The impact of the structuring of hydrotropes in water on the mesoscale solubilisation of a third hydrophobic component

Abstract

In the present contribution, the pre-structuring of binary mixtures of hydrotropes and HO is linked to the solubilisation of poorly water miscible compounds. We have chosen a series of short-chain alcohols as hydrotropes and benzyl alcohol, limonene and a hydrophobic azo-dye (Disperse Red 13) as organic compounds to be dissolved. A very weak pre-structuring is found for ethanol/HO and 2-propanol/HO mixtures. Pre-structuring is most developed for binary 1-propanol/HO and tert-butanol/HO mixtures and supports the bicontinuity model of alcohol-rich and water-rich domains as already postulated by Anisimov et al. Such a pre-structuring leads to a high solubilisation power for poorly water miscible components (limonene and Disperse Red, characterized by high octanol/water partition coefficients, log(P) values of 4.5 and 4.85), whereas a very weak pre-structuring leads to a high solubilisation power for slightly water miscible components (benzyl alcohol). This difference in solubilisation power can be linked to (i) the formation of mesoscale structures in the cases of ethanol and 2-propanol and (ii) the extension of pre-structures in the cases of 1-propanol and tert-butanol. Three different solubilisation mechanisms could be identified: bulk solubilisation, interface solubilisation and a combination of both. These supramolecular structures in binary and ternary systems were investigated by small-and-wide-angle X-ray and neutron scattering, dynamic light scattering and conductivity measurements (in the presence of small amounts of salt).

Chemical signatures of surface microheterogeneity on liquid mixtures

“…62,93 Our findings clearly show that the hydrophobic β-C solute is not randomly dispersed at the air-liquid interfaces of mixtures of water with amphiphilic organic solvents. 12 The strongly non-linear R vs x w dependences in the three W:X solvent mixtures further reveal that the hydration of CIs takes place on intact liquid microjets of well-defined x w values (see Validation Experiments in the supplementary material). Since the hydrophobic C 15 -diolefin should be preferentially associated with the X hydrotropes rather than with water, CIs are expected to be generated in X-rich β-C:X m W n mixed clusters.…”

“…7-10 These concentration (or structural) fluctuations are brief (tens of picoseconds) inhomogeneities in the ∼1 nm scale that happen to peak in the concentration ranges that give rise to thermodynamic and transport properties anomalies in binary mixtures. [11][12][13][14][15][16][17] milliseconds to years) non-covalent aggregates detected by dynamic light scattering experiments in many systems. [18][19][20][21][22] These aggregates also appear in the mass spectra of liquid jets produced by adiabatic expansion of solutions of hydrophobic species in aqueous organic mixtures.…”

“…[23][24][25][26] The universality of this phenomenon opens up the possibility that molecular environments could be engineered in solvent mixtures for steering the course of chemical reactions in unanticipated ways. 12,[27][28][29] It also leads to the realization that spontaneous compartmentalization at the mesoscale in complex liquid mixtures, by potentially supporting disjoint but coupled chemical processes in apparently homogeneous systems, is an alternative mechanism to the reaction-diffusion Turing model of the onset of complexity and pattern formation in Nature. [30][31][32] Water is a major component of natural systems; however, it is rarely present as a neat water, but as a solvent or distributed in diverse materials.…”

Ab initio prediction of structuring/mesoscale inhomogeneities in surfactant-free microemulsions and hydrogen-bonding-free microemulsions

“…Binary mixtures of water/ethanol do not show any correlation functions, indicating a molecularly distributed binary solvent mixture as forwarded recently. 37 Upon the addition of (5, 10, 15, 20 and 25 mol%) anisole to the water/ethanol mixture, a gradual formation of well-defined correlation functions with an increasing shift to higher decay times was observed. DLS measurements on binary water/THF mixtures already show mesoscale compartmentation.…”

“…This pre-structuring gets more and more destroyed upon the addition of a third hydrophobic component, as forwarded recently, resulting in less pronounced correlation functions. 37 For the water/acetone/anisole and water/THF/anisole systems, only weak and unresolved signals -but no mesoscale structuringcould be observed. However, the detected signals might correspond Fig.…”