The Interaction between Surfactants and Lipid Aggregates: a Dissipative Particle Dynamics Study
Date Issued
2009
Date
2009
Author(s)
Chang, Geng-Pei
Abstract
A liposome is a vesicle composed of lipid monomers. Its structure is similar to the permeable cell membrane which exists in organisms. Liposomes are considered ideal drug-carriers due to their high biodegradability and biocompatibility. However, many kinds of biosurfactant existing in human body could decrease vesicle stability, so the reaction between vesicles and biosurfactants has been highly concerned by researchers. In this work, simulations based on dissipative particle dynamics are performed to study the interactions between vesicles and surfactants. The effects of concentration and hydrophobicity of surfactants on the structure and stability of vesicles are investigated.imulation results show that as the concentration of surfactants (s) increases, vesicle increases in size and remains spherical structure at first. Then the surfactants in the bulk solution start to aggregate, i.e. surfactant concentration in the bulk phase reaches its critical micelle concentration (scmc). After that vesicle begins to perforate (sp). Further addition of surfactants results in the beginning of the dissolution process (sd). The lipid molecules start to leave the vesicle and join the surfactant micelles to form mixed micelles. Eventually the total collapse of the vesicle is observed. Therefore, scmc<sp <sd.e have also found that surfactants with greater hydrophilicity can dissolve vesicles more easily. The resulting mixed micelles are comparably small in size and distribute homogeneously with the solution. However, surfactants with excessive hydrophilicity tend to stay in the bulk solution and only a few of them enter into the vesicle. As a consequence, the structure of the vesicle remains intact for all the surfactant concentrations we have studied. On the other hand, surfactants with extreme hydrophobicity tend to go into the vesicle, therefore, the size of the vesicle continues to grow as s increases. Instead of forming discrete mixed micelles, the lipid molecules and surfactants become one large aggregate taking the shapes of cylinders, donuts, bilayers, etc.esicles formed by lipid molecules with higher hydrophobicity possess greater surface tension energy and thus have a smaller inner water zone. The effect of the addition of surfactants on the surface tension of the vesicle membrane is also studied in this work. Two analyses have been made: direct calculation of the surface tension of the vesicle and increase of the pressure inside the vesicle by adding water. It is found that the incorporation of surfactants into the vesicle leads to decrease in the surface tension of the vesicle membrane.e have also studied the physical properties of the lipid-surfactant mixed micelles. As surfactant concentration increases, number of aggregates decreases. This result indicates that the addition of surfactants enhance the aggregation of lipid molecules which is counter-intuitive. In addition, the incompatibility between the hydrophobic tails of surfactants and lipids give rise to segregation within the mixed micelles. The degree of segregation goes through a first-order transition as incompatibility enhances.
Subjects
DPD
lipid
vesicle
surfactant
mixed-micelle
solubilization
surface tension
Type
thesis
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