Adding Salt to an Aqueous Solution of t-Butanol: Is Hydrophobic Association Enhanced or Reduced?

TL;DR

This study combines experiments and simulations to investigate how salt affects hydrophobic associations of t-butanol in water, challenging previous findings and suggesting that salt actually enhances hydrophobic contacts.

Contribution

It provides a joint experimental and theoretical analysis showing that salt increases hydrophobic interactions of t-butanol, contrary to earlier neutron scattering interpretations.

Findings

Salt enhances hydrophobic contacts of t-butanol in water.

Experimental and simulation results are consistent within error margins.

Contradicts previous neutron scattering conclusions about reduced hydrophobic contacts.

Abstract

Recent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney [Phys. Rev. Lett. {\bf 89}, 215508 (2002); J. Chem. Phys. {\bf 118}, 8357 (2003)] suggest the formation of t-butanol pairs, bridged by a chloride ion via ${O}-{H}...{Cl}^-$ hydrogen-bonds, and leading to a reduced number of intermolecular hydrophobic butanol-butanol contacts. Here we present a joint experimental/theoretical study on the same system, using a combination of molecular dynamics simulations and nuclear magnetic relaxation measurements. Both theory and experiment clearly support the more intuitive scenario of an enhanced number of hydrophobic contacts in the presence of the salt, as it would be expected for purely hydrophobic solutes [J. Phys. Chem. B {\bf 107}, 612 (2003)]. Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental errorbars of the Bowron and Finney work.