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Abstract

In the present study, we performed a molecular dynamics simulation of the intercalation of poly(N-isopropyl acrylamide) (NIPAM)₃ and poly(N-vinyl caprolactam) (NVCL)₃ trimers into Na-montmorillonite (Na-Mt) to evaluate their effects on the interlayer structure and the stability of hydrated Na-Mt. The impact of both trimers on the interlayer species and their dynamics properties at different temperatures in a canonical ensemble (NVT) were investigated. The results showed that the electrostatic forces exerted by Na cations on H₂O molecules and the interlayer H₂O molecular arrangement are not affected by the rise in temperature after adding both trimers. Trimer addition reinforced the structure of interlayer H₂O molecules so that the effect of temperature increase on them became negligible. The structural dynamics evolution of the radius of gyration of both trimers showed the existence of conformation changes when temperature increased. These conformational changes are more complex in the case of (NVCL)₃ than (NIPAM)₃ due to its large monomers. Both trimers reduced the mobility of interlayer particles with a better inhibition effect obtained for (NVCL)₃ compared to (NIPAM)₃. The concentration profile of interlayers’ species showed the affinity of Na cations for clay mineral surfaces while H₂O molecules moved away. Compared these two trimers, the most stable state of Na-Mt is achieved with (NVCL)₃. These results could help highlight the inhibition properties of (NIPAM)₃ and (NVCL)₃ on hydrated Na-Mt and to predict its stability against changes in environmental conditions.

Keywords

Na-Mt trimer radial distribution function self-diffusion coefficient radius of gyration concentration profile

Introduction

Polymer–clay nanocomposites (PCNs) are materials composed of a polymer matrix and nanometer-size clay particles. Their production is an active area of research that benefits from the high surface area ratio of nanoclay particles to great improvement of chemical and physical properties of the polymer matrix.^1
-3 Through nanocomposite formulations, an increase of the elastic moduli, better mechanical properties, improve thermal stabilities, desirable barrier properties, and an increase of biodegradability can be achieved.^4,5 These features make them future candidates for applications in oil and gas production such as drilling fluids and flow assurance.⁶ The goal of the preparation of nanocomposites is to achieve a perfect exfoliation of the aggregates of layered silicates which are not easy to be achieved.³ The discovery of various clays and their wide use in many applications resulted in continuous developments in polymer sciences and nanotechnology. Important progress was made through the intercalation of guest organic molecules such as polymers in the interlayer space of clay mineral to obtain new nanostructure composites with desirable properties to meet the expansion of industrial and economic activities.^7,8 PCN is synthesized using in situ polymerization, intercalation of the polymer from a solution, melts intercalation of the polymer, and sol–gel technique.⁷ Poly(N-isopropyl acrylamide) (NIPAM) and poly(N-vinyl caprolactam) (NVCL) are two well-known “smart polymers” belonging to the group of thermosensitive polymers. In aqueous solution, they are sensitive to the environmental conditions (pH, temperature, ionic strength, and electric field, and so on)^9,10 and to chemical changes such as concentration changes, molecular weight, hydrophobic and hydrophilic components, and salt concentration.^11

