Methanol-ethanol “ideal” mixtures as a test ground for the computation of Kirkwood-Buff integrals

The influence of charge ordering in the microscopic structure of monohydroxy alcohols

While radiation scattering data provides insight inside the microstructure of liquids, the Debye relation relating the scattering intensityI(k) to the atom-atom structure factorsSab(k)shows that, ultimately, it is these individual structure correlation functions which contain the relevant information about the micro-structure. However, these quantities are not observables, except in few cases where one can invert the Debye relation to obtain the structure functions. In the majority of other cases, the need for model dependent computer simulations is unavoidable. The resulting calculations reveal that the scattering pre-peak is the result of cancellations between positive pre-peaks and negative anti-peaks contributions from the atom-atom structure factors. What of systems where this cancellation is such that it entirely suppresses the scattering pre-peak? One would be tempted to falsely conclude that there is no underlying micro-heterogeneity. Hence, the structure functions appear as hidden variables, and it is important to understand the relation between their features and the micro-structure of the system. Through the computer simulation study of various mono-ols, ranging from methanol to 1-nonanol, as well as the branched octanols, we show how the features of the atom-atom pair correlation functiongab(r)affect that of the structure factorsSab(k), and reveal that the micro-structure is ultimately the result of the charge ordering between different atoms in the system.

Camel back shaped Kirkwood-Buff Integrals

Some binary mixtures, such as specific alcohol-alkane mixtures, or even water-tbutanol, exhibit two humps "camel back" shaped KBI. This is in sharp contrast with usual KBI of binary mixtures having a single extremum. This extremum is interpreted as the region of maximum concentration fluctuations, and usually occurs in binary mixtures presenting appreciable micro-segregation, and corresponds to where the mixture exhibit a percolation of the two species domains. In this paper, it is shown that two extrema occur in binary mixtures when one species forms "meta-particle" aggregates, the latter which act as a meta-species, and have their own concentration fluctuations, hence their own KBI extremum. This "meta-extremum" occurs at low concentration of the aggregate-forming species (such as alcohol in alkane), and is independant of the other usual extremum observed at mid volume fraction occupancy. These systems are a good illustration of the concept of the duality between concentration fluctuations and micro-segregation.

Cyclodextrin solubilization in hydrated reline: Resolving the unique stabilization mechanism in a deep eutectic solvent

By complexing with hydrophobic compounds, cyclodextrins afford increased solubility and thermodynamic stability to hardly soluble compounds, thereby underlining their invaluable applications in pharmaceutical and other industries. However, common cyclodextrins such as β-cyclodextrin, suffer from limited solubility in water, which often leads to precipitation and formation of unfavorable aggregates, driving the search for better solvents. Here, we study the solvation of cyclodextrin in deep eutectic solvents (DESs), environmentally friendly media that possess unique properties. We focus on reline, the DES formed from choline chloride and urea, and resolve the mechanism through which its constituents elevate β-cyclodextrin solubility in hydrated solutions compared to pure water or dry reline. Combining experiments and simulations, we determine that the remarkable solubilization of β-cyclodextrin in hydrated reline is mostly due to the inclusion of urea inside β-cyclodextrin's cavity and at its exterior surfaces. The role of choline chloride in further increasing solvation is twofold. First, it increases urea's solubility beyond the saturation limit in water, ultimately leading to much higher β-cyclodextrin solubility in hydrated reline in comparison to aqueous urea solutions. Second, choline chloride increases urea's accumulation in β-cyclodextrin's vicinity. Specifically, we find that the accumulation of urea becomes stronger at high reline concentrations, as the solution transitions from reline-in-water to water-in-reline, where water alone cannot be regarded as the solvent. Simulations further suggest that in dry DES, the mechanism of β-cyclodextrin solvation changes so that reline acts as a quasi-single component solvent that lacks preference for the accumulation of urea or choline chloride around β-cyclodextrin.