Fanfan Zhang,Holger Schönherr,Yaowen Xing,Haichang Yang,Xiahui Gui,Yijun Cao
The effects of the dynamic increase of air concentration dissolved in unsaturated water on the interaction between hydrophobic surfaces were investigated by atomic force microscopy (AFM) using a colloidal probe. The experiments were performed at ambient temperature in unsaturated water. Initially, at an air saturation of 57.7%, the hydrophobic interaction was significantly weaker than that observed independently in air saturated water. Consequently, when the concentration of dissolved gas increased to an air saturation of 99.5%, the observed jump-in distances and adhesion were found to increase with time over the course of many hours. Finally, the hydrophobic interaction was stronger than that in air saturated water. The increased forces as a consequence of increasing gas concentration are attributed to the bridging between nanobubble covered surfaces, because under identical conditions initially no surface nanobubbles were detected by AFM on highly oriented pyrolytic graphite surfaces in unsaturated water, while after several hours nanobubbles could be discerned. It is hypothesized that air diffuses into the water in the AFM liquid cell and that nanobubbles are nucleated after the dynamic increase of the gas concentration. This work contributes to the understanding of nanobubble nucleation, interaction forces between nanobubble covered surfaces in aqueous media under variable dynamic conditions and thus to facilitate future applications exploiting nanobubbles.
Calcium carbonate (CaCO 3 ) nanocrystals with controllable polymorph and morphology have been successfully synthesized with the aid of an effective control agent, a halogen-free, low-cost ionic liquid surfactant, 1-butyl-3-methylimidazolium dodecylsulfate ([C 4 mim][C 12 SO 4 ]) in a supersaturated aqueous solution. For the first time, facile preparation of pure lens-like vaterite, sheet-like calcite, and peanut-like aragonite was all achieved in the [C 4 mim][C 12 SO 4 ] aqueous solution through changing the concentration, temperature, and initial pH value and adding magnesium ions. Washed by water and ethanol, all the aggregates were free of [C 4 mim][C 12 SO 4 ] and can be stable at least 1 month in air. The crystal form of the aggregates changed from pure calcite to pure vaterite at room temperature only through increasing [C 4 mim][C 12 SO 4 ] concentration. Formation of the ordered CaCO 3 structures is mainly ascribed to the aggregation of the primary nanoparticles whose formation mechanism is related to the change of supersaturation. This study can provide a facile and environment-friendly method to fabricate CaCO 3 crystal aggregates with various morphologies and polymorphs and can be used for large-scale industrial production and biomimetic synthesis.
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