Effect of surface polarity on the structure and dynamics of liquids at the alumina solid-liquid interface

Molecular dynamics (MD) simulations are used to investigate the structure and dynamics of interfacial water on alumina surfaces as well as mixed liquids confined within alumina pores with varying surface polarities from apolar to polar. Our results demonstrate that the surface polarity has a significant effect on the water layer in the vicinity of the surface, while the internal interactions among water molecules dominate the feature of the water layer far away from the surface. Water molecules on the polar surface occupy all the optimum adsorption sites surrounding the surface oxygen atoms via forming hydrogen bonds, resulting in large residence probability and slow translational mobility as evidenced by free energy estimations. Simulations of liquid ethanol-water mixture within the membrane pore demonstrate that though water molecules are preferentially adsorbed within the pores, the polarization of the alumina pores is in favor of the preferential adsorption of ethanol molecules, thereby compromising the efficiency of ethanol purification. The present results suggest that manipulation of the alumina surface polarity can determine the solid-liquid interactions and the mobile phase transport, and further contribute to improving our knowledge on the basic physical chemistry underlying the electrostatically dominated diffusion behaviors. Our study provides important insight to the polarity modulation of alumina ceramic membranes in future experiments for achieving the designed separation performance in a variety of applications.