Heterogeneity in (2-butoxyethanol + water) mixtures: Hydrophobicity-induced aggregation or criticality-driven concentration fluctuations?

Abstract

Micro-heterogeneity in aqueous solutions of 2-butoxyethanol (BE), a system with closed loop miscibility gap, has been explored via absorption and time-resolved fluorescence measurements of a dissolved dipolar solute, coumarin 153 (C153), in the water-rich region at various BE mole fractions (0 ≤ XBE ≤ 0.25) in the temperature range, 278 ≤ T/K ≤ 320. Evidences for both alcohol-induced H-bond strengthening and subsequent structural transition of H-bond network have been observed. Analyses of steady state and time-resolved spectroscopic data for these aqueous mixtures and comparisons with the results for aqueous solutions of ethanol and tertiary butanol indicate that alcohol aggregation in BE/water mixtures is driven by hydrophobic interaction with no or insignificant role for criticality-driven concentration fluctuations preceding phase separation. Excitation energy dependence of fluorescence emission of C153 confirms formation of aggregated structures at very low BE mole fractions. No asymptotic critical power law dependence for relaxation rates of the type, k ∝ (|T - Tc|/Tc)(γ), with γ denoting universal critical constant, has been observed for both solute's rotational relaxation and population relaxation rates in these mixtures upon either approaching to critical concentration or critical temperature. Estimated activation energies for rotational relaxation rate of C153 and solution viscosity have been found to follow each other with no abrupt changes in either of them at any mixture composition. In addition, measured C153 rotation times at various compositions and temperatures reflect near-hydrodynamic viscosity coupling through the dependence,〈τr〉∝ (η/T)(p), with p = 0.8-1.0, suggesting solute's orientational relaxation dynamics being, on an average, temporally homogeneous.

Molecular Association and Hydration Behavior of 2-<i>n</i>-Butoxyethanol in Aqueous Solution

“…These values are approximately comparable to the rotational relaxation times of fluorescent probe molecules dissolved in aqueous 2BE solutions in the second concentration regime determined by using time-resolved fluorescence spectroscopic techniques13) . These findings obtained by the earlier studies support our assignment of relaxation modes 3 and 4.It is worth writing that the ε 2 value clearly exhibits the maximum at ca.…”

“…Besides, many other studies were published so far, which examined aqueous 2BE systems using various kinds of techniques such as surface tensiometry 6) , calorimetry 7) , viscometry 8) , ultrasound relaxation 9) , dynamic light scattering 10,11) , Taylor dispersion measurement 12) , fluorescent spectroscopy 13) , molecular dynamics simulation 14) , X-ray scattering 15) , neutron scattering [16][17][18] , neutron spin echo 18) , and so on. In many studies listed above, most authors found maxima or inflection points in the measured physical quantities at certain 17 † Corresponding author.…”

Dynamic Solvent Control of a Reaction in Ionic Deep Eutectic Solvents: Time-Resolved Fluorescence Measurements of Reactive and Nonreactive Dynamics in (Choline Chloride + Urea) Melts

“…The anisotropy analysis was performed via the standard protocol described in earlier works. [68][69][70] Time resolved fluorescence anisotropy, ) t ( r was then constructed as follows:…”

Are N-methyl groups of Tetramethylurea (TMU) Hydrophobic? A composition and temperature-dependent fluorescence spectroscopic investigation of TMU/water binary mixtures

“…This three dimensional tetrahedral H-bond network structure of water gets disrupted to two dimensional zig-zag chainlike structure at comparatively larger alcohol concentrations where the tetrahedral network of water can no longer accommodate alcohol molecules. 17,18,20 This effect is more pronounced while moving from small alcohols like ethanol (EtOH) to larger alcohols like tertiary butanol (TBA) because hydrophobic interaction plays a critical role in regulating the structural arrangement of both the specie in a given binary mixture. 18 Consequently, dynamical properties of aqueous solutions exhibit significant changes in presence of co-solvents.…”

“…35,36 Here, we explore whether the hydrophobic part containing four N-methyl groups of TMU acts in the similar manner in aqueous solutions as we had observed before in aqueous alcoholic solutions. [17][18][19][20] In this paper, we have investigated various compositions of TMU in water at different temperatures by UV-Visible absorption and fluorescence spectroscopic measurements. Steady state absorption and fluorescence emission of a nonreactive probe coumarin 153 (C153) in TMU/water binary mixtures have provided important information regarding the structural change in the three dimensional H-bonding network of water upon addition of TMU.…”