Linking excess order to co-solvent aggregation in solvent mixtures

Abstract

Small amphiphilic molecules dissolved in alcohol–water mixtures have been seen to have a tendency to aggregate. Here we present a thorough neutron diffraction experiment, aided by H/D isotopic substitution and empirical potential structure refinement simulations of four mixtures of tert-butyl alcohol (x${}_t$=0.03 molar) in methanol and water (x${}_m$=0.27, 0.54, 0.73, 1.0 molar). The tert-butyl alcohol is found to segregate in the methanol rich regions at all concentrations; at x${}_m$=0.27 the mixture is found to be a bi-percolating mixture, with water forming one interconnected cluster, and methanol and tert-butyl alcohol (apolar–apolar) forming another. At the same x${}_m$=0.27 concentration, tert-butyl alcohol molecules become closer each other at this concentration, forming more dimers than at the other concentrations, and even trimers. Results are discussed in terms of the ability of water molecules to form flexible tetrahedral networks.

GRAPHICAL ABSTRACT

Keywords:

• Neutron scattering
• co-non-solvency
• hydrophobic effect

Acknowledgments

I acknowledge the ISIS Facility Access Panel for allocating beamtime RB1800065. I would also like to thank Prof. Maria Antonietta Ricci for useful suggestions on the present manuscript as well as continuous encouragement over the years. I warmly thank Dr Alex Hannon for careful proof-reading of this manuscript. Finally, I would like to thank Prof. Alan Soper for welcoming me into the Disordered Materials group back in 2007, and then always listening and patiently answering neutrons or EPSR questions.

Disclosure statement

No potential conflict of interest was reported by the author.

ORCID

Silvia Imberti  http://orcid.org/0000-0001-7037-6829

Notes

1 Please note that pure TBA and TBA in water have previously been investigated via neutron diffraction with isotopic substitution by Bowron et al. [16,17].

2 The release date is: Tue Jun 29 09:00:00 BST 2021.