Adding salt to an aqueous solution of t-butanol: Is hydrophobic association enhanced or reduced?

Abstract

Recent neutron scattering experiments on aqueous salt solutions of amphiphilic t-butanol by Bowron and Finney ͓Phys. Rev. Lett. 89, 215508 ͑2002͒; J. Chem. Phys. 118, 8357 ͑2003͔͒ suggest the formation of t-butanol pairs, bridged by a chloride ion via O -H¯Cl − hydrogen bonds, 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 ͑MD͒ simulations and nuclear magnetic relaxation measurements. Both MD simulation and experiment clearly support the more classical scenario of an enhanced number of hydrophobic contacts in the presence of salt, as it would be expected for purely hydrophobic solutes. ͓T. Ghosh et al., J. Phys. Chem. B 107, 612 ͑2003͔͒. Although our conclusions arrive at a structurally completely distinct scenario, the molecular dynamics simulation results are within the experimental error bars of the Bowron and Finney data.

Effect of different sodium halides on the self-association of tertiary butanol in water

“…3 indicate that the relation between the association parameter and the mole ratio of salt to TBA is not linear, as described previously. 23 For NaCl, we observe a strong increase in the association parameter, indicating that NaCl enhances the selfassociation of TBA at all concentrations. For NaBr, the effect appears to be a small decrease of the association.…”

“…Previous studies have highlighted the role of Cl − as a determinant of the association behavior, 21 even though this role is still problematic. [21][22][23] It has been found that an attractive interaction exists between the Br − and Xe in aqueous solution, 33 Xe being a small hydrophobic molecule. This result shows that Br − does not promote the Xe-Xe hydrophobic interaction and might explain the smaller effect of Br − on the self-association.…”

“…To separate the relevant T 1,inter term from the others, we used the method of isotopic dilution. 23,30 Mixing the ͑at the methyl groups͒ fully protonated TBA molecules ͑TBA-d1͒ with fully deuterated molecules ͑TBA-d10͒ yields a variation of the intermolecular relaxation rate. If we parametrize the dilution via the mole fraction…”

“…In contrast to their results, molecular dynamics simulations found no evidence of such structure. 23 In this paper, we describe an experimental investigation of the effect of salt on the association of TBA molecules in water. The basis of our measurements is the dependence of the interaction between nuclear spins on their distance.…”

The Solvent‐Dependent Shift of the Amide I Band of a Fully Solvated Peptide as a Local Probe for the Solvent Composition in the Peptide/Solvent Interface

“…The same value is observed for the case of random bond percolation on a 3D lattice close to the percolation transition, and supports the view of methanol and water as a bipercolating mixture, as proposed by Dougan et al [53] The observation of percolation is a feature apparently not untypical for aqueous solutions and was first observed for the water network in THF/water mixtures by Oleinikova et al, [58] and was also found later for the hydrophobic clustering of tertiary butanol (TBA) in TBA/water mixtures. [59] Quite interestingly, the broad cluster-size distribution also has implications for the structure of the solvent at the peptide/solvent interface. As the picture of the solvation shell of the AK peptide in Figure 3 suggests, the nanoheterogeneous nature is also reflected on the peptide surface and reveals a patchy structure of methanol and water clusters.…”