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Article

Molecular Dynamics Simulation of Clay Mineral–Water Interfaces: Temperature-Dependent Structural, Dynamical, and Mechanical Properties

by
Tong Yang
1,
Chunmei Chu
2,
Yonggang Zhang
3,*,
Zhen Zhang
4 and
Junli Wan
5
1
School of Geological Engineering and Geomatics, Chang’an University, 126 Yanta Road, Xi’an 710054, China
2
School of Geological and Mining Engineering, Xinjiang University, Urumqi 830017, China
3
Engineering Research Institute, China Construction Eighth Engineering Division Corp., Ltd., Shanghai 200122, China
4
Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL 32603, USA
5
Railway Engineering Research Institute, China Academy of Railway Sciences Corporation Limited, Beijing 100081, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(3), 347; https://doi.org/10.3390/w17030347
Submission received: 13 December 2024 / Revised: 15 January 2025 / Accepted: 24 January 2025 / Published: 26 January 2025
(This article belongs to the Special Issue Geotechnical and Underground Engineering Problems Caused by Water Action)
Browse Figures
Figure 1
<p>Schematic representation of simulation models of kaolinite, montmorillonite, and pyrophyllite systems with confined water.</p> "> Figure 2
<p>MSD of water molecules confined in (<b>a</b>) kaolinite; (<b>b</b>) montmorillonite; and (<b>c</b>) pyrophyllite interlayers at different temperatures.</p> "> Figure 3
<p>Comparison of MDS of water molecules confined in kaolinite, montmorillonite, and pyrophyllite interlayers at (<b>a</b>) 298.15 K; (<b>b</b>) 313.15 K; and (<b>c</b>) 363.15 K.</p> "> Figure 4
<p>Density profiles of water molecules confined in the interlayers of (<b>a</b>) kaolinite; (<b>b</b>) montmorillonite; and (<b>c</b>) pyrophyllite at different temperatures (298.15 K, 303.15 K, 313.15 K, 333.15 K, and 363.15 K) along the normalized <math display="inline"><semantics> <mrow> <mi>z</mi> <mo>/</mo> <mi>l</mi> <mi>z</mi> </mrow> </semantics></math> direction.</p> "> Figure 5
<p>Charge density maps of water molecules confined in the interlayers of (<b>a</b>) kaolinite, (<b>b</b>) montmorillonite, and (<b>c</b>) pyrophyllite at three temperatures (298.15 K, 318.15 K, and 368.15 K) in the X−Z plane.</p> "> Figure 6
<p>Temporal evolution and average number of hydrogen bonds formed by water molecules confined in (<b>a</b>) kaolinite, (<b>b</b>) montmorillonite, and (<b>c</b>) pyrophyllite interlayers at various temperatures. (<b>d</b>) The variation of the average hydrogen bond number with temperature for the three clay systems.</p> "> Figure 7
<p>Stress distributions of water molecules confined within the interlayers of (<b>a</b>) kaolinite; (<b>b</b>) montmorillonite; and (<b>c</b>) pyrophyllite at different temperatures. The color scale represents stress magnitudes, with red indicating compressive stress and blue indicating tensile stress.</p> ">
Versions Notes

Abstract

:
Water interacting with clay minerals—such as kaolinite, montmorillonite, and pyrophyllite—fundamentally governs their geotechnical and environmental functions, thereby influencing parameters such as retention, transport, and stability. Understanding the effects of temperature on water behavior within clay mineral interlayers is critical for predicting the performance of clay–water systems under dynamic environmental conditions. This study performed molecular dynamics simulations to investigate the structural, dynamical, and mechanical properties of interlayer water in three representative clay minerals over a temperature range of 298.15–363.15 K. Our analyses focused on mean squared displacement (MSD), density profiles, hydrogen bond dynamics, and stress distributions, thereby revealing the interaction between water structuring and thermal fluctuations. Results indicated distinct temperature-dependent changes in water diffusion and hydrogen bond stability, with montmorillonite consistently exhibiting enhanced water retention and steadier hydrogen bonding networks across the studied temperature spectrum. Density profiles highlighted pronounced confinement effects at lower temperatures that gradually diminish with increasing thermal energy. Concurrently, the stress distributions revealed the mechanical responses of clay–water interfaces, highlighting the interplay between thermal motion of water molecules and their interactions with the clay surfaces. These findings offer valuable insights into how temperature regulates water behavior in clay mineral interlayers and provide a foundation for advancing predictive modeling and the design of engineered systems in water-rich, thermally variable environments.

