Revealing the Molecular Forces that Stabilize Surface Precipitates of Hydroxides by Three-Dimensional Atomic Force Microscopy

Surface charging of solid-liquid interfaces plays a critical role in various chemical processes, such as ion adsorption, particle aggregation, crystal nucleation and growth. Recent work indicates that surface charging can drive the formation of complex laterally ordered surface structures. However, the properties of the resulting heterogenous charge distributions (and their associated electrical double layer structure) are still poorly understood. Modern atomic probe microscopy offers a promising tool for investigating the behaviors at heterogeneously charged surfaces. In situ high speed atomic force microscopy enables directly imaging of the motion of surface adsorbates at molecular scales; while three-dimensional fast force mapping (3D FFM) allows us to track the tip-sample interaction forces above the surface, aiding in the interpretation of surface charging phenomena through DLVO theory.<br/><br/>Here, the adsorption behaviors of multivalent cations (Mg²⁺ and Al³⁺) on muscovite mica has been studied by atomic force microscopy across a range of pH values. At mild pH values, two-dimensional hydroxide films form whose morphology depends on the ion type. For the divalent Mg-hydroxide, it tends to form monolayered large continuous film. For the trivalent Al-hydroxide, it forms monolayered films with a persistent network of gaps, combining positively charged hydroxide islands and negatively charged bare mica regions, which is inconsistent with the prediction of the classical crystal growth model; at even higher pH values, multi-layered gibbsite film growth has been observed, with the upper layers exhibiting similar behavior to Mg hydroxide films that form large continuous sheets. At low pH conditions, it is feasible to image individual ions and clusters and see how the adsorbate populations change with pH. At a pH immediately preceding film nucleation, these ions reveal complex lateral ordering that are controlled by the interplay between ion/ion interactions and ion/substrate interactions.<br/><br/>The links between ion-ordering, film structure, and local surface charge were investigated by using 3D FFM and complementary streaming potential measurements. The results indicate that the coverage and structure of the surface precipitates are strongly influenced by the charged substrate. Also, Al-hydroxide films are more strongly charged than Mg-hydroxide films, supporting the hypothesis that the hydroxide nanostructure is stabilized by electrostatic interactions between positively charged film and negatively charged substrate. The work exemplifies the importance of surface charge in crystal nucleation, while also offering valuable insights on how electric fields can be utilized to control crystallization processes.