Observation of dissipating solvated protons upon hydrogel formation.

Aqueous hyaluronan solutions form an elastic hydrogel within a narrow pH range, around pH 2.4, making this a model system to study the conformational changes of the hydrogen bond network upon gelation. This pH-dependent behavior allows us to probe water surrounding a biologically relevant molecule in different environments (liquid versus elastic state) which change due to an environmental stimulus. Here, we use Terahertz (THz) reflection absorption spectroscopy in attenuated total reflection (ATR) geometry as a tool to study gelation. THz spectroscopy is sensitive to changes in the hydrogen-bonded water network, and here we show that we can correlate changes in macroscopic properties to changes in the solvation of hyaluronan. Above and below the gelation pH, solvated protons are present in the solutions, however, this spectral signature is completely absent between pH 2.4-2.8, which is the pH at which hyaluronan forms a hydrogel. We propose that solvated protons are forming ion pairs with hyaluronan in this pH range. Adding urea or glucose to hyaluronan solutions changes their elasticity, in which an increase or decrease in elasticity can be linked to the formation and destruction of these ion pairs, respectively.

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain.

Hydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. We present here a spectroscopic approach, which gives direct access to the two main contributions: Using THz-spectroscopy to probe the frequency range of the intermolecular stretch (150-200 cm-1 ) and the hindered rotations (450-600 cm-1 ), the local contributions due to cavity formation and hydrophilic interactions can be traced back. We show that via THz calorimetry these fingerprints can be correlated 1 : 1 with the group specific solvation entropy and enthalpy. This allows to deduce separately the hydrophobic (i.e. cavity formation) and hydrophilic contributions to thermodynamics, as shown for hydrated alcohols as a case study. Accompanying molecular dynamics simulations quantitatively support our experimental results. In the future our approach will allow to dissect hydration contributions in inhomogeneous mixtures and under non-equilibrium conditions.

Water reorientation dynamics in colloidal water-oil emulsions.

We study the molecular-scale properties of colloidal water-oil emulsions consisting of 120-290 nm oil droplets embedded in water. This type of emulsion can be prepared with low concentrations of surfactants and is usually kinetically stable. Even though colloidal water-oil emulsions are used ubiquitously, their molecular properties are still poorly understood. Here we study the orientational dynamics of water molecules in these emulsions using polarization resolved pump-probe infrared spectroscopy, for varying surfactant concentrations, droplet sizes, and temperatures. We find that the majority of the water molecules reorients with the same time constant as in bulk water, while a small fraction of the water molecules reorients on a much longer time scale. These slowly reorienting water molecules are interacting with the surface of the oil droplets. The fraction of slowly orienting water molecules is proportional to the oil volume fraction, and shows a negligible dependence on the average droplet size. This finding indicates that the total surface area of the oil droplets is quite independent of the average droplet size, which indicates that the larger oil droplets are quite corrugated, showing large protrusions into the water phase.

Shaping Tin Nanocomposites through Transient Local Conversion Reactions.

Shape-preserving conversion offers a promising strategy to transform self-assembled structures into advanced functional components with customizable composition and shape. Specifically, the assembly of barium carbonate nanocrystals and amorphous silica nanocomposites (BaCO3/SiO2) offers a plethora of programmable three-dimensional (3D) microscopic geometries, and the nanocrystals can subsequently be converted into functional chemical compositions, while preserving the original 3D geometry. Despite this progress, the scope of these conversion reactions has been limited by the requirement to form carbonate salts. Here, we overcome this limitation using a single-step cation/anion exchange that is driven by the temporal pH change at the converting nanocomposite. We demonstrate the proof of principle by converting BaCO3/SiO2 nanocomposites into tin-containing nanocomposites, a metal without a stable carbonate. We find that BaCO3/SiO2 nanocomposites convert in a single step into hydroromarchite nanocomposites (Sn3(OH)2O2/SiO2) with excellent preservation of the 3D geometry and fine features. We explore the versatility and tunability of these Sn3(OH)2O2/SiO2 nanocomposites as a precursor for functional compositions by developing shape-preserving conversion routes to two desirable compositions: tin perovskites (CH3NH3SnX3, with X = I or Br) with tunable photoluminescence (PL) and cassiterite (SnO2)-a widely used transparent conductor. Ultimately, these findings may enable integration of functional chemical compositions into advanced morphologies for next-generation optoelectronic devices.

