NMR monitoring of nonequilibrium aggregation in ionic solutions

Abstract

The complex shaped ¹H NMR signal of [C₁₀mim][Br] has been observed after it was mixed with water. Within minutes the contour transformed into Voigt (V) profile with Gauss (G) contribution as dominant and started to evolve towards Lorentz-shaped (L) contour. Relative G/L contribution in V-profile was proposed as the measure of nonequilibrium aggregation in mesoscopic/supramolecular scale. The equilibration kinetics was monitored by time dependence of the half-widths G/L with waiting time up to 2–4 days. The ‘nonequilibrium’ covers only certain degrees of freedom of the guest (ionic liquid) molecules, whereas the matrix, i.e. the hydrogen-bonded network of water, is equilibrated.

Introduction

Recent studies on the critical behavior of ionic mixtures have revealed some novel features that can be very important in understanding series of anomalies and nontrivial phenomena found in the liquid state, as well as in the presence of ions as in non-ionic systems [1], [2], [3]. The detection of long living micro-heterogeneities and/or mesoscopic nonequilibrium aggregates associated with supramolecular restructuring already brought a certain order and more logical vista in the phenomena associated with criticality and phase transitions. Namely, it allowed to state that the fluid phase transitions driven by Coulomb forces belong to the Ising universality class as in most non-ionic solutions [1]. The apparent mean-field behavior, the crossover between these two regimes was found in the earlier works [4], [5], [6] and Refs. cited therein] and sometimes a bad reproducibility of the experimental data could be caused by the uncontrolled presence and relaxation of such nonequilibrium aggregates. However, the origin and details of these long living mesoscopic structures remain unclear and is the subject of many speculations [2], [3]. This opens new research fields with very challenging goals.

Light scattering (LS) is one of most powerful tools investigating the structuring processes and critical phenomena. On the other hand, by comparing LS versus NMR, it can be noted that if the first technique provides a ‘sum’ effect over the whole system, the NMR spectroscopy (also some other spectroscopic methods) allows to gain more subtle details at the molecular level. Therefore NMR spectroscopy was chosen as a method of choice. Some of our earlier observations using this method [7] provided indirect indications of nonequilibrium character of 3-methylpyridine (3MP)/water/NaBr solution, i.e. one of the ‘classical’ systems used in studies concerning critical phenomena in the ionic mixtures [1], [2], [3], [5].

During the measurements of phase transitions and critical phenomena, a chemical instability of compounds was shown [1] to be one of the major sources of error. The new class of chemical compounds – the salts that are fluid at room temperature, i.e. the so-called room temperature ionic liquids (RTIL), are perfectly stable over a wide range of temperature and are miscible with most solvents, and thus they can be considered as well suited systems for these experiments. One of RTILs, namely, 1-decyl-3-methyl-imidazolium bromide ([C₁₀mim][Br], Figure 1) was chosen for the present studies. The phase diagrams of [C₁₀mim][Br] and other very similar RTILs in water are more or less known [8], [9]; also its NMR spectra have been investigated very recently [10].

In the light of the earlier studies on 3MP/water/NaBr [7], [11], and the latest findings [1], [2], [3], it is clear that detection of supramolecular nonequilibrium aggregates that vanish in time extremely slowly requires particular accuracy and patience. Therefore at the start of the current study we took special care about the protocol that will allow for precise line shape analysis of NMR signals during long observation intervals. The waiting times (i.e. the time intervals between the sample just mixed and the spectrum registered) up to several days can be necessary. Such duration of the equilibration processes has been observed in the light scattering experiments [2], [3]. Because of higher viscosity of [C₁₀mim][Br]/water solutions we also considered a more pessimistic scenario – even indefinitely long waiting time.