Thermodynamics of the hydrophobic effect. II. Calorimetric measurement of enthalpy, entropy, and heat capacity of aggregation of alkylamines and longaliphatic chains

The thermodynamics of long aliphatic chain alkylamine aggregation in aqueou s solution was studied by isothermal titration calorimetry (ITC). Protonate d alkylammonium cations with linear aliphatic chains of 10-14 carbon atoms were fully soluble in aqueous solution at the beginning of titration, but p ractically insoluble after deprotonation by titrating with sodium hydroxide . The alkylamines aggregated and precipitated during the reaction, enabling direct measurement of the enthalpy of aggregation. The enthalpy of aggrega tion became increasingly exothermic upon increasing the chain length. Hydro phobic aggregation was enthalpy-driven and entropy-opposed for alkylamines with 12-14 carbon atoms at room temperature. Direct observation of hydropho bic aggregation by ITC at constant temperature and pressure provided more a ccurate thermodynamic parameters than obtainable from van't Hoff analysis. Aggregation into liquid or solid phases could be distinguished by ITC, but not by van't Hoff analysis of alkylamine solubility data. (C) 2001 Elsevier Science B.V. All rights reserved.