Amphiphilic Water Restructuring and Cooperativity

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In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Dor Ben-Amotz and his research group at Purdue University are investigating the interactions between water and molecules that are both water-loving and oil-loving. Such molecules play a vital role in the self-assembly and orchestration of biological processes, as well as in environmental and geological chemistry. This research addresses open questions concerning the restructuring of water by small molecules such as alcohols and its role in the self-assembly of more complex structures such biological membranes. These questions are quantitatively addressed using a combination of experimental and theoretical methods. The results are used to quantify the water-mediated driving forces that lead to the self-assembly of structures such as those formed in biological and geochemical systems. Raman multivariate curve resolution is used to obtain hydration-shell vibrational spectra of water molecules that are perturbed by amphiphilic solutes. High precision density and sound velocity experiments are used to measure the partial molar volumes and compressibilities of such solutes. Additional molecular/mechanistic information that cannot be unambiguously determined from experiments alone is obtained with the aid of classical and quantum calculations. Water structure changes induced by the formation (self-assembly) of binary and higher-order aggregates are quantified using the concentration dependence of hydration-shell spectra, partial molar volumes, and compressibilities. Solute size dependent water restructuring, and related crossover phenomena, are investigated using temperature and pressure dependent studies extending beyond the normal freezing and boiling points of water. Theoretical calculations are used to elucidate the connection between water structure changes and experimentally measured hydration-shell spectra, partial molar volumes and compressibilities. The results are used to quantify solute-solute, solute-water, and water-water contributions to aggregation potentials of mean force, as well as the associated energy and entropy changes. Broader impacts of this work include the development of experimental methods and theoretical modeling strategies for quantifying molecular interactions in the liquid state, as well as the training of graduate and undergraduate students, and the integration of writing into undergraduate STEM courses.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.