Water structure in aqueous solutions of tetramethylammonium chloride

Turner, J.; Soper, A. K.; Finney, Jl

doi:10.1080/00268979200102521

Cited by 63 publications

(46 citation statements)

References 6 publications

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“…The structure of liquid water is well characterized experimentally in terms of radial distribution functions, g(r), derived from diffraction experiments (2,3). Similar experiments on solutions of alcohols and tetraalkylammonium ions, however, show very little structural differences in g(r) compared with bulk water (2,4). Yet the thermodynamics of insertion of nonpolar solutes into water show an unfavorable free energy, a positive entropy at room temperature, and a large positive heat capacity (5,6).…”

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confidence: 91%

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

Raschke

Levitt²

2005

Proc. Natl. Acad. Sci. U.S.A.

142

145

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The origins of the hydrophobic effect are widely thought to lie in structural changes of the water molecules surrounding a nonpolar solute. The spatial distribution functions of the water molecules surrounding benzene and cyclohexane computed previously from molecular dynamics simulations show a high density first hydration shell surrounding both solutes. In addition, benzene showed a strong preference for hydrogen bonding with two water molecules, one to each face of the benzene ring. The position data alone, however, do not describe the majority of orientational changes in the water molecules in the first hydration shells surrounding these solutes. In this paper, we measure the changes in orientation of the water molecules with respect to the solute through spatial orientation functions as well as radial͞angular distribution functions. These data show that the water molecules hydrogen bonded to benzene have a strong orientation preference, whereas those around cyclohexane show a weaker tendency. In addition, the water-water interactions within and between the first two hydration shells were measured as a function of distance and ''best'' hydrogen bonding angle. Water molecules within the first hydration shell have increased hydrogen bonding structure; water molecules interacting across shell 1 and shell 2 have reduced hydrogen bonding structure.hydrophobic effect ͉ orientation I t is widely understood that the structural changes in water induced by exposure to nonpolar surfaces are responsible for the phenomenon known as the hydrophobic effect (1). The small size, tetrahedral geometry, and hydrogen-bonding (H-bonding) ability of water all play a role in the interactions water takes part in with itself as well as with polar and nonpolar solutes. The structure of liquid water is well characterized experimentally in terms of radial distribution functions, g(r), derived from diffraction experiments (2, 3). Similar experiments on solutions of alcohols and tetraalkylammonium ions, however, show very little structural differences in g(r) compared with bulk water (2, 4). Yet the thermodynamics of insertion of nonpolar solutes into water show an unfavorable free energy, a positive entropy at room temperature, and a large positive heat capacity (5, 6). There are several possible reasons why the g(r) may lack sensitivity in detecting structural changes in the hydration shells of hydrophobic solutes: (i) solubility of such solutes is small, so the majority of the observed signal arises from bulk water; (ii) solutes may be incompletely mixed (7); and (iii) the g(r) is inherently insensitive to orientational structure.The tetrahedral geometry of the water molecule and the directional nature of the H-bond impart a highly oriented structure to water. The orientational correlation function of liquid water has been generated by applying maximum entropy methods to g(r) functions (8,9). The resulting density distribution shows clear density peaks beyond the H-bond donor locations (along the O-H bonds), with a broader distribution...

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mentioning

confidence: 91%

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

Raschke

Levitt²

2005

Proc. Natl. Acad. Sci. U.S.A.

142

145

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“…[45][46][47][48][49][50][51][52] Results indicate that the configuration of water molecules close to nonpolar groups does not differ substantially from that in bulk water, and that no H-bond is significantly strengthened. No clathrate-like water cage structures (often called icebergs, flickering clusters, or clathration shells) leading to an unfavorable excess entropy are formed that could explain hydrophobicity or justify the large positive Gibbs free energy change responsible for the poor aqueous solubility of nonpolar substances.…”

Section: Introductionmentioning

confidence: 99%

The Hydrophobic Effect. 1. A Consequence of the Mobile Order in H-Bonded Liquids

Ruelle

Kesselring

1998

Journal of Pharmaceutical Sciences

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The hydrophobic effect has an entropic nature that cannot be explained by classical multicomponent treatments that do not explicitly take into account both the mobility and the nonergodicity of the H-bonds in amphiphilic liquids. The nonergodic thermodynamics of mobile order in H-bonded liquids based on time fractions rather than on concentrations provides a novel qualitative and quantitative explanation for the molecular origin of the hydrophobic effect. Chiefly, this effect corresponds to the loss of the mobile order entropy of associated molecules by dilution with foreign substances. Not being a unique property of water, the propensity of an amphiphilic solvent to induce a solvophobic effect increases primarily as its structuration factor increases, and secondarity as the solute/solvent molar volume ratio increases. On this basis, it can be expected that in the absence of strong solute-solvent specific interactions, the solubility of nonelectrolytes will generally decrease in the following order: butanol > propanol > ethanol > methanol > propylene glycol > ethylene glycol > formamide > water.

