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Long aliphatic hydrocarbon chains aggregate in aqueous solution due to the hydrophobic effect, forming structures such as micelles and membranes, while amino groups titrate at basic pH. These two biologically important behaviors are linked in alkylamines, in which the pK_a of the amino group is shifted downward by aggregation. In this paper we study the thermodynamics of these coupled processes, following aggregation by observing alkylamine pH titration behavior. The magnitude of the shift depended on the aliphatic chain length and on the concentration of alkylamine: longer chains and higher concentrations lowered the pK_a to a greater extent. Gibbs free energies of protonation and aggregation were calculated from the pK_a shifts. Enthalpies, entropies, and heat capacities were estimated by van't Hoff analysis from the pK_a shift dependencies on temperature. However, the results were less precise than the …