-16 PNIPAM and PNVCL have different structures of the monomeric units but the same reversible low critical solution temperature (LCST) near 32°C. Thermosensitive polymers, which become insoluble or soluble upon heating, possess an LCST or an upper critical solution temperature, respectively.^9,11,17 Recently, thermoresponsive composite materials were synthesized by incorporating montmorillonite (Mt) into PNIPAM gels to improve their mechanical properties.¹⁸ Autieri et al.¹⁹ performed molecular dynamics (MD) and metadynamics simulation to investigate the conformational feature, structure, and dynamics properties of PNIPAM trimer in aqueous solution. They found that the experimentally observed lower hydrophilicity of isotactic PNIPAM in comparison with the syndiotactic one is related to the conformational entropy. Katsumoto et al.²⁰ investigated PNIPAM dimer model compounds with computational and experimental methods to understand the effect of tacticity on the polymer hydrophilicity in H₂O. Their results showed that the dimer model (DNIPA) compounds with the racemo configuration (r-DNIPA) is more soluble in H₂O than DNIPA with the meso configuration (m-DNIPA). Tucker and Stevens²¹ studied the effect of PNIPAM length on the LCST in H₂O using MD simulations. Their results showed that no coil-to-globule transition for shorter oligomers less than 8 repeating units occurred but the conformation transitions were observed for polymers with more than 11 repeating units. The coil-to-globule behavior of poly{γ-2-[2-(2methoxyethoxy)ethoxy]ethoxy-3-caprolactone} (PMEEECL) was investigated by means of MD simulation in H₂O.²² Results showed that the LCST of PMEEECL occurred at 320 K, in a good agreement with previous experimental results. Deshmukh et al.²³ compared the effect of temperature on the hydration and conformational transition of PNIPAM, poly(acrylamide) (PAA), and poly(ethylene glycol) (PEG). They observed that PAA and PEG do not exhibit a coil-to-globule transition due to existing strong H-bonds of their hydrophilic groups. MD simulations were used to investigate the effect of stereochemistry on the LCST.²⁴ De Oliveira et al.²⁵ used MD simulations to study the LCST of different molecular architectures of PNIPAM-based (co)polymers. They found that the collapse of PNIPAM chain upon heating is dependent on the hydration structure around the monomers, the tacticity, and the presence of more hydrophilic acrylamide monomers. Several experimental studies have synthesized and characterized PNVCL for subsequent use as nanofiller, nanocomposite hydrogels, biocompatible thermosensitive PNVCL-clay nanocomposite hydrogels, and drug delivery.^26

-29 Here, we extended the analysis with a molecular approach to the (NIPAM)₃ and (NVCL)₃ to relate the effect of trimers on the stability of hydrated Na-Mt. Mt is widely used in polymer nanocomposites because of its easy availability, well-known intercalation/exfoliation chemistry, high surface area, and high surface reactivity. The swelling properties of clay minerals are influenced by several factors such as thermodynamic variables (temperature, external pressure, H₂O chemical potential, and osmotic pressure) and structural variables (layer charge, charge location, and interlayer cations).³⁰ In contact with H₂O and polar solvent, Na-Mt has a high ability to swell and exhibits two different regimes which are crystalline and osmotic swellings.³⁰ Clay swelling benefits many commercial applications including drilling mud, catalyst in petroleum engineering, PCNs, and landfill lines in environmental engineering.³¹ Many properties of polymer-layered clay have been investigated at macroscopic and microscopic levels, while there still remain significant questions regarding their structure and effect on the interlayer of clay minerals.^{7,18,26,32

-35} Despite their contributions to our understanding of PCN and shale inhibitors, there are relatively few comparable investigations involving interaction between Na-Mt, H₂O molecules, and trimers. Understanding the impact of (NVCL)₃ and (NIPAM)₃ addition on hydrated Na-Mt can elucidate the mechanisms underlining the interaction between the small thermosensitive oligomers and clay mineral for the improvement of drilling fluids performance in the hostile environments and assist the design of shale inhibitor and materials with improved properties. Therefore, in this study, we used MD simulations to analyze the interlayer structure and dynamics properties of Na-Mt with (NIPAM)₃ and (NVCL)₃ intercalation. The effect of both trimers’ addition on the structure of Na-Mt, the interlayer species structures, and dynamics properties is investigated. This study is the first approach to a simulation activity that compares the inhibition effect of (NIPAM)₃ and (NVCL)₃ into hydrated Na-Mt and their effects on its stability.

Model setup

(NIPAM)₃ and (NVCL)₃ models

The molecular architecture of (NIPAM)₃ and (NVCL)₃ consists of a hydrophobic backbone and hydrophilic amide group capped by hydrophobic isopropyl and lactam moieties, as shown in Figure 1. The initial isotactic conformation of the oligomers with a polymerization degree of 3 was constructed using the “Build polymer” module in Material Studio 7.0 software and optimized using the Smart minimizer method.

Figure 1.

Snapshot of (NIPAM)₃ (a) and (NVCL)₃ (b) trimers; the backbone chain is shown in purple, the O atom in red, H atom in white, and N atom in blue.

