Phase Separation Mechanism In The Miscibility Gap And Ion Specific Effects On The Aggregation Of Colloidal And Surfactant

The processes of phase separation in miscibility gap,colloidal gelation and aging of foam are composed of multiple phases that are incompatible,relating to the phase separation and their following stability.The study of phase separation process is necessary as it determines the structure of the final material and their properties.There are two kinetics processes in the miscibilty gap: nucleation-growth?NG?and spinodal decomposition?SD?,however,their effects on the phase separtion are seldom studied.The shell phase selection and the relationship between formation ability of core-shell structure and concentration also need to be clarified.Meanwhile,the macroscopic gelation kinetics and foam stability have been investigated widely,but the salt-specific effects on the gelation process and foam stability are not clear.In this thesis,the transparent immiscible system is used to determine the effects o f NGand SD-type kinetics on phase separation.The shell phase selection and the formation ability of core-shell versus concerntration are also investigated.Meanwhile,different types of salt are used to explore their effects on the colloidal gelation process and foam stability.The main reaults achieved are as below:1)The effects of nucleation growth and spinodal decomposition on the phase separation were determined.SCN-H₂ O transparent system was used to realize the processes of nucleation growth?NG?and spinodal decomposition?SD?.It is found that nder the thermal graidient condition,both types of phase separation can produce layer structure,but their final morphologies are different.This is due to the different initial kinetics and the volume fractions of the minority phase.The structure in the melt spinning ribbons of Fe-Sn alloys showed the different effects of NG and SD to the phase separation,which is similar to that in the transparent system.2)The shell phase selection and the the relationship between formation ability of core-shell structure and the alloy concentrate were examined.Compared to the process of phase separation in transparent system,it is found that the low melting point phase always forms the shell phase owing to its lower surface energy in the metallic system with miscibility gap under the containerless processing condition.This is one key factor to produce double-or triple-layer core-shell structure.In addtion,the formation of this kind of structure is related to the alloy concentration,and it is obtained in the alloys with composition near and on the left side not far away from the critical point.3)The salt-specific effects on the gelation process were investigated through adding NaCl or KCl to a suspension of colloidal silica.It was found that the gelation proceeds much faster in the presence of KCl than NaCl,while the final gels are very similar in structure and properties.The characteristic times of the early aggregation process and the formation of a stress-bearing structure with both salts are shown to collapse onto master curves with single multiplicative constants.The influence of the salt type and concentration is confirmed to be mainly kinetic,as the static structure factors and viscoelastic moduli of the gels are shown to be equivalent at normalized times.This suggests that the differences between gels generated by different salts are only transient in time.The results of light transmission method suggests that at least the scaling behavior with NaCl or KCl can be extended to more monovalent salts and even to divalent ones.In addition,it is found that the ion specific effects on the gelation process following the Hofmeister series no matter monovalent salt or bivalent one.4)The ion specific effects on the foam stability were determined.Ultrastable foams are made very simply by adding salt?NaCl or KCl?to sodium dodecyl sulfate.The addition of high concentrations of salt leads to the precipitatio n of the surfactant on the bubble surfaces and as crystals in the interstices between the bubbles.As a consequence,the ageing of the foams is stopped to make them stable indefinitelyThe use of KCl is shown to be much more effective than that of NaCl beca use potassium dodecyl sulfate has a higher melting temperature and faster rates of crystallization.