1. Introduction

Water plays a fundamental role in shaping the properties and behaviors of layered clay minerals, such as kaolinite, montmorillonite, and pyrophyllite, which are extensively utilized in environmental remediation, energy storage, catalysis, and geotechnical applications. These minerals are composed of stacked layers formed by alternating tetrahedral silicate and octahedral alumina sheets, exhibiting large specific surface areas, high ion exchange capacities, and significant interlayer swelling that govern water retention, transport, and stabilization. Variations in interlayer charge density, layer symmetry, and cationic composition across different clay minerals lead to diverse water–clay interaction mechanisms at the molecular scale. Notably, kaolinite, with its relatively low interlayer charge, offers a non-expanding interlayer region that distinctly modulates water confinement. Understanding these intricate water–clay interactions is critical for predicting the behavior and performance of clay-based materials in naturally occurring and engineered water-rich environments. Meanwhile, montmorillonite, characterized by high cation exchange capacity, exhibits significant swelling and hydration in the presence of water and ions [1,2,3,4]. Conversely, pyrophyllite represents an intermediate case, characterized by a structure that lacks intrinsic interlayer charges but retains comparable basal spacing [5].
The interaction of water and ions with clay surfaces has been extensively studied both experimentally and computationally. Melida Gutierrez’s early work established a theoretical foundation for understanding ion adsorption on surfaces [6]. While previous studies advanced these theories by investigating clay swelling through osmotic processes [7], experimental studies using X-ray diffraction (XRD) and neutron scattering provided crucial insights into the arrangement of water molecules and ions within the interlayers of swelling clays [8].
Molecular dynamics (MD) simulations have complemented these experimental studies by offering atomic-level insights into water and ion dynamics in clay systems [9,10,11]. Cygan et al. [12] analyzed the structural and dynamic properties of montmorillonite, including hydration energy and water diffusion mechanisms. Shen et al. [13] extended these findings by examining the influence of salinity and counterion valence on water diffusion in confined clay environments and highlighted the deviations from the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory at nanometer scales. Similarly, Moussa et al. [14] employed advanced MD techniques to study the thermal effects on water dynamics and ion mobility and demonstrated how temperature alters the hydration layers and diffusion rates. However, comprehensive comparative analyses of water and ion dynamics in kaolinite, montmorillonite, and pyrophyllite under varying thermal conditions remain limited. Existing studies often focus on a single mineral or narrow temperature ranges, resulting in gaps in understanding the role of structural and compositional differences in dynamic behaviors.
The mechanical and thermodynamic stability of clay minerals under varying environmental conditions has also garnered significant attention. Studies demonstrated that montmorillonite exhibits considerable interlayer expansion and contraction with temperature changes through XRD and thermo-mechanical analyses [15,16]. Further research identified the roles of hydrogen bonding and cation interactions in maintaining the structural integrity of clay minerals during hydration [17]. Molecular modeling studies have provided detailed stress profiles and pressure distribution maps for montmorillonite, highlighting the coupling between interlayer ion dynamics and mechanical stability [18,19].
Recent MD simulations have expanded the understanding of clay mechanics under extreme conditions. For example, previous studies investigated the impact of temperature on the stress profiles and swelling pressures of montmorillonite and emphasized the importance of cation–clay Coulomb interactions in stabilizing the interlayer spacing [20]. Despite these advances, the comparative thermodynamic and mechanical responses of kaolinite, montmorillonite, and pyrophyllite to thermal variations are not well documented, particularly regarding stress distribution, hydration behavior, and thermal expansion.
Several critical research gaps remain. One significant gap is the lack of a systematic comparison of the temperature-dependent water dynamics and ion diffusion properties among kaolinite, montmorillonite, and pyrophyllite [21]. Second, the coupling between hydration behavior, stress distribution, and mechanical stability across these clay minerals under thermal variation remains poorly understood. Third, while individual studies have highlighted specific aspects of clay behavior, an integrated approach combining water dynamics, mechanical stability, and thermodynamic properties is necessary to provide a holistic understanding of these minerals [22].
This study addressed these gaps by performing MD simulations to systematically investigate the structural, dynamic, and mechanical properties of kaolinite, montmorillonite, and pyrophyllite across various temperatures. Specific objectives include (1) quantifying water diffusion and mean square displacement (MSD) across clay types, (2) analyzing hydrogen bonding and interlayer density distributions, and (3) characterizing stress profiles and thermodynamic stability under varying thermal conditions. By integrating these analyses, this work provided a comprehensive framework for understanding the coupled hydration-mechanical behavior of clay minerals, which is significant for environmental remediation, advanced material design, and geotechnical applications. This research contributes to the broader understanding of confined water systems and ion dynamics in layered materials. The findings are expected to enhance the design of clay-based systems for applications such as water purification, pollutant adsorption, and barrier systems for waste containment.

2. Modeling Details

In this study, kaolinite, montmorillonite, and pyrophyllite were chosen as representative clay minerals due to their distinct structural, chemical, and interlayer characteristics, which are critical for understanding the behavior of water in confined systems. Kaolinite, a non-swelling clay with a 1:1 layered structure and strong interlayer hydrogen bonding, provides a stable framework for examining confined water. Montmorillonite, a highly swelling clay with a 2:1 layered structure and significant cation exchange capacity, enables the investigation of water dynamics under varying interlayer spacings. Pyrophyllite, also a 2:1 clay, serves as a non-swelling counterpart to montmorillonite, allowing for a comparative analysis of interlayer interactions and surface chemistry. The atomic configurations and dimensions of these clay–water systems are depicted in Figure 1. These three clay minerals together form a comprehensive platform for analyzing the temperature-dependent structural, dynamical, and mechanical properties of water in confined environments.

2.1. Kaolinite

Kaolinite is a 1:1-type clay mineral, characterized by its alternating layers of octahedral alumina and tetrahedral silica sheets, with the chemical formula Al 2 Si 2 O 5 OH 4 [23]. The structural model of kaolinite in this study was constructed based on crystallographic data, with unit cell dimensions of a = 5.1489   Å , b = 8.9340   Å , c = 7.3840   Å , and angles α = 91.93 ° , β = 105.05 ° ,   γ = 89.80 ° [12]. A supercell measuring 10 a × 6 b × 1 c was used, corresponding to lateral dimensions of approximately 51.54   Å   × 53.60   Å .

