Elucidation of temperature-induced water structuring on cellulose surfaces for environmental and energy sustainability.

Optimizing drying energy in the forest products industry is critical for integrating lignocellulosic feedstocks across all manufacturing sectors. Despite substantial efforts to reduce thermal energy consumption during drying, further enhancements are possible. Cellulose, the main component of forest products, is Earth's most abundant biopolymer and a promising renewable feedstock. This study employs all-atom molecular dynamics (MD) simulations to explore the structural dynamics of a small Iß-cellulose microcrystallite and surrounding water layers during drying. Molecular and atomistic profiles revealed localized water near the cellulose surface, with water structuring extending beyond 8 Å into the water bulk, influencing solvent-accessible surface area and solvation energy. With increasing temperature, there was a ∼20 % reduction in the cellulose surface available for interaction with water molecules, and a ∼22 % reduction in solvation energy. The number of hydrogen bonds increased with thicker water layers, facilitated by a "bridging" effect. Electrostatic interactions dominated the intermolecular interactions at all temperatures, creating an energetic barrier that hinders water removal, slowing the drying processes. Understanding temperature-dependent cellulose-water interactions at the molecular level will help in designing novel strategies to address drying energy consumption, advancing the adoption of lignocellulosics as viable manufacturing feedstocks.

Molecular Dynamics of Hydration Shells of Adsorbates in Entropy-Driven Adsorption (Hydrophobic Bonding) to Activated Carbon Surfaces.

Hydrophobic bonding is a phenomenon wherein the adsorption of solutes from aqueous solutions is driven largely by the desire of solvent molecules to interact with each other, thus squeezing out solute molecules onto the adsorbent surface. A novel computational analysis of hydration shell water dynamics was used to study the driving force for the hydrophobic bonding of five small drug molecules to activated carbon. It was demonstrated that the solvation of these drug molecules produced hydration shells of lower density and molecular mobility than bulk water, up to 10.5-14 Å distance. Excellent correlations were found between the simulated water-water hydrogen bonding lifetimes in the hydration shell and the experimental capacity constants of hydrophobic bonding (KHB) obtained from the Two-Mechanism Langmuir-Like Equation. KHB also correlated well with the solute-solvent vdW interaction energies in a manner that could allow future predictions of KHB values from simple simulations. Such correlations were not found with the capacity constant of the well-known enthalpy-driven adsorption. The driving force for hydrophobic bonding has entropic origins due to the elimination of water structuring in the hydration shells. However, unlike a typical entropy-driven process, hydrophobic bonding to activated carbon was also associated with a large exothermic enthalpy change when studied with isoperibol calorimetry.

A study of the permeation and water-structuring behavioural properties of PEG modified hydrated silk fibroin membranes.

The potential of naturally occurring substances as a source of biomedical materials is well-recognised and is being increasingly exploited. Silk fibroin membranes derived fromBombyx morisilk cocoons exemplify this, for example as substrata for the growth of ocular cells with the aim of generating biomaterial-cell constructs for tissue engineering. This study investigated the transport properties of selected silk fibroin membranes under conditions that allowed equilibrium hydration of the membranes to be maintained. The behaviour of natural fibroin membranes was compared with fibroin membranes that have been chemically modified with poly(ethylene glycol). The permeation of the smaller hydrated sodium ion was higher than that of the hydrated calcium ion for all three ethanol treated membranes investigated. The PEG and HRP-modified C membrane, which had the highest water content at 59.6 ± 1.5% exhibited the highest permeation of the three membranes at 95.7 ± 2.8 × 10-8cm2s-1compared with 17.9 ± 0.9 × 10-8cm2s-1and 8.7 ± 1.7 × 10-8cm2s-1for membranes A and B respectively for the NaCl permeant. Poly(ethylene glycol) was used to increase permeability while exploiting the crosslinking capabilities of horseradish peroxidase to increase the compressive strength of the membrane. Importantly, we have established that the permeation behaviour of water-soluble permeants with hydrated radii in the sub-nanometer range is analogous to that of conventional hydrogel polymers.

Interfacial ion specificity modulates hydrophobic interaction.

HYPOTHESIS: Ion specificity is crucial in assembly and aggregation of polymers in water driven by hydrophobic interaction. An increasing number of studies have suggested that specific ion adsorption and consequent impact on interfacial water molecules should play an important role in modulating hydrophobic interaction. EXPERIMENTS: Here, bubble probe atomic force microscopy (AFM) combined with theoretical modeling analysis was applied to quantify hydrophobic interactions involving three model polymers in solutions containing different ions. FINDINGS: For polystyrene, the hydrophobic interaction's decay length D0 was reduced from 0.75 nm to 0.60 nm by introducing weakly hydrated cations (e.g., K+ and NH4+), while varying anion type had little effect. For poly(methyl methacrylate) and polydimethylsiloxane, anion specificity was demonstrated more evident in shortening the hydrophobic interaction range, with D0 decreasing from 0.63 nm to 0.50 nm and from 0.72 nm to 0.58 nm respectively when strongly hydrated F- or Cl- was replaced by weakly hydrated I-. Such results could arise from specific ion adsorption at water/polymer interface which disrupts the water structuring effect. From the nanomechanical perspective, this work has revealed the importance of interfacial ion specificity in modulating hydrophobic interaction, which offers novel implications for tuning assembly behavior of macromolecules in relevant engineering applications such as micelle formation and foam stabilization.

