Study On Structure-Activity Relationship Of Interfacial Activities For Long-Chain Alkyl Imidazolines

Imidazoline and its derivatives are important compounds,which have low critical micelle concentration?CMC?and can effectively reduce interfacial tension,so imidazolines are commonly used as surfactants.In addition,the ability of imidazoline compounds,which can reduce the interfacial tension,is closely related to the molecular structure,such as lipophilic alkyl chain length and hydrophilic group size.Moreover,the molecular structure of imidazoline has significant effects on the interfacial activity,and thus affects its application performance.Therefore,investigation of the influence of the imidazoline structure on the interfacial activity has extremely important theoretical significance and practical application value in revealing the structure-activity relationship of imidazoline surfactants and developing new surfactants.In this paper,experimental and molecular simulation methods were used to systematically study the influence of the imidazoline structure on the interfacial activity.The quantitative structure-activity relationship?QSAR?models were constructed.The influence of imidazoline structure on the microscopic features,such as the micellar aggregation number,micelles configuration in aqueous systems and adsorption arrangement at the interface of oil-water system,were systematically discussed.The influence of imidazoline structure,the oil phase and concentrations on the interfacial thickness,interfacial tension and other interfacial properties,were also investigated to reveal the correlation with the macroscopic interfacial properties.Finally,the synthesized imidazoline as emulsifier were used in asphalt emulsion to study the influence rules of imidazoline structure on the properties of asphalt emulsion.15 kinds of alkyl imidazoline were synthesized by a solvent-vacuum process and the reaction conditions were optimized.The structure of the products were characterized by IR and NMR.The conversion rate of amide in the 15 imidazolines synthesis under the optimizing reaction conditions were all higher than 95%.Experimental results showed that 15 kinds of imidazolines have good interfacial activity.Experimental and computational results indicated that structures of lipophilic and hydrophilic group have a significant effect on the interfacial properties in the oil/water system.The carbon numbers of the lipophilic group T_C increases,the maximum surface excess_?of alkylimidazoline on the surface also increases,and the area occupied by each surfactant molecule S_?and the free energy of micellization?G_m decrease,so CMC and?_cmc decrease.lg CMC and T_C have a negative linear correlation.When the lipophilic groups are the same,as the hydrophilic groups increase,the maximum surface excess_?decreases,S_?and?G_m increase,so CMC and?_cmc increase.Then,54 kinds of molecular structure descriptors were selected as independent variables and the quantitative structure-activity relationship?QSAR?between the CMC and?_cmc of alkylimidazoline and molecular structure was built through regression analysis method of PCA,PLS and GFA.The results showed that the GFA-QSAR model was highly correlated with predicted values and experimental values of the CMC and?_cmc.The correlation coefficient R of CMC and?_cmc model were 0.9991 and 0.9864,respectly.The prediction error of 2 kinds of alkylimidazoline CMCs and?_cmcs in the Validation Set that quantitatively analyzed the influence of the alkylimidazoline molecular structure on the CMC was less than 4%,which would quantitatively analyze the influence of the alkylimidazoline molecular structure on the interfacial performance.Moreover,steady-state fluorescence probe method and dissipative particle dynamics?DPD?were used to investigate the self-aggregation behavior of 15 alkylimidazoline of different structures in aqueous systems.The relationship between microscopic properties of alkyl imidazoline micellar aggregates and macroscopic interfacial properties in aqueous solution was analyzed.DPD simulation and experimental results showed that,N_agg is strongly dependent on alkylimidazoline concentration and also significantly affected by the structure of the hydrophilic and lipophilic groups.The concentration of the alkylimidazoline has a significant influence on the configuration of the micelles generated.More specifically,alkylimidazolines of different concentrations?0.125 mol%to 15.0 mol%?were found to generate different micelles configuration in aqueous solution,such as spherical,rod-shaped,layered and interlaced structures.In addition,there is a linear correlation between the microscopic properties and macroscopic interfacial properties of imidazoline.The CMC and?_cmc have negative linear correlations with micellar aggregates number N₀,which is depended by the structures of imidazolines.Molecular dynamics?MD?and DPD studies showed that imidazoline structure,concentration,the oil structure and oil/water ratios have significant influence on the interfacial properties of the oil/water systems.The interfacial thickness of oil/water system increases while the interface formation energy decrease in the presence of imidazolines.The interfacial thickness also increases with the increasing of the imidazoline concentration and T_C,but decreases while the hydrophilic groups increases.The oil structure affects the interaction between oil and water as well as the the distribution of imidazoline in the oil/water interface and bulk phase,thereby affecting the interfacial properties of the oil/water system.When the oil with a relatively low heteroatom content and degree of unsaturation,the degree of polarity and solubility parameter are relatively smaller,leading to the thinner interfacial thickness and higher interfacial tension.Otherwise the interfacial thickness is thicker,so the interfacial tension is lower.Under the same conditions,the order of the interfacial thickness of the oil/water system is:A₂/water>A₁/water>toluene/water>dodecane/water,which is consistent with the order of the oil polarity.Interfacial tensions obtained by DPD simulation are well accord with the experimental results.?_DPD and?_Exp.decrease with the imidazoline concentration increasing,when the concentration is higher than 2.0 wt%,the interfacial tension changes slightly.Then,alkyl imidazolines were used as asphalt emulsifier to investigate the effect of imidazoline structure on the storage stability of asphalt emulsion.The results showed that it is mainly dependent on the impact of imidazoline structure on the interfacial tension of asphalt/water system.For the asphalt emulsion system,with the increasing of the chain length of the lipophilic group,the interfacial thickness of asphalt/water system increases,the interface formation energy and interfacial tension decrease,leading to better storage stability.On the contrary,with the increase of the hydrophilic group size,the interfacial thickness of asphalt/water system decreases,the interface formation energy and interfacial tension increase,leading to worse storage stability.However,when adding SBR into modified asphalt emulsion system,the change of the interfacial tension is not inconsistent with the change of lipophilic chain length and hydrophilic groups,because of the more complex composition of the oil phase resulting in much more impact factors on the storage stability.There is an associated relationship between the storage stability and interfacial tension of the asphalt emulsion.For asphalt emulsion and SBR modified asphalt emulsion system,storage stability of one day S₁shows good linear correlationship with the interfacial tension?_Exp,but the correlation between storage stability of 5 day and?_Exp is not obvious.Finally,based on the above study results,T₁₁H₅ and modified imidazoline MIM as emulsifiers were used to prepare modified asphalt emulsion for Micro-surfacing.Emulsified asphalt particle size has a homogeneous distribution under the optimum conditions and the performance of the asphalt emulsion can meet the technical requirements of Micro-surfacing.In particular,the asphalt emulsion prepared by MIM can significantly prolong the mixing time with aggregates.