Na-Mt model

The clay mineral used in this study was taken from the model derived by skipper and coworkers.³⁶ The unit cell formula is as follows:

{Na}_{0.75} [{Si}_{7.75} {Al}_{0.25}] ({Al}_{3.25} {Mg}_{0.75}) O_{20} {(OH)}_{4}

Na-Mt model was constructed, and the atomic coordinates were derived from the space group of monoclinic C2/m with a = 5.23 Å, b = 9.06 Å, c = 9.60 Å (with no H₂O in interlayer spaces), c value changed with H₂O molecule content between the layers, α = 90°, β = 99.00°, γ = 90°, and the symmetrical L2PC structure. The clay mineral layers were considered as rigid molecules and mirror image. The atomic charges assigned to each atom sites in the clay were given by Smith.³⁷ Based on the primitive unit cell, the supercell of the model (8a × 4b × 1c) was built (Figure 2). The measure of the three-dimensional, periodic boundary conditions applied to the system was 41.84 × 36.24 Å², which consists of 1280 atoms in the clay mineral layers equivalent to 32 unit cells and 24 atoms of sodium. Based on this formula, our clay sheets with 32 unit cells have 16 isomorphic substitutions of Al by Mg ion in the octahedral sheets, 8 isomorphic substitutions of Si by Al ion in the tetrahedral sheets, and 24 compensating Na cations in the interlayer region.

Figure 2.

A snapshot of Na-Mt 32 unit cells, tetrahedral Si (yellow), O (red), H (white), octahedral Al (pink), octahedral Mg (green), interlayer Na cations (purple).

Computational details

MD simulations were performed under the module “Forcite” of Materials Studio 7.0 software to investigate the effect of (NIPAM)₃ and (NVCL)₃ addition on the hydration process of Na-Mt at temperature changing from 300, 305, 310, 325 to 335 K. The interaction between the atoms was described by Universal Force Field (UFF) which is a wider applicable force field, and its parameters spread all over the entire element of the periodic table.^38
-40 Metal–oxygen interactions were based on a Lennard-Jones potential combined with electrostatics. For H₂O and trimer behavior, the SPC/E model of Berendsen et al.⁴¹ and CVFF force fields were used, respectively. The simulation was performed for two systems (with and without trimer addition) in the canonical ensemble (NVT) in which the volume was constant and temperature changing. In order to analyze the complex system which combines the effect of the confinement (due to the clay) and the effect of interaction (due to the trimers), we built another system (water box) with pure H₂O, containing 24 Na cations and trimers (without Na-Mt) in the similar simulation conditions for comparison (Figure 3). We chose to output the atomic positions every 1.0 fs during production runs, the total simulation time was set to 2000 ps, the number of steps was 2,000,000, and the frame output was obtained every 5000 steps. The short-range van der Waals forces were applied with a 15.5 and 12 Å cutoff distance for Na-Mt and the water box, respectively. To calculate the long-range electrostatics, the Ewald summation method was used. The total potential energy was given by the summation of all interaction sites of the system:

U = \sum_{i} \sum_{j} (\frac{q_{i} q_{j}}{r_{i j}} + 4 ε_{i j} [{(\frac{σ_{i j}}{r_{i j}})}^{12} - {(\frac{σ_{i j}}{r_{i j}})}^{6}])

Figure 3.

Snapshot of Na-Mt and water box with H2O molecules (red dotted lines), Na cations (purple), (NVCL)₃ and (NIPAM)₃ trimers are in the center of Na-Mt and the cubic box; (a) and (c) with (NIPAM)₃ and (b) and (d) with (NVCL)₃.

where q_i and q_j are the charges on atoms $i$ and $j$ , respectively; $r_{i j}$ is the distance between atoms i and $j$ ; and $ε_{i j}$ and $σ_{i j}$ are Lennard-Jones parameters deduced from Lorentz–Berthelot rule. All the interaction terms of Lennard-Jones are from a combination of these relationships³⁷:

σ_{i j} = 1 / 2 (σ_{i} + σ_{j})

ε_{i j} = \sqrt{ε_{i} ε_{j}}

The potential parameters used for oxygen and hydrogen were those used in H₂O.⁴² The method of velocity initialization used for the dynamics calculation was Random. The H₂O contents considered in this study were 800 H₂O molecules for both Na-Mt and the water box. The initial basal spacing was 40 Å for Na-Mt and the size of the cubic box was 30 Å (Figure 3). From the output data obtained, the radial distribution functions (RDFs), radius of gyration, concentration profile, coordination number, hydration radius, and diffusion coefficient profiles were calculated.

