Molecular Dynamics Simulation Of “Qseudo-Gemini” Surfactant

Surfactants have been extensively applied in oil recovery,food processing,drug delivery and detergents due to their unique structures.The interfacial behavior of surfactant molecules has a significant effect on decreasing the surface tension.Generally,the decrease in surface tension of interface is attributed to the interaction between water and hydrophilic headgroup of surfactant at the interface.Therefore,understanding the relationship of surfactant architecture with interfacial properties is helpful in designing of new types highly efficient surfactant.In this paper,the micro aggregation behavior of nonylphenol substituted alkyl sulfonate at the interface was studied by molecular dynamics simulation,and the effect of surfactant structure on the interfacial behavior was systematically simulated.The main research results are as follows:（1）Fully atomistic molecular dynamics simulations have been performed on four isomers of a newly synthesized nonylphenol-substituted dodecyl sulfonate,briefly denoted as x-C₁₂-NPDS,with x=1,2,3,4.The aggregation behavior and the interfacial properties of the surfactant monolayer formed at the air/water interface are investigated by analyzing the density distribution,interfacial thickness,interface formation energy（IFE）,radial distribution function（RDF）,mean square displacement（MSD）and spatial distribution function（SDF）.The results show that the interface properties of 3-C₁₂-NPDS and 4-C₁₂-NPDS are significantly better than those of 1-C₁₂-NPDS and 2-C₁₂-NPDS.3-C₁₂-NPDS leads to the minimum surface tension and the lowest interface formation energy by having the most strong interaction with water.Simulation results suggest that the steric effect of surfactant headgroups plays a dominant role in the interfacial behaviors.The results of this work are useful for designing high performance surfactants.（2）Fully atomistic molecular dynamics simulations have been performed on four isomers of a newly synthesized nonylphenol-substituted dodecyl sulfonate,briefly denoted as x-C₁₂-NPDS,（with x=1,2,3,4）.The aggregation behavior and the interfacial properties of the surfactant monolayer formed at the n-decane/water interface were investigated by analyzing the density distribution,interfacial thickness,interface formation energy（IFE）,interfacial tension,hydrogen bond,the radial distribution function（RDF）,mean square displacement（MSD）,the spatial distribution function（SDF）.The results show that the interface properties of 3-C₁₂-NPDS and 4-C₁₂-NPDS are significantly better than those of 1-C₁₂-NPDS and 2-C₁₂-NPDS.3-C₁₂-NPDS leads to the largest interfacial thickness and the lowest interfacial tension by having the lowest interface formation energy.Simulation results suggest that the steric effect of surfactant headgroups plays a dominant role in the interfacial behaviors.The results of this work are useful for designing high performance surfactants.（3）The all-atomic molecular dynamics method was used to study the aggregation behavior of nonylphenol-substituted series of alkyl sulfonate surfactants（NPAS）at the decane/water interface.The effect of different sulfoalkyl chain lengths on the interfacial properties of nonylphenol-substituted alkyl sulfonate surfactants was investigated by analyzing the interface thickness（IT）,interface formation energy（IFE）,interfacial tension（IFT）,the radical distribution function（RDF）and coordination number.The results show that when the sulfoalkyl chain length is 12,the surfactant has the lowest interfacial tension,and the calculation results are consistent with the experiment.（4）Molecular dynamics simulation was used to investigate the aggregation behavior of monovalent cations（Li⁺,Na⁺,K⁺,Rb⁺,Cs⁺）on nonylphenol-substituted dodecyl sulfonate at the n-decane/water interface.By analyzing the density distribution,calculating the interface thickness,interfacial tension and the interaction between the polar head group of the surfactant and the oxygen atoms in the water,it is found that as the radius of the monovalent cation increases,the interface thickness gradually increases,the coordination number of the polar head group and the oxygen atom in the water is reduced.Studies have also shown that changes in the counter ion radius can affect the interfacial properties of the nonylphenol-substituted dodecyl sulfonate at the n-decane/water interface.