Differences in free surfactant concentration and aggregation properties for amphiphiles with the same critical micelle concentration

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The impact of surfactant architecture on aggregation behavior is investigated using grand canonical Monte Carlo simulations of a lattice surfactant model. Many prior studies have observed differences in macroscopic surfactant system properties or in aggregation behavior for diblock surfactant architectures with varying head length, tail length, and head-to-tail ratio. However, the critical micelle concentration (cmc) also varies with these parameters, thus complicating the comparisons. In the present study, we separate the effects of varying cmc and varying architecture by comparing properties of amphiphiles with the same cmc using a simple lattice nonionic surfactant model in implicit solvent. We find that architectural variables such as the head length and the head-to-tail ratio do have a significant effect on structural micellar properties. We also identify differences in free surfactant concentrations, even when calculated in the volume accessible to them in solution. We modify a previously proposed correction to the free surfactant concentration in order to obtain the cmc from simulations at high concentrations. The differences in aggregation behavior reported using this model can be extended to differences in observed macroscopic properties for real surfactant systems.