Trajectories analysis

The RDF of H₂O molecules around the interlayer cations was calculated as follows⁴³:

g (r) = 1 / (4 π ρ r^2) d N / d r

where $ρ$ is the number density of H₂O molecules and $d N$ is the average number of the particle of H₂O molecules lying at the region of $r + d r$ from a cation. N is the coordination number of H₂O molecules around the cation. The self-diffusion coefficient D of the interlayer species was calculated with a three-dimensional Einstein relation^38,40:

D = 1 / 6 d / d t < r {(t)}^{2} >

where $< r {(t)}^{2} >$ is the mean square displacement of the particle defined by $r (t) = R (t) - R (0)$ , with $R (t)$ is the particle position at time t and $R (0)$ is the initial particle position. The radius of gyration was calculated using the commonly used parameter R_g to describe the size of the flexible molecular chain by the following equation^44,45:

R_{g} = {(\sum_{i} m_{i} {|r_{i}|}^{2} / \sum_{i} m_{i})}^{1 / 2}

where m_i is the mass of atom i and r_i is the position of atom i with respect to the center of mass of the molecule.

The ion hydration number, ion coordination number, and hydration radius were defined as follows^46,47:

n_{i j} = 4 π ρ_{j} \int_{0}^{r} r^{2} g_{i j} (r) d r

n_{hyd} = n_{i j} (r) h

\frac{4}{3} π r_{hyd}^{3} = ϑ n_{hyd} + \frac{4}{3} π r_{eff}^{3}

$n_{hyd}$ is the ion hydration number, $n_{i j} (r)$ is the ion coordination number, $ρ$ is the density, $g_{i j} (r)$ is the RDF and r is the distance, h is the ion hydration factor, which is 0.69 for Na cation, $r_{hyd}$ is the ion hydration radius, $ϑ = 2.99 \times 10^{- 29} m^{3}$ is the volume of H₂O molecules, and $r_{eff}$ is the effective radius, defined as the difference between the effective radius of the first peak of the RDF of H₂O, where the effective radius of H₂O molecule is 0.138 nm.

Results and discussion

Interaction between Na cations and H₂O molecules after trimers addition

The effect of (NIPAM)₃ and (NVCL)₃ addition on the interaction between Na cations and interlayer H₂O molecules was investigated by analyzing the RDF between Na cations and H₂O molecules’ oxygen ( $O_{w}$ ), their coordination number, and hydration radius. The results showed that the attractive forces exerted by Na cations on H₂O molecules remained unchanged into Na-Mt (Figure 4). This effect was observed from the RDF curves of ${Na-O}_{w}$ , which were barely distinguishable between the system with and without (NIPAM)₃ and (NVCL)₃. The highest peak of RDF curves appears near 2.25 Å. At this position, the structure of H₂O molecules around Na cations was almost the same for both systems with and without trimers addition. These results indicate that trimers addition had great effects on the ability of Na cations to be hydrated between the interlayer by keeping the structure of H₂O molecules surrounded Na cations as in their initial state. Their presence between the interlayer space minimized the electrostatic forces exerted by Na cations on H₂O molecules. Their hydrophilic regions aided the binding of Na cations to clay surfaces, minimized their hydration by forming strong H-bond with H₂O molecules and H-bond network around the oligomer compounds. Most of Na cations diffused in the inner- and outer-sphere complexes where they were attracted by the negatively charged clay mineral surface and remained encaged by almost the same amount of H₂O molecules in their hydration shell as in their initial state. Upon the introduction of both trimers, Na cations hydrated to a lesser extent than in the system without trimers addition. To confirm these results, we compared and contrasted the hydration parameters of Na cations in pure H₂O (water box) containing both trimers and those obtained into Na-Mt (Table 1). The hydration parameters analysis showed that the coordination number and hydration radius of Na cations after (NIPAM)₃ and (NVCL)₃ addition are approximately the same into Na-Mt and decrease with the rise in temperature in the water box. This decrease was due to the weakening of the electrostatic forces exerted by Na cations on H₂O molecules when the temperature increased, indicating that both trimers had little or no impact on the structural arrangement of H₂O molecules around Na cations into Na-Mt. From these results, it can be concluded that the structural arrangement of H₂O molecules surrounded Na cations remained intact after adding (NIPAM)₃ and (NVCL)₃ into hydrated Na-Mt. Figure 5 compares the RDF curves of ${Na-O}_{w}$ with the change in temperature after adding (NIPAM)₃ and (NVCL)₃ into Na-Mt. It can be observed that both trimers exhibit almost the same effect on the coordination shell of Na cations when temperature increased. The rise in temperature had no significant effect on the electrostatic forces exerted by Na cations on H₂O molecules. Moreover, the hydration shells of Na cations were not affected by the rise in temperature into Na-Mt compared to those in the water box (Table 1).

