Interfacial forces are modified by the growth of surface nanostructures

Opposing effects of humidity on rhodochrosite surface oxidation

Rhodochrosite (MnCO3) is a model mineral representing carbonate aerosol particles containing redox-active elements that can influence particle surface reconstruction in humid air, thereby affecting the heterogeneous transformation of important atmospheric constituents such as nitric oxides, sulfur dioxides, and organic acids. Using in situ atomic force microscopy, we show that the surface reconstruction of rhodochrosite in humid oxygen leads to the formation and growth of oxide nanostructures. The oxidative reconstruction consists of two consecutive processes with distinctive time scales, including a long waiting period corresponding to slow nucleation and a rapid expansion phase corresponding to fast growth. By varying the relative humidity from 55 to 78%, we further show that increasing humidity has opposing effects on the two processes, accelerating nucleation from 2.8(±0.2) × 10(-3) to 3.0(±0.2) × 10(-2) h(-1) but decelerating growth from 7.5(±0.3) × 10(-3) to 3.1(±0.1) × 10(-3) μm(2) h(-1). Through quantitative analysis, we propose that nanostructure nucleation is controlled by rhodochrosite surface dissolution, similar to the dissolution-precipitation mechanism proposed for carbonate mineral surface reconstruction in aqueous solution. To explain nanostructure growth in humid oxygen, a new Cabrera-Mott mechanism involving electron tunneling and solid-state diffusion is proposed.

Growth of manganese oxide nanostructures alters the layout of adhesion on a carbonate substrate

Nanostructures grown under natural conditions can modify the layout of adhesion on mineral surfaces. Using force-volume microscopy and a silicon-nitride probe, we measure changes in adhesion when a patchy overgrowth of manganese oxide nanostructures forms on the surface of rhodochrosite. For the most part, the observations show that the adhesive force to the nanostructures is dominated by van-der-Waals attraction. Measurements made across an area of the surface provide a frequency distribution of adhesive forces, and the mode of this distribution is 166 pN at pH 5.0, increasing to a maximum of 692 pN at pH 7.1, followed by a decrease to 275 pN at pH 9.7. At a few sampling locations over some nanostructures, electrostatic repulsion overtakes van-der-Waals attraction and thus results in negative adhesive forces (i.e., repulsion). Local roughness causes this effect. In comparison to the oxide nanostructures, the exposed rhodochrosite substrate has negligible adhesive force with the probe over the same pH range, suggesting both weak van-der-Waals attraction and weak electrostatic repulsion over this pH range. The quantitative mapping of adhesive force applied more generally to the study of other nanostructures can lead to an improved mechanistic understanding of how nanostructure growth influences contaminant immobilization and bacterial attachment.