Adsorption of Gadolinium (Gd3+) Ions on the Dibenzo Crown Ether (DBCE) and Dicyclo Hexano Crown Ether (DCHCE) Grafted on the Polystyrene Surface: Insights from All Atom Molecular Dynamics Simulations and Experiments

All atom molecular dynamics simulations and experiments were performed to understand the adsorption behavior of gadolinium (Gd3+) ion on the crown ethers grafted polystyrene (PS) surface. Two different types of crown ethers, viz., dibenzo crown ether (DBCE) and dicyclo hexano crown ether (DCHCE), were grafted separately on the PS surface to understand the adsorption behavior. We investigate the roles of Gd3+ ion concentration and grafting density (rho(s)) of the crown ether on the adsorption behavior of Gd3+ ion on the PS surface. The adsorption of Gd(3+)shows an increasing trend with increasing salt concentration, for all cases of crown ether grafting densities. The adsorption behavior follows the Langmuir isotherm model. The maximum amount of Gd3+ ion adsorption was observed to be 1.83 mg/g for DBCE and 2.02 mg/g for DCHCE at rho(s) = 2.07 mol/nm(2). The maximum amount of Gd3+ ion adsorption on DBCE coated PS beads, in batch experiments, was found to be 1.76 mg/g, which is in good agreement with the theoretical results. The increase in rho(s) from 0.25 to 2.07 mol/nm(2) shows an increase in the q(max) value by similar to 422% and 329% for DBCE and DCHCE, respectively. The optimum value of the crown ether grafting density is found to be 1.25 mol/nm(2), beyond which the q(max) saturates. We further investigate the dynamics of the Gd3+ ion by evaluating the diffusion coefficient (D) and mean residence time (tau). It was found that D decreases with increasing salt concentration for both DBCE and DCHCE. On the contrary, as expected, the tau value of Gd3+ increases with an increase in salt concentration. Overall, a 3-fold increase in tau was seen with increasing salt concentration. The potential of mean force analysis using umbrella sampling reveals favorable binding energy for higher grafting density of DCHCE compared to that of DBCE.