Figure 4.

RDF curves of Na-O_w before and after (NIPAM)₃ and (NVCL)₃ addition at different temperatures into Na-Mt.

Figure 5.

RDF curves comparative chart of Na-O_w after (NIPAM)₃ and (NVCL)₃ addition at different temperatures into Na-Mt.

Table 1.

Hydration parameters of interlayer Na cations before and after trimers addition.

	Temperature (K)	Coordination number			Hydration radius (Å)
	Temperature (K)	No trimer	With (NIPAM)₃	With (NVCL)₃	No trimer	With (NIPAM)₃	With (NVCL)₃
	300	3.877	3.894	3.886	2.715	2.718	2.717
	305	3.874	3.891	3.885	2.714	2.718	2.716
Na-Mt	315	3.862	3.885	3.876	2.712	2.717	2.715
	325	3.860	3.881	3.861	2.711	2.716	2.697
	335	3.852	3.879	3.799	2.709	2.715	2.711
	300	2.115	2.242	2.378	2.549	2.589	2.633
	305	2.114	2.179	2.369	2.547	2.571	2.632
Water box	315	2.084	2.169	2.347	2.539	2.568	2.626
	325	2.049	2.150	2.325	2.527	2.565	2.618
	335	2.047	2.117	2.231	2.527	2.550	2.588

Mt: montmorillonite; NIPAM: poly(N-isopropyl acrylamide); NVCL: poly(N-vinyl caprolactam).

Interaction between interlayer H₂O molecules after trimers addition

The structure of interlayer H₂O molecules is described by the RDF of $O_{w} {-O}_{w}$ , $O_{w} {-H}_{w}$ , and $H_{w} {-H}_{w}$ (Figures 6 to 8). From these figures, it is observed that differences in the RDF curves are barely distinguishable for both systems with and without trimers into Na-Mt.⁴⁸ The first peak of the RDF of $O_{w} {-O}_{w}$ appears close to 3.23 Å with the second nearest neighbor $O_{w} {-O}_{w}$ pair correlation occurring at 6.05 Å. Those of $O_{w} {-H}_{w}$ and $H_{w} {-H}_{w}$ show the first peak near 1 and 1.55 Å in both RDF curves, respectively. The distances between H₂O molecules’ oxygen and hydrogen are almost the same and show the good organization of interlayer H₂O molecules than the bulk H₂O. Similar to the hydration parameters comparison of Na cations in both systems above, we investigated the coordination number of $O_{w} {-O}_{w}$ , $O_{w} {-H}_{w}$ , and $H_{w} {-H}_{w}$ and compared the results (Tables 2 and 3). The coordination numbers of $O_{w} {-O}_{w}$ , $O_{w} {-H}_{w}$ , and $H_{w} {-H}_{w}$ show that the change of the coordination shell of H₂O molecule’s oxygen and hydrogen is minimal after adding (NIPAM)₃ and (NVCL)₃ into Na-Mt and the water box. H₂O molecules remained close to each other, forming hydrogen bonds despite the presence of both trimers when temperature increased. This result suggests that both trimers reacted in the same manner on the structural arrangement of interlayer H₂O molecules into Na-Mt and the water box. Figures 7 and 8 display the RDF curves of $O_{w} {-O}_{w}$ , $O_{w} {-H}_{w}$ , and $H_{w} {-H}_{w}$ into Na-Mt. From these figures, it is observed that the structural arrangement of H₂O molecules is not greatly affected by the increase in temperature after adding (NIPAM)₃ and (NVCL)₃. This phenomenon shows that the hydrogen bonds formed between interlayer H₂O molecules remained as in their initial state upon increasing temperature. Trimer addition reinforced the structure of interlayer H₂O molecules so that the effect of temperature increase on them became negligible. Interlayer H₂O molecules formed H₂O bridge and strong hydrogen bonds between themselves which were not disturbed by the rise in temperature after (NIPAM)₃ and (NVCL)₃ addition. The results obtained from the analysis of the hydration parameters showed that temperature increase has a great effect on the structure of H₂O molecules surrounding Na cations and their rearrangement in the water box (pure H₂O) than into Na-Mt.

Figure 6.

RDF curves of O_w-O_w before and after (NIPAM)₃ and (NVCL)₃ addition at different temperatures into Na-Mt.