2.2. Montmorillonite

Montmorillonite, a 2:1-type clay mineral, is well known for its swelling capacity and negative layer charge, which arises from isomorphic substitutions occurring in its octahedral and tetrahedral sheets. The chemical composition of the montmorillonite model used in this study is represented as Na 0.75 Al 3.25 Mg 0.75 Si 7.25 Al 0.75 O 20 OH 4 , where Mg 2 + replaces Al 3 + in the octahedral sheet, and Al 3 + substitutes Si4+ in the tetrahedral sheet [2]. While montmorillonite typically exhibits a mixture of interlayer cations (e.g., Na+, Ca2+, Mg2+), we assumed Na+ as the sole interlayer cation for simplicity and to focus on the temperature effects on interlayer water dynamics. This approach aligns with previous molecular simulation studies [24], although we acknowledge that this simplification does not fully capture the geochemical diversity of natural montmorillonite. As illustrated in Figure 1, this model employed periodic boundary conditions to ensure continuity in all three spatial directions. The unit cell parameters of the montmorillonite structure are a   = 5.175   Å , b   = 8.896   Å , and c   = 12.59   Å . For the simulations, a supercell of dimensions 10 a   × 6 b   × 2 c was constructed, corresponding to lateral dimensions of approximately 51.75   Å   × 53.38   Å .

2.3. Pyrophyllite

Pyrophyllite, a non-swelling 2:1-type clay mineral, consists of octahedral aluminum sheets sandwiched between layers of tetrahedral silica sheets, with the chemical formula Al 4 Si 8 O 20 OH 4 [25]. In this study, the 1Tc polytype of pyrophyllite was selected due to its smaller unit cell size, which reduces computational cost while maintaining structural accuracy, as illustrated in Figure 1. The unit cell dimensions are a = 5.161 Å, b = 8.941 Å, c = 9.345 Å, with angles α = 90 ° , β = 100.3 ° , and γ = 90 ° [26]. A supercell measuring 10 a × 6 b × 2 c (approximately 51.61   Å   × 53.65   Å   × 18.69   Å ) was constructed to simulate water behavior within the confined interlayer space.
In natural geological environments, kaolinite and pyrophyllite are neutral clay minerals with no significant interlayer charge or cation exchange capacity. Despite their neutrality, they often exist in electrolyte-rich conditions where external ions influence interlayer water dynamics and structural behavior. For montmorillonite, elevated temperatures (e.g., above 90 °C) in such conditions may lead to flocculation, alter the interlayer structural properties, and affect the water extraction processes. However, this study assumed that montmorillonite remains in a stable, non-flocculated state to isolate molecular-scale behavior of interlayer water and focused on the effects of temperature. While simplifying the model, this approach excludes aggregate-level phenomena such as flocculation, which could be explored in future studies. Although our simulations focused on molecular-scale interactions and assumed montmorillonite to be stable and non-flocculated, it is worth noting that flocculation may occur at elevated temperatures under electrolyte-rich conditions. Experimental studies have indicated that flocculation can significantly impact water dynamics and extraction processes at the interlayer scale [27,28,29]. Although this phenomenon is beyond the scope of the current work, our observations of density profile broadening and hydrogen bond weakening provide insights into the molecular precursors to water extraction, which may be influenced by flocculation under more complex conditions.

2.4. Water

In this study, we employed the SPC/E water model [30], which is widely recognized for its ability to accurately capture the structural and dynamical properties of water, including its diffusion coefficient and dielectric constant [31]. This model has been extensively validated and successfully applied to investigations of hydrogen bond dynamics in bulk water, making it particularly suitable for our research. Given the focus on understanding the influence of clay surfaces on the structure and dynamics of hydrogen bonds, the SPC/E model provides a reliable framework. In this model, the water molecules are treated as rigid, with a fixed O–H bond length of 1.0   Å and a H–O–H bond angle of 109.47 ° . To simplify the system and enhance computational efficiency, the clay layers are modeled as rigid structures while ensuring consistent interactions with the confined water molecules.
Interactions among clay atoms, water molecules, and ions are modeled using a combination of pairwise potentials. The total interaction energy includes a Coulomb term representing electrostatic interactions and a Lennard–Jones potential for short-range repulsion and long-range dispersion forces. The interaction energy V i j between two atoms i and j is expressed as [32,33]:
V i j = q i q j 4 π ϵ 0 r i j + 4 ϵ i j σ i j r i j 12 − σ i j r i j 6
where q i and q j are the atomic charges; r i j is the distance between atoms i and j ; ϵ 0 is the permittivity of free space; σ i j is the characteristic distance parameter; and ϵ i j is the depth of the potential well. The Lennard–Jones parameters σ i j and ϵ i j are computed using the Lorentz–Berthelot mixing rules.
The unit cells of the clay minerals were expanded into supercells to construct simulation boxes that are sufficiently large to capture the essential interactions between water molecules and the clay mineral surfaces. The lateral dimensions of these supercells were carefully selected to minimize periodic boundary effects while ensuring computational efficiency. Water molecules were subsequently introduced into the interlayer space using a Monte Carlo placement algorithm, achieving a uniform distribution that reflects realistic hydration conditions. All systems underwent energy minimization after constructing the initial configurations to eliminate steric clashes and achieve a stable initial configuration. This process was conducted using the steepest descent algorithm, and the minimization continued until the total energy converged to a predefined tolerance. Following energy minimization, the systems were equilibrated in the NVT ensemble at a specified temperature for 5 ns to stabilize the thermal conditions. During this stage, periodic boundary conditions were applied in all directions, and the system temperature was controlled using the Nosé–Hoover–Langevin thermostat, with a relaxation time of 0.1 ps. The PPPM method efficiently calculated electrostatic interactions, with an accuracy threshold of 10−6.
The MSD is a fundamental metric for quantifying the diffusion and mobility of particles over time and provides critical insights into molecular dynamics within confined systems. In molecular dynamics simulations, MSD is particularly effective in analyzing the dynamic behavior of water molecules and their interactions with surrounding environments. In this study, MSD was calculated as the time-dependent displacement of water molecules, averaged over all particles in the system. This approach identifies diffusion regimes and the effects of confinement imposed by clay mineral interlayers. The MSD is defined as [34]:
  M S D = 〈 r j t − r j 0 2 〉
where r j t represents the position of a particle j at time t , and r j 0 is its initial position. The angular brackets indicate an ensemble average over all particles. For this work, the MSD was analyzed over a simulation timescale sufficient to capture both short-term molecular motion and long-term diffusive behavior. The plateau in the MSD curve corresponds to the onset of molecular caging, a key feature observed in confined systems.
All MD simulations were performed using the LAMMPS software [35] in the canonical (NVT) ensemble and visualized with VMD [36]. The simulations were performed across a temperature range from 298.15 K to 363.15 K, with a time step of 1 fs. The total runtime for each simulation was 40 ns, with 10 ns for equilibration and 30 ns for data collection and analysis.
Previous studies frequently focus on a single mineral or narrow temperature ranges, which constrains the broader understanding of temperature effects on clay–water systems. Table 1 summarizes these studies, including the specific minerals investigated and the temperature ranges explored.

