Aggregation and re-organization of normal fatty alcohols at the air/water interface: PM3 molecular orbital approximation

The transition state of dimerization at the interface and between the initial and final clusterization products is analyzed. The calculations were performed using the PM3 molecular orbital approximation, implemented in the MOPAC 2000 software. It is shown that the account of the Gibbs energy ΔG leads to a saddle point at the potential energy surface for the two-dimensional cluster formation and re-clusterization in monolayers of normal fatty alcohols (n=8, 12, and 16). The re-organization of a linear trimer and monomer into two dimers was considered. The maximum value ΔG=110 kJ/mol for n-tetradecanol is observed for a distance of 10.5 Å which corresponds to the transition state of re-clusterization reaction. If the distance between the n-tetradecanol trimer and monomer is lower than 10.5 Å, then tetrameric clusters are formed again. For higher distances the monomer is detached from the trimer. The dependencies of the parameters of the activated complex corresponding to the alcohol dimerization reaction on the chain length are analyzed. The position of the maximum on the Gibbs energy curve for dimerization at 298 K corresponds to an intermolecular distances for the dimers in the transition state of 6.31-6.69 Å.