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Surface Charge Heterogeneities Measured by Atomic Force Microscopy

机译：通过原子力显微镜测量的表面电荷异质性

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Unfavorable aggregation and deposition of colloidal particles in natural and engineered systems is still a subject of debate. Complicating factors such as surface roughness, secondary minimum aggregation, and the nature of discrete surface charge and surface potential make it difficult to attribute a specific cause to these phenomena. The presence of surface charge heterogeneity and its influence on interaction forces, which are responsible for aggregation and deposition, are studied in this work through the application of atomic force microscopy (AFM). Force-volume-mode AFM was used to map interaction forces on a surface and relate them to surface charge heterogeneities. The experimental system consisted of a silica plate and a standard silicon nitride AFM tip. Copper ions were used for sorption on the silica surface in order to modify the surface charge and cause charge reversal. Different concentrations of copper ions were selected to identify conditions of partial coverage of the silica surface. The pH and ionic strength of the solutions were varied, and the extension of the surface charge modification and its influence on the resulting interaction forces were monitored via AFM force measurements. Depending on the pH and ionic strength, the interaction force was found to change at certain regions on the surface from attraction to either weak or strong repulsion. Force imaging allowed the visual localization of zones of strong repulsive interaction that diminished in size with increasing ionic strength. X-ray photoelectron spectroscopy analysis was used to confirm the presence of copper on the surface. Local charge differences on a surface result in local differences in surface forces, not only in magnitude but also in direction. This behavior may explain the aggregation, deposition, and transport of colloidal particles under unfavorable chemical conditions.
机译：胶体粒子在自然和工程系统中的不利聚集和沉积仍是争论的话题。诸如表面粗糙度，二次最小聚集以及离散的表面电荷和表面电势的性质等复杂因素使得很难将具体原因归因于这些现象。通过原子力显微镜（AFM）的应用，研究了表面电荷异质性的存在及其对引起聚集和沉积的相互作用力的影响。力-体积-模式原子力显微镜用于绘制表面上的相互作用力，并将它们与表面电荷异质性联系起来。实验系统由二氧化硅板和标准氮化硅AFM尖端组成。铜离子用于吸附在二氧化硅表面上，以改变表面电荷并引起电荷反转。选择不同浓度的铜离子以鉴定二氧化硅表面的部分覆盖的条件。改变溶液的pH和离子强度，并通过AFM力测量监测表面电荷改性的扩展及其对所得相互作用力的影响。根据pH和离子强度，发现相互作用力在表面的某些区域从吸引变为弱排斥或强排斥。力成像使视觉上的排斥力强的区域可视化，该区域随着离子强度的增加而减小。 X射线光电子能谱分析被用来确认表面上铜的存在。表面上的局部电荷差异导致表面力的局部差异，不仅大小，而且方向也不同。此行为可以解释在不利的化学条件下胶体颗粒的聚集，沉积和运输。

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