Henry S. Ashbaugh

Absolute hydration free energies of ions, ion–water clusters, and quasichemical theory

Experimental studies of ion–water clusters have provided insights into the microscopic aspects of hydration phenomena. One common view is that extending those experimental studies to larger cluster sizes would give the single-ion absolute hydration free energies not obtainable by classical thermodynamic methods. This issue is reanalyzed in the context of recent computations and molecular theories of ion hydration, particularly considering the hydration of H+, Li+, Na+, and HO− ions, and thence the hydration of neutral ion pairs. The hydration free energies of neutral pairs computed here are in good agreement with experimental results, whereas the calculated absolute hydration free energies and the excess chemical potentials deviate consistently from some recently tabulated hydration free energies based on ion–water cluster data. We show how the single-ion absolute hydration free energies are not separated from the potential of the phase in recent analyses of ion–water cluster data, even in the limit of la...

A simple molecular thermodynamic theory of hydrophobic hydration

A recently developed microscopic model for associating fluids that accurately captures the thermodynamics of liquid water [Truskett et al., J. Chem. Phys. 111, 2647 (1999)] is extended to aqueous solutions with nonpolar species. The underlying association model incorporates the highly directional and open nature of water’s hydrogen-bond network, and, as a result, captures a number of the distinguishing properties of liquid water, such as the density anomaly. The model for aqueous mixtures developed herein predicts many of the thermodynamic signatures of hydrophobic hydration without resorting to empirical temperature-dependent parameters. The predicted solubility of nonpolar species is slight over a wide range of temperatures, and exhibits a minimum as a function of temperature, in accord with experiment. Hydration is opposed by a dominant entropy and favored by the enthalpy at low temperatures. At elevated temperatures these roles are reversed. Furthermore, the hydration entropies for hydrophobes of vary...

Assessing the thermodynamic signatures of hydrophobic hydration for several common water models

Following the conclusions of an information theory analysis that hydrophobic hydration is dictated by the equation of state of liquid water, we perform simulations of ten different water models to examine the correlation between the fidelity of each model to the experimental density of liquid water and the accuracy of its description of methane hydration. We find that the three- and five-point water models provide an inferior description of both the liquid density and methane solubility compared to the four-point water models. Of the four-point water models, TIP4P/2005 provides the best description of both the aqueous equation-of-state and methane hydration thermodynamics. When the optimized potentials for liquid simulation united-atom description for methane is used, we find that while the entropy and heat capacity of methane hydration are in excellent agreement with experiment, the chemical potential and enthalpy are systematically shifted upwards. We subsequently reoptimize the methane interaction to accurately reproduce the experimental solubilities as a function of temperature by accounting for missing attractive interactions.

Effects of long-range electrostatic potential truncation on the free energy of ionic hydration

The free energy of sodium ion hydration calculated from computer simulations employing both the molecular potential truncation and Ewald summation methods for evaluation of long-range electrostatic interactions disagree by 20%. The discrepancy between the free energies determined by both techniques is found to be methodological, resulting from an imbalance in the molecular potential truncation scheme that biases the interaction of the water–oxygen and hydrogens with the ion unequally. A simple physical model is proposed and an analytical expression derived to correct this discrepancy.

Mesoscale model of polymer melt structure : Self-consistent mapping of molecular correlations to coarse-grained potentials

Development and application of coarse-graining methods to condensed phases of macromolecules is an active area of research. Multiscale modeling of polymeric systems using coarse-graining methods presents unique challenges. Here we apply a coarse-graining method that self-consistently maps structural correlations from detailed molecular dynamics (MD) simulations of alkane oligomers onto coarse-grained potentials using a combination of MD and inverse Monte Carlo methods. Once derived, the coarse-grained potentials allow computationally efficient sampling of ensemble of conformations of significantly longer polyethylene chains. Conformational properties derived from coarse-grained simulations are in excellent agreement with experiments. The level of coarse graining provides a control over the balance of computational efficiency and retention of chemical identity of the underlying polymeric system. Challenges to extension and application of this and similar structure-based coarse-graining methods to model dynamics and phase behavior in polymeric systems are briefly discussed.

Helix stabilization of poly(ethylene glycol)-peptide conjugates.

