M. Costas et al., GENERAL THERMODYNAMIC ANALYSIS OF THE DISSOLUTION OF NONPOLAR MOLECULES INTO WATER - ORIGIN OF HYDROPHOBICITY, Journal of the Chemical Society. Faraday transactions, 90(11), 1994, pp. 1513-1522
Citations number
45
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
The Gibbs energy, enthalpy, entropy and heat capacity of transfer from
the pure non-polar liquid into water are analysed in detail. It is fo
und that if the combinatorial contribution to the Gibbs energy and ent
ropy of transfer is subtracted from the experimental values, all non-p
olar solutes in water behave in a universal manner, i.e. all of their
thermodynamic transfer functions can be studied with their molecular s
urface area as the only parameter. This is illustrated with the alkylb
enzene series, for which experimental Gibbs energies of transfer in a
wide temperature range have been obtained recently. A new interpretati
on scheme for the thermodynamic transfer functions is presented and co
ntrasted with that due to Privalov and Gill. It is considered that wat
er molecules around the solute undergo a relaxation process which lowe
rs the Gibbs energy, enthalpy and entropy of the system and is respons
ible for the large heat capacity of transfer. This relaxation process
is described here using a two-state model for water molecules obtained
from first principles. The negative relaxation contribution to the Gi
bbs energy promotes solubility, but is overcome by a large positive co
ntribution arising from the creation of a cavity in water and the larg
e differences between solute-solute, water-water and solute-water inte
ractions. The origin of hydrophobicity lies then in the high cohesive
energy of water. The proposed interpretation scheme is used to (a) pre
dict the solubility of alkanes in water, (b) understand the origin of
the solubility minimum appearing in aqueous solutions of non-polar sol
utes, (c) rationalize the experimental finding that the enthalpy of tr
ansfer becomes zero in a narrow temperature range for many non-polar s
olutes, (d) discuss the significance of entropy of transfer vs. heat c
apacity of transfer plots often used to understand the nature of the h
ydrophobicity of non-polar solutes and proteins, and (e) account for t
he expected change in sign (with temperature) of the water proton NMR
chemical shifts discussed earlier in the literature.