Understanding the Behavior of Sodium Polyacrylate in Suspensions of Silica and Monovalent Salts
Abstract
:1. Introduction
2. Experimental Methodology
2.1. Materials
2.2. Rheology
3. Computer Simulation
3.1. Molecules and Surfaces
3.2. Force Field
3.3. Initial Setup
- (1)
- Once the quartz surface with dimensions (Sx × Sy × Sz) was constructed, the dimensions Sx and Sy were employed to define the LX and LY of the simulation box. Subsequently, dimension Lz was established to encompass both the surface and an aqueous solution containing the polymer. Lz was set at 12 nm. Consequently, the simulation box dimensions were (LX, LY, LZ) = 9.832 × 11.056 × 12.000 nm³. The selection of these dimensions is critical to prevent interactions between the polymer and its periodic images.
- (2)
- The polymer was placed in the previously generated space, positioned 5 nm above the surface along the z-direction. This polymer consisted of 48 monomers and had an initial length of 12 nm. Although this length is comparable to the box size, it substantially reduced to less than 5 nm during equilibration stages.
- (3)
- Ions corresponding to the salts investigated in this study were randomly distributed within the space occupied by the flocculant, maintaining a separation of 1 nm between them and the polymer. The concentrations utilized in this investigation were 0.001, 0.01, and 0.1 moles/liter. The total number of cations and anions was determined based on the salt concentration and the counterions necessary to neutralize both the quartz surface and the PA.
- (4)
- Finally, water from an equilibrated configuration at 300 K was introduced, and this configuration was replicated throughout the box until completion. Water molecules that coincided with the preexisting elements in the box were removed. The elimination criterion was based on less than 0.3 nm between the water molecules and other atoms (the initial setup can be seen in Figure 1).
3.4. Molecular Simulation
- Minimization of forces in the initial configuration. This step is essential since the initial configuration stems from stable yet vacant configurations, which necessarily alter upon their assembly. For quartz, solely hydrogen atoms were allowed movement.
- NVT-equilibration simulation for 0.1 ns. This simulation was conducted at 300 K, with the positions of the polymer and ions held fixed, while only water molecules exhibited motion.
- NVT-equilibration simulation with annealing for 1 ns. During this simulation, the temperature rapidly increased from 300 to 450 K within 0.0001 ns, followed by a 0.5 ns maintenance at the elevated temperature and then a return to 300 K over 0.5 ns. This annealing process enhanced polymer fluidization in the liquid phase, achieving a stable configuration.
- NPT-equilibration simulation for 2 ns. Conducted at 300 K and 1 bar pressure, adjustments to the simulation box were confined to the z-direction to relax the pressure to the predefined value.
- Lastly, an NVT-production simulation spanning 100 ns captured the time evolution of the PA’s interaction with the quartz surface under varying salt concentrations. Six repetitions were performed to mitigate statistical error.
3.5. Data Processing
4. Results
4.1. Rheological Behavior
- (i)
- An enlargement in cation size precipitated an elevation in yield stress due to the fortification of interparticle bonding. Considering that quartz lends itself to a conceptualization akin to “breaker” salts in certain aspects, it is foreseeable that pulps experience a more pronounced loss of fluidity in the presence of such salts. This observation aligns with the findings reported by Jeldres et al. [57], who proffered an explanation informed by electrostatic and structural considerations of water behavior to characterize rheological conduct.
- (ii)
- Sodium polyacrylate evinced heightened effectiveness in tandem with smaller salts. The augmented effectiveness can be attributed to its superior adeptness in traversing the hydrated layer enveloping surfaces, particularly in scenarios where the ionic milieu comprises “maker” salts.
4.2. Polymer Adsorption
4.3. Polymer Interaction
4.4. Ion Adsorption
4.5. Net Charge
5. Conclusions
- It was confirmed that the yield stress is directly affected by the presence of cations following a specific sequence: Cs > K > Na > Li. This suggests that the adsorbed cations of the “breaker” type have a higher affinity for the quartz surface without the presence of polyacrylate. This observation is related to the weak charge density on the quartz surface, which predisposes it to interact more favorably with low charge density breaking cations.
- The addition of PAA was shown to reduce the yield stress in all cases studied, which indicates its leading role as a dispersant for quartz particles. However, it has been observed that this reduction is more marked in the presence of Li and Na compared to K and Cs. This implies that PA exerts a stronger effect in the presence of Li and Na, suggesting that “maker” ions with PA can overcome the breaker-adsorbed ions in quartz.
- Molecular dynamics simulations supported the experimental results, revealing that the adsorption of the PA polymer on quartz follows the sequence Li > Na > K > Cs. Furthermore, it was observed that higher salt concentrations increase the adsorption of PA on the quartz surface. These results align with the experimental findings, indicating that the presence of cations facilitates PA adsorption.
- Molecular interactions revealed that the formation of cationic bridges was more pronounced on Li and Na, and these bridges contribute to the formation of hydrogen bonds and hydrophobic bridges, increasing adsorption stability on Li and Na. In contrast, the interactions are weak in the cases of K and Cs.
- These results highlight the utility of computational simulations to support experimental observations and provide a deeper understanding of the optimal conditions for admixture rheology modifications in mining processes.
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Quezada, G.R.; Retamal, F.; Jeldres, M.; Jeldres, R.I. Understanding the Behavior of Sodium Polyacrylate in Suspensions of Silica and Monovalent Salts. Polymers 2023, 15, 3861. https://doi.org/10.3390/polym15193861
Quezada GR, Retamal F, Jeldres M, Jeldres RI. Understanding the Behavior of Sodium Polyacrylate in Suspensions of Silica and Monovalent Salts. Polymers. 2023; 15(19):3861. https://doi.org/10.3390/polym15193861
Chicago/Turabian StyleQuezada, Gonzalo R., Francisco Retamal, Matías Jeldres, and Ricardo I. Jeldres. 2023. "Understanding the Behavior of Sodium Polyacrylate in Suspensions of Silica and Monovalent Salts" Polymers 15, no. 19: 3861. https://doi.org/10.3390/polym15193861