Experimental Study On Interfacial Ion Structuring In Highly Concentrated Electrolyte Solutions

Highly concentrated electrolyte solutions are ubiquitous in biology,industrial applications,and geology,meanwhile these industrial and academic applications are basically related to the solid-liquid interface under concentrated electrolyte solutions.Therefore,it is of great significance to study the interfacial ion structuring of high concentration electrolyte solution to guide industrial production and research related to biological processes.Firstly,the formulas for the interaction force between surfaces,including van der Waals force,electric double layer force and hydration force are deduced.It provides theoretical formulas and calculation methods for the quantitative analysis of experimental data in the research work.Then,using the established surface forces apparatus（SFA）experimental platform,the interfacial interactions between the two mica surfaces in different concentrations,different types of co-ions and counterions and different valence electrolyte solutions are measured to study the influencing factors of the ionic distribution structure at the solid-liquid interface.The curve of the interaction force between two mica surfaces in different concentrations of NaCl solution is measured.The results show that the Possion-Boltzmann（PB）theory in the low concentration solution well describes the characteristics of ion and potential distribution near the solid-liquid interface.As the concentration increases,the near surface repulsion between the two mica surfaces prevents the surface from jumping into contacting.The reason for this phenomenon is that when the concentration is higher than a certain value,the hydrated Na⁺replace the hydronium ion in the mica surface binding site and adsorb on the mica surface.When the distance between two surfaces is close to extrusion,dehydration of sodium ions requires a certain amount of energy,resulting in near-repulsive hydration.When the concentration of the solution is large,the PB theory fails,and an additional long-range force observed in the experiment can be explained by the alternating distribution of anion and cation structures.It is noteworthy that the final hard wall value increases with the increased of the solution concentration,and this increasing tendency is caused by a variety of reasons,such as enhanced hydration repulsion and the phenomenon of solidification of the Stern layer.The solid-liquid interface ionic structure shows a strong ionic specificity.This difference in interfacial ion structuring is caused by the difference in the ionic hydration characteristics.the hydration ability of Cs⁺is the weakest,and it is easily dehydrated and embedded in the mica lattice.However,the hydration ability of Li⁺and Na⁺is strong,and it is difficult to dehydrate and embed in the mica lattice,so that the characteristics of the interaction force between the surfaces in the solution are very different.When the concentration is high,the effect of co-ions on the interfacial ion structuring can no longer be neglected.Cl^-and NO₃^-,which have large differences in hydration characteristics,lead to differences in the structure of the solid-liquid interface under the final confinement conditions.The interfacial ion structuring in the high valency solution is very different from that in the low valency solution.When the concentration is high,the surface of the negatively charged mica adsorbs excessive counter ions due to the strong correlation of the ions,resulting in surface charge inversion.After the phenomenon of charge inversion on the mica surface,the counterions and co-ions in the concentrated solution tend to form alternately distributed layering structures,thereby generating an extra long-range force.At the same time,the NO₃^-with weak hydration ability and the dehydrated La³⁺easily form pre-nucleation clusters under confined conditions.Such clusters will eventually remain between the two surfaces,resulting in a large hard wall value.With LabVIEW software,a data processing module suitable for surface force experiments is innovatively designed.The video information recorded in the experiment can be automatically converted into experimental data through the module,and the automatic operation of the corresponding manual operation can be realized,thereby reducing the data processing time and improving the experimental efficiency.