Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements

Abstract

Probing anisotropic surface properties of layer-type mineral is fundamentally important in understanding its surface charge and wettability for a variety of applications. In this study, the surface properties of the face and the edge surfaces of natural molybdenite (MoS2) were investigated by direct surface force measurements using atomic force microscope (AFM). The interaction forces between the AFM tip (Si3N4) and face or edge surface of molybdenite were measured in 10 mM NaCl solutions at various pHs. The force profiles were well-fitted with classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory to determine the surface potentials of the face and the edge surfaces of molybdenite. The surface potentials of both the face and edge surfaces become more negative with increasing pH. At neutral and alkaline conditions, the edge surface exhibits more negative surface potential than the face surface, which is possibly due to molybdate and hydromolybdate ions on the edge surface. The point of zero charge (PZC) of the edge surface was determined around pH 3 while PZC of the face surface was not observed in the range of pH 3-11. The interaction forces between octadecyltrichlorosilane-treated AFM tip (OTS-tip) and face or edge surface of molybdenite were also measured at various pHs to study the wettability of molybdenite surfaces. An attractive force between the OTS-tip and the face surface was detected. The force profiles were well-fitted by considering DLVO forces and additional hydrophobic force. Our results suggest the hydrophobic feature of the face surface of molybdenite. In contrast, no attractive force between the OTS-tip and the edge surface was detected. This is the first study in directly measuring surface charge and wettability of the pristine face and edge surfaces of molybdenite through surface force measurements.

Anisotropic Polymer Adsorption on Molybdenite Basal and Edge Surfaces and Interaction Mechanism With Air Bubbles

“…On the topographic AFM image of freshly polished edge plane shown in Figure 4A, the dark regions were polishing defects that could not be totally avoided, while the bright regions were the aggregates possibly arising from the edge crystal structure. The rms roughness of the polished edge plane was measured to be 1.61 nm, which was smoother than the MoS 2 edge plane obtained by the ultramicrotome cutting technique (1.6–3.3 nm) reported previously (Lu et al, 2015, 2016). Since the edge plane was not as molecularly smooth as the basal plane, the polishing defects and aggregates pre-existed on the edge plane might affect the determination of adsorbed polymer.…”

“…In our previous study, the adsorption of guar gum on MoS 2 basal plane was found to lead to the formation of interconnected polymer network at 5 ppm and a fully covered polymer film at 10 ppm (Xie et al, 2017d). It is evident that the adsorption of CMC is more difficult than guar gum most likely due to the negatively charged carboxyl groups of CMC that could induce stronger electrostatic repulsion with the negatively charged MoS 2 basal plane (surface potential at pH 9: −55 mV in 1 mM NaCl and −44 mV in 10 mM NaCl) (Lu et al, 2015; Xie et al, 2017d). …”

“…Over the last several decades, the anisotropy in MoS 2 basal and edge surfaces has attracted much research interest (Okuhara and Tanaka, 1978; Roxlo et al, 1986; Zhang et al, 2004; Gaur et al, 2014; Jin et al, 2014; Lu et al, 2015; Tan et al, 2015; Govind Rajan et al, 2016; Bentley et al, 2017). The molecular dynamics simulations showed the water contact angle of 54° for the armchair-edge and 24° for the zigzag-edge of MoS 2 , much lower than that of 83° for MoS 2 basal plane (Jin et al, 2014).…”

“…The molecular dynamics simulations showed the water contact angle of 54° for the armchair-edge and 24° for the zigzag-edge of MoS 2 , much lower than that of 83° for MoS 2 basal plane (Jin et al, 2014). The edge plane was found to exhibit more negative surface potential than the basal plane in neutral and alkaline pH conditions, and the hydrophobic interaction was only detected on the basal plane (Lu et al, 2015). In molybdenum-containing ores, water-soluble polymers are widely used as the depressants to alter the surface characteristics of MoS 2 basal planes (Pugh, 1989; Liu et al, 2000; Pearse, 2005; Bulatovic, 2007; Beaussart et al, 2009; Kor et al, 2014; Castro et al, 2016).…”

The surface state of hematite and its wetting characteristics

“…These attractive forces may be due to the presence of nano bubbles present at such hydrophobic surfaces. As suggested in the literature, when two hydrophobic surfaces are brought close to each other, capillary bridging between these nano bubbles takes place leading to attraction which can be considered to be an explanation for hydrophobic interactions [34][35][36][37][38][39][40][41]. One more observation that can be made from Fig.…”