高温下钙蒙脱石膨胀特性的分子动力学模拟

高温下蒙脱石的膨胀特性在核废料深部封存、二氧化碳封存及页岩气开发等应用中有着重要影响,但相关机理尚不明确.本工作使用分子动力学模拟为技术手段计算5 MPa和298—500 K等条件下,1.40—4.00 nm晶面间距(d)的一系列饱和钙蒙脱石的膨胀压力.以模拟所得的数值结果为依据,基于水化效应、双电层效应和离子关联效应等模型推演膨胀压力随温度与d的变化规律,并与相应的实验数据进行对比.模拟结果表明,当d较小时,因为高温会弱化水化力的强度,钙蒙脱石膨胀压力震荡的幅度降低,同时水化力作用的d的范围减小.当d较大时,因为高温强化离子关联效应,膨胀压力降低,同时双电层力的作用的d的范围增加.在较高温度和较大d时,膨胀压力为收缩力,阻碍膨胀.这些膨胀压力的变化规律与前期钠蒙脱石体系的研究类似.然而,通过对比两种蒙脱石体系的模拟结果,发现两种体系存在显著的差异—钙蒙脱石比钠蒙脱石更难膨胀到较大的d.此模拟结果与前人实验观测的结果相符.我们进一步将此差异归于钙蒙脱石的离子关联效应要远大于钠蒙脱石.有别于分子模拟中对于离子关联效应的精确描述,连续化的Poisson-Boltzmann方程因为忽略了离子关联效应,从而无法表达出与两种体系模拟结果都相吻合的膨胀压力变化规律.

Molecular dynamics simulation of swelling properties of Ca-montmorillonite at high temperatures

The swelling of Ca-montmorillonite at elevated temperatures is important for many applications including geological disposal of radioactive waste,subsurface carbon sequestration,and shale gas exploration.However,the experimentally observed swelling behaviors of Ca-montmorillonite contacting liquid water and the temperature effects on the swelling pressure are not well understood.In this work,molecular dynamics simulations are carried out to study the swelling of Wyoming Ca-montmorillonite with a d-spacing(d)range of 1.40–4.00 nm at 5 MPa and various temperatures(298–500 K).The ClayFF and SPC are adopted for modeling Ca-montmorillonite and water,respectively.The simulation box is measured to be 11.15,3.66,and 28.00 nm in the x-,y-,and z-direction.Atomistic pistons are used to control the bulk pressure of the water environment,and the implicit walls are implemented for preventing the ions from leaking from the pore into the water environment.The clay atoms are fixed during the simulation and the swelling pressure is calculated through dividing the force by the area.The equilibrium time is at least 20 ns and the production time falls in a range of 50–88 ns.The swelling pressure results show that for small d,high temperature reduces the magnitude of the oscillating curve of swelling pressure and also reduces the range of d where hydration force dominates the swelling pressure.This temperature effect is due to the weakened hydration force as evidenced from the weakened water density distributions inside the pore.For large d,high temperature reduces the swelling pressure,which is consistent with the experimental result,and increases the range of d where double layer force dominates the swelling pressure.The reduction of the swelling pressure can be explained by the enhanced ion correlation that reduces the double layer force according to the strong coupling theory,given that the calculated coupling parameters at higher temperatures are smaller.The swelling pressures are negative at elevated temperatures and large d,which prevents the clay from further swelling.However,the classical Poisson-Boltzmann(PB)equation predicts the positive double layer force since the ion correlation effect is not considered in the PB equation.Furthermore,the calculated swelling free energy curve shows that at 298 K and 5 MPa,it is difficult for Ca-montmorillonite to swell beyond a d-spacing of around 1.9 nm,which is in good agreement with the experimental result.The energy barrier for Ca-montmorillonite to swell to large d is larger than that for Na-montmorillonite,which means that it is more difficult for Ca-montmorillonite to swell to large d.This behavior is consistent with experimental observation and can be explained by the larger ion correlation effect in the Ca-montmorillonite system.These findings enhance the understanding of swelling of Ca-montmorillonite at elevated temperatures and could help to engineer better barrier materials for nuclear waste storage.