Effect of surface modifier and solvent on the affinity between the surface-modified solid and organic solvent: A molecular dynamics study

To control the dispersibility of surface-modified nanoparticles in fluid, it is important to evaluate the affinity between surface-modified nanoparticles and solvents and elucidate the effect of surface modification on the affinity at the molecular level. In this study, the effect of the chain length of the surface modifier and solvent on the work of adhesion Wadh was investigated for the carboxylic acid-modified Al2O3/n-alkane interface by molecular dynamics simulations using the phantom-wall method. Here, we adopted hexanoic acid, decanoic acid, and octadecanoic acid as the surface modifiers and n-hexane, n-decane, and n-dodecane as the solvents. At a surface coverage of 100%, the effect of the modifier length on Wadh was small because the solvent did not penetrate any modifier layers with a dense packing state. At a surface coverage of 50%, the solvent penetration was good except for the octadecanoic acid modifier, and Wadh for the decanoic acid modifier was higher than that for the hexanoic acid and octadecanoic acid modifiers. For the decanoic acid modifier with a surface coverage of 50%, the effect of the solvent length on Wadh was small because the density distributions of the modifier and solvent near the interface were almost the same in all solvent systems. Solvent penetration was evaluated using the overlap parameter, which is the degree of overlap of the density distribution. A large penetration resulted in high Wadh. We demonstrated that the affinity between surface-modified nanoparticles and solvents can be increased by ensuring high modifier-solvent penetration via adjusting the surface coverage and surface modifier.