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  • Title: Quantum-chemical description of the thermodynamic characteristics of clusterization of melamine-type amphiphiles at the air/water interface.
    Author: Vysotsky YB, Shved AA, Belyaeva EA, Aksenenko EV, Fainerman VB, Vollhardt D, Miller R.
    Journal: J Phys Chem B; 2009 Oct 08; 113(40):13235-48. PubMed ID: 19761199.
    Abstract:
    The semiempiric PM3 method is used to calculate the thermodynamic parameters of the formation of monomers, dimers, trimers, and tetramers of the amphiphilic melamine-type series of 2,4-di(n-alkylamino)-6-amino-1,3,5-triazine (2C(n)H(2n+1)-melamine) with n = 9-16. The most stable conformations are determined, which then are used to construct the clusters. The peculiar feature of these structures is the existence of a bend at one of the alkyl chains. Thus, the formation of infinite films becomes possible because of their spatial arrangement. From the calculation of the relative amount of various conformers in the mixture, it follows that, if the alkyl chain length is lower than 11-12 carbon atoms, the mixture is composed mainly of the monomers that do not contain any intramolecular interactions, whereas for higher alkyl chain lengths the monomers that involve such interactions prevail in the mixture. For all clusters thus considered, the thermodynamic parameters (enthalpies, entropies, and Gibbs' energies) of clusterization are calculated. It is shown that the dependencies of these parameters on the alkyl chain length either exhibit stepwise shape or are represented by the combination of a linear and stepwise function. This depends on the different number of hydrogen-hydrogen interactions in the structures considered. Five types of clusters that are capable of the formation of infinite 2D films are considered in detail. For each of these types, the dependencies of the clusterization enthalpy, entropy, and Gibbs' energy on the alkyl chain length in the constituting monomers are derived. Using these dependencies, it becomes possible to calculate these thermodynamic characteristics for clusters of any size, and also for infinite 2D films. It is shown that the spontaneous clusterization of 2C(n)H(2n+1)-melamine becomes possible if the alkyl chain length exceeds 9 carbon atoms.
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