Computer Studies on the Effects of Long Chain Alcohols on Sodium Dodecyl Sulfate (SDS) Molecules in SDS/Dodecanol and SDS/Hexadecanol Monolayers at the Air/Water Interface He ´ ctor Domı ´nguez* Instituto de InVestigaciones en Materiales, UniVersidad Nacional Auto ´ noma de Me ´ xico (UNAM), Me ´ xico, D.F. 04510 ReceiVed: March 31, 2006; In Final Form: May 5, 2006 Molecular dynamics simulations of sodium dodecyl sulfate (SDS)/dodecanol and SDS/hexadecanol monolayers at the air/water interface were investigated where the monolayer mixtures were prepared by two different configurations. In the first configuration, all of the dodecanol (or hexadecanol) molecules were placed together and also the SDS molecules were placed together in the surface area. In the second configuration, the dodecanol (or hexadecanol) molecules were uniformly distributed with the SDS molecules, forming a homogeneous mixture. The results showed that the alcohol tails are more ordered and thicker than the SDS tails in monolayers where the alcohol molecules are close to each other and separated from the SDS. However, the reverse trend is observed in monolayers where the SDS and alcohol molecules are well mixed; that is, the alcohol tails seem to have less order. Studies of how the SDS tails are affected by the presence of long chain alcohols are also discussed. Basically, by increasing the alcohol chain length, the order and the thickness of the SDS tails increased when those molecules were placed all together in a region of the surface area. When both surfactants were well mixed, the order and thickness of the SDS chains decreased as the alcohol chain length increased. Comparisons of the present results with actual experiments of similar systems were performed, and they showed similar tendencies. 1. Introduction The behavior of surfactant molecules at different interfaces has been extensively studied for the last years not only for the scientific interest but also for the applicability in the industry. Therefore, several experimental techniques have been conducted to study systems of one kind of a surfactant molecule such as fluorescence, resonance Raman scattering, neutron reflection, second harmonic generation, vibrational sum-frequency spec- troscopy, Brewster angle microscopy, atomic force microscopy, and time-resolved quasi-elastic laser. 1-6 However, most of the interesting problems, with applications in commercial products, consist of a mixture of surfactant molecules which have richer properties than individual ones. For instance, nonionic surfac- tants are generally used together with anionic surfactants as active ingredients in products such as shampoo, hand dish washing liquids, and washing powders. Therefore, it is not a surprise to find also several studies in the literature on surfactant mixtures using several experimental techniques such as calo- rimetry, X-ray, neutron scattering, and surface tension measure- ments among others. 7-16 Of particular interest is the study of the structure and composition of the surfactant mixtures such as the extension of the chains or the thickness of the monolayer mixture at interfaces. 14-16 For instance, using neutron reflection experi- ments, 16 some authors have performed studies to investigate differences in the position of the components of the sodium dodecyl sulfate/dodecanol mixture at the air/water interface. Other experiments of anionic/nonionic mixtures have described an increasing change in position of the nonionic molecules at the interface relative to the solvent with increasing solubility of the nonionic molecules. 15 Moreover, surface tension experi- ments have shown that mixtures with similar hydrophobic tail lengths but different headgroups may show different properties which can affect the structure of the monolayer at the interface. Those results suggest that the behavior of different surfactants at interfaces might be attributed to the different polar groups of the molecules. However, the role of the chain length of the molecules in surfactant mixtures is discussed only briefly by some authors. For instance, in the early experimental work of Patist et al., 17 they present studies of sodium dodecyl sulfate with long chain alcohols, showing how the compatibility between the chain length plays an important role in the stabilization of sodium dodecyl sulfate (SDS) micelles. On the other hand, due to the substantial increase in the computational power, computer simulations became an impor- tant tool for the study of such complex interfacial systems. 18-26 Using this computational methodology, it is possible to extract more information about dynamical and structural properties from a molecular level which sometimes are not easy to obtain from real experiments. Therefore, in the present work, we performed a series of computational experiments to investigate the effects of the length of the tails of different surfactants on the structure of monolayer mixtures. In particular, we focused on monolayer mixtures of anionic/nonionic surfactants; moreover, we investigated how two different nonionic molecules which have the same headgroups but different tail lengths affect the behavior of the anionic surfactant or in general the role of the chains of the nonpolar surfactants in the position of the surfactant molecules at the interface. Particularly, the SDS/dodecanol mixture was chosen, since dodecanol seems to be the most common contaminant of SDS caused by the synthesis which makes this system interesting * E-mail: hectordc@servidor.unam.mx. 13151 J. Phys. Chem. B 2006, 110, 13151-13157 10.1021/jp062018n CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006