Introduction Molecular monolayers and thin films have been exten- sively studied and play an important economic and scientific role. They are used extensively, for example, in the mining and oil industry for extraction and separa- tion, or in the household industry for fabric softeners [1]. Clear evidence of the extensive experimental work in this field of chemistry is the appearance in the literature of several reviews [1, 2, 3, 4, 5, 6]. The knowledge of the relationship between the surfactant molecular structure and the macromolecular interfacial structure is important to understand and predict the performance of the surfactant. Yet, only recently the molecular behaviour of the monolayers could be obtained thanks to the advent of new experi- mental techniques, like vibrational sum-frequency-gen- eration spectroscopy [7, 8], neutron reflection [9, 10, 11, 12, 13, 14], or scanning tunnelling microscopy [15, 16], that can provide information on interfacial phenomena at a molecular level. With this new information, the data obtained by computer simulation can now be more thoroughly compared and validated. Extensive theoretical [17, 18, 19] and experimental work (see the reviews already cited and the references therein) has been performed for Langmuir and Lang- muir–Blodgett interfaces but only recently a small number of simulation studies have been performed on Progr Colloid Polym Sci (2004) 126: 68–73 DOI 10.1007/b93977 Ó Springer-Verlag 2004 Daniel J.V.A. dos Santos Jose´ A.N.F. Gomes Chain length effect on the structure of alkyltrimethylammonium chloride monolayers between two immiscible liquids D.J.V.A. dos Santos (&) Jose´ A.N.F. Gomes CEQUP/Departamento de Quı´mica da Faculdade de Cieˆncias da Universidade do Porto, R. Campo, Alegre 687, 4069-007 Porto, Portugal e-mail: dasantos@fc.up.pt Abstract 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 alkyltrimethylammoni- um chloride monolayer adsorbed in the water/isooctane interface. The surfactants studied have 8, 12 and 16 carbon atoms in the chain and are studied at a constant area of 45 A ˚ 2 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 proba- bility of finding a given number of trans conformations per chain is also similar in the three different surfac- tants. 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] concern- ing the structures of the alkyltrime- thylammonium monolayers (they are very similar and independent of the chain length). Keywords Self-assembly Æ Monolayers Æ Molecular dynamics