Why do water molecules around small hydrophobic solutes form stronger hydrogen bonds than in the bulk?

Molecular solutes are known to have a strong effect on the structural and dynamical properties of the surrounding water. In our recent study (PNAS, 114, 322 (2017)) we have identified the presence of strengthened water hydrogen bonds near hydrophobic solutes by using both IR spectroscopy and ab-initio molecular dynamics simulations. The water molecules involved in the enhanced hydrogen bonding have been shown to display extensive structural ordering and restricted mobility. We observed that an individual pair of water molecules can make stronger hydrogen bond to each other if it is not surrounded by intercalating water molecules. Here we present compelling simulation results which unravel a simple mechanistic picture of the emergence of the hydrogen bond (HB) strengthening around solvated methane. We show explicitly that actual absence of water molecules within the excluded volume due to the hydrophobic molecule assures smaller residual torque on neighboring water molecules enabling the formation of stronger HBs between them.

Hydrophobic and hydrophilic effects on water structuring and adhesion in denture adhesives.

Denture adhesives are designed to be moisture-sensitive through the inclusion of a blend of polymer salts with varying degrees of water-sensitivity. This enables the adhesive to mix with saliva in vivo and activate its high tack, through the formation of a mucilaginous layer. We report for the first time, the use of differential scanning calorimetry to study a series of hydrophobic and hydrophilic polymeric systems in order to correlate water-structuring behavior with adhesion strength. Adhesive bonding of the more hydrophobic variants was higher than that of a commercial-based control and a more hydrophilic polymer system in both lap shear and tensile configurations. Water-binding data suggested that increasing the hydrophobicity of the maleic acid copolymer substituents led to decreased levels of freezing water. In comparison, increasing the hydrophilic nature of the polymer backbone gave higher levels of freezing water within the hydrated samples. The results of this study emphasize the importance of varying the levels of hydrophobic and hydrophilic components within denture adhesive formulations, alongside the types of water present within the adhesive systems. This phenomenon has shown the potential to fine-tune the adhesive properties and failure mode against poly(methyl methacrylate), surfaces. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1355-1362, 2018.

Water response to ganglioside GM1 surface remodelling.

BACKGROUND: Gangliosides are biological glycolipids participating in rafts, structural and functional domains of cell membranes. Their headgroups are able to assume different conformations when packed on the surface of an aggregate, more lying or standing. Switching between different conformations is possible, and is a collective event. Switching can be induced, in model systems, by concentration or temperature increase, then possibly involving ganglioside-water interaction. In the present paper, the effect of GM1 ganglioside headgroup conformation on the water structuring and interactions is addressed. METHODS: Depolarized Rayleigh Scattering, Raman Scattering, Quasielastic Neutron Scattering and NMR measurements were performed on GM1 ganglioside solutions, focusing on solvent properties. RESULTS: All used techniques agree in evidencing differences in the structure and dynamics of solvent water on different time-and-length scales in the presence of either GM1 headgroup conformations. CONCLUSIONS: In general, all results indicate that both the structural properties of solvent water and its interactions with the sugar headgroups of GM1 respond to surface remodelling. The extent of this modification is much higher than expected and, interestingly, ganglioside headgroups seem to turn from cosmotropes to chaotropes upon collective rearrangement from the standing- to the lying-conformation. SIGNIFICANCE: In a biological perspective, water structure modulation could be one of the physico-chemical elements contributing to the raft strategy, both for rafts formation and persistence and for their functional aspects. In particular, the interaction with approaching bodies could be favoured or inhibited or triggered by complex-sugar-sequence conformational switch. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Water structuring over the hydrophobic surface of cellulose.

Many important biological solutes possess not only polar and hydrogen-bonding functionalities but also weakly hydrating, or hydrophobic, surfaces. While the aggregation of these hydrophobic surfaces has been shown to play an important role in the aggregation of individual chains of cellulose, it is not known whether the water structuring imposed by these hydrophobic surfaces more closely resembles that associated with small hydrophobic solutes like methane and fats or more closely resembles that associated with extended hydrophobic surfaces like mica or waxy planes. By using molecular dynamics simulations to characterize the water molecule orientations over different regions of the 100 surface of cellulose in contact with water, it was found that the hydrophobic strips of the cellulose crystal are sufficiently narrow that they hydrate like a fatty acid chain, rather than like a more extended surface, suggesting that their aggregation would be dominated by entropy rather than enthalpy.

Physical Modeling of Aqueous Solvation.

We consider the free energies of solvating molecules in water. Computational modeling usually involves either detailed explicit-solvent simulations, or faster computations, which are based on implicit continuum approximations or additivity assumptions. These simpler approaches often miss microscopic physical details and non-additivities present in experimental data. We review explicit-solvent modeling that identifies the physical bases for the errors in the simpler approaches. One problem is that water molecules that are shared between two substituent groups often behave differently than waters around each substituent individually. One manifestation of non-additivities is that solvation free energies in water can depend not only on surface area or volume, but on other properties, such as the surface curvature. We also describe a new computational approach, called Semi-Explicit Assembly, that aims to repair these flaws and capture more of the physics of explicit water models, but with computational efficiencies approaching those of implicit-solvent models.