Center for Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215, USA.
J Chem Phys. 2012 Nov 28;137(20):204502. doi: 10.1063/1.4767355.
We study a coarse-grained model for a water monolayer that cannot crystallize due to the presence of confining interfaces, such as protein powders or inorganic surfaces. Using both Monte Carlo simulations and mean field calculations, we calculate three response functions: the isobaric specific heat C(P), the isothermal compressibility K(T), and the isobaric thermal expansivity α(P). At low temperature T, we find two distinct maxima in C(P), K(T), and ∣α(P)∣, all converging toward a liquid-liquid critical point (LLCP) with increasing pressure P. We show that the maximum in C(P) at higher T is due to the fluctuations of hydrogen (H) bond formation and that the second maximum at lower T is due to the cooperativity among the H bonds. We discuss a similar effect in K(T) and ∣α(P)∣. If this cooperativity were not taken into account, both the lower-T maximum and the LLCP would disappear. However, comparison with recent experiments on water hydrating protein powders provides evidence for the existence of the lower-T maximum, supporting the hypothesized LLCP at positive P and finite T. The model also predicts that when P moves closer to the critical P the C(P) maxima move closer in T until they merge at the LLCP. Considering that other scenarios for water are thermodynamically possible, we discuss how an experimental measurement of the changing separation in T between the two maxima of C(P) as P increases could determine the best scenario for describing water.
我们研究了一个由于存在限制界面（如蛋白质粉末或无机表面）而无法结晶的单层水分子的粗粒度模型。通过蒙特卡罗模拟和平均场计算，我们计算了三个响应函数：等压比热 C(P)、等温压缩率 K(T)和等压热膨胀率 α(P)。在低温 T 下，我们发现 C(P)、K(T)和∣α(P)∣都有两个明显的最大值，它们都随着压力 P 的增加收敛到一个液相-液相临界点 (LLCP)。我们表明，在较高温度 T 下 C(P)的最大值是由于氢键形成的波动引起的，而在较低温度 T 下的第二个最大值是由于氢键之间的协同作用引起的。我们讨论了 K(T)和∣α(P)∣中的类似效应。如果不考虑这种协同作用，较低温度 T 下的最大值和 LLCP 都将消失。然而，与最近关于水在蛋白质粉末中水合的实验进行比较，为较低温度 T 下最大值的存在提供了证据，支持在正 P 和有限 T 下存在假设的 LLCP。该模型还预测，当 P 更接近临界 P 时，C(P)最大值在 T 上的移动距离更近，直到在 LLCP 处合并。考虑到其他水的情况在热力学上是可能的，我们讨论了如何通过实验测量 C(P)的两个最大值之间的 T 间隔随 P 增加的变化，可以确定描述水的最佳情况。