Structure and Dynamics of a Temperature-Sensitive Hydrogel.

Polyisocyanotripeptides (TriPIC) are biomimetic polymers which consist of a ß-helical backbone stabilized by hydrogen bonds between amide groups. Their oligoethylene glycol side chains give aqueous TriPIC solutions a thermoresponsive behavior: at 50 °C the solution becomes a hydrogel. In this paper we study the molecular structure and water dynamics of TriPIC aqueous solutions while undergoing gelation using FT-IR spectroscopy and polarization-resolved femtosecond infrared spectroscopy (fs-IR). We find evidence that the oligoethylene glycol side chains trap part of the water molecules upon gel formation, and we propose that the interaction between the oligoethylene glycol side chains and water plays an essential role in the bundling of the polymers and thus in the formation of a hydrogel.

Slowing Down of the Molecular Reorientation of Water in Concentrated Alkaline Solutions.

It is generally accepted that the hydroxide ion (OH-) is a strong hydrogen bond acceptor and that its anomalously high diffusion constant in water results from a Grotthuss-like structural diffusion mechanism. However, the spatial extent over which OH- ions influence the dynamics of the hydrogen-bond network of water remained largely unclear. Here, we measure the ultrafast dynamics of OH groups of HDO molecules interacting with the deuterated hydroxide ion OD-. For solutions with OD- concentrations up to 4 M, we find that HDO molecules that are not directly interacting with the ions have a reorientation time constant of ∼2.7 ps, similar to that of pure liquid water. When the concentration of OD- ions is increased, the reorientation time constant increases, indicating a strong slowing down of the structural dynamics of the solution.

Hyaluronan biopolymers release water upon pH-induced gelation.

We study the relation between the macroscopic viscoelastic properties of aqueous hyaluronan polymer solutions and the molecular-scale dynamics of water using rheology measurements, differential dynamic microscopy, and polarization-resolved infrared pump-probe spectroscopy. We observe that the addition of hyaluronan to water leads to a slowing down of the reorientation of a fraction of the water molecules. Near pH 2.4, the viscosity of the hyaluronan solution reaches a maximum, while the number of slowed down water molecules reaches a minimum. This implies that the water molecules become on average more mobile when the solution becomes more viscous. This observation indicates that the increase in viscosity involves the expulsion of hydration water from the surfaces of the hyaluronan polymers, and a bundling of the hyaluronan polymer chains.

Strategies To Increase the Thermal Stability of Truly Biomimetic Hydrogels: Combining Hydrophobicity and Directed Hydrogen Bonding.

Enhancing the thermal stability of proteins is an important task for protein engineering. There are several ways to increase the thermal stability of proteins in biology, such as greater hydrophobic interactions, increased helical content, decreased occurrence of thermolabile residues, or stable hydrogen bonds. Here, we describe a well-defined polymer based on ß-helical polyisocyanotripeptides (TriPIC) that uses biological approaches, including hydrogen bonding and hydrophobic interactions for its exceptional thermal stability in aqueous solutions. The multiple hydrogen bonding arrays along the polymer backbone shield the hydrophobic core from water. Variable temperature CD and FTIR studies indicate that, on heating, a better packed polymer conformation further stiffens the backbone. Driven by hydrophobic interactions, TriPIC solutions give fully reversible hydrogels that can withstand high temperatures (80 °C) for extended times. Cryo-scanning electron microscopy (cryo-SEM), small-angle X-ray scattering (SAXS), and thorough rheological analysis show that the hydrogel has a bundled architecture, which gives rise to strain stiffening effects on deformation of the gel, analogous to many biological hydrogels.