A resummed thermodynamic perturbation theory for positive and negative hydrogen bond cooperativity in water

Prediction of water anomalous properties by introducing the two-state theory in SAFT

Water is one of the most abundant substances on earth, but it is still not entirely understood. It shows unusual behavior, and its properties present characteristic extrema unlike any other fluid. This unusual behavior has been linked to the two-state theory of water, which proposes that water forms different clusters, one with a high density and one with a low density, which may even form two distinct phases at low temperatures. Models incorporating the two-state theory manage to capture the unusual extrema of water, unlike traditional equations of state, which fail. In this work, we have derived the framework to incorporate the two-state theory of water into the Statistical-Associating-Fluid-Theory (SAFT). More specifically, we have assumed that water is an ideal solution of high density water molecules and low density water molecules that are in chemical equilibrium. Using this assumption, we have generalized the association term SAFT to allow for the simultaneous existence of the two water types, which have the same physical parameters but different association properties. We have incorporated the newly derived association term in the context of the Perturbed Chain-SAFT (PC-SAFT). The new model is referred to as PC-SAFT-Two-State (PC-SAFT-TS). Using PC-SAFT-TS, we have succeeded in predicting the characteristic extrema of water, such as its density and speed of sound maximum, etc., without loss of accuracy compared to the original PC-SAFT. This new framework is readily extended to mixtures, and PC-SAFT-TS manages to capture the solubility minimum of hydrocarbons in water in a straightforward manner.

Seeking Solvation: Exploring the Role of Protein Hydration in Silk Gelation

The mechanism by which arthropods (e.g., spiders and many insects) can produce silk fibres from an aqueous protein (fibroin) solution has remained elusive, despite much scientific investigation. In this work, we used several techniques to explore the role of a hydration shell bound to the fibroin in native silk feedstock (NSF) from Bombyx mori silkworms. Small angle X-ray and dynamic light scattering (SAXS and DLS) revealed a coil size (radius of gyration or hydrodynamic radius) around 12 nm, providing considerable scope for hydration. Aggregation in dilute aqueous solution was observed above 65 °C, matching the gelation temperature of more concentrated solutions and suggesting that the strength of interaction with the solvent (i.e., water) was the dominant factor. Infrared (IR) spectroscopy indicated decreasing hydration as the temperature was raised, with similar changes in hydration following gelation by freezing or heating. It was found that the solubility of fibroin in water or aqueous salt solutions could be described well by a relatively simple thermodynamic model for the stability of the protein hydration shell, which suggests that the affected water is enthalpically favoured but entropically penalised, due to its reduced (vibrational or translational) dynamics. Moreover, while the majority of this investigation used fibroin from B. mori, comparisons with published work on silk proteins from other silkworms and spiders, globular proteins and peptide model systems suggest that our findings may be of much wider significance.

Theoretical insight into the hybridization effect of donor and acceptor atoms on the cooperativity of C-H···N hydrogen bonds

In the present work, the influence of hybridization on cooperativity between C-H···N hydrogen bonds is theoretically investigated. Here, C₂H₆, C₂H₄, and C₂H₂ are considered as hydrogen bonding donor while NH₃, N₂H₄, N₂H₂, and N₂ act as the hydrogen bonding acceptor. The calculations are performed at MP2/aug-cc-pVTZ level. It is observed that the stability of systems is amplified as C(sp) > C(sp²) > C(sp³) and also N(sp³) > N(sp²) > N(sp). The role of interaction and deformation energies on the stability of the systems is examined. The results indicate the contribution of interaction energy is dominant in all complexes. The strength of C-H···N hydrogen bond is estimated using interaction energy. In agreement with cooperative energies, the C-H···N hydrogen bond is respectively weakened/strengthened in the triads containing C(sp) and C(sp²)/C(sp³) where two hydrogen bonds coexist. On the other hand, the C-H···N hydrogen bond is strengthened in the ternary systems including N(sp³) and N(sp²) while an opposite behavior is obtained in the triad having N(sp).