Thermodynamics of the hydrophobic effect. 1. Coupling of aggregation and pK(a) shifts in solutions of aliphatic amines

Long aliphatic hydrocarbon chains aggregate in aqueous solution due to the hydrophobic effect, forming structures such as micelles and membranes, whil e amino groups titrate at basic pH. These two biologically important behavi ors are linked in alkylamines, in which the pK(a) of the amino group is shi fted downward by aggregation. In this paper we study the thermodynamics of these coupled processes, following aggregation by observing alkylamine pH t itration behavior. The magnitude of the shift depended on the aliphatic cha in length and on the concentration of alkylamine: longer chains and higher concentrations lowered the pK(a) to a greater extent. Gibbs free energies o f protonation and aggregation were calculated from the pK(a) shifts. Enthal pies, 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 calorimetrically measured values, as described in th e following article. A model to calculate titration curves, pK(a) shifts, a nd aggregation of uncharged alkylamines as a function of aliphatic chain le ngth, concentration, and temperature is presented. (C) 2001 Elsevier Scienc e B.V. All rights reserved.