Ca/Na Montmorillonite: Structure, Forces and Swelling Properties
Ca/Na Montmorillonite: Structure, Forces and Swelling Properties
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DOI：
10.1021/la9036293
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发表时间：
2010-04-20
期刊：
影响因子：
3.9
通讯作者：
Cabane, B.
中科院分区：
文献类型：
--
作者：
Segad, M.;Jonsson, Bo;Cabane, B.
对钙钠型蒙脱石和天然怀俄明州膨润土（MX-80）进行了实验和理论研究。对于与纯水平衡的粘土系统，Monte Carlo模拟预测当粘土抗衡离子是单价时的大溶胀，而在二价抗衡离子的存在下，获得了有限的溶胀，其中粘土片晶之间的水层为约10埃。后者的结果是在良好的协议与X-射线散射数据，而透析实验给出了一个显着较大的溶胀钙蒙脱石在纯水中。显然，有一个“层内”和第二个“层外”肿胀。与同时含有Na+和Ca 2+的盐储层接触的蒙脱石将仅显示出适度的溶胀，除非在船体中的Na+浓度比浓度大几个数量级。在二价抗衡离子的存在下，粘土的有限溶胀是相关性的结果，这降低了熵排斥，并在总渗透压中产生了有吸引力的组分。离子-离子相关性也有利于二价抗衡离子与一价竞争的情况下。离子-离子相关性的一个更基本的结果是，作为粘土片分离的函数的渗透压变得非单调。这表明相分离成浓缩和稀释粘土相的可能性，这将对应于透析实验中发现的“层外”溶胀。这一观点也得到了X射线散射谱的支持，在X射线散射谱中有时会出现两个对应于不同片层间距的峰。
Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 angstrom. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one "intra-lamellar" and a second "extra-lamellar" swelling. Montmorillonite in contact with a salt reservoir containing both Na+ and Ca2+ counterfoils will only show a modest swelling unless the Na+ concentration in the hulk is several orders of magnitude larger than the concentration. The limited swelling of clay in presence of divalent counterions is a consequence of correlations, which reduce the entropic repulsion is well as give rise to an attractive component in the total osmotic pressure. Ion ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic. which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the "extra-lamellar" swelling found in dialysis experiments. This idea also rinds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.