Study On Solid-liquid Interface Structure Of Electrolyte Aqueous Solution Based On Surface Forces Apparatus

The solid-liquid interface is ubiquitous in our daily life and industrial technology,controling many physical and chemical processes,such as colloidal dispersion,ion exchange,crystal growth and dissolution,friction and lubrication.It is necessary to understand the interface structure to develop the basic physical chemistry of solid-liquid interface.In this paper,surface force apparatus(SFA)technology is used to measure the interaction forces between mica surfaces in the electrolyte solutions.By analyzing the functional relationship between the interaction force of surface and distance,we can obtain the comprehensive information regarding the solid-liquid interface structure.In the first part,the solid-liquid interface structure was studied in highly concentrated electrolyte solutions.It was found that when the salt concentration is 0.1 mol/L(M),the decay length of electric double layer force is consistent with the theoretical calculation,and the distribution of ions in the diffusion layer can be predicted by Poisson-Boltzmann theory.Above0.1 M,the decay length and the range of interaction force were not reduced,while an abnormal long-range repulsion occured.The results indicate that anions affect the ions distribution near the negatively charged surface.The surface charge is screened by the specific adsorption of anions;on the other hand,anions and cations may form stable ion pairs.Due to Specific ion effects,the anions have different probability to form contact ion pairs.In addition,when the salt concentration is 0.1 M,in a highly confined environment,some hydrated ions have not been squeezed out of the surface,resulting in a phenomenon called Hardwall.The adsorption of hydrated ions between the surfaces increase along with the salt concentration(the Hardwall distance increases).When the salt concentration continues to increase,the compressive effect of osmotic pressure is too obvious to be ignored,and the thickness of the adsorption layer will decrease(Hardwall distance decreases).Then,the paper explored the structural changes of Nafion absorbed on mica in solution environment.Fristly,it was measured that the interaction force between the Nafion film and the mica surface in water.With the higher load,Nafion still adsorbed on the mica substrate stably,and the absorbed water would be squeezed out of the Nafion film.In low-concentration salt solutions,water molecules combined the sulfonic acid groups of nafion directly.In highconcentration solutions,cations directly binded to Nafion’s sulfonic acid groups.In addition,cations with different hydration capacities have different effects on the structure of Nafion membranes.It is difficult for strongly hydrated cations and Nafion to form ion pairs.So the strongly hydrated cations have poor electrostatic screened effect on Nafion.