Studies of the Effects of Surfactants on the Physical Properties and Fusion Process of Vesicles

Vesicles are spherical aggregates composed of lipid monomers. Its structure is similar to the cell membrane which exists in organisms. Vesicles are widely used for drug delivery carriers and simulation of cell membrane behavior due to their high biodegradability and biocompatibility. However, vesicles are affected by biosurfactants when they enter organisms, so the interactions between vesicles and surfactants have been an important work in studying vesicles. In this work, simulations based on dissipative particle dynamics are performed to study the interactions between vesicles and surfactants. The effects of surfactants on the physical properties and fusion process of vesicles in low surfactant concentration are investigated.
Simulation results show that surfactants distribute in the bulk solution and the bilayer of the vesicle. As the concentration of surfactants (φs) increases, the amount of surfactants into the membrane increases. A surfactant is a kind of amphiphile, and the incorporation of surfactants into the vesicle causes decrease in the surface tension of the vesicle membrane until the surface tension becomes zero. The order parameter and thickness of vesicle increase as surfactants enter into the vesicle initially, and the trend corresponds to that of spontaneously formed vesicles. After the surface tension of membrane equals zero, adding more surfactants disrupts the arrangement of lipid tails, and then makes the vesicle become deformed and ductile, so the thickness of membrane decreases.
In our research of surfactants with different hydrophobicity, surfactants with excessive hydrophobicity tend to enter into the vesicle, and therefore the effect on the vesicle is more severe. Surfactants with excessive hydrophobicity cause the surface tension of vesicle to become zero at relatively low concentration. In addition, the trend of order parameter and thickness of vesicle which increases initially and then decreases is more obvious, and the maximum occurs at relatively low concentration. Furthermore, we have found that there is a certain relationship between the physical properties of vesicle and the amount of surfactants on the membrane, i.e. the physical properties of vesicle depend on the amount of surfactants on the membrane irrespective of the surfactant''s hydrophobicity.
The number of surfactants per lipids is more in small vesicle. By fixing the number of lipids in the system and increasing the pressure inside the vesicle by adding water, it is found that the amount of surfactants on the membrane increases as the surface tension increases. As a consequence, the surface tension is a driving force of surfactants to go into the vesicle. Adding surfactants leads to decrease in the surface tension of vesicle. When the number of surfactants is enough for the surface tension to become zero, the order parameter and thickness decrease compared to the original vesicle. On the other hand, the order parameter and thickness increase when the number of surfactants is not enough for the surface tension to become zero. This phenomenon corresponds to the trend of spontaneously formed vesicle.
Fusion results show that the fusion time of vesicles which is affected by surfactants is longer as surfactants concentration increases. The surface tension of vesicle decreases as surfactants concentration increases, and this result is consistent to the previous studies, that is, vesicles of high surface tension fuse faster than those of low surface tension.