3. Results

3.1. Temperature-Dependent Diffusion Behavior

We first investigated the temperature-dependent dynamics of water confined within clay mineral interlayers. By examining the MSD trends across a temperature range of 298.15 K to 363.15 K for kaolinite, montmorillonite, and pyrophyllite, we aimed to elucidate how the structural and chemical properties of these clay minerals influence water diffusion. This analysis serves as the foundation for exploring the relationship among temperature, clay type, and the dynamical properties of confined water.
Figure 2 illustrates the temperature-dependent diffusion behavior of water molecules confined within kaolinite, montmorillonite, and pyrophyllite interlayers. The MSD increased over time for all systems, reflecting continuous water diffusion. It exhibited a strong positive correlation with temperature, as higher thermal energy enhanced water mobility. Among the three clay minerals, montmorillonite showed the highest MSD values at all temperatures, reflecting its dynamic interlayer environment. This behavior can be attributed to montmorillonite’s swelling nature and relatively weak interlayer interactions.
In contrast, kaolinite demonstrated the lowest water mobility due to its tightly packed interlayer structure and strong hydrogen bonding, which restricted molecular motion. Pyrophyllite exhibited intermediate diffusion behavior, and its MSD values exceeded that of kaolinite but remained below that of montmorillonite, which was consistent with its moderate interlayer interactions and non-swelling characteristics. The sharp increase in MSD at higher temperatures, particularly at 333.15 K and 363.15 K, underscored the significant impact of thermal energy on water diffusion, especially in montmorillonite [3,40].
As the temperature increased to 318.15 K and 368.15 K, the differences in MSD among the three clay minerals diminished, indicating a convergence in water mobility. This convergence was particularly notable for pyrophyllite, which exhibited MSD values comparable to those of montmorillonite at higher temperatures. While montmorillonite’s significantly higher cation exchange capacity (CEC) and specific surface area generally facilitated enhanced water mobility, the observed convergence indicated the effects of increased thermal energy, which reduced the structural differences between the clays. Additionally, the rigid layer assumption for montmorillonite may underestimate its dynamic interlayer behavior at elevated temperatures, potentially affecting the observed MSD trends. It is important to note that montmorillonite’s high specific surface area and CEC, which are temperature-independent properties, generally promoted enhanced water mobility across all temperatures. However, the convergence observed at elevated temperatures highlighted the role of thermal agitation in overcoming structural constraints. Future studies could explore the effects of more flexible interlayer representations and higher cation concentrations to better capture montmorillonite’s unique properties. This trend is reflected in the charge density maps, where the distributions become increasingly diffuse across all clay systems. Montmorillonite, which initially exhibits the lowest mobility, underwent the most significant change in charge density, transitioning to a more uniform and widespread distribution. In contrast, kaolinite retained localized regions of higher charge density but showed a notable reduction in structural organization. Pyrophyllite, exhibiting intermediate behavior, balanced localized ordering and thermal dispersion, reflecting its moderate interlayer interactions [41,42].
To further explore the temperature-dependent differences in water diffusion across the interlayers of kaolinite, montmorillonite, and pyrophyllite, we analyzed the MSD of water molecules at three representative temperatures, as illustrated in Figure 3. At 298.15 K (Figure 3a), the MSD of water molecules in montmorillonite was the lowest among the three clay minerals, indicating the strongest confinement effect at this temperature. This behavior underscored montmorillonite’s ability to tightly bind water molecules within its interlayer at low thermal energies [41]. In contrast, kaolinite and pyrophyllite exhibited slightly higher MSD values, suggesting a weaker confinement effect compared to montmorillonite under these conditions.
As the temperature rose to 313.15 K, water diffusion became more pronounced across all systems due to enhanced thermal motion (Figure 3b), At this temperature, the differences in MSD among the three clay minerals narrowed. Pyrophyllite exhibited relatively high MSD values at elevated temperatures, which could be attributed to the increased mobility of water molecules on its external surfaces. Unlike montmorillonite, pyrophyllite interacted with water primarily through external surfaces due to its low cation exchange capacity (CEC) and specific surface area. While its water content was significantly lower than that of smectites, thermal agitation enhanced the mobility of the limited water present, leading to the observed MSD trends. It is important to note that, unlike the interlayer and external surface interactions observed in montmorillonite, pyrophyllite primarily operated through external surface interactions. While this characteristic limited its overall water retention capacity, the mobility of water molecules on its external surfaces increased significantly at elevated temperatures due to thermal effects. This dynamic behavior may explain the observed MSD trends, even though pyrophyllite has inherently lower water content compared to smectites. In contrast, kaolinite and montmorillonite displayed comparable MSD values, suggesting that the increased thermal energy began to overcome the structural constraints imposed by their interlayer configurations.
When the temperature reached the highest level studied (Figure 3c), the thermal effects dominated, resulting in significant increases in MSD across all three clay minerals. At elevated temperatures, thermal agitation weakened hydrogen bonds and enhanced the mobility of water molecules. While hydrogen bonding is assumed to play a major role in the interlayer space, other forces such as ion–dipole interactions, van der Waals forces, and Coulombic forces may also contribute to the dynamics. These forces, influenced by factors such as pH and ionic strength, could shift the relative importance of each interaction as temperature increases. For montmorillonite, this complex interplay highlights the necessity for further investigations into the contributions of these forces under varying conditions. The role of pH in governing the properties of clay minerals is an important consideration that was not explicitly addressed in this study. Variations in pH can alter surface charge, interlayer ion distribution, and the strength of hydrogen bonds, particularly in montmorillonite, which exhibits high cation exchange capacity. Additionally, while our study assumes hydrogen bonding as a primary interaction, ion–dipole forces, van der Waals forces, and Coulombic forces may also play significant roles in the interlayer space. The relative dominance of these forces is influenced by factors such as temperature, ionic strength, and pH. As temperature increases, the weakening of hydrogen bonds may enhance the prominence of ion–dipole interactions, thereby driving the observed changes in water dynamics. Future research could investigate these effects in systems with varying pH to fully understand the action mechanisms.
Pyrophyllite consistently exhibited the highest MSD values, while the differences between kaolinite and montmorillonite were further reduced. This convergence suggested that, at high temperatures, the structural effects of the clay minerals became less influential, and the dynamics of confined water were primarily governed by thermal motion. In conclusion, the trends highlight how the interplay between temperature and interlayer properties dictates water mobility. Montmorillonite demonstrated the strongest confinement effects at low temperatures, while pyrophyllite allowed for the highest diffusion at elevated temperatures. Kaolinite consistently showed an intermediate behavior, which balanced structural rigidity and thermal adaptability.

