Research On Electrical Double-layer Structure And Mechanical Properties Of Ionic Liquids At Solid-liquid Interface

In this paper,the electrical double-layer structure of ionic liquid at solid-liquid interface was studied by surface force apparatus.The measurement of force and distance was made,based on the principle of multi-beam interferometry.The force-distance curves was obtained by analyzing the FECO（fringes of equal chromatic order）.The electrical double-layer structure was revealed,using the classical DLVO（Derjguin-Landau-Verwey-Overbeek）theory and the Poisson Boltzmann equation to analyze the force-displacement curve measured in the experiment.The innovative results were obtained as follows.1)The influence of the change of cationic group（length of alkyl carbon chain）on the related characteristics of the structure at the solid-liquid interface was studied using ionic liquids with[BF₄^-]and[PF₆^-]as anions.It is observed that as the length of the alkyl carbon chain of the cationic group（volume of cation）increases,the decaying length and Hardwall distance both increase.The observed correlated phenomena is related to the influence towards interfacial arrangement and electrostatic interactions（distance between ions）between the charges,which is caused by the change of the structure of cationic group.For ionic liquids with different anions（[BF₄^-],[PF₆^-],[NTf₂^-]）,different temperatures were selected for multiple experiments in which we find that the decaying length of the force curve decreases with the increase of temperature.The related phenomena is analyzed from the perspective of dissociation combining with the model of ion distribution and related formulas.The force curves measured at different temperatures using same ionic liquid as the medium have no significant difference when the distance between the two surface is small,which reflect that the change of temperature has little effect on the ions’structure and arrangement of near-wall regions.In addition,it was observed that the adhesion force of curves of separation between two surfaces decrease as the temperature increased.2)Experiment were conducted by passing the air with ambient humidity of different duration into the cavity or passing nitrogen gas after adding water and relaxation,using[C₄mim⁺][NTf₂^-],[C₄mim⁺].[BF₄^-],[C₄mim⁺].[PF₆^-]as the medium between the two surfaces.It was observed that the Hardwall and decaying length increased with the increase of the degree of moisture introduced in a certain range,and the change trend of the adhesive force when two surfaces are separated is exactly the opposite.The phenomenon is explained by the hydrogen bonding interactions between water molecules and ions,hydrophobicity of anion,arrangement of ions and charge screening,interaction between surface charge and ionic liquid charge,and distribution of surface charge.In addition,the effects of the way of absorbing moisture from ambient humidity air and the method of mixing water with ionic liquids directly are compared.In order to simplify the loading and unloading of the upper and lower silica disks and reduce the influence of the unsealed solid-liquid interface environment,the modularization liquid pool structure was designed based on the clamping devices of upper and lower surfaces load-carrying.The relevant parameters are measured and the materials of the selected parts was chosen.The work of this article deepens the related research of ionic liquids and is of great significance for its application as an electrolyte of electric double layer capacitors.