High temperature end of the so-called "Koga line": anomalies in temperature derivatives of heat capacities

Connections between the Anomalous Volumetric Properties of Alcohols in Aqueous Solution and the Volume of Hydrophobic Association

The partial molar volumes of alcohols in water exhibit a non-monotonic dependence on concentration at room temperature, initially decreasing with increasing concentration before passing through a minimum and rising to the pure liquid plateau. This anomalous behavior is associated with hydrophobic interactions. We report molecular simulations of short chain alcohols and alkanes in water to examine the volumetric properties of these mixtures at infinite dilution over a range of temperatures. Our simulations find this anomaly disappears at a crossover temperature, above which the solute volume only varies monotonically with concentration. A Voronoi volume analysis of solution configurations finds that solutes in clusters take up less space than individual solutes at low temperature and more space at elevated temperatures. These changes in cluster volumes are subsequently shown to correlate with the derivative of the solute partial molar volume with respect to solute concentration. The changes in solute volume upon nonpolar solute association impact the response of molecular-scale hydrophobic interactions for assembly with increasing pressure.

Mixing scheme of an aqueous solution of tetrabutylphosphonium trifluoroacetate in the water-rich region

The enthalpic interaction of this particular ionic liquid is extremely high, 16 000 kJ mol⁻¹!

Is there a third order phase transition for supercritical fluids?

We prove that according to Molecular Dynamics (MD) simulations of liquid mixtures of Lennard-Jones (L-J) particles, there is no third order phase transition in the supercritical regime beyond Andrew's critical point. This result is in open contrast with recent theoretical studies and experiments which instead suggest not only its existence but also its universality regarding the chemical nature of the fluid. We argue that our results are solid enough to go beyond the limitations of MD and the generic character of L-J models, thus suggesting a rather smooth liquid-vapor thermodynamic behavior of fluids in supercritical regime.

Influence of urea on tert-butyl alcohol aggregation in aqueous solutions

Ternary solutions consisting of urea, tert-butyl alcohol (TBA), and water are investigated employing molecular dynamics simulations. The main purpose of the present paper is to investigate the effect of urea on TBA aggregation and by extension its influence on hydrophobic interactions. The aggregation of TBA can be detected from the concentration dependence of structural properties such as first-shell TBA-water coordination numbers and TBA-TBA hydrogen-bond numbers, as well as through changes in the translational diffusion coefficients of TBA. It is found that urea acts to delay the association of TBA to concentrations greater than those required to cause TBA aggregation in binary TBA-water systems. It is shown that urea acts through a direct mechanism, whereby it preferentially binds to TBA replacing water from the first coordination shell. TBA-urea hydrogen bonds can be as strong as, or stronger than, those of TBA-water, and urea binds to both the hydrophilic and hydrophobic moieties of TBA. Our observations are qualitatively consistent with experimental results for urea-TBA-water solutions and with recent simulation studies of urea's action as a protein denaturant.

Association and microheterogeneity in aqueous 2-butoxyethanol solutions

Molecular dynamics simulations are employed to investigate aggregation and microheterogeneity in dilute solutions of 2-butoxyethanol (BE) in water. The BE concentration is varied from near infinite dilution to the mole fraction X(BE) = 0.04. It was found that large systems (32,000 molecules) are necessary to accommodate the BE aggregates that form in this concentration range. Simulations were performed with two different force fields, and similar results were obtained. At very low concentration, BE aggregation is not observed, but evidence is found for intramolecular hydrogen bonds (between the hydroxyl hydrogen and the ether oxygen of the same BE molecule) that form five-member ring configurations, similar to those reported in experimental studies of BE in nonaqueous solvent. Initial signs of BE association appear at X(BE) ≈ 0.005, after which aggregation occurs rapidly, with aggregates that can be described as micelle-like being fully formed at X(BE) ≈ 0.02. This concentration agrees well with many experimental studies of aggregation in BE-water solutions. Between X(BE) = 0.02 and 0.04, the aggregates appear to grow a little in size, but the basic structure remains the same. At long range, the various pair correlation functions show clear density oscillations associated with BE aggregation. This allows us to identify the length scales of the existing microheterogeneity and to estimate the size of the BE aggregates. If we assume spherical aggregates, then our estimate of the radius at X(BE) = 0.04 (~42 Å) is close to estimates obtained from light scattering and small-angle neutron scattering experiments.