Molecular Theory and the Effects of Solute Attractive Forces on Hydrophobic Interactions (original) (raw)
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Molecular Hydrophobic Attraction and Ion-Specific Effects Studied by Molecular Dynamics †
Langmuir, 2008
Much is written about "hydrophobic forces" that act between solvated molecules and nonpolar interfaces, but it is not always clear what causes these forces and whether they should be labeled as hydrophobic. Hydrophobic effects roughly fall in two classes, those that are influenced by the addition of salt and those that are not. Bubble adsorption and cavitation effects plague experiments and simulations of interacting extended hydrophobic surfaces and lead to a strong, almost irreversible attraction that has little or no dependence on salt type and concentration. In this paper, we are concerned with hydrophobic interactions between single molecules and extended surfaces and try to elucidate the relation to electrostatic and ion-specific effects. For these nanoscopic hydrophobic forces, bubbles and cavitation effects play only a minor role and even if present cause no equilibration problems. In specific, we study the forced desorption of peptides from nonpolar interfaces by means of molecular dynamics simulations and determine the adsorption potential of mean force. The simulation results for peptides compare well with corresponding AFM experiments. An analysis of the various contributions to the total peptide-surface interactions shows that structural effects of water as well as van der Waals interactions between surface and peptide are important. Hofmeister ion effects are studied by separately determining the effective interaction of various ions with hydrophobic surfaces. An extension of the Poisson-Boltzmann equation that includes the ion-specific potential of mean force yields surface potentials, interfacial tensions, and effective interactions between hydrophobic surfaces. There, we also analyze the energetic contributions to the potential of mean force and find that the most important factor determining ion-specific adsorption at hydrophobic surfaces can best be described as surface-modified ion hydration.
The Journal of Chemical Physics, 2007
The hydrophobic interaction that is characterized by a potential of mean force (PMF) between spherical apolar solutes immersed in the simple point charge (SPCE) model for water was studied using an interaction site model integral equation based on a density-functional theory for molecular fluids. For comparison with the PMFs for various size solutes in the SPCE model, the PMFs in a Lennard-Jones (LJ) model for a solvent whose diameter is same as the SPCE model were also studied using a hypernetted chain integral equation. It is noted in the LJ model that the hydrogen bond and its network structure are completely ignored, but the translational entropy is taken into account. Both PMFs obtained from the SPCE model and from the LJ model have a large first minimum at a contact distance of solutes. In the case that the solute size is about three times larger than water, these PMFs also have a large maximum at a longer distance than the contact position. The strong attraction is attributed to the translational entropy of the solvent, and that the large activation barrier is arising from the weak attraction between the solute and the solvent. The comparison between the SPCE model and the LJ solvent model suggests that the qualitative description of the hydrophobic interaction is possible by using the LJ solvent model. On the other hand, the dewetting tendency on the surface of the apolar solute in a room condition is observed only on the SPCE model. These results indicate that the characteristics of water such as the hydrogen bond affect rather the hydrophobic hydration than the hydrophobic interaction.
Statistical Analyses of Hydrophobic Interactions: A Mini-Review
The Journal of Physical Chemistry B, 2016
This review focuses on the striking recent progress in solving for hydrophobic interactions between small inert molecules. We discuss several new understandings. Firstly, the inverse temperature phenomenology of hydrophobic interactions, i.e., strengthening of hydrophobic bonds with increasing temperature, is decisively exhibited by hydrophobic interactions between atomic-scale hard sphere solutes in water. Secondly, inclusion of attractive interactions associated with atomic-size hydrophobic reference cases leads to substantial, non-trivial corrections to reference results for purely repulsive solutes. Hydrophobic bonds are weakened by adding solute dispersion forces to treatment of reference cases. The classic statistical mechanical theory for those corrections is not accurate in this application, but molecular quasi-chemical theory shows promise. Finally, because of the masking roles of excluded volume and attractive interactions, comparisons that do not discriminate the different possibilities face an interpretive danger.
One-dimensional model for water and aqueous solutions. IV. A study of “hydrophobic interactions”
The Journal of Chemical Physics, 2008
The solute-solute pair correlation function and the potential of mean force (PMF) between two hard-rod solutes in two solvents are studied in one-dimensional systems. One solvent consists of particles interacting via square well (SW) potential. The second consists of particles interacting via “hydrogen-bond-like” (HB) pair potential. It was found that the first minimum of the solute-solute PMF at infinite dilution in the two solvents grows deeper as we increase the strength of the solvent-solvent interaction. In the SW (but not in the HB) solvent, we found that the range of solute-solute pair correlation is larger at lower temperatures (or at larger εBB∕kBT). The relevance of this finding to the problem of hydrophobic interactions is discussed.
