Monte Carlo simulation of the self-assembly process of nonionic surfactants (type H _X T _Y ) considering second-neighbor interaction between surfactant molecules

Abstract

Monte Carlo simulation for the self-assembly process of nonionic surfactant type H _X T _Y in water on a two-dimensional lattice, considering second-neighbor interactions, was carried out. The model surfactant molecules selected for the simulation were H ₁ T ₃, H ₁ T ₄,H ₂ T ₄,H ₂ T ₅,H ₃ T ₄, H ₃ T ₆ and H ₄ T ₇, and their aggregation behavior was evaluated between 0.4 and 3 vol% at a reduced temperature between 1 and 3. By setting second-neighbor interactions between surfactant tails at values lower than 20% of the interaction energy magnitude (ɛ) and a reduced temperature of t = 1, no surfactant aggregation was observed. At a value of 20% of ɛ, micelles were formed with aggregation numbers between 2 and 27. Higher values for the second-neighbor interactions induced a micellation process where dimers were the main species. The values of the critical micelle concentration obtained in the simulation were dependent on the head and tail length of the surfactant molecule following the tendencies reported in the literature. Increasing the reduced temperature to over 1.25 induced the destruction of the surfactant aggregates, suggesting that the present model is highly sensitive to such changes. The simulation also permitted calculation of Gibbs free energies for the micellation process from the micelle size distribution function.

Acknowledgements: We thank the Comité para el Desarrollo de la Investigación of the Universidad de Antioquia, and Instituto Colombiano para el Desarrollo de las Ciencias.