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

Abstract

Articles you may be interested inSolvent effects on the UV-visible absorption spectrum of benzophenone in water: A combined Monte Carlo quantum mechanics study including solute polarization Mesoscale structure and fractal nature of 1-propanol aqueous solution: A reverse Monte Carlo analysis of small angle neutron scattering intensity High-pressure neutron diffraction on fluid carbon tetrafluoride and interpretation by reverse Monte Carlo simulations Neutron diffraction measurements from 2.4 mole% solutions of tetrapropylammonium bromide ͑TPrABr͒, ͑C 3 H 7 ͒ 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 Å, with structure in the 2.5-4 Å 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 ϩ ions is discussed in relation to previous results on the structure of water in TPrABr solution at the same concentration.

Microscopic probing of the size dependence in hydrophobic solvation

“…The ionic radii of Me 4 N + and Et 4 N + cations are about 3.5 and 4 Å, falling into the category of small solutes with respect to the proposed crossover, while those of Pr 4 N + and Bu 4 N + with the alkyl chains fully stretched have been estimated as about 4.5 and 5 Å, respectively. 28,33,45 Although their conformations in solution are not fully clear, their ionic radii still provide good estimates of their dimension as solvated cations. The crossover observed in Figs.…”

“…On the other hand, hydrophobic association was observed for aromatic ions by small-angle neutron scattering 32 and close contacts between tetraalkyl-ammonium cations without intervening water molecules were suggested by neutron diffraction. 33 However, no crossover behavior was detected by either technique.…”

“…The cation-cation distance estimated from the diffraction peak position is found to be 8 Å for Bu 4 NBr and 7 Å for Pr 4 NBr, slightly smaller than the diameters of the cations, 10 and 9 Å, respectively. 28,33,45 possibly attributable to the close contact of cations with more compact conformation or to the penetration of long hydrophobic chains into nearby cations. 46 Such a strong association occurs for large hydrophobic objects as suggested by theory 7,11,14 and also observed experimentally.…”

Simulation and Neutron Diffraction Studies of Small Biomolecules in Water

“…40 In recent years, diffraction methods have been applied to more complex molecular species, including tert -butyl alcohol, 41,42 ethanol, phenol, D-glucose, 43 , and amino acids. 44–46 Unfortunately, in the case of large asymmetric biological solutes like sugars, containing multiple atoms of the type being isotopically substituted, with each of these atoms in a different local environment, the structure factor still remains too complex to be interpreted even in a conventional NDIS experiment.…”

A new structural technique for examining ion-neutral association in aqueous solution

“…2 In the pyridine example cited above, such a first-order difference experiment can be performed using H/D substitution on the Hnon atoms of the pyridine, reducing the 15 correlations in the total scattering measurement to just five: Hnon-C, Hnon-N, Hnon-Hex, Hnon-O and Hnon-Hnon. Such experiments have been effective in the study of the solution structure of many simple solutes such as rare gases 3 , mono-atomic ions 4,5 , and approximately spherical molecular solutes 6 . In recent years, NDIS methods has been applied to much more complex molecular species including tert -butyl alcohol 7,8 , ethanol, phenol, D-glucose 9 , amino acids 10–12 , and polymer electrolytes.…”