重叠双电层粘土体系中粘土的膨胀

SWELLING OF CLAY IN CLAY SYSTEM WITH OVERLAPPING ELECTRIC DOUBLE LAYER

本文采用Ag-AgCl电极判断平衡的方法,在25℃恒温条件下探讨了重叠双电层粘土体系中Upton蒙脱粘土的膨胀压与粘粒板层间距离的关系;同时应用双电层理论计算了该体系中粘土的膨胀压,并对膨胀压的理论值与实验值进行了比较和讨论。实验结果表明:不同电解质浓度下,膨胀压P与粘粒板层间距λ呈较好的指数关系;在相同的λ距离下,膨胀压随电解质浓度的增加而减小;由双电层理论导出的渗透膨胀公式所计算得到的粘土的膨胀压与实验值偏差甚大,渗透膨胀压仅占实际粘土膨胀压的很小比例,完全可忽略不计。看来,双电层理论可能不适用于讨论粘土的膨胀。

The experimental apparatus which is applied to study the swelling of clay has been designed and made. The experimental apparatus is mainly composed of five parts which are (1) nitrogen tank, (2) gas valve, (3) pressure gauge, (4) temperature control box, and (5) pressure membrane system. The fifth part is made of polymethyl methacrylate and consists of two chambers, i. e., upper chamber and lower chamber in which two Ag-AgCl electrods are respectively installed to indicate the equilibrium.The swelling of clay in the upton montmorillonite clay system with overlapping electric double layer was studied at 25℃ by using the method of Ag-AgCl electrod to set the equilibrium. The clay suspensions with different electrolyte concentrations were made up by montmorillonite and NaCl solution of given concentration. When the experiment began, the given pressure was applied to the clay suspension surface in upper chamber by nitrogen gas. When the difference of millivolt between two Ag-AgCl electrods did not vary ith time, the equilibrium had been obtained. The experimental data, such as swelling pressure, P, di stance between two clay plates, λ, Cl^- concentration in clay cake of the upper chamber and in the bulk solution of lower chamber were measured. The experimental results shown that the swelling pressure, P, was the exponential function of the distance between two clay plates, λ, and the variation of swelling pressure with the distance between two clay plates could be described by empirical equation,ln(P+1)=(2α)/(ρ_w^s)(1/λ-1/((?)λ_0),where α is constant related to the kinds of clays, specific surface area and cation exchange capacity, ρ_w the density of water and s the specific surface area of clay. A plot of ln (P+1) versus 1/λ produced a straight line having a slope equal to (2α)/(ρ_w^s) but the straight line was divided into two parts because of the change of α (Fig.3). This empirical equation early proposed by Low could be used to describe the results under several different experimental conditions in this study.The electric potential at the midplane, φ_λ, was calculated by the combination of anion negative adsoption equation derived by author and the overlapping electric double layer theoretical equation. The theoretical value of swelling pressure, i. e., osmotic pressure was calculated by Langmuir swelling equation according to the electric double layer theory. It is concluded that the osmotic pressure of clay calculsted by electric double layer theory occupies the small fraction of the experimental swelling pressure value through comparision of theoretical swelling pressure value with the experimental value and that the theory of electric double layer may not be applied to discuss the swelling pressure of clay.