Thermodynamics of the hydrophobic effect. III. Condensation and aggregation of alkanes, alcohols, and alkylamines

Knowledge of the energetics of the low solubility of non-polar compounds in water is critical for the understanding of such phenomena as protein foldi ng and biomembrane formation. Solubility in water can be considered as one leg of the three-part thermodynamic cycle - vaporization from the pure liqu id, hydration of the vapor in aqueous solution, and aggregation of the subs tance back into initial pure form as an immiscible phase. Previous studies on the model compounds n-alkanes, 1-alcohols, and 1-aminoalkanes have noted that the thermodynamic parameters (Gibbs free energy, DeltaG; enthalpy, De ltaH; entropy, DeltaS; and heat capacity, DeltaC(P)) associated with these three processes are generally linear functions of the number of carbons in the alkyl chains. Here we assess the accuracy and limitations of the assump tion of additivity of CH2 group contributions to the thermodynamic paramete rs for vaporization, hydration, and aggregation. Processes of condensation from pure gas to liquid and aqueous solution to aggregate are compared. Hyd roxy, amino, and methyl headgroup contributions are estimated, liquid and s olid aggregates are distinguished. Most data in the literature were obtaine d for compounds with short aliphatic hydrocarbon tails. Here we emphasize l ong aliphatic chain behavior and include our recent experimental data on lo ng chain alkylamine aggregation in aqueous solution obtained by titration c alorimetry and van't Hoff analysis. Contrary to what is observed for short compounds, long aliphatic compound aggregation has a large exothermic entha lpy and negative entropy. (C) 2001 Elsevier Science B.V. All rights reserve d.