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We have used dissipative particle dynamics (DPD) to simulate surfactant monolayers on the interface between oil and water. With a simple surfactant model, we investigate how variations in size and structure of surfactants influence their ability to reduce the interfacial tension. In particular, we studied the effect of branching of the hydrophobic tail. We found that branched surfactants are more efficient at the interface than linear ones only if the head groups are sufficiently hydrophilic to prevent the molecules from staggering. By combining DPD with a Monte Carlo method, we have imposed constant surfactant chemical potential and (normal) pressure in separate simulations of bulk and interface. From this, we can determine the bulk concentration needed to obtain a given interfacial tension. We found that higher concentrations of branched surfactants are required to obtain the same reduction of the interfacial tension. We argue that the stronger excluded volume interactions which make branched surfactants more efficient at the interface compared to their linear isomers at the same time make them less inclined to adsorb at the interface.

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