3.2. Temperature Effects on Density and Charge Distribution

Figure 4 illustrates the evolution of density profiles of water molecules within the interlayers across different temperatures. At lower temperatures, kaolinite and pyrophyllite exhibited sharp, well-defined peaks near the clay surfaces, as shown in Figure 4a,c. This suggested that water molecules are strongly influenced by spatial confinement provided by the rigid clay layers. Such ordering was particularly evident in systems with strong interlayer interactions, where thermal energy was insufficient to disrupt the water structure [43]. Although pyrophyllite is non-swelling, it exhibited similar sharp density peaks, highlighting its moderate capacity for maintaining ordered water layers. In contrast, montmorillonite displayed broader and less pronounced density peaks under the same conditions, as illustrated in Figure 4b. This phenomenon was attributed to its swelling nature and relatively weaker interlayer interactions, which facilitated greater water mobility and a more diffuse distribution. The broader peaks also reflect montmorillonite’s ability to accommodate water molecules with varying degrees of freedom, resulting in a less rigid structural organization.
As the temperature rose to 313.15 K, thermal motion began to disrupt the structural organization of confined water. In kaolinite and pyrophyllite, the density peaks remained relatively pronounced, though slightly less sharp, indicating that their interlayer environments continued to exert significant confining effects on water molecules [5]. Conversely, montmorillonite exhibited further broadening of density peaks, signifying increased water mobility and a higher degree of structural disorder within its interlayer space.
At higher temperatures (333.15 K and 363.15 K), the sharp density peaks near the surfaces of kaolinite and pyrophyllite diminished significantly, reflecting the disruptive effects of thermal agitation on hydrogen bond networks and the ordering of water molecules. Nevertheless, kaolinite maintained a relatively higher degree of structural organization compared to montmorillonite and pyrophyllite, which could be attributed to its stronger hydrogen bonding and rigid interlayer configuration. In contrast, montmorillonite displayed an almost uniform density profile across the interlayer region at these elevated temperatures, underscoring its adaptability to thermal motion and its capacity to support a more dynamic and less ordered water environment.
In this section, we explored the impact of temperature on the spatial distribution of water molecules within clay interlayers. The charge density maps of kaolinite, montmorillonite, and pyrophyllite were examined, as depicted in Figure 5. At 298.15 K, the charge density distributions were tightly localized in all three clay minerals, which is consistent with the limited molecular motion observed at lower temperatures. Montmorillonite exhibited the most restricted water mobility and a more ordered charge density distribution near the clay surfaces, as observed in Figure 5b. This behavior suggested that montmorillonite initially provided stronger confinement of water molecules, likely due to its specific interlayer characteristics. In contrast, kaolinite (Figure 5a) and pyrophyllite (Figure 5c) demonstrated more distributed charge densities even at this temperature, aligning with their higher MSD values, which suggested relatively less restricted water mobility. Kaolinite’s hydrogen bonding network and pyrophyllite’s moderate interlayer interactions contribute to this observed difference in mobility compared to montmorillonite [11,23].
As the temperature increased to 318.15 K, the differences in water mobility among the clay minerals began to diminish, as indicated by the convergence of MSD curves. The corresponding charge density maps reflected this change, showing that montmorillonite’s initially localized charge density became more dispersed. This transition signified the increasing influence of thermal motion on water molecules, which weakened the confining effects of the clay interlayers [44,45]. While kaolinite and pyrophyllite still exhibited some localized charge regions, they also showed a broader distribution compared to their behavior at lower temperatures. This observation indicated that even for clays with stronger hydrogen bonding or less flexible interlayers, thermal effects progressively disrupt the structural ordering of confined water molecules.
At the highest temperature of 368.15K, thermal motion dominated across all systems, effectively minimizing structural differences among the clays. This finding indicated that temperature, rather than clay-specific interlayer interactions, primarily governs water mobility. In the charge density maps, this is reflected by the highly diffuse distributions in all three systems. Montmorillonite, which initially exhibited the lowest water mobility, now displays a charge density distribution comparable to those of kaolinite and pyrophyllite, emphasizing the loss of structural constraints. However, kaolinite maintained some localized high-density regions near the clay surfaces, indicating a residual confining effect from its strong hydrogen bonding network [3,23]. As expected, pyrophyllite demonstrated an intermediate behavior, balancing structural stability and increased water mobility. These results collectively highlighted the dynamic relationship among temperature, clay type, and the behavior of confined water molecules [43,46]. At lower temperatures, the inherent structural characteristics of the clays dominated, with montmorillonite showing the most vigorous confinement and lowest water mobility. As the temperature increased, thermal agitation reduced the influence of interlayer interactions, leading to a more uniform charge density and convergence in water mobility among the clays. This transition was particularly significant for montmorillonite, where the initially strong confinement effect was almost entirely overridden at higher temperatures.
