled with water were rejected, and registered value of W used to construct the histograms are associated to much lower insertion probabilities. This enables us to conclude that in the frame of our model, electrolyte does not penetrate inside the zeolites. On the microscopic scale, a silanol defect is not singularly different from a water molecule because silanol hydroxil group play the same role in solvating Cl⁻ anion than those of water molecule. The intrusion and extrusion of various electrolyte (NaCl, NaBr and CaCl₂) at 2 and 3 M in pure-silica chabazite has been investigated in detail in an earlier work to be discussed in the context of our simulations.²¹ Characterizations of chabazite samples before and after intrusion using ²⁹Si solid-state NMR spectroscopy confirm formation of silanol defects upon intrusion of NaCl electrolyte solution, resulting in a lowering of the intrusion pressure in the second and third intrusion cycle, from 54 to 47 and 44 to 38 MPa for respectively 2 and 3 M solution. This indicates that despite increasing amount of silanol defects in the chabazite, it remains hydrophobic.

A detailed investigation of the nature of the intruded fluid obtained by Rietveld analysis shows that water molecules penetrate first chabazite at the intrusion pressure of 54 and 60 MPa for 2 M NaCl and CaCl₂ respectively, followed by electrolyte intrusion at a much higher pressure of ~250 and ~600 MPa and the concentration inside the chabazite is found to be much higher than those of the bulk, closed to the saturation concentration. Once intruded, it is shown that electrolyte interpher with silanol defect, but it does not mean in absolute that silanol defects would results in itself on electrolyte intrusion at a higher concentration than the bulk. The non-intrusion of electrolyte is consistent with experiments
reporting only penetration of water molecules at the intrusion pressure, but the reported
intrusion of electrolyte at a much higher pressure may not be accessible due to limits of both
forcefield and classical simulations. First, the force field used to model interactions within
the zeolites are tailored to reproduce the intrusion of TIP4P water in silicalite-1 and may not
be relevant to describe properly interaction with water at very high pressure. Second, the use
of our current empirical force field to model electrolyte by a simple electrostatic potential
coupled with Lennard-Jones interaction does not take into account polarizability, charge
transfer or use of unreduced Coulomb interactions, that may relevant at higher pressure.⁶⁴

Conclusion

In this work, we have investigated in detail how the hydration thermodynamics and elec-
trolyte intrusion in silicalite-1 and chabazite zeolites is affected by introducing various
amount of attractive point defects (modeled by silanol nests) into the parent pure-silica
zeolites. First, we reproduced qualitatively the evolution of hydration as a function of the
strenght of silanol defects introduced in the zeolites. In presence of “weak” defects, the ad-
sorption isotherms for both chabazite and silicalite-1 are of type V (according to IUPAC
classification). The effect of weak defects is to make the framework more attractive as shown
by a slight shift of the condensation transition, although the frameworks remain hydropho-
bic. As for defect-free zeolites, the condensation transition occurs through a homogeneous
nucleation process. In case of “strong” defects, the isotherms shift to type IV. The first stage
of water loading is attributed to a heterogeneous nucleation of water around the defects,
forming small water clusters. The next stage consists of the growth of these clusters of water
acting as a seed to trigger condensation transition.

Then, we probed the influence of defects on electrolyte intrusion. Our simulations in
the osmotic ensemble reveal that the alchemical work associated with electrolyte intrusion
is similar to that computed in defect-free zeolites, and that the probability of electrolyte
intrusion is very low. Consequently, silanol defects may not explain electrolyte intrusion in the approximation of our force field and classical simulations. The non-penetration of electrolyte observed in our simulations is consistent with experimental reporting, with water as the only species intruded in the chabazite at the intrusion pressure. The intrusion of electrolyte occurs at several order of magnitudes higher than the intrusion pressure and our simulations may not grasp this phenomenon: our forcefield describing the “rigid host” framework within the Kiselev approximation is specifically tailored to reproduce the intrusion of TIP4P water in silicalite-1 and electrolyte ions are simply described by charged beads ignored polarizability or charge transfer. The present study shows, in any case, that the attractive nature of the defects does not compensate the thermodynamic cost of desolvation of the ions from the bulk electrolyte. While it could be possible with defects of a stronger nature, they would then turn the zeolite hydrophilic overall.

## Acknowledgement

This work was funded by Agence Nationale de la Recherche under project MESAMM (ANR-19-CE05-0031). Access to high-performance computing platforms was provided by GENCI grant A0150807069.

## Supporting Information Available

Force field parameters, Monte Carlo snapshots of silicalite-1 containing 1 weak defect per cell and raw data of alchemical work extracted from MC simulations in the osmotic ensemble.

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