Adsorption of different PAM structural units on kaolinite (001) surface: Density functional theory study

The objectives of this study are to explore the adsorption mechanisms for how the polyacrylamide (PAM) interact with kaolinite (0 0 1) surface. Three structural unit models (P-AM, P-AA, and P-DAC) were developed from the structure of PAM. The adsorption energies were calculated with density functional theory (DFT) to determine the optimal adsorption system of PAM structural units on the kaolinite (0 0 1) surface. From the Mulliken population analysis, electron density difference, electronic density of states, and electron localization function of the optimal adsorption system, the adsorption mechanism of each adsorption system was derived and their differences were understood. The results show that these reactive sites of the three PAM structural units are O atoms. P-AM and P-AA can be adsorbed on the kaolinite (0 0 1) surface by forming hydrogen bonds. The interaction of kaolinite (0 0 1) surface using P-DAC is driven by both hydrogen bonds and electrostatic attraction, electrostatic attraction as a prevailing role is determined simultaneously. The strength of the hydrogen bonds of the P-AM adsorbed on the kaolinite (0 0 1) surface is higher than that of P-DAC and P-AA. However, the order of the adsorption interaction of the PAM structural units adsorbed onto the kaolinite (0 0 1) surface is P-DAC > P-AM > P-AA.