NMR evidence of a sharp change in a measure of local order in deeply supercooled confined water

Abstract

Using NMR, we measure the proton chemical shift ␦, of supercooled nanoconfined water in the temperature range 195 K < T < 350 K. Because ␦ is directly connected to the magnetic shielding tensor, we discuss the data in terms of the local hydrogen bond geometry and order. We argue that the derivative ؊(٢ ln ␦/٢T)P should behave roughly as the constant pressure specific heat CP(T), and we confirm this argument by detailed comparisons with literature values of CP(T) in the range 290 -370 K. We find that ؊(٢ ln ␦/٢T)P displays a pronounced maximum upon crossing the locus of maximum correlation length at Ϸ240 K, consistent with the liquid-liquid critical point hypothesis for water, which predicts that CP(T) displays a maximum on crossing the Widom line.configurational specific heat ͉ nuclear magnetic resonance ͉ proteins ͉ proton chemical shift U nlike most fluids, water displays anomalies in thermodynamical properties such as compressibility, isobaric heat capacity, and thermal expansion coefficient, and their explanation on molecular basis remains a challenge (1-3). One hypothesis that has received support from various theoretical studies (4-7) is the liquid-liquid (LL) critical point hypothesis, but a proper test can be obtained only by studying the properties of liquid water well below its homogeneous nucleation temperature, T H ϭ 231 K. This is made possible by confining water inside nanoporous structures so small that the liquid cannot freeze.Among recent findings concerning water's dynamical properties at these low temperatures are (8-13): a fragile-to-strong crossover and the violation of the Stokes-Einstein relation, related to the crossing of the Widom line and to the existence of a low-density-liquid-like (LDL-like) local structure. The Widom line is the locus of maximum correlation length in the one-phase region beyond the liquid-liquid critical point, where thermodynamic response functions take their maximum values (12, 13). Scattering experiments (using neutrons and x-rays) have given precise values of the pair correlation function (PCF), providing important benchmarks for testing models of its structure. The PCF represents only an isotropically averaged measure of structure. Thus, in many cases, PCFs may not faithfully reproduce the subtle hydrogen bond geometry responsible for water's thermal anomalies. Our goal in this study is to provide additional information on the local hydrogen bond geometry and, in particular, the average number of the possible configurations of the local molecular hydrogen bonding geometry, by measuring the NMR proton chemical shift ␦. If a water molecule in a dilute gas is taken to be an isolated-state reference, the chemical shift ␦ accounts for the change of the value of the magnetic shielding with respect to that of such a reference. Hence the chemical shift is related to the ''non-dilute'' or ''virial'' interaction of a water molecule with its surroundings, providing a picture of the intermolecular geometry (14-19). Originally, it has been proposed, especially in the high ...

Structural Properties of High and Low Density Water in a Supercooled Aqueous Solution of Salt

“…There * Author to whom correspondence should be addressed, e-mail: gallop@fis.uniroma3.it are also indications from experiments of the existence of the LLCP in bulk water [16][17][18][19] . It would be approximately located at T ∼ 220 K at P ∼ 100 MPa.…”

Model of waterlike fluid under confinement for hydrophobic and hydrophilic particle-plate interaction potentials

“…In these systems the presence of criticality in the bulk has been associated with a dynamic transition between liquids of different viscosities [10][11][12][13] .…”

The liquid water polymorphism

“…(i) a fragile-to-strong crossover (9,(14)(15)(16), (ii) the violation of the Stokes-Einstein relation (17,22), (iii) the observation of the predominance of LDL on the HDL local structure (18,21), (iv) local maxima in the configurational specific heat (23) and in the absolute value of the thermal expansion coefficient (19,20).…”