61st Annual Report on Research 2016

The principal goal of this project is to understand the dynamics of hydration water aqueous solutions of polymers. Hydraulic fracturing has enabled a paradigm shift in our nation's petroleum reserves, enabling extraction of oil and gas from shale as well as decreasing costs, which provides critical insulation from the manipulation of oil prices by gulf nations. Although clearly effective, commercial formulations of hydraulic fracturing fluids are based on essentially empirical considerations, and very little work has been done to study these complex, non-Newtonian fluids using modern spectroscopic methods. The approach our group has taken during this project has been to use water soluble transition metal carbonyl complexes as dynamical probes of polymer solutions in order to characterize ultrafast water dynamics. Over the period of the project we have needed to switch from studying the natural guar polysaccharide polymer used in industrial hydraulic fracturing due to our inability to prepare samples of sufficient optical quality to perform coherent non-linear spectroscopy. The natural products contain too many particles that scatter light and ruin two-dimensional infrared (2D-IR) spectra. Nevertheless, as is well known from polymer physics, many properties of polymers are transferable, and only depend weakly on the precise chemical nature of the polymer itself. While that remains to be seen in our case, we opted to characterize the hydration dynamics of a model polymer (polyetheylene glycol/oxide) in aqueous solutions. PEG (PEO for large molecular weights) are convenient because they are soluble at a wide range of concentrations and commercial sources of numerous molecular weights are readily available. We also studied the water/surfactant interface in reverse micelles, which are key components of oil extraction. In the case of micelles, we discovered that the thiocyanate anion (SCN-) is a surface-specific probe of hydration and head-group dynamics. This report summarizes our findings for the PEG solutions, due to the length restrictions. In previous work, we studied the influence of macromolecular crowding on interfacial hydration dynamics using a site-specifically surface labeled protein (hen lysozyme). In experiments that accessed dynamics of water reported by the metal carbonyl vibrational probe, we found that below a specific crowding concentration, the water was essentially uninfluenced by the presence of the crowder, despite a general increase in macroscopic viscosity. Above a particular concentration (depending on the nature of the crowder), the hydration dynamics appear to slow, but reach a plateau and do not slow further with additional crowder. We predicted we would see similar effects in polymer solutions due to the influence of multiple interfaces presented by the polymer, causing a collective slowdown of water's reorientational motion. In an extensive study of several molecular weights (MW) of PEG or PEO, we did not see significant dynamical slowdown except for the low-MW PEG-400. Even in 55% (wt/wt) PEG-20000 solution, which has a very high viscosity, we find bulk-like water dynamics. There two explanations for the lack of a slowdown. One possibility is that there truly is no slowdown due to the unusual topology of the PEG molecule, which is described below. The other possibility is that there would be a slowdown if it were possible to dissolve PEG at higher concentrations, but that we cannot achieve the necessary concentration to observe the slowdown. The solubility limitation hypothesis is supported by the clear observation of a slowdown in PEG-400, where we can study up to 90% PEG, and observed a pronounced slowdown above ~50% PEG by volume. We did even observe the slight appearance of a high-PEG concentration plateau. Most interesting is that we find an abrupt dependence of water dynamics time scale (determined using 2D-IR spectroscopy) at the so-called critical overlap concentration, which is the polymer concentration where different polymer molecules begin to contact each other. This abrupt change is revealed when considering the experimental dynamics timescale as a function of the number of water molecules per ethylene oxide monomer. There is a possibility, however, that for the longer PEGs there is no actual dynamical slowdown even if we could dissolve the polymer at arbitrary concentrations. PEG is well-known to alter the hydration of other species when in heterogeneous solution, and is used to assist protein crystallization. Although PEG is largely soluble in water, polymethylene and polypropylene oxide/glycol are completely insoluble. Two thermodynamic parameters explain why PEG is so unusual. The enthalpy of hydration is negative, indicating more net hydrogen bonds in solution than in the separated species. Since PEG has almost no hydrogen bonding alone, water must form numerous hydrogen bonds with PEG. The entropy of hydration is also negative, which means that the water must be more constrained in solution. Indeed, water adopts clathrate hydrate like structure in the vicinity of the ethylene units. From these thermodynamic considerations, it is clear that PEG essentially 'fits' into the hydrogen bonding network of water. The O-O spacing is also consistent with O-O distances in the radial distribution function of water. Putting all of these ideas together we can conclude that the water directly hydrating PEG is highly hydrogen bonded, but quite constrained. Presumably the dynamics, such as translational diffusion, becomes altered as well. However, since the water is essentially able to maintain a bulk-compatible structure, the second hydration shell sees a bulk-templated first hydration shell. Since perturbations to water dynamics at interfaces is largely determined by the increased entropic barrier to hydrogen bond rearrangements, the templated first hydration shell does not deplete the availability of appropriately positioned partners. When the polymers start to overlap, however, the templating is disrupted, and we see a pronounced slowdown due to the collective hydration leading to longer-range hydrogen bonding defects. As far as we know this is the first detailed experimental study of hydrated polymer dynamics that is able to connect to the prevailing understanding of water dynamics. The implications are numerous for polymer solution rheology, and that was the main goal of this project.