Quantification of molecular aggregation equilibria using spectroscopic measurements and random mixing modeling
Abstract
Molecular aggregation equilibria, such as the binding of ligands to a central solute molecule, are prevalent throughout biological processes and energy storage devices. However, both the sign and magnitude of hydrophobic and ionic interactions remains a subject of theoretical debate, and has yet to be experimentally determined. Here, Raman vibrational spectroscopy is combined with multivariate curve resolution (Raman-MCR) to experimentally quantify both the number of hydrophobic contacts between alcohol molecules in water and the affinity of ions for molecular hydrophobic interfaces. Furthermore, a generalized theoretical model is developed based on random statistics in which it is assumed that the concentration of each chemical species is everywhere identical to its bulk concentration. Solute-ligand (direct) and ligand-ligand (cooperative) interactions are incorporated into the RM model and validated against an exact finite lattice (FL) model. Comparison of the Raman-MCR experimental results and random mixing predictions imply that there are no more hydrophobic contacts in aqueous solutions of alcohols ranging from methanol to tertiary butyl alcohol than in random mixtures of the same concentration. This suggests that the interaction between small hydrophobic groups in water is weaker than thermal energy fluctuations. Thus, the corresponding water-mediated hydrophobic interaction must be repulsive, with a magnitude sufficient to negate the attractive direct van der Waals interaction between the hydrophobic groups. Additional Raman-MCR experimental results imply that the interaction between aqueous sodium or fluoride ions and molecular hydrophobic groups is repulsive. In contrast, the sign and magnitude of the interaction energy between iodide ions and molecular hydrophobic groups depends on the methyl group partial charge. However, the interaction energies do not significantly compete with thermal energy fluctuations.
Degree
Ph.D.
Advisors
Ben-Amotz, Purdue University.
Subject Area
Chemistry|Physical chemistry
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