Interfacial Adsorption And Phase Behavior Of Liquid Mixtures

The interfacial adsorption of liquid mixtures is one of the basic phenomena studied in the field of colloid and interfacial chemistry.However,due to its complexity,there is still a lack of feasible thermodynamic models to accurately predict or describe the interfacial adsorption behavior.It is essential to establish a general thermodynamic model for the interfacial adsorption to deepen the understanding of the interfacial phenomena.Surfactant-free microemulsions(SFMEs)are a new type of colloidal dispersions,which are formed from oil(apolar component),water(polar component),and "amphi-solvent" in the absence of traditional surfactants.The understanding of SFMEs is very superficial so far,and some basic issues such as their formation law,microstructure,and thermodynamic nature need to be explored.Studying the interfacial adsorption of liquid mixtures can provide information for revealing the formation mechanism and microstructure of SFMEs.In this work,the interfacial adsorption of liquid mixtures was studied and the thermodynamic adsorption model was established.The phase behaviors of the n-butanol/ethanol/water ternary system and the n-dodecane/(n-butanol-ethanol)/water pseudo-ternary system were investigated,and special emphasis was placed on the formation and microstructure of SFMEs.It is expected to deepen the understanding of the interfacial adsorption phenomena and the SFMEs,to provide information for revealing the formation mechanism of SFMEs.Main research contents and conclusions:(1)Analysis on the surface tension isotherm types and the existing adsorption models for liquid mixturesAccording to the change of surface tension(a)with the bulk phase composition,the surface tension isotherms(σ-isotherms)of liquid mixtures were divided into Langmuir-type(Ltype)and sigmoid-type(S-type),the former including LⅠ-type and Ln-type,and the latter including Si-type and SⅡ-type.Different types of isotherms have different limiting slopes at both ends of the composition range.The application scope of existing thermodynamic adsorption models was analyzed,and the results showed that all of them can describe the L-type isotherms,but most of them cannot describe the S-type isotherms.The extended Langmuir model can describe the S-type isotherms,but the agreement between its prediction and the experimental data is not good for some binary liquid mixtures.Therefore,it is necessary to develop new thermodynamic models to accurately describe the S-type isotherms.(2)Surface aggregation adsorption model for binary liquid mixturesBased on the adsorption equilibrium principle and the surface micellization model,a thermodynamic adsorption model,called the surface aggregation adsorption(SAA)model.was developed for binary liquid mixtures.By coupling the SAA model with the modified Eberhart model,a two-parameter equation was derived,which correlates σ and the surface composition with the bulk composition.Its two parameters,namely,the adsorption equilibrium constant(K)and the average aggregation number(n).have rigorous physical meanings and can be estimated by linear fitting of experimental σ data to obtain unique values.The rationality of the SAA model was examined using the σ data of binary mixtures.The results indicate that the new model can accurately describe the S-and L-type isotherms of binary liquid mixtures,showing a good universality.In addition,this model suggests that the Sand LⅡ-type isotherms arise from the surface aggregation(n≠1).Analysis of the standard molar Gibbs free energy of surface adsorption suggests that the order of adsorption tendency is LⅠtype>>SⅠ-type≈SⅡ-type>LⅡ-type.(3)A model for predicting the surface tension and composition of multicomponent liquid mixturesBased on the SAA model and the assumptions that surface aggregation adsorption occurs and the adsorptions of various components are independent of each other,a thermodynamic adsorption model,also called the SAA model,was developed for multicomponent liquid mixtures.It relates the surface composition(φi,s)with the bulk composition(φi,b).By coupling with the extended Eberhart equation,the model can predict the surface tension(σ)and φi,s of multicomponent liquid mixtures using the model parameters of corresponding binary systems.The model was examined by using σ data of ternary systems.A good agreement between the model prediction and the experimental data was observed,indicating the model is reasonable(at least for the ternary systems).The model can be used to analyze the change of the surface concentration of each component with the bulk composition to obtain information on the surface adsorption.(4)Phase behavior and interfacial adsorption of n-butanol/ethanol/water ternary systemThe phase behavior of the ternary mixture of n-butanol(oil phase),ethanol(amphisolvent),and water was investigated.The ternary phase diagram shows a single-phase homogeneous region and a multiphase region.The mesoscale structuring was observed within the macroscopically homogeneous single-phase zone near the phase boundary using dynamic light scattering(DLS)and small-wide angle X-ray scattering(SWAXS)techniques,indicating that SFMEs are formed in the ternary mixture.In addition,the electrical conductivity and micropolarity of the single-phase mixtures were determined,suggesting the existence of three types of microstructures,i.e.,oil-in-water(O/W),bicontinuous(BC),and water-in-oil(W/O),for the SFMEs,which can be transformed into each other depending on the system composition,similar to the case of traditional surfactant-based microemulsions(SBMEs).The closer the system composition is to the phase boundary and critical point,the larger the size of droplets formed.The W/O droplets exist in the form of ethanol-water aggregates.The region of SFMEs and the subregions corresponding to the three microstructures were roughly identified in the ternary phase diagram.n-Butanol is one of the extremely weak hydrophobic substances,and the detection of microemulsions in the mixture indicates that the formation of SFMEs is a universal phenomenon.The SAA model was used to analyze the surface tension of n-butanol/ethanol/water ternary homogeneous system.and an obvious systematic deviation of the model prediction from the experimental data was observed,which corresponds to the formation of SFMEs,indicating that the SAA model can be used to distinguish the formation of micro/mesoscopic structures in bulk phases.In addition,based on the SAA model and the assumption that the adsorptions of various components are independent of each other,a interfacial aggregation adsorption(IAA)model was established for the two-phase equilibrium system of ternary liquid mixtures(containing oil.water,and amphi-solvent).The model can predict the change of the interfacial composition with the bulk composition using the model parameter values of corresponding binary homogeneous systems.obtaining information on the interfacial adsorption(enrichment).The interfacial adsorption of the two-phase equilibrium system of n-butanol,ethanol,and water was analyzed using the IAA model,showing an obvious enrichment of ethanol at the oil/water interface,which is conducive to the formation and stability of SFMEs.The results provide information for studying the formation mechanism of SFMEs.(5)Phase behavior of n-dodecane/(ethanol/n-butanol)/water pseudo-ternary systemThe phase behavior of the pseudo-ternary mixture of strongly hydrophobic n-dodecane(oil),the ethanol/n-butanol solution(called mixed-alcohol,used as the amphi-solvent),and water was investigated.Interestingly,it was found that the pseudo-ternary mixture can form the three-phase equilibrium systems,namely,containing a lower,a middle,and an upper phase.The microstructure of each phase was analyzed,showing that the microemulsion is formed in the middle phase.This indicates that the Winsor Ⅲ microemulsion(i.e.,the middle-phase microemulsion)can be formed from oil,water,and amphi-solvent(at least mixed-alcohol amphi-solvent)in the absence of traditional surfactants.The middle-phase SFMEs have not been reported in the literature,and the finding expands the understanding of SFMEs.However,the formation conditions and mechanism of the middle-phase microemulsions remain to be further studied.