Understanding the adsorption of air entraining agents on portlandite: A combined DFT and molecular dynamics investigation

Abstract

The molecular activity of four air entraining agents (AEA) widely used in cement and concrete systems is firstly calculated by DFT method and then adsorption behaviors including adsorption conformation, adsorption interaction and adsorption stability at portlandite-water interface are investigated by the classical atomistic molecular dynamics (MD) and non-equilibrium MD (named SMD) simulations. The molecular activity decreases as Rosin>SDBS>CDEA>SDS and the negative electrostatic potential (ESP) charges of oxygen atoms of adsorption groups decrease as Rosin>CDEA>SDBS>SDS, which is agree with the trends of adsorption interaction and adsorption stability of four AEAs at portlandite-water interface. The adsorption interaction and adsorption stability are determined by the electrostatic interaction and assembly density of AEAs at portlandite-water interface has less effect on the adsorption and adsorption stability. Conformation of SDS exhibit noticeable orientation reversal and a bilayer structure tends to form. The angles of the hydrophobic tail chains of SDS with z-axis are smallest, indicating the more perpendicular orientation and a denser assembly of the hydrophobic chains. Strong interaction of water molecules with portlandite surface causes competitive adsorption between AEA and water molecules on the portlandite surface.