pH effects on adsorption behavior and self-aggregation of dodecylamine at muscovite/aqueous interfaces

In this work, molecular dynamics simulation was used to examine the effect of solution pH on the adsorption behavior and self-aggregation of dodecylamine hydrochloride (DDA) on the muscovite (0 0 1) surface. The properties of surfactants are assessed in terms of density profiles in the direction perpendicular to the muscovite surface. Results show that although DDA can adsorb at muscovite at all pH we discussed, the self-aggregation of DDA varies significantly at different pH values. At pH 10, a compact hydrophobic monolayer forms on the muscovite surface. At pH 3, hemi-micelle aggregated structure forms with several DDA cations far away from muscovite surface. At pH 12, it has been confirmed that adsorption of DDA neutral molecules occurs with only a few DDA molecules adsorbing on muscovite directly and acting as a bridge linking the rest DDA molecules, which exists nearby muscovite surface irregularly. Density profiles revealed that at pH 10, DDA cations play a dominant role in the interaction between DDA surfactants and muscovite. While DDA molecules have difficulty in forming a hydrogen bond with the oxygen atom on the muscovite surfaces, and they co-adsorb onto muscovite through the electrostatic interactions with muscovite and hydrophobic force with DDA cations. Therefore, the hydrophobization of muscovite in the presence of DDA are higher at pH 10 than that at pH 3 and pH 12. Our results indicate that molecular dynamics simulation can be a power tool in charactering adsorption behavior of surfactants onto mineral surfaces at different pH values.