Bonding between polyacrylamide and smectite

In various applications of polyacrylamide (PAM) in soil erosion control, soil conditioning, water treatment, and oil recovery, the polymer mainly reacts with clay minerals. The objective of this study was to investigate the probable bonding mechanisms between PAM and smectite, a common clay mineral that occurs in soils and sediments. Three polyacrylamides – nonionic PAM 903N, cationic PAM 494C, and anionic PAM 836A – were investigated. Additionally, the impact of exchangeable cation, Na+, K+, Mg2+, Al3+, Cu2+, and Ni2+, on bonding strength was investigated. The PAM-smectite complexes were analyzed by Fourier transform infrared spectroscopy and X-ray diffraction. After adsorption on Ca2+-smectite, the asymmetric and symmetric stretching vibrations of the NH2 groups of nonionic PAM blue-shifted 140 and 186 cm−1, respectively, suggesting that the amino group was less H-bonded in the adsorbed PAM than in the bulk polymer. The CO band showed only a 3 cm−1 blue-shift, suggesting that the carbonyl oxygen was still bonded by other atoms or ions. The IR-band shifts were affected by the exchangeable cation. For example, the stretching vibrations of NH2 and CO bonds of adsorbed PAM in complexes containing transition-metal cations (Cu2+ or Ni2+) occurred at frequencies 20 and 9 cm−1 lower than those containing exchangeable alkali cations (Na+ or K+). The IR bands of amide groups of the complexes containing exchangeable Ca2+, Mg2+, and Al3+ fell between those containing transition-metal cations and those containing alkali cations. The IR band shifts suggest that carbonyl (CO) oxygens contributed more than the amino group (NH2) to the bonding between PAMs and smectite. The IR-band shifts associated with the exchangeable cation suggest that: (1) an ion-dipole interaction or coordination occurs between the amide groups of the PAMs and the exchangeable cations and (2) hydrogen bonding exists between the amide groups and water molecules in the hydration shells of exchangeable cations. Similar IR band shifts were observed in the cationic PAM 494C- and anionic PAM 836A-smectite complexes, suggesting that the nonionic acrylamide units in these copolymers reacted similarly with smectite as the nonionic PAM 903N.