Agregação entre copolímeros dibloco negativamente carregados e surfactantes catiônicos: um estudo computacional

Abstract

In this work Langevin Dynamics simulations have been performed to study the complexation between oppositely charged diblock copolymers and cationic surfactants, in solution with or without monovalent salt. Our methodology is based on using the ESPResSo open source software package. We use a model where the hydrophobic e↵ect present in solutions containing surfactants is mapped by a short-ranged e↵ective Lennard-Jones 12-6 attraction, between the tail monomers that are located at least on two di↵erent surfactant molecules. The intensity of this attraction is defined by the parameter " , which has the value adjusted in our simulations in order to reproduce the experimental critical micellar concentration of cationic surfactant DTAB. With the model and computational methodology, we were able to reproduce qualitatively the observed experimental copolymer/surfactant aggregates. Specifically, we have obtained the aggregate morphologies, analyzing their composition in terms of the surfactant to copolymer charge ratio Z. With no salt, for low Z values the aggregates that are formed are consistent with the experimental observations, that is, single surfactant micelles with one or a few adsorbed copolymers on their surfaces. Increasing Z (higher surfactant concentrations) new aggregates were observed. In the region close to the isoelectric point, Z =1,wefoundlargecopolymer/surfactant aggregates, produced by the aggregation of di↵erent micelles connected by the copolymer charged blocks that are adsorbed on their surfaces. We attribute to the responsible mechanisms for the formation of these aggregates the same basic principles for obtaining the experimental copolymer/surfactant complexes with core-shell structure. In the presence of monovalent salt, although the necessary mechanisms for large complexes formation have been found, even in a qualitatively perspective our results were not conclusive. For the simulated systems and salt concentrations that we analyzed, the salt e↵ect over the morphology of the complexes is not clearly defined, specially for the high Z region and high salt concentrations, since we were not able to reproduce the experimental observation of copolymer/surfactant complex dissolution.