Hybrid polymer-peptide conjugates offer the potential for incorporating biological function into synthetic materials. The secondary structure of short helical peptides, however, frequently becomes less stable when expressed independent of longer protein sequences or covalently linked with a conformationally disordered synthetic polymer. Recently, new amphipathic peptide-poly(ethylene glycol) conjugates were introduced (Shu, J., et al. Biomacromolecules 2008, 9, 2011), which displayed enhanced peptide helicity upon polymer functionalization while retaining tertiary coiled-coil associations. We report here a molecular simulation study of peptide helix stabilization by conjugation with poly(ethylene glycol). The polymer oxygens are shown to favorably interact with the cationic lysine side chains, providing an alternate binding site that protects against disruption of the peptide hydrogen-bonds that stabilize the helical conformation. When the peptide lysine charges are neutralized or poly(ethylene glycol) is conjugated with polyalanine, the polymer exhibits a negligible effect on the secondary structure. We also observe the interactions of poly(ethylene glycol) with the amphipathic peptide lysines tends to segregate the polymer away from the nonpolar face of the helix, suggesting no disruption of the interactions that drive tertiary contacts between helicies.

Temperature dependence of hydrophobic hydration and entropy convergence in an isotropic model of water

We have investigated temperature dependence of hydrophobic hydration and molecular-scale density fluctuations in an isotropic single-site model of water originally devised by Head-Gordon and Stillinger [J. Chem. Phys. 98, 3313 (1993)] using Monte Carlo simulations. Our isotropic model of water, HGS water, has the same oxygen–oxygen radial distribution function as that of the simple point charge (SPC) water at room temperature and water density. For HGS water, we find that non-Gaussian occupancy fluctuations lead to cavity formation probabilities that are considerably lower than in SPC water. Wetting of a hard-sphere solute by HGS water is also found to be significantly greater than that by SPC water. These observations can be understood in terms of differences in Hamiltonians of the two water models. Despite these differences in the details of hydration, small hydrophobic solutes display many of the well-known thermodynamic finger prints of hydrophobic hydration once the variation of density with temperat...

The hydrophobic effect

Computer simulation and theoretical studies have improved significantly our understanding of the connection between the structural organization of water surrounding hydrophobic solutes and anomalous thermodynamic behavior associated with the ‘hydrophobic effect’. Recent studies have yielded the quantitative temperature dependence of hydrophobic interactions and the dependence of hydration free energy on solute size and shape. The success of new proximity approximations, which assume that water organization is only locally sensitive to solute structure, has encouraged the study of the hydration of complex hydrophobic solutes.

An analysis of molecular packing and chemical association in liquid water using quasichemical theory

We calculate the hydration free energy of liquid TIP3P water at 298 K and 1 bar using a quasi-chemical theory framework in which interactions between a distinguished water molecule and the surrounding water molecules are partitioned into chemical associations with proximal (inner-shell) waters and classical electrostatic-dispersion interactions with the remaining (outer-shell) waters. The calculated free energy is found to be independent of this partitioning, as expected, and in excellent agreement with values derived from the literature. An analysis of the spatial distribution of inner-shell water molecules as a function of the inner-shell volume reveals that water molecules are preferentially excluded from the interior of large volumes as the occupancy number decreases. The driving force for water exclusion is formulated in terms of a free energy for rearranging inner-shell water molecules under the influence of the field exerted by outer-shell waters in order to accommodate one water molecule at the center. The results indicate a balance between chemical association and molecular packing in liquid water that becomes increasingly important as the inner-shell volume grows in size.

Flow improvement of waxy oils mediated by self-aggregating partially crystallizable diblock copolymers

Precipitation and gelation of long chain paraffins from oil presents a challenge to the recovery and processing of waxy crude oils and fuel stocks. Diblock polymers consisting of a crystallizable polyethylene (PE) block and an amorphous poly(ethylenepropylene) (PEP) block self-assemble in oil to form expansive plate-like aggregates, consisting of a PE core within a PEP brush layer. In the presence of crystallizable paraffins the crystalline PE core can promote nucleation of solubilized long chain paraffins or may cocrystallize with the paraffin phase with the soluble PEP brush providing steric stabilization of the wax crystals. We examine the effect of PE-b-PEP additives of varying PEP brush length (5 and 11 K) on the yield stresses of straight chain paraffin gels (ranging in length from 24 to 36 carbons) in decane. PE-b-PEP addition at levels as low as 500 ppm produce reductions of wax gel yield stresses by factors of 3000. At higher PE-b-PEP addition levels gels can be formed with higher yield values than solutions without polymer. The location of the minimum in the yield stress with respect to polymer addition depends on the molecular weight of the paraffin and the PEP brush length. Microscopic crystal dimensions and mobility correlate with the observed rheological results. Potential underlying mechanisms for the observed efficiencies are discussed.