Figure 7.

RDF curves comparative chart of O_w-O_w after (NIPAM)₃ and (NVCL)₃ addition at different temperatures into Na-Mt.

Figure 8.

RDF curves comparative chart of O_w-H_w and H_w-H_w after (NIPAM)₃ and (NVCL)₃ addition at different temperature into Na-Mt.

Table 2.

Ion coordination number of O_w-O_w before and after (NIPAM)₃ and (NVCL)₃ addition.

		Coordination number
	Temperature (K)	No trimer	With (NIPAM)₃	With (NVCL)₃
Na-Mt	300	3.215	3.228	3.242
	305	3.215	3.233	3.238
	315	3.213	3.232	3.235
	325	3.213	3.232	3.235
	335	3.213	3.230	3.234
Water box	300	2.464	2.541	2.561
	305	2.462	2.538	2.556
	315	2.461	2.536	2.552
	325	2.457	2.537	2.549
	335	2.456	2.528	2.546

Mt: montmorillonite; NIPAM: poly(N-isopropyl acrylamide); NVCL: poly(N-vinyl caprolactam).

Table 3.

Ion coordination number of O_w-H_w and H_w-H_w before and after (NIPAM)₃ and (NVCL)₃ addition.

		Coordination number (O_w-H_w)			Coordination number (H_w-H_w)
	Temperature (K)	No trimer	With (NIPAM)₃	With (NVCL)₃	No trimer	With (NIPAM)₃	With (NVCL)₃
	300	3.3981	3.4073	3.4112	0.7090	0.7097	0.7119
	305	3.3983	3.4074	3.4117	0.7091	0.7112	0.7123
Na-Mt	315	3.3987	3.4077	3.4117	0.7093	0.7114	0.7121
	325	3.3989	3.4077	3.4126	0.7093	0.7112	0.7123
	335	3.3994	3.4077	3.4128	0.7094	0.7112	0.7124
	300	2.7388	2.2512	2.5266	0.5978	0.5004	0.5473
	305	2.7383	2.2486	2.5245	0.5915	0.4953	0.5469
Water box	315	2.7364	2.2478	2.5240	0.5905	0.4920	0.5439
	325	2.7352	2.2460	2.5230	0.5918	0.4881	0.5426
	335	2.7345	2.2496	2.5210	0.5898	0.4875	0.5424

Mt: montmorillonite; NIPAM: poly(N-isopropyl acrylamide); NVCL: poly(N-vinyl caprolactam).

Diffusion coefficient of Na cations and H₂O molecules after trimers addition into Na-Mt

Unlike the RDF curves studied above, the mobility of Na cations and H₂O molecules is greatly affected by the change in temperature,^23,40 and the type of trimers increased with the rise in temperature in both systems of Na-Mt (Figure 9). The decrease of the diffusion coefficient of Na cations and H₂O molecules in the system with trimers addition compared to that without trimers addition was observed due to the inhibition effect of both trimers on their mobility.⁴⁸ (NVCL)₃ exerted a good inhibition effect on the mobility of Na cations and H₂O molecules than (NIPAM)₃. Trimers exerted strong attractive forces on interlayer H₂O molecules which increased the affinity of Na cations for clay surfaces. In the presence of both trimers, most of Na cations diffused in the inner sphere complexes because of the strong electrostatic forces exerted by the negatively charged layers on them. In this position, they were less hydrated by H₂O molecules and coordinated by clay surface’s oxygen and H₂O molecules. The interactive forces exerted by both trimers on Na cations reduced their mobility and minimized the solvation of clay surfaces. With the rise in temperature, the attractive forces exerted by H₂O molecules and clay surfaces on Na cations became weak. Their affinity to clay surface decreased leading to the rise in their mobilities. The diffusion of H₂O molecules into Na-Mt depends on the presence of both trimers, Na cations, and the charges on clay mineral surface. (NIPAM)₃ and (NVCL)₃ have two stages in aqueous solution which are the hydrogen bonding formation and hydrophobic interaction at low and high temperatures, respectively.⁴⁹ At low temperatures, the interlayer H₂O solution had a higher viscosity due to the solubility of both trimers. Strong hydrogen bonds were formed between H₂O molecules and clay mineral surfaces, between H₂O molecules and trimers, and between trimers and clay mineral surfaces.⁵⁰ As the temperature increased from 300 to 305 K, the hydrogen bonds formed between trimers and H₂O molecules became weak leading to the increase in the mobility of H₂O molecules. Above 305 K, the interlayer fluid viscosity became lower, the trimers became insoluble, and attractive forces exerted by clay mineral surface on H₂O molecules became weak. Hydrogen bonds between H₂O molecules and both trimers are broken causing the release of H₂O molecules from them, thus increasing the mobility of H₂O molecules.⁵¹ As temperature increased, the mobility of H₂O molecules in the system with (NIPAM)₃ addition remained higher than that of (NVCL)₃, because H₂O molecules associated with the side isopropyl moieties are released more easily than that of lactam moieties of (NVCL)₃ due to the weaker intramolecular and hydrophobic interaction between (NIPAM)₃ residues than those of (NVCL)₃.

