水中土clay-water（bound water-free water）、双电层、结合水-自由水
一、结合水与自由水的定义
2.2. Free water – bound water, saturation, porosity
In general liquid water in soils and aggregates can be classified as: 1) adsorption water, also called hygroscopic water,(吸附水，又称吸湿水)—--对应结合水；
2) viscous water, or capillary water (粘性水，或毛细水)；
3) free water (自由水).
A more simpler classification divides water into two forms：
a) bound water (结合水)：强结合水（固定层或者叫吸附层）、弱结合水（扩散层），构成双电层
b) free water（自由水）：毛细水、重力水（土力学分类）
Terminology of water in soil profile. （土壤剖面中水的术语）
2.2.1. Bound water
Adsorption water and viscous water around a soil particle with a negatively charged mineral surface. A meniscus is the surface or interface of water and air. Well arranged water molecules in a meniscus can increase tensile strength of the water. (0.002um = 2 nm = 20 A)
（吸附水和粘性水周围的土壤颗粒带有带负电荷的矿物表面。半月板是水和空气的表面或界面。在半月板中排列良好的水分子可以增加水的抗拉强度。）
a. Adsorption water（吸附水）
Because the water molecule is polar, and because the greatest part of mineral surfaces have a negative charge, the water molecules closest to the mineral surface are very well arranged. This adsorption water consists of two layers; tightly and loosely bound layers. The thickness of the tightly bound adsorption water layer is about 0.002 um. Adsorption water condenses on the surface of soil particles straight from water vapour in the air. Around the tightly bound layer is the loosely bound adsorption water layer. This layer thickness varies from the 0.002 um to 0.006 um. Salt reduces the thickness of this layer, and thus helps compaction of the material.
Adsorption water can also be called bound water as it can act as a “binder” between soil particles producing the tensile strength of the dry material. That is why in some countries dry unbound materials are also called as “water bound” materials.
The amount of adsorption water is also controlled by the specific surface area of minerals. The higher the specific surface area, the higher is the adsorption water content. However not all the adsorption water is harmful to material performance. For instance, iron oxides can adsorb great amounts of water but this water does not cause performance problems for the aggregates.
（1）吸附水包含两个水层：强结合水（约厚2 nm）、弱结合水（约厚2-6 nm）；
（2）吸附水也可以称为结合水，因为它可以作为土壤颗粒之间的“粘合剂”，产生干材料的抗拉强度。
（3）吸附水量也受矿物比表面积的控制。比表面积越高，吸附含水量越高。
（4）并不是所有的吸附水都对材料性能有害。例如，氧化铁可以吸附大量的水，但这些水不会对骨料的性能造成影响。
b.Viscous water or capillary water (粘性水，或毛细水)
The moisture in soil which is not bound around mineral grains as hygroscopic water, and does not respond to gravity, is usually called as viscous water, or capillary water. Capillary water can also be divided into “inner” and “outer” layers. When compacting aggregates the optimum water content in road materials is where the inner capillary layer changes to the outer capillary layer. Capillary forces are also very important factors in the frost heave process on roads.
（1）毛细水又可分为“内”层和“外”层。
（2）压实集料时，路面材料的最佳含水量是内毛细层向外毛细层的变化。毛细力也是影响路面冻胀的重要因素。
c. Menisci or contractile skin （半月板或收缩皮肤）
Capillary menisci, also called contractile skin, form between the particles and air in unsaturated soils and aggregates. This air-water interface is only a few molecular layers thick but its presence is very important in soil mechanics because of its property of exerting tensile pull. This ability is called surface tension. The magnitude of surface tension depends on temperature; if the temperature increases the magnitude decreases.
Menisci or contractile skin is an important factor in matric suction （基质吸力）, which is described later in this lesson.
毛细半月板，又称收缩皮，是非饱和土壤和聚集体中颗粒与空气之间形成的。这种气-水界面虽然只有几个分子层厚，但由于其具有拉力作用的特性，在土力学中具有重要意义。这种能力被称为表面张力。表面张力的大小取决于温度;如果温度升高，幅度减小。
Hysteresis effect can be seen when placing a water drop to a glass plate and tilting the plate slightly（在玻璃板上放置一滴水滴，并将玻璃板稍微倾斜，就可以看到滞后效应）
2.2.2. Free water
Free water (also called gravitation water) moves through the soil voids under the force of gravity. It is important to be conversant with free water as road drainage systems can be affected by the amount of this water type. The amount of free water has an immediate influence in decreasing bearing capacity. It also weakens the stability of road edges and causes edge settlements and erosion. Free water is an important factor in the freeze-thaw process. In the fall when the temperature of mineral aggregates or soil drops below 0°C, free water freezes first forming hexagonal crystals so increasing its volume and causing frost heave.
自由水(也叫重力水)在重力的作用下穿过土壤的空隙。熟悉自由水是很重要的，因为道路排水系统可能会受到这种水类型的数量的影响。
（1）自由水量对承载力的降低有直接的影响。它还削弱了道路边缘的稳定性，造成边缘沉降和侵蚀。
（2）自由水是冻融过程中的一个重要因素。在秋天，当矿物聚集物或土壤的温度降至0°C以下时，自由水首先结冰，形成六边形晶体，从而增加其体积，造成冻胀。
二、黏土-水体系（教材：固体表面与界面）
https://wenku.baidu.com/view/05460c20f12d2af90242e6e2?pcf=2&bfetype=new
水分子的直径大约3A，3个水分子层大约9-10A (1 nm), 所以，3~10个水分子层大约是1-3nm
三、高等土力学---双电层
三、Determining the bound water content of montmorillonite from molecular simulations
Zhang, S. and Pei, H. Determining the bound water content of montmorillonite from molecular simulations. Eng. Geol.2021: 106353. http://doi.org/10.1016/j.enggeo.2021.106353.
结合水厚度根据密度分布和shear plane来划分，结论得到：
（1）固定水层的厚度是4.3A。
（2）（紧密）强结合水的厚度大约是6.0A，对应两个水分子层。
（3）总结合水（强、弱结合水）厚度分别为10.5 Å和11.8 Å接近水密度剖面的第三或第四个峰。这表明钠基蒙脱石的总结合水可能有三层或四层。
三、Wang, H., Qian, H., Gao, Y. and Li, Y. Classification and physical characteristics of bound water in loess and its main clay minerals. Eng. Geol.2020: 105394. http://doi.org/10.1016/j.enggeo.2019.105394.
（1）蒙脱石的结合水密度是1.41-1.19g/cm3，最大的结合水层厚度是10.65A (3个水分子层)。
（2）蒙脱石总结合水层厚度是3.1-10.65A，对应相对湿度（RH）11%-100%；其中强结合水厚度约为0-4.6A（69%），4.6-10.65A是弱结合水（扩散层）。
只有强（紧密）结合水条件：RH小于69% for MMT。大于69%，就会有弱结合水（扩散层）。
（3）伊利石结合水层厚度是2.8-5.25A，对应11%-100%。黄土：1.5-2A，对应11%-100%。
（4）MMT上的总结合水有三层以上的多层吸附，IL伊利石和ML黄土上的结合水分别可视为单层吸附和双层吸附。