Thermodynamics and mechanism of hydrophobic interaction
Australian Journal of Chemistry, 1977
In the mixed solvent, 0.1 mole fraction ethanol-water, long-chain decyltrimethylammonium carboxylates form ion pairs. Ion-pair association constants (and hence the free energy of ion-pair formation) can be measured conductometrically. It is possible to separate the hydrophobic from the electrostatic contribution to the free energy of ion-pair formation by systematically varying the hydrocarbon chain length. We report measurements of the free energy of hydrophobic interaction (AG&) over the temperature range 278-328 K. The value of AG& becomes more negative (stronger hydrophobic interaction) with increasing temperature. The temperature coefficient of AG& was used to calculate the enthalpy (AH,",) and entropy (AS&) of hydrophobic interaction. At low temperature the entropic contribution to the free energy is the larger but AH,", dominates at temperatures above c. 324 K. The volume change of hydrophobic interaction was similarly estimated from the volume change of ion-pair formation. We obtained values of apparent molar volume of the decyltrimethylammonium carboxylates (over a range of concentrations) from very precise density measurements. These could then be combined with the appropriate ion-pair association constant (from the conductance measurements) to give the partial molar volumes of the free ions and the ion pair. Hydrophobic interaction was found to be accompanied by a substantial increase in volume amounting to 10.2 + 0.3 ml mol-' for each pair of interacting methylene groups. Our results support the view that hydrophobic interaction occurs with a further ordering of water molecules over and above that which exists in the hydrophobic hydration layer surrounding an isolated hydrophobic molecule.
Water's Hydrogen Bonds in the Hydrophobic Effect: A Simple Model
The Journal of Physical Chemistry B, 2005
We propose a simple analytical model to account for water's hydrogen bonds in the hydrophobic effect. It is based on computing a mean-field partition function for a water molecule in the first solvation shell around a solute molecule. The model treats the orientational restrictions from hydrogen bonding, and utilizes quantities that can be obtained from bulk water simulations. We illustrate the principles in a 2-dimensional Mercedes-Benz-like model. Our model gives good predictions for the heat capacity of hydrophobic solvation, reproduces the solvation energies and entropies at different temperatures with only one fitting parameter, and accounts for the solute size dependence of the hydrophobic effect. Our model supports the view that water's hydrogen bonding propensity determines the temperature dependence of the hydrophobic effect. It explains the puzzling experimental observation that dissolving a nonpolar solute in hot water has positive entropy.
Towards a microscopic theory of hydrophobic solutions
Journal of the Chemical Society, Faraday Transactions 2, 1978
The distinguishing features of water and apolar solute molecular potentials responsible for the behaviour of hydrophobic solutions are inferred from a consideration of the thermodynamic properties of bulk water. These molecular properties are built into exactly soluble models in one dimension, and their necessity underlined by a comparison of models which give normal and hydrophobic solution thermodynamics. The form of solute-solvent and solute-solute molecular distribution functions is explored, and used to infer the nature of solute induced structure and the solventmediated hydrophobic interaction between apolar molecules in water.
Proceedings of the National Academy of Sciences of the United States of America, 2013
The osmotic second virial coefficients, B2, for atomic-sized hard spheres in water are attractive (B2 < 0) and become more attractive with increasing temperature (ΔB2/ΔT < 0) in the temperature range 300 K ≤ T ≤ 360 K. Thus, these hydrophobic interactions are attractive and endothermic at moderate temperatures. Hydrophobic interactions between atomic-sized hard spheres in water are more attractive than predicted by the available statistical mechanical theory. These results constitute an initial step toward detailed molecular theory of additional intermolecular interaction features, specifically, attractive interactions associated with hydrophobic solutes.
Toward a simple molecular theory of hydrophobic hydration
Journal of Molecular Liquids, 2014
A perturbation theory of water is extended to mixtures, and its application to aqueous solutions of noble gases is presented. The present approach is based on the thermodynamic perturbation theory of the primitive models of associating fluids, substituting the reference pseudo-hard-body term by a hard-sphere/pseudo-hard-body mixture term and introducing appropriate corrections. The primitive models are constructed in a rigorous way from realistic parents. The procedure yields equations of state allowing for the determination of all residual properties. The residual chemical potential is expressed and subsequently the Henry's law constants of noble gases, from He to Xe, are evaluated as functions of temperature showing qualitative agreement with experimental data. Throughout the procedure, no experimental data are used to adjust the parameters or to fine-tune the results.