Interactions Between Oppositely Charged Polyelectrolytes And Surfactants:Molecular Dynamics Simulations

Polyelectrolytes and oppositely charged surfactants can form complexes which have many phase states such as solution, gel, solid and potential applications in many areas, such as rheological control, drug delivery, protein separation, functional materials preparation, detergency and pharmaceutical formulations, etc.The effect of hydrophilicity or hydrophobicity of polyelectrolyte on the interaction between polyelectrolyte and oppositely charged surfactants was investigated by using coarse-grained molecular dynamics simulations. The aggregation of surfactants on the hydrophilic polyelectrolyte is significantly different from that on the hydrophobic polyelectrolyte. The complexes evolve from the "bottle brush", through the "necklace", then to the micelle. However, the rod-like micelle, in which polyelectrolyte wraps around the micelle surface, only appears in the hydrophilic polyelectrolyte system. For the hydrophobic polyelectrolyte system, the spherical micelle is formed, and the polyelectrolyte penetrates into the hydrophobic core of complexes. The hydrophobic nature of the surfactant tails induces the surfactant’s tendency to depart from the hydrophilic polyelectrolyte and point toward the bulk phase, but it is apt to combine with the hydrophobic polyelectrolyte, leading to a parallel configuration between the surfactants and the polyelectrolyte. When the charge ratio (Z) of surfactant to polylelectrolyte is lower, the polyelectrolyte shows extended structure, and with the increase of Z, the polyelectrolyte collapse undergoes either a continuous or an abrupt change depending on if it is a hydrophobic or hydrophilic polyelectrolyte. At higher charge density of the hydrophilic polyelectrolyte, there is a synergistic effect of the electrostatic interaction between surfactant and polyelectrolyte, with the hydrophobic interaction among the adsorbed surfactants. For the hydrophobic polyelectrolyte system, no synergistic effect is observed.The effects of hydrophilicity or hydrophobicity of neutral block of the block polyelectrolyte on the structure of block polyelectrolyte/surfactant complex were also investigated using this model. In the diblock polyelectrolyte systems, the presence of the hydrophilic neutral block of the polyelectrolyte has no obvious effect on the structure of the complex. With increasing the concentration of the surfactant, the structure of complex formed by the charged block of the polyelectrolyte and surfactant varied from bottle-brush to micelle. However, for the polyelectrolyte with hydrophobic block system, the polyelectrolyte and surfactant formed3layers core-shell structure. For the triblock polyelectrolyte with charge block distributed in the two ends system, both the polyelectrolyte with hydrophilic and hydrophobic neutral block show the asymmetric adsoption ability to surfactant although the two charged block of the polyelectrolyte are symmetric. The asymmetry of adsorption ability disappears until the adsorption to the surfactant attained saturation. In the polyelectrolyte with hydrophilic neutral block system, the polyelectrolyte and surfactants formed complex with "basket" structure. However, in the polyelectrolyte with hydrophobic neutral block system, the polyelectrolyte and surfactants formed3layers core-shell structure which is similar to the diblock polyelectrolyte system. For the triblock polyelectrolyte with charge block distributed in the middle system, if both ends of it are hydrophilic block, the surfactants adsorpted in charge block of polymer and at high concentration the surfactants formed spherical micelle.If one end is hydrophilic and another is hydrophobic, the complex are "tadpole" like and the head of "tadpole" is3layers core-shell structure and the tail are hydrophilic block. If both ends of polymer are hydrophobic, the complex is3layers core-shell structure and the polymer show loop structure. Finally, the difference of adsorption ability to surfactant of alternative copolymer and random copolymer was investigated. It was found that for the polyelectrolyte with hydrophilic neutral block system they showed the similar adsorption ability to the surfactants, while in the polyelectrolyte with hydrophobic neutral block system notable differences was observed.The effects of charge number, diameter, surface charge density as well as concentration of macroions on the structure of poly electrolyte/surfactants complex were investigated using this model. It was found that the macroion with the same charges as polyelectrolyte has no obvious effect on the structure of the polyelectrolyte/surfactants complex. However, the macroion carried opposite charges to the polyelectrolyte can induce the release of surfactant from the polyelectrolyte, even lead to the complete disassembly of the polyelectrolyte/surfactants complex and the formation of the macroion/polyelectrolyte complex, and the inducing effect increases with the increase of the charge number, while for the microions with the same number of charges, the samller ones are much easier to cause the desorption of surfactant. However the opposite effect was found for the diameter at fixed charge density. The concentration of macroions also affects the structure of the complex greatly, the surfactants released from the polyelectrolyte increase with the macroions concentration and finally a macroion/polyelectrolyte complex with reversed charges can be formed.