Atomic Force Microscopy of Hydrolysed Polyacrylamide Adsorption onto Calcium Carbonate

Abstract

In this work, the interaction of hydrolysed polyacrylamide (HPAM) of two molecular weights (F3330, 11-13 MDa; F3530, 15-17 MDa) with calcium carbonate (CaCO₃) was studied via atomic force microscopy (AFM). In the absence of polymers at 1.7 mM and 1 M NaCl, good agreement with DLVO theory was observed. At 1.7 mM NaCl, repulsive interaction during approach at approximately 20 nm and attractive adhesion of approximately 400 pN during retraction was measured, whilst, at 1 M NaCl, no repulsion during approach was found. Still, a significantly larger adhesion of approximately 1400 pN during retraction was observed. In the presence of polymers, results indicated that F3330 displayed higher average adhesion (450-625 pN) and interaction energy (43-145 aJ) with CaCO₃ than F3530's average adhesion (85-88 pN) and interaction energy (8.4-11 aJ). On the other hand, F3530 exerted a longer steric repulsion distance (70-100 nm) than F3330 (30-70 nm). This was likely due to the lower molecular weight. F3330 adopted a flatter configuration on the calcite surface, creating more anchor points with the surface in the form of train segments. The adhesion and interaction energy of both HPAM with CaCO₃ can be decreased by increasing the salt concentration. At 3% NaCl, the average adhesion and interaction energy of F3330 was 72-120 pN and 5.6-17 aJ, respectively, while the average adhesion and interaction energy of F3530 was 11.4-48 pN and 0.3-2.98 aJ, respectively. The reduction of adhesion and interaction energy was likely due to the screening of the COO^- charged group of HPAM by salt cations, leading to a reduction of electrostatic attraction between the negatively charged HPAM and the positively charged CaCO₃.

Keywords: atomic force microscopy (AFM); calcium carbonate; force spectroscopy; hydrolysed polyacrylamide; molecular weight.

Figure 1

Schematics of polymer adsorption model with attached segments (trains) separated by unattached segments (tails and loops), adapted from Al-Hashmi et al. [18].

Figure 2

Molecular formula of HPAM.

Figure 3

Force–distance curve of calcite crystal incubated in 1.7 mM NaCl solution.

Figure 4

Force–distance curve of calcite crystal incubated in 1 M NaCl solution. Inset: extend force distance curve in red-squared region.

Figure 5

The two commonest forms of retraction profiles observed for F3330 in 0.1% NaCl. (a) Type I; (b) Type II.

Figure 6

The two commonest forms of retraction force curve shape at F3530 in 0.1% NaCl. (a) Type III; (b) Type IV.

Figure 7

Histogram plot of adhesion (left) and interaction energy (right) of calcite immersed in F3330S in 0.1% NaCl. Green curve shows normal distribution plot.

Figure 8

Histogram plot of adhesion (left) and interaction energy (right) of calcite immersed in F3530S in 0.1% NaCl. Green curve shows normal distribution plot.

Figure 9

Cartoon representation of the possible configurations adopted by F3330 (left) and F3530 (right) on the calcite surface and their interactions with a CaCO₃ particle attached to an AFM tip.

Figure 10

Force–distance curve of a single HPAM molecule (black) with probable HPAM detachment mechanisms from the substrate surface, adapted from Zhang et al. [51]. Anchor points between polymers with the surface are emphasized in red.

Figure 11

Force–distance curve for multiple detachment from more than one F3330 molecule (top) with proposed illustrations (bottom). Here two polymer molecules are coloured differently (blue and black) for the sake of clarity. Anchor points are emphasized in red. Different stages of polymer detachment are as followed: (a) Contact of CaCO₃ particles with both polymer molecules at rest; (b) detachment of majority of the anchor points of polymer molecules from the surface; (c) even detachment of anchor points of polymer molecules from the surface; (d) full detachment of polymer molecules from the surface.

Figure 12

Illustration of possible single HPAM molecule (black) detachment that forms zigzag retraction force curve. Adapted from Zhang et al. [51]. Anchor points are emphasized in red. Different stages of polymer detachment are as followed: (a) tension due to detachment of polymer’s first anchor point from the surface produced the first adhesion peak; (b) stretching of the polymer molecule before second anchor point with no tension observed; (c) tension due to detachment of polymer’s third anchor point from the surface produced the third adhesion peak; (d) complete detachment of polymer molecule from the surface.

Figure 13

Specific viscosity plot of F3330 in 0 M (DI water) and 0.5 M salinity. CEC of F3330 in 0 M and 0.5 M are approximately 75 ppm and 2250 ppm, respectively.

Figure 14

Histogram plot of adhesion (left) and interaction energy (right) of calcite immersed in F3330 in 3% NaCl. Green curve shows normal distribution plot.

Figure 15

Histogram plot of adhesion (left) and interaction energy (right) of calcite immersed in F3530 in 3% NaCl. Green curve shows normal distribution plot.