Research On AFM Method For The Determination Of Soil Mineral Interaction Forces And Surface Electrochemical Properties

The interaction forces between soil minerals determine the aggregation and dispersion of soil particles,which affect the formation and stability of soil aggregates and soil pores,and thus affect the occurrence of macroscopic processes such as soil water movement and soil particle migration.The direct measurement of the interaction forces between the particle surfaces in solutions is of great significance for understanding the electrical double layer structure,colloidal aggregation and dispersion process,specific ion effects(Hofmeister effects),etc.Since the introducing of the DLVO theory describing the mechanism of colloidal aggregation and dispersion,a large number of direct determination experiments of the interaction forces have been used to verify its correctness.However,in the existing literature on the study of the interactions between soil mineral surfaces,the experimental results obtained have major problems in terms of their reproducibility and the rationality of the measured values,mainly in:(1)The force,surface potential and surface charge density of the same material measured under the same condition(electrolyte concentration or pH)are very different;(2)The absolute values of the surface potential and surface charge density obtained under each condition are too small,and the surface potential is sometimes even smaller than the measured value of zeta potential under the same condition.These results are obviously unreasonable,but the existing literature did not give an explanation for these unreasonable results.In fact,in the existing researches,when using mechanical measuring instruments(atomic force microscope(AFM),surface force meter(SFA),etc.)to directly measure the forces between particles,the experimental material is considered to have an absolutely regular geometric shape(ideal cylinder,sphere or plane)and an absolutely smooth surface.However,the actual shape of the experimental material is often not absolutely regular,and the surface is not completely smooth.Therefore,in the determination of the interaction forces,if the influence of these factors is not considered,the analysis and fitting of the experimental results may be seriously distorted.Based on this,this study assumes that the main reason for the above-mentioned problems is that the existing studies have not considered the influence of the shape and surface roughness of the experimental materials.Therefore,in this study,a different approach was taken.First,the influence of the shape and surface roughness of the experimental material on the determination of the interaction forces was quantitatively characterized,and then a feasible solution was proposed.The study achieved the following results:(1)Correction method in the measurement of mica interaction forces.As a special material mica has an atomically smooth surface and is widely used in the determination of interaction forces.However,in the previous literature related to the research on the forces of mica,the obtained surface potential and surface charge density generally deviated seriously from theoretical expectations.Obviously,the possible cause is not the problem of the surface roughness of mica,but the shape of the mica material in the measurement.Therefore,in this study,the parameter "effective radius" was used to adjust the apparent radius value of cylindrical mica used in the literature to refit the experimental data,and it was found that the obtained results were consistent with the theoretical values of surface potential and surface charge density.The refitted surface charge density is a constant value,consistent with the "constantly charged surface";the refitted surface potential is much higher than the zeta potential,which is consistent with the actual situation.(2)Correction method for silica interaction force measurement.Most materials not only have irregular shapes,but also rough surfaces,so it is necessary to correct the shape and surface roughness at the same time.In this study,using silica as the material,the interaction force between the "silica sphere probe and silica wafer" was measured,and the distance offset and effective radius parameters of the interaction were directly calculated.The results show that the shape and surface roughness of the silica material directly affect the interaction force measurement;the influence of the material shape and surface roughness on the interaction forces can be quantified by the two parameters "distance offset" and "effective radius";based on the material shape and surface roughness correction,the correct surface potential and surface charge density values can be fitted through the force curves.(3)Since the shape and surface roughness of the material directly affect the interaction force measurement,this study further proposes a method for correcting the shape and surface roughness in AFM analysis of natural montmorillonite particles with very irregular shapes and surfaces.This method successfully measured the interaction forces between montmorillonite and mica.After correcting the shape and surface roughness,the surface electrochemical properties of soil minerals can be obtained by fitting the force curves measured by AFM.Through AFM measurement,direct evidence of the influence of the Hofmeister effects on the metal-cation interface reactions and soil mineral interaction forces was obtained.Therefore,AFM will be very important in revealing the mechanism of Hofmeister effects in soil system.To sum up,this study proved that the shape and surface roughness of materials will directly affect the measurement of the interaction forces between particles and the surface electrochemical properties of materials.In the process of measuring the interaction forces,it is necessary to first correct the shape and surface roughness of the material.Through the theoretical method established in this study,the shape and surface roughness of the material can be corrected well,and the correct surface electrochemical properties of the material can be obtained by fitting the determined force curves.Since the biggest difficulty in AFM analysis of soil particles is the shape and surface roughness of soil particles,the principles and methods established in this study have laid the foundation for the application and promotion of AFM in soil system.