A Molecular Mechanism of Hysteresis in Clay Swelling

摘要

Clay minerals are an important class of materials because they are extensively present in many soils and sediments. They are also used in many very different applications, such as construction material, additives to mud drilling, or catalysis. In these applications it is important to control the swelling properties of clay minerals. For example, the extreme swelling capacity of these materials can cause severe difficulties for construction on clay soils. Depending on their structure, certain clay minerals are capable of swelling as a function of the relative humidity, salt concentration, or temperature. From a fundamental point of view the swelling behavior is only partially understood. Experiments showed that swelling proceeds stepwise, forming layers of water molecules (layered hydrates) between the clay-mineral platelets. Interestingly, this swelling shows hysteresis; the adsorption and swelling proceeds differently from desorption and shrinking. In the literature there is little consensus about the molecular origin of this hysteresis. Explanations are given in terms of structural rearrangements in the clay or changes in interactions between layers upon expansion or contraction. In this work, we use molecular simulation to study the mechanism of Na-montmorillonite swelling hysteresis in detail. In particular, we demonstrate that this hysteresis has a thermodynamic origin and its molecular mechanism is a free-energy barrier separating the transition between stable layered hydrates. Our simulations also predict how swelling hysteresis depends on the external pressure.