纳米多孔固体中的吸附滞后理论：II。分子凝聚
Theory of Sorption Hysteresis in Nanoporous Solids: II. Molecular condensation
摘要：
出于波特兰水泥混凝土或水泥浆体的吸附滞后的困惑，我们在本研究的第二部分中开发了一个一般的理论，从纳米多孔固体的蒸汽吸附和解吸，其属性滞后受阻的分子凝聚有吸引力的横向相互作用。应用经典的范德瓦尔斯平均场论，采用Cahn和Hilliard的正则溶液模型和梯度能，预测了滞后与温度和孔径的关系。开发了一个简单的薄纳米孔“分层润湿”模型来描述第一个单层的强润湿，然后是本体中纳米液滴和纳米气泡的冷凝的情况。该模型预测了一个更大的滞后临界温度和增强滞后的分子凝聚在纳米孔在高蒸气压比在低蒸气压的单层内。对于非均质孔，该理论预测的吸附/解吸序列类似于在分子动力学模拟中看到的那些，其中在纳米孔接合处的界面能（或梯度罚分）充当用于快速通过不稳定性的自由能势垒。该模型有助于定量地了解最近的实验数据，混凝土或水泥浆的润湿和干燥周期，并建议在不同的温度和湿度扫描速率的新实验。
英文摘要：
Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple "hierarchical wetting" model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.
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