Molecular Dynamics Study of the Green Solvent Polyethylene Glycol with Water Impurities

Polyethylene glycol (PEG) is one of the environmentally benign solvent options for green chemistry. It readily absorbs water when exposed to the atmosphere. The Molecular Dynamics (MD) simulations of PEG200, a commercial mixture of low molecular weight polyethyelene glycol oligomers, as well as di-, tetra-, and hexaethylene glycol are presented to study the effect of added water impurities up to a weight fraction of 0.020, which covers the typical range of water impurities due to water absorption from the atmosphere. Each system was simulated a total of four times using different combinations of two force fields for the water (SPC/E and TIP4P/2005) and two force fields for the PEG and oligomer (OPLS-AA and modified OPLS-AA). The observed trends in the effects of water addition were qualitatively quite robust with respect to these force field combinations and showed that the water does not aggregate but forms hydrogen bonds at most between two water molecules. In general, the added water causes overall either no or very small and nuanced effects in the simulation results. Specifically, the obtained water RDFs are mostly identical regardless of the water content. The added water reduces oligomer hydrogen bonding interactions overall as it competes and forms hydrogen bonds with the oligomers. The loss of intramolecular oligomer hydrogen bonding is in part compensated by oligomers switching from inter- to intramolecular hydrogen bonding. The interplay of the competing hydrogen bonding interactions leads to the presence of shallow extrema with respect to the water weight fraction dependencies for densities, viscosities, and self-diffusion coefficients, in contrast to experimental measurements, which show monotonous dependencies. However, these trends are very small in magnitude and thus confirm the experimentally observed insensitivity of these physical properties to the presence of water impurities.

On the Behavior of the Ethylene Glycol Components of Polydisperse Polyethylene Glycol PEG200

Molecular dynamics (MD) simulations are reported for [polyethylene glycol (PEG)200], a polydisperse mixture of ethylene glycol oligomers with an average molar weight of 200 g·mol^-1. As a first step, available force fields for describing ethylene glycol oligomers were tested on how accurately they reproduced experimental properties. They were found to all fall short on either reproducing density, a static property, or the self-diffusion coefficient, a dynamic property. Discrepancies with the experimental data increased with the increasing size of the tested ethylene glycol oligomer. From the available force fields, the optimized potential for liquid simulation (OPLS) force field was used to further investigate which adjustments to the force field would improve the agreement of simulated physical properties with experimental ones. Two parameters were identified and adjusted, the (HO)-C-C-O proper dihedral potential and the polarity of the hydroxy group. The parameter adjustments depended on the size of the ethylene glycol oligomer. Next, PEG200 was simulated with the OPLS force field with and without modifications to inspect their effects on the simulation results. The modifications to the OPLS force field significantly decreased hydrogen bonding overall and increased the propensity of intramolecular hydrogen bond formation at the cost of intermolecular hydrogen bond formation. Moreover, some of the tri- and more so tetraethylene glycol formed intramolecular hydrogen bonds between the hydroxy end groups while still maintaining strong intramolecular interactions with the ether oxygen atoms. These observations allowed the interpretation of the obtained RDFs as well as structural properties such as the average end-to-end distances and the average radii of gyration. The MD simulations with and without the modifications showed no evidence of preferential association of like-oligomers to form clusters nor any evidence of long-range ordering such as a side-by-side stacking of ethylene glycol oligomers. Instead, the simulation results support the picture of PEG200 being a random mixture of its ethylene glycol oligomer components. Finally, additional MD simulations of a binary mixture of tri-and hexaethylene glycol with the same average molar weight as PEG200 revealed very similar structural and physical properties as for PEG200.

Foam Silica Films Synthesized by Calcium Chloride-Assisted Emulsification

The development of porous films with an accessible high specific surface area is important for designing new adsorbents, sensors, or catalyst supports. Here, we describe a simple method to prepare a silica foam coating using a calcium chloride-assisted evaporation-induced emulsification method. An alcoholic silica sol containing calcium chloride and a poly(ethylene oxide)-based polymer is deposited on a substrate by dipping. The evaporation of the alcohol induces a phase separation between the silica-rich phase and the calcium-rich one. The size of the droplets increases via a coalescence process until the gelation of the sol, which determines the final pore size between 100 nm and 3 μm. Thermal analysis and monitoring of droplet evaporation confirm that the departure of the solvent is delayed by the presence of calcium chloride in the sol. The influence of the nature of the polymer on the porosity is discussed. The use of a block copolymer such as the Pluronic F-127, which strongly stabilizes the emulsion, allows to reach a low pore size (400 nm), while on the contrary, we propose to use a short poly(ethylene glycol) (PEG) such as PEG-400, which weakly stabilizes it, leading to larger pores (2-3 μm). Furthermore, we show that the addition of a zirconium salt (ZrOCl₂·8H₂O) to the silica sol accelerates the condensation step of the silica and leads to the decrease in the pore size.

Microwave Assisted Surfactant-Thermal Synthesis of Metal-Organic Framework Materials

This research explores the application of surfactants as green solvents for the rapid microwave synthesis of metal-organic framework (MOF) materials. Lead, copper, and iron metal-organic framework materials were synthesized using surfactant-thermal microwave synthesis. The MOF materials were characterized by powder X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis. With the exception of the iron MOF, which was expected to be amorphous, the MOF materials are crystalline, though generally exhibiting smaller crystals than those achieved with traditional solvothermal synthesis. This green synthetic method reduced synthesis time and reduced the use of traditional organic solvents as the reaction media.