60th Annual Report on Research 2015

This project has developed spectroscopic probes of aqueous polymer solutions that exemplify the non-Newtonian fluids used in many industrial applications, most notably hydraulic fracturing drilling fluids. Inspired by our group’s work to understand biomolecular hydration dynamics, particularly under crowding conditions, we sought to identify the composition and concentration dependence of hydration dynamics in a wide range of aqueous polymer solutions. The basic approach combined ultrafast two-dimensional infrared spectroscopy with a water-soluble transition metal carbonyl probe which serves as a dynamical sensor. We used a carbon monoxide releasing molecule (CORM), referred to in the literature as “CORM-2” as a hydration dynamics sensor. CORM-2 is a ruthenium carbonyl complex which transforms in water into a stable species that absorbs very strongly in the infrared. Surprisingly, this chemical product is not fully characterized in the literature, and we are using numerous analytical methods to determine the precise identity and structure of the CORM-2 product. Nevertheless, our extensive experience using 2D-IR spectroscopy to study transition metal carbonyl molecules in solution indicates that the measured dynamics are rarely if ever dependent on the microscopic nature of the probe’s structure. Rather, regardless of the metal carbonyl complex, we obtain essentially the same dynamical time scales.

Using the CORM-2 product, we performed an extensive survey of dynamics in aqueous polymer solutions. We first attempted to study guar, which is the key additive in many hydraulic fracturing fluid formulations. Unfortunately the solutions were highly scattering, and precluded careful investigation with 2D-IR spectroscopy. Seeking a highly controllable and well-defined polymer system, we turned to polyethylene glycol of various molecular weights (up to 1 million Dalton). With this series of five different molecular weight PEGs, we systematically varied the concentration and used 2D-IR spectroscopy of the CORM-2 product to determine the hydration dynamics. In total, we obtained 55 dynamical time constants, and found a remarkable observation: in all but the shortest PEG (PEG400), we observed only bulk-like water dynamics. That is, we found no concentration dependent dynamics in any of the longer PEGs. This finding suggests that the water in aqueous polymer solutions is bulk-like and is not subject to the interfacially mediated slowdown found at extended surfaces such as those of proteins and membranes. Indeed, there are broad implications for a microscopic understanding of non-Newtonian fluids, in some sense confirming the picture dominated by the sliding of extended chains under, for example, shearing conditions. We expected the entangled polymers to create small pools or pockets of water that would be constrained on multiple interfaces and exhibit slowdowns similar to what we found in protein solutions. Our data clearly refute this idea entirely. We suspect the short PEG, for which we did find a marked concentration dependence, is able to wrap around the probe complex. We will support this picture with simulations studies in the near future. It is highly intriguing to consider that even saturated aqueous polymer solutions contain essentially bulk-like water, with no dynamical influence of the macromolecule on the orientational dynamics of the solvent. We expect to investigate this phenomenon in considerably more detail in the future, thanks to the support of this grant.