Chain length effect on the structure of alkyltrimethylammonium chloride monolayers between two immiscible liquids
The main purpose of this article is to understand at the microscopic level the possible effect of the chain length on the properties of an adsorbed monolayer between two immiscible liquids. We report a molecular dynamics simulation study of an alkyltrimethylammonium chloride monolayer adsorbed in the d/li interface. The surfactants studied have 8, 12 and 16 carbon atoms in the chain and are studied at a constant area of 45 Å2 per surfactant. It was found that the chain widths of the number density profile distributions increase with the chain length and the tilt angle distribution does not change with the chain length. There is also no correlation between the percentage of trans conformations in an alkyltrimethylammonium chain and the length of the chain. The probability of finding a given number of trans conformations per chain is also similar in the three different surfactants. An increase of four methylene groups in the central part of the chain just displaces this distribution by about two trans conformations. The mean-square displacement of the terminal methyl group of the alkyltrimethylammonium chains is also very similar in the different surfactant types. Our results support the conclusions of Bell et al. [(1998) J. Phys. Chem. B 102: 218] concerning the structures of the alkyltrimethylammonium monolayers (they are very similar and independent of the chain length).
D.J.V.A. dos Santos*, José A.N.F. Gomes
CEQUP/Departamento de Quıímica da Faculdade de Ciências da Universidade do Porto,
R. Campo, Alegre 687, 4069-007 Porto, Portugal
http://dx.doi.org/10.1007/b93977