PH-responsive Adsorption At Air-water Interface And Self-assembly In Aqueous Solution Of Monorhamnolipids: Molecular Dynamics Simulations Study

Rhamnolipid is one of the widely used glycolipids in many applications due to the complex physicochemical properties.The carboxylic acid adds to the possibility to adjust the properties of rhamnolipids at different pH values.Rhamnolipids can exhibit the pH-responsive adsorption at air-water interface and self-assembly in aqueous solution.The molecular orientation,density distribution and Hydrogen bonding interaction are all changeable with the change of pH.In addition,the equilibrate structures of rhamnolipids are transformed at different pH values,which increases the complexity and functionality of rhamnolipids and makesthem promising precursors for application in advanced material science.Molecular dynamics simulation has been used to study the effect of pH on the molecular orientation,density distribution,hydrogen bonding interaction and surface tension of monorhamnolipids(monoRL).Results show monoRLs move into deeper liquid phase and the interfacial thickness increases due to the increased hydrophilicity.The orientation of rhamnose rings becomes disordered and scloping with increasing pH.This is because the number of hydrogen bonds between rhamnose rings and water molecules lessens with the increase of pH.The title and twist angles of hydrophobic tails become larger due to the effect of rhamnose rings on the orientation of hydrophobic tails.The surface tension increases with increasing pH due to the uneven force of monorhamnolipids and low interfacial density at the air-water interface.Dissipative particle dynamics(DPD)simulation has been used to study the effect of pH on the morphology transition of micelles assembled by monorhamnolipids(monoRLs).Results show that micellar structures and transition modes with increasing mass concentrations are multiform due to the changeable hydrophilicity of pH-responsive beads at different pH levels.Various chaotic multilayer aggregations of monoRLs are observed at low pH(pH < 4.0)whereas well-ordered single-layer structures are obtained at high pH(pH >7.4).At medium pH region(4.0 < pH < 7.4),morphologies with semi-chaotic structures will transform from multilayer to single layer with increasing pH due to the hydrophilicity increase of micelles.In addition,three micellar transition modes are built by varying the hydrophilicity of pH-responsive beads and validated by the growth rates of solvent accessible surface areas(SASAs)as a function of mass concentrations.