Figure 9.

Diffusion coefficient of Na cations (a) and H2O molecules (b) before and after (NIPAM)₃ and (NVCL)₃ addition at different temperatures.

Concentration profile of H₂O molecules and Na cations into Na-Mt after trimers addition

At different temperatures for both systems with and without trimers addition, it was observed that both particles diffused close to Na-Mt surfaces and in the mid-plane after trimers addition (Figures 10 and 11). The distribution of H₂O molecules at each temperature after adding trimers remained the same and that of Na cations undergone some changes near the clay mineral surface.⁴⁸ These results indicate that Na cations diffusion was not uniform in the interlayer space of Na-Mt. The highest peaks were observed close to Na-Mt surface, indicating that H₂O molecules and Na cations diffused close to Na-Mt surface than the mid-plane. In this position, they were subjected to the attractive forces exerted by Na-Mt surface when the temperature was increased. The hydrophilic and hydrophobic components of both trimers remained surrounded by H₂O molecules causing the repulsion of Na cations from the center of the interlayer to the position close to clay mineral surface. For the two trimers at different temperatures, the slight difference between their distribution was observed in both systems when temperature increased. This observation shows that temperature increase has no significant effect on the distribution of the trimers in the interlayer space. However, reduced temperature effect resulted on the distribution of H₂O molecules and Na cations between the layers of Na-Mt due to the increased in affinity of Na cations for clay mineral surfaces and the strong hydrogen bond formation in H₂O molecules grouped in the interlayer.

Figure 10.

Concentration profile of Na cations and H2O molecules before and after (NVCL)₃ and (NIPAM)₃ addition at different temperatures into Na-Mt.

Figure 11.

Concentration profile of Na cations (a) and H2O molecules (b) after (NVCL)₃ and (NIPAM)₃ addition at different temperatures into Na-Mt.

Dynamic evolution of the radius of gyration of (NVCL)₃ and (NIPAM)₃ into Na-Mt

Figure 12 shows the radius of gyration (R_g) of both trimers into Na-Mt. The decrease in R_g of both trimers with the rise in temperature was observed. This decrease indicates a distinct conformational transition of both trimers into Na-Mt when the temperature increased. The values of R_g obtained for (NVCL)₃ remained larger than those of (NIPAM)₃, indicating that (NIPAM)₃ was more sensitive to temperature, related to the thermodynamic and molecular mechanisms underlying their phase transition. The large value of R_g of (NVCL)₃ was due to its molecular structure which contained NVCL monomers. These monomers make difficult the flexibility of its structure, the hydrophobic interaction between (NVCL)₃ residues, and the hydration of its hydrophilic compounds compared to those of (NIPAM)₃. Large value of R_g was obtained between 300 and 305 K; at these temperatures, both trimers were soluble in the interlayer H₂O solution and existed in a coil conformation. Under this conditions, the trimers were surrounded by H₂O molecules to form a homogenous liquid phase, thus contributing to the formation of strong hydrogen bonds between them and H₂O molecules and between H₂O molecules. These interactions promoted their stretching when they laid into the hydrated Na-Mt. Strong hydrogen bonds were formed between H₂O molecules around (NVCL)₃ than (NIPAM)₃. These hydrogen bonds in their surroundings and the electrostatic forces exerted by Na cations maintained them in the extended form in the interlayer.^27,52 With increasing temperature from 305 to 335 K, the hydrogen bonds formed between the trimers and H₂O molecules and the attractive forces in the interlayer became weak. The intramolecular and hydrophobic interactions between monomer units of both trimers increased, resulting in the change of their conformation which was observed by the decrease of the R_g curves. The abrupt change from H₂O soluble to the insoluble state was due to a relevant rearrangement of H₂O molecules in the proximity of both trimers.²⁵ Below 305 K, the R_g curve of (NVCL)₃ remained higher than that of (NIPAM)₃. This result indicates that less conformational changes and more interaction between (NVCL)₃ and H₂O molecules, and between (NVCL)₃ and clay mineral surfaces were observed compared to (NIPAM)₃ when temperature increased. The trimer (NVCL)₃ remained hydrated than (NIPAM)₃ during the increase in temperature. Both trimer chains displayed the conformational change in the interlayer of hydrated Na-Mt, but these transitions are more significant for the polymers containing a large enough chain.^8,25,53