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“…Recent advances in the application of isotope substitution neutron scattering measurements have given us detailed insight into the structures of aqueous twocomponent amphiphile systems, showing us how amphiphiles interact in aqueous solution as temperature and concentration are varied [8][9][10][11]. We present here a significant extension of these techniques to an aqueous solution of tertiary butanol to which a salting-out agent has been added.…”

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confidence: 99%

“…These latter views argue that the saltingout agents do not interact with the protein and hence question the classical view. Recent NMR studies have argued in favor of direct salt-nonpolar group interactions [5,6], while other views favor an indirect interaction mediated by the hydration shells of the ions and the nonpolar solutes [7].Recent advances in the application of isotope substitution neutron scattering measurements have given us detailed insight into the structures of aqueous twocomponent amphiphile systems, showing us how amphiphiles interact in aqueous solution as temperature and concentration are varied [8][9][10][11]. We present here a significant extension of these techniques to an aqueous solution of tertiary butanol to which a salting-out agent has been added.…”

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confidence: 99%

Anion Bridges Drive Salting Out of a Simple Amphiphile from Aqueous Solution

Bowron

Finney

2002

Phys. Rev. Lett.

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Neutron diffraction with isotope substitution has been used to determine the structural changes that occur on the addition of a simple salting-out agent to a dilute aqueous alcohol solution. The striking results obtained demonstrate a relatively simple process occurs in which interamphiphile anionic salt bridges are formed between the polar groups of the alcohol molecules. These ion bridges drive an increase in the exposure of the alcohol molecule nonpolar surface to the solvent water and hence point the way to their eventual salting out by the hydrophobic effect. DOI: 10.1103/PhysRevLett.89.215508 PACS numbers: 61.20.Qg, 61.12.-q, 64.75.+g, 82.30.Nr Ever since the early studies of Hofmeister [1] the effect of salts on the solubility of amphiphilic molecules has been of intense interest. Despite its regular use for the concentration and crystallization of biological molecules such as proteins, the phenomenon is not well understood. The ''classical view'''(see, e.g., [2]) explains the effect in terms of competition between the salt and the protein for water of solvation. Arakawa and Timasheff 's experiments [3] indicate that proteins are preferentially hydrated in the presence of salting-out agents, while Chick and Martin [4] suggest that the salt is excluded from the protein precipitate. These latter views argue that the saltingout agents do not interact with the protein and hence question the classical view. Recent NMR studies have argued in favor of direct salt-nonpolar group interactions [5,6], while other views favor an indirect interaction mediated by the hydration shells of the ions and the nonpolar solutes [7].Recent advances in the application of isotope substitution neutron scattering measurements have given us detailed insight into the structures of aqueous twocomponent amphiphile systems, showing us how amphiphiles interact in aqueous solution as temperature and concentration are varied [8][9][10][11]. We present here a significant extension of these techniques to an aqueous solution of tertiary butanol to which a salting-out agent has been added.To extract the structural correlations in this system a series of isotopically labeled solutions were prepared without and with sodium chloride, a known and simple salting-out agent. The stoichiometric ratio of alcohol to water for all solutions was 1:50, corresponding to a mole fraction concentration of 0:02. This concentration is close to the minimum in the relative partial molar volume of t-butanol (TBA) in t-butanol-water mixtures [12], a region that is traditionally associated with the maximum in hydrophobic effects observed in these solutions [13]. In the solutions containing NaCl, the NaCl:water ratio was fixed at 1:100, so that the concentration of ions (irrespective of type) is then equal to that of the alcohol molecules.To allow extraction of information relating to the hydration of the alcohol molecules and the structure of the water solvent, five isotopic mixtures were prepared for each of the salt and no-salt cases, namely, (1) CD 3 3 COH in ...

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Water structure in aqueous solutions of tetramethylammonium chloride

Cited by 63 publications

References 6 publications

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

Nonpolar solutes enhance water structure within hydration shells while reducing interactions between them

The Hydrophobic Effect. 1. A Consequence of the Mobile Order in H-Bonded Liquids

Anion Bridges Drive Salting Out of a Simple Amphiphile from Aqueous Solution

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