2︰1和1︰1型黏土矿物胶体凝聚中Hofmeister效应的比较研究

Hofmeister Effects of 2︰1 and 1︰1 Clay Minerals in Agglomeration:A Comparative Study

土壤胶体是土壤具备肥力与生态功能的物质基础,土壤胶体凝聚与分散影响着土壤中一系列微观过程和宏观现象。采用动态光散射技术比较研究三种碱金属阳离子(Li+、Na+、K+)引发不同类型黏土矿物(2︰1型蒙脱石和1︰1型高岭石)胶体凝聚中的Hofmeister效应。研究发现,Li+、Na+、K+作用下蒙脱石、高岭石胶体的凝聚速率、临界聚沉浓度及活化能都存在明显差异,表现出强烈的Hofmeister效应。当电解质浓度为20 mmol·L–1时,K+引发蒙脱石胶体凝聚的速率为66.61 nm·min–1,远高于Na+、Li+引发蒙脱石胶体凝聚速率(5.93、4.41 nm·min–1);而与之对应的临界聚沉浓度则呈现K+(蒙脱石21.8 mmol·L–1、高岭石34.6 mmol·L–1)低于Na+(蒙脱石57.6 mmol·L–1、高岭石85.8 mmol·L–1)低于Li+(蒙脱石81.8 mmol·L–1、高岭石113.9 mmol·L–1)规律,胶体凝聚中活化能可合理解释此现象。电解质浓度为25 mmol·L–1时,Li+、Na+、K+引发蒙脱石、高岭石胶体凝聚的活化能分别为1.97 kT、1.43 kT、0 kT和2.94 kT、1.71 kT、0.49 kT,说明蒙脱石、高岭石胶体凝聚过程中Hofmeister效应序列均为Li+<Na+<K+。并且,在给定阳离子及相同浓度条件下,蒙脱石胶体凝聚的活化能低于高岭石。考虑到蒙脱石表面电荷密度(0.1227 C·m–2)高于高岭石(0.0583 C·m–2),本研究明确了强外电场中离子非经典极化作用是导致带电胶体颗粒凝聚中Hofmeister效应的根本原因。

【Objective】Soil colloid is the material foundation of soil fertility and soil ecological function.Agglomeration and dispersion of soil colloids influences a series of microscopic processes and macroscopic phenomena in soil.The purpose of this paper was to compare Hofmeister effects of 2︰1-typed montmorillonite and 1︰1-typed kaolinite mineral colloids in agglomeration triggered by alkali cations(Li+,Na+and K+),and analyze sources of the Hofmeister effects.【Method】To that end,ultrasonic dispersion and centrifugation of soil samples was performed to extract montmorillonite and kaolinite colloids,and dynamic light scattering was to determine separately,effects of Li+,Na+and K+on agglomeration of the mineral colloidal particles.And then comparison was made between agglomerations of the mineral colloids in three different alkali solutions in kinetic process and activation energy.【Result】Results show that the agglomeration varied sharply in rate,critical coagulation concentration and activation energy as affected by Li+,Na+or K+,demonstrating strong Hofmeister effects.In solutions with electrolyte concentration being 20 mmol·L–1,the aggregation of montmorillonite colloid triggered by K+reached 66.61 nm·min–1 in rate,much higher than that triggered by Na+(5.93 nm·min–1)and by Li+(4.41 nm·min-1).And in solutions with electrolyte concentration being 30 mmol·L–1,the agglomeration of kaolinite colloid activated by K+reached 32.43 nm·min–1 in rate,which was far too much higher than that triggered by Na+(7.28 nm·min–1)and by Li+(1.90 nm·min–1).The effect of Hofmeister effects on aggregation rate of montmorillonite and kaolinite colloids varied with the cation in the solution in an order of Li+<Na+<K+,its effect on critical coagulation concentration did in an order of K+(montmorillonite 21.8 mmol·L–1,kaolinite 34.6 mmol·L–1)<Na+(montmorillonite 57.6 mmol·L–1,kaolinite 85.8 mmol·L–1)<Li+(montmorillonite 81.8 mmol·L–1,kaolinite 113.9 mmol·L–1),which could be reasonably explained by activation energy needed in agglomeration of colloids.In solutions,25 mmol·L–1 in electrolyte concentration for montmorillonite agglomeration activation energy varied with the cation in the solution in an order of Li+(1.97 kT)>Na+(1.43 kT)>K+(0 kT),while for kaolinite it did in the same order(2.94 kT,1.71 kT and 0.49 kT).All demonstrated that the Hofmeister effects in agglomeration of montmorillonite and kaolinite did vary with the cation in the solution in the order of Li+<Na+<K+.A strong external electric field could greatly amplify the difference in deflection of ion outer layer electron cloud,thus producing strong polarization,i.e.non-classical polarization effect.With the ion radius of Li+,Na+and K+increasing the power of the central atom binding the outer electron cloud decreased step by step.The deflection of the electron cloud configuration varied in magnitude with the ion in the solution in an order of Li+<Na+<K+.Additionally,in the solutions with a given concentration of a given ion,montmorillonite needed lower activation energy than kaolinite for agglomeration.Montmorillonite(0.1227 C·m–2)was higher than kaolinite(0.0583 C·m–2)in surface charge density,and significantly higher too in surface electric field strength in the same system.Therefore,the strong polarizing capability of Li+,Na+and K+was more obvious in the montmorillonite colloidal system,and the stronger the polarizing action,the more significantly the surface potential lowered,which led to higher agglomerability of montmorillonite colloids than that of kaolinite colloids.【Conclusion】All the findings described above demonstrate that the nonclassical polarization of ions in the strong external electric field is the fundamental cause of the Hofmeister effects in agglomeration of charged colloidal particles.