Hydrophobic and Electrostatic Driving Forces for Ion Adsorption to Polymers and Extended Interfaces

The Macromolecular, Supramolecular, and Nanochemistry Program in the Chemistry Division supports Professor Paul S. Cremer and his students at the Pennsylvania State University to explore the ways that salts influence oily polymer molecules dissolved in water. Oil and water do not readily mix. In salad dressing, for example, an oily layer is typically found floating on top of a water-based layer. Moreover, dissolved salts reside primarily within the water phase rather than in the oil phase. Specifically, this study helps to determine when negatively or positively charged ions interact with polymer molecules or separate from them. The attraction of the polymer for salts in water dictates whether the polymers can remain dissolved in solution or deposit as a solid from the solution. The solubility of polymers in salt water is important in a surprisingly wide variety of areas ranging from the biological and food sciences to atmospheric chemistry, the self-assembly of novel materials, and even the cleaning of laundry. This research not sheds light on problems of direct relevance to society, but also trains students in interdisciplinary fields involving chemistry, biology, materials science, and engineering. Professor Cremer and his group are working with high school chemistry teachers to create inexpensive and easily accessible experiments for 10th and 11th graders. The instructors will develop teaching modules on principles of polymer solubility and the central role of salt ions in everyday life.

The Cremer group is exploring how weakly hydrated anions, strongly hydrated cations, as well as hydronium and hydroxide influence the behavior of thermoresponsive polymers in aqueous solutions. Their experimental approach involves correlating ion binding and hydrophobic collapse behavior with spectroscopic signatures of the hydration shell. This information can be obtained by using techniques like Raman multivariate curve resolution spectroscopy and vibrational sum frequency spectroscopy. These thermodynamic and spectroscopic experiments are performed in collaboration with theoreticians performing molecular dynamic simulations. Measurements are made in bulk solution and with surfactant monolayers at the air/water interface. For example, results are being explored as a function of the size of the macromolecules as well as their net charges and the presence of specific negatively and positively charged functional groups. Special attention is focused on whether ions in solution can interact with polymer molecules via contact ion pairing or through solvent shared ion pairing. Moreover, the context in which charged moieties are presented is explored. These studies provide information as to how the number density of charged moieties as well as the configuration of surrounding hydrophobic sites influence ion-polymer interactions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.