The temporal evolution of hydrogen bonds highlights distinct differences in stability and average bond numbers among the three clay minerals. As illustrated in Figure 6a, kaolinite maintained a stable number of hydrogen bonds at lower temperatures, indicating strong and consistent hydrogen bonding interactions within its confined interlayer space [23]. However, as the temperature increased, a gradual decline in hydrogen bonds was observed, which was attributed to the weakening of interlayer hydrogen bonding caused by intensified thermal motion.
In contrast, montmorillonite exhibited a higher and more stable hydrogen bond count across the temperature range, as depicted in Figure 6b. This behavior is due to its expansive interlayer space, which accommodates more water molecules and promotes the formation of more significant hydrogen bonds. The system exhibited remarkable structural resilience, with hydrogen bond numbers remaining largely unaffected up to 333.15 K. However, at 363.15 K, a slight reduction in bond number occurred, reflecting the onset of thermal disruption in the hydrogen bonding network.
Pyrophyllite exhibited a markedly lower and more temperature-sensitive hydrogen bond count, as shown in Figure 6c. At lower temperatures, the limited interlayer space and weaker interactions resulted in minimal hydrogen bonds. As thermal motion intensified, the bond count decreased sharply, particularly at 363.15 K, where thermal agitation dominated, leading to near-complete disruption of hydrogen bonding. The overall trend in Figure 6d demonstrated that montmorillonite consistently exhibits the highest hydrogen bond counts across all temperatures, followed by kaolinite and pyrophyllite. These differences underscored the influence of clay mineral structure and interlayer characteristics on hydrogen bonding behavior [40,41]. Montmorillonite’s swelling capacity and larger interlayer space facilitated extensive bonding, while the stronger yet more confined hydrogen bonding interactions in kaolinite resisted thermal disruption. In contrast, pyrophyllite, characterized by its limited capacity for hydrogen bonding, is highly susceptible to thermal effects, highlighting the significant impact of temperature on less constrained systems [47].
The stress distributions within the interlayers of kaolinite, montmorillonite, and pyrophyllite demonstrated the complex interaction among thermal motion, interlayer interactions, and the structural behavior of confined water, as illustrated in Figure 7. At lower temperatures, stress was highly localized near the interlayer surfaces in kaolinite (Figure 7a) and pyrophyllite (Figure 7c), reflecting the structural rigidity that confines water molecules [25]. This behavior is consistent with the limited water mobility observed in these clays, the pronounced charge density peaks, and stable hydrogen bonding networks. In contrast, montmorillonite exhibited a more uniform stress profile at the same temperature, highlighting its weaker interlayer constraints and the greater flexibility of its interlayer environment (Figure 7b).
As temperature increased, thermal motion disrupted the structural order of the interlayer region. This disruption was most evident in montmorillonite, where stress patterns became increasingly diffuse, indicating enhanced water mobility and reduced structural organization [32]. The decline in hydrogen bond counts further supported this observation, demonstrating that montmorillonite’s flexible interlayer environment was more susceptible to thermal effects. Pyrophyllite, due to its moderate structural rigidity, partially lost localized stress regions, thereby balancing structural order and adaptability. Despite a reduction in stress localization, kaolinite retained distinct areas of higher stress due to its robust hydrogen-bonding network, which imposed significant constraints on water molecules.
At higher temperatures, thermal agitation dominated, leading to dramatic changes in stress distribution across all clay minerals. Montmorillonite achieved a highly uniform stress profile, reflecting its capacity to accommodate increased water mobility and its inherent adaptability to thermal conditions. This uniformity aligned with the significant reduction in its hydrogen bonding network and the high mobility of water molecules, as indicated by the MSD trends. While pyrophyllite exhibited a significant increase in stress uniformity, it maintained some localized stress regions, demonstrating its ability to balance thermal adaptability and structural resistance. In contrast, kaolinite retained the most distinct localized stress regions, underscoring its structural rigidity and strong hydrogen bonding interactions, which can withstand complete thermal disruption.

