Singh Rakesh S, Bagchi Biman
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Chem Phys. 2014 Apr 28;140(16):164503. doi: 10.1063/1.4871388.
The well-known classical nucleation theory (CNT) for the free energy barrier towards formation of a nucleus of critical size of the new stable phase within the parent metastable phase fails to take into account the influence of other metastable phases having density/order intermediate between the parent metastable phase and the final stable phase. This lacuna can be more serious than capillary approximation or spherical shape assumption made in CNT. This issue is particularly significant in ice nucleation because liquid water shows rich phase diagram consisting of two (high and low density) liquid phases in supercooled state. The explanations of thermodynamic and dynamic anomalies of supercooled water often invoke the possible influence of a liquid-liquid transition between two metastable liquid phases. To investigate both the role of thermodynamic anomalies and presence of distinct metastable liquid phases in supercooled water on ice nucleation, we employ density functional theoretical approach to find nucleation free energy barrier in different regions of phase diagram. The theory makes a number of striking predictions, such as a dramatic lowering of nucleation barrier due to presence of a metastable intermediate phase and crossover in the dependence of free energy barrier on temperature near liquid-liquid critical point. These predictions can be tested by computer simulations as well as by controlled experiments.
著名的经典成核理论（CNT）用于描述母亚稳相内形成新稳定相临界尺寸核的自由能垒，该理论未能考虑其他亚稳相的影响，这些亚稳相的密度/有序度介于母亚稳相和最终稳定相之间。这一缺陷可能比CNT中所做的毛细管近似或球形假设更为严重。这个问题在冰成核中尤为重要，因为液态水在过冷状态下呈现出由两个（高密度和低密度）液相组成的丰富相图。对过冷水热力学和动力学异常的解释常常援引两个亚稳液相之间液 - 液转变的可能影响。为了研究过冷水中热力学异常的作用以及不同亚稳液相的存在对冰成核的影响，我们采用密度泛函理论方法来寻找相图不同区域的成核自由能垒。该理论做出了一些惊人的预测，例如由于亚稳中间相的存在导致成核势垒急剧降低，以及在液 - 液临界点附近自由能垒对温度的依赖性发生转变。这些预测可以通过计算机模拟以及控制实验来检验。
