Simulating the effect of surfactant structure on bending moduli of monolayers

~Received 30 October 2003; accepted 10 December 2003!We have used dissipative particle dynamics to simulate amphiphilic monolayers on the interfacebetween oil and water. An ultralow interfacial tension is imposed by means of Monte Carlo toresemble the amphiphilic films that separate oil and water regions in microemulsions. We calculatethe bending modulus by analyzing the undulation spectrum. By varying the surfactant chain lengthand topology we investigate the effect of surfactant structure and composition of the monolayer onthe bending moduli. We find that increasing the thickness has a larger effect than increasing thedensity of the layer. This follows from the observations that at a given interfacial tension, thebending modulus increases with chain length and is larger for linear than branched surfactants. Theincrease with chain length is approximately linear, which is slower than the theoretical predictionsat a fixed area. We also investigated a binary mixture of short and long surfactants compared to purelayers of the same average chain length. We find a roughly linear decrease in bending modulus withmole fraction of short surfactants. Furthermore, the mixed film has a lower bending modulus thanthe corresponding pure film for all mole fractions. Linking the bending moduli to the structure of thesurfactants is an important step in predicting the stability of microemulsions. © 2004 AmericanInstitute of Physics. @DOI: 10.1063/1.1645509#