Ranieri Umbertoluca, Formisano Ferdinando, Gorelli Federico A, Santoro Mario, Koza Michael Marek, De Francesco Alessio, Bove Livia E
Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, Roma, 00187, Italy.
Centre for Science at Extreme Conditions and School of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3FD, UK.
Nat Commun. 2024 May 16;15(1):4142. doi: 10.1038/s41467-024-47961-7.
According to textbooks, no physical observable can be discerned allowing to distinguish a liquid from a gas beyond the critical point. Yet, several proposals have been put forward challenging this view and various transition boundaries between a gas-like and a liquid-like behaviour, including the so-called Widom and Frenkel lines, and percolation line, have been suggested to delineate the supercritical state space. Here we report observation of a crossover from gas-like (Gaussian) to liquid-like (Lorentzian) self-dynamic structure factor by incoherent quasi-elastic neutron scattering measurements on supercritical fluid methane as a function of pressure, along the 200 K isotherm. The molecular self-diffusion coefficient was derived from the best Gaussian (at low pressures) or Lorentzian (at high pressures) fits to the neutron spectra. The Gaussian-to-Lorentzian crossover is progressive and takes place at about the Widom line intercept (59 bar). At considerably higher pressures, a liquid-like jump diffusion mechanism properly describes the supercritical fluid on both sides of the Frenkel line. The present observation of a gas-like to liquid-like crossover in the self dynamics of a simple supercritical fluid confirms emerging views on the unexpectedly complex physics of the supercritical state, and could have planet-wide implications and possible industrial applications in green chemistry.
根据教科书的说法，在临界点以上，不存在任何可观察到的物理现象能够用于区分液体和气体。然而，已有多项提议对这一观点提出挑战，并且有人提出了各种类气态与类液态行为之间的转变边界，包括所谓的维登线和弗伦克尔线，以及逾渗线，以界定超临界状态空间。在此，我们报告通过对超临界流体甲烷沿着200K等温线随压力变化进行非相干准弹性中子散射测量，观测到从类气态（高斯型）到类液态（洛伦兹型）的自动态结构因子的转变。分子自扩散系数由对中子谱的最佳高斯拟合（在低压下）或洛伦兹拟合（在高压下）得出。从高斯型到洛伦兹型的转变是渐进的，大约发生在维登线截距（59巴）处。在高得多的压力下，一种类液态跳跃扩散机制能够恰当地描述弗伦克尔线两侧的超临界流体。在一种简单超临界流体的自动力学中观测到从类气态到类液态的转变，证实了关于超临界状态意外复杂的物理学的新观点，并且可能对全球产生影响，以及在绿色化学领域具有潜在的工业应用价值。
