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. 2024 Feb 10;16(4):494.
doi: 10.3390/polym16040494.

Atomic Force Microscopy and Molecular Dynamic Simulation of Adsorption of Polyacrylamide with Different Chemistries onto Calcium Carbonate

Jin Hau Lew  1 , Keat Yung Hue  1 , Omar K Matar  1 , Erich A Müller  1 , Paul F Luckham  1 , Adrielle Sousa Santos  1 , Maung Maung Myo Thant  2
Affiliations

Atomic Force Microscopy and Molecular Dynamic Simulation of Adsorption of Polyacrylamide with Different Chemistries onto Calcium Carbonate

Jin Hau Lew et al. Polymers (Basel). .

Abstract

This study investigates the interaction of polyacrylamide (PAM) of different functional groups (sulfonate vs. carboxylate) and charge density (30% hydrolysed vs. 10% hydrolysed) with calcium carbonate (CaCO3) via atomic force microscopy (AFM) and partly via molecular dynamic (MD) simulations. The PAM used were F3330 (30% hydrolysed), AN125 (25% sulfonated), and AN910 (% hydrolysed). A total of 100 ppm of PAMs was prepared in 0.1% NaCl, 3% NaCl, and 4.36% NaNO3 to be employed in AFM experiments, while oligomeric models (30 repeating units) of hydrolysed polyacrylamide (HPAM), sulfonated polyacrylamide (SPAM), and neutral PAM (NPAM) were studied on a model calcite surface on MD simulations. AFM analysis indicated that F3330 has a higher average adhesion and interaction energy with CaCO3 than AN125 due to the bulky sulfonate side group of AN125 interfering with SPAM adsorption. Steric repulsion of both PAMs was similar due to their comparable molecular weights and densities of the charged group. In contrast, AN910 showed lower average adhesion and interaction energy, along with slightly longer steric repulsion with calcite than F3330, suggesting AN910 adopts more loops and tails than the slightly flatter F3330 configuration. An increase in salt concentration from 0.1% to 3% NaCl saw a reduction in adhesion and interaction energy for F3330 and AN125 due to charge screening, while AN910 saw an increase, and these values increased further at 4.36% NaNO3. MD simulations revealed that the salt ions in the system formed salt bridges between PAM and calcite, indicating that the adhesion and interaction energy observed from AFM are likely to be the net balance between PAM charged group screening and salt bridging by the salt ions present. Salt ions with larger bare radii and smaller hydrated radii were shown to form stronger salt bridges.

Keywords: adsorption; atomic force microscopy (AFM); force spectroscopy; hydrolysed polyacrylamide; molecular dynamic (MD) simulation; salt; sulfonated polyacrylamide.

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Conflict of interest statement

Author Maung Maung Myo Thant is employed by the company “Petronas. (Malaysia)”. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest. The company “Petronas. (Malaysia)” provided the funding. They were not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication.

Figures

Figure 1
Figure 1
Chemical formula of HPAM [23].
Figure 2
Figure 2
Structure of sulfonated polyacrylamide (SPAM) [29].
Figure 3
Figure 3
Histogram plot of (a) adhesion; (b) interaction energy of calcite immersed in AN125 in 0.1% NaCl. Green curve shows normal distribution plot; (c) most recurring retract force curve shape at AN125 in 0.1% NaCl.
Figure 4
Figure 4
Proposed HPAM (left) and SPAM (right) configurations on calcite crystal surface.
Figure 5
Figure 5
Histogram plot of (a) adhesion; (b) interaction energy of calcite immersed in AN910 in 0.1% NaCl. Green curve shows normal distribution plot; (c) most recurring retract force curve shape at AN910 in 0.1% NaCl.
Figure 6
Figure 6
Snapshot of the adsorption equilibrium of (a) HPAM 33%, (b) SPAM 33%, and (c) NPAM onto calcite in the presence of NaCl; and (d) detail of the positioning of Na ions (purple) between O group (red) of polyacrylamides and calcite surface. Legend for colour-coding the atoms in MD simulation is displayed on the right.
Figure 7
Figure 7
Snapshots of NPAM adsorption onto calcite in the presence of (a) NaCl (green particles are the Cl− ions and (b) NaNO3 (yellow and brown particles are the NO3− ions); (c) detail of the orientation of NO3− ions (yellow–brown atoms) between NPAM and calcite surface. NO3− ions have been enlarged to emphasise their orientation. Other atoms follow same colour scheme as Figure 6.
Figure 8
Figure 8
Simulation time profile of adsorbed amount of NPAM onto calcite surface in different salt from MD simulation. The average adsorbed amount is taken from last 5 ns of data. Translucent light colour area around the darker colour curves represents the standard error of each curve from 5 different realisations.
Figure 9
Figure 9
Polymer adsorption number density profile on calcite surface with distance distribution along z-direction, averaged over last 5 ns of production and run from 5 different realisations. (a) NaCl adsorption. Blue: chloride ion as salt anion, orange: sodium ion as salt cation, and green: oxygen atom from polymer carbonyl group (C=ONH2); (b) NaNO3 adsorption. Blue: nitrogen atom from nitrate ion as salt anion, orange: sodium ion as salt cation, and green: oxygen atom from polymer carbonyl group.
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