Dipartimento di Matematica e Fisica, Università Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy and INFN, Sezione di Roma Tre, Via della Vasca Navale 84, I-00146 Rome, Italy.
J Chem Phys. 2013 Nov 28;139(20):204503. doi: 10.1063/1.4832382.
We study by molecular dynamics simulations the dynamical properties of an aqueous solution of NaCl at a concentration of 0.67 mol/kg upon supercooling. In a previous study of the same ionic solution, we have located the liquid-liquid critical point (LLCP) and determined the Widom line connected to the liquid-liquid transition. We present here the results obtained from the study of the self-intermediate scattering function in a large range of temperatures and densities approaching the LLCP. The structural relaxation is in agreement with the mode coupling theory (MCT) in the region of mild supercooling. In the deeper supercooled region the α-relaxation time as function of temperature deviates from the MCT power law prediction showing a crossover from a fragile to a strong behavior. This crossover is found upon crossing the Widom line. The same trend was found in bulk water upon supercooling and it appears almost unchanged by the interaction with ions apart from a shift in the thermodynamic plane toward lower pressures and higher temperatures. These results show that the phenomenology of supercooled water transfers from bulk to solution where the study of the supercooled region is experimentally less difficult.
我们通过分子动力学模拟研究了浓度为 0.67mol/kg 的 NaCl 水溶液在过冷时的动力学性质。在对同一离子溶液的先前研究中，我们定位了液-液相变临界点 (LLCP) 并确定了与液-液转变相关的 Widom 线。在这里，我们展示了在接近 LLCP 的较大温度和密度范围内研究自中间散射函数所得到的结果。结构弛豫在轻度过冷区域与模式耦合理论 (MCT) 一致。在更深的过冷区域，α-弛豫时间随温度的变化偏离 MCT 幂律预测，表现出从脆弱到强的转变。这种转变发生在穿过 Widom 线时。在过冷的情况下，在 bulk water 中也发现了同样的趋势，除了在热力学平面上向更低的压力和更高的温度移动之外，与离子的相互作用几乎没有改变这种趋势。这些结果表明，过冷水的现象从 bulk 转移到溶液中，在溶液中，过冷区域的研究在实验上较为容易。