4. Discussion

This study provides valuable insights into the temperature-dependent behavior of water confined within the interlayers of kaolinite, montmorillonite, and pyrophyllite. By analyzing structural, dynamic, and mechanical properties, we emphasize the critical role of clay type and interlayer characteristics in governing water behavior. These findings complement previous research and deepen the understanding of the interplay among temperature, clay structure, and confined water dynamics.
As indicated by the MSD, water mobility reveals notable differences among the three clay minerals. Montmorillonite, characterized by its flexible and expansive interlayer, exhibited the highest water mobility across all temperatures, which is consistent with its swelling behavior and weak interlayer interactions [9,48]. In contrast, kaolinite displayed the lowest water mobility, reflecting its tightly confined interlayer structure and strong hydrogen bonding, which is consistent with earlier studies on 1:1 clay mineral. Pyrophyllite, characterized by its non-swelling properties, demonstrated intermediate behavior, underscoring the importance of interlayer interactions in controlling water dynamics.
Charge density maps and hydrogen bond analysis further highlight the impact of thermal motion on the structural organization of confined water. At lower temperatures, kaolinite and pyrophyllite maintained well-defined hydrogen bonding networks, while montmorillonite exhibited more diffuse interactions due to its weaker confining forces. As the temperature increased, hydrogen bond strength diminished across all systems, with montmorillonite experiencing the most significant reduction. This finding reinforces the hypothesis that clay minerals with stronger interlayer interactions, such as kaolinite, offer more excellent resistance to thermal disruption. Conversely, montmorillonite’s weaker interactions facilitate a more rapid adaptation to increasing temperatures [33].
Stress distributions provide additional insights into the influence of clay type on water behavior. At lower temperatures, localized stress patterns in kaolinite and pyrophyllite reflect their structural rigidity, whereas montmorillonite exhibits a more uniform stress distribution, indicating its flexible interlayer environment. As the temperature increases, stress diffusion becomes evident across all systems, with montmorillonite displaying the most pronounced changes. These observations underscore the mechanical implications of thermal effects on confined water, offering a mechanistic basis for understanding the thermal stability of clay–water systems [40,49,50]. Pyrophyllite is highly susceptible to thermal effects, as reflected in the observed trends of water mobility and structural dynamics. Specifically, at elevated temperatures, the mobility of water molecules interacting with pyrophyllite increases significantly compared to kaolinite and montmorillonite. This is evidenced by the higher MSD values for pyrophyllite at higher temperatures, as shown in Figure 4. Unlike montmorillonite, where interlayer water is constrained, and kaolinite, which has weaker surface–water interactions, pyrophyllite allows water to diffuse relatively unrestrictedly along its external surfaces.
Additionally, the density profiles presented in Figure 5 revealed a broader distribution of water molecules near pyrophyllite surfaces at elevated temperatures. This broadening indicates reduced structural ordering and increased thermal motion, further supporting the conclusion that pyrophyllite’s water dynamics are highly sensitive to thermal effects. These findings align with the inherent characteristics of pyrophyllite, such as its lower cation exchange capacity and lack of interlayer water, which make it more responsive to thermal agitation.
In a broader context, the findings of this study hold significant implications for fields such as environmental engineering, geoscience, and materials science. The ability of montmorillonite to retain dynamic water at high temperatures makes it a promising candidate for efficient water transport applications. In contrast, the thermal stability of kaolinite makes it particularly suitable for scenarios necessitating water immobilization under extreme conditions.

5. Conclusions

This study systematically investigated the temperature-dependent structural, dynamic, and mechanical properties of water confined within kaolinite, montmorillonite, and pyrophyllite interlayers using molecular dynamics simulations. The results underscored the pivotal role of clay mineral type in determining the behavior of confined water. Montmorillonite exhibited the highest water mobility due to its weak interlayer interactions, while kaolinite displayed the strongest structural confinement. Pyrophyllite, characterized by its non-swelling nature, demonstrated intermediate behavior, balancing mobility and stability.
Analysis of hydrogen bonding and stress distribution revealed distinct thermal responses among the clays, which are determined by their unique structural and interlayer properties. Montmorillonite readily adapted to thermal motion, kaolinite maintained significant structural stability, and pyrophyllite exhibited a transitional response, reflecting its moderate interlayer interactions.
These findings provide valuable insights into clay–water interactions and their implications for environmental engineering, geoscience, and materials science. Additionally, they establish a foundation for future investigations into more complex conditions, such as the influence of pressure, ionic species, or multi-component systems.

