| The electrostatic properties of the surface of soil minerals directly affect the interaction force between minerals,which strongly affects the aggregation of soil colloids and the formation,stabilization or dispersion of soil aggregates,and controls the occurrence of a series of macroscopic phenomenon such as water movement,soil erosion,and agricultural non-point source pollution.Atomic Force Microscopy(AFM)as a mechanical measuring instrument is widely used in the determination of the force between soil minerals.Based on the interaction force between the tip and the mineral determined by AFM,the Derjaguin-Landau-Verwey-Overbeek(DLVO)theory fitting the force curve,the electrostatic properties of the mineral surface can be further obtained.However,the surface potential and surface charge density measured by AFM in the existing literature are far lower than the true value.The reasons may be as follows.On the one hand,some studies use an approximate model of the interaction between the tip and the sample without considering the rationality of the approximate model.On the other hand,some studies have used Scanning Electron Microscope(SEM)to obtain the tip half-angle,radius and other parameters.The tip parameters obtained in this way usually have a large error.Therefore,the reason for the low surface potential and surface charge density fitted in the literature may be related to the unreasonable tip-sample interaction model and the inaccuracy of the tip parameters.Therefore,this article first established the tip-sample interaction model for the AFM probe to determine the electrostatic properties of the soil mineral surface.This model was used to determine the force between the tip and silica,and the force curve was fitted considering the real situation.And get the tip parameters,so as to verify the correctness of the model established in this paper,and perfect the method of AFM tip to measure the electrostatic properties of soil mineral surface.The research got the following results:(1)Establish the tip-sample interaction model in the determination of the electrostatic properties of the soil mineral surface by the AFM tip.In the literature,tip-sample interaction models of a cone with a spherical end interacting with a flat surface and a cone with a flat circular end interacting with a flat surface are usually used for AFM tips to determine the surface electrostatic properties of soil minerals.The calculation and analysis found that on the one hand,the tip-sample interaction model used in the literature is usually over-approximate:the tip-sample interaction model in the existing literature usually uses an over-approximate electrostatic force formula,which will cause great deviations in the calculation results.In the literature,the tip is often directly approximated as a spherical surface or a flat surface.Through calculations,it is found that the conventional radius and half-angle can only be approximated at a higher concentration(greater than0.01 or 0.1 mol L-1).In other cases,it can be approximated directly.Will deviate from the actual situation to a large extent.On the other hand,the tip parameters used in the literature are not accurate,the common cone tips on the market can be equivalent to the right cone model,but the equivalent cone half-angle is different from the side half-angle directly observed by SEM.The influence of the tip tilt angle and the tilt angle of the tip holder can be included in the influence on the cone half-angle,ignoring the tilt angle will cause deviations in the results.Under the usual electrolyte concentration and charge density conditions,the deviation of the radius and half-angle will affect the charge density The fitting result has a great influence,so the method of using SEM to directly observe the tip radius and half angle in the literature will bring great deviation.Compared the cone with a spherical end interacting with a flat surface and cone with a flat circular end interacting with a flat surface,the radius parameter of the cone with a flat circular end interacting with a flat surface is more accurate,The overall noise interference is smaller.In general,the tip-sample interaction model used in the literature has excessive approximation and inaccurate tip parameters,which will greatly affect the calculation results.Therefore,in the process of applying AFM tips to determine the electrostatic properties of soil mineral surfaces,it is necessary to use a more accurate flat-bottomed cone-plane action model,and to improve the accuracy of the tip parameters through effective methods.(2)The model of the cone with a flat circular end interacting with a flat surface was used to measure the force between the tip and silica.According to the established tip-sample interaction model,by measuring the interaction force between the tip-silica wafer under a certain solution condition,the force curve under different electrolyte concentrations shows a good law.The tip parameters of the tip can be obtained in situ from the force curves of different electrolyte concentrations at high pH,and then the surface electrostatic properties of the sample can be further fitted according to the tip parameters.Therefore,the surface roughness of the tip and the silicon dioxide wafer and the correction error of the tip's elastic coefficient can affect the measured force.In this regard,in the process of applying the established tip-sample interaction model to determine the force,and then inferring the electrostatic properties of the sample surface,it is also necessary to consider the deviation caused by the tip and sample surface roughness and the tip elastic coefficient.(3)The tip parameters are determined in situ by force.The deviation of the force caused by the surface roughness and the elastic coefficient of the tip can be corrected by the two correction parameters of the distance offset?D and the system error coefficient m.By introducing two correction parameters,the distance offset and the system error coefficient,the theoretical equation is used to refit the force curve between the tip and the silicon dioxide wafer in different electrolyte concentrations at high pH,and the corresponding tip is obtained.The tip parameters(equivalent cone half angle 39.8°,equivalent bottom radius 33nm)and correction parameters(distance offset 4.7nm,system error coefficient 105%).After considering the correction parameters,the theoretical equation can be completely consistent with the experimental force curve,which further proves the rationality of the correction method.In summary,this study has established a reasonable tip-sample interaction model for the AFM tip to measure the electrostatic properties of mineral surfaces,and proposed a method to obtain the tip parameters in situ,which perfected the method of AFM tip to determine the surface properties of minerals.In the process of AFM tip measuring the surface force of soil minerals and then calculating the electrostatic properties of the soil mineral surface,the model of a cone with a flat circular end interacting with a flat surface should be selected first,and the force on the surface roughness of the material and the elastic coefficient of the tip should be selected.The deviation is corrected,and the force curve between the tip and the soil minerals in different electrolyte concentrations at high pH is fitted by theoretical equations,and then the needle tip parameters are directly obtained,and then the correct surface potential and surface charge density values of the minerals are further obtained. |
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