Structural, rheological and dynamic aspects of hydrogen-bonding molecular liquids: Aqueous solutions of hydrotropic tert-butyl alcohol

Abstract

Hypothesis: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small-and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the 'complemented-system approach' method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular and supramolecular structures at the intra-, inter-, and supra-molecular scales. Experiments: The SWAXS and x-ray diffraction experiments and MD simulations were performed for aqueous t-butanol solutions at 25 °C. Literature viscosity and self-diffusion data were also used. Findings: The interpretive power of the proposed scheme was demonstrated by the extensive and diverse results obtained for aqueous t-butanol solutions across the whole concentration range. Four composition ranges with qualitatively different structures and viscosity trends were revealed. The experimental and calculated zero-shear viscosities and molecular self-diffusion coefficients were successfully related to the corresponding structural details. The hydrogen bonds 2 that were, along with hydrophobic effects, recognized as the most important driving force for the formation of t-butanol aggregates, show intriguing lifetime trends and thermodynamic properties of their formation. Binary System.Recently, attention has turned to substances with small molecules exhibiting only a weak amphiphilic character that are sometimes addressed as 'amphiphilic solvents' or 'solvosurfactants' [9, 10]. They can be seen as liquids exhibiting the properties of surfactants, such as 3 surface activity, self-assembly in water, co-micellization, etc. The term 'hydrotrope' is also very commonly used, even though, as reviewed by Kunz et al. [11], it comprises a broader range of amphiphilic compounds [12][13][14]. Their ternary systems are often called 'surfactant-free microemulsions' [15] or 'detergentless microemulsions' [16]. More recently Zemb et al.proposed the term 'ultraflexible microemulsions' [17], as such systems exhibit low bending rigidities.The binary aqueous solutions of such substances are especially interesting due to their strong non-ideal behavior, e.g., in terms of their viscosity, partial molar volume, excess thermodynamic properties, compressibility, and sound-attenuation coefficient [18][19][20][21][22][23]. Such behavior arises because the hydrophilic part of the molecule forms strong hydrogen-bonds (HBs) with water, while the hydrophobic part introduces a perturbation to the water's hydrogen-bonded structure and induces cooperative ordering, stemming from the hydrophobic hydration effects.However, a full understanding of the hydration structure of such systems and their non-ideal behavior is still incomplete.In our recent research we have strived to learn m...

Chloride ions as integral parts of hydrogen bonded networks in aqueous salt solutions: the appearance of solvent separated anion pairs

“…17,18) have first been determined; these are shown in Figure 1. Robust hydrogen bonded networks, like those in most alcohol-water mixtures, percolate 18,24,25 , i.e. the largest H-bonded cluster is comparable in its size with the system size.…”

Universal features in lifetime distribution of clusters in hydrogen bonding liquids

“…Intuitively, one would expect a broad distribution of H-bonding life times, as function of both time and H-bonding distance, centered around some mean representative life time, which could be subsequently searched in the various experimental techniques investigating the relaxation processes. What we uncover here, is that while the distribution of H-bonding distances are indeed as hypothesized, the life time distributions exhibit several specific times, which vary from 1 at very short distances to 3 at larger ones, where the distances are picked around the main peak of the oxygen-oxygen distribution function, typically in the range 2.5 Å to 3.5 Å, this latter value being often used in the literature [46,47,48,49], as in the Luzar paper mentioned above [19]. These specific times, and related distribution, appear as common to all three associated liquids we have investigated herein, namely water, alcohols and amine, and across different force field representation.…”

Camel back shaped Kirkwood-Buff Integrals

“…In addition, the equivalent of the micelle extrema -in the present case it would be tbutanol micellar aggregates, occurs for very small tbutanol concentrations in the interval 0.1 < x < 0.2, which is similar to that observed in the present work for an entirely different system. Interestingly, the aggregation in aqueous tbutanol is not clearly understood to date, despite several experimental [61,62] and simulation [31,32,34,63] investigations.…”