Figure 12.

Radius of gyration of (NVCL)₃ and (NIPAM)₃ at different temperatures into Na-Mt.

Conclusion

We have performed MD simulations to investigate the effect of (NVCL)₃ and (NIPAM)₃ on the structure of hydrated Na-Mt and to analyze the hydration parameters and the conformation change of both trimers in the interlayer to understand their impacts on Na-Mt structure when the temperature increases. The hydration parameters calculated into Na-Mt were compared with those obtained in the pure H₂O for validation. The results obtained show that RDF curves of ${Na-O}_{w}$ , $O_{w} {-O}_{w}$ , $O_{w} {-H}_{w}$ , and $H_{w} {-H}_{w}$ are barely distinguishable between the system with and without (NVCL)₃ and (NIPAM)₃ into Na-Mt. Temperature increase had a more significant effect on the hydration shell of Na cations in the pure H₂O than into Na-Mt when trimers were added. The structural arrangement of H₂O molecules remained the same as in their initial state when the temperature increased. This result confirmed that the electrostatic forces exerted by Na cations on H₂O molecules and between H₂O molecules were not affected by the rise in temperature when (NVCL)₃ and (NIPAM)₃ were added into Na-Mt. The structural analysis (R_g) of both trimers reproduced the conformation changes when temperature increased. Our investigations add novel information, showing that because of the poor solubility of NVCL monomers in aqueous solution, the chain rearrangement transition is more difficult in the case of (NVCL)₃ than (NIPAM)₃. The decrease of the mobility of Na cations and H₂O molecules in the system with trimers addition was observed. But lower mobility of H₂O molecules and Na cations was obtained in the system with (NVCL)₃ addition, indicating that (NVCL)₃ has a good inhibition effect on the mobility of interlayer species than (NIPAM)₃. The concentration profile of Na cations and H₂O molecules showed an increase of the affinity of Na cations for clay surface while H₂O molecules moved away. Both trimers exhibited good inhibition effect on Na-Mt in terms of reducing interlayer particle mobility, maintaining the structural arrangement of interlayer H₂O molecules and protecting clay mineral surfaces. Compared with (NIPAM)₃, (NVCL)₃ had a good performance on inhibiting clay swelling and reducing the effect of temperature on the stability of hydrated Na-Mt.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51874343, U1762212, 51874332), the Natural Science Foundation of Shandong Province (ZR2017MEE027), and the PCSIRT (IRT_14R58).

ORCID iD

Moussa Camara

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Comparison of the effect of poly( N -vinyl caprolactam) and poly( N -isopropyl acrylamide) trimers on the stability of hydrated Na-montmorillonite: A molecular dynamics study

Abstract

Keywords

Introduction

Model setup

(NIPAM)3 and (NVCL)3 models

Na-Mt model

Computational details

Trajectories analysis

Results and discussion

Interaction between Na cations and H2O molecules after trimers addition

Interaction between interlayer H2O molecules after trimers addition

Diffusion coefficient of Na cations and H2O molecules after trimers addition into Na-Mt

Concentration profile of H2O molecules and Na cations into Na-Mt after trimers addition

Dynamic evolution of the radius of gyration of (NVCL)3 and (NIPAM)3 into Na-Mt

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

(NIPAM)₃ and (NVCL)₃ models

Interaction between Na cations and H₂O molecules after trimers addition

Interaction between interlayer H₂O molecules after trimers addition

Diffusion coefficient of Na cations and H₂O molecules after trimers addition into Na-Mt

Concentration profile of H₂O molecules and Na cations into Na-Mt after trimers addition

Dynamic evolution of the radius of gyration of (NVCL)₃ and (NIPAM)₃ into Na-Mt