Unconventional solubilization of sparingly water soluble (hydrophobic) substances using inorganic ions.

Ioanna Chazapi, Olivier Diat, Coralie Pasquier, Amélie Cartier, Pierre Bauduin

ICSM, CEA, CNRS, ENCSM, Univ Montpellier, Marcoule, France

I. Chazapi et al, Journal of Colloid and Interface Science, 638 (2023) pp 561-568

The process of making hydrophobic chemicals dissolve in water is essential for their use in different fields like medicine, fragrance, cleaning products, and agriculture [1]. Scientists usually achieve this by using substances such as surfactants, macrocycles or co-solvents.
In a recent paper, we described a new method to dissolve partially hydrophobic substances using small inorganic ions with nanometric size (nano-ions), called cobalta-bis-dicarbollides (COSAN) and their derivatives. These nano-ions do not have the usual hydrophilic-hydrophobic amphiphilic structure of surfactants but still have some surfactant-like properties [2]. They can self-associate in water and interact with hydrophobic compounds. This offers opportunity because COSAN nano-ions are inorganic and carry a negative charge. We found that the COSAN nanoions are very good at dissolving certain organic compounds with low logP values in water. For example, we tested their ability to dissolve 2-butanol, and they worked well. Surprisingly, even a small amount of COSAN nano-ions was enough to dissolve the compound, unlike traditional surfactants that require higher concentrations to work. Spectroscopic and scattering techniques were used in order to decipher this unconventional solubilization mechanism at the molecular and supramolecular levels. As revealed by SWAXS in laboratory and SANS (using contrast variation*), the alcohol solubilization by COSANs is made by the formation of thermodynamically stable (COSAN/water/alcohol rich) anisotropic nano-droplets (2 to 10 nm in size). Furthermore, we discovered that these droplets can also dissolve more hydrophobic organic compounds, with higher logP values, including dyes, medicines, perfumes, and solvents.
This new method of using COSAN nano-ions to dissolve hydrophobic compounds in water has opened up exciting possibilities for various applications. However, more research is needed to fully understand and optimize this process.

[1] S. H. Yalkowsky, American Chemical Society, 1999.
[2] Bauduin, P. et al. Angew. Chem. Int. Ed. 50 (2011) 5298 – 5300. Matějíček, P. et al. Langmuir 22 (2006) 575 – 581. Gassin, P.M. et al. Langmuir, 31 (2015) 2297 – 2303. Brusselle, D. et al. Angew. Chem.Int. Ed., 52 (2013) 12114 –12118. Vinas, C. et al. Dalton Transactions, 43 (2014) 5062-5068.

* a special thank to the deuteration service at the Jülich centre for Neutron Science (Dr K. Schwärzer) for 2-BuOH deuteration – proposal #ID13 in 2022 with the support of the MLZ user office.

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