Study On The Aggregation Of Montmorillonite Particles And Interaction Force Between Particles By AFM

The mineral,organic and microorganism nano-colloidal particles in the soil are the most active component,which play ubiquitous critical role in soil fertility and ecological functions.Its size is mainly concentrated in the range of tens to hundreds of nanometers,which has special interface scale effect.The aggregation?dispersion and the interaction of nano-colloidal particles have a profoundly impact on the macro-phenomena occurring in the soil such as the erosion and the pollution of the soil.However,the study on microscopic effects of soil nano-colloidal particles interactions has not yet aroused enough attention due to the limitation of the measuring method.Due to the atomic force microscope?AFM?is an intuitive microscopic tool for studying aggregation and interaction of montmorillonite nanoparticles,thus in this study,aggregation of montmorillonite particles and the interaction force in gas/liquid interface are observed by the AFM,and we also analyze the ion specific effect occurring in the aggregation of the particles;At the same time,we establish a theoretical model to describe the electrostatic repulsion between particles considering the ion specific effect under the liquid phase.The study will provide experimental support to theoretical studies on interactions between ions and particles,and give references for further studies on ion-specific effects of aggregating particles in strong electric fields.The main results are obtained as follows:?1?The concentration of electrolyte solution significantly affects the aggregation of Montmorillonite nanoparticles,and the montmorillonite particles aggregation degree exhibite obvious ion specific effects.With the increasing of the electrolyte concentration in the system,the montmorillonite particles aggregate horizontally first and then aggregate vertically.The height of the montmorillonite particles in the 10mmol L^-1 of the different electrolyte system are all about 1 nm;In the Cs⁺system,the particles aggregate horizontally because the diameter of the particles is up to 300 nm,while those in the other ionic systems are merely 100 nm,and has not aggregated horizontally;As the electrolyte concentration is up to 30 mmol L^-1,the diameter of the particles in the Li⁺,Na⁺and K⁺systems vary in the range of 80¹60 nm,and the height of the particles in those systems are 1¹.5 nm,which indicates that the aggregation of the montmorillonite particles has not obviously found,while in the Rb⁺system,the diameter reaches 400 nm and the maximum height is about 4 nm,which indicates that the particles aggregate horizontally,and small parts of the particles aggregate vertically;and in the Cs⁺system,the maximum height reaches about 600nm and maximum height reaches about 27 nm,which indicates that the particles aggregate horizontally and vertically;In the 50 mmol L^-1 of the electrolyte,the height of the montmorillonite particles in the Li⁺,Na⁺,K⁺,Rb⁺and Cs⁺systems is approximate 0.7 nm,0.8 nm,2 nm,2¹4 nm and 60 nm,respectively,which indicates that superposition is weak in the K⁺system,strong aggregation process is occurred in the Rb⁺system and the particles are up to 10 layers vertically;the aggregation in the Cs⁺system is most obvious.?2?The ion specific effect is found obviously in the interaction force between the montmorillonite particles.The force spectrum characteristics curve of montmorillonite particles treated with NaCl solution of 10 mmol L^-1,30 mmol L^-1 and50 mmol L^-1 are observed.It shows that the interaction force between montmorillonite particles is attractive force at 2 nm,and the attractive force between particles increases with the increasing of the electrolyte concentration.When the electrolyte concentration is 10 mmol L^-1,the maximum attractive force of montmorillonite particles treated with LiCl,NaCl,KCl,RbCl and CSCl solutions is about 1² nN.In spite of the difference of interaction force between the particles is little,but it also show an obvious ion specific effect.When the electrolyte concentration is 30 mmol L^-1,the maximum attractive force of the montmorillonite particles is about 1³ nN.The maximum attractive force between the montmorillonite particles and the probe increases with the increasing of electrolyte concentration,at the same time the difference of maximum attractive force also increases at different electrolytes.The difference of maximum attractive force between the montmorillonite particles treated with RbCl and CsCl and probe is much greater,and the interaction of montmorillonite particles in each electrolyte system shows strong ion specific effects.The maximum adhesion force between Montmorillonite particles and the probe varies in the order of Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺,showing significant ion specific effects.The difference of maximum attractive force is little at the lower electrolyte concentration,but it is greater at higher electrolyte concentration.?3?A theory/method for direct prediction of electrostatic interaction force between particles is established as considering the ion specific effects.The theoretical expressions of the surface charge density and the midpoint potential of particles are obtained,and further the electrostatic repulsion force between particles can be calculated,thereby the net resultant force between particles can be calculated by DLVO theory.According to the results of atomic force measurement,it is deduced that the charge coefficient of Li⁺is about 1,and the charge coefficient of Na⁺is about1.07.The theory and method provide a theoretical basis for further exploration of soil water movement,soil erosion and agricultural non-point source pollution caused by soil particle interaction.By the AFM,strong ion specific effect is found in the aggregation of montmorillonite particles and varies in the sequence of Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺.The colloid particles aggregate mainly horizontally in the lower electrolyte concentration,and then begin to aggregate vertically with the increasing of ionic concentration.The higher the particle aggregation,the higher the adhesion force of the particles.In a word,the surface charge density of the particles affects the electrostatic repulsion between the particles,further affects the interaction force between the particles.