Deuteron NMR relaxation, phase diagrams, and isotope effects in liquid mixtures of tetrahydrofuran/d₂O/salt

Abstract

NMR spin‐lattice relaxation time T1 and phase diagrams of deuterosubstituted tetrahydrofurans (THF‐hg, THF‐h4d4, THF‐d8) in D2O and D2O/salt (Salt = MeCl, Me = Na, K, Rb, Cs; CaCl2, AlCl3; MeOH, Me = Na, K) mixtures have been studied close to their lower critical solution points. It has been found that deuteration of THF leads to a shrinkage of the closed‐loop phase diagram THF‐h8‐xdx/D2O. It is an opposite and much stronger effect to the well‐known replacement of H2O by D2O in the system THF‐h8/H2O (D2O). The lower and the upper critical solution points (TC,L, TC,U) shift from 63.7 °C and 143.5 °C in THF‐h8/D2O to 84.5 °C and 128.5 °C in THF‐h4d4/D2O respectively, and they disappear in THF‐d8/D2O mixtures. This phenomena has been studied by precise (±0.01 °C) measurements of dependencies of T1 (2H) of THF and D2O deuterons on temperature. An increase of activation energies (Ea) of reorientational motion of THF‐d8 has been found by introduction of ions into the solutions. The theoretical modeling of phase diagrams using renormalization‐group and decorated lattice methods shows that the observed changes of TC,L can not be considered as a pure energetical effect where only Ea changes due to additional interaction of polar molecules with the electric field of ions. The entropic contribution, viz. the appearance of additional orientations has to be taken into account as well.

Structural changes of the molecular complexes of pyridines with water and demixing phenomena in aqueous solutions

“…48 It is reasonable to suppose that anions Cl Ϫ and SO 4 2Ϫ cause strengthening of the H bonds in water, whereas anions I Ϫ weakens them. As was obtained by NMR investigations, 23 the activation energy of the reorientational motion of the tetrahydrofuran molecule in water increases due to the addition of the salt KCl. It is evident that the H bonds between water and tetrahydrofuran molecules strengthen due to the presence of the Cl Ϫ anions.…”

Isotope Effect on Eutectic and Hydrate Melting Temperatures in the Water-THF System

“…Maximum melting points for THF hydrate have been reported in the range of (277. 15-278.15) K [4][5][6][7][8]. Because of the ease with which THF hydrate forms, and the fact that at room temperature water and THF are completely miscible, the rather complex phase behavior taking place in this system is not apparent.…”

“…Matouš et al [4] identified a broad, closed loop with lower and upper consolute temperatures of approximately 344 K (53.5 wt%) and 410 K (48 wt%), respectively. Their study, and a more recent one by Balevicius et al [5], were undertaken to characterize the critical behavior in the system and clarify and describe substantial isotope effects on liquid miscibility. Rosso and Carbonnel [6] and Carbonnel and Rosso [7] determined the phase diagram at low temperatures and found a miscibility gap with a critical temperature of approximately 286 K and a peritectic reaction at 278 K forming the structure II clathrate hydrate at a composition of 17 : 1 H 2 O : THF.…”

NMR probing of structural peculiarities in ionic solutions close to critical point

“…6,9 Recently, the presence of multiple critical ͑hypercritical͒ points has also been identified in this system. We have previously investigated THF/ H 2 O / NaCl solution using various techniques such as Fourier transform infrared ͑FTIR͒, NMR, and critical opalescence, 5,6,9 and we have found that it exhibited unique temperature versus composition region of a weak turbidity, a long equilibration time, and fascinating isotopic effects.…”

“…6,[17][18][19][20] In particular, the difference of chemical shifts of nuclei involved in equilibrium association processes along the phase boundary may be taken as an order parameter. 6,[17][18][19][20] In particular, the difference of chemical shifts of nuclei involved in equilibrium association processes along the phase boundary may be taken as an order parameter.…”