Author Contributions

Project administration, T.Y. and C.C.; data curation, T.Y.; investigation, T.Y. and Z.Z.; writing—original draft, T.Y.; writing—review and editing, T.Y. and C.C.; submission suggestions, Y.Z. and J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Xinjiang Uygur Autonomous Region (2021D01C043).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The author would like to thank the other authors for their very helpful support in the writing of this research.

Conflicts of Interest

Yonggang Zhang was employed by China Construction Eighth Engineering Division Corp., Ltd. Junli Wan was employed by China Academy of Railway Sciences Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic representation of simulation models of kaolinite, montmorillonite, and pyrophyllite systems with confined water.
Figure 1. Schematic representation of simulation models of kaolinite, montmorillonite, and pyrophyllite systems with confined water.
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Figure 2. MSD of water molecules confined in (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite interlayers at different temperatures.
Figure 2. MSD of water molecules confined in (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite interlayers at different temperatures.
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Figure 3. Comparison of MDS of water molecules confined in kaolinite, montmorillonite, and pyrophyllite interlayers at (a) 298.15 K; (b) 313.15 K; and (c) 363.15 K.
Figure 3. Comparison of MDS of water molecules confined in kaolinite, montmorillonite, and pyrophyllite interlayers at (a) 298.15 K; (b) 313.15 K; and (c) 363.15 K.
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Figure 4. Density profiles of water molecules confined in the interlayers of (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite at different temperatures (298.15 K, 303.15 K, 313.15 K, 333.15 K, and 363.15 K) along the normalized z / l z direction.
Figure 4. Density profiles of water molecules confined in the interlayers of (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite at different temperatures (298.15 K, 303.15 K, 313.15 K, 333.15 K, and 363.15 K) along the normalized z / l z direction.
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Figure 5. Charge density maps of water molecules confined in the interlayers of (a) kaolinite, (b) montmorillonite, and (c) pyrophyllite at three temperatures (298.15 K, 318.15 K, and 368.15 K) in the X−Z plane.
Figure 5. Charge density maps of water molecules confined in the interlayers of (a) kaolinite, (b) montmorillonite, and (c) pyrophyllite at three temperatures (298.15 K, 318.15 K, and 368.15 K) in the X−Z plane.
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Figure 6. Temporal evolution and average number of hydrogen bonds formed by water molecules confined in (a) kaolinite, (b) montmorillonite, and (c) pyrophyllite interlayers at various temperatures. (d) The variation of the average hydrogen bond number with temperature for the three clay systems.
Figure 6. Temporal evolution and average number of hydrogen bonds formed by water molecules confined in (a) kaolinite, (b) montmorillonite, and (c) pyrophyllite interlayers at various temperatures. (d) The variation of the average hydrogen bond number with temperature for the three clay systems.
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Figure 7. Stress distributions of water molecules confined within the interlayers of (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite at different temperatures. The color scale represents stress magnitudes, with red indicating compressive stress and blue indicating tensile stress.
Figure 7. Stress distributions of water molecules confined within the interlayers of (a) kaolinite; (b) montmorillonite; and (c) pyrophyllite at different temperatures. The color scale represents stress magnitudes, with red indicating compressive stress and blue indicating tensile stress.
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Table 1. Overview of previous studies on clay minerals and temperature ranges.
Table 1. Overview of previous studies on clay minerals and temperature ranges.
Study ReferenceMineral TypeTemperature Range (K)
Mohammed and Nigussa, 2023 [37]Fluorohectorite293–350
Zhang and Song, 2021 [38]Pyrophyllite, Montmorillonite298–350
Chen et al., 2024 [20]Na-Montmorillonite298–500
Tang and Cui, 2007 [39]MX80 Clay293–333
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Yang, T.; Chu, C.; Zhang, Y.; Zhang, Z.; Wan, J. Molecular Dynamics Simulation of Clay Mineral–Water Interfaces: Temperature-Dependent Structural, Dynamical, and Mechanical Properties. Water 2025, 17, 347. https://doi.org/10.3390/w17030347

AMA Style

Yang T, Chu C, Zhang Y, Zhang Z, Wan J. Molecular Dynamics Simulation of Clay Mineral–Water Interfaces: Temperature-Dependent Structural, Dynamical, and Mechanical Properties. Water. 2025; 17(3):347. https://doi.org/10.3390/w17030347

Chicago/Turabian Style

Yang, Tong, Chunmei Chu, Yonggang Zhang, Zhen Zhang, and Junli Wan. 2025. "Molecular Dynamics Simulation of Clay Mineral–Water Interfaces: Temperature-Dependent Structural, Dynamical, and Mechanical Properties" Water 17, no. 3: 347. https://doi.org/10.3390/w17030347

APA Style

Yang, T., Chu, C., Zhang, Y., Zhang, Z., & Wan, J. (2025). Molecular Dynamics Simulation of Clay Mineral–Water Interfaces: Temperature-Dependent Structural, Dynamical, and Mechanical Properties. Water, 17(3), 347. https://doi.org/10.3390/w17030347

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