Transition State for Aggregation and Reorganization of Normal Fatty Alcohols at the Air/Water Interface.

The transition state between the initial and final clustering products is analyzed with special reference to the two-dimensional cluster formation and reclustering in the monolayers of normal fatty alcohols (n = 8-16). The calculations were performed using the PM3 molecular orbital approximation implemented in the Mopac 2000 software. It is shown that taking into account the enthalpy factor only does not lead to any saddle point at the potential energy surface. This point is reproduced if the entropy factor is assumed. A linear dependence of enthalpy, entropy, and Gibbs free energy on the number of the carbon atoms n is shown to exist for any positions of the system at the reaction coordinate, including the positions of the transition state. This fact was used to construct an additive scheme for the description of transition states in clustering and reclustering reactions in fatty alcohol monolayers for any arbitrary size of the initial cluster. It is shown that in the kinetic formalism the formation of linear clusters is more probable as compared with the formation of star clusters, whereas the attachment of any linear cluster to a star cluster is preferential both from the kinetic and thermodynamic considerations. The dependence of the parameters of the activated complex, corresponding to the dimerization reaction of n-alcohols, on alkyl chain length and temperature is analyzed. The results of the theoretical calculations are in qualitative agreement with experimental data obtained for tetradecanol monolayers. [ABSTRACT FROM AUTHOR]