Hydrophobic interaction of tetrapropylammonium ions in water: A neutron diffraction and reverse Monte Carlo study

Neutron diffraction measurements from 2.4 mole% solutions of tetrapropylammonium bromide (TPrABr), (C3H7)(4)NBr in water, using hydrogen/deuterium isotope substitution on the alkyl groups of the cation, is used to investigate the distribution of cations in the solution. The intermolecular partial pair correlation function for the alkyl group hydrogens is obtained, which shows a broad peak with a maximum at about 11 Angstrom, with structure in the 2.5-4 Angstrom region, suggesting that some of the alkyl chains of neighboring ions may not be separated by water molecules. The three-dimensional structure is modeled using the reverse Monte Carlo method to fit the HH partial structure factor. The simulation results show that at least half of the cations are not separated by water from one other cation and that a few clusters of three or four cations also occur. The close contact found between the TPrA(+) ions is discussed in relation to previous results on the structure of water in TPrABr solution at the same concentration. (C) 1997 American Institute of Physics.