Quantum chemical semiempirical approach to the structural and thermodynamic characteristics of fluoroalkanols at the air/water interface

Yu B. Vysotsky, V. S. Bryantsev, F. L. Boldyreva, V. B. Fainerman, D. Vollhardt

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Abstract

In the framework of quantum chemical PM3 approximation, the geometrical structure and thermodynamic functions characteristics of the formation of monomers (n = 1-14, 34), dimers (n = 1 -14, 34), and trimers and tetramers (n = 1-8) of fluoroalkanols with the composition CnF2n+1CH 2CH2OH are calculated. It is shown that, in contrast to the fatty alcohols, which have a flat zigzag structure, the fluoroalkanol monomers are helical with an average backbone torsion angle equal to 162°. For the minimum-energy structure of dimers, the self-organization of the molecules in a dimer was observed; that leads to an opposite alternation of the torsion angles corresponding to the matching atoms in the two molecules that form the dimer. This results in the fact that the most stable conformation of the dimer is the double helix. The lead (39.5 Å) and diameter (7.3 A) of the double helix are determined from the calculations of C34F 69CH2CH2OH dimers. Enthalpy, entropy, and Gibbs energy of the clusterization are shown to be linearly dependent on the length of the fluorinated chain. From the analysis of these thermodynamic quantities, it is concluded that dimerization of fluoroalkanols at the air/water interface takes place if the hydrocarbon link number exceeds 6, whereas for ordinary alcohols this characteristic number is 11. These calculated values agree with experimental data. The additive scheme for the evaluation of the clusterization free energies for arbitrary clusters is developed and applied to obtain the estimate of the Gibbs clusterization energy for infinitely large clusters.

Original languageEnglish
Pages (from-to)454-462
Number of pages9
JournalJournal of Physical Chemistry B
Volume109
Issue number1
DOIs
StatePublished - Jan 13 2005
Externally publishedYes

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