arXiv subjects

M. E. Paulaitis

Publications and source records attributed to M. E. Paulaitis.

6 recordsLinked to original sources

arXiv2014

Concentration dependence of the Flory-Huggins interaction parameter in aqueous solutions of capped PEO chains

The dependence on volume fraction $φ$ of the Flory-Huggins $χ_{\mathrm{wp}}\left(φ\right)$ describing the free energy of mixing of polymers in water is obtained by exploiting the connection of $χ_{\mathrm{wp}}\left(φ\right)$ to the chemical potential of the water, for which quasi-chemical theory is satisfactory. We test this theoretical approach with simulation data for aqueous solutions of capped PEO oligomers. For CH$_3$(CH$_2$-O-CH$_2$)$_m$CH$_3$ ($m$=11), $χ_{\mathrm{wp}}\left(φ\right)$ depends strongly on $φ$, consistent with experiment. These results identify coexisting water-rich and water-poor solutions at $T$ = 300K and $p$ = 1atm. Direct observation of the coexistence of these two solutions on simulation time scales supports that prediction for the system studied. This approach directly provides the osmotic pressures. The osmotic second virial coefficient for these chains is positive, reflecting repulsive interactions between the chains in the water, a good solvent for these chains.

cond-mat.soft↗

arXiv2014

Loop-Closure and Gaussian Models of Collective Structural Characteristics of Capped PEO Oligomers in Water

Parallel-tempering MD results for a CH$_3$(CH$_2$-O-CH$_2$)$_m$CH$_3$ chain in water are exploited as a data-base for analysis of collective structural characteristics of the PEO globule with a goal of defining models permitting statistical thermodynamic analysis of dispersants of Corexit type. The chain structure factor, relevant to neutron scattering from a deuterated chain in neutral water, is considered specifically. The traditional continuum-Gaussian structure factor is inconsistent with the simple $k \rightarrow \infty$ behavior, but we consider a discrete-Gaussian model that does achieve that consistency. Shifting-and-scaling the discrete-Gaussian model helps to identify the low-$k$ to high-$k$ transition near $k \approx 2π/0.6 \mathrm{nm}$ when an empirically matched number of Gaussian links is about one-third of the total number of effective-atom sites. This short distance-scale boundary of 0.6 nm is directly verified with the $r$-space distributions, and this distance is thus identified with a natural size for coarsened monomers. The probability distribution of $R_g{}^2$ is compared with the classic predictions for both Gaussian model and freely-jointed chains. $\left\langle R_g{}^2(j)\right\rangle$, the contribution of the $j$-th chain segment to $\left\langle R_g{}^2\right\rangle$, depends on contour index about as expected for Gaussian chains despite significant quantitative discrepancies which express the swelling of these chains in water. Monomers central to the chain contour occupy the center of the chain globule. The density profiles of chain segments relative to their center of mass can show distinctive density structuring for smaller chains due close proximity of central elements to the globule center. But that density structuring washes-out for longer chains where many chain elements additively contribute to the density profiles.

physics.chem-ph↗

arXiv2008

Quasi-chemical theory with a soft cutoff

In view of the wide success of molecular quasi-chemical theory of liquids, this paper develops the soft-cutoff version of that theory. This development has important practical consequences in the common cases that the packing contribution dominates the solvation free energy of realistically-modeled molecules because treatment of hard-core interactions usually requires special purpose simulation methods. In contrast, treatment of smooth repulsive interactions is typically straightforward on the basis of widely available software. This development also shows how fluids composed of molecules with smooth repulsive interactions can be treated analogously to the molecular-field theory of the hard-sphere fluid. In the treatment of liquid water, quasi-chemical theory with soft-cutoff conditioning doesn't change the fundamental convergence characteristics of the theory using hard-cutoff conditioning. In fact, hard cutoffs are found here to work better than softer ones.

physics.chem-ph↗

arXiv2007

Balancing Local Order and Long-Ranged Interactions in the Molecular Theory of Liquid Water

A molecular theory of liquid water is identified and studied on the basis of computer simulation of the TIP3P model of liquid water. This theory would be exact for models of liquid water in which the intermolecular interactions vanish outside a finite spatial range, and therefore provides a precise analysis tool for investigating the effects of longer-ranged intermolecular interactions. We show how local order can be introduced through quasi-chemical theory. Long-ranged interactions are characterized generally by a conditional distribution of binding energies, and this formulation is interpreted as a regularization of the primitive statistical thermodynamic problem. These binding-energy distributions for liquid water are observed to be unimodal. The gaussian approximation proposed is remarkably successful in predicting the Gibbs free energy and the molar entropy of liquid water, as judged by comparison with numerically exact results. The remaining discrepancies are subtle quantitative problems that do have significant consequences for the thermodynamic properties that distinguish water from many other liquids. The basic subtlety of liquid water is found then in the competition of several effects which must be quantitatively balanced for realistic results.

physics.chem-ph↗

arXiv2006

Gaussian Models for the Statistical Thermodynamics of Liquid Water

A gaussian distribution of binding energies, but conditioned to exploit generally available information on packing in liquids, provides a statistical-thermodynamic theory of liquid water that is structurally non-committal, molecularly realistic, and surprisingly accurate. Neglect of fluctuation contributions to this gaussian model yields a mean-field theory that produces useless results. A refinement that accounts for sharper-than-gaussian behavior at high binding energies recognizes contributions from a discrete number of water molecules and permits a natural matching of numerically exact results. These gaussian models, which can be understood as vigorous simplifications of quasi-chemical theories, are applicable to aqueous environments where the utility of structural models based on geometrical considerations of water hydrogen bonding have not been established.

physics.bio-ph↗

arXiv1998

Hydrophobic Effects on a Molecular Scale

A theoretical approach is developed to quantify hydrophobic hydration and interactions on a molecular scale, with the goal of gaining insight into the molecular origins of hydrophobic effects. The model is based on the fundamental relation between the probability for cavity formation in bulk water resulting from molecular-scale density fluctuations, and the hydration free energy of the simplest hydrophobic solute, a hard particle. This probability is estimated using an information theory (IT) approach, incorporating experimentally available properties of bulk water -- the density and radial distribution function. The IT approach reproduces the simplest hydrophobic effects: hydration of spherical nonpolar solutes, the potential of mean force between methane molecules, and solvent contributions to the torsional equilibrium of butane. Applications of this approach to study temperature and pressure effects provide new insights into the thermodynamics and kinetics of protein folding. The IT model relates the hydrophobic-entropy convergence observed in protein unfolding experiments to the macroscopic isothermal compressibility of water. A novel explanation for pressure denaturation of proteins follows from an analysis of the pressure stability of hydrophobic aggregates, suggesting that water penetrates the hydrophobic core of proteins at high pressures. This resolves a long-standing puzzle, whether pressure denaturation contradicts the hydrophobic-core model of protein stability. Finally, issues of ``dewetting'' of molecularly large nonpolar solutes are discussed in the context of a recently developed perturbation theory approach.

physics.chem-ph↗